A new study has found that ultrafine particle concentration decreased by 48 percent during the COVID-19 state-of-emergency period, which corresponded with aircraft and road traffic reductions of 74 percent and 51 percent, respectively.A new study has found that ultrafine particle concentration decreased by 48 percent during the COVID-19 state-of-emergency period, which corresponded with aircraft and road traffic reductions of 74 percent and 51 percent, respectively.