Worldwide biogenic soil NOx emissions inferred from OMI NO2 observations

G. C. M. Vinken, K. F. Boersma, J. D. Maasakkers, M. Adon, R. V. Martin
2014 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> Biogenic NO<sub>x</sub> emissions from soils are a large natural source with substantial uncertainties in global bottom-up estimates (ranging from 4 to 15 Tg N yr<sup>−1</sup>). We reduce this range in emission estimates, and present a top-down soil NO<sub>x</sub> emission inventory for 2005 based on retrieved tropospheric NO<sub>2</sub> columns from the Ozone Monitoring Instrument (OMI). We use a state-of-science soil NO<sub>x</sub> emission inventory (Hudman et
more » ... entory (Hudman et al., 2012) as a priori in the GEOS-Chem chemistry transport model to identify 11 regions where tropospheric NO<sub>2</sub> columns are dominated by soil NO<sub>x</sub> emissions. Strong correlations between soil NO<sub>x</sub> emissions and simulated NO<sub>2</sub> columns indicate that spatial patterns in simulated NO<sub>2</sub> columns in these regions indeed reflect the underlying soil NO<sub>x</sub> 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<sub>2</sub> columns. We find that responses of simulated NO<sub>2</sub> columns to changing NO<sub>x</sub> emissions are suppressed over low NO<sub>x</sub> 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<sub>x</sub> 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<sup>−1</sup>). We evaluate NO<sub>2</sub> concentrations simulated with our new OMI top-down inventory against surface NO<sub>2</sub> 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<sub>x</sub> emissions on all major continents using OMI NO<sub>2</sub> columns. Our results rule out the low end of reported soil NO<sub>x</sub> emission estimates, and suggest that global emissions are most likely around 12.9 ± 3.9 Tg N yr<sup>−1</sup>.</p>
doi:10.5194/acp-14-10363-2014 fatcat:jgbu3fxworhrnh6gah64m5p4ui