Surface-Atmosphere fluxes
Outline
Alex
Guenther
• Introduction
Atmospheric Chemistry Division
• Major cycles
National Center for Atmospheric Research
• Recent scientific
advances
Boulder
CO, USAand
challenges
1. Introduction
What is in the atmosphere? How did
it get there? How does it leave?
What is in the Atmosphere?
Well mixed
Variable
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%), CFCs
H2O, O3, CO, non-methane VOC, NOy, NH3, NO3,
NH4, OH, HO2, H2O2, CH2O, SO2, CH3SCH3,
CS2, OCS, H2S, SO4, HCN
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.
Where does the atmosphere come from?
Cosmos
Earth
1.
2.
3.
4.
Original atmosphere
Dead planet
Living planet
Anthropocene
Global Biogeochemical Cycles
Air Quality: Cloud processesWeather/Climate:
Organic
Photoozone and
Temperature,
aerosol
oxidant
particles
sunshine,
processes
processes
precipitation
Biological
CO2
NOy
Latent and H2O
particles and
NO/NH3
sensible heat
VOC
NH3
emission
emissions
Water &
Carbon
Nitrogen
Energy
Cycle
Cycle
Cycles
Precipitation and
solar radiation
Natural
Ecosystem Health:
Productivity,
diversity, water
availability
Ozone and N
deposition
Anthropogenic
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.
Eddy Covariance Flux Data
)
Concentration and wind speed
measurements above a forest canopy
Sampling rate = 10 Hz
C ( g m
-3
20
10
0
10
w (m s
-1
)
0
s )
-1
( g m
-2
20
10
20
0
40
50
60
70
30
40
50
60
70
40
50
Flux
C
12
6
0
-6
30
Vertical wind speed
B
3
1 .5
0
-1 .5
0
w 'C '
Concentration
A
10
20
30
Time (seconds)
60
70
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.
Surface layer gradients
K: eddy diffusivity coefficient
Flux = K dC/dz
dz: vertical height difference
dC: concentration difference
inertial sublayer
dC
HEIGHT
dz
Concentration
Profile
roughness
sublayer
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
Imaginary box
HEIGHT
zi
May need to consider
MIXED LAYER
Conc.
Profile
- chemical loss/production
- horizontal advection
- non-stationary
0
0
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).
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).
THE NITROGEN CYCLE
ATMOSPHERE
fixation
N2
combustion
lightning
NO
oxidation
HNO3
biofixation
orgN
BIOSPHERE
SOIL/OCEAN
burial
denitrification
deposition
decay
assimilation
NH3/NH4+
nitrification
NO3weathering
LITHOSPHERE
Daniel Jacob 2008
Natural
Anthropogenic
Atmospheric ammonia sources and sinks (Tg per year)
Sources
Domestic animals:
Human excrement:
Industry:
Fertilizer losses:
Fossil fuel combustion:
Biomass Burning:
Soil:
Wild animals:
Ocean:
21
2.6
0.2
9
0.1
5.7
6
0.1
8.2
Sinks
Wet precipitation (land): 11
Wet precipitation (ocean): 10
Dry deposition (land):
11
Dry deposition (ocean):
5
Reaction with OH:
3
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
Atmospheric NOx sources and sinks (Tg per year)
Sources
Aircraft:
Fossil fuel combustion:
Biomass Burning:
Soil:
Lightning:
NH3 oxidation:
Stratosphere:
Ocean:
0.5
20
12
20
8
3
0.1
<1
Sinks
Wet precipitation (land): 19
Wet precipitation (ocean): 8
Dry deposition:
11
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
The Sulfur Cycle
Atmosphere
SO2, SO4
H2S, DMS, OCS, CS2, DMDS
Vegetation and
soils 0.4 to 1.2
Tg of H2S,
DMS, OCS,
CS2, DMDS
Volcanoes
7-10 Tg of
H2S, SO2,
Biomass
OCS
burning 2-4
Tg of H2S,
Ocean 10SO2, OCS
40 Tg of
DMS, OCS,
CS2, H2S
Wet
