We have investigated the impact of reduced emissions due to COVID-19 lockdown measures in spring 2020 on air quality in Canada’s four largest cities: Toronto, Montreal, Vancouver, and Calgary. Observed daily concentrations of NO₂, PM_2.5, and O₃ during a “pre-lockdown” period (15 February–14 March 2020) and a “lockdown” period (22 March–2 May 2020), when lockdown measures were in full force everywhere in Canada, were compared to the same periods in the previous decade (2010–2019). Higher-than-usual seasonal declines in mean daily NO₂ were observed for the pre-lockdown to lockdown periods in 2020. For PM_2.5, Montreal was the only city with a higher-than-usual seasonal decline, whereas for O₃ all four cities remained within the previous decadal range. In order to isolate the impact of lockdown-related emission changes from other factors such as seasonal changes in meteorology and emissions and meteorological variability, two emission scenarios were performed with the GEM-MACH air quality model. The first was a Business-As-Usual (BAU) scenario with baseline emissions and the second was a more realistic simulation with estimated COVID-19 lockdown emissions. NO₂ surface concentrations for the COVID-19 emission scenario decreased by 31 to 34% on average relative to the BAU scenario in the four metropolitan areas. Lower decreases ranging from 6 to 17% were predicted for PM_2.5. O₃ surface concentrations, on the other hand, showed increases up to a maximum of 21% close to city centers versus slight decreases over the suburbs, but O_x (odd oxygen), like NO₂ and PM_2.5, decreased as expected over these cities.

我们调查了 2020 年春季因 COVID-19 封锁措施而减少排放对加拿大四个最大城市（多伦多、蒙特利尔、温哥华和卡尔加里）空气质量的影响。在“封锁前”期间（2020 年 2 月 15 日至 3 月 14 日）和“封锁”期间（2020 年 3 月 22 日至 5 月 2 日）观察到的 NO ₂、PM _2.5和 O ₃的每日浓度，此时封锁措施已全面实施与前十年（2010-2019）的同期相比，加拿大各地的部队。在 2020 年封锁前至封锁期间，观察到平均每日 NO ₂的季节性下降高于往常。对于 PM _2.5，蒙特利尔是唯一一个季节性下降高于往常的城市，而对于 O ₃所有四个城市都保持在之前的十年范围内。为了将锁定相关排放变化的影响与气象和排放的季节性变化以及气象变化等其他因素隔离开来，使用 GEM-MACH 空气质量模型执行了两种排放情景。第一个是具有基线排放的一切照旧 (BAU) 情景，第二个是估计 COVID-19 锁定排放的更现实的模拟。与四个大都市地区的 BAU 情景相比，COVID-19 排放情景的NO ₂表面浓度平均下降了 31% 至 34%。_{预计 PM 2.5}的降幅较低，为 6% 至 17% 。氧₃另一方面，地表浓度在靠近市中心的地方最多增加了 21%，而在郊区则略有下降，但 O _x（奇氧），如 NO ₂和 PM _2.5，在这些城市中如预期的那样下降。