Worldwide biogenic soil NO x emissions inferred from OMI NO 2 observations
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 NO 2 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 NO 2 columns are dominated by soil NO x emissions. Strong correlations between soil NO x emissions and simulated NO 2 columns indicate that spatial patterns in simulated NO 2 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 NO 2 columns. We find that responses of simulated NO 2 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 NO 2 concentrations simulated with our new OMI top-down inventory against surface NO 2 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 NO 2 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.