The formation mechanism of aerosol sulfate during wintertime haze events in China is still largely unknown. As companions, SO₂ and transition metals are mainly emitted from coal combustion. Here, we argue that the transition metal-catalyzed oxidation of SO₂ on aerosol surfaces could be the dominant sulfate formation pathway and investigate this hypothesis by integrating chamber experiments, numerical simulations and in-field observations. Our analysis shows that the contribution of the manganese-catalyzed oxidation of SO₂ on aerosol surfaces is approximately one to two orders of magnitude larger than previously known routes, and contributes 69.2% ± 5.0% of the particulate sulfur production during haze events. This formation pathway could explain the missing source of sulfate and improve the understanding of atmospheric chemistry and climate change.

Composite materials, containing magnetic nanoparticles and cellulose, were synthesized by one-step co-precipitation using NaOH–thiourea–urea aqueous solution for cellulose dissolution. The NaOH in cellulose solution acted as the precipitant of iron oxide nanoparticles, and low-cost cellulose was used as the template to promote the growing of nanoparticles in the cellulose matrix. The method provided a facile, “green” pathway for the fabrication of magnetic nanomaterials. The synthesized cellulose@iron oxide nanoparticles were characterized by FTIR, XRD, SEM, TEM, XPS, TG and VSM. The FTIR, XRD and XPS results demonstrated the formation of Fe₂O₃ nanoparticles in the composite materials after the co-precipitation. SEM and TEM characterization showed that the Fe₂O₃ nanoparticles were dispersed in the cellulose matrix due to the synergistic effect. Magnetometric measurements revealed that the resultant composites of cellulose@Fe₂O₃ nanoparticles exhibited a sensitive magnetic-induced behavior and could be easily separated from aqueous solution through the external magnetic field. The composite materials were applied to remove arsenic from aqueous solution. The results showed that the magnetic nanoparticle composites displayed excellent adsorption efficiency of arsenic compared with other magnetic materials reported, and the Langmuir adsorption capacities of the composites for the removal of arsenite and arsenate were 23.16 and 32.11 mg g⁻¹, respectively.

The Atmospheric Pollution and Human Health in a Chinese Megacity (APHH-Beijing) programme is an international collaborative project focusing on understanding the sources, processes and health effects of air pollution in the Beijing megacity. APHH-Beijing brings together leading China and UK research groups, state-of-the-art infrastructure and air quality models to work on four research themes: (1) sources and emissions of air pollutants; (2) atmospheric processes affecting urban air pollution; (3) air pollution exposure and health impacts; and (4) interventions and solutions. Themes 1 and 2 are closely integrated and support Theme 3, while Themes 1–3 provide scientific data for Theme 4 to develop cost-effective air pollution mitigation solutions. This paper provides an introduction to (i) the rationale of the APHH-Beijing programme and (ii) the measurement and modelling activities performed as part of it. In addition, this paper introduces the meteorology and air quality conditions during two joint intensive field campaigns – a core integration activity in APHH-Beijing. The coordinated campaigns provided observations of the atmospheric chemistry and physics at two sites: (i) the Institute of Atmospheric Physics in central Beijing and (ii) Pinggu in rural Beijing during 10 November–10 December 2016 (winter) and 21 May–22 June 2017 (summer). The campaigns were complemented by numerical modelling and automatic air quality and low-cost sensor observations in the Beijing megacity. In summary, the paper provides background information on the APHH-Beijing programme and sets the scene for more focused papers addressing specific aspects, processes and effects of air pollution in Beijing.

Based on our comprehensive observations in Beijing, we show that 80–90% of PM _2.5 was formed via atmospheric reactions during haze days and over 65% of the number concentration of haze particles resulted from new particle formation.