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Secondary organic aerosol (SOA) formation originating from the emission of anthropogenic volatile organic compounds (VOCs) makes a significant contribution to fine particulate matter (PM 2.5 ) pollution in urban areas. Investigation on the SOA formation potential (SOAFP) can help us understand the contribution of different sources to SOA formation. To characterize the SOAFP of ambient air from anthropogenic VOCs in the urban area of Beijing, field observation was implemented using a twin oxidation flow reactor (Twin-OFRs) system in the winters of 2016 and 2017. Compared to the winter of 2016, the seasonal-average SOAFP in the winter of 2017 was found to decrease by about 74% (18.6 to 4.9 μg/m 3 ), which is more than that of PM 1 (59%, 48.7 to 20.2 μg/m 3 ), PM 2.5 (61%, 114.4 to 44.8 μg/m 3 ) and CO (57%, 2.1 to 0.9 mg/m 3 ) that mainly comes from the combustion of fossil fuels, suggesting complex affecting factors on SOAFP. The results of wind decomposition mathematical modeling showed that anthropogenic factors and favorable meteorological conditions both contributed significantly to the decrease in SOAFP. The reduction of emissions from scatter coal combustion, which is the key VOCs source for SOAFP, is probably the most important anthropogenic factor affecting SOAFP. In the winter of 2016, the ratio of benzene to toluene is 1.45 that was close to 1.54 representing coal combustion emission; however, it decreased dramatically to 1.05 in the winter of 2017, suggesting considerable reduction of VOC emissions from scatter coal combustion in the latter year due to the coal-to-gas transition in Beijing and surrounding regions. The SOAFP measured in this study considers all ambient VOCs that can react with OH radical, providing another representative method for estimating it. These results could be beneficial to understanding the factors driving SOAFP and its contribution to PM 2.5 , especially in regions with high-intensity anthropogenic emissions. Synopsis: This study reported the sharp decline of secondary organic aerosol formation potential (SOAFP) between two consecutive winters in Beijing and analyzed the reasons., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

The understanding at a molecular level of ambient secondary organic aerosol (SOA) formation is hampered by poorly constrained formation mechanisms and insufficient analytical methods. Especially in developing countries, SOA related haze is a great concern due to its significant effects on climate and human health. We present simultaneous measurements of gas-phase volatile organic compounds (VOCs), oxygenated organic molecules (OOMs), and particle-phase SOA in Beijing. We show that condensation of the measured OOMs explains 26-39% of the organic aerosol mass growth, with the contribution of OOMs to SOA enhanced during severe haze episodes. Our novel results provide a quantitative molecular connection from anthropogenic emissions to condensable organic oxidation product vapors, their concentration in particle-phase SOA, and ultimately to haze formation.

Secondary organic aerosol (SOA) plays a critical role in sustained haze pollution in megacities. Traditional observation of atmospheric aerosols usually analyzes the ambient organic aerosol (OA) but neglects the SOA formation potential (SOAFP) of precursors remaining in ambient air. Knowledge on SOAFP is still limited, especially in megacities suffering from frequent haze. In this study, the SOAFP of ambient air in urban Beijing was characterized at different pollution levels based on a two-year field observation using an oxidation flow reactor (OFR) system. Both OA and SOAFP increased as a function of ambient pollution level, in which increasing concentrations of precursor volatile organic compounds (VOCs) and decreasing atmospheric oxidation capacity were found to be the two main influencing factors. To address the role of the atmospheric oxidation capacity in SOAFP, a relative OA enhancement ratio (ER OA = 1 + SOAFP/OA) and the elemental composition of the OA were investigated in this study. The results indicated that the atmospheric oxidation capacity was weakened and resulted in higher SOAFP on more polluted days. The relationship found between SOAFP and the atmospheric oxidation capacity could be helpful in understanding changes in SOA pollution with improving air quality in the megacities of developing countries.

Understanding the atmospheric new particle formation (NPF) process within the global range is important for revealing the budget of atmospheric aerosols and their impacts. We investigated the seasonal characteristics of NPF in the urban environment of Beijing. Aerosol size distributions down to ∼1 nm and H 2 SO 4 concentration were measured during 2018-2019. The observed formation rate of 1.5 nm particles ( J 1.5 and NPF frequency in urban Beijing show a clear seasonal variation with maxima in winter and minima in summer, while the observed growth rates are generally within the same range around the year. We show that ambient temperature is a governing factor driving the seasonal variation of J 1.5 and NPF frequency in urban Beijing show a clear seasonal variation with maxima in winter and minima in summer, while the observed growth rates are generally within the same range around the year. We show that ambient temperature is a governing factor driving the seasonal variation of J 1.5 . In contrast, the condensation sink and the daily maximum H 2 SO 4 concentration show no significant seasonal variation during the NPF periods. In all four seasons, condensation of H 2 SO 4 and (H 2 SO 4 ) n (amine) n clusters contributes significantly to the growth rates in the sub-3 nm size range, whereas it is less important for the observed growth rates of particles above 3 nm. Therefore, other species are always needed for the growth of larger particles.

Atmospheric particulate matters have a crucial impact on climate change, visibility and human health. In this study, a detailed characteristic of summertime PM 1 was real-time measured in south suburb of Beijing from 16 th August to 16 th September 2018. Averaged PM 1 concentration of 24.1 ± 18.0 μg m -3 was observed, consisting of OM (50.8%), SO 4 2- (16.0%), BC (13.2%), NO 3 - (10.2%), NH 4 + (9.2%), and Cl - (0.6%). There was an accumulation mode with a peak diameter of ∼500 nm for all the species (except BC), and OM was additionally characterized by a smaller mode of <100 nm. Elemental analysis of OM showed that the diurnal variations of H/C, O/C, N/C, and S/C were correlated to the photochemical and aqueous-phase process. Four organic factors including one hydrocarbon-like (HOA) and three oxygenated (LO-OOA, SV-OOA and MO-OOA) organic aerosol factors were identified by positive matrix factorization (PMF) analysis. The contributions of these factors varied with PM 1 concentration and their average values were 31%, 30%, 14%, and 25%, respectively. Contribution of HOA was RH-independent but decreased with the increasing PM 1 concentration, while OOA factors were a combined result of RH and O x (=O 3 +NO 2 ), revealing the important role of photochemical and aqueous-phase process in OA evolution. The contribution of SV-OOA with the highest S/C increased significantly with RH, indicating a certain number of S-containing organics. Our results also showed that secondary OA was the dominant species, as well as increased with the pollution level, implicating VOCs and NO x should be controlled to relieve the secondary OA pollution., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Secondary organic aerosol (SOA) is an important component of atmospheric fine particles (PM 2.5 ), while the key factors controlling SOA formation in ambient air remain poorly understood. In this work, the SOA formation in Beijing urban ambient air was investigated using an oxidation flow reactor (OFR) with high concentrations of OH radicals. The SOA formation potential increased significantly with the increase of ambient PM 2.5 concentration during the observation. The optimum ambient exposure time, which is the aging time equivalent to atmospheric oxidation (with similar OH exposure) associated with the peak SOA formation, varied between 2 and 4 days in this study. The OA enhancement in this study was much higher than that of developed countries under different environmental conditions. The higher OA enhancement is probably due to the higher concentrations of volatile organic compounds (VOCs) in the urban air of Beijing. This might also have occurred because fragmentation did not dominate in the oxidation of OA, and did not result in negative OA enhancement on highly polluted days compared to relatively clean days with similar exposure time. These results suggested that under typical ambient conditions, high concentrations of VOC precursors might contribute to sustained organic aerosol growth and long duration haze events in Beijing.