Source apportioning of primary and secondary organic carbon in summer PM_2.5 in Hong Kong using positive matrix factorization of secondary and primary organic tracer data

The major inorganic constituents and organic tracer compounds in PM _2.5 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 components of SOA among seven factors resolved by PMF. One SOA component was mainly associated with secondary sulfate and nitrate. The other SOA component, 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 (SOC_PMF) was on average 6.84 μgC m^-3 (65% of PM_2.5 OC) on high pollution days under influence of significant regional transport (i.e., regional days) and 0.70 μgC m^-3 (25% of PM_2.5 OC) on days under the influence of mainly local emissions (i.e., local days). The biomass burning-induced SOA accounted for 20% of the total SOA on the regional days, underlining the importance of biomass burning aerosol source in this region. The average uncertainty for the SOC_PMF estimates was ∼20% on the regional days and ∼120% on the local days. SOC_PMF was compared with SOC determined by CMB (SOC_CMB, i.e., unapportioned OC by CMB analysis) and a tracer-based method (SOC_TBM) that apportioned SOC contributions by four hydrocarbon precursors including isoprene, monoterpenes, -caryophyllene, and toluene. The three estimates of SOC closely tracked with each other among individual samples. The SOC_CMB and SOC_PMF estimates on the majority of the regional days differed from each other by less than 25%. Good correlations between contributions of SOC and individual primary OC sources apportioned by PMF and CMB further added to the credence to the PMF-derived estimation of secondary and primary OC source contributions by using secondary and primary aerosol organic tracers as the fitting species.