Characterization and source identification of sub-micron particles at the HKUST Supersite in Hong Kong

Differences in aerosol chemistry at a regional background site in Hong Kong before and during the COVID-19 pandemic

Restrictions on human-related activities implemented in Hong Kong to curb the spread of the coronavirus disease 2019 (COVID-19) pandemic provided an opportunity to investigate the anthropogenic impact on organic aerosols (OA) composition. In this study, we conducted a comparative analysis of online measurements of non-refractory submicron particulate matters (NR-PM1) at a regional background site in Hong Kong, covering the periods before the COVID-19 control (November 2018) and during the COVID-19 control (October to November 2020), to investigate changes in OA sources and formation mechanisms. Among the measured NR-PM1 components, organics were the most dominant species with an average percentage of 51.0 ± 0.5 %, exceeding pre-control levels of 44.0 ± 0.7 %. Moreover, 88 % of the organics were attributed to oxygenated OA (OOA). Diurnal variations of all bulk components in NR-PM1 consistently showed afternoon peaks, indicating photochemical processes during COVID-19 control. Similar to the pre-restriction period, the positive matrix factorization (PMF) model showed that OOA was composed of three factors, including two less-oxidized oxygenated factors (LO-OOA1 and LO-OOA2) and one more-oxidized oxygenated factor (MO-OOA). The contribution of the LO-OOA2 factor remained small and stable during both sampling campaigns, which might imply background levels of OOA at this site. The formation of the two predominant components of organics (e.g., LO-OOA1 and MO-OOA) was further discussed. Compared with before control, observational evidence showed that the levels of MO-OOA exceeded LO-OOA1 during the control period, and the average concentration of odd oxygen (Ox = ozone + nitrogen dioxide) increased by 53 % during the COVID-19 control. Besides, the results showed that both LO-OOA1 and MO-OOA exhibited similar diurnal variations to Ox, and their concentrations generally enhanced with increasing Ox levels. This suggested that the formation of OOA was closely related to the photochemical oxidation processes when anthropogenic emissions were reduced. By correlating LO-OOA1 and MO-OOA with speciated OA markers, we found that the formation of LO-OOA1 remained associated with anthropogenic sources, while biogenic emissions contributed to the formation of MO-OOA during the COVID-19 control. Our findings highlight the interplay between emissions, atmospheric conditions, and aerosol composition, providing valuable insights to guide strategic decisions for future air quality improvement.

Seasonal variations of C1-C4 alkyl nitrates at a coastal site in Hong Kong: Influence of photochemical formation and oceanic emissions

Five C₁-C₄ alkyl nitrates (RONO₂) were measured at a coastal site in Hong Kong in four selected months of 2011 and 2012. The total mixing ratios of C₁-C₄ RONO₂ (Σ₅RONO₂) ranged from 15.4 to 143.7 pptv with an average of 65.9 ± 33.0 pptv. C₃-C₄ RONO₂ (2-butyl nitrate and 2-propyl nitrate) were the most abundant RONO₂ during the entire sampling period. The mixing ratios of C₃-C₄ RONO₂ were higher in winter than those in summer, while the ones of methyl nitrate (MeONO₂) were higher in summer than those in winter. Source analysis suggests that C₂-C₄ RONO₂ were mainly derived from photochemical formation along with biomass burning (58.3-71.6%), while ocean was a major contributor to MeONO₂ (53.8%) during the whole sampling period. The photochemical evolution of C₂-C₄ RONO₂ was investigated, and found to be dominantly produced by the parent hydrocarbon oxidation. The notable enrichment of MeONO₂ over C₃-C₄ RONO₂ was observed in a summer episode when the air masses originating from the South China Sea (SCS) and MeONO₂ was dominantly derived from oceanic emissions. In order to improve the accuracy of ozone (O₃) prediction in coastal environment, the relative contribution of RONO₂ from oceanic emissions versus photochemical formation and their coupling effects on O₃ production should be taken into account in future studies.

A multicomponent kinetic model established for investigation on atmospheric new particle formation mechanism in H2SO4-HNO3-NH3-VOC system

Secondary new particle formation (NPF) plays a significant role in atmospheric particulate matters (e.g., PM2.5), and has been studied over the past decades. However, the mechanism of NPF still remains ambiguous, setting significant barrier for PM2.5 mitigations, especially in complex atmosphere with multi-pollutants. Since the NPF process can hardly be observed directly by experiment methods due to the measuring limitations, a multicomponent kinetic model (MKM), which can be used to analyze the process and the mechanism of NPF in H₂SO₄-HNO₃-NH₃-VOC (Volatile Organic Compounds) system, has been developed in this paper. According to MKM, seven cases with initial concentrations of total precursor vapors (CPV) in the range of 10⁷-10⁸cm^-3 were calculated to analyze the NPF process. Firstly, the 3nm particle (PM_3nm) formation rate was calculated via MKM, which showed a good agreement with the previous measurements. Moreover, according to MKM calculation, it is found that the peak value of PM_3nm formation rate, i.e., J_m, is proportional to [CPV]², while the time at which J_m occurred, i.e., t_m, is proportional to [CPV]^-1/3, indicating that the increases in CPV would lead to a significant increase of J_m and decrease of t_m. That's why NPF bursts immediately and PM2.5 pollution occurs suddenly in heavily pollutant areas. Afterwards, the roles of precursors in H₂SO₄-HNO₃-NH₃-VOC system were identified. It indicates that H₂SO₄, NH₃ and VOC mainly contribute to the early stage of the NPF, while the growth of the nuclei is mainly driven by HNO₃ and NH₃. And HNO₃ makes increasing contributions at the early stage of NPF with CPV rising (especially above 10⁸cm^-3). Thus in high CPV areas, especially for China, HNO₃ should be paid the same attention as H₂SO₄, NH₃ and VOC. The findings provide important implications for haze mitigations in China and other industrializing countries with multi-pollutant emission sources.

New particle formation in China: Current knowledge and further directions

New particle formation (NPF) studies have been conducted in China since 2004. Formation of new atmospheric aerosol particles has been observed to take place in diverse environments, even under the circumstances of high pre-existing particle loading, challenging the traditional and present understanding of the physicochemical nucleation mechanisms, which have been proposed based on the investigations in clean environments and under laboratory experimental conditions. This paper summarizes the present status and gaps in understanding NPF in China and discusses the main directions opening for future research.