Impact of tropospheric nitrogen dioxide on the regional radiation budget

Abstract. Following the launch of several satellite ultraviolet and visible spectrometers including the Ozone Monitoring Instrument (OMI), much has been learned about the global distribution of nitrogen dioxide (NO₂). NO₂, which is mostly anthropogenic in origin, absorbs solar radiation at ultraviolet and visible wavelengths. We parameterized NO₂ absorption for fast radiative transfer calculations. Using this parameterization with cloud, surface, and NO₂ information from different sensors in the NASA A-train constellation of satellites and NO₂ profiles from the Global Modeling Initiative (GMI), we compute the global distribution of net atmospheric heating (NAH) due to tropospheric NO₂ for January and July 2005. The globally-averaged NAH values due to tropospheric NO₂ are very low: they are about 0.05 W/m². While the impact of NO₂ on the global radiative forcing is small, locally it can produce instantaneous net atmospheric heating of 2–4 W/m² in heavily polluted areas. We assess the impact of clouds and find that they reduce the globally-averaged NAH values by 5–6% only. However, because most of NO₂ is contained in the boundary layer in polluted regions, the cloud shielding effect can significantly reduce the net atmospheric heating due to tropospheric NO₂ (up to 50%). We examine the effect of diurnal variations in NO₂ emissions and chemistry on net atmospheric heating and find only a small impact of these on the daily-averaged heating (11–14% at the most). We also examine the sensitivity of NO₂ absorption to various geophysical conditions. Effects of the vertical distributions of cloud optical depth and NO₂ on net atmospheric heating and downwelling radiance are simulated in detail for various scenarios including vertically-inhomogeneous convective clouds observed by CloudSat. The maximum effect of NO₂ on downwelling radiance occurs when the NO₂ is located in the middle part of the cloud where the optical extinction peaks.