Air pollution has a negative impact on the ecological environment as well as on the health of people. Setting off fireworks and firecrackers lead to a significant deterioration in air quality in a short period of time. The firework prohibition policy of 2016 in Zhengzhou City provides an opportunity to investigate the effect of banning fireworks on air quality during the Spring Festival. The Air Quality Index and the concentrations of PM_2.5, PM₁₀, SO₂, NO₂, CO and O₃ during the Spring Festival between 2014 and 2019 in Zhengzhou City were analyzed. The results show that: There were small fluctuations in precipitation, air pressure, wind speed, temperature and relative humidity during the Spring Festival from 2014 to 2019. The air quality index in the Spring Festival in 2016 was significantly lower than that in 2014 and 2015, and the air quality in 2018 and 2019 showed greater improvements. The sudden increases in pollutants concentrations has been weakened, showing an obvious “peak-shaving” effect during the traditional heavy pollution period, New Year’s Eve to the first day of the first lunar month. In 2014 and 2015 years, the concentrations of PM_2.5 and PM₁₀ reached their peaks at 1:00 a.m. on the first day of the new year, and fluctuated in the ranges of 259–271 μg/m³ and 380–384 μg/m³, respectively. The concentrations of PM_2.5 and PM₁₀ were 44 and 100 μg/m³, respectively in 2016, 40 and 80 μg/m³ in 2018 during the same period. In addition, the ban implemented has the most obvious impact on PM_2.5 and PM₁₀. Compared with the Spring Festival in 2014, the concentrations of PM_2.5 and PM₁₀ increased by 19.15 and 18.64% in 2015, decreased by 18.38% and 15.90% in 2016, and decreased by 16.83% and 26.05% in 2018. Therefore, banning fireworks and firecrackers will help to improve the air quality during the Spring Festival in Zhengzhou City to a certain extent.

Despite significant improvements in air quality during and after COVID-19 restrictions, haze continued to occur in Zhengzhou afterwards. This paper compares ionic compositions and sources of PM_2.5 before (2019), during (2020), and after (2021) the restrictions to explore the reasons for the haze. The average concentration of PM_2.5 decreased by 28.5% in 2020 and 27.9% in 2021, respectively, from 102.49 μg m⁻³ in 2019. The concentration of secondary inorganic aerosols (SIAs) was 51.87 μg m⁻³ in 2019, which decreased by 3.1% in 2020 and 12.8% in 2021. In contrast, the contributions of SIAs to PM_2.5 increased from 50.61% (2019) to 68.6% (2020) and 61.2% (2021). SIAs contributed significantly to PM_2.5 levels in 2020–2021. Despite a 22~62% decline in NO_x levels in 2020–2021, the increased O₃ caused a similar NO₃⁻ concentration (20.69~23.00 μg m⁻³) in 2020–2021 to that (22.93 μg m⁻³) in 2019, hindering PM_2.5 reduction in Zhengzhou. Six PM_2.5 sources, including secondary inorganic aerosols, industrial emissions, coal combustion, biomass burning, soil dust, and traffic emissions, were identified by the positive matrix factorization model in 2019–2021. Compared to 2019, the reduction in PM_2.5 from the secondary aerosol source in 2020 and 2021 was small, and the contribution of secondary aerosol to PM_2.5 increased by 13.32% in 2020 and 12.94% in 2021. In comparison, the primary emissions, including biomass burning, traffic, and dust, were reduced by 29.71% in 2020 and 27.7% in 2021. The results indicated that the secondary production did not significantly contribute to the PM_2.5 decrease during and after the COVID-19 restrictions. Therefore, it is essential to understand the formation of secondary aerosols under high O₃ and low precursor gases to mitigate air pollution in the future.

To explore the characteristics of PM_2.5 and assess the health risks to residents in Shijiazhuang before and during the heating period in 2019, 2020 and 2021, the hourly concentrations of PM_2.5 and its nine selected trace elements were determined. The results showed that the mass concentrations of PM_2.5 were 80.32 ± 50.21 μg m⁻³ (2019), 69.97 ± 41.91 μg m⁻³ (2020) and 58.70 ± 41.97 μg m⁻³ (2021) during the heating period, representing greatly improved air quality. The PM_2.5 levels in the heating period were 1.04~1.60 times greater than those before the heating period, while the total selected trace element concentrations were about 1.44~1.97 times higher, indicating that strict control for PM_2.5 in the heating period should be imposed. The overall hazard quotient (HQ) of the nine selected trace elements in the heating period were 1.08~1.42 times higher than those before the heating period, while the total cancer risks (CR) were decreased by 29.04% (2020) and 3.50% (2021). There were high health risks not only in local areas, but also in the south of Hebei, the north of Henan, and southern and central Shanxi. The health risks increased by 1.21~2.26 times from clean levels to heavy pollution levels. The leading element of HQ was Mn, while the dominant elements of CR varied from As to Co. Increases in PM_2.5 concentrations and HQ from before the heating period to during the heating period were observed, and there was even an inverse CR change between before the heating period and during the heating period, further identifying that air pollution control was efficient.