Dry deposition
50-75 Tg of deposition
50-75 Tg of
SO2, SO4
SO2, SO4
Anthropogenic
88-92 Tg of SO2,
sulfates
The Carbon Cycle
Atmosphere
CO2
VOC, CH4, CO
Dry deposition
and
photosynthesis
Vegetation
and soils
VOC, CH4,
CO2, CO
Ocean
VOC, CH4,
CO2, CO
Biomass
burning VOC,
CH4, CO2, CO
Anthropogenic
VOC, CH4, CO2,
CO
Wet
precipitation
Carbon Emissions: Methane
There are hundreds cytoplasm/chloroplast
of BVOCs
C1-C3 metabolites
emitted from
resin ducts Vegetation
/ glands
terpenoid VOCs
chloroplast
terpenoid VOCs
phytohormones
e.g. ethylene,
DMNT
cell walls
MeOH, HCHO
cell membranes
fatty acid peroxidation
wound-induced OVOCs
flowers
~100’s of VOCs
Halogens
Atmosphere
Br-, I-, Cl-
CH3Cl, CH3Br, CH3I
Dry deposition
and soil
microbe uptake
Vegetation
and soils
Anthropogenic
Biomass
burning
Ocean
3. Surfaceatmosphere
exchange: Recent
scientific advances
and challenges
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
NCAR CCSM Future Landcover
Change Predictions
Current
Future
(2100)
Percent land cover changes
Snow or Ice
-100%
Mix Shrub/Grass
1461%
Mixed Tundra
-100%
Bare Sparse Veg.
1317%
Wooden Tundra
-100%
Dryland Crop.
267%
Wooded Wetland
-100%
Urban
205%
Evergrn. Broadlf.
-100%
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.
Landcover change could result in a large regional increases and
decreases in U.S. isoprene emissions
High = 5600
(Future Isoprene – Current Isoprene
Emission factors g m-2 h-1)
The overall impact is a large
decrease in U.S. average
isoprene emission factor
(~800 g m-2 h-1)
Low = -5900
This is mostly due to a
predicted decrease in
broadleaf tree coverage
Broadleaf
tree change
High = 0%
Low = 30%
BVOC emissions will increase with
increasing temperatures
Isoprene emission activity
3
Short-term and Long-term
response
2.5
2
1.5
Short-term
response
30
35
40
Temperature (oC)
45
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 longterm component
Guenther et al. 2006
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
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
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
Resistance Model
CA
for estimating dry deposition
Fd  v d  C A
RA
Aerodynamic resistance
(turbulent diffusion)
Boundary layer resistance
(molecular diffusion)
RB
CU
C
CLC
RM
RS
RL
vd 
CS
U
RML RSL
1
R A  R B  RC
: RC
RAG
Canopy resistance
RGS
CG
CC
We evaluated model performance for oxyVOC
with measurements at a wide range of field sites
Our field flux measurements indicated that model
Rc for oxygenated VOC is too high.
Why are we underestimating
VOC deposition?
traditional
model
modified
model
The models assume
that oVOC
deposition is just a
physical process
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
Exposure Experiments
MVK fumigation
O3 fumigation
acetaldehyde
methyl vinyl ketone
pre-fum
fum
acetaldehyde
methyl vinyl ketone
post- fum
pre-fum
fum
post- fum
qPCR (quantitative polymerase chain reaction)
biotic and a-biotic
stress markers
100.0
conversion of carbonyls (AAO2, ALDH2)
and oxidative stress repair (MsrA)
a-carbonic acid synthase (ACS)
carboxylic acid oxidase (ACO1)
ROS
10.0
This tells us
that the plants
turned on
these genes to
actively take
up oVOC
2-folding
ozone
mvk
wound
significance level
1.0
0.1
MsrA AAO2 ALDH2 SOD
APX
ACS
ACO1 p450
DHQ WRKY
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%
Any Questions?