Complex pollution with high aerosol and ozone concentrations has recently been occurring in several densely populated cities in China, raising concerns about the influence of meteorological factors, including synoptic circulation and local conditions. In this study, comprehensive analyses on the associations between PM_2.5, PM₁₀, and O₃ and meteorological conditions were conducted based on observations from radar wind profiler, microwave radiometer, automatic weather station, and air quality monitoring sites in Beijing during the spring of 2019. The results showed that the boundary layer height and temperature inversion were negatively (positively) correlated with PM (O₃) concentrations, modulating the degree of air pollution. Five identified synoptic patterns were derived using geopotential height data of the ERA5 reanalysis, among which Type 1, characterised by south-westerly prevailing winds with high pressure to the south, was considered to be associated with severe PM and O₃ contamination. This indicates that air pollutants originating from southern regions exert a major influence on Beijing through the transportation effect. In addition, high temperature, relative humidity, and low wind velocity exacerbate pollution. Overall, this study provides significant information for understanding the vital roles played by meteorological elements at both the regional and local scales in regulating air contamination during spring in Beijing.

Climate policy plays an important role in keeping global temperature rises below the target of 1.5–2 °C above pre-industrial levels, and technological innovations are key to determining the effectiveness of climate policy. In this study, we investigated the climate impact of the USA’s policy choices using the enforced multi-factor regional climate and economy system (EMRICES,) and the Earth system model from Beijing Normal University (BNU-ESM). Three emission scenarios were designed based on the assumption of whether or not the US follows its proposed nationally determined contribution (NDC) and makes use of technological innovations. The results showed that if the US does not implement the NDC and had no technological progress, there would be an extra 176.7 Gt of cumulative carbon emissions by the end of the 21st century compared to that of all the countries that follow their NDC. The additional emissions would lead to an increase of 62 ppm in CO₂ concentration and a 0.4 °C increase in global warming by 2100. It would also lead to a 2% loss for the US and Chinese economies, compared to the NDC scenario. The Earth system model results also show that even if all the countries follow the DNC, it would be difficult to keep the temperature from increasing less than 1.5 °C. This study implies that the US withdrawal from the Paris Agreement and their refusal to adopt technological progress is not conducive to achieving the 1.5 °C goal, and more stringent emission reduction targets or technology innovations would be required for the world to control global warming to a level below 1.5 °C.

With the abrupt and significant drop of PM _2.5 concentrations during the lockdown in 2020, hourly direct radiation ( R _dir ) at surface substantially increased in East China, such as Zhengzhou, Wuhan and Baoshan, with the maximum enhancement of 86% at Wuhan. Most of these stations had decreased diffuse radiation ( R _dif ) except Zhengzhou. Zhengzhou had both enhanced R _dir and R _dif , as well as reduced but still high PM _2.5 concentrations, indicating atmospheric particles were more scattering in this region. At Beijing and Harbin in North and Northeast China, intensification of aerosol pollution led to hourly R _dir ( R _dif ) falling (rising) up to −28% (59%) and −23% (40%), respectively. By contrast, surface solar radiation (SSR) in West China was also greatly influenced by the elevated dust/smoke layers, revealed by aerosol layer vertical distribution and the reduction of SSR and PM _2.5 concentrations. This study highlighted the importance of aerosol optical properties and vertical structures in aerosol–radiation interactions.

Anthropogenically induced climate warming will not stop as long as carbon dioxide (CO₂) emissions continue to increase. As positive feedback from the cryosphere can amplify climate warming in higher latitudes, adaptation to and mitigation of climate change in the cryosphere have become particularly important. In this study, we quantified the contributions of climate policies of the developed and developing worlds to moderating decreases in Northern Hemisphere sea ice and snow, using the Earth System Model of Beijing Normal University (BNU‐ESM). The results indicate that the sea ice extent and snow cover would continue to shrink as CO₂ concentrations continue to increase, and the combined efforts of the developed and developing worlds could successfully reduce losses of Arctic sea ice and snow. In the medium term (before 2060), contributions of the developing world to reducing the loss of Arctic sea ice extent are expected to be 65% in JFM and 60% in JAS, while the corresponding contributions to reducing snow cover loss are 46 and 70%. These contributions from the developed world are smaller: 44% in JFM and 22% in JAS for sea ice extent and 34% in JFM and 20% in JAS for snow cover. Over the long term (until 2100), the developing world is expected to contribute 71% in JFM and 77% in JAS to reducing the Arctic sea ice losses, and its contribution to reducing snow cover loss is 73 and 66%. Similarly, the corresponding contribution for the developed world is also smaller: 36% in JFM and 24% in JAS for sea ice extent and 32% in JFM and 30% in JAS for snow cover. However, the developed and developing world achieves a combined mitigation effect of more than 90% in the near and long term.