1. Surface-Atmosphere fluxes
Alex Guenther Atmospheric Chemistry Division
National Center for Atmospheric Research
Boulder CO, USA
Outline
Introduction
Major cycles
Recent scientific advances and challenges

2. 1. Introduction
What is in the atmosphere? How did it get there? How does it leave?

3. What is in the Atmosphere?
N2 (78.084%), O2 (20.948%), Ar (0.934%), CO2 (0.039%), Ne (0.0018%), He ( %), CH4 ( %), H2 ( %), N2O ( %), Halogens ( %), CFCs
H2O, O3, CO, non-methane VOC, NOy, NH3, NO3, NH4, OH, HO2, H2O2, CH2O, SO2, CH3SCH3, CS2, OCS, H2S, SO4, HCN
Well mixed
Variable

4. What is in the atmosphere?
1950s: Atmosphere is % 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.

5. Where does the atmosphere come from?
Original atmosphere
Dead planet
Living planet
Anthropocene
Cosmos
Atmosphere
Earth

6. Global Biogeochemical Cycles
Atmosphere
Air Quality:
ozone and particles
Weather/Climate:
Temperature, sunshine, precipitation
Ecosystem Health:
Productivity, diversity, water availability
Cloud processes
Organic aerosol processes
Photo-oxidant processes
Biological particles and VOC emissions
H2O
CO2
NOyNH3
Latent and sensible heat
NO/NH3 emission
Water & Energy Cycles
Carbon Cycle
Nitrogen Cycle
Precipitation and solar radiation
Ozone and N deposition
Earth
Anthropogenic
Natural
Surface

7. 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.

8. Eddy Covariance Flux Data
Concentration 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
Time (seconds)

9. Surface layer gradients
K: eddy diffusivity coefficient
dz: vertical height difference
dC: concentration difference
Flux = K dC/dz
inertial sublayer dC dz
Concentration
Profile
HEIGHT
roughness sublayer

10. Enclosure measurements
Mass Balance Budgets
Enclosure measurements
Emission (deposition) rate is related to the increase (decrease) in mass
Static: change with time
Dynamic: difference between inflow and outflow
Boundary Layer Budget zi
Imaginary box
May need to consider
- chemical loss/production
- horizontal advection
- non-stationary
MIXED LAYER
HEIGHT
Conc.
Profile

11. 2. The Cycles From 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).

12. Water 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).

13. THE NITROGEN CYCLE N2 NO HNO3 orgN NH3/NH4+ NO3- ATMOSPHERE fixation
combustion
lightning N2 NO
oxidation
HNO3
denitri-
fication
biofixation
deposition
orgN
decay
BIOSPHERE
NH3/NH4+
NO3-
assimilation
nitrification
SOIL/OCEAN
burial
weathering
LITHOSPHERE
Daniel Jacob 2008

14. Atmospheric ammonia sources and sinks (Tg per year)
Anthropogenic
Sources
Domestic animals: 21
Human excrement: 2.6
Industry: 0.2
Fertilizer losses: 9
Fossil fuel combustion: 0.1
Biomass Burning: 5.7
Soil: 6
Wild animals: 0.1
Ocean: 8.2
Sinks
Wet precipitation (land): 11
Wet precipitation (ocean): 10
Dry deposition (land):
Dry deposition (ocean):
Reaction with OH:
Natural
Does it add up?
Sources: 52.9 Tg
Sinks: 40 Tg
This is good agreement considering the uncertainties of factors of 2 or more
From Brasseur et al. 1999

15. Atmospheric NOx sources and sinks (Tg per year)
Aircraft: 0.5
Fossil fuel combustion: 20
Biomass Burning: 12
Soil: 20
Lightning: 8
NH3 oxidation: 3
Stratosphere: 0.1
Ocean: <1
Sinks
Wet precipitation (land): 19
Wet precipitation (ocean): 8
Dry deposition:
Does it add up?
Sources: 64 Tg
Sinks: 43 Tg
This is good agreement considering the uncertainties of factors of 2 or more
From Brasseur et al. 1999

16. The Sulfur Cycle Atmosphere SO2, SO4 H2S, DMS, OCS, CS2, DMDS
Wet deposition Tg of SO2, SO4
Dry deposition
50-75 Tg of SO2, SO4
Vegetation and soils 0.4 to 1.2 Tg of H2S, DMS, OCS, CS2, DMDS
Volcanoes 7-10 Tg of H2S, SO2, OCS
Anthropogenic Tg of SO2, sulfates
Biomass burning 2-4 Tg of H2S, SO2, OCS
Ocean Tg of DMS, OCS, CS2, H2S

17. The Carbon Cycle Atmosphere CO2 VOC, CH4, CO
Dry deposition and photosynthesis
Wet precipitation
Vegetation and soils VOC, CH4, CO2, CO
Anthropogenic VOC, CH4, CO2, CO
Biomass burning VOC, CH4, CO2, CO
Ocean VOC, CH4, CO2, CO

