![]() These articles were soon followed by scientific publications for different regions around the world. Many media articles appeared in the spring of 2020 reporting on satellite images released by several space agencies that revealed a marked drop in air pollutants in different regions during lockdown periods (ESA 2020a, b NASA 2020 Schindler 2020). Such societal responses to the pandemic around the world make this crisis a natural emissions-reduction experiment that provides a unique opportunity to study anthropogenic impacts on air quality (AQ) in many countries and the potential of future air pollution controls (Henneman et al. 2020 NASA 2020 Rodríguez-Urrego and Rodríguez-Urrego 2020). These measures led to significant reductions in road traffic, air travel, industrial operations, construction, and commercial business operations, which in turn reduced anthropogenic emissions of air pollutants and resulted in cleaner air quality worldwide (Bauwens et al. ![]() To control the rapid spread of the virus, most countries were forced to adopt emergency control measures, including travel restrictions, regional lockdowns, social distancing, stay-at-home and shelter-in-place orders, and shutdowns of non-essential businesses. ![]() On 11 March 2020, with cases of COVID-19 reported in 114 countries, the World Health Organization (WHO) declared the outbreak of this new coronavirus a pandemic (WHO 2020). O 3 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 NO 2 and PM 2.5, decreased as expected over these cities.Ī novel coronavirus disease 2019 (COVID-19) was initially reported in the city of Wuhan, China, on 31 December 2019 and then began to spread around the world. Lower decreases ranging from 6 to 17% were predicted for PM 2.5. NO 2 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. 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. 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. For PM 2.5, Montreal was the only city with a higher-than-usual seasonal decline, whereas for O 3 all four cities remained within the previous decadal range. Higher-than-usual seasonal declines in mean daily NO 2 were observed for the pre-lockdown to lockdown periods in 2020. Observed daily concentrations of NO 2, PM 2.5, and O 3 during a “pre-lockdown” period (15 February–14 March 2020) and a “lockdown” period (22 March–), when lockdown measures were in full force everywhere in Canada, were compared to the same periods in the previous decade (2010–2019). 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.
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