Summertime and wintertime atmospheric processes of secondary aerosol in Beijing

Abstract

Abstract. Secondary aerosol constitutes a large fraction of fine particles in urban air of China. However, its formation mechanisms and atmospheric processes remain largely uncertain despite considerable study in recent years. To elucidate the seasonal variations in fine-particle composition and secondary aerosol formation, an Aerodyne quadrupole aerosol chemical speciation monitor (Q-ACSM), combined with other online instruments, was used to characterize the sub-micrometer particulate matter (diameter < 1 µm, PM1) in Beijing during summer and winter 2015. Our results suggest that photochemical oxidation was the major pathway for sulfate formation during summer, whereas aqueous-phase reaction became an important process for sulfate formation during winter. High concentrations of nitrate (17 % of the PM1 mass) were found during winter, explained by enhanced gas-to-particle partitioning at low temperature, while high nitrate concentrations (19 %) were also observed under the conditions of high relative humidity (RH) during summer, likely due to the hydrophilic property of NH4NO3 and hydrolysis of N2O5. As for organic aerosol (OA) sources, secondary OA (SOA) dominated the OA mass (74 %) during summer, while the SOA contribution decreased to 39 % during winter due to enhanced primary emissions in the heating season. In terms of the SOA formation, photochemical oxidation perhaps played an important role for summertime oxygenated OA (OOA) formation and less-oxidized wintertime OOA (LO-OOA) formation. The wintertime more-oxidized OOA (MO-OOA) showed a good correlation with aerosol liquid water content (ALWC), indicating a more important contribution of aqueous-phase processing over photochemical production to MO-OOA. Meanwhile, the dependence of LO-OOA and the mass ratio of LO-OOA to MO-OOA on atmospheric oxidative tracer (i.e., Ox) both degraded when RH was greater than 60 %, suggesting that RH or aerosol liquid water may also affect LO-OOA formation.

Quantification of solid fuel combustion and aqueous chemistry contributions to secondary organic aerosol during wintertime haze events in Beijing

“…heating season, and one episode bridged the transition date. Consistent with previous studies (Duan et al, 2020;Duan et al, 2019;Zhao et al, 2019;Xu et al, 2019;Sun et al, 2016a;Sun et al, 2016b), alternating haze episodes and clean periods corresponded systematically to changing meteorological conditions. Haze build-up was associated with stagnant air masses with slow wind speed (< 1.5 m s -1 ) mainly from the south or southwest, and terminated by air masses with high wind speed (> 3 m s -1 ) from the north or northwest (Fig.…”

“…As discussed in Sect. 3.1, meteorological conditions are responsible for an alternating occurrence of haze and clean periods and these effects from meteorology are well-understood (Duan et al, 2020;Duan et al, 2019;Zhao et al, 2019;Xu et al, 2019;Sun et al, 2016a;Sun et al, 2016b). In addition, https://doi.org/10.5194/acp-2020-835 Preprint.…”

“…Also, extreme haze events are still very frequent in northern China (An et al, 2019). Extensive studies conducted in Beijing in recent years investigated the chemical composition and sources of fine particles by different online and offline analytical methods (Duan et al, 2020;Duan et al, 2019;Xu et al, 2019;Zhao et al, 2019;Äijälä et al, 2017;Elser et al, 2016;Hu et al, 2016;Sun et al, 2016a;Huang et al, 2014;Zhang et al, 2014;Sun et al, 2013). Among these studies, aerosol mass spectrometers (AMS) equipped with a PM1 aerodynamic lens are widely used due to their robust quantification ability.…”

Characteristics, primary sources and secondary formation of water soluble organic aerosols in downtown Beijing

“…During the whole sampling period, the primary emissions of biomass burning contributed 23.0 % to the total WSOC, with a higher contribution to its hydrophilic fraction (37.2 %) than to the hydrophobic section (15.7 %), which was probably due to the large amounts of saccharides with high water-solubility from biomass burning (Yan et al, 2019;Xu et al, 2020). The total contribution of primary and secondary biomass burning to WSOC (35.3 %) was slightly lower than that previously reported in Beijing (Cheng et al, 2013;Duan et al, 2020), likely owing to the effective control of the open biomass burning activities in the surrounding areas of Beijing in recent years. Other primary combustion sources (Source 3) also contributed considerably to WSOC (14.4 %), most of which only contributed to the hydrophobic fraction (19.1 %).…”

“…Primary organic tracers. The average CO-scaled concentration of levoglucosan in summer was much lower than in the other three seasons (only 15.4 % of that in winter), suggesting that biomass burning was not active in summer (Sun et al, 2018;Duan et al, 2020). Levoglucosan exhibited similar CO-scaled concentrations in seasons other than summer, indicating that the emission strength from biomass burning was relatively constant in these seasons and open burning of crop residues, which has distinct seasonality, was not a significant source of levoglucosan.…”