Environmental Engineering Reference
In-Depth Information
Depending on composition, aerosols can heat or cool the atmosphere by
absorbing or scattering solar radiation (“direct effects”). For example, the overall
impact of absorption by black carbon is atmospheric warming, whereas sulfates
cool by scattering solar radiation back to space. By interacting with the hydrologic
cycle and changing cloud properties, aerosols also affect transmission of both
solar and terrestrial radiation (“indirect effects”). The overall impact of aerosols is
believed to be a cooling influence, estimated to offset ~75% of the positive radi-
ative forcing from CO
2
from 1750 to 2005, though large uncertainties surround
these estimates (Forster et al., 2007).
Since warm temperatures and stagnant air masses are conducive to O
3
pollution
episodes, changes in climate will likely affect air quality. A variety of modeling
approaches have been applied to project how local air quality will respond to
climate change: sensitivity studies in which individual meteorological parameters
are perturbed (e.g., Steiner et al., 2006); using observed historical correlations
between meteorological variables and air quality indices to statistically downscale
predictions of future meteorology from climate models (e.g., Holloway et al.,
2008); and dynamical downscaling, which links a suite of climate and atmospheric
chemistry models from global to regional scales (e.g., Hogrefe et al., 2004). In an
analysis of studies examining how a warmer climate will influence air pollution at
northern mid-latitudes, Jacob and Winner (2009) conclude that projected increases
in temperature and stagnation will exacerbate O
3
pollution in urban areas, parti-
cularly in the northeastern United States and southern and central Europe, regions
where climate models show consistency in predicted meteorological changes
(Christenson et al., 2007). Studies of the aerosol response to climate change disagree
in sign, reflecting discrepancies in model projections for changes in precipitation
frequency and ventilation in many polluted regions (Christenson et al., 2007;
Jacob and Winner, 2009 and references therein).
We present two examples of air pollutants influencing climate: (1) CH
4
and O
3
,
and (2) sulfate and black carbon aerosols, including potential impacts of aerosols
on precipitation. We then briefly review the key processes through which climate
is expected to affect air quality, mainly focusing on the more widely studied O
3
response to climate change (Jacob and Winner, 2009). Finally, we suggest steps
towards building confidence in model simulations of these processes.
2. Air Pollutants Influence Climate: Methane, Ozone,
and Aerosols
Methane is relatively well-mixed in the troposphere, reflecting its lifetime of
approximately a decade. In contrast to the O
3
precursors currently regulated to
abate O
3
pollution (NO
x
, NMVOC, and CO), the contribution from CH
4
to surface
O
3
is fairly uniform globally, though largest in high-NO
x
polluted regions (Fiore
et al., 2002, 2008). A multi-model study indicates that this result is robust, with a
20% decrease in global CH
4
abundances yielding roughly a 1.2 ppb decrease over
