Worldwide biogenic soil NO_x emissions inferred from OMI NO₂ observations

Abstract. Biogenic NO_x emissions from soils are a large natural source with substantial uncertainties in global bottom-up estimates (ranging from 4 to 15 Tg N yr⁻¹). We reduce this range in emission estimates, and present a top-down soil NO_x emission inventory for 2005 based on retrieved tropospheric NO₂ columns from the Ozone Monitoring Instrument (OMI). We use a state-of-science soil NO_x emission inventory (Hudman et al., 2012) as a priori in the GEOS-Chem chemistry transport model to identify 11 regions where tropospheric NO₂ columns are dominated by soil NO_x emissions. Strong correlations between soil NO_x emissions and simulated NO₂ columns indicate that spatial patterns in simulated NO₂ columns in these regions indeed reflect the underlying soil NO_x emissions. Subsequently, we use a mass-balance approach to constrain emissions for these 11 regions on all major continents using OMI observed and GEOS-Chem simulated tropospheric NO₂ columns. We find that responses of simulated NO₂ columns to changing NO_x emissions are suppressed over low NO_x regions, and account for these non-linearities in our inversion approach. In general, our approach suggests that emissions need to be increased in most regions. Our OMI top-down soil NO_x inventory amounts to 10.0 Tg N for 2005 when only constraining the 11 regions, and 12.9 Tg N when extrapolating the constraints globally. Substantial regional differences exist (ranging from −40% to +90%), and globally our top-down inventory is 4–35% higher than the GEOS-Chem a priori (9.6 Tg N yr⁻¹). We evaluate NO₂ concentrations simulated with our new OMI top-down inventory against surface NO₂ measurements from monitoring stations in Africa, the USA and Europe. Although this comparison is complicated by several factors, we find an encouraging improved agreement when using the OMI top-down inventory compared to using the a priori inventory. To our knowledge, this study provides, for the first time, specific constraints on soil NO_x emissions on all major continents using OMI NO₂ columns. Our results rule out the low end of reported soil NO_x emission estimates, and suggest that global emissions are most likely around 12.9 ± 3.9 Tg N yr⁻¹.