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This study presents one of the first long term datasets including a statistical summary of PM 2.5 concentrations obtained from one-year monitoring in 190 cities in China. We found only 25 out of 190 cities could meet the National Ambient Air Quality Standards of China and the population-weighted mean of PM 2.5 in Chinese cities are 61 μg/m 3 , ~3 times as high as global population-weighted mean, highlighting a high health risk. PM 2.5 concentrations are generally higher in north than in south regions due to relative large PM emissions and unfavorable meteorological conditions for pollution dispersion. A remarkable seasonal variability of PM 2.5 is observed with the highest during the winter and the lowest during the summer. Due to the enhanced contributions from dust particles and open biomass burning, high PM 2.5 abundances are also found in the spring (in Northwest and West Central China) and autumn (in East China), respectively. In addition, we found the lowest and highest PM 2.5 often occurs in the afternoon and evening hours, respectively, associated with daily variation of the boundary layer depth and anthropogenic emissions. The diurnal distribution of the PM 2.5 -to-CO ratio consistently displays a pronounced peak during the afternoon periods, reflecting a significant contribution of secondary PM formation.

Particulate nitrate (NO3-) not only influences regional climates but also contributes to the acidification of terrestrial and aquatic ecosystems. In 2016 and 2017, four intensive online measurements of water-soluble ions in PM2.5 were conducted in Nanjing City in order to investigate the potential formation mechanisms of particulate nitrate. During the sampling periods, NO3- was the predominant species, accounting approximately for 35 % of the total water-soluble inorganic ions, followed by SO42- (33 %) and NH4+ (24 %). Significant enhancements of nitrate aerosols in terms of both absolute concentrations and relative abundances suggested that NO3- was a major contributing species to high-PM2.5 events (hourly PM2.5≥150 µg m-3). High NO3- concentrations mainly occurred underNH4+-rich conditions, implying that the formation of nitrate aerosols in Nanjing involved NH3. During the high-PM2.5 events, the nitrogen conversion ratios (Fn) were positively correlated with the aerosol liquid water content (ALWC; R>0.72 and p<0.05). Meanwhile, increasing NO3- concentrations regularly coincided with increasing ALWC and decreasing Ox (Ox=O3+NO2). These results suggested that the heterogeneous reaction was probably a major mechanism of nitrate formation during the high-PM2.5 events. Moreover, the average production rate of NO3- by heterogeneous processes was estimated to be 12.6 % h-1 (4.1 µg m-3 h-1), which was much higher than that (2.5 % h-1; 0.8 µg m-3 h-1) of gas-phase reactions. This can also explain the abrupt increases in nitrate concentrations during the high-PM2.5 events. Utilizing the ISORROPIA II model, we found that nitrate aerosol formation in Nanjing during the high-PM2.5 events was HNO3 limited. This indicated that control of NOx emissions will be able to efficiently reduce airborne particulate nitrate concentrations and improve the air quality in this industrial city.

Isocoumarins and dihydroisocoumarins are important skeletons with a wide range of biological activities, such as anti-bacterial, anti-allergy, anti-fungal, anti-tumor, and anti-HIV properties. Herein, we demonstrated divergent syntheses of isocoumarins and 3,4-dihydroisocoumarins by intramolecular dehydrogenative cyclization of 2-(3-oxobutyl) benzoic acids. This transformation undergoes Csp3–H bonds and O–H bonds coupling in air using copper salt. The reactions may undergo free radical process.

