Size Distributions of Molecular Markers for Biogenic Secondary Organic Aerosol in Urban Beijing

Abstract

To track the sources and explore the generation processes of biogenic secondary organic aerosol (BSOA), a nine-stage cascade impactor was utilized to collect particulate samples in Beijing, China from 2017 to 2018. Concentrations of isoprene- and monoterpene-derived SOA tracers exhibited a identical seasonal variation with summer maxima, while a contradictory trend was found for the sesquiterpene-derived SOA tracer that dominated in winter. Particle size distribution of BSOA tracers displayed various patterns with seasons. Different from most of the BSOA tracers that showed a fine-mode peak (0.7 – 1.1 μm), cis-pinonic acid and pinic acid presented the coarse-mode enrichment (4.7 – 5.8 μm) for their volatile nature. The bimodal pattern with a fine- and a coarse-mode peak was observed in summer for isoprene-derived SOA tracers, which indicates that they were likely formed as gaseous species and subsequently converted to aerosols. In spring dust storm period, bimodal pattern of isoprene-derived SOA tracers may be explained by the alkaline nature of mineral particles, which led to the adsorption of gaseous species onto pre-existing particles. Regardless of seasons, β-caryophyllinic acid showing a unimodal size pattern with a fine-mode peak (0.7-1.1 µm) that resulted from biomass burning emission. The contributions of isoprene, monoterpene, and sesquiterpene to SOC formation were investigated using tracer-yield method. The contribution of fine-mode SOC was greater than that of coarse-mode SOC. The fine-mode isoprene SOC proved dominant in summer (1.61 µg m-3) and accounted for 3.83% of OC. These results suggest that SOA tracers are promising in tracking the source and understanding the process of BSOA formation.

Keywords: Biogenic secondary organic aerosols, tracer-based method, 2-methyltetrols, monoterpenes, bimodal pattern