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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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