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Stable isotopic constraint and observationally coupled atmospheric chemistry model are two robust means to identify the oxidation mechanisms for atmospheric particulate nitrate (NO3−) production. However, large discrepancies between the two approaches have been noted in the heavily polluted environments of North China Plain (NCP). Herein, daily PM2.5 samples (n = 418) were collected at five urban cities across NCP during the winter of 2017–2018; the ionic species (e.g., NO3− and methanesulfonic acid: MSA) and the dual‐isotopic signatures of NO3− were quantified. In this study, we revisited the O isotopic signatures of atmospheric O‐bearing molecules, especially that of OH radical in accordance with the up‐to‐date field measurements conducted in NCP regions. In brief, the δ18O‐OH was determined by the competition between the OH loss reaction rate and its isotopic equilibration reaction with H2O. Our improved isotopic results suggested that the daytime OH oxidation pathway was as important as the nocturnal channels in wintertime nitrate production in NCP (46.4% vs. 53.6%), whereas the exceedingly high loadings of NO3− in haze days were attributed to the heterogeneous chemistry of N2O5 hydrolysis. Chemical analysis also confirmed the importance of active photochemistry in secondary components production, as the MSA (a photooxidation product) linearly correlated with the NO3− (R2 = 0.86, p < 0.05). Our preliminary results compared well with the recent model simulations (e.g., sophisticated 3D model and observationally constraint model). To alleviate the winter haze in NCP, future emission regulations should aim to reduce the photochemical oxidants, particularly their precursors of NOx and VOCs simultaneously. Key Points: The predicted δ18O‐OH values are on average of −29.9 ± 8.2‰ and +13.5 ± 1.4‰ in clean and polluted days, respectivelyThe determined PM2.5 nitrate is significantly and positively correlated with MSA (R2 = 0.86, p < 0.05), a potential photooxidation indicatorResults suggest the wintertime photochemistry plays an important role in the production of particulate nitrate [ABSTRACT FROM AUTHOR]

Stable nitrogen and oxygen isotopic signatures of nitrate in atmospheric fine-mode particulate (as δ15N–NO 3 - and δ18O–NO 3 - in PM 2.5) was proposed to be useful in distinguishing the sources and oxidation chemistry of NO x (NO x = NO + NO 2). In the present study, the chemical oxidation processes of atmospheric NO x in urban Nanchang, the capital of Jiangxi province, southeast of China were estimated based on the δ15N-δ18O space of NO 3 -. Daily PM 2.5 samples (n = 91) were collected during wintertime of 2017–2018 (1 November to 31 January), the major water-soluble inorganic ions and the dual isotopic signatures of NO 3 - were measured. During the observations, the NO 3 - concentrations in PM 2.5 varied widely from 0.8 μg/m3 to 57.7 μg/m3, on average of 15.5 ± 6.7 μg/m3. The δ15N–NO 3 - and δ18O–NO 3 - also ranged widely, from −1.9‰ to +12.1‰ (+6.5 ± 3.7‰) and +69.1‰ to +95.5‰ (+85.9 ± 17.7‰), respectively. The daily δ15N–NO 3 - was observed to be independent with the corresponding δ18O–NO 3 - (R 2 = 0.06, p > 0.05), which contrasted with many previous reports. By linking the δ15N–NO 3 - to the NO x oxidation chemistry, we tried to explore the environmental significance of the δ15N-δ18O space of NO 3 -. Our results suggested that the nocturnal pathways (e.g., N 2 O 5 hydrolysis and NO 3 radical reacted with hydrocarbons: NO 3 +HCs) dominated the chemical conversion of NO x to NO 3 - in the wintertime of Nanchang, with an average fractional contribution of 60%. Interestingly, results also indicated the importance of the NO 3 +HCs channel in NO x oxidation (on average of 33%), which can reach 45% during extreme nitrate aerosol polluted days. Our observations highlighted the importance of NO 3 radical in NOx oxidation and particulate nitrate formation in polluted urban environment. [Display omitted] The δ15N–NO 3 - and δ18O–NO 3 - ranged from −1.9‰ to +12.1‰ and +69.1‰ to +95.5‰, respectively. The daily δ15N–NO 3 - was independent with the corresponding δ18O–NO 3 - (R2 = 0.06, p > 0.05). Results highlighted the importance of NO 3 radical in NO x oxidation in polluted urban environment. [ABSTRACT FROM AUTHOR]

