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The major inorganic constituents and organic tracer compounds in PM2.5 were used in positive matrix factorization(PMF)and chemical mass balance(CMB)models to apportion the primary and secondary source contributions to organic carbon(OC)in Hong Kong during the summer of 2006.Secondary organic aerosol(SOA)tracers of several biogenic and anthropogenic hydrocarbons were included in the PMF analysis.Their inclusion allowed the identification of two sources of SOA among seven sources resolved by PMF.One SOA source was mainly associated with secondary sulfate and nitrate.The other SOA source, characterized by biogenic SOA tracers and mixed with biomass burning and vegetative detritus particles, was biomass burning-induced SOA.Secondary OC(SOC)apportioned by PMF(SOCPMF)was on average 6.84μgC m-3(65%of PM 2.5 OC)on high pollution days under influence of significant regional transport (i.e.,regional days)and 0.70μgC m-3(25%of PM 2.5 OC)on days under the influence of mainly local emissions(i.e.,local days).SOCPMF was compared with SOC determined by CMB(SOCCMB,i.e., unapportioned OC by CMB analysis)and a tracer-based method(SOCTBM)that apportioned SOC contributions by four hydrocarbon precursors including isoprene,monoterpenes,β-caryophyllene,and toluene.The three estimates of SOC closely tracked with each other among individual samples.The SOCCMB and SOCPMF estimates on the majority of the regional days were within±16%.Good correlations between contributions of SOC and individual primary OC sources apportioned by PMF and CMB further demonstrated that PMF provides a reliable estimation of secondary and primary OC source contributions by using secondary and primary aerosol organic tracers as the fitting species.