ABSTRACT
Activity restrictions implemented to control the spread of the COVID-19 virus imposed a significant effect on the air quality of cities across the world. The initiative of the World Meteorological Organisation/Global Atmospheric Watch for studying effects of the 2020 COVID-19 lockdowns on air quality has produced two sets of analysis results for cities across the world, based on observational data and modelling, respectively. The modelling study aims to evaluate the modelling tools in a regime involving significant changes of activity, and at the same provide insights on the effect of selectively reduced emissions on the chemistry and composition of urban pollutants. For most of the cities, a reduction on NOx average concentrations between 11% and 70% was calculated for the lockdown period, while PM10 was reduced by 8% up to 35% in a good agreement with measured reductions observed during the 2020 lockdown period compared to the corresponding period of 2019. Taking advantage of an operational Air Quality Modelling System, which is in continuous application in the cities of Thessaloniki and Nicosia, the contribution of sectoral emissions and the role of meteorology over the observed concentration reductions was assessed. The study reveals that in both cities, observed reductions of urban PM2.5, PM10 and NOx concentration patterns can be mainly attributed to the corresponding emissions reductions in the transport and heating sectors, while O3 is strongly affected by titration near the city centre. At the same time, meteorological patterns appear to strongly influence and even mask these effects in terms of daily averages, while the impact of imposed large-scale boundary conditions on the modelling results can also be significant. © British Crown Copyright (2022)