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The adverse effects of acidic ultrafine particles (AUFPs) have been widely recognized in scientific communities. However, a handful of studies successfully acquired the concentrations of AUFPs in the atmosphere. To explore the AUFPs pollution, six extensive measurements were for the first time conducted in the roadside, urban and rural areas in Hong Kong, and the urban area in Shanghai between 2017 and 2020. The concentrations of AUFPs and UFPs, and the proportions of AUFPs in UFPs were obtained. The concentration of UFPs was the highest at the roadside site, followed by the urban site and the rural site, while the proportion of AUFPs in UFPs showed a contrary trend. The difference, on one hand, indicated the potential transformation of AUFPs from non-acidic UFPs during the transport and aging of air masses, and on the other hand, suggested the minor contribution of anthropogenic sources to the emission of AUFPs. In addition, the urban area in Hong Kong suffered from heavier pollution of UFPs and AUFPs than that in Shanghai. As for size distribution, the proportion of AUFPs in UFPs peaked in the size range of 35-50 nm and 50-75 nm in roadside and urban area, respectively. In rural area, the peak was observed in the size range of 5-10 nm, which might indicate the stimulation of new particle formation with the AUFPs as seeds. Furthermore, in the urban areas of Hong Kong and Shanghai, no significant difference was found for the geometric mean diameters of UFPs and AUFPs (p > 0.05). At last, the sulfuric acid proxy was positively correlated with the proportions of AUFPs in UFPs but not well correlated with the AUFPs levels. The results suggested the important roles of interaction between sulfuric acid vapor and non-acidic UFPs in AUFPs formation. Due to the significant reduction of sulfur dioxide in China during the last decade, the pollution of AUFPs in urban areas was alleviated., 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.)

Reducing the amount of organic aerosol (OA) is crucial to mitigation of particulate pollution in China. We present time and air-origin dependent variations of OA markers and source contributions at a regionally urban background site in South China. The continental air contained primary OA markers indicative of source categories, such as levoglucosan, fatty acids, and oleic acid. Secondary OA (SOA) markers derived from isoprene and monoterpenes also exhibited higher concentrations in continental air, due to more emissions of their precursors from terrestrial ecosystems and facilitation of anthropogenic sulfate for monoterpenes SOA. The marine air and continental-marine mixed air had more abundant hydroxyl dicarboxylic acids (OHDCA), with anthropogenic unsaturated organics as potential precursors. However, OHDCA formation in continental air was likely attributable to both biogenic and anthropogenic precursors. The production efficiency of OHDCA was highest in marine air, related to the presence of sulfur dioxide and/or organic precursors in ship emissions. Regional biomass burning (BB) was identified as the largest contributor of OA in continental air, with contributions fluctuating from 8% to 74%. In contrast, anthropogenic SOA accounted for the highest fraction of OA in marine (37 ± 4%) and mixed air (31 ± 3%), overriding the contributions from BB. This study demonstrates the utility of molecular markers for discerning OA pollution sources in the offshore marine atmosphere, where continental and marine air pollutants interact and atmospheric oxidative capacity may be enhanced.

A sampling campaign was conducted at an urban roadside site in Hong Kong from Nov. to Dec. in 2017 using a suite of state-of-the-art instruments to monitor compositions of non-refractory sub-micron particulate matter (NR-PM 1) and gaseous pollutants. Results showed that the average NR-PM 1 concentration was 26.1 ± 0.7 μg/m3 (average ± 95% confidence interval) and organic aerosol (OA) contributed the most to NR-PM 1 with a proportion of 57.7 ± 0.2%. The aerosol size distributions of bulk composition of NR-PM 1 presented a peak at ~600 nm with internal mixtures of the organic and inorganic components, while there were a larger proportion of primary organic particles at < 200 nm, indicating intensive emissions of primary organics at this site. Positive matrix factorization (PMF) analysis was applied to the measurement data and four OA components were identified, including a hydrocarbon-like OA (HOA) factor, a cooking organic aerosol (COA) factor and two oxygenated OA (OOA) factors of different oxidation levels: less oxidized OOA (LO-OOA) and more oxidized OOA (MO-OOA). Strikingly, the contribution of MO-OOA was the highest (30.9 ± 0.4%), suggesting high oxidation degree and/or high regional background in the roadside environment. Moreover, the proportion of COA reached 25.4 ± 0.3% at this roadside site with heavy traffic fleet, which was even higher than the percentage of HOA (p < 0.01). The average ratio of C 3 H 3 O + / C 3 H 5 O + (2.01 ± 0.01) and the opposite pattern of C 3 H 3 O + / C 3 H 5 O + to O x during daytime hours suggested that the COA was oxidized to some extent when transported to the site. The findings implied that cooking activities are a significant source of organic aerosols in Hong Kong, even at a busy road. Control measures should focus on both cooking and traffic emissions in Hong Kong. [Display omitted] • The proportion of COA in NR-PM 1 was higher than HOA at this traffic-busy site. • The pattern of C 3 H 3 O + / C 3 H 5 O + ratio was opposite to the trend of O x during daytime hours. • Cooking activities are a significant source of organic aerosols in Hong Kong. [ABSTRACT FROM AUTHOR]