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2.5. PM2.5 Composition, Sources, and Air Concentrations
Composition of PM
Industrial emissions Major sources
Composition of ultrafines Motor vehicles and elemental
carbon Secondary vs primary aerosol
Local vs distant sources Exceedances in SE
Diurnal and seasonal variability
Radiative and optical properties
Internal mixing of constituents
Urban plumes under stagnation conditions
Urban plumes under advective conditions
Ozone transport from urban to rural areas
2.5. PM2.5 COMPOSITION, SOURCES, AND AIR CONCENTRATIONS Paul
Solomon and David Allen
SOS was selected by EPA’s Office of Air Quality
Planning and Standards (EPA/OAQPS) to establish the
first of two initial Supersites. The other initial Supersite
was in Fresno-Bakerfield, CA. Both sites were part of
EPA's effort under the leadership of Paul Solomon and
Richard Scheffe to increase the nation's capacity to
monitor both coarse (PM10) and fine (PM2.5) fractions in
a reliable way in various parts of the United States.
The Atlanta Supersite Experiment was established
under the leadership of SOS' Chief Scientist, Bill
Chameides. SOS brought together in Atlanta during
August 1999, the most comprehensive array of
particulate-matter measurement instruments ever
assembled in the US and about 150 of the nation’s most
competent aerosol research scientists and a few from
abroad.
David Allen of the University of Texas in Austin, TX and Matt
Fraser of Rice University in
Houston, TX also were selected by EPA/OAQPS to lead the Houston
Supersite, one of eight
EPA Supersites around the United States that are part of EPA’s
PM Supersite Program – a
cooperative endeavor with other carefully selected public-health
and regional-haze investigators.
Here very detailed, chemical and physical characterization
measurements of ambient coarse and
fine aerosols were made in all months of the year (2001-2002) at
multiple, carefully selected
urban and rural locations in southeast Texas.
The objectives of both the Atlanta and Houston Supersite
programs were to “advance
scientific understanding of atmospheric processes regarding
formation and accumulation of PM.”
The Atlanta Supersite Experiment was located on Jefferson Street
in Atlanta, GA at a mixed
commercial and industrial area about 8 km west of the city
center. Here detailed PM
characterization measurements had been made routinely as a part
of EPRI’s and the Southern
Company’s SEARCH and ARIES programs for more than a year. An
extraordinarily diverse
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variety of instruments were assembled at this site for
comparison purposes and to determine the
mass, particle size distribution, chemical composition of
individual particles, and chemical
composition of hourly-collected filter samples collected on all
days of the week, and
simultaneous gas and particle measurements.
Scientific findings are summarized below – mainly from the
Atlanta Supersite Experiment in
August 1999, year-round, rural and urban measurements at the
Houston Supersite in southeast
Texas, and both January and July measurements at a rural site
near Anderson, SC.
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2.5.1. Composition of PM in the Southeastern US The major
components of total PM mass on average for the urban Atlanta
Supersite study
[and both rural and urban locations within the Houston Supersite
program] were: organic
material (~35%) [25-30%], sulfate (~34%) [30-40%]; ammonium
(~12%) [7-10%], elemental
carbon (~3%) [2-5%], nitrate (~2%) [1-4%], and crustal material
(~3%) [
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• Sulfate, ammonium ion (which neutralizes the sulfate ion),
organic carbon, and elemental carbon are the major constituents of
PM2.5; the annual average concentrations of these major components
were spatially homogeneous across southeast Texas.
• Nevertheless, localized events with high mass fractions of
sulfate or carbon occurred frequently at many monitors in this
region.
• Concentrations of sulfate were slightly higher in the spring
and late fall than in the summer; carbon concentrations were
highest in the late fall.
• High organic-carbon to elemental-carbon ratios suggest that
much of the carbonaceous material in PM2.5 in southeast Texas is
not emitted directly, but is formed in the air through reactions
involving both gaseous biogenic and anthropogenic VOC
emissions.
KEY CITATIONS: Husain, H. and C. Christoforou. 2003.
Concentration and Chemical Composition of PM2.5 Particles at a
Rural Site
in South Carolina. SOS Final Report. Clemson University. 35 pp.
Modey, W.K., E.J. Eatough, Y. Pang, and N.L. Eatough. 2004.
Performance and evaluation of the PC-BOSS for
fine PM2.5 sampling during the summer EPA Supersite Program in
Atlanta. J. Air Waste Manage. Assoc. (in press).
Russell, M.M. and D.T. Allen. 2004. Seasonal and spatial trends
in primary and secondary organic carbon concentrations in southeast
Texas. Atmos Environ. 38:3225-3239.
Russell, M.M., D.T. Allen, D.R. Collins, and M.P. Fraser. 2004.
Daily, seasonal and spatial trends in PM2.5 mass and composition in
southeast Texas. Aerosol. Sci. Technol. 38(S1):14-26,
doi:10.1080/02786820390229138.
Solomon, P.A., W. Chameides, R. Weber, A. Middlebrook, C.S.
Kiang, A.G. Russell, A. Butler, B. Turpin, D. Mikel, R. Scheffe, E.
Cowling, E. Edgerton, J. St. John, J. Jansen, P. McMurry, S.
Hering, and T. Bahadori. 2003b. Overview of the 1999 Atlanta
Supersite project. J. Geophys. Res. 108(D7), 8413,
doi:10.1029/2001JD001458.
Weber, R., D. Orsini, A. Sullivan, M. Bergin, C.S. Kiang, M.
Chang, Y.N. Lee, P. Dasgupta, J. Slanina, B. Turpin, E. Edgerton,
S. Hering, G. Allen, P. Solomon, and W. Chameides. 2003. Transient
PM2.5 aerosol events in metro Atlanta: Implications for air quality
and health. J. Air Waste Manage. Assoc. 53:84-91.
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2.5.2. Sources of Fine Particles in Houston, TX Fresh
hydrophobic ultrafine particles are emitted by industrial sources
in the Ship Channel
area of Houston; they grow in size and become more hydrophilic
as they grow.
Particle growth within the VOC-rich ship channel plume exceeded
that expected solely from
SO2 oxidation. But particle growth within the plume of the
Parish power plant was generally
consistent with condensation of the oxidation products of SO2
when the plume did not pass over
substantial sources of VOCs.
There are five major sources of primary PM2.5 emissions in
southeast Texas. They include:
1) mobile sources, 2) cooking of foods, 3) point sources, 4)
geological sources, and 5) wild fires
and open burning.
• Primary mobile-source emissions are significant; evidence
suggests that these emissions account for about 25-35% of PM2.5
mass in SE Texas. In fact, diesel engines in heavy duty trucks,
trains, and farm or construction equipment, and gasoline engines in
cars, trucks, boats, and hand tools, as well as jet-fueled
aircraft, account for most primary emissions of PM2.5 in southeast
Texas.
• Primary emissions from cooking of foods are significant in all
urban areas; evidence suggests that these emissions account for
about 10-15% of PM2.5 mass in urban areas.
• Point sources of primary PM10 particles are significant, but
point-sources of primary PM2.5 particles have not yet been
quantified. Thus, additional research is needed to determine the
importance, size distributions, and chemical compositions of these
PM2.5 primary emissions.
• Geological sources (wind-blown dust) are a relatively minor
contributor to the total mass of PM2.5.
• Fires are a sporadic, but significant source of primary PM2.5
emissions in Texas. On an annual average basis, they contribute
about 1-2% of the total mass of fine particles in the
Houston-Galveston area; but these emissions tend to be concentrated
on specific days with fire events.
KEY CITATIONS: Allen, D.T. 2002. Particulate Matter
Concentrations, Compositions, and Sources in Southeast Texas: State
of the
Science and Critical Research Needs. Report to the Texas
Environmental Research Consortium. 93 pp.
http://www.harc.edu/harc/Projects/AirQuality/Projects/Status/Reports.aspx
Brock, C.A., M. Trainer, T.B. Ryerson, J.A. Neuman, D.D.
Parrish, J.S. Holloway, D.K. Nicks, Jr., G.J. Frost, G. Hübler,
F.C. Fehsenfeld, J.C. Wilson, J.M. Reeves, B.G. Lafleur, H.
Hilbert, E.L. Atlas, S.G. Donnelly, S.M. Schauffler, V.R. Stroud,
and C. Wiedinmyer. 2003. Particle growth in urban and industrial
plumes in Texas. J. Geophys. Res. 108(D3), 4111,
doi:10.1029/2002JD002746.
NOAA Aeronomy Laboratory. 2003. Texas 2000 Air Quality Study -
Phase II Analysis of NOAA Data. Final Report to Texas Commission on
Environmental Quality Houston/Galveston Air Quality Science
Evaluation. 158 pp.
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30 June 2004 | State of SOS-3: 1995 - 2003 | 84
ftp://ftp.tnrcc.state.tx.us/pub/OEPAA/TAD/Modeling/HGAQSE/Contract_Reports/Data_Analysis/TexAQS2000_NOAA_Data_Analysis.pdf
2.5.3. Emission Sources of PM2.5 and PM10 in Atlanta, GA A source
identification technique called positive matrix factorization was
used with daily
integrated particulate matter data on mass concentration and
composition collected in Atlanta
between August 1998 and August 2000. For PM2.5 particles, eight
major types of emission
sources were identified: 1) SO42--rich secondary aerosol sources
(56%), 2) motor vehicle sources
(22%), 3) wood smoke sources (11%), 4) NO3--rich secondary
aerosol sources (7%), 5) mixed
cement kiln and organic carbon sources (2%), 6) airborne soil
sources (1%), 7) metal recycling
facilities (0.5%), and 8) a miscellaneous source that includes
bus stations and metal processing
facilities (0.3%). Invariably, NH4+ [presumably mainly from
agricultural sources] was
associated with both the SO42--rich and NO3--rich secondary
aerosols.
For PM10 particles, five major types of sources were identified:
1) airborne soil sources
(60%), 2) NO3--rich secondary aerosol sources (16%), 3)
SO42--rich secondary aerosol sources
(12%), 4) cement kiln facilities (11%), and 5) metal recycling
facilities (1%).
Summary finding from this work can be stated:
• Sulfate-rich secondary aerosol was the primary contributor to
Atlanta PM2.5 mass; airborne soil was the largest primary source of
PM10 particle mass in Atlanta.
