Environmental Engineering Reference
In-Depth Information
NO
x
emissions by 95% compared to precontrol (U.S.,
pre-1971) levels. They also reduced CO emissions by
96% and hydrocarbons by 99% (MVMA 2000). Atmo-
spheric oxidation converts the emitted SO
x
and NO
x
rather rapidly into sulfates and nitrates whose small size
(0.1-1 m) is responsible for most of the light scattering.
There are no accurate estimates of global anthropo-
genic aerosol fluxes, but leaving aside dust generated by
field cultivation and enhanced soil erosion, it appears
that sulfates are the largest source (@150 Mt SO
4
/a) fol-
lowed by nitrates (@100 Mt NO
3
/a), organic carbon in
smoke from biomass combustion (@50 Mt/a), and fly
ash from coal-fired boilers (30 Mt/a). Annual global
emissions of black carbon were put at only 14 Mt, split
between fossil fuel and biomass combustion (Ramana-
than et al. 2001). Natural sources of aerosols, also diffi-
cult to quantify, mainly because of the total flux of
desert dust, are undoubtedly much larger than anthropo-
genic emissions, but because they are composed of larger
and hence fewer particles, the optical depths and hence
global atmospheric effects of the two kinds of particulates
are very similar.
Anthropogenic aerosols can have both negative (cool-
ing) and positive (warming) temperature forcing. Those
with diameters above 50 nm (sulfates in particular)
are excellent condensation nuclei, and their presence
increases cloud drop density (and hence cloud albedo)
and produces surface cooling. In contrast, black carbon
absorbs the incoming radiation and contributes to global
warming, and its effect may be disproportionate to its
emitted mass because it alters regional atmospheric sta-
bility, the water cycle, and climate. The leading emitters
are China, mostly from inefficient coal combustion, and
India, where biomass and coal combustion contribute
roughly equally (Venkataraman et al. 2005). Large-scale
effects of anthropogenic aerosols were noted first in
northern polar latitudes, where they are the primary con-
stituents of the Arctic haze, a thick (up to 3 km) brown-
ish or orange pall covering a circumpolar area as large as
North America (Nriagu, Coker, and Barrie 1991).
More dramatically, anthropogenic emissions, made
up mostly of sulfates, organics, nitrates, black carbon,
and fly ash, have been creating seasonal (January to
April) anthropogenic haze above the equatorial Indian
Ocean and reducing insolation by as much as 30 W/m
2
(Ramanathan et al. 2001). Global satellite measurements
indicate that by 2005 direct radiative forcing by aerosols
amounted to about
1.9 W/m
2
, significantly stronger
than standard model estimates, and that 47%G9% of
the aerosol optical thickness over land was due to anthro-
pogenic particles (Bellouin et al. 2005). This cooling ef-
fect is unevenly distributed, with pronounced peaks in
eastern North America, Europe, and Southeast and East
Asia, the regions with the highest sulfate levels. This ef-
fect also means that if aerosol emissions continue to de-
cline (in large part because of reduced emissions from
fossil and biomass fuel combustion) future atmospheric
warming could be greater than currently anticipated.
Yet the decline of aerosol emissions is desirable be-
cause they have long been implicated in aggravating
chronic respiratory diseases and, more recently, heart
attacks. Kaiser (2005) found that aerosols with diameter
less than 2.5 mm increased the risk of heart attacks and
caused as many as 60,000 premature deaths per year in
the United States alone. Another health risk arises from
photochemical smog, whose formation requires emis-
sions of nitrogen oxides (NO
x
: NO and NO
2
) generated
during high-temperature combustion, which breaks
the N
2
bond. NO
x
originates mostly in densely popu-
lated regions of the Northern Hemisphere, which have
