ABSTRACT

We investigated the summertime formation of secondary organic aerosols (SOA) via the oxidation of isoprene, α/β-pinene and β-caryophyllene in a Quercus crispula and Picea glehnii mast mixed forest located at Hokkaido University Uryu Experimental Forest in Japan. Biogenic SOA tracers and other polar organic compounds (e.g., sugars and aromatic acids) in time-resolved (4 h) aerosol samples (13–15 August, 2001) were characterized using gas chromatography-mass spectrometry. Isoprene SOA tracers including 2-methyltetrols and C₅-alkane triols were found to be the most abundant compound class (32–219 ng m^–3, average 113 ng m^–3), followed by sugars/sugar alcohols. A strong diurnal variation of isoprene oxidation products was observed with higher concentrations during late afternoon-early evening. However, there were no clear trends for α/β-pinene and β-caryophyllene oxidation products. The daytime formation of isoprene SOA correlated well with increased temperature and solar radiation, suggesting a temperature- and/or light-dependent emission of isoprene in the forest followed by photochemical oxidation. Levoglucosan, a biomass burning tracer, showed no correlation with biogenic SOA tracers, indicating that biomass burning contribute little to the formation of biogenic SOA at the sampling site. A significant decrease in the concentrations of biogenic SOA tracers and other polar organic tracers was found during a fog event. Using a tracer-based method, we conclude that the contributions of secondary organic carbon (SOC) from isoprene oxidation products to organic carbon were more significant than those of α/β-pinene and β-caryophyllene oxidation products. The total SOC accounts for 5.7–34% (average 17%) of OC. This suggests that the emission of biogenic volatile organic compounds followed by subsequent oxidation plays an important role in the formation of SOA over the Quercus and Picea mixed forest.