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Changes in Light Absorptivity of Molecular Weight Separated
BrownCarbon Due to Photolytic AgingJenny P. S. Wong,*,† Athanasios
Nenes,†,‡,§,⊥ and Rodney J. Weber†
†School of Earth and Atmospheric Sciences and ‡School of
Chemical and Biomolecular Engineering, Georgia Institute of
Technology,Atlanta 30331, United States§Institute of Chemical
Engineering Sciences, Foundation for Research and
Technology-Hellas, Patras GR-26504, Greece⊥Institute for
Environmental Research and Sustainable Development, National
Observatory of Athens, Palea Penteli GR-15236,Greece
*S Supporting Information
ABSTRACT: Brown carbon (BrC) consists of those organiccompounds
in atmospheric aerosols that absorb solar radiationand may play an
important role in planetary radiative forcing andclimate. However,
little is known about the production and lossmechanisms of BrC in
the atmosphere. Here, we study how thelight absorptivity of BrC
from wood smoke and secondary BrCgenerated from the reaction of
ammonium sulfate withmethylglyoxal changes under photolytic aging
by UVA radiationin the aqueous phase. Owing to its chemical
complexity, BrC isseparated by molecular weight using size
exclusion chromatog-raphy, and the response of each molecular
weight fraction toaging is studied. Photolytic aging induced
significant changes inthe light absorptivity of BrC for all
molecular weight fractions;secondary BrC was rapidly photoblenched,
whereas for woodsmoke BrC, both photoenhancement and photobleaching
were observed. Initially, large biomass burning BrC molecules
wererapidly photoenhanced, followed by slow photolysis. As a
result, large BrC molecules dominated the total light absorption of
agedbiomass burning BrC. These experimental results further support
earlier observations that large molecular weight BrCcompounds from
biomass burning can be relatively long-lived components in
atmospheric aerosols, thus more likely to havelarger impacts on
aerosol radiative forcing and could serve as biomass burning
tracers.
1. INTRODUCTION
Organic aerosols (OA) are a major component of fine
ambientparticles and affect the Earth’s radiative balance by
directlyinteracting with solar radiation or indirectly via
theirinteractions with clouds. These aerosol effects on
climaterepresent the largest uncertainty in global radiative
forcingassessments.1 While OA was originally thought to only
scattersolar radiation, recent studies demonstrate that components
inOA can absorb UV−visible radiation.2 This class of lightabsorbing
OA, collectively termed brown carbon (BrC), canpotentially shift
the direct radiative forcing of OA from netcooling to net
warming.3,4 Additionally, modeling studies haveobserved that
absorption of near UV solar radiation by BrC canresult in decreased
photolysis rates for NO2 and O3, indicatingthat BrC can influence
tropospheric photochemistry.5,6
Characterizing the sources and aging processes of BrC iscritical
to evaluate its atmospheric impacts and to understandthe persistent
signatures in biomass burning aerosols.Multiple sources of BrC have
been identified, including
emissions from biomass burning,7−9 fossil fuel
combustion,10,11
and release of biogenic matter, such as soils and
bioaerosols.12
While many studies have established that biomass burning is
likely to be an important source of atmospheric BrC, only asmall
fraction of organic chromophores has been identified,such as
nitrophenols.13−16 Production of secondary BrC inaerosols and
clouds has also been proposed.13,17 Althoughsecondary BrC formation
from the reactions of carbonyl oraromatic compounds with
nitrogen-containing compounds hasbeen studied extensively in the
laboratory,13 its contribution toatmospheric BrC remains unclear.
