The recent severe and frequent PM2.5 (i.e., fine particles smaller than 2.5 µm) pollution in China has aroused unprecedented public concern. The first two years of PM2.5 measurements in China are reported and compared with those of Europe and the United States. The average PM2.5 concentration in China is approximately 5 times that over Europe and America. The contribution of atmospheric dispersion to such air quality is evaluated in this study. Air stagnation or its absence is a good indicator of the atmosphere’s capability to disperse its pollutants, but the NOAA definition of an air stagnation event is found to not be applicable to China since it depends on vertical mixing that is weakened in China by the effects of terrain. To address this deficiency, a new threshold for air stagnation events is proposed that depends on the 10-m wind speed, boundary layer height, and occurrence of precipitation. This newly defined air stagnation closely tracks the day-to-day variation of PM2.5 concentrations. Such events are more frequent over China than over Europe and the United States during autumn and winter, especially over the Sichuan basin and Jing-Jin-Ji region of China. If China had the same frequency of air stagnation as the United States or Europe, 67% and 82% of its stations would improve their current air quality during autumn and winter (e.g., an average of 12% decrease in PM2.5 concentrations for the Jing-Jin-Ji region in wintertime). Its severe pollution and frequent air stagnation conditions make controls on emission less effective in China than elsewhere.

Observations show that the surface incident solar radiation (R s ) decreased over land from the 1950s to the 1980s and increased thereafter, known as global dimming and brightening. This claim has been questioned due to the inhomogeneity and low spatial-temporal coverage of R s observations. Based on direct comparisons of~200 observed and sunshine duration (SunDu) derived R s station pairs, meeting data record lengths exceeding 60 months and spatial distances less than 110 km, we show that meteorological observations of SunDu can be used as a proxy for measured R s . Our revised results from~2,600 stations show global dimming from the 1950s to the 1980s over China (À1.90 W/m 2 per decade), Europe (À1.36 W/m 2 per decade), and the United States (À1.10 W/m 2 per decade), brightening from 1980 to 2009 in Europe (1.66 W/m 2 per decade) and a decline from 1994 to 2010 in China (À1.06 W/m 2 per decade). Even if 1994-2010 is well known as a period of global brightening, the observed and SunDu-derived R s over China still exhibit declining trends. Trends in R s from 1923 to 1950 are also found over Europe (1.91 W/m 2 per decade) and the United States (À1.31 W/m 2 per decade), but the results in Europe may not well represent the actual trend for the European continent due to poor spatial sampling.Plain Language Summary Ground-based observations of the surface incident solar radiation (R s ) reveal the phenomena known as global dimming and brightening, that is, a downtrend over land from the 1950s to the 1980s and an uptrend thereafter. However, R s observations suffer from inhomogeneity issues and low spatial-temporal coverage. Sunshine duration-derived R s is not present above problems and was utilized here to compare with observed R s from China, Europe, and the United States over the 1950-2010 common period. Results show a good agreement between two data sets except for the dimming period in China, mainly due to instrument sensitivity drift of R s observations. Therefore, using more extensive sunshine duration-derived R s data set at approximately 2,600 stations over China, Europe, and the United States since 1901, a revisit of global dimming and brightening has been reasonably conducted, including the early period prior to the 1950s.

This study compared the diurnal and seasonal cycles of atmospheric and surface urban heat islands (UHIs) based on hourly air temperatures (Ta) collected at 65 out of 262 stations in Beijing and land surface temperature (Ts) derived from Moderate Resolution Imaging Spectroradiometer in the years 2013–2014. We found that the nighttime atmospheric and surface UHIs referenced to rural cropland stations exhibited significant seasonal cycles, with the highest in winter. However, the seasonal variations in the nighttime UHIs referenced to mountainous forest stations were negligible, because mountainous forests have a higher nighttime Ts in winter and a lower nighttime Ta in summer than rural croplands. Daytime surface UHIs showed strong seasonal cycles, with the highest in summer. The daytime atmospheric UHIs exhibited a similar but less seasonal cycle under clear‐sky conditions, which was not apparent under cloudy‐sky conditions. Atmospheric UHIs in urban parks were higher in daytime. Nighttime atmospheric UHIs are influenced by energy stored in urban materials during daytime and released during nighttime. The stronger anthropogenic heat release in winter causes atmospheric UHIs to increase with time during winter nights, but decrease with time during summer nights. The percentage of impervious surfaces is responsible for 49%–54% of the nighttime atmospheric UHI variability and 31%–38% of the daytime surface UHI variability. However, the nighttime surface UHI was nearly uncorrelated with the percentage of impervious surfaces around the urban stations.

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Land surface temperature Ts provides essential supplementary information to surface air temperature, the most widely used metric in global warming studies. A lack of reliable observational Ts data makes assessing model simulations difficult. Here, the authors first examined the simulated Ts of eight current reanalyses based on homogenized Ts data collected at ~2200 weather stations from 1979 to 2003 in China. The results show that the reanalyses are skillful in simulating the interannual variance of Ts in China (r = 0.95) except over the Tibetan Plateau. ERA-Interim and MERRA land versions perform better in this respect than ERA-Interim and MERRA. Observations show that the interannual variance of Ts over the north China plain and south China is mostly influenced by surface incident solar radiation Rs, followed by precipitation frequency, whereas the opposite is true over the northwest China, northeast China, and the Tibetan Plateau. This variable relationship is well captured by ERA-Interim, ERA-Interim land, MERRA, and JRA-55. The homogenized Ts data show a warming of 0.34°C decade−1 from 1979 to 2003 in China, varying between 0.25° and 0.42°C decade−1 for the eight reanalyses. However, the reanalyses substantially underestimate the warming trend of Ts over northwest China, northeast China, and the Tibetan Plateau and significantly overestimate the warming trend of Ts over the north China plain and south China owing to their biases in simulating the Rs and precipitation frequency trends. This study provides a diagnostic method for examining the capability of current atmospheric/land reanalysis data in regional climate change studies.