The impact of COVID-19 on air quality levels in Portugal: A way to assess traffic contribution

The pandemic caused by coronavirus COVID-19 is having a worldwide impact that affects health, the economy and indirectly affects the air pollution in cities. In Portugal, the number of cases increased continually (32700 confirmed cases as of May 31, 2020), which has affected the health system and caused movement restrictions which in turn affects the air pollution in the country. This article analyses the indirect effect produced by this pandemic on air pollution in Portugal, by comparison of data from a period of movement restriction of the citizens by the government – COVID lockdown period (March–May 2020) with data from baseline conditions (mean of the mirrored periods from the five previous years (March–May from 2015 to 2019)). Air quality data – in particular NO2 and PM10 hourly concentration - from more than 20 monitoring stations spread over mainland Portugal was used to perform this evaluation. The mean reduction observed on pollutant concentrations was higher for NO2 (41%) than for PM10 (18%). For NO2, mean reductions were more significant in traffic (reaching values higher than 60% in some monitoring stations) and background urban sites than in rural stations. The reduction of NO2 concentration observed in traffic sites were compared to the estimation of traffic contribution by the incremental method, suggesting that this latter approach is not consistent (lower in same sites and higher in others) and alerting to the careful use of this approach in future works. [Display omitted] •The effect of COVID lockdown on air quality (NO2 and PM10) in Portugal was assessed.•Average reductions of 41% and 18% were observed for NO2 and PM10, respectively.•The NO2 and PM10 reductions occurred mainly at urban areas.•Based on observed reductions, NO2 concentrations traffic contribution reaches >60%.•Incremental method was tested and inconsistencies on traffic contribution were found.