Simulated tropospheric NOx: Its evaluation, global distribution and individual source contributions

Using the 11‐level Geophysical Fluid Dynamics Laboratory global chemical transport model, we simulate global tropospheric fields of NO x , peroxyacetyl nitrate (PAN), HNO 3 , and NO y , as well as the deposition of nitrate, extensively evaluate them against available observations from surface stations and aircraft missions, and quantify the contributions of individual natural and anthropogenic sources. The patterns and magnitudes of simulated and observed HNO 3 wet deposition are generally in good agreement around the globe. Scatterplots of model simulations versus aircraft observations for NO x and NO y find ∼50% of the points within ±25%, find ∼75% within ±50%, and show no systematic global biases. Both simulated and observed vertical profiles have similar shapes with high levels (∼1 ppbv or greater) in the polluted boundary layer (BL), very low values in the remote BL, and values increasing from the middle to the upper troposphere. Simulated NO y HNO 3 , and NO x + PAN are also in good agreement with extensive lower free tropospheric (FT) observations made at Mauna Loa Observatory. In general, the level of agreement between simulation and observation is as good as the agreement between separate, but simultaneous, observations of NO, NO x . or NO y . As previous studies have shown, fossil fuel combustion and biomass burning control NO x levels in most of the lower half of the troposphere with a significant contribution from biogenic emissions. The exceptions are the remote low‐NO x regions where BL and FT sources make comparable contributions. Unlike most previous studies, we find that the much smaller in situ FT sources generally dominate in the upper half of the troposphere. Lightning dominates in the tropics and summertime midlatitudes, and stratospheric injection is the major source in the summer high latitudes. The exception is transported emissions from fossil fuel combustion, which dominate in winter high latitudes. Though seldom dominant, aircraft emissions do have a significant impact on the upper troposphere and lower stratosphere of the northern hemisphere extratropics.