The emissions profile of BrCis poorly understood, but how aging
modulates BrC levels andproperties in the atmosphere is still
unclear. Part of this limitedunderstanding arises from the low mass
fraction ofchromophores in the organic aerosol, as well as the
uncertainand complex nature of their chemical identity.13
Most studies that have investigated BrC aging focused
onsecondary BrC, which was observed to undergo rapidphotobleaching
with atmospheric lifetimes on the order ofminutes to several
hours.18−21 Despite the growing evidence
Received: April 19, 2017Revised: June 8, 2017Accepted: June 22,
2017Published: June 22, 2017
Article
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that aged secondary BrC rapidly photobleaches in theatmosphere,
laboratory studies investigating the effects ofaging on primary BrC
have observed that biomass burning BrCcan undergo both
photoenhancement and photobleach-ing.20,22,23 The results from
these studies illustrated thedynamic nature of biomass burning BrC
due to aging, butthe mechanisms leading to these observations
remain unclear.For example, it is unknown whether all classes of
compounds inbiomass burning BrC respond to photolytic aging in the
samemanner (e.g., initial photoenhancement followed by
photo-bleaching), or that different classes of compounds
exhibitdifferent photolytic aging effects, or a combination
thereof.The effects of aging on biomass burning BrC have also
been
observed from ambient measurements. By following theevolution of
a biomass burning plume in Western U.S.A.,airborne observations
suggested that while the majority ofprimary BrC from biomass
burning have short atmosphericlifetimes of 9 to 15 h, a persistent
fraction may remain evenafter 50 h following emission, although the
conclusion isuncertain since there are few data points for more
aged BrC(>20 h).24 Other ambient measurements of aged
(approx-imately 2 days of atmospheric transport) biomass
burningaerosols indicated that large molecular weight
organiccompounds contributed significantly to the total
organicaerosol mass25 and total light absorption.26 Collectively,
theseobservations suggest that atmospheric aging of biomass
burningBrC decreases the light absorptivity of smaller
chromophoresconsiderably more than for larger chromophores.
Largerchromophores may therefore be the most persistent BrCspecies
in the atmosphere, hence most influential for perturbingthe
planetary radiative balance.27 The atmospheric processesleading to
these observations remain unknown, and it is unclearwhether the
reactivity of secondary BrC is also dependent onits molecular
weight.The objective of this study is to systematically investigate
the
effects of photolytic aging on the light absorptivity of
differentmolecular weight BrC components. Size exclusion
chromatog-raphy was coupled to UV−vis absorption spectroscopy in
orderto characterize the molecular weight distributions
ofchromophores in different types of BrC and to determinetheir
photolytic reactivity. The photolysis of two types of BrCwere
investigated: primary BrC from pyrolysis-generated woodsmoke
emissions and secondary BrC generated from thereaction of ammonium
sulfate with methylglyoxal (AS-MGL).Results demonstrated that both
types of BrC undergosignificant changes in their optical and
molecular weightsproperties due to photolytic aging. Rapid
photobleaching wasobserved for AS-MGL BrC, whereas initial
photoenhancement,followed by photobleaching, was observed for
primary BrCfrom wood smoke emissions. These contrasting
observationsillustrate that the atmospheric evolution of BrC is
highlyvariable and dynamic.
2. EXPERIMENTAL METHODS2.1. Preparation of BrC Samples. Wood
smoke BrC
samples, chosen to represent biomass burning BrC, weregenerated
in the laboratory via controlled wood pyrolysis usingthe method of
Chen and Bond that simulates the thermaldecomposition of solid
organic fuel during biomass burning.28
An electronically heated combustor, with an internal volume
of950 cm3, was continually flushed with 2000 sccm of N2 gas,where
the lack of oxygen suppresses black carbon formationduring wood
pyrolysis. For each pyrolysis event, a rectangular
piece of dry hardwood (cherry of size 3 × 2 × 2 cm,approximately
5 g) was placed in the bottom center of thecombustor, where the
exterior temperature was measured. Thesmoke stream was further
diluted by HEPA-filtered air (1500sccm) in a mixing volume (0.01
m3), following which particleslarger than 1.0 μm were removed using
an impactor. Once thecombustor reached 210 °C, the emitted organic
carbon wascollected on polytetrafluoroethylene filters (47 mm, 2 μm
poresize, Pall Corporation) at 3500 sccm for 100 min.
Theseconditions represent the smoldering phase of the
combustionprocess. Some low volatility components of the smoke
emissionmay not be measured by this method as a viscous substance
wasobserved to accumulate on the tubing walls. Immediately
aftercollection, the filters were stored in a freezer at −10 °C.
