Nitrogen oxides in the free troposphere: implications for tropospheric oxidants and the interpretation of satellite NO₂ measurements

Abstract. Satellite-based retrievals of tropospheric NO2 columns are widely used to infer NOx (≡ NO + NO2) emissions. These rely on model information for the vertical distribution NO2. The free background above 2 km is particularly important because sensitivity increases with altitude. Free also has a strong effect OH and ozone concentrations. Here we use observations from three aircraft campaigns (SEAC4RS, DC3, ATom) four atmospheric chemistry models (GEOS-Chem, GMI, TM5, CAMS) evaluate capabilities simulating in troposphere attribute it sources. measurements during Studies Emissions Atmospheric Composition, Clouds, Climate Coupling by Regional Surveys (SEAC4RS) Deep Convective Clouds Chemistry (DC3) over southeastern U.S. summer show increasing concentrations upper 10 km, which not replicated GEOS-Chem, although consistent measurements. Using concurrent NO, NO2, DC3 flight thunderstorm outflow, that biased high, plausibly due interference thermally labile reservoirs such as peroxynitric acid (HNO4) methyl peroxy nitrate (MPN). We find calculated NO–NO2 photochemical steady state (PSS) more reliable profiles models. GEOS-Chem reproduces shape PSS-inferred throughout SEAC4RS but overestimates about factor 2. underestimates MPN alkyl concentrations, suggesting missing organic chemistry. On other hand, standard Tomography Mission (ATom) Pacific Atlantic oceans, indicating source oceans. can account this including photolysis particulate sea salt aerosols at rates inferred laboratory studies field nitrous (HONO) Atlantic. median column density ATom campaign 1.7 ± 0.44 × 1014 molec. cm−2, simulated range 1.4–2.4 implying uncertainty using modeled clean areas stratosphere–troposphere separation 1 cm−2. lightning main primary tropics southern midlatitudes, emissions dominate northern midlatitudes winter Particulate up 5 ppbv (parts per billion volume) extratropics model, would largely correct low bias relative ozonesonde observations. Global increase 19 %. contribution observed satellites contiguous 25 11 % 65 9 summer, according profiles. This needs be accounted when deriving satellite