18.104.22.168 Nitrogen Oxides
The short lifetime and complex nonlinear chemistry, which cause two opposing indirect effects through ozone enhancements and CH4 reductions, make calculations of GWP for NOx emissions very uncertain (Shine et al., 2005a). In addition, the effect of nitrate aerosol formation (see Section 22.214.171.124), which has not yet been included in model studies calculating GWPs for NOx, can be significant. Due to the nonlinear chemistry, the net RF of NOx emissions will depend strongly on the location of emission and, with a strict definition of a pulse emission for the GWP, also on timing (daily, seasonal) of the emissions (Fuglestvedt et al., 1999; Derwent et al., 2001; Wild et al., 2001; Stevenson et al., 2004; Berntsen et al., 2005, 2006). Due to the lack of agreement even on the sign of the global mean GWP for NOx among the different studies and the omission of the nitrate aerosol effect, a central estimate for the 100-year GWP for NOx is not presented.
Chlorine- and bromine-containing halocarbons lead to ozone depletion when the halocarbon molecules are broken down in the stratosphere and chlorine or bromine atoms are released. Indirect GWPs for ozone-depleting halocarbons are estimated in Velders et al. (2005; their Table 2.7). These are based on observed ozone depletion between 1980 and 1990 for 2005 emissions using the Daniel et al. (1995) formulation. Velders et al. (2005) did not quote net GWPs, pointing out that the physical characteristics of the CFC warming effect and ozone cooling effect were very different from each other.