Significance: We illustrate the similarity and difference in particulate matter (PM) formation between Beijing and other world regions. The periodic cycle of PM events in Beijing is regulated by meteorological conditions. While the particle chemical compositions in Beijing are similar to those commonly measured worldwide, efficient nucleation and growth over an extended period in Beijing are distinctive from the aerosol formation typically observed in other global areas. Gaseous emissions of volatile organic compounds and nitrogen oxides from urban transportation and sulfur dioxide from regional industry are responsible for large secondary PM formation, while primary emissions and regional transport of PM are insignificant. Reductions in emissions of the aerosol precursor gases from transportation and industry are essential to mediate severe haze pollution in China.

Significance: Exceedingly high levels of fine particulate matter (PM) occur frequently in China, but the mechanism of severe haze formation remains unclear. From atmospheric measurements in two Chinese megacities and laboratory experiments, we show that the oxidation of SO ₂ by NO ₂ occurs efficiently in aqueous media under two polluted conditions: first, during the formation of the 1952 London Fog via in-cloud oxidation; and second, on fine PM with NH ₃ neutralization during severe haze in China. We suggest that effective haze mitigation is achievable by intervening in the sulfate formation process with NH ₃ and NO ₂ emission control measures. Hence, our results explain the outstanding sulfur problem during the historic London Fog formation and elucidate the chemical mechanism of severe haze in China.

Significance: Although black carbon (BC) represents a key short-lived climate forcer, its direct radiative forcing remains highly uncertain. The available results from available studies of absorption enhancement of BC particles during atmospheric aging are conflicting. Using a novel environmental chamber method, we have, for the first time to our knowledge, quantified the aging and variation in the optical properties of BC particles under ambient urban conditions representative of developed and developing countries. Our results indicate that BC under polluted urban environments could contribute significantly to both pollution development and large positive radiative forcing, implying that reduction of BC emissions under polluted environments achieves a cobenefit in simultaneously controlling air pollution and protecting climate, especially for developing countries.

Significance: High concentrations of ultrafine particles (UFPs), approaching 1 million/cm ³ , are frequently produced from new particle formation under urban environments, but the fundamental mechanisms regulating nucleation and growth for UFPs are poorly understood. From simultaneous ambient and environmental chamber measurements, we demonstrate remarkable formation of UFPs from urban traffic emissions. By replicating ambient conditions using an environmental chamber method, we elucidate the roles of existing particles, photochemistry, and synergy of multipollutant photooxidation in nucleation and growth of UFPs. Our results reveal that synergetic oxidation of vehicular exhaust leads to efficient formation of UFPs under urban conditions. Recognition of this large urban source for UFPs is essential to accurately assessing their impacts and to effectively developing mitigation policies.