长三角地区大气污染物对新冠肺炎封城的时空响应特征

利用2020年1月1日—2月29日上海、南京、合肥和杭州4个城市常规污染物的逐时监测资料，结合卫星反演的NO₂垂直柱浓度信息，探讨了新冠肺炎封城的前、中、后期长三角地区城市大气污染物的污染水平及响应特征.结果显示：除O₃外，其余大气污染物的平均浓度在时间上的整体变化趋势均表现为封城前>封城中（1月24日—2月10日）>封城后，表明空气质量并非完全受封城导致的污染减排控制.封城期间的PM_2.5/PM₁₀比值高于封城前和封城后，表明气溶胶二次生成对封城期间仍出现的颗粒物污染可能有重要贡献.O_x浓度在封城期间也有显著上升（p<0.01），表明大气氧化性可能在NO₂减少的背景下得到强化，从而促进二次气溶胶的生成.从空间看，O₃分布呈以城市为中心的包围式往内聚集分布，表明以局地生成为主.PM_2.5、PM₁₀、CO、SO₂和NO₂分布特征为北高南低，表明冬季自北向南的区域传输对封城期间的空气污染有重要贡献.卫星反演结果进一步证实华北平原是污染的主要源区，这也得到轨迹来源分析的佐证.

Spatial-temporal responses of atmospheric pollutants to the COVID-19 lockdown across the Yangtze River Delta region

From January 1 to February 29, 2020, using hourly monitoring data of criteria pollutants in Shanghai, Nanjing, Hefei and Hangzhou, combined with the vertical columns of SO₂ and NO₂ from satellite inversions, we explore the pollution levels and response characteristics of urban atmospheric pollutants in the Yangtze River Delta region to the COVID-19 lockdown. The results show that, except O₃, the average concentrations of air pollutants during the lockdown (from January 24 to February 10) were lower than that before the lockdown but higher than that after the lockdown, indicating that emission reduction was not the only factor in terms of governing air quality. The ratio of PM_2.5/PM₁₀ during the lockdown period was higher than before and after the lockdown, indicating that the secondary generation of aerosols has an important contribution to the fine particle pollution that still occurs during the lockdown period. We observed a significant increase of O_x levels (p < 0.01) during the lockdown, suggesting that atmospheric oxidation capacity may be substantially enhanced in the context of NO₂ reduction. Spatially, the O₃ typically centered in individual city as hotspots, indicating its nature of local emission and formation. However, other pollutants (including PM_2.5, PM₁₀, CO, SO₂, and NO₂) demonstrated a gradual decrease gradient from north to south, satellite observations pinpointed that North China Plain was the major source of regional transport, which can be further validated by back-trajectory analysis.