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

[摘要] Satellite-based retrievals of tropospheric NO 2 columns arewidely used to infer NO x ( ≡  NO  +  NO 2 ) emissions. These retrievals rely on model information for the vertical distribution ofNO 2 . The free tropospheric background above 2 km is particularlyimportant because the sensitivity of the retrievals increases with altitude. Free tropospheric NO x also has a strong effect on tropospheric OH and ozone concentrations. Here we use observations from three aircraft campaigns (SEAC 4 RS, DC3, and ATom) and four atmospheric chemistry models (GEOS-Chem, GMI, TM5, and CAMS) to evaluate the model capabilities for simulating NO x in the free troposphere and attribute it to sources. NO 2 measurements during the Studies of Emissions andAtmospheric Composition, Clouds, and Climate Coupling by Regional Surveys(SEAC 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 2 , and ozone observations from a DC3 flight in a thunderstorm outflow, we show that the NO 2 measurements in the upper troposphere are biased high, plausibly due to interference from thermally labile NO 2 reservoirs such as peroxynitric acid (HNO 4 ) and methyl peroxy nitrate (MPN). We find that NO 2 concentrations calculated from the NO measurements andNO–NO 2 photochemical steady state (PSS) are more reliable to evaluatethe vertical profiles of NO 2 in models. GEOS-Chem reproduces the shapeof the PSS-inferred NO 2 profiles throughout the troposphere forSEAC 4 RS and DC3 but overestimates NO 2 concentrations by about afactor of 2. The model underestimates MPN and alkyl nitrate concentrations,suggesting missing organic NO x chemistry. On the other hand, thestandard GEOS-Chem model underestimates NO observations from the Atmospheric Tomography Mission (ATom) campaigns over the Pacific and Atlantic oceans, indicating a missing NO 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 2 column density for the ATom campaign is 1.7  ±  0.44  ×  10 14  molec. cm −2 , and the NO 2 column density simulated by the four models is in the range of 1.4–2.4  ×  10 14  molec. cm −2 , implying that the uncertainty from using modeled NO 2 tropospheric columns over clean areas in the retrievals for stratosphere–troposphere separation is about 1  ×  10 14  molec. cm −2 . We find from GEOS-Chem that lightning is the main primary NO x 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 (parts per billion by volume) 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 NO 2 columns observed by satellites over the contiguous U.S. increases from 25  ±  11 % in winter to 65  ±  9 % in summer, according to the GEOS-Chem vertical profiles. This needs to be accounted for when deriving NO x emissions from satellite NO 2 column measurements.

[学科分类] 医学（综合）