6.3.4. Nitrogen Oxides and Volatile Organic Compounds

6.3.4.1. Nitrogen Oxides Emissions

The 1990 NO_x emissions in the six SRES models range between 26.5 and 34.2 MtN, but not all the models provide a comprehensive description of NO_x missions. Some models do not estimate NO_x emissions at all (MARIA, MiniCAM), whereas others only include energy-related sources (MESSAGE) and have adopted other sources of emissions from corresponding scenarios derived from other models (i.e. MESSAGE uses corresponding AIM scenarios). Standardized (see Box 5-1 on Standardization) 1990 NO_x emissions in the SRES scenarios, measured as nitrogen, amount to 31 MtN (Figure 5-9 in Chapter 5).

As mentioned in Chapter 4, the volume of fossil fuels used for various energy purposes varies widely in the SRES scenario families. In addition, the level and timing of emission controls, inspired by local air quality concerns, is assumed to differ. As a result the spread of NO_x emissions is largest within the A1 scenario family (28-151 MtN by 2100), almost as large as the range across all 40 SRES scenarios (see Table 6-2b). Only in the highest emission scenarios (the fossil fuel intensive A1C and A1G scenario groups within the A1 scenario family and the high population, coal intensive A2 scenario family) do emissions rise continuously throughout the 21 st century. In the A1 ("balanced") scenario group and in the B2 scenario family, NO_x emission levels rise less. NO_x emissions tend to increase up to 2050 and stabilize thereafter, the result of a gradual substitution of fossil fuels by alternatives as well as of the increasing diffusion of NO_x control technologies. Low emission futures are described by various B1 family scenarios, as well as in the A1T scenario group that describe futures in which NO_x emissions are controlled because of either local air quality concerns or rapid technological change away from conventional fossil technologies. Overall, the SRES scenarios describe a similar upper range of NO_x emissions as the previous IS92 scenarios (151 MtN versus 134 MtN, respectively, by 2100), but extend the IS92 uncertainty range toward lower emission levels (16 versus 54 MtN by 2100 in the SRES and IS92 scenarios, respectively).

6.3.4.2. Volatile Organic Compounds, Excluding Methane

Non-methane volatile organic compounds (NMVOCs) arise from fossil fuel combustion (as with NO_x , wide ranges of emission factors are typical for internal combustion engines), and also from industrial processes, fuel storage (fugitive emissions), use of solvents (e.g., in paint and cleaners), and a variety of other activities. As chemical reactivities of the various substances grouped under the NMVOCs category are very different, so are their roles in ozone formation and the (potential) health hazards associated with them. In this report, NMVOCs are discussed as one group. In 1990, the estimated emissions range was between 83 and 178 Mt NMVOC, which after standardization (see Box 5-1) translates into 140 Mt NMVOC. As for NO_x emissions, not all models include this emissions category or all of its sources; the most detailed treatment of NMVOC emissions is given in the ASF model.

A relatively robust trend across all 40 scenarios (see Figure 5-10 in Chapter 5) is a gradual increase in NMVOC emissions up to about 2050, with a range of between 190 and 260 Mt. Beyond 2050, uncertainties increase with respect to both emission levels and trends. By 2100, the range is between 58 and 552 Mt, which extends the IS92 scenario range of 136 to 403 Mt by 2100 toward both higher and lower emissions (see Table 6-2b). As for NO_x emissions, the upper bounds of NMVOC emissions are formed by fossil fuel intensive scenario groups within the A1 scenario family (A1C, A1G, combined into one fossil intensive scenario group A1FI in the Summary for Policymakers, see also footnote 2), and the lower bounds by the scenarios within the B1 scenario family. Characteristic ranges are between 60 and 90 Mt NMVOC by 2100 in the low emissions cluster and between 370 and 550 Mt NMVOC in the high emissions cluster. All other scenario families and individual scenarios fall between these two emissions clusters; the B2 marker scenario (B2-MESSAGE) closely tracks the median of global NMVOC emissions from all the SRES scenarios (see Figure 5-10 in Chapter 5).

6.3.4.3. Carbon Monoxide

CO emissions in 1990 are estimated to range between 752 and 984 Mt CO (880 Mt CO after standardization) across the models used to derive the SRES scenarios. The same caveats as for NO_x and NMVOC emissions also apply to CO emissions - the number of models that represent all the emission source categories is limited and modeling and data uncertainties, such as emission factors, are considerable. As a result, CO emission estimates across scenarios are highly model specific and future emission levels overlap considerably between the four SRES scenario families (see Table 6-2b). By 2100, emissions range between 363 and 3766 Mt CO, a considerably larger uncertainty range, particularly toward higher emissions, than in IS92, for which the 2100 emission range was between 450 and 929 Mt CO (see Table 6-2b).

As for the NO_x and NMVOC emissions discussed above, the highest CO emission levels are associated with the high-growth fossil fuel intensive scenarios (A1C and A1G scenario groups, combined into one fossil intensive scenario group A1FI in the Summary for Policymakers; see also footnote 2) within the A1 scenario family, and the lowest emission levels are generally associated with the B1 and B2 scenario families. However, inter-model variability is considerable, which indicates that uncertainties are equally large with respect to scenario driving forces (such as energy demand and supply growth) and other factors that influence CO emissions (such as local air quality concerns or technological change).

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