Abstract
During the global COVID-19 pandemic, anthropogenic emissions of air pollutants and greenhouse gases (GHGs), especially traffic emissions in urban areas, have declined. Long-term measurements of trace gas concentrations in urban areas can be used to quantify the impact of emission reductions on GHG mole fractions. Open-path Fourier transform infrared (OP-FTIR) spectroscopy is a non-intrusive technique that can be used to simultaneously measure multiple atmospheric trace gases in the boundary layer. This study investigates the reduction of mole fractions and mole fraction enhancements above background for surface CO, CO2, and CH4 in downtown Toronto, Canada (the fourth largest city in North America) during the 2020 and 2021 COVID-19 stay-at-home periods. Mean values obtained from these periods were compared with mean values from a reference period prior to the 2020 restrictions. Mean CO mole fraction enhancement declined by 51 ± 23% and 42 ± 24% during the 2020 and 2021 stay-at-home periods, respectively. The mean afternoon CO2 mole fraction enhancement declined by 3.9 ± 2.6 ppm (36 ± 24%) and 3.5 ± 2.8 ppm (33 ± 26%) during the stay-at-home periods in 2020 and 2021. In contrast, CH4 mole fraction enhancement did not show any significant decrease. Diurnal variation in CO during the stay-at-home period in 2020 was also significantly reduced relative to the reference period in 2020. These reductions in trace gas mole fraction enhancements coincide with the decline of local traffic during the stay-at-home periods, with an estimated reduction in CO and CO2 enhancements of 0.74 ± 0.15 ppb and 0.18 ± 0.05 ppm per percentage decrease in traffic, respectively.
Highlights
The observed reductions of carbon monoxide (CO) and ∆CO in this study during the two stay-at-home periods are close to previously reported urban surface CO reduction during similar periods, including a 17% reduction of mole fraction at a roadside station in Seattle, USA [30], a 25% average reduction of mole fraction enhancement from ground-based observations in California, USA [9], a 23% reduction of surface concentration in Wuhan, China [27], and a 35% reduction of surface concentration at a site located in the historical center of Athens, Greece [18]
The Open-path Fourier transform infrared (OP-FTIR) system used in this study detected changes in surface mole fractions and enhancements above background of CO and CO2 in downtown Toronto during the COVID-19 stay-at-home periods in 2020 and 2021
2020 stay-at-home period compared to the reference period in 2020, indicating that the COVID-19 stay-at-home restrictions in Toronto in 2020 altered the diurnal variations of CO and CO2
Summary
Emissions of air pollutants related to travel and industrial activities have declined, especially for urban regions, during the COVID-19 restrictions. These impacts are of interest to air quality and climate research, and have been investigated in many cities around the world [1]. Global data show that fossil fuel carbon dioxide (CO2 ) and nitrogen oxides (NOx = NO + NO2 ) emissions declined during the COVID-19 pandemic restrictions in 2020, with a maximum of 30% in mid-April 2020, due to reduced activity in the transport sector [2]. Nitrogen dioxide (NO2 ) is an important air pollutant in urban and industrial regions. Due to the short atmospheric lifetime of NO2 and fuel combustion as its major anthropogenic source, NO2 reductions have been observed and quantified over urban regions around the world using satellite observations [3,4,5,6,7,8,9,10,11,12]
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