Biomass burning activities are ubiquitous in China, especially in northern China, where there is a large rural population and winter heating custom. Biomass burning tracers (i.e., levoglucosan, mannosan and potassium (K⁺)), as well as other chemical components, were quantified at a rural site (Gucheng, GC) in northern China from 15 October to 30 November, during a transition heating season, when the field burning of agricultural residue was becoming intense. The measured daily average concentrations of levoglucosan, mannosan and K⁺ in PM_2.5 (particulate matter with aerodynamic diameters less than 2.5 µm) during this study were 0.79 ± 0.75, 0.03 ± 0.03 and 1.52 ± 0.62 µg m⁻³, respectively. Carbonaceous components and biomass burning tracers showed higher levels during nighttime than daytime, while secondary inorganic ions were enhanced during daytime. An episode with high levels of biomass burning tracers was encountered at the end of October 2016, with high levoglucosan at 4.37 µg m⁻³. Based on the comparison of chemical components during different biomass burning pollution periods, it appeared that biomass combustion can obviously elevate carbonaceous component levels, whereas there was essentially no effect on secondary inorganic aerosols in the ambient air. Moreover, the levoglucosan $/$ mannosan ratios during different biomass burning pollution periods remained at high values (in the range of 18.3–24.9); however, the levoglucosan $/$ K⁺ ratio was significantly elevated during the intensive biomass burning pollution period (1.67) when air temperatures were decreasing, which was substantially higher than in other biomass burning periods (averaged at 0.47).

The oxidation of sulfur dioxide (SO₂) to sulfate in the atmosphere is an important concern in regional air quality, global climate change, and human health. While gas-phase and liquid-phase oxidation of SO₂ are widely regarded as important sources of sulfate, the contribution of the heterogeneous oxidation process on particle surfaces is controversial. Recently, this heterogeneous chemistry has been considered to be an important mechanism that is missing in current models to explain sulfate concentrations observed in haze episodes in East Asia. Therefore, the heterogeneous oxidation of SO₂ on particles under the conditions of complex air pollution needs to be reassessed. This review summarizes the fundamental understanding of the heterogeneous reactions of SO₂ on solid particles such as mineral dust, black carbon, sea salts, organic aerosol, and so on. The factors affecting the mechanism and kinetics of the heterogeneous reactions of SO₂, including coexisting components (O₃, NO₂, H₂O₂, NH₃, and VOCs), reactive sites, surface properties, relative humidity, and illumination, are reviewed. Reactive oxygen species involved in the heterogeneous oxidation of SO₂ on particles are discussed. To our knowledge, while previous reviews have appeared on the oxidation of SO₂ in the aqueous-phase, this is the first review on the atmospheric heterogeneous reactions of SO₂ on the surface of solid particles, which can be of help in understanding the sulfur cycle in the atmosphere and its environmental impacts. A number of recommendations for future research are also presented. Graphical abstract

The structure and activity of surface hydroxyl groups determined the performance of TiO₂ during NO₂ reaction.

Complex aerosol and photochemical pollution (ozone and peroxyacetyl nitrate, PAN) frequently occur in eastern China, and mitigation strategies to effectively alleviate both kinds of pollution are urgently needed. Although the effectiveness of powerful control measures implemented by the Chinese State Council has been comprehensively evaluated in terms of reducing atmospheric primary pollutants, the effectiveness in mitigating photochemical pollution is less assessed and therefore the underlying mechanisms are still poorly understood. The stringent emission controls implemented from 24 August to 6 September 2016 during the summit for the Group of Twenty (G20) provide us a unique opportunity to address this issue. Surface concentrations of atmospheric O₃, PAN, and their precursors including volatile organic compounds (VOCs) and nitrogen dioxides (NO_x), in addition to the other trace gases and particulate matter, were measured at the National Reference Climatological Station (NRCS) (30.22^∘ N, 120.17^∘ E, 41.7 m a.s.l) in urban Hangzhou. We found significant decreases in atmospheric PAN, NO_x, total VOCs, PM_2.5, and sulfur dioxide (SO₂) under the unfavorable meteorological conditions during G20 (DG20) relative to the adjacent period before and after G20 (BG20 and AG20), indicating that the powerful control measures were effective in reducing the pollutant emissions in Hangzhou. Unlike with the other pollutants, daily maximum 8 h average (DMA8) O₃ exhibited a slight increase and then decrease from BG20 to AG20, which was mainly attributed to the variation in the solar irradiation intensity and regional transport in addition to the contribution from the implementation of stringent control measures. Results from an observation-based chemical model (OBM) indicated that acetaldehyde and methylglyoxal (MGLY) were the most important second-generation precursors of PAN, accounting for 37.3 %–51.6 % and 22.8 %–29.5 % of the total production rates including the reactions of OVOCs, propagation of other radicals, and other minor sources. Moreover, we confirmed the production of PAN and O₃ was sensitive to VOCs throughout the whole period, specifically dominated by aromatics in BG20 and DG20 but by alkenes in AG20. These findings suggested that reducing emissions of aromatics, alkenes, and alkanes would mitigate photochemical pollution including PAN and O₃. Source appointment results attributed the reductions of VOC source and ozone formation potentials (OFPs) during G20 to the effective emission controls on traffic (vehicle exhaust) and industrial processes (solvent utilization and industrial manufacturing). However, fuel combustion and biogenic emissions both weakened such an effect with a sizable contribution to the VOC mixing ratios (18.8 % and 20.9 %) and OFPs (25.6 % and 17.8 %), especially during the latter part of G20 (G20 II) when anthropogenic VOCs were substantially reduced. This study highlights the effectiveness of stringent emission controls in relation to traffic and industrial sources, but a coordinated program related to controlling fuel combustion and biogenic emissions is also required to address secondary pollution.