Sudden changes in nitrogen dioxide emissions over Greece due to lockdown after the outbreak of COVID-19.

The unprecedented order, in modern peaceful times, for near-total lockdown of the Greek population, as means of protection against the Severe Acute Respiratory Syndrome CoronaVirus-2, commonly known as COVID-19, infection, has brought unintentional positive side-effects to the country's air quality levels. S5P/TROPOMI monthly mean tropospheric nitrogen dioxide (NO₂) observations show an average decrease of -3 % to -26 % [-1 % to -27 %] with an average of -22 % [-11 %] for March and April 2020 respectively, compared to the previous year, over the six larger Greek metropolitan areas, attributable mostly to vehicular emission reductions. Furthermore, significant effects for shipping emissions over the Aegean Sea as well as the areas surrounding major Greek ports were observed, of approximately -12 % [-5 %]. For the capital city of Athens, weekly analysis was possible and it revealed a marked decline in NO₂ load between -8 % and -43 % for seven of the eight weeks studied. Chemical transport modelling, provided by the LOTOS-EUROS CTM, shows that the magnitude of these reductions cannot solely be attributed to the difference in meteorological factors affecting NO₂ levels during March and April 2020 and the equivalent time periods of the previous year. Taking this factor into account, the resulting decline was estimated to range between 0 % and -37 % for the five largest cities, with an average of ~ -10 %. As transport is the second largest sector that affects Greece's air quality, this occasion may well help policy makers in enforcing more targeted measures to aid Greece in further reducing emissions according to international air quality standards. [ABSTRACT FROM AUTHOR]