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

Abstract. Satellite-based retrievals of tropospheric NO2 columns are used to infer NOx (NO+NO2) emissions at the surface. These retrievals rely on model information for the vertical distribution of NO2. The free tropospheric background above 2 km is particularly important because the sensitivity of the retrievals increases with altitude. Free tropospheric NOx also has a strong effect on tropospheric OH and ozone concentrations. Here we use observations from three aircraft campaigns (SEAC4RS, DC3, and ATom) and four atmospheric chemistry models (GEOS-Chem, GMI, TM5, and CAMS) to evaluate the model capabilities for simulating background NOx and attribute this background to sources. NO2 measurements over the southeast US during SEAC4RS and DC3 show increasing concentrations in the upper troposphere above 10 km, which is not replicated by GEOS-Chem although the model is consistent with the NO measurements. Using concurrent NO, NO2 and ozone observations from a DC3 flight in a thunderstorm outflow, we show that NO2 measurements in the upper troposphere are biased high, plausibly due to interference from thermally labile NO2 reservoirs, such as peroxynitric acid (HNO4) and methyl peroxy nitrate (MPN). We find that NO2 concentrations calculated from the NO measurements and NO-NO2 photochemical steady state (PSS) are more reliable to evaluate the vertical profiles of NO2 in models. GEOS-Chem reproduces the shape of the PSS-inferred NO2 profiles throughout the troposphere for SEAC4RS and DC3 but overestimates NO2 concentrations by about a factor of 2. The model underestimates MPN and alkyl nitrate concentrations, suggesting missing organic NOx chemistry. On the other hand, the standard GEOS-Chem model underestimates NO observations from the ATom campaigns over the Pacific and Atlantic Oceans, indicating a missing NOx 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 average NO2 column density for the ATom campaign in the GEOS-Chem simulation is 2.4×1014 molec cm-2 with particulate nitrate photolysis and 1.5×1014 molec cm-2 without, compared to 1.9×1014 molec cm-2 in the observations (using PSS NO2) and 1.4–2.4×1014 molec cm-2 in the GMI, TM5 and CAMS models. We find from GEOS-Chem that lightning is the main primary NOx source in the free troposphere over the tropics and southern midlatitudes, but aircraft emissions dominate at northern midlatitudes in winter and in summer over the oceans. Particulate nitrate photolysis increases ozone concentrations by up to 5 ppbv in the free troposphere in the northern extratropics in the model, which would largely correct the low model bias relative to ozonesonde observations. Global tropospheric OH concentrations increase by 19 %. The contribution of the free tropospheric background to the tropospheric NO2 columns observed by satellites over the contiguous US increases from 25 % in winter to 65 % in summer according to the GEOS-Chem vertical profiles. This needs to be accounted for when deriving NOx emissions from satellite NO2 column measurements.