Priorto each photolysis experiment, water-soluble BrC (WS BrC)was
extracted from one particle filter by adding 15 mL ofpurified water
(18.2 mΩ) in a sealed glass vial and sonicated for60 min. After the
water extract was removed, 15 mL ofmethanol (HPLC grade, Merck) was
added and sonicated for60 min to extract the water-insoluble BrC
(WI BrC). Eachextract was filtered using a new 0.2 μm PTFE syringe
filter(Fisher).Ammonium sulfate-methylglyoxal (AS-MGL) BrC was
prepared using a similar method employed by previouslaboratory
studies, which simulates BrC formed by secondaryprocesses.18,20,29
The AS-MGL stock solution was prepared bycombining 98 mL of an
aqueous solution of ammonium sulfate(Fisher Scientific) and 3 mL of
methylglyoxal (Sigma-Aldrich,40% in water) in sealed amber bottles.
The final concentrationsin the stock solution were ∼1.5 M of
ammonium sulfate and∼0.17 M of methylglyoxal. The resulting
solution was kept inthe dark at room temperature for 10 days.
During this period oftime, the color of the solution turned dark
yellow/brown froma pale yellow color. Prior to each photolysis
experiment, thestock solution was diluted by a factor of 7.
2.2. Photolysis of BrC. All photolysis experiments wereconducted
in a photoreactor, with a slowly rotating vial rack (4rpm, 40 vials
capacity) placed in the center that was surroundedby 8 UVA lamps
(F-25T8BL, Sylvania) and maintained close tonear-room temperature
by continuous chamber ventilation withtwo fans. With all the UVA
lamps turned on, the temperatureinside the photoreactor increased
by 6° (from 24 to 30 °C).The integrated photon flux inside the
photoreactor wascharacterized by chemical actinometry using
2-nitrobenzalde-hyde,30 and the wavelength dependent photon flux
was directlymeasured using a spectroradiometer (StellaNet Inc.).
Thechemical actinometry method is discussed in Section S1, andthe
photon fluxes determined using both approaches are shownin Figure
S1 (Supporting Information). Most of the radiationemitted by the
UVA lamps fell in the 300−400 nm range with amaximum at 355
nm.30
Photolysis experiments using wood smoke BrC and AS-MGLBrC were
conducted separately. For each experiment, multiple2 mL
borosilicate glass vials (sealed with Telfon-lined caps),each
containing 0.75 mL of the filter extract or dilute solution,were
placed on the rotating vial rack. At different illuminationtimes,
one vial was removed for offline measurements(discussed below). For
the wood smoke BrC samples, filterextracts were illuminated up to
130 h in the photoreactor andup to 40 h for AS-MGL BrC. To ensure
reproducibility,photolysis experiments using wood smoke BrC and
AS-MGLBrC were repeated four and five times, respectively.
Addition-ally, control experiments were conducted; no changes in
BrC
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properties were observed when the vials were completelycovered
by aluminum foil (i.e., exposed to only the elevatedtemperature
conditions and not UVA radiation).2.3. BrC Measurements. Changes in
the water-soluble
organic carbon (WSOC) concentration due to photolysis
weremonitored offline using a Sievers Total Organic Carbon
(TOC)Analyzer (Model 900, GR Analytical Instruments).
