北京冬季疫情期间空气质量及气象影响分析

针对北京地区2020年冬季疫情防控期（1月24至2月29）的空气质量及两次持续性重污染过程进行分析，探究了该时段的大气污染特征及其气象影响.与过去5a同期相比，2020年疫情防控期间北京冷空气强度偏弱，活动频次偏少50%，气温偏高0.73℃，风速和混合层高度偏低17.8%和32.5%，相对湿度和露点温度增加60.9%和48.1%，偏北风频率减少7.5%，而偏南风和偏东风频率均增大6.0%；气象条件较历史同期明显转差；虽然降水量偏多，但整体降水强度弱、时次集中，因而颗粒物的整体清除作用有限.两次重污染过程（1月24~29日和2月8~13日）分别维持59和75h，两个过程累积阶段（1月24~25日和2月9~11日）均受区域输送影响较大，输送占比为70%和58%，分偏东和偏南两个通道.针对污染过程的源解析显示，本地污染贡献占比为67%和48%，可见在维持和加重阶段颗粒物的吸湿增长和二次生成占比增加.经分析，“高湿静稳”的不利气象背景下，大气垂直动力和水平辐合的叠加使PM_2.5和水汽在北京平原累积，将其压制在边界层内快速增长；升高的污染物也与静稳的边界层气象因子双向反馈，导致污染进一步加重.根据EMI指数计算，2020年冬季疫情防控期的气象条件约引起70.1%的PM_2.5浓度增加；而与过去5a同期相比，疫情防控期间排放的减少抵消了约53%的不利气象条件影响；两次污染过程与过去5a同期的9次过程相比，EMI分别偏大26.9%和19.7%，但PM_2.5浓度基本持平或略有降低.可见，在目前的排放基数上，即使出现特殊情况下的城市封锁，排放量的减少将削减污染浓度峰值，但仍不足以完全抵消不利气象条件的影响.

Effect analysis of meteorological conditions on air quality during the winter COVID-19 lockdown in Beijing

The influence of meteorological conditions on the pollution processes was investigated in this study by analyzing the changes of air quality as well as the characteristics of two persistent heavy pollution episodes during the Coronavirus Disease 2019 (COVID-19) prevention (January 24 to February 29) of 2020 winter compared with the same period of 2015~2019. Cold air intensity in 2020 winter was weaker with the cold surges frequency decreased by 50%. Air temperature was 0.73℃ higher, and wind speed and mixed layer height were 17.8% and 32.5% lower, respectively. Relative humidity and dew point temperature increased by 60.9% and 48.1%, respectively. Northerly wind frequency reduced 7.5% while both of southerly and easterly wind increased 6.0%. As shown above, all meteorological conditions in 2020 winter were significantly more favorable for air pollution than the same historical period. Moreover, two heavy pollution episodes (January 24~29 and February 8~14) lasted for 59 and 75 hours were analyzed. At the cumulative stage, regional transport that can be divided into east and south channel greatly affected PM_2.5, with the contribution of 70% and 58% for two episodes. By contrast, the contribution of local pollution was 67% and 48%, respectively, indicating the increased proportion of hygroscopic growth and secondary generation in the maintenance and aggravation stages. Under the meteorological background of "high humidity and high atmospheric stability", the combined effects of atmospheric vertical dynamics and horizontal convergence accumulated PM_2.5 and water vapor in Beijing plain and prevented them from spreading beyond the boundary layer. Further bidirectional feedback between increased pollutants and meteorological factors in stable boundary layer resulting in aggravation of pollution. According to EMI index, meteorological conditions during the epidemic prevention in 2020 winter caused an increase of 70.1% in PM_2.5 concentration compared to pre-COVID-19. Emissions reduction caused by emergency measures for COVID-19lockdown offset 53% of the adverse impact induced by meteorological conditions. As for the two episodes in 2020 winter, EMI was 26.9% and 19.7% larger than the average of other nine episodes in the corresponding period of 2015~2019, and PM_2.5 concentration was basically unchanged or slightly reduced. Overall, if the current social emission level is not changed, emission reduction caused by city blockade under special circumstances can only partially reduce the pollution concentration, however, cannot completely offset the adverse impact of meteorological conditions.