Air quality in megacities is significantly impacted by emissions from vehicles and other urban-scale human activities. Amid the outbreak of Coronavirus (COVID-19) in January 2020, strict policies were in place to restrict people’s movement, bringing about steep reductions in pollution activities and notably lower ambient concentrations of primary pollutants. In this study, we report hourly measurements of fine particulate matter (i.e., PM_2.5) and its comprehensive chemical speciation, including elemental and molecular source tracers, at an urban site in Shanghai spanning a period before the lockdown restriction (BR) (1 to 23 Jan. 2020) and during the restriction (DR) (24 Jan. to 9 Feb. 2020). The overall PM_2.5 was reduced by 27% from 56.2 ± 40.9 (BR) to 41.1 ± 25.3 μg m⁻³ (DR) and the organic carbon (OC) in PM_2.5 was similar, averaged at 5.45 ± 2.37 (BR) and 5.42 ± 1.75 μgC m⁻³ (DR). Reduction in nitrate was prominent, from 18.1 (BR) to 9.2 μg m⁻³ (DR), accounting for most of the PM_2.5 decrease. Source analysis of PM_2.5 using positive matrix factorization modeling of comprehensive chemical composition, resolved nine primary source factors and five secondary source factors. The quantitative source analysis confirms reduced contributions from primary sources affected by COVID-19, with vehicular emissions showing the largest drop, from 4.6 (BR) to 0.61 μg m⁻³ (DR) and the percentage change (−87%) in par with vehicle traffic volume and fuel sale statistics (−60% to −90%). In the same time period, secondary sources are revealed to vary in response to precursor reductions from the lockdown, with two sources showing consistent enhancement while the other three showing reductions, highlighting the complexity in secondary organic aerosol formation and the nonlinear response to broad primary precursor pollutants. The combined contribution from the two secondary sources to PM_2.5 increased from 7.3 ± 6.6 (BR) to 14.8 ± 9.3 μg m⁻³ (DR), partially offsetting the reductions from primary sources and nitrate while their increased contribution to OC, from 1.6 ± 1.4 (BR) to 3.2 ± 2.0 μgC m⁻³ (DR), almost offset the decrease coming from the primary sources. Results from this work underscore challenges in predicting the benefits to PM_2.5 improvement from emission reductions of common urban primary sources.

中文翻译：

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受COVID-19锁定影响的上海市基于PM2.5和有机碳源变化的示踪剂表征

汽车和其他城市规模的人类活动所排放的污染物对特大城市的空气质量产生了重大影响。在2020年1月的冠状病毒（COVID-19）爆发期间，制定了严格的政策来限制人们的活动，从而大大减少了污染活动，并显着降低了主要污染物的环境浓度。在这项研究中，我们报告了在锁定限制（BR）之前的一段时间内，上海某城市每小时对细颗粒物（即PM _2.5）及其综合化学形态（包括元素和分子源示踪剂）进行的每小时测量（1至2020年1月23日）和限制期间（DR）（2020年1月24日至2月9日）。整体PM _2.5从56.2±40.9（BR）降低27％至41.1±25.3μgm ^-3（DR），PM _2.5中的有机碳（OC）相似，平均分别为5.45±2.37（BR）和5.42±1.75μgCm ⁻³（DR）。硝酸盐的减少显着，从18.1（BR）降至9.2μgm ^-3（DR），占PM _2.5下降的大部分。使用全面化学成分的正矩阵分解模型对PM _2.5进行来源分析，解决了9个主要来源因子和5个次要来源因子。定量来源分析证实，受COVID-19影响的主要来源的贡献减少，汽车排放量下降幅度最大，从4.6（BR）降至0.61μgm ^-3（DR）和百分比变化（−87％）与车辆交通量和燃油销售统计（−60％至−90％）相当。在同一时间段内，发现次要来源因锁定而导致的前驱物减少而变化，其中两个来源显示出一致的增强，而其他三个来源显示出减少，突出了次要有机气溶胶形成的复杂性以及对广泛的主要前驱物的非线性响应污染物。两个次要来源对PM _2.5的总贡献从7.3±6.6（BR）增加到14.8±9.3μgm ^-3（DR），部分抵消了主要来源和硝酸盐的减少，而它们对OC的贡献从1.6± 1.4（BR）至3.2±2.0μgCm ^-3（DR），几乎抵消了来自主要来源的减少。这项工作的结果突显了在预测普通城市主要排放源的减排对PM _2.5改善的收益方面所面临的挑战。