Precursors and Pathways Leading to Enhanced Secondary Organic Aerosol Formation during Severe Haze Episodes

Abstract

Molecular analyses help to investigate the key precursors and chemical processes of secondary organic aerosol (SOA) formation. We obtained the sources and molecular compositions of organic aerosol in PM 2.5 in winter in Beijing by online and offline mass spectrometer measurements. Photochemical and aqueous processing were both involved in producing SOA during the haze events. Aromatics, isoprene, long-chain alkanes or alkenes, and carbonyls such as glyoxal and methylglyoxal were all important precursors. The enhanced SOA formation during the severe haze event was predominantly contributed by aqueous processing that was promoted by elevated amounts of aerosol water for which multifunctional organic nitrates contributed the most followed by organic compounds having four oxygen atoms in their formulae. The latter included dicarboxylic acids and various oxidation products from isoprene and aromatics as well as products or oligomers from methylglyoxal aqueous uptake. Nitrated phenols, organosulfates, and methanesulfonic acid were also important SOA products but their contributions to the elevated SOA mass during the severe haze event were minor. Our results highlight the importance of reducing nitrogen oxides and nitrate for future SOA control. Additionally, the formation of highly oxygenated long-chain molecules with a low degree of unsaturation in polluted urban environments requires further research.

Weakened Gas-to-Particle Partitioning of Oxygenated Organic Molecules in Liquified Aerosol Particles

“…In Beijing, haze episodes in winter are generally associated with a low wind speed (e.g., WS < 1.5 m s –1 herein), while clean episodes are dominated by the northern wind with high WS . In this campaign, the most polluting events occurred from March 9 to 12, featured by the maximum NR-PM 2.5 mass loading of 186.6 μg m –3 , which is close to the PM 2.5 mass concentration of 191 μg m –3 in Beijing in 2017, along with high concentrations of NO x , SO 2 , and O 3 (Figure ). In addition, high RH and weak irradiation were observed during the pollution events.…”

“…This can be ascribed to an increased contribution of nitrate recently in the NCP. , For example, the mean mass fractions of nitrate, ammonium, sulfate, and chloride in NR-PM 2.5 were 44.3%, 18.0%, 10.5%, and 1.2%, respectively. In particular, the mass ratio of sulfate to nitrate (0.2) was much lower than that in 2017 (1.6) and in 2018 (2.1) in Beijing. This implies a decreased aerosol acidity, which might indirectly affect SOA formation. , …”

“…Haze pollution, characterized by high levels of fine particulate matter (PM 2.5 ), is still a major concern in the densely populated Northern China Plain (NCP), although the government has implemented the Clean Air Action Plans since 2013. − Meanwhile, as the emissions of sulfur dioxide (SO 2 ) and nitrous oxides (NO x ) have decreased greatly in the past 10 years, the contribution of organic aerosol (OA), dominated by secondary OA (SOA), became more and more vital to PM 2.5 in the NCP. − Most SOA is produced by the oxidation of anthropogenic or biogenic volatile organic compounds (VOCs) in the atmosphere. Primary OA (POA) in the atmosphere could be oxidized to SOA at the interfaces of liquefied particles or fog/cloud droplets. , Recently, extensive studies have already reported that the uptake of oxygenated VOCs (OVOCs) by liquefied aerosols and fog/clouds, and subsequently, transformation to low-volatility products, could also contribute to SOA formation. − However, the mechanisms of SOA formation are poorly understood, which in turn hindered us from evaluating and simulating the responses of SOA mass loading to changes in VOC emissions and other factors.…”

“…Here continuous measurements of the gas- and particle-phase composition of organics were conducted by an iodine-adduct time-of-flight chemical ionization mass spectrometer (I – -TOF-CIMS) coupled with a filter inlet for gases and aerosols (FIGAERO) in Beijing . The gas-to-particle partitioning and equilibrium were explored.…”

Impact of fossil and non-fossil sources on the molecular compositions of water soluble humic-like substance in PM2.5 at a suburb site of Yangtze River Delta, China

“…The organonitrates formation from NO3 oxidation of biogenic or anthropogenic VOCs can affect the interactions between anthropogenic and natural emissions (He et al, 2021;Shen et al, 2021;Wang et al, 2020). Organonitrates were found to be important species contributing to SOA formation in polluted urban environment, which were enhanced under high NOx level (Zheng et al, 2021). The significant higher relative intensities of CHON compounds in winter indicated that the high NOx environment in winter promoted the formation of organonitrates and highlighted the importance of orgnonitrates for SOA control in polluted environment.…”