Chemical evolution of secondary organic aerosol tracers during high-PM_2.5 episodes at a suburban site in Hong Kong over 4 months of continuous measurement

Abstract

Abstract. Secondary organic aerosol (SOA) makes a sizable contribution to fine-particulate-matter (PM2.5) pollution, especially during high-PM episodes. Past studies of SOA evolution at the episode scale mainly rely on measurements of bulk SOA mass, with few studies probing individual SOA molecular tracers. In this study, we continuously monitored (at a bi-hourly resolution) SOA tracers specific to a few common volatile organic compound (VOC) precursors at a suburban site in Hong Kong for a 4-month period from the end of August to December 2020. The SOA molecules include tracers for SOA derived from biomass burning (BB) emissions, monoaromatics, naphthalene/methylnaphthalenes, and three biogenic VOCs (isoprene, monoterpene, and sesquiterpene). Generally, the SOA tracers showed regional characteristics for both anthropogenic and biogenic SOA as well as for the BB-derived SOA. This work focused on the seasonal variation and evolution characteristics of SOA tracers during 11 city-wide PM2.5 episodes, which are defined as periods with PM2.5 concentrations exceeding 35 µg m−3 at 3 or more of the 15 general air quality monitoring stations cross the city. Mass increment ratios (MIR), calculated as the ratio of the mass concentration prior to an episode to that during an episode, were examined for individual species during each episode. During most episodes, the SOA tracer concentrations were enhanced (i.e. MIR >1), and the 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 the higher importance of SOA to the 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 the increased presence of the early-generation products. Thus, 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 high-time-resolution organic marker measurement at multiple sites in order to fully capture the spatial heterogeneity of haze pollution at the city scale.

Ozone Chemistry on Greasy Glass Surfaces Affects the Levels of Volatile Organic Compounds in Indoor Environments

“…Recent studies reported O 3 mixing ratios of more than 100 ppb in Guangdong-Hong Kong urban area. 7,92 Hundreds of O 3 pollution days exceeding O 3 levels over 100 ppb were also observed in Shanghai metropolitan area. 93 These increasing O 3 levels in the ambient air in China will certainly have implications on O 3 reactivity within indoor environments, where we spent most of our time.…”

Molecular and Elemental Tracers‐Based Receptor Modeling of PM2.5 in Suburban Hong Kong With Hourly Time‐Scale Air Quality Considerations

“…The sampling site is ∼5–10 km away from the nearby urban commercial centers and surrounded by residential neighborhoods, with relatively low influences of local anthropogenic sources except for a construction site and small student canteen, which is representative of a clean suburban environment in Hong Kong. For a detailed description of the sampling site, we refer readers to our recent work (Wang et al., 2022a). The monitored parameters include hourly PM 2.5 , its major chemical composition, and molecular and elemental markers, as well as the meteorological parameters.…”

“…All the mentioned online data at even hours were chosen to match the TAG-measured data and used for further analysis. The SHARP PM 2.5 and RH data were corrected due to the measurement bias, see more discussion in Wang et al (2022a) and Text S1 in Supporting Information S1. Details of the data validation and quality assessment are provided in Text S2 in Supporting Information S1.…”

“…Previous studies reported significant reductions of 48% both in China during 2013–2020 and Hong Kong during 2010–2020 (Hong Kong Environment Bureau, 2021; Xiao et al., 2022). Though the major mainland cities, as well as Hong Kong, have complied with the national annual air quality objectives of PM 2.5 (35 μg m −3 ) in recent years, episodic PM 2.5 pollution events happened frequently, especially during the winter season (Li et al., 2019; Wang et al., 2022a; Zhong et al., 2018). Consequently, the short‐term (several hours to few days) PM 2.5 episodes has increasingly aroused public concerns.…”

Time‐Resolved Measurements of PM2.5 Chemical Composition and Brown Carbon Absorption in Nanjing, East China: Diurnal Variations and Organic Tracer‐Based PMF Analysis

“…The commercial thermal‐desorption aerosol GC‐MS has recently been used for hourly observations of organic molecular markers (OMMs) in eastern China (X. He et al., 2020; R. Li et al., 2020; Q. Wang et al., 2022). The observational results provide valuable information on the sources and evolution of primary and secondary organic aerosols (SOA) during pollution episodes.…”