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Here we present a field measurement of ClNO 2 (nitryl chloride) and N 2 O 5 (dinitrogen pentoxide) by a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) with the Filter Inlet for Gas and AEROsols (FIGAERO) at a regional site in the Pearl River Delta during a photochemical pollution season from 26 September to 17 November 2019. Three patterns of air masses are sampled during this campaign, including the dominating air masses from the north and northeast urban regions (Type A), the southeast coast (Type B), and the South China Sea (Type C). The concentration of ClNO 2 and N 2 O 5 was observed to be much higher in Type A and B than in Type C, indicating that the urban nighttime chemistry is more active than the background marine regions. The N 2 O 5 uptake coefficient and ClNO 2 production yield were estimated based on the field measurement, and the performance of the previously derived parameterizations was assessed. The nighttime ClNO 2 correlated with particulate chloride and the mass concentration of fine particles (most likely due to aerosol surface area) suggested that the ClNO 2 formation was limited by the N 2 O 5 uptake at this site. By examining the relationship between particulate chloride and other species, we implied that anthropogenic emissions (e.g., biomass burning) rather than sea salt particles dominate the origin of particulate chloride, although the site was only about 100 km away from the ocean. A box model with detailed chlorine chemistry is used to investigate the impacts of ClNO 2 chemistry on atmospheric oxidation. Model simulations showed that the chlorine radical liberated by ClNO 2 photolysis during the next day had a slight increase in concentrations of OH, HO 2 , and RO 2 radicals, as well as minor contributions to RO 2 radical and O 3 formation (< 5 %, on daytime average), in all the three types of air masses. Relatively high contributions were observed in Type A and B. The overall low contributions of ClNO 2 to atmospheric oxidation are consistent with those reported recently from wintertime observations in China (including Shanghai, Beijing, Wangdu, and Mt. Tai). This may be attributed to the following: (1) relatively low particle mass concentration limited ClNO 2 formation; (2) other reactions channels, like nitrous acid (HONO), oxygenated volatile organic compounds (OVOCs, including formaldehyde), and ozone photolysis had a more significant radical formation rate during the ozone pollution episodes and weakened the ClNO 2 contribution indirectly. The results provided scientific insights into the role of nighttime chemistry in photochemical pollution under various scenarios in coastal areas. [ABSTRACT FROM AUTHOR]

Many methods have been developed to verify the correlation between meteorological conditions and air pollutants; however, all have limitations that lead to biased or incomplete conclusions. Hence, improved methods are urgently required to describe this correlation comprehensively and accurately. In this study, we demonstrated the ability of the Copula function to apply time‐varying correlations between meteorological factors and atmospheric pollutants. A mixed Copula model was constructed using meteorological monitoring data for Beijing and Guangzhou from 2014 to 2019 to dynamically analyse the correlation characteristics and tail dependence between these factors. We then performed a correlation analysis for the data from the average, lower, and upper tails to obtain a more accurate and comprehensive correlation description. Dynamic analysis results demonstrated significant seasonal fluctuations between meteorological conditions and pollutants relationships. Moreover, the correlation coefficient variations differ according to their average and tail values. High humidity is more likely to be accompanied by increased NO2 compared with average summer humidity. Our proposed model represents a novel application of the Copula function for determining the factors influencing air pollution. This model emphasizes the tail dependence between meteorological conditions and air pollutant concentrations and can be used to guide more targeted prevention and control strategies. [ABSTRACT FROM AUTHOR]

In the past decade, average PM 2.5 concentrations decreased rapidly under the strong pollution control measures in major cities in China; however, ozone (O3) pollution emerged as a significant problem. Here we examine a unique (for China) 12-year data set of ground-level O3 and precursor concentrations collected at an urban site in Beijing (PKUERS, campus of Peking University), where the maximum daily 8 h average (MDA8) O3 concentration and daytime Ox (O3+NO2) concentration in August increased by 2.3±1.2 ppbv (+3.3±1.8 %) yr -1 and 1.4±0.6 (+1.9±0.8 %) yr -1 , respectively, from 2005 to 2016. In contrast, daytime concentrations of nitrogen oxides (NOx) and the OH reactivity of volatile organic compounds (VOCs) both decreased significantly. Over this same time, the decrease of particulate matter (and thus the aerosol optical depth) led to enhanced solar radiation and photolysis frequencies, with near-surface J(NO2) increasing at a rate of 3.6±0.8 % yr -1. We use an observation-based box model to analyze the combined effect of solar radiation and ozone precursor changes on ozone production rate, P(O3). The results indicate that the ratio of the rates of decrease of VOCs and NOx (about 1.1) is inefficient in reducing ozone production in Beijing. P(O3) increased during the decade due to more rapid atmospheric oxidation caused to a large extent by the decrease of particulate matter. This elevated ozone production was driven primarily by increased actinic flux due to PM 2.5 decrease and to a lesser extent by reduced heterogeneous uptake of HO2. Therefore, the influence of PM 2.5 on actinic flux and thus on the rate of oxidation of VOCs and NOx to ozone and to secondary aerosol (i.e., the major contributor to PM 2.5) is important for determining the atmospheric effects of controlling the emissions of the common precursors of PM 2.5 and ozone when attempting to control these two important air pollutants. [ABSTRACT FROM AUTHOR]

During the period 2012–2015, photolysis frequencies were measured at the Peking University site (PKUERS), a site representative of Beijing. We present a study of the effects of aerosols on two key photolysis frequencies, j (O1D) and j (NO2). Both j (O1D) and j (NO2) display significant dependence on aerosol optical depth (AOD; 380 nm) with a non-linear negative correlation. With the increase in AOD, the slopes of photolysis frequencies vs. AOD decrease, which indicates that the capacity of aerosols to reduce the actinic flux decreases with AOD. The absolute values of slopes are equal to 4.2– 6.9×10-6 and 3.4×10-3 s -1 per AOD unit for j (O1D) and j (NO2) respectively at a solar zenith angle (SZA) of 60 ∘ and AOD smaller than 0.7, both of which are larger than those observed in a similar, previous study in the Mediterranean. This indicates that the aerosols in Beijing have a stronger extinction effect on actinic flux than absorptive dust aerosols in the Mediterranean. Since the photolysis frequencies strongly depended on the AOD and the SZA, we established a parametric equation to quantitatively evaluate the effect of aerosols on photolysis frequencies in Beijing. According to the parametric equation, aerosols lead to a decrease in seasonal mean j (NO2) by 24 % and 30 % for summer and winter, respectively, and a corresponding decrease in seasonal mean j (O1D) by 27 % and 33 %, respectively, compared to an aerosol-free atmosphere (AOD =0). Based on an observation campaign in August 2012, we used a photochemical box model to simulate the ozone production rate (P (O3)). The simulation results shows that the monthly mean daytime net ozone production rate is reduced by up to 25 % due to the light extinction of aerosols. Through further in-depth analysis, it was found that particulate matter concentrations maintain a high level under the condition of high concentrations of ozone precursors (volatile organic compounds, VOCs, and NOx), which inhibits the production of ozone to a large extent. This phenomenon implies a negative feedback mechanism in the atmospheric environment of Beijing. [ABSTRACT FROM AUTHOR]