TOCmeasurements were conducted using the bulk BrC samples(i.e., not
molecular weight separated), since the use of organiccompounds in
the eluent for the chromatographic molecularweight separation
technique (discussed below) resulted in veryhigh background
signals. Additionally, quantification of WI-BrCwas not possible due
to the use of methanol as an extractionsolvent. The TOC analyzer
was routinely calibrated usingsolutions of dissolved sucrose of
known concentrations. BrCsamples were diluted by up to a factor of
1000 to ensure themeasured TOC concentrations were in the linear
responserange of the instrument. From the TOC measurements,
eachsample vial (i.e., 0.75 mL of filter extract or dilute
solution) ofunreacted WS smoke BrC contained 342 ± 91 μg of WSOCand
for the unphotolyzed AS-MGL BrC 386 ± 40 μg ofWSOC.Changes in
molecular weight distributions of BrC due to
photolysis were measured using a high performance
liquidchromatography (HPLC) system (GP40 pump with AS40autosampler,
Dionex), equipped with a size exclusionchromatography (SEC) column
(discussed below), coupledto an UV−vis spectrometer, consisting of
a liquid waveguidecapillary (1 m optical path-length, World
Precision Instru-ment), a deuterium tungsten halogen light source
(DT-Mini-2,Ocean Optics), and an absorption spectrometer
(USB4000,Ocean Optics) that continuously monitored all
wavelengthsbetween 200 and 800 nm. The long optical path length
waschosen to increase detection sensitivity.Separations were
achieved by operating an aqueous size
exclusion/gel filtration chromatography column (Polysep
GFCP-3000, Phenomenex). Briefly, separation by size
exclusionchromatography (SEC) is controlled by differences in
theextent of permeation into the pores of the column
packingmaterial by analyte molecules, where larger molecules
areeluted first due to weaker interactions with the packing
materialcompared to smaller molecules.31 The chromatographicmethod
used is similar to that developed by Di Lorenzo andYoung for the
analysis of atmospheric particles;26 however, thecomposition of the
mobile phase was modified to optimize theseparation of weakly
interacting molecules. The chromatog-raphy system was operated in
isocratic mode using a 90:10 v/vmixture of water and methanol with
25 mM ammonium acetateas the mobile phase, at a flow rate of 1
mL/min and a sampleinjection volume of 20 μL. Ammonium acetate, a
pH buffer,was added to the mobile phase to minimize
electrostaticinteractions between the analytes and the column,
which caninterfere with the column’s ability to separate by
molecular size.If electrostatic interactions are negligible, SEC
separatesanalytes based solely on their hydrodynamic volume, which
isa function of both molecular weight and density of
thecompound.32,33 The relationship between elution volume
andmolecular weight was empirically determined using thefollowing
standards with known molecular weights (Sigma-Aldrich): blue
dextran (2 M Da), bovine serum albuminum (66kDa), horseradish
peroxidase (44 kDa), myoglobin (16.9 kDa),lysozyme (14.3 kDa),
apotinin (6.5 kDa), tannic acid (1.7kDa), vitamin B12 (1.4 kDa),
dichlorofluorescene (401 Da),
uridine (244 Da), and 2-nitrobenzaldehyde (151 Da).
Thecalibration curve is shown in Figure S2 (SupportingInformation),
where the linear region of the relationshipbetween elution volume
and molecular weight represents therange of molecules that had weak
interactions with the packingcolumn material. This calibration
method only providesestimates of the molecular weights for BrC
compounds sinceit remains unknown whether the molecular densities
of thestandards are representative of that of the BrC molecules
ofinterest.
3. RESULTS AND DISCUSSION3.1. Wood Smoke BrC. The change in
water-soluble
organic carbon (WSOC) concentration in smoke BrC uponUV
irradiation is shown in Figure 1a. Decreases in WSOC due
to photolysis were observed, resulting in a net loss in 30%
ofWSOC after 125 h of UV exposure. Absorption of UV radiationby
chromophores can initiate photolysis, leading to theformation of
products having higher volatility (e.g., fewercarbon numbers).
Evaporation of these volatile products canlead to the observed loss
in WSOC. In addition, the loss ofWSOC due to photolysis exhibited
an initial decay (i.e., first 8 hof UV exposure) that was rapid,
followed by a slower decay,suggesting that WS smoke BrC contains
multiple chromo-phores of varying degrees of photolability.In
addition to changes in WSOC, changes in the absorption
per mass of water-soluble carbon (mass absorption
coefficient,MAC) provide insight into the effects of photolysis on
the lightabsorptivity of water-soluble chromophores. The
calculationmethod for MAC at 365 and 400 nm is discussed in Section
S2.Shown in Figure 1b, exposure to UV light leads to an
initialincrease in MAC values at both wavelengths,
indicatingphotoenhancement (i.e., increased absorptivity of near
UV−vis radiation by BrC). Given that a loss in WSOC was
observedduring this photoenhancement period, we speculate that
thephotolysis of WS smoke BrC leads to the formation of productsof
higher volatility that evaporate to the gas phase, as well
asproducts that remain in the aqueous BrC solution, but are
morelight absorbing. Previous studies have shown that aqueous-phase
photo-oxidation of phenolic compounds34−37 and nitro-aromatic
compounds,20 both of which have been identified inbiomass burning
organic aerosols,38−40 lead to increasedabsorption of near UV−vis
radiation. The proposed mecha-nisms leading to the increased
absorption were attributed to the
Figure 1. Time series profile of the (a) changes in
WSOCconcentration (normalized to initial values) and (b) WSOC
massabsorption coefficients at 365 nm (black circles) and 400 nm
(redsquares) for the photolysis of WS smoke BrC. The error bars
representthe variability (±1σ) of multiple experiments (n = 4).