KEY CITATION: Kim, E., P.K. Hopke, and E.S. Edgerton. 2003.
Source identification of Atlanta aerosol by positive matrix
factorization. J. Air Waste Manage. Assoc. 53:731-739.
2.5.4. Composition of Ultrafine Particles in Atlanta, GA • The
composition of the ultrafine (less than 100 nm) particles was
dominated by carbon
compounds. The major composition classes (expressed as
percentage of particle mass) were: organic carbon (~74%), potassium
(~8%), iron (~3%), calcium (~2%), nitrate (~2%), elemental carbon
(~1.5%), and sodium (~1%).
• The total mass of ultrafine particles (
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Bahadori. 2003b. Overview of the 1999 Atlanta Supersite project.
J. Geophys. Res. 108(D7), 8413, doi:10.1029/2001JD001458.
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2.5.5. Elemental Carbon in PM2.5 Time-resolved ambient
particulate organic and elemental carbon were measured during
the
Atlanta Supersite Study using five different instruments, in
order to investigate temporal trends
of carbon-containing aerosols and to determine the contributions
of primary and secondary
organic carbon to particulate organic carbon. Major findings of
this work are summarized
below.
• In spite of the large uncertainties inherent in measuring
carbon-containing particulate matter, which is very complex in
composition, and in utilizing different operational techniques for
measurement, there was generally good agreement between measurement
systems.
• Organic matter and elemental carbon comprised ~40% and ~8%,
respectively, of PM2.5 mass on average during the August 1999
Atlanta Supersite Study.
• Motor vehicles were indicated as a primary emission source of
elemental carbon in Atlanta, using carbon monoxide as a tracer.
Elemental carbon concentrations tended to peak at 0600-0900 EST,
which is probably indicative of motor vehicle emissions, and had a
much smaller peak in the evening.
KEY CITATIONS: Lim, H.-J. and B.J. Turpin. 2002. Origins of
primary and secondary organic aerosol in Atlanta: Results of
time-resolved
measurements during the Atlanta Supersite Experiment. Environ.
Sci. Technol. 36:4489-4496. Lim, H.-J., B.J. Turpin, E. Edgerton,
S. Hering, G. Allen, H. Maring, and P. Solomon. 2003.
Semicontinuous aerosol
carbon measurements: Comparison of Atlanta Supersite
measurements. J. Geophys. Res. 108(D7), 8419,
doi:1029/2001JD001214.
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2.5.6. Secondary Formation of Organic Aerosols Contributions of
primary emissions and secondary aerosol formation to measured
organic
carbon in Atlanta in August 1999 were estimated to be roughly
equal. Secondary organic carbon
estimates are estimated to have a + 10% uncertainty.
Poor correlation between elemental carbon and organic carbon in
Atlanta in summer is
indicative of secondary organic aerosol formation. The ratio of
organic carbon to elemental
carbon closely tracked daytime variations in ozone
concentrations. Thus, the ratio increased
during the afternoon in correspondence to photochemical
activity.
Ratios of organic carbon to elemental carbon suggest that much
of the carbonaceous material
in southeast Texas is secondary organic aerosol, that is, it is
formed in the atmosphere as the
result of the reactions of gas phase VOC emissions. Point
sources and area/non-road emissions
are the dominant contributors to formation of anthropogenic
secondary organic aerosol in the
Houston urban area, but on a regional basis, biogenic emissions
of secondary organic aerosol
precursors overwhelm anthropogenic sources.
Radiocarbon dating (14C/13C ratios) indicates that at some
suburban and rural locations in
southeast Texas, formation of secondary organic aerosol is
dominated by reactions involving
biogenic emissions. These locations are primarily north and
southwest of Houston’s urban core.
• Secondary formation of organic aerosols tended to be large
compared to primary emissions. This was true at both the Atlanta
and Houston Supersites
KEY CITATIONS: Dechapanya, W., M.M. Russell, and D.T. Allen.
2002. Estimates of anthropogenic secondary organic aerosol
formation in Houston, Texas. Aerosol Sci. Technol.
38(1):156-166, doi:10.1080/02786820390229462. Lemire, K.R., D.T.
Allen, G.A. Klouda, and C.W. Lewis. 2002. Fine particulate matter
source attribution for
Southeast Texas using 14C/13C ratios. J. Geophys. Res. 107(D22),
4613, doi:10.1029/2002JD002339. Lim, H.-J. and B.J. Turpin. 2002.
Origins of primary and secondary organic aerosol in Atlanta:
Results of time-
resolved measurements during the Atlanta Supersite Experiment.
Environ. Sci. Technol. 36:4489-4496. Lim, H.-J., B.J. Turpin, E.
Edgerton, S. Hering, G. Allen, H. Maring, and P. Solomon. 2003.
Semicontinuous
aerosol carbon measurements: Comparison of Atlanta Supersite
measurements. J. Geophys. Res. 108(D7), 8419,
doi:1029/2001JD001214.
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2.5.7. Local and Regional Sources of PM2.5 in Southeast
Texas
Sulfate makes up about 30-40% and organic carbon and elemental
carbon make up about 25-
30% of the total, annual average, PM2.5 mass in southeast Texas.
Point sources of SO2 emissions
are the dominant source of locally generated sulfate in PM2.5.
There is also evidence for
significant local sources of the carbonaceous material found in
PM2.5. The sulfate in PM2.5 often
is not completely neutralized; thus ammonia emissions influence
the total mass of PM2.5. The
dominant source of ammonia in Texas is cattle and other
livestock; in most of the state’s urban
areas, on-road sources and domestic activities (use of cleaning
products, human perspiration and
respiration, human wastes and pet wastes) dominate ammonia
emissions.
But it appears that not all of the sulfate or carbonaceous
material observed in PM2.5 in
southeast Texas is emitted or formed locally. Examination of the
spatial distributions of PM2.5
and composition and analysis of air parcel back trajectories
indicate that:
• When high concentrations of fine particulate matter mass,
sulfate and organic carbon are observed throughout southeast Texas,
back-trajectory analyses of these air parcels often indicate high
concentrations of background sulfate and organic carbon in PM2.5
that extend far upwind. This suggests that much that much sulfate
and carbonaceous aerosol is transported into southeast Texas from
the eastern half of North America.
• But, high concentrations of fine particulate matter mass and
organic carbon are observed at isolated monitors in southeast
Texas, suggesting local source contributions are important on some
days.
KEY CITATION: Russell, M.M., D.T. Allen, D.R. Collins, and M.P.
Fraser. 2004. Daily, seasonal and spatial trends in PM2.5 mass
and composition in southeast Texas. Aerosol. Sci. Technol.
38(S1):14-26, doi:10.1080/02786820390229138.
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2.5.8. Concentrations of PM2.5 Mass in the Southeastern US Data
collected and analyzed in connection with the Atlanta Supersite
Experiment lead to the
following conclusions:
• The average air concentration of PM2.5 mass during the August
1999 Atlanta Supersite Experiment was 31.3 µg m-3, with a peak
value of 47.2 µg m-3. Thus, the 24-hour PM2.5 standard was not
exceeded. Interestingly, the 1-hr ozone standard was exceeded on
several days, for multiple hours, during the study. Sulfate and
ammonium ion concentrations were well correlated with PM2.5 mass;
but organic carbon and elemental carbon concentrations were not
very well correlated.
• Monitoring data from January 2001 to February 2002 at the
Atlanta Speciation Trends Network site showed an average total
PM2.5 concentration in the winter of 12 µg m-3, and 20 µg m-3 in
the summer, with the highest 24-hour average totaling µg m-3.
• Annual PM2.5 mass concentrations measured from March 1999 to
February 2000 in the ASACA study in Atlanta exceeded the annual
NAAQS of 15 µg m-3 at all four monitoring sites, with annual
averages ranging from 19.3 to 21.2 µg m-3. One site violated the
daily NAAQS of 65 µg m-3.
Analyses of PM measurements at a rural site during summer and
winter indicate:
• At a rural site near Anderson, SC, the average PM2.5 mass
during July 2001 was 20.9 µg m-3, with a high of 41.2 µg m-3 on
July 18 and a low of 4.4 µg m-3 on July 25. The overall average in
January 2002 was 9.4 µg m-3, with a high of 18.2 µg m-3 on January
18 and a low of 3.7 µg m-3 on January 25. Across all sampling
events, the average annual mass concentration was 15.1 µg m-3, just
above the new NAAQS annual standard of 15 µg m-3.
Data on air concentrations of PM2.5 mass in southeast Texas
indicate that:
• Annual average mass concentrations of PM2.5, over wide regions
of eastern and southeastern Texas, range from approximately 10 µg
m-3 to 15 µg m-3, which is close to the annual average NAAQS of 15
µg m-3.
• When averaged over long time periods, PM2.5 mass
concentrations are spatially homogeneous throughout southeast
Texas.
• Local events with moderately high air concentrations of PM2.5
mass occur at many monitors in this region but only rarely exceed
the annual average NAAQS.
KEY CITATIONS: Butler, A.J., M.S. Andrew, and A.G. Russell.
2003. Daily sampling of PM2.5 in Atlanta: Results of the first
year
of the Assessment of Spatial Aerosol Composition in Atlanta
study. J. Geophys. Res. 108(D7), 8415,
doi:10.1029/2002JD002234.
Husain, H. and C. Christoforou. 2003. Concentration and Chemical
Composition of PM2.5 Particles at a Rural Site in South Carolina.
SOS Final Report. Clemson University. 35 pp.
Russell, M.M., D.T. Allen, D.R. Collins, and M.P. Fraser. 2004.
Daily, seasonal and spatial trends in PM2.5 mass and composition in
southeast Texas. Aerosol. Sci. Technol. 38(S1):14-26,
doi:10.1080/02786820390229138.
Solomon, P.A., W. Chameides, R. Weber, A. Middlebrook, C.S.
Kiang, A.G. Russell, A. Butler, B. Turpin, D. Mikel, R. Scheffe, E.
Cowling, E. Edgerton, J. St. John, J. Jansen, P. McMurry, S.
Hering, and T. Bahadori. 2003b. Overview of the 1999 Atlanta
Supersite project. J. Geophys. Res. 108(D7), 8413,
doi:10.1029/2001JD001458.
