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Abstract:
To investigate the seasonal characteristics of submicron aerosol (PM1)
in Beijing urban areas, a high-resolution time-of-flight
aerosol-mass-spectrometer (HR-ToF-AMS) was utilized at an urban site in
summer (August to September 2011) and winter (November to December
2010), coupled with multiple state of the art online instruments. The
average mass concentrations of PM1 (60-84 mu gm(-3)) and its chemical
compositions in different campaigns of Beijing were relatively
consistent in recent years. In summer, the daily variations of PM1 mass
concentrations were stable and repeatable. Eighty-two percent of the PM1
mass concentration on average was composed of secondary species, where
62% is secondary inorganic aerosol and 20% secondary organic aerosol
(SOA). In winter, PM1 mass concentrations changed dramatically because
of the different meteorological conditions. The high average fraction
(58%) of primary species in PM1 including primary organic aerosol (POA),
black carbon, and chloride indicates primary emissions usually played a
more important role in the winter. However, aqueous chemistry resulting
in efficient secondary formation during occasional periods with high
relative humidity may also contribute substantially to haze in winter.
Results of past OA source apportionment studies in Beijing show 45-67%
of OA in summer and 22-50% of OA in winter can be composed of SOA. Based
on the source apportionment results, we found 45% POA in winter and 61%
POA in summer are from nonfossil sources, contributed by cooking OA in
both seasons and biomass burning OA (BBOA) in winter. Cooking OA,
accounting for 13-24% of OA, is an important nonfossil carbon source in
all years of Beijing and should not be neglected. The fossil sources of
POA include hydrocarbon-like OA from vehicle emissions in both seasons
and coal combustion OA (CCOA) in winter. The CCOA and BBOA were the two
main contributors (57% of OA) for the highest OA concentrations (>100 mu
gm(-3)) in winter. The POA/CO ratios in winter and summer are 11 and 16
mu gm(-3)ppm(-1), respectively, similar to ratios from western cities.
Higher OOA/O-x (=NO2+O-3) ratio (0.49 mu gm(-3)ppb(-1)) in winter study
than these ratios from western cities (0.03-0.16 mu gm(-3)ppb(-1)) was
observed, which may be due to the aqueous reaction or extra SOA
formation contributed by semivolatile organic compounds from various
primary sources (e.g., BBOA or CCOA) in Beijing. The evolution of oxygen
to carbon ratio (O/C) with photochemical age allows to estimate an
equivalent rate constant for chemical aging of OA in summer as
k(OH)similar to 4.1x10(-12)cm(3)molecule(-1)s(-1), which is of the same
order as obtained in other anthropogenic influenced areas and may be
useful for OA modeling.
