Nitrogen oxides in the free troposphere: implications for tropospheric oxidants and the interpretation of satellite NO.sub.2 measurements

Satellite-based retrievals of tropospheric NO.sub.2 columns are widely used to infer NO.sub.x (â¡ NO + NO.sub.2) emissions. These retrievals rely on model information for the vertical distribution of NO.sub.2 . The free tropospheric background above 2 km is particularly important because the sensitivity of the retrievals increases with altitude. Free tropospheric NO.sub.x also has a strong effect on tropospheric OH and ozone concentrations. Here we use observations from three aircraft campaigns (SEAC.sup.4 RS, DC3, and ATom) and four atmospheric chemistry models (GEOS-Chem, GMI, TM5, and CAMS) to evaluate the model capabilities for simulating NO.sub.x in the free troposphere and attribute it to sources. NO.sub.2 measurements during the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC.sup.4 RS) and Deep Convective Clouds and Chemistry (DC3) campaigns over the southeastern U.S. in summer show increasing concentrations in the upper troposphere above 10 km, which are not replicated by the GEOS-Chem, although the model is consistent with the NO measurements. Using concurrent NO, NO.sub.2, and ozone observations from a DC3 flight in a thunderstorm outflow, we show that the NO.sub.2 measurements in the upper troposphere are biased high, plausibly due to interference from thermally labile NO.sub.2 reservoirs such as peroxynitric acid (HNO.sub.4) and methyl peroxy nitrate (MPN). We find that NO.sub.2 concentrations calculated from the NO measurements and NO-NO.sub.2 photochemical steady state (PSS) are more reliable to evaluate the vertical profiles of NO.sub.2 in models. GEOS-Chem reproduces the shape of the PSS-inferred NO.sub.2 profiles throughout the troposphere for SEAC.sup.4 RS and DC3 but overestimates NO.sub.2 concentrations by about a factor of 2. The model underestimates MPN and alkyl nitrate concentrations, suggesting missing organic NO.sub.x chemistry. On the other hand, the standard GEOS-Chem model underestimates NO observations from the Atmospheric Tomography Mission (ATom) campaigns over the Pacific and Atlantic oceans, indicating a missing NO.sub.x source over the oceans. We find that we can account for this missing source by including in the model the photolysis of particulate nitrate on sea salt aerosols at rates inferred from laboratory studies and field observations of nitrous acid (HONO) over the Atlantic. The median PSS-inferred tropospheric NO.sub.2 column density for the ATom campaign i