Global Photochemical Model Evaluation using GOME Tropospheric Column Data

The focus of our TROPOSAT-related work has been mainly on comparisons of tropospheric NO₂ with output from our global chemistry-transport model MATCH-MPIC. Global monthly NO₂ distributions from MATCH-MPIC were compared with GOME NO₂ columns in von Kuhlmann (2001). A closer focus on NO₂ over Asia has been taken by a joint PhD student with the University of Bremen (T. Kunhikrishnan); the first part of this study has been aimed at improving our understanding of NO_X in the troposphere over Asia (especially India), and at developing an improved estimate of the NO_X emissions rate from this region, through the use of sensitivity runs with MATCH-MPIC along with GOME tropospheric NO₂ columns. We first verified that the retrieval assumptions from GOME (stratospheric zonal symmetry and negligible troposphere NO₂ in the Pacific reference sector) hold well for MATCH-MPIC. We then showed that the model simulates the mean NO₂ amounts in the region fairly well, and that the NO_X lifetime of about 15 hours computed by the model, agrees well with the lifetime derived from the decay of NO₂ off the west coast of India. Taken together, these imply that the current total magnitude of NO_X emissions in MATCH-MPIC is reasonable. While 60–70 % of the NO_X in the lower troposphere over India comes from Indian emissions, in the upper troposphere the NO_X comes mainly from outside the region during the winter months and from convective transport and lightning during the summer monsoon.