“…According to their findings, NO 2 levels decreased by 47.9, 36.7, and 23.9%, PM 2.5 levels decreased by 18.1, 17.3, and 2.6%, and O 3 levels increased by 34.1, 7.4, and 0.1% at urban traffic, urban background, and rural sites, respectively. Furthermore, several other papers reported similar results, for example, [ 31 , 32 , 33 ] analysed the effect of the COVID-19 lockdown on air quality and reported that the concentrations of NO 2 and PM 2.5 had decreased, whereas the concentration of O 3 had significantly increased.…”