Water-soluble inorganic ions (WSIIs) in PM 2.5 from different cities have been studied in previous studies. However, the studies of WSIIs in plateau cities are relative deficient. Due to differences in topography, climate and emission sources, the WSIIs characteristics of plateau cities are expected to be different. Here, we determined the concentrations of WSIIs (SO 4 2−, NO 3 −, NH 4 +, Ca2+, Mg2+, Na+, K+, F−, Cl−, NO 2 −) in PM 2.5 from different seasons of Kunming, a typical plateau city in southwest China. The data will improve our understanding of the chemical characterization and source of PM 2.5 in plateau environment. Our results showed that the secondary aerosols were the main pollutants (contributing >50% to PM 2.5) in PM 2.5 of Kunming, which mainly from coal combustion, agricultural activities and vehicle exhaust. Seasonally, high volatility of the NO 3 − and NH 4 + and washout effects of rainfall in hot months (wet seasons) were favorable for the decreased of pollutants, while high emission, poor dispersion conditions and low removal rate could lead to the increased of pollutants in cold months (dry seasons). It suggested that adequate NH 3 and intense solar radiation promotes the photochemical reactions of SO 2 , NO x and NH 3 to form NH 4 HSO 4 , (NH 4) 2 SO 4 and NH 4 NO 3. High temperature in hot months would promote the volatilization of NH 4 NO 3 in Kunming. Unlabelled Image • The sources of WSIs in PM 2.5 in a plateau city at different seasons were identified. • Seasonal climate change and special weather conditions have influence on sources of WSIs. • Solar radiation and temperature are important factors affecting secondary aerosols in plateau region. [ABSTRACT FROM AUTHOR]

Schematic of the nitrate evolution with the sampling days during haze period (A), which was controlled by the Rayleigh fractionation model (B). • The δ15N-NO 3 - values of PM 2.5 in Beijing-Tianjin-Shijiazhuang ranged from -3.1‰ to +11.4 ‰. • Rayleigh fractionation model was used to track NO x emission sources. • δ15N-NO 3 - and the corresponding p-NO 3 - were negatively correlated during the haze period. • Non-fossil NO x and fossil NO x contributed equally to NO 30 - in PM 2.5. Despite the implementation of some effective measures to control emissions of nitrogen oxides (NO x = NO + NO 2) in recent years, the ambient NO x concentration in urban cities of China remains high. Therefore, a quantitative understanding of NO x emission sources is critical to developing effective mediation policies. In the present study, the dual isotopic compositions of nitrate (p -NO 3 −) in fine-particle aerosol (PM 2.5) collected daily at a regional scale (Beijing-Tianjin-Shijiazhuang) were measured to better constrain the NO x emission sources. The specific focus was on a typical haze episode that occurred simultaneously in the three urban cities (October 22–29, 2017). It was found that the nitrogen isotopic values of nitrate in PM 2.5 (hereafter as δ15N-NO 3 −) ranged widely from −3.1‰ to + 11.4‰, with a mean value of 3.5 ± 3.7‰. Furthermore, a negative relationship between the δ15N-NO 3 − values and the corresponded p -NO 3 − concentrations during the haze period was observed. This implied the preferential formation of 15N-enriched NO 3 − into a fine-particle aerosol. Taking a different approach to previous publications, the Rayleigh fractionation model was used to characterize the initial isotopic signatures of ambient NO x. After accounting for the δ15N difference between p -NO 3 − and the source NO x , the estimated initial δ15N-NO x ranged from −20‰ to 0‰, which indicated a cosniderable contribution of non-fossil fuel emissions. The individual contributions of potential sources were further quantified using the Bayesian mixing model, revealing that NO x from coal or natural gas combustion, vehicle exhausts, biomass burning, and the microbial activity contributed 17.9 ± 11.4%, 29.4 ± 19.6%, 29.1 ± 18.9% and 23.5 ± 12.7% to NO 3 − in PM 2.5 , respectively. These results highlighted that tightening controls of gaseous NO x emissions from non-fossil sources may represent an opportunity to mitigate PM 2.5 pollution in urban cities of China. [ABSTRACT FROM AUTHOR]