Biomass burning can significantly impact the chemical and optical properties of carbonaceous aerosols. Here, the biomass burning impacts were studied during wintertime in a megacity of Nanjing, eastern China. The high abundance of biomass burning tracers such as levoglucosan (lev), mannosan (man), galactosan (gal) and non-sea-salt potassium (nss-K+) was found during the studied period with the concentration ranges of 22.4–1476 ng m−3, 2.1–56.2 ng m−3, 1.4–32.2 ng m−3 and 0.2–3.8 µg m−3, respectively. The significant contribution of biomass burning to water-soluble organic carbon (WSOC; 22.3±9.9 %) and organic carbon (OC; 20.9±9.3 %) was observed in this study. Backward air mass origin analysis, potential emission sensitivity of elemental carbon (EC) and MODIS fire spot information indicated that the elevations of the carbonaceous aerosols were due to the transported biomass-burning aerosols from southeastern China. The characteristic mass ratio maps of lev∕man and lev∕nss-K+ suggested that the biomass fuels were mainly crop residuals. Furthermore, the strong correlation (p < 0.01) between biomass burning tracers (such as lev) and light absorption coefficient (babs) for water-soluble brown carbon (BrC) revealed that biomass burning emissions played a significant role in the light-absorption properties of carbonaceous aerosols. The solar energy absorption due to water-soluble brown carbon and EC was estimated by a calculation based on measured light-absorbing parameters and a simulation based on a radiative transfer model (RRTMG_SW). The solar energy absorption of water-soluble BrC in short wavelengths (300–400 nm) was 0.8±0.4 (0.2–2.3) W m−2 (figures in parentheses represent the variation range of each parameter) from the calculation and 1.2±0.5 (0.3–1.9) W m−2 from the RRTMG_SW model. The absorption capacity of water-soluble BrC accounted for about 20 %–30 % of the total absorption of EC aerosols. The solar energy absorption of water-soluble BrC due to biomass burning was estimated as 0.2±0.1 (0.0–0.9) W m−2, considering the biomass burning contribution to carbonaceous aerosols. Potential source contribution function model simulations showed that the solar energy absorption induced by water-soluble BrC and EC aerosols was mostly due to the regionally transported carbonaceous aerosols from source regions such as southeastern China. Our results illustrate the importance of the absorbing water-soluble brown carbon aerosols in trapping additional solar energy in the low-level atmosphere, heating the surface and inhibiting the energy from escaping the atmosphere.

Water-soluble organic carbon (WSOC) is a significant fraction of organic carbon (OC) in atmospheric aerosols. WSOC is of great interest due to its significant effects on atmospheric chemistry, the Earth's climate and human health. The stable carbon isotope (δ13C) can be used to track the potential sources and investigate atmospheric processes of organic aerosols. However, the previous methods measuring the δ13C values of WSOC in ambient aerosols require a large amount of carbon content, are time-consuming and require labor-intensive preprocessing. In this study, a method of simultaneously measuring the mass concentration and the δ13C values of WSOC from aerosol samples is established by coupling the GasBench II preparation device with isotopic ratio mass spectrometry. The precision and accuracy of isotope determination is better than 0.17 ‰ and 0.5 ‰, respectively, for samples containing WSOC amounts larger than 5 µg. This method is then applied for the aerosol samples collected every 3 h during a severe wintertime haze period in Nanjing, eastern China. The WSOC values vary between 3 and 32 µg m−3, whereas δ13C−WSOC ranges from −26.24 ‰ to −23.35 ‰. Three different episodes (Episode 1, Episode 2 and Episode 3) are identified in the sampling period, showing a different tendency of δ13C−WSOC with the accumulation process of WSOC aerosols. The increases in both the WSOC mass concentrations and the δ13C−WSOC values in Episode 1 indicate that WSOC is subject to a substantial photochemical aging during the air mass transport. In Episode 2, the decline of the δ13C−WSOC is accompanied by the increase in the WSOC mass concentrations, which is associated with regional-transported biomass burning emissions. In Episode 3, heavier isotope (13C) is exclusively enriched in total carbon (TC) in comparison to WSOC aerosols. This suggests that the non-WSOC fraction in total carbon may contain 13C-enriched components such as dust carbonate, which is supported by the enhanced Ca2+ concentrations and air mass trajectory analysis. The present study provides a novel method to determine the stable carbon isotope composition of WSOC, and it offers a great potential to better understand the source emission, the atmospheric aging and the secondary production of water-soluble organic aerosols.

