OCTOBER 2003: Variations in HCHO During the Growing Season over North America

Monthly-mean HCHO columns over North America for March-October 1997. Data from the GOME satellite instrument (left) are compared to the GEOS-CHEM chemical transport model (right) sampled along the GOME orbit tracks at the GOME observation time. The GOME data have been mapped onto the GEOS-CHEM 2x2.5 degree model grid. The color scale is capped at 2.5x10¹⁶ molec cm^-2; values as high as 3x10¹⁶ molec cm^-2 are found over the southeastern United States in GEOS-CHEM in July and August. Negative HCHO columns in the GOME data are typically within the 4x10¹⁵ molec cm^-2 fitting uncertainty [Chance et al., 2000]. Values for November-February are generally below the GOME fitting uncertainty, both in the model and the observations, and are not included here. We exclude observation scenes with cloud or snow cover fraction >40% [Kurosu et al., 1999]. White areas denote regions with persistent cloud or snow cover fraction greater than this cut-off value.

The interannual variation in HCHO columns was investigated as well as the seasonal variation. Over North America during the growing season, isoprene is the main regional contributor to measureable HCHO enhancement [Lee et al., 1998; Sumner et al., 2001]. Temperature was found to have a strong influence on HCHO variation, as expected from isoprene's strong dependence on temperature [Guenther et al., 1995]. For details refer to Abbot et al., 2003