As the most important gas-phase alkaline species, atmospheric ammonia (NH 3) contributes considerably to the formation and development of fine-mode particles (PM 2.5), which affect air quality and environmental health. Recent satellite-based observations suggest that the North China Plain is the largest agricultural NH 3 emission source in China. However, our isotopic approach shows that the surface NH 3 in the intraregional urban environment of Beijing-Tianjin-Shijiazhuang is contributed primarily by combustion-related processes (i.e., coal combustion, NH 3 slip, and vehicle exhaust). Specifically, the Batch fractionation model was used to describe the partitioning of gaseous NH 3 into particles and to trace the near-ground atmospheric NH 3 sources. With the development of haze pollution, the dynamics of δ15N-NH 4 + were generally consistent with the fractionation model. The simulated initial δ15N-NH 3 values ranged from −22.6‰ to −2.1‰, suggesting the dominance of combustion-related sources for urban NH 3. These emission sources contributed significantly (92% on hazy days and 67% on clean days) to the total ambient NH 3 in urban cities, as indicated by a Bayesian mixing model. Based on the Batch fractionation model, we concluded the following: 1) δ15N-NH 4 + can be used to model the evolution of fine-mode aerosols and 2) combustion-related sources dominate the near-ground atmospheric NH 3 in urban cities. These findings highlight the need for regulatory controls on gaseous NH 3 emissions transported from local and surrounding industrial sources. Image 1 • δ15N-NH 4 + values of PM 2.5 in urban cities of NCP were higher on clean days than on hazy days. • Simulated initial δ15N-NH 3 values ranged from −22.6‰ to −2.1‰ based on a Batch model. • Surface NH 3 in the urban environment is dominated by combustion-related processes. [ABSTRACT FROM AUTHOR]

In this study, we measured the daily water-soluble inorganic ions (WSIIs) concentration (including SO42−, NO3−, NH4+, Ca2+, K+, Cl−, Na+, Mg2+, and F−) of PM2.5 (particulate matter with a diameter smaller than 2.5 μm) throughout the year in Nanning (a typical subtropical monsoon climate city in southwestern China) to explore the influence of seasonal climate change on the properties of PM2.5 pollution. This suggested that SO42−, NO3−, and NH4+ were the main component of WSIIs in Nanning. Secondary inorganic ions from fossil fuel combustion, agricultural activities, and automobile emissions were the main contributors to PM2.5, contributing more than 60% to PM2.5. Compared with the wet season, the contributions of different sources increased in the dry season (including pollution days); of these sources, automobile emissions and coal combustion emissions increased the most (about nine times and seven times, respectively). Seasonal weather and climate change affected the concentration level of WSIIs. During the wet season, higher temperatures and abundant rainfalls contributed to the volatilization and removal of WSIIs in PM2.5, while in the dry season and on pollution days, lower temperatures and less precipitation, higher emissions, and poor diffusion conditions contributed to the accumulation of WSIIs in PM2.5. NH4HSO4, (NH4)2SO4 and NH4NO3 were the main chemical forms of secondary inorganic ions. Sufficient NH3, intense solar radiation, and moist particulate matter surface promoted the formation of secondary inorganic ions. The higher temperature contributed to the volatilization of secondary inorganic ions. [ABSTRACT FROM AUTHOR]