Volatile organic compounds (VOCs) are key components of tropospheric chemistry. We investigated ambient VOCs in an industrial area in Nanjing, China, between July 2018 and May 2020. The sum of the suite of measured total VOC (TVOC) concentrations was 59.8 ± 28.6 ppbv (part per billion by volume) during the investigation period. About twice the TVOC concentrations were observed in the autumn (83 ± 20 ppbv) and winter (77.5 ± 16.8 ppbv) seasons compared to those in spring (39.6 ± 13.1 ppbv) and summer (38.8 ± 10.2 ppbv). In previous studies in Nanjing, oxygenated VOCs (OVOCs) and halocarbons were not measured, and the current TVOC concentration without halocarbons and OVOCs was similar to the previous investigation in the same study area. However, it was twofold higher than the one reported in the nonindustrial suburban area of Nanjing. Due to the industrial influence, the halocarbons VOC group (14.3 ± 7.3 ppbv, 24 %) was the second-largest contributor to the TVOCs after alkanes (21 ± 7 ppbv, 35 %), which is in contrast with the previous studies in Nanjing and also in almost all other regions in China. Relatively high proportions of halocarbons and aromatics were observed in autumn (25.7 % and 19.3 %, respectively) and winter (25.8 % and 17.6 %, respectively) compared to those in summer (20.4 % and 11.8 %, respectively) and spring (20.3 % and 13.6 %, respectively). According to the potential source contribution function (PSCF), short-distance transport from the surrounding industrial areas and cities was the main reason for the high VOC concentrations in the study area. According to positive matrix factorization (PMF) model results, vehicle-related emissions (33 %–48 %) contributed to the major portion of the ambient VOC concentrations. Aromatics, followed by alkenes, were the top contributors to the loss rate of OH radicals (LOH; 37 % and 32 %, respectively). According to the empirical kinetic modelling approach (EKMA) and relative incremental reactivity (RIR) analysis, the study area was in the VOC-sensitive regime for ozone (O3) formation during all measurement seasons. Therefore, alkenes and aromatics emissions from automobiles need to be decreased to reduce secondary air pollution formation in the study area.

Environmentally persistent free radicals (EPFRs) are a new type of substance with potential health risks. EPFRs are widely present in atmospheric particulates, but there is a limited understanding of the size-resolved health risks of these radicals. This study reports the exposure risks and source of EPFRs in atmospheric particulate matter (PM) of different particle sizes (<10 µm) in Linfen, a typical coal-burning city in China. The type of EPFRs in fine particles (< 2.1 µm) is different from that in coarse particles (2.1–10 µm) in both winter and summer. However, the EPFR concentration is higher in coarse particles than in fine particles in summer, and the opposite trend is found in winter. In both seasons, combustion sources are the main sources of EPFRs, with coal combustion as the major contributor in winter, while other fuels are the major source in summer. Dust contributes part of the EPFRs, and it is mainly present in coarse particles in winter and the opposite in summer. The upper respiratory tract was found to be the area with the highest risk of exposure to EPFRs of the studied aerosols, with an exposure equivalent to that of approximately 21 cigarettes per person per day. Alveolar exposure to EPFRs is equivalent to 8 cigarettes per person per day, with combustion sources contributing the most to EPFRs in the alveoli. This study helps us to better understand the potential health risks of atmospheric PM with different particle sizes.

Atmospheric humic-like substances (HULIS) affect the global radiation balance due to their strong light absorption at the ultraviolet wavelength. The potential sources and molecular compositions of water-soluble HULIS at a suburban site in the Yangtze River Delta from 2017 to 2018 were discussed, based on the results of the radiocarbon (14C) analysis and combining the Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) technique in this study. The 14C results showed that the averaged non-fossil-fuel source contributions to HULIS were 39 ± 8 % and 36 ± 6 % in summer and winter, respectively, indicating significant contributions from fossil fuel sources to HULIS. The Van Krevelen diagrams obtained from the FT-ICR-MS results showed that the proportions of tannin-like and carbohydrate-like groups were higher in summer, suggesting significant contribution of HULIS from biogenic secondary organic aerosols (SOAs). The higher proportions of condensed aromatic structures in winter suggested increasing anthropogenic emissions. Molecular composition analysis on the CHO, CHON, CHOS, and CHONS subgroups showed relatively higher intensities of high O-containing macromolecular oligomers in the CHO compounds in summer, further indicating stronger biogenic SOA formation in summer. High-intensity phenolic substances and flavonoids, which were related to biomass burning and polycyclic aromatic hydrocarbon (PAH) derivatives indicating fossil fuel combustion emissions, were found in winter CHO compounds. Besides, two high-intensity CHO compounds containing condensed aromatic ring structures (C9H6O7 and C10H5O8) identified in the summer and winter samples were similar to those from off-road engine samples, indicating that traffic emissions were one of the important fossil fuel sources of HULIS at the study site. The CHON compounds were mainly composed of nitro compounds or organonitrates with significantly higher intensities in winter, which were associated with biomass burning emissions, in addition to the enhanced formation of organonitrates due to high NOx in winter. However, the high-intensity CHON molecular formulas in summer were referring to N-heterocyclic aromatic compounds, which were produced from the atmospheric secondary processes involving reduced N species (e.g., ammonium). The S-containing compounds were mainly composed of organosulfates (OSs) derived from biogenic precursors, namely long-chain alkane and aromatic hydrocarbon, which illustrate the mixed sources of HULIS. Generally, different policies need to be considered for each season due to the different seasonal sources (i.e., biogenic emissions in summer and biomass burning in winter for non-fossil-fuel sources, traffic emissions and anthropogenic SOA formation in both seasons, and additional coal combustion in winter). Measures to control emissions from motor vehicles and industrial processes need to be considered in summer. Additional control measures on coal power plants and biomass burning should be applied in winter. These findings add to our understanding of the interaction between the sources and the molecular compositions of atmospheric HULIS.

