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Peroxyacetyl nitrate (PAN) is an important reservoir of atmospheric nitrogen, modulating reactive nitrogen cycle and ozone (O 3) formation. To understand the origins of PAN, a field measurement was conducted at Tung Chung site (TC) in suburban Hong Kong from October to November 2016. The average level of PAN was 0.63 ± 0.05 ppbv, with a maximum of 7.30 ppbv. Higher PAN/O 3 ratio (0.043–0.058) was captured on episodes, i.e. when hourly maximum O 3 exceeded 80 ppbv, than on non-episodes (0.01), since O 3 production was less efficient than PAN when there was an elevation of precursors (i.e. volatile organic compounds (VOCs) and nitrogen oxide (NO x)). Model simulations revealed that oxidations of acetaldehyde (65.3 ± 2.3%), methylglyoxal (MGLY, 12.7 ± 1.2%) and other oxygenated VOCs (OVOCs) (8.0 ± 0.6%), and radical cycling (12.2 ± 0.8%) were the major production pathways of peroxyacetyl (PA) radical, while local PAN formation was controlled by both VOCs and nitrogen dioxide (NO 2). Among all VOC species, carbonyls made the highest contribution (59%) to PAN formation, followed by aromatics (26%) and biogenic VOCs (BVOCs) (10%) through direct oxidation/decomposition. Besides, active VOCs (i.e. carbonyls, aromatics, BVOCs and alkenes/alkynes) could stimulate hydroxyl (OH) production, thus indirectly facilitating the PAN formation. Apart from primary emissions, carbonyls were also generated from oxidation of first-generation precursors, i.e. , hydrocarbons, of which xylenes contributed the most to PAN production. Furthermore, PAN formation suppressed local O 3 formation at a rate of 2.84 ppbv/ppbv, when NO 2 , OH and hydroperoxy (HO 2) levels decreased and nitrogen monoxide (NO) value enhanced. Namely, O 3 was reduced by 2.84 ppbv per ppbv PAN formation. Net O 3 production rate was weakened (∼36%) due to PAN photochemistry, so as each individual production and loss pathway. The findings advanced our knowledge of atmospheric PAN and its impact on O 3 production. Image 1 • Higher PAN/O 3 ratio on high-pollution episodes was attributable to more efficient PAN production. • Carbonyls contributed the most to PAN via oxidation/decomposition & stimulation of OH production. • PAN production reduced HO x and NO 2 , increased NO, thus suppressing O 3 formation. Local O 3 formation was weakened by in-situ PAN formation, mostly driven by carbonyls through their direct decomposition and facilitation on OH concentration in suburban Hong Kong. [ABSTRACT FROM AUTHOR]

Investigating the long-term trends of alkyl nitrates (RONO 2) is of great importance for evaluating the variations of photochemical pollution. Mixing ratios of C 1 –C 5 RONO 2 were measured in autumn Hong Kong from 2002 to 2016, and the average level of 2-butyl nitrate (2-BuONO 2) always ranked first. The C 1 –C 4 RONO 2 all showed increasing trends (p < 0.05), and 2-BuONO 2 had the largest increase rate. The enhancement in C 3 RONO 2 was partially related to elevated propane, and dramatic decreases (p < 0.05) in both nitrogen monoxide (NO) and nitrogen dioxide (NO 2) also led to the increased RONO 2 formation. In addition, an increase of hydroxyl (OH) and hydroperoxyl (HO 2) radicals (p < 0.05) suggested enhanced atmospheric oxidative capacity, further resulting in the increases of RONO 2. Source apportionment of C 1 –C 4 RONO 2 specified three typical sources of RONO 2 , including biomass burning emission, oceanic emission, and secondary formation, of which secondary formation was the largest contributor to ambient RONO 2 levels. Mixing ratios of total RONO 2 from each source were quantified and their temporal variations were investigated. Elevated RONO 2 from secondary formation and biomass burning emission were two likely causes of increased ambient RONO 2. By looking into the spatial distributions of C 1 –C 5 RONO 2 , regional transport from the Pearl River Delta (PRD) was inferred to build up RONO 2 levels in Hong Kong, especially in the northwestern part. In addition, more serious RONO 2 pollution was found in western PRD region. This study helps build a comprehensive understanding of RONO 2 pollution in Hong Kong and even the entire PRD. Image 1 • C 1 –C 4 RONO 2 levels all kept increasing during the past 15 years in Hong Kong. • Enhanced atmospheric oxidative capacity facilitated photochemical formation of RONO 2. • Elevated biomass burning emissions built up primary RONO 2. • Regional transport from inland China possibly contributed to the elevated RONO 2 as well. Elevated secondary formation and biomass burning emission caused RONO 2 increase from 2002 to 2016. [ABSTRACT FROM AUTHOR]

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]