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Black carbon (BC) and PM 2.5 were studied for nine years from 2005 to 2013 in the Beijing urban area. The overall weekly average mass concentrations of BC and PM 2.5 were 4.3 and 66.8 μg/m³. PM 2.5 annual means of the nine years are around 2 times of the standard (GB3095-2012) in China, and are 5–7 times higher than the WHO standard. The Beijing Olympic Games in 2008 was a milestone to mitigate aerosol pollution. Temporal distribution of BC shows a distinct declining trend, and annual mean mass concentrations of PM 2.5 after 2008 were lower than those before 2008 but increased from 2011 to 2013. Wind rose plots show that high BC concentrations are usually associated with low wind speed of northeastern or southwestern winds, generally causing poor visibility. Governmental mitigation measures such as traffic restriction despite increased motor vehicle numbers and gasoline consumption and industry relocation with declining consumption of coal and coke were successful in reducing BC emissions. Annual mean of BC was reduced by 38% in 2013 compared to 2005. However, BC contamination in Beijing is still severe when compared to other urban areas around the world. [ABSTRACT FROM AUTHOR]

Cirrus clouds are ice clouds. Our understanding of the basic microscale physics of ice clouds is not yet adequate. In this study, interior physical properties of cirrus clouds over Beijing are investigated based on four years of continuous measurements of a Ka-band polar Doppler radar, which performs long-term observations with higher temporal resolution than the aircraft and the satellite. The probability distribution functions of radar reflectivity at various temperatures are presented and a positive correlation relationship between reflectivity and temperature is also found. In addition, the distributions of the particle Doppler velocity and particle movements in cirrus clouds are also revealed. According to the cloud-base temperature (CBT), all cirrus clouds are divided into three types [ C ih : CBT < 273 K and CBH (cloud-base height) ≥ 258 K; C im : CBT < 258 K and CBT > 235 K; C il : CBT ≤ 235 K] in order to understand their formation mechanisms and possible differences. The results of investigation show that C il has the lowest mean depth, the lowest mean and maximum reflectivity, and the lowest inhomogeneity among the three types of cirrus clouds, while C ih has the highest and C im lies between the two. The reflectivity–temperature relationship is also different in the three types. In particular, for C il , reflectivity varies little with the variation in temperature. The significant difference in microphysical properties and interior structures between C il and C ih proves their different formation mechanisms. C il , illustrating higher vertical uniformity, is likely generated by radiation cooling, while the other cirrus cloud types are generated differently. The results presented and analyzed in this study provide new knowledge regarding cirrus clouds in Beijing, a mid-latitude region, and a reference for related parameterization in global climate models. • Probability distribution function of reflectivity with temperature is presented. • Cirrus clouds at various temperature levels show different vertical structures. • Downward movements dominate in cirrus cloud with base temperatures above 235 K. • Cirrus clouds below 235 K show vertically homogeneous features. • Occurrence time further testifies different formation origins between cirrus clouds. [ABSTRACT FROM AUTHOR]