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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]

Nitrate (NO 3 −) is one of the most important inorganic ions in fine particulate (PM 2.5) and drives regional haze formation; however, the NO 3 − sources and formation mechanisms in different seasons and regions are still debated. Here, PM 2.5 samples were collected from Kunming and Nanning in southwestern China from September 1, 2017, to February 28, 2018 (spanning warm and cold months). We measured the daily O and N isotopic compositions of NO 3 − (δ18O–NO 3 − and δ15N–NO 3 −), estimated the δ18O–HNO 3 values produced by different oxidation pathways, and quantified the NO 3 − formation pathways based on the isotope mass-balance equation. Our results showed that the δ18O–NO 3 − values in Kunming (65.3 ± 7.6‰) and Nanning (67.7 ± 10.1‰) are close to the δ18O–HNO 3 values arising from the OH radical pathway (P OH , 54.7 ± 1.2‰ to 61.2 ± 1.8‰), suggesting that the δ18O–NO 3 − values are mainly influenced by P OH , which showed a contribution greater than 74%. Stronger surface solar radiation and higher air temperatures in low-latitude regions and warm months increased the amount of HNO 3 produced by P OH and reduced the amount of HNO 3 produced by P N2O5 , which produced low δ18O–NO 3 − values. Increased air pollution emissions decreased the contribution from P OH and increased the contribution from N 2 O 5 and NO 3 pathways (P N2O5+NO3). The δ15N–NO 3 − values of PM 2.5 in Kunming (7.3 ± 2.8‰) were slightly higher than those in Nanning (2.8 ± 2.7‰). The increased NO x emissions with positive isotopic values led to high δ15N–NO 3 − values in northern China and during cold months. A higher f NO2 (f NO2 = NO 2 /(NO + NO 2), temperature, and contribution of P OH produced lower N isotope fractionation between NO x and δ15N–NO 3 −, which was found to further decrease the δ15N–NO 3 – values in southwestern China and during warm months. • Daily δ18O–NO3 and δ15N– NO3 in PM2.5 were analyzed in Kunming and Nanning. • OH radical oxidation was the main oxidation pathway (>74%) of NOx. • NOx oxidation pathways were changed with latitude and season. • Sources and isotope fractionation affected regional and seasonal δ15N–NO3 values. [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]