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2.5.9. Variability in Concentrations of PM2.5 in the
Southeastern US Measurements of the air concentrations of PM2.5
mass in Atlanta and surrounding rural areas
indicate that:
• Winter and summer data from a rural site near Anderson, SC
showed higher mass concentrations in summer than in winter. Sulfate
ion and ammonium ion concentrations increased in summer, but
nitrate ion concentrations decreased in summer at this site.
Comparison of these SC air concentration data with those for
similar rural sites in GA and NC showed that the NC sites generally
had higher and the GA sites generally had lower air concentrations
of PM2.5 mass during late 2001 and early 2002.
• ASACA data in Atlanta showed that most PM2.5 constituents
peaked during summer months; but nitrate, metals, and elemental
carbon usually showed some enhancement during winter due mainly to
lower inversion heights. Diurnally, there were discernible early
morning and late night peaks that corresponded to rush-hour traffic
patterns and inversion heights, respectively.
• Comparison of data from Atlanta, GA and Fresno, CA showed that
seasonal differences in meteorology and amounts of emissions have
significant influences on seasonal variability in the composition
of PM2.5 at both locations.
Data on the air concentrations of PM2.5 mass in southeast Texas
indicate that:
• Throughout the region, concentrations are slightly higher in
the spring and late fall than in summer.
• A consistent and strong morning peak in PM2.5 mass
concentrations is observed throughout the region and a weaker and
slightly less consistent peak in mass concentration is observed in
the late afternoon to early evening.
KEY CITATIONS: Butler, A.J., M.S. Andrew, and A.G. Russell.
2003. Daily sampling of PM2.5 in Atlanta: Results of the first
year
of the Assessment of Spatial Aerosol Composition in Atlanta
study. J. Geophys. Res. 108(D7), 8415,
doi:10.1029/2002JD002234.
Chu, S.-H., J.W. Paisie, and B.W.-L. Jang. 2003. PM data
analysis – A comparison of two urban areas: Fresno and Atlanta. J.
Geophys. Res. (in review)
Husain, H. and C. Christoforou. 2003. Concentration and Chemical
Composition of PM2.5 Particles at a Rural Site in South Carolina.
SOS Final Report. Clemson University. 35 pp.
Russell, M.M., D.T. Allen, D.R. Collins, and M.P. Fraser. 2004.
Daily, seasonal and spatial trends in PM2.5 mass and composition in
southeast Texas. Aerosol. Sci. Technol. 38(S1):14-26,
doi:10.1080/02786820390229138.
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2.5.10. Radiative Forcing by Aerosols in Atlanta During the
Atlanta Supersite Experiment, studies were made of both the optical
and radiative
forcing properties of the aerosols observed in Atlanta. These
studies showed that:
• Light scattering was dependent on a wide range of chemical
components of the aerosols. • Light absorption is most strongly
linked to the elemental carbon component. • The average direct
aerosol radiative forcing properties of the Atlanta was about minus
11
+ 6 watts m-2; this value is about 10 times larger than global
mean estimates for aerosols.
KEY CITATION: Carrico, C.M., M.H. Bergin, J. Xu., K. Baumann,
and H. Maring. 2003. Urban aerosol radiative properties:
Measurements during the 1999 Atlanta Supersite experiment. J.
Geophys. Res. 108(D7), 8422, doi:10.1029/2001JD001222.
2.5.11. Composition of Aerosols in Atlanta Chemical composition
of single particles was measured using the Particle Analysis by
Laser
Mass Spectrometry (PALMS) instrument during the Atlanta
Supersite Study. Particle
composition was generally internally mixed. The predominant
particle consisted of organic
species and sulfate, and often contained other components such
as nitrate, ammonium, halogens,
metals, soot/hydrocarbon, and aluminosilicates. More than 20% of
the negative ion spectra of
single particles contained nitrate ion peaks, which showed a
clear maximum during the morning
at high relative humidity and a smaller maximum in the
afternoon. About 45% of the negative
spectra contained ions indicative of oxidized organics, with
similar morning and afternoon
maxima. At high relative humidity, the nitrate peaks were often
mixed internally with sulfate.
The single particle data also indicated the presence of organic
sulfur containing compounds,
which might account for 10 — 15% of sulfur observed in Atlanta
PM2.5.
• The composition of particles measured during the Atlanta
Supersite Study was generally internally mixed, with components of
organic matter, sulfate, nitrate, ammonium and other
constituents.
KEY CITATIONS: Lee, S.-H., D.M. Murphy, D.S. Thomson, and A.M.
Middlebrook. 2002. Chemical components of single
particles measured with particle analysis by laser mass
spectrometry (PALMS) during the Atlanta Supersite Experiment: Focus
on organic/sulfate, lead, soot, and mineral particles. J. Geophys.
Res. 107(D1), 4003, doi:10.1029/2000JD000011.
Solomon, P.A., W. Chameides, R. Weber, A. Middlebrook, C.S.
Kiang, A.G. Russell, A. Butler, B. Turpin, D. Mikel, R. Scheffe, E.
Cowling, E. Edgerton, J. St. John, J. Jansen, P. McMurry, S.
Hering, and T. Bahadori. 2003b. Overview of the 1999 Atlanta
Supersite project. J. Geophys. Res. 108(D7), 8413,
doi:10.1029/2001JD001458.
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2.6. Particle Phase Measurement Technologies - Development and
Intercomparison of Techniques
Integrated Filter Methods for Fine Particle Composition
Integrated Filter Measurements of Semi-Volatile Fine
Particulates
Semi-Continuous Methods for Measuring Fine Particle Chemical
Composition–Atlanta Supersite Intercomparison
Mass Spectroscopic Methods for Measuring Fine Particle Chemical
Composition
Methods for Measuring Fine Particle Density
2.6. PARTICLE PHASE MEASUREMENT TECHNOLOGIES - DEVELOPMENT AND
INTERCOMPARISON OF TECHNIQUES
Rodney Weber
Measurements of the physical and chemical properties
of aerosols are needed to gain insight into sources, atmospheric
transformations, and impacts of ambient
aerosol particles on human health and the environment. Aerosol
particle properties of interest include size
distributions, density, optical properties, total mass,
chemical composition, and the aerosol particle mixing state.
Methods capable of fast sampling rates and
automated operation are especially useful since they provide
large data sets that are better integrated with
meteorological and gas phase measurements and thus
have the potential to provide greater insights than highly
time-integrated measurements. Although improvements
are still being made, automated instrumentation to measure
aerosol physical properties, such as number
concentrations, size distributions, and optical properties
have existed for some time. It is only recently that significant
progress has been achieved in developing online methods for
measuring particle chemical composition, much of it with SOS
support. This section focuses on recent developments in aerosol
measurement instrumentation
that has been aided by the SOS research program. Following the
approach of the Atlanta Supersite Study, instrumentation is divided
into three categories, integrated methods, semi-
continuous methods, and methods based on mass spectrometry.
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2.6.1.1 Assessment of Integrated Filter Methods for Measuring
Fine Particle Chemical Composition
Collection of particles onto filter substrates for extended
integration periods, followed by off-line
extraction and analysis, is the standard method for measuring
fine particle total mass and chemically speciated mass
concentrations. One of the findings from the Atlanta Supersite
Study
was gained from a unique experiment in which side-by-side
comparisons were made between 12 different integrated filter
methods for measuring PM2.5 mass and chemical composition. The
results indicate:
• Integrated filter methods showed good agreement for PM2.5 mass
(most samplers within ±20%), and sulfate and ammonium (most
samplers within 10%).
• There were larger discrepancies between methods (±3 0 to 35%)
for nitrate, possibly due to the low ambient concentrations. Higher
variability also was found for the organic (OC) and elemental (EC)
carbonaceous fractions of PM2.5. For all OC samplers the
variability ranged between 35 and 45%. EC variability was high,
especially between the different analytical methods.
KEY CITATION: Solomon, P., K. Baumann, E. Edgerton, R. Tanner,
D. Eatough, W. Modey, H. Maring, D. Savoie, S.
Natarajan, M. B. Meyer, and G. Norris. 2003. Comparison of
integrated samplers for mass and composition during the 1999
Atlanta Supersites project. J. Geophys. Res. 108(D7), 8423,
doi:10.1029/2001JD001218.
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2.6.1.2 Integrated Filter Measurements of Semi-Volatile Fine
Particulates by the PC-BOSS
Semi-volatile compounds associated with aerosol particles
include ammonium nitrate and
semi-volatile organics that are mainly from secondary organic
aerosol formation. Although these compounds can comprise a
significant fraction of the PM2.5 mass, they are difficult to
measure accurately due to evaporation loss during sampling.
Standard single filter methods, such as the methods currently
accepted by the US EPA, including the PM2.5 Federal Reference
Method (FRM), will underestimate semi-volatile compounds. The
PC-BOSS sampler has been
developed to quantify the fine particle composition, including
the semi-volatile compounds. The instrument has been deployed in a
number of studies, including the Atlanta Supersite Study, to
quantify the semi-volatile aerosol compounds. A key finding from
the range of studies with the PC-BOSS is that:
• An estimated 10% to 50% of the fine particulate mass was not
measured with the PM2.5 FRM sampler due to the loss of
semi-volatile organic material and ammonium nitrate during
sampling.
These types of methods are necessary to accurately characterize
PM2.5 chemical components and
to aid in setting relevant policy standards. KEY CITATIONS:
Eatough, D.J., R.W. Long, W.K. Modey, and N.L. Eatough. 2003.
Semi-volatile secondary organic aerosol in urban
atmospheres: Meeting a measurement challenge. Atmos. Environ.
37:1277-1292. Modey, W.K., Y. Pang, N.L. Eatough, and D.J. Eatough.
2001. Fine particulate (PM2.5) composition in Atlanta, USA:
Assessment of the particle concentrator-Brigham Young University
organic sampling system, PC-BOSS, during the EPA supersite study.
Atmos. Environ. 35(36):6493-6502.
Modey, W.K., E.J. Eatough, Y. Pang, and N.L. Eatough. 2004.
Performance and evaluation of the PC-BOSS for fine PM2.5 sampling
during the summer EPA Supersite Program in Atlanta. J. Air Waste
Manage. Assoc. (in press).