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polymerization of phenolic compounds35,37 and
OH-function-alization of nitroaromatric compounds.20 Additionally,
photo-enhancement has been previously observed for aged
biomassburning BrC emitted from the pyrolysis of hickory, pine,
andoak wood,22,23 as well as from the combustion of
kaoliangstalk.20 After this initial period of photoenhancement (up
to 20h), continual exposure to UV lights led to the
photobleaching,behaviors previously observed from the
aforementionedstudies.20,23
In addition to total light absorptivity, the molecular
weightdistributions of BrC (provided by SEC) offer additional
insightson the molecular nature of chromophores and the effects
ofphotolysis. Typical image plots of the molecular weightseparated
BrC absorption spectra from the WS and water-insoluble (WI)
components of wood smoke are shown inFigure 2. Two main populations
of chromophores can be
observed for unreacted BrC smoke: highly absorbing
largechromophores and less absorbing smaller
chromophores.Comparison of light absorption by the unreacted WS and
WIcomponents indicated that the majority of light absorption canbe
attributed to water-soluble chromophores, at all illuminationtimes
(shown in Figure S3). On average, WI BrC contributed23 ± 9% of the
total light absorption at 365 nm by wood smokeBrC (i.e., sum of
absorption by both WS and WI BrC). Whilethe discussion below
primarily focuses on the results from thephotolysis of WS BrC,
similar trends in results were observedfor the WI BrC component
(shown in Figure S4).To illustrate the evolution of chromophores
with different
molecular weights, the changes in the total absorption at 365nm
(Abs365) for different molecular weight fractions are shownin
Figure 3. Here the total Abs365 is binned according to thestrength
of interaction with the column packing material, wherea high
molecular weight fraction (high-MW) is defined aschromophores that
had weak interactions with the SEC column(i.e., the linear region
of the calibration curve shown in FigureS2) and have approximate
molecular weights between 66 kDaand 401 Da. The small molecular
weight fraction (small-MW)is defined as chromophores that had
strong interactions withthe SEC volume and have approximate
molecular weights
smaller than approximately 400 Da. Note that the molecularweight
values are only estimates, as it remains unknownwhether the
molecular densities of the calibration standards arerepresentative
of the molecular densities of BrC molecules. Forboth molecular
weight fractions, initial photoenhancement wasobserved, followed by
photobleaching with prolonged UV lightexposure. These initial
increases and subsequent decays inabsorption by different molecular
weight fractions exhibitedfirst-order kinetics. Shown in Table 1,
the rates of photo-
enhancement (kpe) were determined by fitting first-ordergrowth
curves to the first 4 h of absorption data. Forphotobleaching rates
(kpb), first-order decay curves were fittedto the initial decay in
absorption (e.g., between 20 and 52 h ofUV exposure for WS BrC and
between 8 and 40 h of UVexposure for WI BrC), where kpb represents
the rate of decayfor more photolabile species. In general,
photoenhancementwas more significant for the high-MW fraction of
smoke BrC,whereas the kinetics of photobleaching is similar for
both high-MW and low-MW fractions. Faster photoenhancement by
thehigh-MW fraction may be due to these chromophores beingmore
photoreactive (e.g., larger absorption cross sections and/or
quantum yields) compared to chromophores in low-MWfraction.
