MAY 2005: Constraints on seasonal and interannual variability of North American isoprene emissions from satellite observations of formaldehyde column

Our previous work has shown that observations of formaldehyde (HCHO) column from the Global Ozone Monitoring Experiment (GOME) satellite instrument provide information to estimate biogenic VOC emissions, specifically isoprene emissions, on a global scale [Palmeret al, 2003]. Quantifying isoprene emissions using GOME HCHO columns is subject to errors involving the column retrieval and the assumed relationship between HCHO columns and isoprene emissions, taken here from the GEOS-CHEM chemical transport model. In this paper we use a six-year (1996-2001) HCHO column dataset from the GOME satellite instrument to (1) quantify these errors, (2) evaluate GOME-derived isoprene emissions with in situ flux measurements and a process-based emission inventory (MEGAN), and (3) investigate the factors driving the seasonal and interannual variability of North American isoprene emission.

Figure:Monthly mean isoprene emissions (10¹² atom C cm^-2 s^-1) over North America at 10-12 local time for May-September 2001 with cloud cover < 40%. MEGAN (left) and GOME (right) isoprene emissions are for averaged over the 2x2.5^o GEOS-CHEM grid. North American monthly totals are shown inset.

The seasonal variation of North American isoprene emissions during 2001 inferred from GOME is similar to MEGAN (Figure), with GOME emissions 10-30% higher. GOME and MEGAN both show a maximum over the southeastern United States but differ in the precise locations of maximum emissions. The MEGAN isoprene emission dependence on surface air temperature explains 80% of the month-to-month HCHO column variability observed over the southeastern United States during May-September, 1996-2001.