Insignificant Impact of the "Stay-At-Home" Order on Ambient Air Quality in the Memphis Metropolitan Area, USA

The lockdowns due to the COVID-19 pandemic have been reported to reduce ambient air pollution in many cities globally. This study aims to examine whether air pollution dropped in Memphis, a typical U.S. metropolitan city and transportation hub, during the lockdown from 25 March to 4 May, 2020. Daily air pollution data measured at five representative monitoring stations in the Memphis Metropolitan Area were downloaded from the U.S. Environmental Protection Agency's Air Quality System. The mean concentrations of fine particulate matter (PM2.5), nitrogen dioxide (NO2), and ozone during the lockdown were compared with the baseline concentrations measured during the same periods in 2017-2019 using linear regression models. The average vehicle miles traveled (VMT) reduced by 57% in this region during the lockdown compared to that during 1-24 March, 2020. The mean (+/- standard deviation) concentrations of PM2.5, NO2, and ozone were 7.5 +/- 2.6 mu g/m(3), 16.5 +/- 9.4 ppb, and 44.5 +/- 8.4 ppb, respectively, during the lockdown. They did not statistically differ from the baseline concentrations, nor were they lower than the mean concentrations in the prior month (25 February-24 March, 2020), after accounting for meteorological conditions. The lack of effect could be explained by the small contribution of traffic emissions to air pollution. The results suggest that the "stay-at-home" order had an insignificant impact on reducing air pollution in Memphis.