18. Carbon Emissions: Methane

19. There are hundreds of BVOCs emitted from Vegetation
flowers
~100’s of VOCs
cell walls
MeOH, HCHO
phytohormones
e.g. ethylene, DMNT
cell membranes
fatty acid peroxidation
wound-induced OVOCs
resin ducts / glands
terpenoid VOCs
cytoplasm/chloroplast
C1-C3 metabolites
chloroplast 19

20. Dry deposition and soil microbe uptake
Halogens
Atmosphere
Br-, I-, Cl-
CH3Cl, CH3Br, CH3I
Dry deposition and soil microbe uptake
Vegetation and soils
Anthropogenic
Biomass burning
Ocean

21. 3. Surface-atmosphere exchange: Recent scientific advances and challenges

22. How will biogenic VOC emissions respond to future 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

23. NCAR CCSM Future Landcover Change Predictions
Current
Future (2100)
One unique aspect in this study is the consideration LULC changes in the future. Under the A2 scenario we estimated climate and human influence have large impacts on future LULC in the US.
Percent land cover changes
Snow or Ice
-100%
Mixed Tundra
Wooden Tundra
Wooded Wetland
Evergrn. Broadlf.
Mix Shrub/Grass
1461%
Bare Sparse Veg.
1317%
Dryland Crop.
267%
Urban
205%

24. USDA predictions of tree species composition changes in the eastern U
USDA predictions of tree species composition changes in the eastern U.S.
Large increase in oak trees which have very high isoprene emissions
USDA climate change tree atlas
Current estimates are based on observations (FIA dist. Data). Future is based on 2x CO2 equil. climate vars from 3 GCMs (PCM, GFDL, HAD)
Provides future state level estimates of 135 tree species for eastern U.S.

25. (Future Isoprene – Current Isoprene Emission factors mg m-2 h-1)
Landcover change could result in a large regional increases and decreases in U.S. isoprene emissions
High = 5600
Low = -5900
(Future Isoprene – Current Isoprene Emission factors mg m-2 h-1)
This is mostly due to a predicted decrease in broadleaf tree coverage
High = 0%
Low = 30%
Broadleaf tree change
The overall impact is a large decrease in U.S. average isoprene emission factor (~800 mg m-2 h-1)

26. BVOC emissions will increase with increasing temperatures3
but we don’t know if the response will be similar to what is observed for short-term variations or if there will be an additional long-term component
Short-term and Long-term response
2.5 2
Isoprene emission activity
1.5
Short-term response 30 35 40 45
Guenther et al. 2006
Temperature (oC)

27. Decreasing emissions are expected for increasing CO2
but the magnitude is uncertain and there may be indirect CO2 effects (increasing LAI, changing species composition)
Heald et al. 2008

28. of future changes in biogenic VOC emissions
As a result of these uncertainties: Different models have substantially different predictions
of future changes in biogenic VOC emissions
Year 2050 BVOC – Year 2000 BVOC (g/m2/day)
These differences have a large impact on predicted future ozone and particles
Weaver et al. 2009

29. Why do recent “state-of-the-art” estimates of secondary organic aerosol (SOA) production differ by a factor of 5?
Goldstein and Galbally, ES&T, 2007
Hallquist et al., ACP, 2009
SOA: 134 TgC/yr
large uncertainty in estimates of Volatile Organic Carbon (VOC) deposition

30. for estimating dry deposition
Resistance Model
for estimating dry deposition CA RA
Aerodynamic resistance (turbulent diffusion)
Boundary layer resistance (molecular diffusion) RB RM RS
CUC
RLU CS
RML
RSL
CLC
: RC
RAG
Canopy resistance
RGS CG CC

31. We evaluated model performance for oxyVOC with measurements at a wide range of field sites

32. Why are we underestimating VOC deposition?
Our field flux measurements indicated that model Rc for oxygenated VOC is too high.
Why are we underestimating VOC deposition?
traditional model
modified model

33. The models assume that oVOC deposition is just a physical process
FL0 growth chamber experiments with Populus trichocarpa x deltoides

34. FL0 growth chamber experiments with Populus trichocarpa x deltoides
Stomata ~20-30 μm
We suspected that the high deposition rates were due to a biological process.
FL0 growth chamber experiments with Populus trichocarpa x deltoides

35. Exposure Experiments O3 fumigation MVK fumigation acetaldehyde
methyl vinyl ketone
acetaldehyde
methyl vinyl ketone
pre-fum
fum
post- fum
pre-fum
fum
post- fum

36. qPCR (quantitative polymerase chain reaction)
biotic and a-biotic stress markers
This tells us that the plants turned on these genes to actively take up oVOC
conversion of carbonyls (AAO2, ALDH2)
and oxidative stress repair (MsrA)
a-carbonic acid synthase (ACS)
carboxylic acid oxidase (ACO1)
ROS

37. Change in oVOC dry deposition when we put the new model in NCAR/MOZART model
This has a significant impact on regional atmospheric chemistry
Global increase in dry deposition: ~36%
Global decrease in wet deposition: ~7%

38. Any Questions?