Aerosol‐planetary boundary layer (PBL) interactions have been found to enhance air pollution in megacities in China. We show that black carbon (BC) aerosols play the key role in modifying the PBL meteorology and hence enhancing the haze pollution. With model simulations and data analysis from various field observations in December 2013, we demonstrate that BC induces heating in the PBL, particularly in the upper PBL, and the resulting decreased surface heat flux substantially depresses the development of PBL and consequently enhances the occurrences of extreme haze pollution episodes. We define this process as the “dome effect” of BC and suggest an urgent need for reducing BC emissions as an efficient way to mitigate the extreme haze pollution in megacities of China.

Atmospheric aerosols have been found to influence the development of planetary boundary layer (PBL) and hence to enhance haze pollution in megacities. Previous works on aerosol‐PBL interaction was mainly based on model simulation for short‐term cases, so far there is a lack of long‐term observational evidences. In this study, based on multi‐year measurements and reanalysis meteorological data, we give observational evidences on aerosol‐PBL interaction and its impact on pollution aggravation. We found a significant heating in upper‐PBL with maximum temperature change about 0.7 ^oC on average, and a substantial dimming near surface with a mean temperature drop of ‐2.2 ^oC under polluted condition. By integrating Eulerian forward simulation and Lagrangian backward trajectory calculation, we demonstrated that the atmospheric heating was mainly induced by light‐absorbing aerosols like black carbon. Then an index representing such effect was proposed, which could well characterize aerosol‐PBL interaction and its impact on air pollution.

Recent evidence shows that carbon emissions in China are likely to peak ahead of 2030. However, the social and economic impacts of such an early carbon peak have rarely been assessed. Here we focus on the economic costs and health benefits of different carbon mitigation pathways, considering both possible socio-economic futures and varying ambitions of climate policies. We find that an early peak before 2030 in line with the 1.5 °C target could avoid ~118,000 and ~614,000 PM_2.5 attributable deaths under the Shared Socioeconomic Pathway 1, in 2030 and 2050, respectively. Under the 2 °C target, carbon mitigation costs could be more than offset by health co-benefits in 2050, bringing a net benefit of $393–$3,017 billion (in 2017 USD value). This study not only provides insight into potential health benefits of an early peak in China, but also suggests that similar benefits may result from more ambitious climate targets in other countries.

Haze pollution caused by PM_2.5 is the largest air quality concern in China in recent years. Long-term measurements of PM_2.5 and the precursors and chemical speciation are crucially important for evaluating the efficiency of emission control, understanding formation and transport of PM_2.5 associated with the change of meteorology, and accessing the impact of human activities on regional climate change. Here we reported long-term continuous measurements of PM_2.5, chemical components, and their precursors at a regional background station, the Station for Observing Regional Processes of the Earth System (SORPES), in Nanjing, eastern China, since 2011. We found that PM_2.5 at the station has experienced a substantial decrease (−9.1 % yr⁻¹), accompanied by even a very significant reduction of SO₂ (−16.7 % yr⁻¹), since the national “Ten Measures of Air” took action in 2013. Control of open biomass burning and fossil-fuel combustion are the two dominant factors that influence the PM_2.5 reduction in early summer and winter, respectively. In the cold season (November–January), the nitrate fraction was significantly increased, especially when air masses were transported from the north. More NH₃ available from a substantial reduction of SO₂ and increased oxidization capacity are the main factors for the enhanced nitrate formation. The changes of year-to-year meteorology have contributed to 24 % of the PM_2.5 decrease since 2013. This study highlights several important implications on air pollution control policy in China.