Regional NOx emission inversion through a four-dimensional variational approach using SCIAMACHY tropospheric NO2 column observations

In this paper, the NOx emission scaling factors applied over the 2001 National Emissions Inventory (NEI) are estimated through a four-dimensional variational (4D-Var) approach using SCIAMACHY (Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY) tropospheric NO2 columns measured during summer 2004. In the “top-down” approach, two-month average NO2 columns are assimilated into a regional chemical transport model (CTM), STEM, using different assimilation setups. In a basic setup, NOx emissions are adjusted by assimilating the NO2 columns. A more general setup of emission inversion allows the initial O3 concentrations be adjusted along with the NOx emissions. A final case is set up to assimilate both the NO2 columns and O3 measurement from various platforms while allowing adjustments of both the NOx emissions and the initial O3 concentrations. It is found that the addition of O3 measurements did not improve the NOx emission inversion. With the NOx emission at surface and upper levels being adjusted separately, results from four cases show that the elevated NOx emission reduction ranges from 8.9% to 11.4%, and the surface NOx emission reduction is up to 6.6%. All the cases show NOx emission reduction in Ohio valley and Washington, District of Columbia areas.