Mixing layer transport flux of particulate matter in Beijing, China
Quantifying the transport flux (TF) of atmospheric pollutants plays an important role in understanding the causes of air pollution and in making decisions regarding the prevention and control of regional air pollution. In this study, the mixing layer height (MLH) and wind profile were measured by a ceilometer and Doppler wind radar, respectively, and the characteristics of the atmospheric dilution capability were analyzed using these two datasets. The ventilation coefficient (VC) appears to be the highest in the spring (3940±2110 m2 s−1) and lower in the summer (2953±1322 m2 s−1), autumn (2580±1601 m2 s−1) and winter (2913±3323 m2 s−1). Combined with the backscatter measured by the ceilometer, vertical profiles of the PM2.5 concentration were obtained and the PM2.5 TF in the mixing layer was calculated. The TF was the highest in the spring at 4.33±0.69 mg m−1 s−1 and lower in the summer, autumn and winter, when the TF values were 2.27±0.42, 2.39±0.45 and 2.89±0.49 mg m−1 s−1, respectively. Air pollutants transport mainly occurs between 14:00 and 18:00 LT. The TF was large in the pollution transition period (spring: 5.50±4.83 mg m−1 s−1; summer: 3.94±2.36 mg m−1 s−1; autumn: 3.72±2.86 mg m−1 s−1; winter: 4.45±4.40 mg m−1 s−1) and decreased during the heavy pollution period (spring: 4.69±4.84 mg m−1 s−1; summer: 3.39±1.77 mg m−1 s−1; autumn: 3.01±2.40 mg m−1 s−1; winter: 3.25±2.77 mg m−1 s−1). Our results indicate that the influence of the air pollutants transport in the southern regions should receive more focus in the transition period of pollution, while local emissions should receive more focus in the heavy pollution period.