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2.6.2 Advances in Semi-Continuous Methods for Measuring Fine
Particle Chemical Composition–Atlanta Supersite Intercomparison
A significant outcome of SOS efforts in instrument development
was the assistance given to
a range of methods for automated measurements of particle
chemical composition. Five relatively new methods for measurement
of bulk fine particle ionic composition, and five for
measurement of carbonaceous compounds were intercompared as part
of the Atlanta Supersite Study. These methods included:
Inorganic composition: • Aerosol Dynamics, Inc Integrated
Collection and Vaporization Cell (ICVC) for nitrate
and sulfate, now commercially available through R&P (e.g.,
Rupprecht and Patashnick Sulfate and Nitrate Monitors, Albany NY).
(Hering and Stolzenburg, 1997; Stolzenburg and Hering, 2000)
• Netherlands Energy Research Foundation (ECN): nitrate and
sulfate. (Slanina et al., 2001) • Georgia Tech/Brookhaven National
Lab Particle Into Liquid Sampler (PILS). (Orsini et al.,
2003; Weber et al., 2001). • Atmospheric Research and Analysis
(ARA) nitrate. • Texas Tech University (TT): nitrate and sulfate.
(Boring et al., 2002)
Carbonaceous Composition • Rutgers University/Oregon Graduate
Institute Thermal optical carbon analyzer (OC, EC,
TC). A similar instrument is commercially available (Sunset
Labs) • Rupprecht and Patashnick (R&P) 5400 ambient carbon
particulate monitor (OC, EC, TC)
(Turpin et al., 1990). • Radiance Research particle soot
absorption photometer (PSAP, Seattle WA) (EC). • Aerosol Dynamics
Inc integrated collection and vaporization cell (ICVC) for
carbon
(OC). • Magee Scientific AE-16 aethalometer (EC).
Two separate publications detailed the performance of the
semi-continuous methods based on an unprecedented side-by-side
ambient study.
1) Ionic Compounds: Five semi-continuous PM2.5 instruments for
measurements of fine particle (PM2.5) nitrate and sulfate were
compared. It was found that most instruments were in close
agreement with r-squared values between instrument pairs typically
ranging from 0.7 to 0.94. Based on comparison between individual
semi-continuous devices and 24-hour integrated filter measurements,
most instruments were within 20 to 30% for nitrate (~0.1-0.2 µg
m-3) and 10 to 15% for sulfate (1-2 µg m-3). Within 95% confidence
intervals, linear regression fits suggest no biases existed between
the semi-continuous techniques and the 24-hour integrated filter
measurements of nitrate and sulfate, however, for nitrate, the
semi-continuous intercomparisons showed significantly less
variability than intercomparisons amongst the 24-hour integrated
filters. (Weber et al., 2003b, JGR)
2) Carbonaceous Compounds: As observed in the integrated filter
inter-comparisons, the measurement of the aerosol carbonaceous
component is subject to larger uncertainties than the inorganic
compounds. Semicontinuous aerosol carbon measurements made
during
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30 June 2004 | State of SOS-3: 1995 - 2003 | 95
Atlanta using five different samplers showed moderate
correlations between pairs of organic carbon measurements and high
correlations between pairs of elemental carbon measurements. The
two semi-continuous samplers capable of a total carbon (TC)
measurement were in good agreement, within 5% based on the slope of
a Deming least squares fit, and r-squared of 0.83. Regression
slopes between pairs of the OC measurements were within 22%, but in
some cases intercepts as large as 2.3 ugC/m3 suggest there were
significant systematic differences. Large differences were found
between pairs of EC measurements. Regression slopes indicate
differences ranged from 4 to 45%. The study demonstrates the
successful operation of automated semi-continuous carbon analyzers
and illustrates the need for standards to decrease uncertainties in
current OC-EC measurements. (Lim et al., 2003, JGR) Many of the
semi-continuous methods developed with SOS assistance have had a
significant
impact on other federally funded projects, and are likely to
impact regulation of PM2.5. Key findings are summarized below.
Data from new semi-continuous instrumentation deployed at the
Atlanta Supersite have
enabled the following new insights into aerosol sources and
atmospheric processing: • Secondary organic aerosol, which
comprised about 46% of the measured organic
carbon, was from a combination of in situ photochemical
production and transport of more aged secondary organic aerosol.
This is based on diurnal patterns and correlations with ozone and
carbon monoxide and estimates of the fraction of OC from secondary
organic aerosol formation processes from mean 1-hour fine particle
OC and EC data.
• Transient PM2.5 episodes in which particle mass rapidly rises
and falls over a period of a few hours but which go undetected with
traditional time-integrated measurements are ubiquitous. Continuous
highly time resolved measurements of fine particle mass and
chemical composition at the EPA Atlanta Supersite Study, August
1999, revealed the transient episodes. Speciated composition data
show that these events are driven by sudden increases of two
specific aerosol chemical components that dominate at different
times, carbonaceous events in the early morning and sulfate events
in late afternoon. Apart from providing insights into sources, the
unique chemical nature of these transient events may have specific
health effects that previous epidemiologic studies based on highly
averaged aerosol data could not readily resolve.
• Partitioning between the gaseous and condensed phases was in
reasonable agreement with predictions of a thermodynamic
equilibrium model. Application of the thermodynamic equilibrium
model ISORROPIA to near real-time measurements of fine particle
sulfate, nitrate, and ammonium, and gas phase ammonia and nitric
acid showed good agreement, with an indication of potential bias in
estimates of acidity/alkalinity.
KEY CITATIONS: Lim, H.-J. and B.J. Turpin. 2002. Origins of
primary and secondary organic aerosol in Atlanta: Results of
time-resolved
measurements during the Atlanta Supersite Experiment. Environ.
Sci. Technol. 36:4489-4496.
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30 June 2004 | State of SOS-3: 1995 - 2003 | 96
Lim, H.-J., B.J. Turpin, E. Edgerton, S. Hering, G. Allen, H.
Maring, and P. Solomon. 2003. Semicontinuous aerosol carbon
measurements: Comparison of Atlanta Supersite measurements. J.
Geophys. Res. 108(D7), 8419, doi:1029/2001JD001214.
Weber, R., D. Orsini, Y. Duan, K. Baumann, C.S. Kiang, W.
Chameides, Y.-N. Lee, F. Brechtel, P. Klotz, P. Jongejan, H. ten
Brink, J. Slanina, C.B. Boring, Z. Genfa, P. Dasgupta, S. Hering,
M. Stolzenburg, D.D. Dutcher, E. Edgerton, B. Hartsell, P. Solomon,
and R. Tanner. 2003a. Intercomparison of near real time monitors of
PM2.5 nitrate and sulfate at the U.S. Environmental Protection
Agency Atlanta Supersite. J. Geophys. Res. 108(D7), 8421,
doi:10.1029/2001JD001220.
Weber, R., D. Orsini, A. Sullivan, M. Bergin, C.S. Kiang, M.
Chang, Y.N. Lee, P. Dasgupta, J. Slanina, B. Turpin, E. Edgerton,
S. Hering, G. Allen, P. Solomon, and W. Chameides. 2003b. Transient
PM2.5 aerosol events in metro Atlanta: Implications for air quality
and health. J. Air Waste Manage. Assoc. 53:84-91.
Zhang, J., W.L. Chameides, R. Weber, G. Cass, D. Orsini, E.
Edgerton, P. Jongejan, and J. Slanina. 2002. Validity of
thermodynamic equilibrium assumption for fine particulate
composition: Nitrate and ammonium during the 1999 Atlanta Supersite
Experiment. J. Geophys. Res. 108(D7), 8414,
10.1029/2001JD001592.
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2.6.3 Advances in Mass Spectroscopic Methods for Measuring Fine
Particle Chemical Composition
Four particle mass spectrometers operated side-by-side during
the Atlanta Supersite Study:
1) Particle Analysis by Laser Mass Spectrometer (PALMS) of the
National Oceanic and Atmospheric Administration, 2) University of
California at Riverside’s Aerosol Time-of-Flight
Mass Spectrometer (ATOFMS), 3) University of Delaware’s Rapid
Single-Particle Mass Spectrometer II (RSMS-II), and 4) Aerodyne’s
Aerosol Mass Spectrometer (AMS). Of these,
the PALMS and ATOFMS were fairly well established techniques,
whereas the RSMS-II and
AMS were in earlier stages of development and thus the Atlanta
Supersite Study provided a valuable opportunity to assess these
instrument’s performance. Key findings are summarized
below.
• Particle sizes were measured most accurately with ATOFMS,
RSMS-II, and AMS. The RSMS-II system can obtain composition
information on individual particles as small as 15 nm. The three
systems that utilize laser desorption/ionization, (PALMS, ATOFMS,
and RSMS-II), produce mass spectra that are qualitative and
representative of individual particles. The AMS instrument, which
uses a two-step volatilization on a heated surface and ionization
by electron impact, can produce quantitative results representative
of the ensemble of particles measured.
• Single-particle positive ion classifications from the Atlanta
data by the laser-based instruments are broadly consistent and
revealed similar trends as a function of size for organic, sulfate,
and mineral particles. The AMS, which is the most quantitative of
the mass spectrometers compared, had nitrate to sulfate molar
ratios that were highly correlated with those of the
semi-continuous instruments discussed above. Based on insights from
the Atlanta study, subsequent studies, such as those undertaken at
the New York EPA Supersite in the summer of 2002 (PEMTACS),
demonstrated the quantitative measurement capabilities of the AMS.
Overall, the strength and primary focus of the laser-based
instruments are their ability to find associations between chemical
species in individual particles with high time resolution.
KEY CITATIONS: Drewnick, F., J.J. Schwab, O. Hogrefe, S. Peters,
L. Husain, D. Diamond, R. Weber, and K. Demerjian. 2003.
Intercomparison and evaluation of four semi-continuous PM2.5
sulfate instruments. Atmos. Environ. 37(2003):3335-3350.
Middlebrook, A.M., D.M. Murphy, S.-H. Lee, D.S. Thomson, K.A.
Prather, R.J. Wenzel, D.-Y. Liu, D.J. Phares, K.P. Rhoads, A.S.
Wexler, M.V. Johnston, J.L. Jimenez, J.T. Jayne, D.R. Worsnop, I.
Yourshaw, J.H. Seinfeld, and R.C. Flagan. 2003. A comparison of
particle mass spectrometers during the 1999 Atlanta Supersite
Project. J. Geophys. Res. 108(D7), 8424,
doi:10.1029/2001JD000660.
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2.6.4 Advances in Methods for Measuring Fine Particle Density A
new technique that measures the density of aerosol particles in the
diameter range 0.1 to 0.3
um was developed and used during the Atlanta Supersite Study.
