The process of frost formation has been studied as an important deposition pathway for removing atmospheric pollutants. However, the removal of organic species during frost formation remains unknown. Gas, PM_2.5 and frost samples were collected in Wangdu, a severely polluted rural site in the North China Plain, during the winter of 2018. The concentrations, distributions, sources, and ecological and health risks of the 16 PAHs in the samples were determined. The 24 h-averaged concentrations of PAHs in the air (gas + PM_2.5) reached 719 ng m^-3, indicating that the local atmosphere was heavily contaminated by PAHs in winter. The average concentrations of PAHs in the frost insoluble reached 27.5 μg g^-1, indicating the strong ambient PAHs deposition during the frost process. The distribution patterns of individual PAH species indicated that the PAHs in the gas influenced the frost samples, and the sources of PAHs in frost samples were highly similar to those in PM_2.5 samples. The calculated lifetime lung cancer risk (LLCR) via inhalation of ambient PAHs was classified as high risks. The average total Benzo(a)pyrene toxic equivalent quotient (TEQ) of PAHs in the frost samples was estimated as 2.50 μg TEQ g^-1, posing a high threat to the ecological environment and health. After the melting of frost, the PAHs are released into the soil. Although the concentrations are diluted, the PAH concentrations could adversely affect the growing vegetation or agricultural activities. The extremely high PAH concentrations and their ecological and health risks require special attention and strict regulation.

Metals are important components of PM2.5, with significant implications for the ecological environment and human health. This study comprehensively investigated the characteristics of 19 metals in PM2.5 samples collected in Jinan from 16 December 2020 to 15 December 2021. The total concentration of the studied metals accounted for 5.3% of PM2.5 (41.7 μg m–3) in Jinan. The seasonal variation of PM2.5-bound metals followed the order of spring (3.5 μg m–3) > winter (2.2 μg m–3) > summer (1.4 μg m–3) > autumn (1.1 μg m–3). Four sources could be extracted using the positive matrix factorization (PMF) model, namely resuspended dust (51.9%), oil combustion (19.9%), factories (16.5%), and traffic emissions (11.7%). According to the results of the enrichment factor (EF) and geo-accumulation (Igeo), Se, Sb, Tl, Ag, Cd, Zn, V, Pb, and Mo were mainly from anthropogenic sources, which was consistent with the PMF results. Applying the conditional bivariate probability function (CBPF), traffic emissions were found to be the main local contributor, with large contributions when the wind speed was lower than 2 m s–1. The environmental influence of PM2.5-bound metals was evaluated in terms of the ecological risk index, which was found to be higher than 600, indicating high ecological risk in Jinan. Regarding the human health risk, children were possibly more exposed to non-carcinogenic risks than adults, with hazard index (HI) values of 47.9 and 6.7, respectively. Moreover, the residents of Jinan were also exposed to moderate carcinogenic risk (children: 4.8 × 10–5, adults: 4.9 × 10–5), with Cr(VI), Pb, and Co as the dominant high-risk metals. Therefore, PM2.5-bound metal pollution in Jinan cannot be overlooked, and appropriate measures should be implemented.

To better understand the pollution characteristics and formation mechanisms of atmospheric carbonyl compounds, continuous measurements of carbonyl compounds in Jinan were taken for one month at a sampling frequency of 2h. The sources, pollution characteristics, and concentration changes of carbonyl compounds during the summers of 2018 and 2020 were compared. The total concentrations of carbonyl compounds were 10.51±0.13ppbV and 6.30±1.08ppbV in 2018 and 2020, respectively. In both years, formaldehyde, acetone, and acetaldehyde were the major carbonyls. Diurnal variations and correlation analyses showed that exhaust emissions from motor vehicles during peak traffic periods significantly contributed to the concentrations of carbonyl compounds in Jinan, with formaldehyde exhibiting net production. The ratio of formaldehyde/acetaldehyde (C₁/C₂) was 2.64 in 2018 and 2.03 in 2020, indicating that carbonyl compounds are jointly affected by anthropogenic sources and photochemical reactions. Master Chemical Mechanism model analyses showed that the formation of formaldehyde in Jinan was controlled by RO+O₂ reactions, and formaldehyde was mainly consumed via photolysis and its reaction with the hydroxyl radical. In situ photochemistry can further promote formaldehyde production. The comparison of the reactivities of different carbonyl compounds revealed that formaldehyde, acetaldehyde, butyraldehyde, and propionaldehyde play an important role in hydroxyl radical reactions and ozone generation. Among all the measured carbonyl compounds, benzaldehyde contributed the most to secondary organic aerosols (SOAs). Overall, this study provides new insights into the formation mechanisms of carbonyl compounds as well as their pollution characteristics.

A one-year observation campaign from December 2020 to December 2021 was carried out in Jinan, a city that previously suffered from severe fine particulate matter (PM_2.5) pollution. The meteorological parameters and ambient concentrations of the air pollutants were recorded, and PM_2.5 samples were collected during the campaign. The inorganic chemical compositions of the PM_2.5 samples were measured, identifying 8 water-soluble ions (WSIs) and 15 trace metals. Pollution events, including 7 dust events (DEs) and 19 secondary inorganic events (SIEs), were identified and classified. The inorganic chemical compositions of the pollution events were characterized, summarized, and concluded. The low ratio of WSIs to PM_2.5, high concentrations of Ca²⁺ and Mg²⁺, and significant increases in trace metals were the dominant characteristics of PM_2.5 during DEs. The high concentrations of SO₄²⁻, NO₃⁻, and NH₄⁺; the high ratio of secondary ions to WSIs; and the high ratio of WSIs to PM_2.5 were the dominant characteristics of PM_2.5 during SIEs. Even though, recently, the PM_2.5 pollution in Jinan has not been as severe as pollution from a dozen years ago, it still exists, just at lower frequencies and pollution levels. This investigation provides the characteristics of PM_2.5 pollution under dramatically improved conditions and various ideas for the management and control of PM_2.5 pollution.

Secondary organic aerosol (SOA) contributes greatly to atmospheric environmental pollution, and SOA tracers provide direct insights into the chemical composition, source identification, and formation pathways of SOA. Observations of SOA tracers were performed in Tai’an, a relatively clean city located in the North China Plain. Seventeen SOA tracers in PM2.5, which originated from isoprene (SOAI), monoterpene (SOAM), β-caryophyllene (SOAC), and aromatic hydrocarbon (SOAA), were measured and investigated. Characteristics of SOA tracers in different seasons and during different pollution days were revealed. The concentrations of these four kinds of tracers in summer were 28.5±7.49 ng m-3, 41.4±11.2 ng m-3, 1.94±0.72 ng m-3, and 1.26±0.51 ng m-3, respectively, indicating strong oxidation processes under high biogenic emissions. In winter, SOAI and SOAM decreased significantly by more than 65%, but the SOAA concentration increased by a factor of 2.65. These opposing trends collectively resulted in the same secondary organic carbon concentration (0.60 μg m-3) in summer and winter. Compared with non-pollution days, SOA was found to be fresher during O3 pollution days in summer and haze days in winter, but the underlying mechanisms were different. Through the combined analysis of malic acid, levoglucosan, and SO42‒, the importance of biomass burning and secondary inorganic formation to SOA was revealed. This study provides insights into SOA characteristics in a mountain city under relatively clean conditions.