Working Group I: The Scientific Basis

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6.6.3 Indirect Forcing by NOx Emissions

Through production of tropospheric O3, emissions of nitrogen oxides (NOx = NO + NO2) lead to a positive radiative forcing of climate (warming), but by affecting the concentration of OH they reduce the levels of CH4, providing a negative forcing (cooling) which partly offsets the O3 forcing. Due to non-linearities in O3 photochemical production together with differences in mixing regimes and removal processes, the O3 and OH changes strongly depend on the localisation of the NOx surface emission perturbation, as calculated by Hauglustaine and Granier (1995), Johnson and Derwent (1996), Berntsen et al. (1996), Fuglestvedt et al. (1996, 1999) and Gupta et al. (1998). The CH4 and O3 forcings are similar in magnitude, but opposite in sign, as calculated by Fuglestvedt et al. (1999). Due to differences in CH4 and O3 lifetimes, the NOx perturbation on the CH4 forcing acts on a global scale over a period of approximately a decade, while the O3 forcing is of regional character and occurs over a period of weeks. Based on three-dimensional model results, Fuglestvedt et al. (1999) have calculated that the O3 radiative forcing per change in NOx emission (10-2 Wm-2 per TgN/yr) is 0.35 and 0.29 for the USA and Scandinavia, respectively, and reaches 2.4 for Southeast Asia. The CH4 forcing per change in NOx emission ranges from -0.37 (Scandinavia) and -0.5 (USA) to -2.3 (Southeast Asia) in the same units. Additional work is required to assess the impact of NOx on the radiative forcing of climate.

Deep convection can remove pollutants from the lower atmosphere and inject them rapidly into the middle and upper troposphere and, occasionally, into the stratosphere. Changes in convective regimes associated with climate changes have therefore the potential to significantly modify the distribution and the photochemistry of O3 in a region where its impact on the radiative forcing is the largest. Based on the tropospheric O3 column derived from satellite during the 1997 to 1998 El Niño, Chandra et al. (1998) reported a decrease in O3 column of 4 to 8 Dobson Units (DU) in the eastern Pacific and an increase of 10 to 20 DU in the western Pacific, largely as a result of the eastward shift of the tropical convective activity. Lelieveld and Crutzen (1994) showed that convective transport can change the budget of O3 in the troposphere. Berntsen and Isaksen (1999) have also indicated that changes in convection can modify the sensitivity of the atmosphere to anthropogenic perturbations as aircraft emissions. Their study indicates that reduced convective activity leads to a 40% increase in the O3 response to aircraft NOx emissions due to modified background atmospheric concentrations. In addition to that, lightning is a major source of NOx in the troposphere and thus contributes to the photochemical production of O3 (Huntrieser et al., 1998; Wang et al., 1998; Hauglustaine et al., 2001; Lelieveld and Dentener, 2000). In the tropical mid- and upper troposphere, modelling studies by Lamarque et al. (1996), Levy et al. (1996), Penner et al. (1998a), and Allen et al. (2000) have calculated a contribution of lightning to the NOx levels of 60 to 90% during summer. Direct observations by Solomon et al. (1999) of absorption of visible radiation indicate that nitrogen dioxide can lead to local instantaneous radiative forcing exceeding 1 Wm-2. These enhancements of NO2 absorption are likely to be due both to pollution and to production by lightning in convective clouds. Further measurements are required to bracket the direct radiative forcing by NO2 under a variety of storm and pollution conditions. On the basis of two-dimensional model calculations, Toumi et al. (1996) calculated that for a 20% increase of lightning the global mean radiative forcing by enhanced tropospheric O3 production is about 0.1 Wm-2. Based on an apparent correlation between lightning strike rates and surface temperatures, Sinha and Toumi (1997) have suggested a positive climate feedback through O3 production from lightning NOx in a warmer climate. Improved modelling and observations are required to confirm this hypothesis.

Aircraft emissions also have the potential to alter the composition of the atmosphere and induce a radiative forcing of climate. According to IPCC (1999), in 1992 the NOx emissions from subsonic aircraft are estimated to have increased O3 concentrations at cruise altitudes in the Northern Hemisphere mid-latitude summer by about 6% compared with an atmosphere without aircraft. The associated global mean radiative forcing is 0.023 Wm-2. In addition to increasing tropospheric O3, aircraft NOx are expected to decrease the concentration of CH4 inducing a negative radiative forcing of -0.014 Wm-2 for the 1992 aircraft fleet. Again, due to different O3 and CH4 lifetimes, the two forcings show very different spatial distribution and seasonal evolution (IPCC, 1999). The O3 forcing shows a marked maximum at northern mid-latitudes, reaching 0.06 Wm-2, whilst the CH4 forcing exhibits a more uniform distribution, with a maximum of about -0.02 Wm-2 in the tropical regions. Based on IPCC (1999), the O3 forcing calculated for 2050 conditions is 0.060 Wm-2 and the CH4 forcing is -0.045 Wm-2.

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