Secondary organic aerosol tracers and malic acid in Hong Kong: Seasonal trends and origins

Fine particle samples collected at an urban location in Hong Kong during winter were analysed by gas chromatography-ion trap mass spectrometry with prior chemical derivatisation. In total, 15 secondary organic aerosol (SOA) tracers from isoprene, monoterpenes, β-caryophyllene and toluene, and 24 other polar oxygenated compounds, were identified and quantified. Monoterpenes and isoprene SOA tracers showed lower levels on winter long-range transport (LRT) days than summer regional days, the latter being reported in our previous study. Opposite seasonal trends were observed for SOA tracers of β-caryophyllene and toluene. The averaged total secondary organic carbon (SOC) apportioned to these four volatile organic compounds (VOCs) was estimated to be 4.73 μg C m^-3 on winter LRT days, lower than that on summer regional days (5.21 μg C m^-3). β-caryophyllene and monoterpenes were found to be the most significant SOC contributors to PM_2.5 in Hong Kong in both winter and summer, and their averaged SOC contributions on winter LRT days were 2.24 and 1.59 μg C m^-3. Toluene and isoprene had relatively minor contributions to SOC in Hong Kong in both seasons, with averaged SOC contributions of 0.81 and 0.08 μg C m^-3 on winter LRT days. Malic acid was well correlated with biogenic SOA tracers and oxalate in both seasons, whereas correlation between malic and succinic acid was only found in winter. Based on the seasonal characteristic inter-species correlations in the region, we hypothesise that malic acid could be formed mainly by the aqueous-phase photodegradation of SOA products of biogenic VOCs during summer. In winter, emissions of biogenic VOCs are greatly reduced and succinic acid then becomes the predominant contributor to malic acid.