SO 2 emissions and lifetimes: Estimates from inverse modeling using in situ and global, space‐based (SCIAMACHY and OMI) observations

Top‐down constraints on global sulfur dioxide (SO 2 ) emissions are inferred through inverse modeling using SO 2 column observations from two satellite instruments (SCIAMACHY and OMI). We first evaluated the SO 2 column observations with surface SO 2 measurements by applying local scaling factors from a global chemical transport model (GEOS‐Chem) to SO 2 columns retrieved from the satellite instruments. The resulting annual mean surface SO 2 mixing ratios for 2006 exhibit a significant spatial correlation (r = 0.86, slope = 0.91 for SCIAMACHY and r = 0.80, slope = 0.79 for OMI) with coincident in situ measurements from monitoring networks throughout the United States and Canada. We evaluate the GEOS‐Chem simulation of the SO 2 lifetime with that inferred from in situ measurements to verify the applicability of GEOS‐Chem for inversion of SO 2 columns to emissions. The seasonal mean SO 2 lifetime calculated with the GEOS‐Chem model over the eastern United States is 13 h in summer and 48 h in winter, compared to lifetimes inferred from in situ measurements of 19 ± 7 h in summer and 58 ± 20 h in winter. We apply SO 2 columns from SCIAMACHY and OMI to derive a top‐down anthropogenic SO 2 emission inventory over land by using the local GEOS‐Chem relationship between SO 2 columns and emissions. There is little seasonal variation in the top‐down emissions (<15%) over most major industrial regions providing some confidence in the method. Our global estimate for annual land surface anthropogenic SO 2 emissions (52.4 Tg S yr −1 from SCIAMACHY and 49.9 Tg S yr −1 from OMI) closely agrees with the bottom‐up emissions (54.6 Tg S yr −1 ) in the GEOS‐Chem model and exhibits consistency in global distributions with the bottom‐up emissions (r = 0.78 for SCIAMACHY, and r = 0.77 for OMI). However, there are significant regional differences.