The technique offers an
alternative to calculating density from measured aerosol
composition. Particles are selected based on known electrical
mobility and then measurements of their mass are made with an
aerosol particle mass analyzer; density is determined based on
geometric diameter, which is
equal to electrical mobility equivalent diameter for spherical
particles, and mass. • Particles measured in August 1999 in urban
Atlanta typically included a major
mass peak with a density in the ~1.5 to 1.7 g cm-3 range at 3-6%
relative humidity. These data agreed with calculated densities
based on measured size-resolved composition within about 5%.
KEY CITATION: McMurry, P.H., X. Wang, K. Park, and K. Ehara.
2002. The relationship between mass and mobility for
atmospheric
particles: A new technique for measuring particle density.
Aerosol Sci. Techol. 36:227-238.
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30 June 2004 | State of SOS-3: 1995 - 2003 | 99
2.7. Gas Phase Measurement Technologies - Development and
Intercomparison of Techniques
Chemical ionization & proton transfer mass spectrometry
Odd hydrogen radical measurements
NMHC measurements Formaldehyde measurements
Carbonyl measurements NO2 and NOy measurements
Organic nitrate measurements CO measurements
2.7. GAS PHASE MEASUREMENT TECHNOLOGIES - DEVELOPMENT AND
INTERCOMPARISON OF TECHNIQUES
Eric Apel and David Parrish
Full understanding of the photochemistry that
produces ozone and PM2.5 requires ambient
measurements of the precursors, intermediates (radicals
as well as more stable species), and products. These
measurements must be made with high and well-defined
sensitivity, accuracy, precision, and specificity, and with
fast time response (particularly ~ 1 Hz for aircraft
measurements). The achievement of these goals is an
evolutionary process requiring careful instrument
development and operation in the field. This section
summarizes the major progress of the SOS research
program in this area.
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2.7.1 Major Advance: Chemical Ionization and Proton Transfer
Mass Spectrometry The combination of mass spectrometry with
chemical ionization (CIMS) potentially provides
a sensitive and specific measurement technique for many
atmospheric species. SOS-sponsored
work includes the development, testing, and deployment of CIMS
techniques for the
measurement of HNO3, isoprene, and ammonia as well as the
deployment and testing of proton
transfer (PTR-MS) instruments. The PTR-MS instruments can detect
most gas-phase organic
species with excellent time response, but their sensitivity and
specificity are limited for many
compounds by the manifold of organic species and fragment ions
with similar masses.
Highlights of this work are summarized below.
• Specific HNO3 Measurement Developed. A CIMS instrument to
measure gas-phase HNO3 was developed and demonstrated to be
sensitive with fast response (detection limit of approximately 10
pptv at 1 Hz), accurate, precise, and interference-free. It has
been tested in a ground-based intercomparison and deployed on
aircraft during SOS 1999 and 2000 field intensives.
• PTR-MS Deployed and Tested. PTR-MS instruments have been
deployed at ground sites and on aircraft along with other
instruments for measurement of several organic species.
Intercomparison of the coincident measurements will help to define
the capabilities of the PTR-MS instruments.
• CIMS Isoprene and Ammonia Measurements Developed. CIMS
techniques for measurement of isoprene and ammonia have been
developed and tested in ground-based intercomparisons. These
techniques promise to provide sensitive and fast aircraft
measurements of those species. The selectivity of the isoprene
technique must be tested by comparison to other techniques.
KEY CITATIONS: Huey, L.G., E.J. Dunlea, E.R. Lovejoy, D.R.
Hanson, R.B. Norton, F.C. Fehsenfeld, and C.J. Howard. 1998.
Fast time response measurement of HNO3 in air with a chemical
ionization mass spectrometer. J. Geophys. Res. 103:3355-3360.
Fehsenfeld, F.C., L.G. Huey, D.T. Sueper, R.B. Norton, E.J.
Williams, F.L. Eisele, R.L Mauldin, III, and D.J. Tanner. 1998.
Ground-based intercomparison of nitric acid measurement techniques.
J. Geophys. Res. 103:3343-3353.
Leibrock, E., and L.G. Huey. 2000. Ion chemistry for the
detection of isoprene and other volatile organic compounds in
ambient air. Geophys. Res. Letters 19:1763-1766.
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2.7.2 Major Advances: Odd Hydrogen Radical Techniques The
hydroxyl radical (OH), the main oxidant in atmospheric chemistry,
cycles rapidly with
the hydroperoxyl radical (HO2), initiating the production of
ozone and other pollutants. A now
widely accepted measurement technique for OH and HO2, together
called HOX, is laser-induced
fluorescence in detection chambers at low pressure. This
technique was applied for the first time
to continuous, 24-hour measurements on 10-meter tall towers in
Nashville in 1999 and at
TEXAQS 2000 in Houston. In addition, a new, unique instrument
was developed to measure the
OH loss rate due to reactions with other atmospheric chemicals.
This new instrument, the Total
OH Loss-rate Measurement (TOHLM), was successfully deployed for
the first time in Nashville
in 1999. The combination of HOX measurements and TOHLM provides
powerful new
diagnostics for understanding and testing the oxidation
chemistry of any environment.
• First continuous OH and HO2 measurements in urban
environments. The measurements, when compared to models, test the
fundamental atmospheric chemistry that underpins chemical transport
models. For Nashville SOS, OH and HO2 measurements agree to within
a factor of two with model calculations near midday, but tend to be
larger than models in the evening, at night, and for periods when
nitrogen oxides are especially abundant. These observations
indicate unidentified HOx sources and questions about HOx-NOx
chemistry.
• Total OH Loss-rate Measurement tests the completeness of
measured VOC inventories. The presence of unmeasured VOC is
indicated if TOHLM-measured OH loss rates are greater than those
calculated from the sum of VOC measurements and OH reaction rate
coefficients. Preliminary Nashville observations indicate that OH
loss rates are about twice those calculated, suggesting unmeasured
VOC.
KEY CITATION: Kovacs, T.A. and W.H. Brune. 2000. Total OH loss
rate measurement. J. Atmos. Chem. 39(2):105-122,
doi:10.1023/A:1010614113786.
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2.7.3 Non-Methane Hydrocarbons (NMHCs): Intercomparisons of
Techniques NMHCs, emitted from a variety of sources, are precursors
to ozone and PM2.5, and some
species are considered toxins. NMHCs are the primary
photochemical fuel in urban and many
rural areas whereas carbon monoxide and methane play this role
in regions remote from sources.
Biogenic NMHCs dominate the VOC chemistry in many highly
forested regions and even in
urban areas situated in regions surrounded by forests, as is the
case in most cities located in the
SOS domain (southeastern U.S.) [e.g., Chameides et al., 1997].
Anthropogenic NMHCs
dominate in most other urban areas.
• Accuracy of NMHC Measurements Tested. Because of the large
role of NMHCs in ozone formation, it is imperative that
measurements accurately reflect the true atmospheric composition.
SOS has taken a leadership role in the atmospheric science
community and has partnered with the NOAA Climate and Global Change
Program in conducting “The Non Methane Hydrocarbon InterComparison
Experiment” (NOMHICE). This experiment was designed to assess the
accuracy of analytical methods used to determine mixing ratios of
atmospheric non-methane hydrocarbons (NMHCs).
• NMHC Measurements Intercompared in the SOS Network. To help
ensure quality measurements and to understand where there are
problems, intercomparisons of NMHC measurements are conducted in
the SOS network before and during every field study. Prior to the
Nashville 99 Field Study nine measurements were intercompared. For
each measurement, a set of canisters were simultaneously collected
at the Cornelia Ft. site in Nashville, TN and distributed to 4
groups for analysis. Figure 2.6.1 shows the mean ratio of each
laboratory’s analysis of a given compound to that of the reference
laboratory (EPA-Lonneman). It is apparent that for some species,
the accuracy of the analyses is good (agreement to within 11% for
all groups), but for some species there were errors up to a factor
of 5. Some of the problems could be corrected during the
experiment. For example, the discrepancy of the Argonne data for
i-pentane was determined to be due to an overlap with another peak
and corrected. The precision of the measurements of the individual
species by the different laboratories is indicated by the standard
deviations of the ratios. These standard deviations ranged from
very good (±3%) to quite poor (factor of 3).
• High Quality Multi-Component Standards Developed. Eight
identical standards containing 100 NMHC were developed and tested
for the TexasAQS 2000 study. These standards were used as a common
calibration source for all participants in the study.
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0.1
2
3
4
5
6
1
2
3
4
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Figure 2.7.1. Comparison of the mean ratios of selected NMHC
measurements from 4 different SOS measurement groups using
EPA-Lonneman results as reference.
KEY CITATIONS: Chameides, W.L., R.D. Saylor, and E.B. Cowling.
1997. Ozone pollution in the rural United States and the new
NAAQS. Policy Forum. Science 276:916. Apel, E.C., J.G. Calvert,
T.M. Gilpin, F.C. Fehsenfeld, D.D. Parrish, and W.A. Lonneman.
1999. The Non-
Methane Hydrocarbon Intercomparison Experiment (NOMHICE): Task
3. J. Geophys. Res. 104:26069-26086.
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30 June 2004 | State of SOS-3: 1995 - 2003 | 104
2.7.4 Formaldehyde Technique Development and Intercomparisons
Formaldehyde (CH2O) is a primary emission product from internal
combustion engines and
is produced in the atmosphere by the photochemical oxidation of
methane and non-methane
hydrocarbons (NMHCs). It is the most abundant gas-phase carbonyl
compound in both urban
areas and the remote troposphere. Formaldehyde is extensively
connected with the odd
hydrogen (HOx = H+OH+HO2) and odd nitrogen (NOx =NO + NO2)
chemical cycles. It is also a
major source for HO2 and for CO in air not strongly perturbed by
anthropogenic sources.
Consequently, accurate measurements of formaldehyde are critical
to understanding the overall
tropospheric chemistry leading to hydrocarbon oxidation, the
processes controlling the odd
hydrogen cycles and nitrogen cycles, and the global budgets of
OH and CO.
