Abstract
Abstract. The unprecedented order, in modern peaceful times, for a near-total lockdown of the Greek population as a means of protection against severe acute respiratory syndrome coronavirus 2, commonly known as COVID-19, has generated unintentional positive side-effects with respect to the country's air quality levels. Sentinel-5 Precursor/Tropospheric Monitoring Instrument (S5P/TROPOMI) monthly mean tropospheric nitrogen dioxide (NO2) observations show an average change of −34 % to +20 % and −39 % to −5 % with an average decrease of −15 % and −11 % for March and April 2020 respectively, compared with the previous year, over the six larger Greek metropolitan areas; this is mostly attributable to vehicular emission reductions. For the capital city of Athens, weekly analysis was statistically possible for the S5P/TROPOMI observations and revealed a marked decline in the NO2 load of between −8 % and −43 % for 7 of the 8 weeks studied; this is in agreement with the equivalent Ozone Monitoring Instrument (OMI)/Aura observations as well as the ground-based estimates of a multi-axis differential optical absorption spectroscopy ground-based instrument. Chemical transport modelling of the NO2 columns, provided by the Long Term Ozone Simulation European Operational Smog (LOTOS-EUROS) chemical transport model, 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 ∼ −25 % and −65 % for 5 of the 8 weeks studied, with the remaining 3 weeks showing a positive average of ∼ 10 %; this positive average was postulated to be due to the uncertainty of the methodology, which is based on differences. As a result this analysis, we conclude that the effect of the COVID-19 lockdown and the restriction of transport emissions over Greece is ∼ −10 %. As transport is the second largest sector (after industry) affecting Greece's air quality, this occasion may well help policymakers to enforce more targeted measures to aid Greece in further reducing emissions according to international air quality standards.
Highlights
In this work, we aim to quantify the decline in tropospheric nitrogen dioxide (NO2) levels over Greece during the ongoing severe acute respiratory syndrome coronavirus 2 (COVID-19) pandemic, as sensed by the spaceborne Sentinel-5 Precursor (S5P)/TROPOMI instrument
We should note at this point that as the main findings in this work refer to relative differences between different time periods, absolute differences to standard instruments do not affect our findings because the stability of the TROPOMI datasets is assured
We first show the effect on monthly NO2 levels over the entire domain, the six Greek cities with the largest number of inhabitants, and we present a more in depth analysis, on a weekly basis, for the city of Athens, examining the long-term variability of tropospheric NO2 levels over the capital city using 15 years of space-borne observations by the Ozone Monitoring Instrument (OMI/Aura) as well as the air quality in situ measurements of the Greek Ministry Environment and Energy network, reporting to the European Environment Agency
Summary
We aim to quantify the decline in tropospheric nitrogen dioxide (NO2) levels over Greece during the ongoing severe acute respiratory syndrome coronavirus 2 (COVID-19) pandemic, as sensed by the spaceborne S5P/TROPOMI (hereafter referred to as TROPOMI) instrument. By comparing the relative levels for the months of March and April for the years 2020 and 2019, while properly accounting for the differences in meteorology using the simulations of a chemical transport model (CTM), we quantify the improvement in local and regional air quality due to the reduced nitrogen oxides (NOx) emissions. Koukouli et al.: Sudden changes in nitrogen dioxide emissions over Greece with respect to sensing abrupt atmospheric content changes, and provide the dates when the different lockdown measures were enforced nationwide in Greece. It is well accepted that NO2 fluxes will remain relatively close to their source which, first of all, makes it possible for NOx emissions to be well detected from space (e.g. Stavrakou et al, 2008; Lamsal et al, 2010; van der A et al, 2008) and precludes any transboundary pollution effects which might otherwise hinder this study
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