The first nitrous acid (HONO) observation was conducted in Kunming of Yunnan Province, from April to May 2021 with frequent ozone (O₃) pollution events. To evaluate the impact of HONO on O₃, the WRF-Chem model simulations were conducted, with two cases including or excluding potential HONO sources. Based on the comparison between simulations and the corresponding observations, it was found that the application of the MEIC inventory (version 2020) could reasonably simulate HONO, nitrogen dioxide (NO₂), O₃ and fine particulate matter (PM_2.5) in urban Kunming. The additional HONO sources noticeably enhanced atmospheric oxidation capacity in Kunming and accelerated the hydroxyl radical (OH) production rate, leading to an O₃ enhancement of (2–6)×10⁻⁹ with the corresponding percentage enhancement of 2%–8% at the Kunming site within the height of 0–4 km and an O₃ enhancement of (4–7)×10⁻⁹ near the ground at the Kunming site and its surrounding area. The O₃-VOCs-NO_x sensitivity was also influenced by the potential HONO sources. This study deepened the understanding of HONO, atmospheric oxidation capacity and O₃ formation in the Yunnan-Guizhou Plateau, and could be helpful for regional O₃ pollution control. 

To explore the characteristics and sources of water-soluble inorganic ions in PM_2.5 of Kunming, simultaneously on-line measurements of major water-soluble inorganic ions and gaseous pollutants were performed from April 15 to May 20, 2021 in Xishan district, Kunming city using a Gas-aerosol Collector and Ion Chromatograph (GAC-IC). The results showed that the average mass concentration of PM_2.5 was 25.0±15.0 μg/m³, indicating that the atmosphere of Kunming was at a relatively clean level, and the average mass concentration of water-soluble ions was 8.32±4.83 μg/m³, which accounted for 32.1% of the PM_2.5 concentration. The water-soluble components of PM_2.5 and gaseous precursors showed obvious diurnal variation, with the increase of inorganic ion concentrations at night and the decrease of inorganic ion concentrations during the day after reaching their peak in the morning. The average SOR and NOR were 0.55 and 0.042, indicating that there was an obvious secondary transformation process of sulfate, but not of nitrate, which might be accompanied by the formation of nitrate and the decomposition of NH₄NO₃. The result of positive matrix factorization (PMF) indicated that there were five main contribution sources of water-soluble ions in PM_2.5, which were fossil fuel combustion source and industry (36%), secondary sulfate (27%), biomass combustion (18%), secondary nitrate (16%) and sea salt (3%).

Carbon components, water-soluble ions and their gaseous precursors in atmospheric PM_2.5 in Taian were monitored online from May 10 to June 10, 2019. The mass concentration and component composition of PM_2.5 (carbonaceous species and water-soluble ions) were analyzed. The results showed that the average mass concentration of PM_2.5 was 37.7 μg/m³ during the sampling period, which was 1.1 times higher than the standardary limit (35 μg/m³) regulated by of Chinese Ambient air quality standards (GB 3095–2012). For instance, the content of water-soluble ions in PM_2.5 was the highest with 47.3%. The water-soluble components of PM_2.5 and gaseous precursors had obvious diurnal variation, with a peak (single peak) at 7:00 am. OC/EC ratio of Taian in summer ranged from 1.1 to 17.5, indicated that Taian was mainly affected by a mixture of biomass combustion, coal combustion and exhaust emissions. Positive matrix factor analysis (PMF) showed that the proportion of secondary nitrate, biomass combustion source, secondary sulfate and coal combustion soure in PM_2.5 were 22.0%, 46.7%, 29.9% and 1.4%, respectively.

In order to explore the impact of large-scale circulations on O₃ pollution in Chengdu, based on the 700 hPa geopotential height field in the Sichuan Basin (SCB) and using the T-PCA method, the atmospheric circulations in the summer of 2015–2019 (June–August) were classified into five circulation patterns, and the characteristics of O₃ pollution and O₃ sources in Chengdu under different circulation patterns were identified. Among the five types of circulation patterns, the northeast-high pressure (NEH) pattern and the under-high pressure field (UNF) pattern generally led to higher air temperature, and lower humidity and cloud cover in the SCB. Air temperature change compared to the previous days showed an obvious increasing trend in the SCB under the UNF pattern, kept stable under the NEH pattern, and indicated cooling effects with varying intensities under the other three circulation patterns (the southeast-high pressure pattern, the northwest-low pressure pattern and the northwest-high pressure pattern). The meteorological conditions under NEH and UNF were more conducive to the photochemical production of O₃ in the SCB. During the study period, O₃ concentrations and the percentages of over-standard days of O₃ in Chengdu under NEH and UNF were significantly higher than those under the other three circulation patterns, and thus NEH and UNF were the main circulation patterns that were related to O₃ pollution in summer. Months with more severe O₃ pollution in Chengdu tended to have higher proportion of days under the NEH and UNF patterns. The simulation studies of O₃ sources in Chengdu in summer suggested that O₃ was mainly derived from the SCB (55.0%), with local contribution of Chengdu (31.6%), and other important contributing cities including Deyang (5.4%), Chongqing (4.0%), Ziyang (3.9%) and Meishan (2.2%). However, O₃ sources in Chengdu under different circulation patterns showed different characteristics. Under the NEH pattern, O₃ transport within the Chengdu Plain had notable impact on O₃ pollution in Chengdu, with a significant increasing contribution from Deyang (14.9%). By contrast, under the UNF pattern, O₃ pollution in Chengdu became more localized, with nearly half (46.8%) of O₃ contributed by local emissions.