• Two Measurement Techniques Developed. SOS has encouraged the
development of fast and sensitive techniques to measure
formaldehyde. Two techniques, tunable diode laser absorption
spectroscopy (TDLAS) and coil/2,4-dinitrophenylhydrazine (CDNPH),
have emerged which satisfy the necessary criteria for ground-based
and aircraft-based measurements. These techniques have been used
extensively in SOS field studies. The TDLAS technique can provide
1-second averages, while the CDNPH technique has been limited to
averages of at least 1 minute.
• Two Field Intercomparisons Completed. A blind intercomparison
of six ambient formaldehyde measurement techniques was conducted at
the National Center for Atmospheric Research (NCAR) in Boulder, CO,
from May 29 to June 3, 1995. It was concluded that gas phase
standards should be employed with any of the measurement
techniques, and the cartridge measurement methods are limited by
long collection periods, generally lower precision, and the
incomplete understanding of potential interferences from ozone and
possibly other compounds. Airborne CH2O measurements by TDLAS and
CDNPH techniques indicated that, on average, both instruments
measured identical ambient CH2O concentrations to better than
0.1-ppbv over the 0 to 0.8-ppbv-concentration range. However,
significant differences, larger than the combined 2σ total
uncertainty estimates, were observed in 29% of data set. It is
clear that careful attention must be paid to the behavior of CH2O
in the inlet for accurate airborne measurements.
• CH2O Gas Phase Standards Developed. Through SOS support,
formaldehyde standards have been produced in high-pressure
cylinders at the ppmv level. Techniques have been developed,
including FTIR and GC-FID, to verify the concentration of the
standard. Long-term stability tests are presently being
conducted.
KEY CITATION: Gilpin, T., E. Apel, A. Fried, B. Wert, J.
Calvert, Z. Genfa, P. Dasgupta, J.W. Harder, B. Heikes, B.
Hopkins,
H. Westberg, T. Kleindienst, Y.-N. Lee, X. Zhou, W. Lonneman,
and S. Sewell. 1997. Intercomparison of six ambient [HCHO]
measurement techniques. J. Geophys. Res. 102:21,161-21,188.
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2.7.5 Carbonyl Technique Development and Intercomparisons
Aldehydes and ketones and other oxygenates are produced by the
oxidation of hydrocarbons,
and some are emitted directly. Until recently, this compound
class has received much less
attention than its VOC counterpart, the NMHCs. This is largely
due to difficulties encountered
in measuring these compounds. SOS has taken a leading role in
developing carbonyl standards,
carbonyl measurement techniques, and evaluating the techniques
with intercomparisons.
Through these studies, it is becoming apparent that the
carbonyls and other oxygenates may be
more ubiquitous than previously thought, and hence contribute
significantly to photochemical
processes in the troposphere.
• The first quantified and verified carbonyl standards. Carbonyl
standards have been prepared gravimetrically with both calibrated
permeation sources and in specially treated aluminum cylinders.
Techniques such as atomic emission detection (AED) (Apel et al.,
1998a) and FTIR have been applied to verify the accuracy of the
standards. Table 2.6.1. demonstrates the verification of a prepared
standard.
Table 2.6.1. Quantification of Standards Results Compound High
Pressure Cylinder
gravimetric (ppm) High Pressure Cylinder
analyzed (ppm)* FTIR (ppm)
Methanol 3.00 ± 0.03 3.0 ± 0.1 2.9 ± 0.2 Ethanol 3.02 ± 0.03
n.d. 2.9 ± 0.3 Acetone 3.03 ± 0.03 3.03 ± 0.08 3.2 ± 0.2 MEK 3.04 ±
0.03 n.d 2.5 ± 0.3 Acetaldehyde 3.03 ± 0.03 n.d 3.2 ± 0.4 Propanal
3.03 ± 0.03 n.d n.d. Butanal 3.02 ± 0.03 n.d n.d.
*analysis based on calibration factors from permeation tubes
n.d. - not determined • Intercomparison completed. Cartridge–based
(Si-Gel and C18) and GC-MS
measurements have been intercompared through SOS. Serious
discrepancies were found and more work is needed to resolve these
differences.
• New techniques developed. A new relatively fast-response (15
minute cycle) GC-FID technique has been developed to measure
carbonyls and other oxygenates aboard aircraft. A new GC-MS
technique is currently being developed to measure carbonyls with a
5-minute time response.
KEY CITATIONS: Apel, E.C., J.G. Calvert, J.P. Greenberg, D.
Riemer, R. Zika, T.E. Kleindienst, W.A. Lonneman, K. Fung,
and E. Fujita. 1998a. Generation and validation of oxygenated
volatile organic carbon standards for the 1995 Southern Oxidants
Study Nashville Intensive. J. Geophys. Res. 103:22,281-22,294.
Apel, E.C., J.G. Calvert, D. Riemer, W. Pos, R. Zika, T.E.
Kleindienst, W.A. Lonneman, K. Fung, E. Fujita, P.B. Shepson, T.K.
Starn, and P.T. Roberts. 1998b. Measurements comparison of
oxygenated volatile organic compounds at a rural site during the
1995 SOS Nashville Intensive. J. Geophys. Res.
103:22,295-22,316.
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30 June 2004 | State of SOS-3: 1995 - 2003 | 106
2.7.6 NO2 and NOy Technique Development and Intercomparisons.
The oxides of nitrogen (NO and NO2) are precursors of ozone and
PM2.5, and the total
oxidized nitrogen family (NOy) in an air parcel represents the
total emissions of these precursors
that remain in the atmosphere. Improvements in the measurement
of these species are
summarized below.
• Improved NO2 measurements by photolysis-chemiluminescence. A
new photolytic converter utilizing the focused UV output from a
high-pressure mercury (Hg) arc lamp was developed and tested. The
new configuration permits simple and accurate retrieval of ambient
NO2 data at very high time resolution, is more specific, provides
increased sensitivity, and is less expensive to operate than
previous photolytic converter designs.
• Validation of an aircraft inlet for HNO3 and NOy. Rapid and
quantitative sampling of NOy species, including HNO3, has been
demonstrated using a short, heated Teflon inlet. In flight,
standard addition calibrations of HNO3 at the aircraft inlet
demonstrate freedom from significant surface adsorption of HNO3,
which has significantly compromised measurements through other
aircraft inlets.
• Intercomparisons of ground-based NO2 and NOY measurements.
Intercomparisons during SOS field intensives have demonstrated that
laser-induced fluorescence, differential optical absorption
spectroscopy, and photolysis-chemiluminescence techniques are all
capable of accurately quantifying atmospheric NO2 above 1 ppbv.
Further, MoO and Au converters were shown to be capable of
accurately measuring NOy above 2 ppbv in the urban and suburban
environments typical of the SOS region. These studies further
concluded that generation of reliable NO2 or NOy data still demands
skilled operators and dedicated, critical oversight during the
measurement process.
KEY CITATIONS: Ryerson, T. B., E.J. Williams, and F.C.
Fehsenfeld. 2000. An efficient photolysis system for fast-response
NO2
measurements,. J. Geophys. Res. 105:26,447-26,461. Ryerson,
T.B., L.G. Huey, K. Knapp, J.A. Neuman, D.D. Parrish, D.T. Sueper,
and F.C. Fehsenfeld. 1999.
Design and initial characterization of an inlet for gas-phase
NOy measurements from aircraft, J. Geophys. Res. 104:5483-5492.
Williams, E.J., K. Baumann, J.M. Roberts, S.B. Bertman, R.B.
Norton, F.C. Fehsenfeld, S.R. Springston, L.J. Nunnermacker, L.
Newman, K. Olszyna, J. Meagher, B. Hartsell, E. Edgerton, J.R.
Pearson, and M.O. Rodgers. 1998. Intercomparison of ground-based
NOy measurement techniques. J. Geophys. Res. 103:22,261-22,280.
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30 June 2004 | State of SOS-3: 1995 - 2003 | 107
2.7.7 Organic Nitrate Technique Development Several advances
have been made in the measurements of organic nitrates through the
course
of SOS. Emphasis has been put on improving separation and
quantitation of compounds that are
of biogenic origin, and on developing rapid methods for aircraft
measurements.
• Measurements of peroxyacyl nitrates (PANs). Gas
chromatographic methods for PANs have been refined to provide rapid
and sensitive measurement. Aircraft-based instrumentation was
developed to measure four of the major compounds of interest, PAN,
PPN, PiBN, and MPAN, every 3.5 min. The measurement of PANs by
proton-transfer reaction mass spectrometry (PTR-MS) was deployed
during the Nashville 99 intensive (Hansel and Wisthaler, 2000).
While still in the development stage, this method has the potential
to provide rapid (10 sec) measurements of PAN aboard aircraft.
• Development of PAN calibration systems. Two different
calibration methods for PAN have been developed: a diffusion source
and photochemical production of PAN in acetone/air/CO/NO mixtures.
The diffusion system relies on an NOy measurement for calibration,
while the photochemical source relies on a known, efficient
conversion of an NO standard to PAN. The two were compared during
the TEXAQS 2000 study and were found to agree within 5%.
• Development of organic nitrate measurements. An automated
system for the measurement of the organic nitrates produced from OH
radical attack on isoprene was developed and deployed at the
Dickson site during SOS 99. These compounds result when the peroxy
radicals derived from OH reaction with isoprene, react with NO to
produce a set of 8 isomeric RONO2 species. The maximum
concentrations of the sum of these species were in the 100-200 pptv
range, much higher than observed in a previous study. Comparison of
these two data sets provides a good opportunity to examine the
NOx-dependence of this aspect of isoprene photochemistry.
KEY CITATIONS: Williams, J., J.M. Roberts, S.B. Bertman, C.A.
Stroud, F.C Fehsenfeld, K. Baumann, M.P. Buhr, K. Knapp,
P.C. Murphy, M. Nowick, and E.J. Williams. 2000. A method for
the airborne measurement of PAN, PPN and MPAN. J. Geophys. Res.
105(D23):28,943–28,960.
Hansel, A. and A. Wisthaler. 2000. A method for real-time
detection of PAN, PPN, and MPAN in ambient air. Geophys. Res. Lett.
27:895-898.
Grossenbacher, J.W., P.B. Shepson, T. Thornberry, M.
Witmer-Rich, M.A., Carroll, I. Faloona, D. Tan, W. Brune, E. Apel,
D. Riemer, and H.A. Westberg. 2000. Measurements of isoprene
nitrates above a forest canopy. J. Geophys. Res. 109, D11311,
doi:10.1029/2003JD003966.
