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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]

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]

Particulate ammonium (NH 4 +) is one of the most important inorganic components in aerosol. The concentrations of NH 4 + in PM 2.5 significantly increased when PM 2.5 levels elevated. Increased understanding of the atmospheric processes and sources of ambient ammonia is an effective way to control atmospheric ammonia and tackle air pollution problems. This study focused on the concentration and nitrogen stable isotopic composition of particulate NH 4 + in PM 2.5 in a southwest typical plateau city, Kunming. The trend in NH 4 +concentrations was parallel to the trend in PM 2.5 levels, with obviously increased concentrations observed in November. Aerosol pH and liquid water content (ALWC) were synchronously simulated by the ISORROPIA-II model. And the ammonia gas-particle conversion ratio (f) was calculated for each day. Then, we proposed that a self-amplifying feedback mechanism of NH 4 + formation was associated with the variations of ALWC, pH, and the ammonia gas-particle conversion ratio. Based on the inverse analysis of the nitrogen isotopic composition of particulate NH 4 +, the corresponding δ15N values of initial ambient NH 3 were estimated to be −27.4‰–15.3‰, with an average of −8.1 ± 8.3‰. Results from Back trajectory analysis, PSCF analysis, and isotope-based source apportionment of NH 3 shed light on source compositions and potential source regions, indicating that, in the study area, ambient NH 3 during slightly polluted days were dominated by biomass burning emissions, which might have been originated from local emissions and regional transport process in late autumn. [Display omitted] • A self-amplifying loop was proposed when the concentration of NH 4 + in PM 2.5 elevated in late autumn in Kunming. • Biomass burning related ammonia emissions significantly enhanced during slightly polluted days. • Increased biomass burning emissions in Kunming were related to ammonia emissions from local area and regional transport. [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]