Spatial and seasonal variations of primary and secondary organic aerosols at urban areas and continental background site of China: Ambient levels, size distribution, and sources

Organic aerosols are important constituents of particulate matter (PM) in the atmosphere, however, the studies on their size distribution, particularly for special primary and secondary organic aerosols (POA, SOA), are relatively limited. In this work, the size-segregated PM was collected from five megacities (Shanghai, Guangzhou, Xi'an, Chengdu, Urumqi) and a continental background site (Qinghai Lake, Tibetan Plateau) of China during 2012–2014. Special POA (plant wax n-alkanes, fossil fuel n-alkanes, PAHs, levoglucosan, and trehalose) and biogenic SOA (BSOA, i.e. isoprene oxidants, monoterpene oxidants, and β-caryophyllenic acid) in PM were measured using gas chromatography/mass spectrometry. n-Alkanes were highest in autumn in the Southeast of China and highest in summer in the Northwest of China during the four seasons. PAHs and levoglucosan were higher in the cold season, mainly influenced by residential heating. Isoprene and monoterpene oxidants were higher in the warm season, while β-caryophyllenic acid was found to be much higher in cold seasons. n-Alkanes showed a monomodal distribution, with a peak at the fine mode (<1.1 µm). PAHs and levoglucosan were characterized by an accumulation mode, revealing the peak at <1.1 µm. However, only trehalose displayed coarse mode at the six sites, peaking at a size of 3.3–9.0 µm, because it was mostly derived from biota in desert regions. Isoprene oxidants including 2-methylglyceric acid, 2-methyltetrols, and C5-alkene triols exhibited a unimodal size distribution pattern with a peak in the fine mode. Monoterpene oxidants such as cis-pinic acid, pinonic acid, 3-hydroxyglutaric acid, and 3-methyl-1,2,3-butanetricarboxylic acid exhibited a bimodal size distribution pattern in the whole sampling time, peaking at the size range of <1.1 µm and 3.3–9.0 µm, respectively. Only β-caryophyllenic acid displayed a fine mode at the six sites during the whole sampling time. These findings can be used to support research into the origin and formation mechanisms of POA and BSOA.