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30 June 2004 | State of SOS-3: 1995 - 2003 | 108
2.7.8 Carbon Monoxide Technique Development and Intercomparisons
Carbon monoxide is a long-lived gas primarily emitted in automobile
exhaust, which makes
it a useful tracer for urban pollution plumes. The result of an
SOS effort to develop an
instrument for aircraft measurements of this species is
summarized below.
• Development of an instrument based upon vacuum UV resonance
fluorescence of CO. An instrument was developed that is capable of
fast (~1 Hz), accurate (~5%), and precise (~1 ppbv) measurement of
CO from an aircraft platform. Intercomparisons with other
techniques demonstrate that it is highly specific with no
identified interferences (see Figure 2.7.2).
7
6
5
4
3
2
1
0
Altitu
de
(km
)
14:00 16:00 18:00 20:00
UTC 2 September 1997
300
250
200
150
100
50
0
CO
(p
pb
v)
1
2 3
4
Figure 2.7.2. Time series of coincident 5-second-average
measurements of CO. The solid lines give tunable diode laser
absorption (darker) and the vacuum ultraviolet fluorescence
(lighter) results and the dotted line indicates the aircraft
altitude. The features labeled 1 through 4 are intercepted urban
plumes. KEY CITATION: Holloway, J. S., R.O. Jacoubek, D.D. Parrish,
C. Gerbig, A. Volz-Thomas, S. Schmitgen, A. Fried, B. Wert, B.
Henry, and J.R. Drummond. 2000. Airborne intercomparison of
vacuum ultraviolet fluorescence and tunable diode laser absorption
measurements of tropospheric carbon monoxide. J. Geophys. Res.
105:24,251-24,261.
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30 June 2004 | State of SOS-3: 1995 - 2003 | 109
2.8. Atmospheric Dynamics and Mixing on Urban and Regional
Scales
Daytime transport processes Nighttime transport processes
Variations in mixing height Vertical distribution of
ozone, precursors, and aerosols
Off- to on-shore flow reversal Heterogeneity of industrial
plumes
2.8. ATMOSPHERIC DYNAMICS AND MIXING ON URBAN AND REGIONAL
SCALES
Allen White and Christoph Senff
While a thorough understanding of the atmospheric
chemistry associated with the formation and destruction
of atmospheric pollutants is critical in air quality
research, knowledge of atmospheric dynamics and
mixing processes is equally important to gain a full
understanding of the problem. Vertical and horizontal
transport mechanisms as well as the temporal and spatial
evolution of mixed layer height often play a critical role
in the distribution of pollutants on urban and regional
scales. In this section, we summarize key scientific
findings from the SOS’ Nashville ’95 and Nashville ‘99
studies, and TexAQS 2000 that pertain to atmospheric dynamics
and mixing processes.
2.8.1. Daytime Transport Processes During SOS field studies, a
number of meteorological processes contributed to horizontal
and vertical transport of pollutants during the daytime. A
schematic summarizing many of these
processes is shown in Fig. 2.8.1. SOS used a variety of
ground-based and airborne observing
systems to study transport mechanisms. Key findings are
summarized below.
• Horizontal Advection. We found that synoptically driven winds
were the dominant daytime horizontal transport mechanism. Mesoscale
circulations caused by topography or land use differences also
contribute to daytime transport. During TexAQS 2000, synoptic flow
exported the Houston/Ship Channel and Texas City pollution plumes
to rural, source-free areas, resulting in ozone concentrations well
above the ozone standard far downwind of the Houston metropolitan
area. Many of these ozone exceedances were missed by the surface
monitoring network due to sparse network coverage in rural
areas.
• Boundary-layer Venting. Under light wind conditions, we
observed substantial (~40%) horizontal variations in daytime mixing
height due to the urban-rural contrast in the surface energy
balance (the “urban dome”). The dome allowed venting of urban
emissions aloft, making them available for horizontal transport but
unavailable for vertical mixing downwind of the dome during the day
(refer to D in Fig. 2.8.1).
• Convection. Cumulus clouds vented pollutants from the boundary
layer and reduced the sunlight available for photochemistry.
Because direct measurements of cumulus venting are difficult to
obtain experimentally, we were unable to quantify this process
during SOS’ Nashville ‘95 and Nashville ’99 studies or during
TexAQS 2000. Deep vertical
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30 June 2004 | State of SOS-3: 1995 - 2003 | 110
mixing associated with convective storms may have resulted in
stratosphere/troposphere exchange.
Figure 2.8.1. Transport between the surface and the atmospheric
boundary layer (ABL) and between the ABL and free troposphere: A.
fluxes from nearly homogeneous surfaces to ABL, B. fluxes from
inhomogeneous surfaces to ABL, C. transport across capping
inversion (Zi) by entrainment/detrainment and cumulus venting, and
D. direct injection to troposphere caused by horizontal variations
in boundary-layer depth.
• Subsidence. Synoptic scale subsidence associated with high
pressure strengthened the boundary-layer capping inversion, thereby
inhibiting vertical transport of momentum and pollutants and
cumulus convection. This behavior, combined with the stagnant
conditions resulting from relaxation of the synoptic-scale pressure
gradient, allowed pollutants to accumulate locally during the day
(Banta et al., 1998).
• Morning Transition. The morning transition caused
photochemically aged pollutants from the residual layer to interact
with pollutants emitted at night into the shallow nocturnal
boundary layer. During the 1999 SOS Nashville Intensive, the
breakup of the nocturnal inversion occurred at an urban site 1-2 h
earlier than at three rural sites. In the humid environment in the
SOS field studies, surface water vapor mixing ratio was often an
excellent meteorological tracer for the timing of the morning
transition (Fig. 2.8.2).
Figure 2.8.2. 1-min time series of CO and NOy concentrations,
the concentration ratio of NOx to NOy, and water vapor mixing ratio
(rmix) measured near the surface on 15 July 1999 at the Dickson,
Tennessee site. The early morning increase in rmix is due to
surface evaporation. The sharp transition in rmix near 0920 CDT
occurs after the nocturnal inversion is fully eroded and as the
mixed layer grows rapidly through the residual layer and entrains
drier air from aloft. The CO and NOy concentrations decrease
because nocturnal surface emissions of these gases were confined to
the shallow nocturnal boundary layer. The NOx/NOy ratio decreases
because the residual layer contains photochemically aged air from
the previous day. KEY CITATIONS: Banta, R.M., C.J. Senff, A.B.
White, M. Trainer, R.T. McNider, R.J. Valente, S.D. Mayor, R.J.
Alvarez, R.M.
Hardesty, D. Parrish, and F.C. Fehsenfeld. 1998. Daytime buildup
and nighttime transport of urban ozone in the boundary layer during
a stagnation episode. J. Geophys. Res. 103:22,519–22,544.
White, A.B., B.D. Templeman, W.M. Angevine, R.J. Zamora, W.W.
King, C.A. Russell, R.M. Banta, W.A. Brewer, and K.J. Olszyna.
2002. Regional contrast in morning transitions observed during the
1999 Southern Oxidants Study Nashville/Middle Tennessee Intensive.
J. Geophys. Res. 107(D23), 4726, doi:10.1029/2001JD002036.
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30 June 2004 | State of SOS-3: 1995 - 2003 | 111
2.8.2. Nighttime Transport Processes During the night, the
effect of surface friction is confined to a shallow layer (10s of
meters)
near the surface. In general, vertical motions not associated
with convective storms are
considerably weaker at night than during the day. Thus, during
SOS, nocturnal transport tends to
redistribute pollutants horizontally rather than vertically. Key
features are summarized below.
• Low-Level Jet. At night, the winds above a shallow (10s of
meters) layer at the surface accelerated as the atmosphere
decouples from the surface.
• Inertial Oscillation. The nocturnal winds rotated in time in
accordance with the principles of the inertial oscillation. McNider
et al. (1998) demonstrated the persistent nature of this phenomenon
using wind spectra obtained from wind profilers deployed during the
SOS95 Nashville Intensive. Under sufficiently weak synoptic
forcing, the low-level jet and inertial oscillation dominated
nocturnal transport. Trajectories derived from the wind profiler
network deployed during SOS95 demonstrated the combined effect of
these important features (see Fig. 2.8.3).
• Vertical transport is suppressed at night. In the absence of
convective storms, the atmosphere stabilized at night, which
suppressed any significant vertical transport. In many cases,
intermittent turbulence has been observed in the nocturnal boundary
layer, which may be linked to wind shear associated with the
low-level jet. The effect of intermittent turbulence on pollution
levels at the surface is an important topic of current SOS
research.
Figure 2.8.3. Overnight forward trajectories calculated from the
network of 915-MHz boundary-layer wind profilers deployed during
SOS95. Winds were averaged over the 400-m vertical intervals shown
in the key. Trajectories were calculated from an origin centered on
Nashville, at 36.2Ε N, 86.8Ε W (after Banta et al., 1998).
KEY CITATIONS: Banta, R.M., C.J. Senff, A.B. White, M. Trainer,
R.T. McNider, R.J. Valente, S.D. Mayor, R.J. Alvarez, R.M.
Hardesty, D. Parrish, and F.C. Fehsenfeld. 1998. Daytime buildup
and nighttime transport of urban ozone in the boundary layer during
a stagnation episode. J. Geophys. Res. 103:22,519–22,544.
McNider, R.T., W.B. Norris, R.I. Clymer, S. Gupta, R.M. Banta,
R.J. Zamora, A.B. White, and M. Trainer. 1998. Meteorological
conditions during the 1995 Southern Oxidants Study Nashville/Middle
Tennessee Field Intensive. J. Geophys. Res. 103:22,225–22,243.
-160 -140 -120 -100 -80 -60 -40 -20 0 20 40
East-West Distance (km)
-60
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-20
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20
40
60
80
100
120
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Nort
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nce
(km
)
Profiler ForwardTrajectories
7/12/95 2000 CDT -7/13/95 1100 CDT
400-800 m
800-1200 m
1200-1600 m
1600-2000 m
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30 June 2004 | State of SOS-3: 1995 - 2003 | 112
2.8.3. Variations in Mixing Height During the SOS95 and SOS99
studies, it was found that the daytime mixing height can vary
considerably in the Nashville, TN area, especially when
comparing the urban and adjacent rural
areas. As mixing height is an important parameter affecting air
pollution concentrations, this
finding has significant implications for the regional
distribution of ozone and particulates. Key
results are summarized below.