Comparison of the photoreactivity of WS and WI BrCsuggests that WI
BrC of all molecular weight fractionsundergoes more rapid
photoenhancement during the first 4 h
Figure 2. Typical molecular weight separated absorption spectra
ofunreacted water-soluble (WS) smoke BrC (top) and
water-insoluble(WI) smoke BrC (bottom). Arrows indicate the elution
volumes (Ve)of some calibration standard: bovine serum albumin (Ve
= 7.6 mL, 66kDa), aprotinin (Ve = 10.3 mL, 6.5 kDa), and
dichlorofluorescene (Ve= 15.1 mL, 401 Da). Note that molecular
weight increases withdecreasing elution volume.
Figure 3. Time series profile of the change in absorption at
awavelength of 365 nm for the high molecular weight (red circles)
andlow molecular weight (black triangles) fractions in WS smoke BrC
dueto photolysis. The insert is a zoomed-in view of the changes
observedat longer UV illumination times.
Table 1. Observed Rate Constants for thePhotoenhancement (kpe)
and Photobleaching (kpb) ofWater-Soluble (WS) Smoke BrC,
Water-Insoluble (WI)Smoke BrC, and Ammonium Sulfate-Methylglyoxal
(AS-MGL) BrC at 365 nma
smoke BrC fraction kpe (s−1) kpb (s
−1)
WS high-MW (9.2 ± 1.4) × 10−5 (1.5 ± 0.6) × 10−5
low-MW (5.3 ± 1.5) × 10−5 (1.8 ± 0.4) × 10−5
WI high-MW (2.0 ± 1.2) × 10−4 (1.2 ± 0.7) × 10−5
low-MW (1.7 ± 1.0) × 10−4 (2.8 ± 2.0) × 10−5
AS-MGL BrC fraction kpe (s−1) kpb (s
−1)
high-MW (8.2 ± 2.4) × 10−5
low-MW (1.6 ± 0.4) × 10−4
aRate constants for the photoenhancement of smoke BrC
weredetermined from the first 4 h of UV exposure, and
photobleaching rateconstants were determined between 8 and 40 h of
UV exposure for WIBrC and between 20 and 52 h of UV exposure for WS
BrC. Rateconstants for the photobleaching of AS-MGL BrC were
determinedfrom the first 4 h of UV exposure. Uncertainties in the
rate constantsrepresent the variability (±1σ) between multiple
experiments.
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of UV light exposure (Figure S4). Additionally,
maximumabsorption by the WI BrC was observed at this time, while
WSBrC exhibited longer photoenhancement (up to 20 h of UVlight
exposure). Note that the concentration of organic carbonin WI BrC
was not quantified in this study (see Section 2.3).Owing to their
rapid photoenhancement, chromophores in
the high-MW fraction were persistent and dominated total
lightabsorption at 365 nm (with an increasing contribution with
UVexposure; Figure S5). This result is consistent with
previousambient measurement of molecular weight separated
agedbiomass burning organic aerosols (from a boreal forest
fire),where the majority of water-soluble BrC absorption
wasattributed to molecules larger than 500 Da.26 It is possible
thatthe allocation of a molecular weight cutoff value
foratmospherically stable chromophores is dependent on thebiomass
fuel type and burning conditions, as emissions areknown to depend
on both factors.41
3.2. AS-MGL BrC. Although the previous set of
experimentsdemonstrated that the effects of photolytic aging on the
lightabsorbing properties of BrC from wood smoke are dependenton
the molecular weight of its components, it remains unclearwhether
other types of BrC exhibit this type of behavior. Forthe current
study, we examined the changes in molecularweight distributions due
to the photolysis of chromophoresgenerated from the reaction of
ammonium sulfate andmethylglyoxal (AS-MGL BrC), as this reaction
system iscommonly used as laboratory surrogates of
secondaryBrC.18,20,29,42,43
Shown in Figure 4, two populations of chromophores wereobserved
for unphotolyzed AS-MGL BrC: a population of
larger chromophores that strongly absorbs radiation at 365 nmand
a less absorbing population of smaller chromophores.Upon exposure
to UV lights, rapid photobleaching wasobserved for all chromophores
(Figure 5).Rate constants for the photobleaching of AS-MGL BrC
were
determined by fitting the observed Abs365 during the first 4 h
ofUV exposure to first-order decay curves (Table 1). Similar towood
smoke BrC, the change in light absorption due tophotolytic aging
exhibited a molecular weight dependence,where the fastest decay was
observed for the smallestmolecules. Additionally, the rate of
