Since early 2020, the COVID-19 pandemic has led to lockdowns at national scales. These lockdowns resulted in large cuts of atmospheric pollutant emissions, notably related to the vehicular traffic source, especially during spring 2020. As a result, air quality changed in manners that are still currently under investigation. The robust quantitative assessment of the impact of lockdown measures on ambient concentrations is however hindered by weather variability. In order to circumvent this difficulty, an innovative methodology has been developed. The Analog Application for Air Quality (A³Q) method is based on the comparison of each day of lockdown to a group of analog days having similar meteorological conditions. The A³Q method has been successfully evaluated and applied to a comprehensive in situ dataset of primary and secondary pollutants obtained at the SIRTA observatory, a suburban background site of the megacity of Paris (France). The overall slight decrease of submicron particulate matter (PM₁) concentrations (−14 %) compared to business-as-usual conditions conceals contrasting behaviors. Primary traffic tracers (NO_x and traffic-related carbonaceous aerosols) dropped by 42 %–66 % during the lockdown period. Further, the A³Q method enabled us to characterize changes triggered by NO_x decreases. Particulate nitrate and secondary organic aerosols (SOAs), two of the main springtime aerosol components in northwestern Europe, decreased by −45 % and −25 %, respectively. A NO_x relationship emphasizes the interest of NO_x mitigation policies at the regional (i.e., city) scale, although long-range pollution advection sporadically overcompensated for regional decreases. Variations of the oxidation state of SOA suggest discrepancies in SOA formation processes. At the same time, the expected ozone increase (+20 %) underlines the negative feedback of NO titration. These results provide a quasi-comprehensive observation-based insight for mitigation policies regarding air quality in future low-carbon urban areas.

中文翻译：

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巴黎地区（法国）春季大气成分对 COVID-19 封锁措施的响应

自 2020 年初以来，COVID-19 大流行已导致全国范围内的封锁。这些封锁导致大气污染物排放量大幅减少，特别是与车辆交通源有关，尤其是在 2020 年春季。因此，空气质量的变化方式目前仍在调查中。然而，对封锁措施对环境浓度影响的稳健定量评估受到天气变化的阻碍。为了规避这个困难，已经开发了一种创新方法。空气质量模拟应用 (A ³ Q) 方法基于将锁定的每一天与一组具有相似气象条件的模拟天进行比较。A ³Q 方法已成功评估并应用于在 SIRTA 天文台（巴黎（法国）大城市的郊区背景站点）获得的一次和二次污染物的综合原位数据集。 与一切照旧的条件相比，亚微米颗粒物 (PM ₁ ) 浓度 ( -14 %)的整体轻微下降掩盖了对比行为。在封锁期间，主要交通示踪剂（NO _x和交通相关的碳质气溶胶）下降了 42%–66%。此外，A ³ Q 方法使我们能够表征由 NO _x触发的变化减少。颗粒硝酸盐和二次有机气溶胶 (SOA) 是欧洲西北部春季气溶胶的两种主要成分 ，分别减少了-45  % 和-25 %。NO _x关系强调了 区域（即城市）尺度的 NO _x减缓政策的兴趣，尽管长期污染平流偶尔会过度补偿区域减少。SOA 氧化态的变化表明 SOA 形成过程存在差异。与此同时，预计臭氧增加（+20 %) 强调了 NO 滴定的负反馈。这些结果为未来低碳城市地区的空气质量缓解政策提供了基于观察的准综合见解。