Haze occurred in Zhengzhou, a megacity in the northern China, with the PM_2.5 as high as 254 μg m⁻³ on 25 December 2019, despite the emergency response measure of restriction on the emission of anthropogenic pollutants which was implemented on December 19 for suppressing local air pollution. Air pollutant concentrations, chemical compositions, and the origins of particulate matter with aerodynamic diameter smaller than 2.5 µm (PM_2.5) between 5–26 December were investigated to explore the reasons for the haze occurrence. Results show that the haze was caused by efficient SO₂-to-suflate and NO_x-to-nitrate conversions under high relative humidity (RH) condition. In comparison with the period before the restriction (5–18 December) when the PM_2.5 was low, the concentration of PM_2.5 during the haze (19–26 December) was 173 µg m⁻³ on average with 51% contributed by sulfate (31 µg m⁻³) and nitrate (57 µg m⁻³). The conversions of SO₂-to-sulfate and NO_x-to-nitrate efficiently produced sulfate and nitrate although the concentration of the two precursor gases SO₂ and NO_x was low. The high RH, which was more than 70% and the consequence of artificial water-vapor spreading in the urban air for reducing air pollutants, was the key factor causing the conversion rates to be enlarged in the constriction period. In addition, the last 48 h movement of the air parcels on 19–26 December was stagnant, and the air mass was from surrounding areas within 200 km, indicating weather conditions favoring the accumulation of locally-originated pollutants. Although emergency response measures were implemented, high gas-to-particle conversions in stagnant and moisture circumstances can still cause severe haze in urban air. Since the artificial water-vapor spreading in the urban air was one of the reasons for the high RH, it is likely that the spreading had unexpected side effects in some certain circumstances and needs to be taken into consideration in future studies.

Fireworks burning (FB) constitutes a major but short-lived source of PM_2.5 during the Chinese Spring Festival, significantly deteriorating air quality in certain regions. This study was conducted to evaluate its impact through real-time monitoring of PM_2.5 chemical compositions in a forestry city (Xinyang) during the pre-fireworks and fireworks periods at the Spring Festival of 2023–2025. During the fireworks period, PM_2.5 concentrations increased by 10.5–226.4% compared to pre-fireworks levels, of which the concentrations of secondary inorganic aerosols (SIA), K and Cl⁻ rose by 1.6–4.8, 1.9–14.7 and 1.5–8.1 times, and they accounted for 33.2–47.7%, 6.7–12.5% and 3.8–6.4% of PM_2.5, respectively. Correspondingly, PM_2.5/CO and SIA/CO ratios in 2023–2025 elevated by factors of 1.4–2.3 and 1.1–3.4, indicating distinct enhancements in secondary inorganic aerosols formation. Additionally, acidity of PM_2.5, RH and O_x also increased during fireworks. Collectively, higher sulfur and nitrogen oxidation ratios (SOR and NOR) during the fireworks period under the combined effects of high RH, O_x and acidity conditions indicated a greater conversion of secondary inorganic aerosols. Positive Matrix Factorization (PMF) analysis confirmed that FB and secondary aerosols (SA) source levels during fireworks increased by 2.5–19.3 and 1.9–4.4 times compared to pre-fireworks values. This study underscores the need for implementing stringent management of fireworks and secondary formation mitigation to reduce PM_2.5 concentrations during the Spring Festival.

To highlight the characteristics of PM_2.5–O₃ pollution in the Central Plains Urban Agglomeration, spatial and temporal characteristics, key meteorological factors, and source pollution data for the area were analyzed. These data from the period 2014–2020 were obtained from state-controlled environmental monitoring stations in seven major cities of the agglomeration. The results revealed the following: (1) Spatially, the PM_2.5–O₃ pollution days were aggregated in the central area of Xinxiang and decreased toward the north and south. Temporally, during the 2014–2020 period, 50 days of PM_2.5–O₃ pollution were observed in the major cities of the Central Plains Urban Agglomeration, with an overall decreasing trend. (2) A low-temperature, high-pressure environment appeared unfavorable for the occurrence of PM_2.5–O₃ pollution days. Wind speeds of 2.14–2.19 m/s and a southerly direction increased the incidence of PM_2.5–O₃ pollution days. (3) The external transport range in summer was smaller and mainly originated from within Henan Province. These results can provide important reference information for achieving a synergistic control of PM_2.5–O₃ pollution, determining the meteorological causes, as well as the potential sources, of PM_2.5–O₃ pollution in polluted areas and promoting air pollution control.