Interactions between air quality and climate change - Air Pollution Modeling and Its Application - page 423

populated regions in the northern hemisphere ( Fig. 1 ; Fiore et al., 2009). As CH 4
is not currently regulated for air quality, its contribution to surface O 3 , along with
that from foreign (and natural) emissions of the other O 3 precursors, is typically
considered part of the “background”. While combined reductions of NO x , NMVOC,
and CO within a region are more effective than equivalent percentage reductions
of CH 4 at decreasing surface O 3 within that same region, “background” surface O 3
responds roughly equivalently to foreign anthropogenic emission reductions of
CH 4 and NO x +NMVOC+CO ( Fig. 2 ).

Fig. 1. Decrease in surface O 3 (ppb) resulting from a 20% decrease in global CH 4 abundances
(from 1,760 to 1,408 ppb) in 18 global or hemispheric chemical transport models over
continental-scale source regions: North America (NA; 15-55°N, 60-125°W), Europe (EU; 25-
65°N; 10°W-50°E), East Asia (EA; 15-50 N; 95-160°E) and South Asia (SA; 5-35°N; 50-
95°E) (Fiore et al., 2009). Model ensemble mean (blue) and range across individual models
(black lines) are shown

°

Fig. 2. Model ensemble surface O 3 decrease (ppb), annually and spatially averaged over the
regions in Fig. 1 from 20% decreases in anthropogenic emissions of NO x + CO + NMVOC (red)
versus 20% decreases in anthropogenic CH 4 (blue). Influence of each source region on surface
O 3 within the same region (termed “domestic”; left panel). Sum of the O 3 responses to emission
changes within the three foreign source regions; these components are generally considered to be
“background” O 3 (right panel) (Adapted from Figure 3 of Fiore et al. 2009)

The availability of low-cost options suggests that CH 4 emission controls are
feasible for jointly addressing climate and O 3 air quality goals (West and Fiore,
2005). Full-chemistry transient simulations (2005-2030) in the GFDL MOZART-
2 global tropospheric chemistry model indicate that cost-effective CH 4 controls (at
a marginal cost of ~$15/t CO 2 equivalent) would offset the positive climate forcing
from CH 4 and O 3 that would otherwise occur from increases in NO x and CH 4
emissions in the baseline CLE (Current Legislation) scenario ( Fig. 3a ; Fiore et al.,
2008). Cost-effective controls (scenario B in Fig. 3) are sufficient to decrease the
incidence of O 3 events above 70 ppb in the model to below the number of occur-
rences in 2005 over Europe in summer; over the United States, only the simulation
where CH 4 is set to pre-industrial levels (CH 4 -700) achieves this result, reflecting
differences in regional projections for NO x emissions under the baseline scenario
(Fig. 3b; Fiore et al., 2008). In all cases, however, CH 4 reductions decrease O 3
relative to the 2030 CLE baseline scenario.