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. SO2 and NO2 observations from the Ozone Mapping and Profiler Suite (OMPS) sensor are used for the first time in conjunction with the GEOS-Chem adjoint model to optimize both SO2 and NOx emission estimates over China for October 2013. Separate and joint (simultaneous) optimizations of SO2 and NO2 emissions are both conducted andcompared. Posterior emissions, compared to the prior, yield improvements insimulating columnar SO2 and NO2, in comparison to measurementsfrom the Ozone Monitoring Instrument (OMI) and OMPS. The posterior SO2 and NOx emissions fromseparate inversions are 748 Gg S and 672 Gg N, which are 36 % and 6 %smaller than prior MIX emissions (valid for 2010), respectively. In spite of the large reduction of SO2 emissions over the North China Plain, the simulated sulfate–nitrate–ammonium aerosol optical depth (AOD) only decrease slightly, which can be attributed to (a) nitrate rather than sulfate as the dominant contributor to AOD and (b) replacement of ammonium sulfate with ammonium nitrate as SO2 emissions are reduced. For joint inversions, both data quality control and the weight given to SO2 relative toNO2 observations can affect the spatial distributions of the posterioremissions. When the latter is properly balanced, the posterior emissionsfrom assimilating OMPS SO2 and NO2 jointly yield a difference of−3 % to 15 % with respect to the separate assimilations for totalanthropogenic SO2 emissions and ±2 % for total anthropogenicNOx emissions; but the differences can be up to 100 % for SO2and 40 % for NO2 in some grid cells. Improvements on SO2 andNO2 simulations from the joint inversions are overall consistent withthose from separate inversions. Moreover, the joint assimilations save∼ 50 % of the computational time compared to assimilating SO2and NO2 separately in a sequential manner of computation. Thesensitivity analysis shows that a perturbation of NH3 to 50 %(20 %) of the prior emission inventory can (a) have a negligible impact on theseparate SO2 inversion but can lead to a decrease in posterior SO2emissions over China by −2.4 % (−7.0 %) in total and up to −9.0 %(−27.7 %) in some grid cells in the joint inversion with NO2 and (b) yield posterior NOx emission decreases over China by −0.7 %(−2.8 %) for the separate NO2 inversion and by −2.7 % (−5.3 %) intotal and up to −15.2 % (−29.4 %) in some grid cells for the joint inversion. The large reduction of SO2 between 2010 and 2013, however, only leads to ∼ 10 % decrease in AOD regionally; reducingsurface aerosol concentration requires the reduction of emissions ofNH3 as well.

atmospheric chemistry and physics

Inverse modeling of SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; emissions over China using multisensor satellite data – Part 1: Formulation and sensitivity analysis