Secondary organic aerosol of PM2.5 in a mountainous forest area in southeastern China: Molecular compositions and tracers implication

Abstract Secondary organic aerosol (SOA) plays an important role in global climate change and air quality. PM 2.5 (particles with aerodynamic diameters ≤2.5 μm) samples were collected at a mountainous forest site (Mt. Wuyi) in southeastern China between November 2015 and July 2016. Fourteen PM 2.5 -bound SOA tracers, including isoprene, α / β ‑pinene, β ‑caryophyllene, and toluene, were measured using the gas-chromatography–mass-spectrometry method. The total concentrations of the isoprene, α / β ‑pinene, β ‑caryophyllene, and toluene SOA tracers were 45.28 ± 65.52, 30.66 ± 24.44, 5.99 ± 7.25, and 0.62 ± 0.72 ng m −3 , respectively. The isoprene SOA tracers exhibited the highest concentration (145.97 ± 53.78 ng m −3 ) and accounted for 76 ± 9% of the total concentration of SOA tracers in summer. In fall-winter, the mass fraction of 2‑methylglyceric acid was significantly enhanced because of the lower temperature and higher NO x level. As later-generation products of α / β ‑pinene tracers, high proportions of 3‑hydroxyglutaric acid and 3‑methyl‑1,2,3 butanetricarboxylic acid were observed on Mt. Wuyi, suggesting that the aerosols were highly oxidized. Biomass burning events affected by local and regional sources were identified by analyzing typical SOA tracers. Significant positive correlation ( R 2  = 0.74) was found between the β ‑caryophyllene tracer and levoglucosan. The average concentration of secondary organic carbon (SOC) as estimated from SOA tracers was 1.46 μgC m −3 . The isoprene SOC accounted for 70% of the total SOC in summer, whereas the β ‑caryophyllene SOC was the predominant component in winter. Meanwhile, the estimated toluene SOC accounted for 11.6% of the total SOC during the study period. The study helps understanding the characteristics and the formation of SOA in a mountainous forest area of southeastern China.