Spatial distribution of isoprene emissions from North America derived from formaldehyde column measurements by the OMI satellite sensor

Abstract

Space‐borne formaldehyde (HCHO) column measurements from the Ozone Monitoring Instrument (OMI), with 13 × 24 km 2 nadir footprint and daily global coverage, provide new constraints on the spatial distribution of biogenic isoprene emission from North America. OMI HCHO columns for June‐August 2006 are consistent with measurements from the earlier GOME satellite sensor (1996–2001) but OMI is 2–14% lower. The spatial distribution of OMI HCHO columns follows that of isoprene emission; anthropogenic hydrocarbon emissions are undetectable except in Houston. We develop updated relationships between HCHO columns and isoprene emission from a chemical transport model (GEOS‐Chem), and use these to infer top‐down constraints on isoprene emissions from the OMI data. We compare the OMI‐derived emissions to a state‐of‐science bottom‐up isoprene emission inventory (MEGAN) driven by two land cover databases, and use the results to optimize the MEGAN emission factors (EFs) for broadleaf trees (the main isoprene source). The OMI‐derived isoprene emissions in North America (June–August 2006) with 1° × 1° resolution are spatially consistent with MEGAN (R 2 = 0.48–0.68) but are lower (by 4–25% on average). MEGAN overestimates emissions in the Ozarks and the Upper South. A better fit to OMI (R 2 = 0.73) is obtained in MEGAN by using a uniform isoprene EF from broadleaf trees rather than variable EFs. Thus MEGAN may overestimate emissions in areas where it specifies particularly high EFs. Within‐canopy isoprene oxidation may also lead to significant differences between the effective isoprene emission to the atmosphere seen by OMI and the actual isoprene emission determined by MEGAN.