Surface-Atmosphere fluxes Surface-Atmosphere fluxes Alex Guenther Alex Guenther Atmospheric Chemistry Division Atmospheric Chemistry Division National Center for Atmospheric Research National Center for Atmospheric Research Boulder CO, USA Boulder CO, USA Outline Outline • Introduction Introduction • Major cycles Major cycles • Recent scientific advances and Recent scientific advances and challenges challenges
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Surface-Atmosphere fluxes Surface-Atmosphere fluxes Alex Guenther Atmospheric Chemistry Division National Center for Atmospheric Research National Center.
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Atmospheric Chemistry DivisionAtmospheric Chemistry Division National Center for Atmospheric ResearchNational Center for Atmospheric Research
Boulder CO, USA Boulder CO, USA
OutlineOutline• IntroductionIntroduction• Major cyclesMajor cycles• Recent scientific advances and Recent scientific advances and challengeschallenges
1. IntroductionWhat is in the atmosphere? How did it get there? How does it leave?
What is in the Atmosphere?What is in the Atmosphere?
N2 (78.084%), O2 (20.948%), Ar (0.934%), CO2 (0.039%), Ne (0.0018%), He (0.000524%), CH4 (0.00018%), H2 (0.000055%), N2O (0.000032%), Halogens (0.0000003%), CFCsH2O, O3, CO, non-methane VOC, NOy, NH3, NO3, NH4, OH, HO2, H2O2, CH2O, SO2, CH3SCH3, CS2, OCS, H2S, SO4, HCN
Well mixed
Variable
What is in the atmosphere?What is in the atmosphere?
• 1950s: Atmosphere is 99.999% composed of N2, O2, CO2, H2O, He, Ar, Ne. All are inert! (no chemistry). O3 in the stratosphere. Trace CH4, N2O
• 1960s: Recognized that reactive compounds in the atmosphere were important even at extremely low levels.
• 1970s: Regional air quality becomes a major research topic.
• 1980s: Global atmospheric chemistry becomes a major research topic.
Earth
Cosmos
Where does the atmosphere come from?Where does the atmosphere come from?
1. Original atmosphere2. Dead planet3. Living planet4. Anthropocene
Organic aerosol
processes
Photo-oxidant
processes
Cloud processes
Global Biogeochemical Cycles
Carbon Cycle
Nitrogen Cycle
Water & Energy Cycles
Ozone and N deposition
NO/NH3 emission
CO2H2O NOy
NH3
Precipitation and solar radiation
Latent and sensible heat
Biological particles and VOC emissions
Air Quality:ozone and particles
Weather/Climate:Temperature,
sunshine, precipitation
Ecosystem Health:Productivity,
diversity, water availability
AnthropogenicNatural
How do we measure surface exchange?How do we measure surface exchange?
• Eddy covariance: The flux is related to the product of fluctuations in vertical wind and concentration. This is the only direct measurement.
• Gradient: The flux is related to vertical concentration gradient.
• Mass balance (Inverse Model): The flux is related to a concentration or concentration change.
01020
0 10 20 30 40 50 60 70C (
g m
-3) A
-1.50
1.53
0 10 20 30 40 50 60 70
w (
m s
-1) B
-606
12
0 10 20 30 40 50 60 70
w'C
'
(g
m-2
s-1
) C
Time (seconds)
Eddy Covariance Flux DataEddy Covariance Flux DataConcentration and wind speed measurements above a forest canopy Sampling rate = 10 Hz
The flux of a trace gas is calculated as the covariance between the instantaneous deviation of the vertical wind velocity (w’) and the instantaneous deviation of the trace gas (c’) for time periods between 30 min and an hour.
Concentration
Vertical wind speed
Flux
roughness sublayer
ConcentrationProfile
HE
IGH
TSurface layer gradientsSurface layer gradients
Flux = K dC/dz K: eddy diffusivity coefficient
dz: vertical height difference
dC: concentration difference
inertial sublayerdC
dz
Enclosure measurements
0
zi
MIXED LAYER Conc.Profile
0
HE
IGH
T
Emission (deposition) rate is related to the increase (decrease) in mass
Static: change with time
Dynamic: difference between inflow and outflow
Boundary Layer Budget
Imaginary box
May need to consider
- chemical loss/production
- horizontal advection
- non-stationary
Mass Balance Budgets
2. The CyclesFrom the earth surface to the atmosphere and back again
Chapter 5. Trace Gas Exchanges and Biogeochemical Cycles. In: Atmospheric Chemistry and Global Change (1999). Brasseur et al. (editors).
Water Cycle: source of OH in the atmosphereWater Cycle: source of OH in the atmosphere
Separating evapotranspiration into evaporation and transpiration components is an active area of research
Atmospheric Chemistry and Global Change (1999). Brasseur et al. (editors).
There are hundreds of BVOCs emitted from Vegetation
flowers~100’s of VOCs
cell wallsMeOH, HCHO
phytohormonese.g. ethylene,
DMNTcell membranes
fatty acid peroxidationwound-induced OVOCs
resin ducts / glandsterpenoid VOCs
cytoplasm/chloroplastC1-C3 metabolites
chloroplastterpenoid VOCs
Vegetation and soils
HalogensAtmosphere Br-, I-, Cl-
Dry deposition and soil microbe
uptake
CH3Cl, CH3Br, CH3I
Anthropogenic
Ocean
Biomass burning
3. Surface-atmosphere exchange: Recent scientific advances and challenges
How will biogenic VOC emissions respond to future How will biogenic VOC emissions respond to future changes in landcover, temperature and CO2? changes in landcover, temperature and CO2?
• Landcover, temperature and CO2 are changing
• Biogenic VOC (BVOC) emissions are very sensitive to these changes
• But it is difficult to even predict the sign of future changes in BVOC emissions