PM2.5 aerosol samples were collected from the Chinese megacity of Nanjing (32.21∘ N, 118.73∘ E) during winter and analyzed for a total of 127 compounds from 12 organic compound classes. The most abundant classes of compounds were n-alkanes (mean concentration of 205 ng m−3), followed by fatty acids (76.3 ng m−3), polycyclic aromatic hydrocarbons (PAHs; 64.3 ng m−3), anhydrosugars (56.3 ng m−3), fatty alcohols (40.5 ng m−3) and phthalate esters (15.2 ng m−3), whereas hydroxy-/polyacids (8.33 ng m−3), aromatic acids (7.35 ng m−3), hopanes (4.19 ng m−3), primary sugars and sugar alcohols (4.15 ng m−3), lignin and resin products (2.94 ng m−3), and steranes (2.46 ng m−3) were less abundant. The carbon preference index of n-alkanes (0.83–1.38) indicated that they had a strong fossil fuel combustion origin. Diagnostic concentration ratios of organic tracers suggested that PAHs and hopanes originated mostly from coal burning and traffic emissions, respectively, in the Nanjing urban area. Positive matrix factorization analysis demonstrated that fossil fuel combustion was the major pollution source (28.7 %), followed by emissions from biomass burning (17.1 %), soil dust (14.5 %) and plastic burning (6.83 %) for Nanjing winter aerosols, although the contribution of secondary oxidation products (32.9 %) was the most abundant. Most of the compounds generally showed higher concentrations at nighttime compared with daytime; this was due to the accumulation process associated with inversion layers and the enhancement of emissions from heavy trucks at night. We conclude that fossil fuel combustion largely influences the winter organic aerosols in urban Nanjing. Based on the comparison of this study's results with previous research, we found that pollution levels in organic aerosols have decreased in the urban Nanjing atmosphere over the last decade.

The North China Plain (NCP) is becoming one of the most polluted areas characterized by a high frequency of haze pollution. However, the spatial and temporal evolutions of aerosol chemical compositions in such a highly polluted region are not well understood due to the lack of a long-term and comprehensive observation-based network. China’s National Aerosol Composition Monitoring Network (NACMON) has conducted comprehensive offline and online measurements of compositions and optical properties of airborne aerosols in order to systematically investigate the formation process, source apportionments of haze, and interactions between haze pollution and climate change. The objective of the observations is to provide information for policy makers to make strategies for the alleviation of haze occurrence. In this paper, we present instrumentations and methodologies as well as the preliminary results of the offline observations in NACMON stations over the NCP region. The implications and future perspectives of the network are also summarized. Benefiting from simultaneous observations from this network, we found that secondary aerosols were the dominant component in haze pollution. High anthropogenic emissions, low wind speed, and high relative humidity (RH) facilitated gas-to-particle transformation and resulted in high PM 2.5 formation (PM 2.5 is particulate matter that is smaller than 2.5 μ m in diameter). Sulfate-dominant or nitrate-dominant aerosols during the haze period were driven by ambient RH. Moreover, the contributions of coal combustion and biomass burning to PM 2.5 revealed downward trends, whereas secondary aerosols showed upward trends over the last decade. Thus, we highlighted that strict control of anthropogenic emissions of precursor gases, such as NO x , NH 3 , and volatile organic compounds (VOCs), will be an important way to decrease PM 2.5 pollution in the NCP region.