Dylan Millet

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HCHO as a Proxy for Reactive VOCs
Dylan Millet University of Minnesota
Aura Science Meeting Columbia, MD October 27, 2008
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HCHO A Key Photochemical Intermediate
SOA
NO2
OH
VOCs
HO2 RO2
O3
NO
HCHO
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Mapping Isoprene Emissions from Space
Formaldehyde (HCHO) is a high-yield oxidation
product of isoprene
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Isoprene Drives OHCHO Variability over US in
Summer
INTEX-A
Measured HCHO production rate vs. column amount
Millet et al., 2006
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OHCHO Distribution over the US
Jul-Aug 2006
HCHO
NO2
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Relating HCHO Columns to Isoprene Emission
OMI HCHO Columns July-August 2006
OHCHO m?Eisoprene B
GEOS-Chem global 3D model of atmospheric chemistry
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Top-Down Constraints on Isoprene Emissions
MEGAN (bottom-up)
MEGAN - OMI
Bottom-up emissions 70 too high over dominant
source regions
Emissions too low in Deep South
Bias 100
Large modeled fluxes driven by oak tree cover,
high T

• Underestimate of understory broadleaf tree
  coverage?
• Evergreen trees /crop emissions underestimated?

Broadleaf Tree Cover
CLM Fineleaf Evergreens
CLM Crops

? Broadleaf tree isoprene emissions currently
overestimated
Millet et al., 2008
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Separating Biogenic, Pyrogenic, Anthropogenic VOC
Sources
Asia
Amazon
Barkley et al., 2008.
Fu et al., 2007.
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What is the Useful Resolution of the Data?
Std. Error in OMI HCHO (3-Day Mean, 0.25 grid)
V002
V003
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Links to Air Quality Aerosol Formation
Summer Winter AOT
Goldstein et al., submitted.
OMI HCHO (Summer)
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Links to Air Quality Photochemistry
Boundary Layer VOC OH Reactivity vs. HCHO
HCHO as a Proxy for VOC OH Reactivity?
INTEX-A
VOC OH Reactivity (s-1)
Measure of air mass photochemical reactivity
INTEX-B (Mexico)
Measurements Fried, Blake, Singh, Sachse,
Diskin, Heikes
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Testing Bottom-Up Emissions
OMI isoprene emission
Best Bottom-Up Estimate
MEGAN model, CLM land cover
Difference (MEGAN OMI)
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Top-Down Constraints on Isoprene Emission Factors
Regress OMI isoprene emissions against PFT
coverage
Constant EFs More consistent with OMI Optimum
broadleaf tree EF 13 x 1012 atomsC/cm2/s
similar to MEGAN mean rejects MEGANs use of
3-4x higher EFs in certain locations
Possible explanation for OMI-MEGAN
discrepancy Fast chemical loss within forest
canopies? Farmer and Cohen 2008 , Kuhn et al.
2008
Millet et al., JGR (2008).
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Mapping Isoprene Emissions from Space
HCHO vertical columns from OMI July August 2006
OH, h? ki, Yi
OH, h? kHCHO
HCHO
VOCs
Local OHCHO-Ei Relationship