• Remote sensors provide reliable measurements of mixing height.
A comparison of mixed layer depth estimates deduced from wind
profiler and airborne lidar data showed very good agreement under
clear or partly cloudy conditions (White et al., 1999). This result
confirmed that radar wind profilers and lidars are well suited to
measure the depth of the mixed layer and its variability.
• Mixing height variability is tied to land use differences.
Variations in mixing height are related to differences in surface
characteristics, such as soil and vegetation type as well as
surface moisture (see Fig. 2.8.4). During SOS99 Nashville
Intensive, these different surface characteristics were reflected
in varying energy, ozone, and carbon fluxes at the surface.
Figure 2.8.4. Time height cross section of aerosol backscatter
gradient indicating the top of the mixed layer measured with the
NOAA/ETL airborne lidar during midday on 12 July 1995 during a
northwest to southeast transect over Nashville, TN (city of
Nashville is in center of panel). The urban heat island over
Nashville is clearly visible at flight times near 11:30 LST. Over
forested areas to the northwest of Nashville (left side of panel)
the mixed layer is strongly suppressed, while over suburban and
agricultural terrain to the southeast (right side of panel) the
mixed layer is only slightly shallower than over the city.
• Differences in mixing height are most pronounced under light
wind conditions. Under stagnant conditions, air parcels tend to
dwell over regions of one surface type, which allows surface
heating differences to express themselves as variations in mixing
height. Stronger flow moves air parcels over many surface types,
thus producing a more uniform mixing height
• Urban heat island. The strong differences in surface heating
between the Nashville urban area and the surrounding agricultural
and forested areas resulted in significantly deeper mixed layers
over the city, especially under stagnant conditions. We frequently
observed urban mixing heights of 2 km or more, which was as much as
800 m higher than the mixing heights over adjacent rural areas (see
Figs. 2.8.4 and 2.8.5).
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30 June 2004 | State of SOS-3: 1995 - 2003 | 113
Figure 2.8.5. Hourly measurements of mixed layer depth on 4 July
1999 from the wind profiler network deployed in and around
Nashville, TN during SOS 99. The wind profiler at Cornelia Fort
airport shows a maximum mixed layer depth of about 2.1 km while all
other profilers deployed in rural areas around Nashville detect
peak mixed layer heights of only about 1.5 km.
Time (LST) • Ozone concentrations are anti-correlated with
mixing height. Peak ozone
concentrations in the Houston/Ship Channel pollution plume
downwind of the sources were found to be anti-correlated with
mixing height. In the Houston area, mixing depth typically
increases with distance away from the coast. Thus, transport of the
Houston/Ship Channel pollution plume to coastal areas tends to
produce higher ozone peak values than transport to inland
areas.
• Model prediction of mixing depth. We found that the
Pennsylvania State National Center for Atmospheric Research
Mesoscale Model 5 (PSU/NCAR MM5) had difficulty accurately
predicting mixing depths under both stable and unstable atmospheric
conditions. Deficiencies in the atmospheric radiation
parameterizations caused excess energy in the model’s surface layer
(see Fig. 2.8.6). MM5’s response to this additional input resulted
in daytime mixed layers that were too deep.
Figure 2.8.6. Solar radiative flux predicted by MM5 (line) and
measured with a spectral pyranometer (asterisks) at New
Hendersonville, Tennessee during SOS95. The model bias leads to
model errors in the surface energy budget which impact the depth
and strength of vertical mixing and thermally driven circulations
such as the land-sea breeze. KEY CITATIONS: Angevine, W.M., A.B.
White, C.J. Senff, M. Trainer, R.M. Banta, and M.A. Ayoub. 2003.
Urban-rural contrasts
in mixing height and cloudiness over Nashville in 1999. J.
Geophys. Res. 108(D3), 4092, doi:1029/2001DJ0001061.
White, A.B., C.J. Senff, and R.M. Banta. 1999. A comparison of
mixing depths observed by ground-based wind profilers and an
airborne lidar. J. Atmos. Oceanic Technol. 16, No. 5:584-590.
Zamora, R.J., S. Solomon, E.G. Dutton, J.W. Bao, M. Trainer,
R.W. Portmann, A.B. White, D.W. Nelson, and R.T. McNider. 2003.
Comparing MM5 radiative fluxes with observations gathered during
the 1995 and 1999 Nashville Southern Oxidants Studies. J. Geophys.
Res. 108(D2), 4050, doi:10.1029/2002JD002122.
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30 June 2004 | State of SOS-3: 1995 - 2003 | 114
2.8.4. Vertical Distribution of Ozone, Precursors, and Aerosols
The vertical distribution of ozone, precursors, and aerosols is
influenced to a large extent by
the dynamical processes described above. The key findings
are:
• The vertical distribution of pollutants in the daytime mixed
layer. Mixing processes due to convective or mechanical turbulence
acted to smooth out vertical inhomogeneities in the daytime
boundary layer. Figure 2.8.7 shows a high-ozone layer near the
surface mixing vertically as convective turbulence increased over
the course of the morning.
Figure 2.8.7. Series of vertical profiles of ozone concentration
measured with the NOAA/ETL airborne ozone lidar on the morning of
12 July 1995. The ozone profiles are each spaced about 50 min
apart, starting at 7:30 CDT and show the vertical redistribution of
ozone as the mixed layer grows.
• The vertical distribution of pollutants at night. Due to a
lack of vertical mixing (see
Section 2.8.2) pollutants tended to form horizontal layers or
patches that persisted throughout the night until they were mixed
out by the growing boundary layer the next morning. Fig. 2.8.8
depicts the cross section of a power plant plume that had been
injected into the stabilizing evening atmosphere. In the absence of
any significant vertical mixing the power plant plume stayed
confined to a thin vertical layer.
Figure 2.8.8. Cross section of Cumberland power plant plume
measured with the NOAA/ETL airborne ozone lidar on the evening of 4
July 1999 at about 10 km downwind of the power plant. The plume is
easily identified by its low-ozone signature. Note that the plume
is confined to the layer between 800 and 1200 m ASL.
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30 June 2004 | State of SOS-3: 1995 - 2003 | 115
520 560 600 640 680
Solar Irradiance (W m-2)
0
0.2
0.4
0.6
0.8
1
Aero
so
l O
pti
ca
l D
ep
th
Durham, NH (41.0o zenith angle) August 11-16, 2002
Observed Irradiance
ETA Forecast Irradiance
Average Bias ~ 84.4 W m-2
Y = -0.019 * X + 15.7R-squared = 0.52
Y = -0.0089 * X + 6.8R-squared = 0.99
• Pollutant concentrations in the free troposphere are affected
by long-range transport or stratosphere-troposphere exchange
processes. Ozone sonde and aircraft measurements from the Nashville
‘95 and ‘99 campaigns showed that concentrations of ozone and other
pollutants in the free troposphere were highly variable and were
primarily affected by regional to continental-scale advection of
clean or polluted air masses. Another significant process
contributing to high tropospheric ozone concentrations is the
intrusion of stratospheric air. Through entrainment processes (see
Section 2.8.1) pollutant concentrations in the lower free
troposphere can impact the air quality in the atmospheric boundary
layer and at the surface.
• The improper treatment of aerosols in models contributed to
forecast errors in solar radiation reaching the surface. Aerosol
absorption and scattering reduce the amount of sunlight that
reaches the Earth’s surface. During the 2002 New England Air
Quality Study, the correlation between observed aerosol optical
depth and incoming solar radiation measured at Durham, NH was
greater than the correlation between observed aerosol optical depth
and predicted incoming solar radiation from the ETA model (see Fig.
2.8.9). The model also was unable to produce the observed slope for
the line of regression between these two variables. This improper
treatment of aerosols in the ETA model contributed to a large
radiation bias (see also Fig. 2.8.6).
Figure 2.8.9. Correlation between observed solar irradiance and
aerosol optical depth measured at Thompson Farm in Durham, NH
during the 2002 New England Air Quality Study along with
correlation between the observed optical depth and solar irradiance
predicted by the ETA model.
KEY CITATIONS: Banta, R.M., C.J. Senff, A.B. White, M. Trainer,
R.T. McNider, R.J. Valente, S.D. Mayor, R.J. Alvarez, R.M.
Hardesty, D. Parrish, and F.C. Fehsenfeld. 1998. Daytime buildup
and nighttime transport of urban ozone in the boundary layer during
a stagnation episode. J. Geophys. Res. 103:22,519–22,544.
Senff, C.J., R.M. Hardesty, R.J. Alvarez II, and S.D. Mayor.
1998. Airborne lidar characterization of power plant plumes during
the 1995 Southern Oxidants Study. J. Geophys. Res.
103(D23):31,173-31,189.
Wotawa, G. and M. Trainer. 2000. The influence of Canadian
forest fires on pollutant concentrations in the United States.
Science 288:324-328.
Zamora, R.J., E.G. Dutton, M. Trainer, S.A. McKeen, J.M.
Wilczak, and Y.-T. Hou. 2004. The accuracy of solar irradiance
calculations used in medium range forecast models. Mon. Wea. Rev.
(accepted for publication).
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30 June 2004 | State of SOS-3: 1995 - 2003 | 116
2.8.5. Off- to On-Shore Flow Reversal In coastal areas, the land
- sea breeze circulation can act to focus large concentrations
of
ozone and other pollutants. Very high concentrations of
pollutants can be expected when the
morning offshore flow is followed by a period of stagnant winds
and the sea breeze recirculates
the aged pollutants released in the morning back over the
sources areas. Key features are
summarized below.
• Off- to on-shore flow reversal was observed in Houston during
TexAQS 2000 in conjunction with very large accumulations of ozone.
Severe ozone exceedances on flow-reversal days were linked to a
combination of two meteorological factors: (1) Light-wind
conditions that allow buildup of ozone plumes over source areas
during the middle of the day, and (2) Afternoon sea breeze that
transports aged pollutants back over source areas, thus reinforcing
the already high ozone concentrations (see Fig. 2.8.10). The
distribution of pollutants and the severity of the ozone event
depend on the morning offshore flow regime, the timing of the sea
breeze onset, and the strength of the sea breeze.
Figure 2.8.10. Cross section of ozone measured with the NOAA/ETL
airborne ozone lidar during TexAQS 2000 in the late afternoon on 30
August. Very high ozone concentrati