absorption decay decreaseswith time, suggesting that this type of
BrC containschromophores with different photoreactivity. To date,
studiesinvestigating the photolysis of this type of BrC have
notobserved photoenhancement.18,20
Observed decay rates for bulk absorption (i.e., sum of
allmolecular weight fractions) at 400 nm for the current study[(1.2
± 0.1) × 10−4 s−1] resulted in a half-life of 95 min
againstphotolysis (Figure S6). This value is generally consistent
withphotolysis half-life determined by Zhao et al. using
bulkabsorbance measurements (∼13 min), considering differencesin
the following experimental conditions between the twostudies:
concentrations of BrC precursors and the photon fluxesinside the
respective photoreactors.20
4. ATMOSPHERIC IMPLICATIONSUVA light exposure of BrC molecules
led to significant changesin their light absorptivity and molecular
weight distributions;the extent of photoenhancement and
photobleaching dependedon the molecular weight fraction and source
of BrC. Inparticular, the largest molecules in biomass burning BrC
(i.e.,high-MW fraction) contributed to the majority of total
lightabsorption, due to rapid photoenhancement of these
molecules.These results indicate that molecular weight
separatedtechniques, such as SEC, can be useful tools to elucidate
theaging mechanisms of large molecular weight substances in
theatmosphere. However, the molecular weights of BrC reportedin
this work are only approximate values, as the accuracy of theSEC
calibration approach depends on whether the moleculardensities of
the calibration standards are representative of thatof the BrC
molecules. Further work to verify the molecularweights of BrC, such
as coupling SEC-UV absorptionspectroscopy with light scattering
techniques, which havebeen employed to determine the absolute
molecular weights oflignin and its byproducts,44,45 is
warranted.From the observed decay rate for the high-MW fraction
of
WS BrC, the initial atmospheric lifetime with respect
tophotolysis is estimated to be approximately 14−36 h at solarnoon
(calculation method discussed in Section S3). Given thatthe average
lifetime of particles in the atmosphere isapproximately 1 week with
respect to deposition, this veryrough estimate suggests that large
water-soluble BrC moleculesfrom biomass burning could remain
throughout the majority ofthe particles’ lifespan and so could be
ubiquitous in theatmosphere, as observed.27 We stress that there
areuncertainties in this estimate, as it assumes that the
photolysisquantum yield is wavelength-independent and that
photolysisof BrC in the atmosphere is restricted to the wavelength
rangeconsidered (300−400 nm). The wavelengths responsible forBrC
photolysis (i.e., photolysis quantum yields) are
currentlyunknown.Nonetheless, these experimental results further
support
earlier observations that large molecular weight BrC speciesfrom
biomass burning can be long-lived components in
Figure 4. An image plot of the molecular weight separated
absorptionspectra of unphotolyzed AS-MGL BrC. Arrows indicate the
elutionvolumes (Ve) of some calibration standard: bovine serum
albumin (Ve= 7.6 mL, 66 kDa), aprotinin (Ve = 10.3 mL, 6.5 kDa),
anddichlorofluorescene (Ve = 15.1 mL, 401 Da). Note that
molecularweight increases with decreasing elution volume.
Figure 5. Time series profile of the 365 nm wavelength
absorptionchange compared to initial values for the high-molecular
weight (redcircles) and low molecular weight (black triangles)
fractions in AS-MGL BrC due to photolysis.
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atmospheric aerosols.25,26 This means they are more likely
tohave larger impacts on aerosol direct radiative forcing
onregional scales, whereas the contribution of smaller species
toBrC, either emitted from biomass burning or formed fromsmall
carbonyl compounds, are likely to be most important nearsource
regions. In addition, the observed rapid photoenhance-ment of
water-soluble biomass burning BrC suggests thatsecondary production
of BrC in atmospheric aqueous media(e.g., wet aqueous, fog and
cloud droplets) can be an importantsource of BrC in the atmosphere.