In most of the urban atmosphere, the formation of ammonium (NH 4 +) is mainly controlled by the initial isotope value of ammonia (δ15N-NH 3 values) and the corresponding ammonia (NH 3) conversion ratio (NH 4 +/(NH 4 + + NH 3)), as N isotope fractionation may occur under an NH 3 -sufficient environment. The significant influence of δ15N-NH 3 values on the formation of NH 4 + in the urban atmosphere has been demonstrated by many previous studies; however, exploration of the effect of the NH 3 conversion ratios on NH 4 + formation has been limited, especially during pollution episodes. To better understand the NH 3 sources and the effect of the NH 3 conversion ratios on the formation of NH 4 + in the urban environment, NH 4 + concentrations (0.2 to 13.4 μg·m−3, average 3.9 ± 2.8 μg·m−3) and their δ15N values (−4.0‰ to 29.2‰, average 20.6‰ ± 6.1‰) in PM 2.5 were determined in the urban area of Nanning from September 1 to December 31, 2017. Based on the measured δ15N-NH 4 + values and the isotope fractionation that occurred during gas-to-particle partitioning, we estimated that the initial δ15N-NH 3 values were − 22.8‰ to 3.1‰ (average − 8.8‰ ± 5.5‰). These initial δ15N-NH 3 values were influenced by agricultural emission sources (−31.0‰ to −4.4‰), fossil fuel-related NH 3 emission sources (−14.6‰ to 10.1‰) and biomass combustion sources (12‰ and 23‰). Source apportionment indicated that these three potential NH 3 emission sources contributed 43%, 33% and 24% to the ambient NH 3 , respectively. On clean days, similar trends in initial δ15N-NH 3 and δ15N-NH 4 + values were observed, and there was an apparent positive linear correlation between these values (R2 = 0.70, p < 0.01). This suggested that the δ15N-NH 4 + signatures recorded changes in the NH 3 sources and that NH 4 + formation was controlled by the NH 3 sources on clean days. However, during different pollution episodes the variations in δ15N-NH 4 + values were consistent with the NH 4 + concentrations and the NH 4 +/(NH 4 + + NH 3) values, and there were negative linear correlation between these values (R2 > 0.6, p < 0.01). This suggested that the δ15N-NH 4 + signatures recorded NH 4 + evolution processes and that the increased NH 3 conversion ratios was the main reason for the development of NH 4 + and PM 2.5 in pollution episodes. We inferred that the decreased air temperature in cold months may stimulate the increased NH 3 conversion ratios, based on the negative correlation of NH 4 +/(NH 4 + + NH 3) values with air temperature (r = −0.63, p < 0.01). Unlabelled Image • Agricultural emissions were the main NH 3 sources (account for 43%) in Nanning. • Fossil fuel-related and biomass combustion emissions increased in cold months. • Increased NH 3 conversion ratio promoted NH 4 + and PM 2.5 evolution in pollution episode. • Decreased temperature in cold months might increase the NH 3 conversion ratio. [ABSTRACT FROM AUTHOR]

The source of fine particulate matter (PM 2.5) has been a longstanding subject of debate, the nitrogen-15 isotope (δ15N) has been used to identify the major sources of atmospheric nitrogen. In this study, PM 2.5 samples (n = 361) were collected from September 2017 to August 2018 in the urban area of Guiyang (SW, China), to investigate the chemical composition and potential sources of PM 2.5. The results showed an average PM 2.5 of 33.0 μg m−3 ± 20.0 μg m−3. The concentration of PM 2.5 was higher in Winter, lower in Summer. The major water resolved inorganic ions (WSIIs) were Ca2+, NH 4 +, Na+, SO 4 2−, NO 3 −, Cl−. Nitrogen-containing aerosols (i.e., NO 3 − and NH 4 +) suddenly strengthened during the winter, when NO 3 − became the dominant contributor. Over the sampling period, the molar ratio of NH 4 +/(NO 3 − + 2 × SO 4 2−) ranged from 0.1 to 0.9, thus indicating the full fixation of NH 4 + by existing NO 3 − and SO 4 2− in PM 2.5. The annual value of NOR was 0.1 while rised to 0.5 in Winter. The variations of NOR (Nitrogen oxidation ratio) (0.1–0.5) values suggest that the secondary formation of NO 3 − occurred every season and was most influential during the winter. The total particulate nitrogen (TN) δ15N value of PM 2.5 ranged from −5.9‰ to 25.3‰ over the year with annual mean of +11.8‰ ± 4.7‰, whereas it was between −5.9‰ and 14.3‰ during the winter with mean of 7.0‰ ± 3.8‰. A Bayesian isotope mixing model (Stable Isotope Analysis in R; SIAR) was applied to analyze the nitrogen sources. The modeling results showed that 29%, 21%, and 40% of TN in PM 2.5 during the winter in Guiyang was due to nitrogen-emissions from coal combustion, vehicle exhausts, and biomass burning, respectively. Our results demonstrate that biomass burning was the main contributor to PM during the winter, 80% of the air mass comes from rural areas of Guizhou border, this transport process can increase the risk of particulate pollution in Guiyang. Unlabelled Image • Nitrogen isotopes in PM2.5 were characterized in Guiyang. • Seven source factors were identified for PM2.5 using the SIAR model. • Source of biomass burning played a pivotal role in PM 2.5 during winter. [ABSTRACT FROM AUTHOR]

