Comment on acp-2022-302

Abstract

Secondary organic aerosol (SOA) makes a sizable contribution to fine particulate matter (PM2.5) pollution, especially during episodic hours. Past studies of SOA evolution at episode-scale mainly rely on measurements of bulk SOA mass and few studies probe individual SOA molecular tracers. In this study, we continuously monitored at a bihourly resolution SOA tracers specific to a few common volatile organic compound (VOC) precursors at a suburban site in Hong Kong for four-month from the end of Aug. to Dec. 2020. The SOA molecules include tracers for SOA derived from biomass burning emissions, monoaromatics, naphthalene/methylnaphthalenes, and three biogenic VOCs (i.e., isoprene, monoterpene and sesquiterpene). Generally, the SOA tracers showed regional characteristics for both anthropogenic and biogenic SOA, as well as the biomass burning-derived SOA. This work focused on the seasonal variation and evolution characteristics of SOA tracers during eleven city-wide PM2.5 episodes, which are defined to be periods of PM2.5 exceeding 35 μg/m3 at three or more of the 15 general air quality monitoring stations cross the city. Mass increment ratios (MIR), calculated as the ratio of mass concentration between before and during an episode, were examined for individual species in each episode. During most episodes, the SOA tracers were enhanced in their concentrations (i.e., MIR > 1) and maximum MIR values were in the range of 5.5–11.0 for SOA tracers of different precursors. Episodes on summer and fall days showed notably larger MIR values than those falling on winter days, indicating a higher importance of SOA to formation of summer/fall PM2.5 episodes. Simultaneous monitoring of six tracers for isoprene SOA revealed the dominance of the low-NOx pathway in forming isoprene SOA in our study region. The multiple monoterpene SOA products suggested fresher SOA in winter, evidenced by an increased presence of the early generation products. The current study has shown by example the precursor-specific SOA chemical evolution characteristics during PM2.5 episodes in different seasons. This study also suggests the necessity to apply the high time resolution organic marker measurement at multiple sites to fully capture the spatial heterogeneity of the haze pollution at the city scale.