Abstract
Abstract. The mitigation of air pollution in megacities remains a great challenge because of the complex sources and formation mechanisms of aerosol particles. The 2014 Asia-Pacific Economic Cooperation (APEC) summit in Beijing serves as a unique experiment to study the impacts of emission controls on aerosol composition, size distributions, and oxidation properties. Herein, a high-resolution time-of-flight aerosol mass spectrometer was deployed in urban Beijing for real-time measurements of size-resolved non-refractory submicron aerosol (NR-PM1) species from 14 October to 12 November 2014, along with a range of collocated measurements. The average (±σ) PM1 was 41.6 (±38.9) μg m−3 during APEC, which was decreased by 53 % compared with that before APEC. The aerosol composition showed substantial changes owing to emission controls during APEC. Secondary inorganic aerosol (SIA: sulfate + nitrate + ammonium) showed significant reductions of 62–69 %, whereas organics presented much smaller decreases (35 %). The results from the positive matrix factorization of organic aerosol (OA) indicated that highly oxidized secondary organic aerosol (SOA) showed decreases similar to those of SIA during APEC. However, primary organic aerosol (POA) from cooking, traffic, and biomass-burning sources were comparable to those before APEC, indicating the presence of strong local source emissions. The oxidation properties showed corresponding changes in response to OA composition. The average oxygen-to-carbon level during APEC was 0.36 (±0.10), which is lower than the 0.43 (±0.13) measured before APEC, demonstrating a decrease in the OA oxidation degree. The changes in size distributions of primary and secondary species varied during APEC. SIA and SOA showed significant reductions in large accumulation modes with peak diameters shifting from ~ 650 to 400 nm during APEC, whereas those of POA remained relatively unchanged. The changes in aerosol composition, size distributions, and oxidation degrees during the aging processes were further illustrated in a case study of a severe haze episode. Our results elucidated a complex response of aerosol chemistry to emission controls, which has significant implications that emission controls over regional scales can substantially reduce secondary particulates. However, stricter emission controls for local source emissions are needed for further mitigating air pollution in the megacity of Beijing.
Highlights
Atmospheric aerosols, especially fine particles of particulate matter (PM) with aerodynamic diameters less than 2.5 μm, play significant roles in human health hazards (Pope et al, 2009) and visibility reduction (Chow et al, 2002)
We present a detailed investigation of the impacts of emission controls on the changes in chemical composition, oxidation properties, and size distributions of submicron aerosols
The average mass concentration of PM1 showed a substantial decrease from 88.0 μg m−3 before Asia-Pacific Economic Cooperation (APEC) to 41.6 μg m−3 during APEC
Summary
Atmospheric aerosols, especially fine particles of particulate matter (PM) with aerodynamic diameters less than 2.5 μm, play significant roles in human health hazards (Pope et al, 2009) and visibility reduction (Chow et al, 2002). The severe haze pollution, which is characterized by high concentrations of fine particles, has become a significant concern in China (Zhang et al, 2010). Recent studies further highlighted the important roles of SIA and secondary organic aerosols (SOA) in the formation of severe haze pollution (Sun et al, 2014; Huang et al, 2014; Jiang et al, 2015; Zheng et al, 2015). The substantial emissions from primary sources and rapid secondary aerosol formation coupled with stagnant meteorological conditions lead to frequent haze pollution in China, during winter (Sun et al, 2014). Recent real-time measurements of aerosol composition have improved our understanding of the evolutionary processes of haze pollution, most of them focus on chemical composition and source analysis, and the oxidation properties of aerosol particles remain less understood
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