In particular, Gilardoni et al.recently reported ambient
observations of light absorbingsecondary organic aerosol formation
from the processing ofbiomass burning emissions in the aqueous
phase.17
Also, we note that the majority of total light absorption at365
nm observed in this study was contributed by the water-soluble
component of wood smoke BrC (77 ± 9%), which isconsistent with the
observations by Di Lorenzo and Young.26
However, dominant contributions to total light absorption at365
nm by BrC extractable in methanol or acetone wereobserved in the
atmosphere,14,46 as well as from laboratorygenerated smoke BrC from
the pyrolysis of pine and oak.28
These differences may be due to fuel type and burn conditions,or
that only primary smoke aerosol, in isolation from otheratmospheric
species, was studied here. Additionally, the samplepreparation
approach employed in this study (sequential filterextraction with
water, followed by methanol) does not take intoaccount the
contribution of water-insoluble BrC compounds onsuspended particles
that may have been removed duringfiltration of the water sample
extract. Given these contrastingresults, additional work
investigating the relative contributionsof water-soluble and
insoluble BrC using a wide range of BrCprecursors and burn
conditions is necessary.While our results continue to support the
view that the
majority of AS-MGL BrC undergo rapid photobleaching, asmall
fraction of these chromophores may persist in theatmosphere (e.g.,
in Figure 5, 5−10% of the initial absorptionby AS-MGL BrC remains
after 40 h of UV exposure). As such,it is important to quantify the
relative contribution of differentsources to background BrC. We
note that not all secondaryBrC undergo rapid photobleaching, as the
atmospheric lifetimeof secondary BrC formed from the
photo-oxidation ofnaphthalene (under high NOx-conditions) has been
estimatedto be approximately 20 h.19
Further investigations on the effects of other atmosphericaging
processes on the light absorptivity and chemicalcomposition of
different molecular weight BrC fractions areneeded. In particular,
the current study examined the photolyticaging of BrC dissolved in
bulk solutions. This type of approachdoes not simulate aging
processes occurring on or withinsuspended particles, where
parameters such as gas-particlecollision frequencies, aerosol phase
state, and solute concen-trations (including pH) are different. For
example, the effects ofaging processes such as cloud/fog droplets
evaporation47 andheterogeneous reactions48 on the physio-chemical
properties ofBrC have been demonstrated.Sources of ambient fine
particle OA remains an open
question since the components all tend to evolve to a
similarhighly oxygenated state49,50 and specific chemical
sourcetracers, including those for biomass burning, can have
aconsiderably shorter atmospheric lifetimes than aerosol.51−53
Because of this, the mass fraction of aerosol attributed
tobiomass burning may be grossly underestimated for agedaerosol,
leading to the view that biomass burning may be a
much more important contributor to global than
currentlybelieved.53,54 The unique stability of the high-MW
fraction ofBrC may provide an alternative to traditional biomass
burningmarkers and enable a better estimate of the true impact
ofbiomass burning emissions on the atmospheric aerosol burden.
■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting
Information is available free of charge on theACS Publications
website at DOI: 10.1021/acs.est.7b01739.
Experimental procedure for the determination of photonflux
inside the photoreactor; details on the calculation ofmass
absorption coefficients; method to convertobserved decay rates to
equivalent atmospheric lifetimes;six supporting figures (photon
flux inside photoreactor;molecular weight vs retention time
calibration curve;evolution of light absorption by WS and WI-BrC
fromwood smoke; time series of the change in absorption
fordifferent molecular weight fractions of WS and WI woodsmoke BrC;
time series of the change in WSOC andmass absorption coefficients
of AS-MGL BrC at 365 and400 nm; table listing the estimated
atmospheric lifetimesof BrC (PDF)
■ AUTHOR INFORMATIONCorresponding Author*Phone: 404-894-1750.
Fax: 404-894-5638. E-mail: [email protected].
ORCIDJenny P. S. Wong: 0000-0002-8729-8166NotesThe authors
declare no competing financial interest.
■ ACKNOWLEDGMENTSFunding for this work was provided by the
Electric PowerResearch Institute (EPRI) through contract
#00-10003806.Additional support was also provided by NASA
throughcontract NNX14A974G. A.N. acknowledges support from aGeorgia
Power Faculty Scholar chair and a Johnson FacultyFellowship.
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