受不利气象因素影响，北京正经历一次污染过程

Core Viewpoint - The air pollution process in the Beijing-Tianjin-Hebei region began on December 15, with PM2.5 concentrations reaching significant levels, indicating a deterioration in air quality due to unfavorable diffusion conditions and regional transmission of pollutants [1][2]. Overall Situation - On December 15, PM2.5 concentrations in cities near the Taihang and Yanshan mountains rose to light pollution levels, with Langfang experiencing moderate pollution. On December 16, under the influence of northern winds, pollutant concentrations decreased in the northern region while accumulating in the south, leading to widespread light pollution and moderate pollution at the Hebei-Shandong-Henan border. By December 17, pollution spread to the Yangtze River basin, reaching moderate to heavy pollution levels, and by December 18, severe pollution was reported in most of Henan and southern Hebei [1]. Pollution Cause Analysis - The pollution process was primarily driven by unfavorable diffusion conditions and local accumulation, compounded by regional transmission, which collectively raised PM2.5 concentrations in Beijing. Recent weak surface pressure systems and persistent warming in the middle atmosphere contributed to the formation of regional pollution [2]. Regional Transmission Impact - During the pollution event, regional transmission accounted for 72% of the PM2.5 concentration increase in Beijing, with coal combustion sources contributing significantly, as evidenced by a 6.6-fold increase in coal source tracers. The local contribution was only 28%, indicating that the majority of pollution was transported from surrounding areas, particularly from Hebei province [3]. PM2.5 Composition - The composition of PM2.5 during the pollution event showed a significant presence of nitrates, with nitrate, ammonium, and sulfate accounting for 39%, 16%, and 11% of the total PM2.5, respectively. Secondary components made up over 60% of the total, highlighting the need for enhanced control of emissions from mobile sources in Beijing [3]. Protective Recommendations - Forecast models indicate that the pollution process will only be alleviated after the arrival of cold air on the afternoon of December 19, improving diffusion conditions. Citizens are advised to minimize unnecessary travel and wear masks for protection until conditions improve [3].