Kunming, a Chinese southwestern tourist city which has not large local pollution sources, has found to have an increasing tendency of haze pollution in recent years. But the pollution sources are unclear. In order to identify them, daily PM 2.5 samples (n = 346) were collected from September 2017 to August 2018 in the urban area. And the major water-soluble inorganic ions (WSIIs) were determined to better understand the chemical characteristics, source categories and potential region of sources. Our study showed that the mass concentration of PM 2.5 in Kunming ranged from 7.61 to 91.83 μg m−3, with an annual average value of 33.59 ± 15.71 μg m−3. Positive matrix factorization (PMF) model identified five factors including secondary aerosol (the contributions of 36.3%), coal combustion (26.0%), biomass burning (19.2%), dust (12.5%) and sea salt (6.0%). And coal combustion played a leading role in the source contribution of PM 2.5 in winter while biomass combustion was dominant in spring. Being located between two severe haze zones in the world, northern-central China and north of South Asia, and affected by India monsoon and East Asia monsoon in summer and prevailing westerlies in winter, we found that air masses from South Asia (especially India) contained pollutants could be brought to Kunming by prevailing westerlies in winter. In spring, however, the sources of PM 2.5 in Kunming were mainly affected by biomass burning from South Asia and Southeast Asia when prevailing westerlies gradually weakened. Image 1 • PM 2.5 and its major water-soluble inorganic ions were characterized in Kunming. • Five source factors were identified for PM 2.5 by PMF. • Coal combustion played a leading role in the source contribution of PM 2.5 in winter. • In winter, air pollutants from India could be brought to Kunming by prevailing westerlies. • In spring, the sources of PM 2.5 were mainly affected by biomass burning from South Asia and Southeast Asia. [ABSTRACT FROM AUTHOR]

Little updated observation of background aerosols in Central China is available since the clean air actions, despite increasing aerosols pollution in this region. PM 2.5 was sampled at Dongting Lake during September 2017 to August 2018 to probe the chemical characteristics of background PM 2.5 in Central China. We sampled aerosols at an urban site simultaneously. Annual mean PM 2.5 level reached 43.4 μg m−3, and SO 4 2−, NO 3 −, and NH 4 + (sulfate–nitrate–ammonium, simplified as SNA) accounted for 22.8%, 26.8%, and 15.0%, respectively. PM 2.5 and most ions levels were highest during winter and lowest during summer. PM 2.5 mass was driven by NO 3 − during cold months (October–March) or SO 4 2− during warm months (September, April–August), suggesting that different emission control measures should be applied in different seasons in future air quality policy of this region. Both SO 4 2− and NO 3 − were almost completely neutralized by NH 4 +. Low temperature increased the SO 4 2− concentration and sulfur oxidation ratio. Low temperature also facilitated the partition of NO 3 − from gas phase into particle phase. Severe particulate pollutions occurred in cold months when the diffusion and wet scavenging of aerosols and volatilization of NH 4 NO 3 were weak, and the NO 2 concentration was high. An interesting finding was the highest NO 3 − concentration and conversion ratios for SNA as well as lowest SO 4 2− concentration in Dongting Lake among major background sites, despite low levels of local gaseous precursors. Industry, secondary SO 4 2−, and NO 3 − were indentified as main sources of PM 2.5 mass based on PMF analysis. Concurrent occurrences of PM 2.5 and SNA pollution at Dongting Lake and Wuhan sites as well as back trajectories verified the regional effect. The results will be helpful for investigating water–soluble ions and the major sources as well as geographical origins for PM 2.5 at Central China. • Severe PM 2.5 pollution occurred in Dongting Lake site and was driven by NO 3 −. • NO 3 − concentration and conversion ratios for SNA were highest in Dongting Lake among major background sites. • Impacts of regional transport on the background areas of Central China can not be ignored. [ABSTRACT FROM AUTHOR]