Quantifying the impact of the COVID-19 pandemic restrictions on CO, CO2, and CH4 in downtown Toronto using open-path Fourier transform spectroscopy

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 surface CO, CO2, and CH4 mole fractions during the stay-at-home periods in downtown Toronto, Canada, which is the fourth largest city in North America. Mean CO mole fraction enhancement above background declined by 51+/-23% and 42+/-24% during the 2020 and 2021 stay-at-home periods, respectively, relative to a reference period before the 2020 restrictions started. The mean afternoon CO2 mole fraction enhancement above background 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 relative to the 2020 reference period. CH4 mole fraction enhancement above background did not show significant decline during the 2020 stay-at-home period relative to the 2020 reference period. Diurnal variation in CO during the stay-at-home period in 2020 was also significantly reduced relative to the reference period in 2020. Reductions in trace gas mole fraction enhancements are coincident with the decline of local traffic during the stay-at-home periods, with reduction in CO and CO2 enhancements of 0.74+/-0.15 ppb and 0.18+/-0.05 ppm per percentage decrease in traffic, respectively.