PM2.5 concentrations have
decreased remarkably in China
in recent years, coinciding with a more rapid decrease in SO2 concentrations and a slower decrease in NO2 concentrations,
while O3 concentrations increased. Correlations between
PM2.5 and key gaseous pollutants were studied to identify
linked trends as a means of understanding the impacts of air pollution
control in China. In most cities, the PM2.5–NO2 correlation coefficients were higher than the PM2.5–SO2 correlation coefficients, and the gap tended
to expand as air quality improved. Multiple linear regression also
indicated that PM2.5 concentrations were more sensitive
to changes in NO2 than in SO2. The rate of decrease
in the PM2.5 concentration with a decreasing NO2 concentration is nearly 3 times higher than that with SO2. These results support the priority of controlling NO
x
to further reduce PM2.5 pollution in
China. The chemistry behind this was twofold: (1) NO
x
can be converted into nitrate, and (2) NO
x
contributes to atmospheric oxidation capacity. The decrease
in PM2.5 concentration always coincided with an increase
in O3 concentration when the PM2.5 concentration
was higher than 50 μg m–3. However, the correlation
between PM2.5 and O3 tended to change from negative
to positive as air quality improved, indicating O3 and
PM2.5 control could both benefit from reducing the concentrations
of gas precursors.
Abstract. Gasoline evaporative emissions have become an important anthropogenic source
of urban atmospheric volatile organic compounds (VOCs) and secondary organic
aerosol (SOA). These emissions have a significant impact on regional air
quality, especially in China where car ownership is growing rapidly.
However, the contribution of evaporative emissions to secondary aerosol
(SA) is not clear in an air pollution complex in which a high concentration of
SO2 and NH3 was present. In this study, the effects of SO2
and NH3 on SA formation from unburned gasoline vapor were investigated
in a 30 m3 indoor smog chamber. It was found that an increase in SO2
and NH3 concentrations (0–151 and 0–200 ppb, respectively)
could linearly promote the formation of SA, which could be enhanced by a
factor of 1.6–2.6 and 2.0–2.5, respectively. Sulfate was most sensitive
to the SO2 concentration, followed by organic aerosol, which was due
not only to the acid catalytic effect, but was also related to the formation of
organic sulfur-containing compounds. In the case of an increasing NH3
concentration, ammonium nitrate increased more significantly than organic
aerosol, and nitrogen-containing organics were also enhanced, as revealed by
the results of positive matrix factorization (PMF) analysis. New particle
formation (NPF) and particle size growth were also significantly enhanced in
the presence of SO2 and NH3. This work indicates that gasoline
evaporative emissions will be a significant source of SA, especially in the
presence of high concentrations of SO2 and NH3. Meanwhile, these
emissions might also be a potential source of sulfur- and
nitrogen-containing organics. Our work provides a scientific basis for the
synergistic emission reduction of secondary aerosol precursors, including
NOx, SO2, NH3, and particularly VOCs, to mitigate particulate matter (PM) pollution
in China.
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