Assessing and quantifying the impact of the lockdown measures is no easy feat, because air quality is linked not just to emissions of pollutants accumulating in the atmosphere, but also to weather conditions, which can help to disperse them to a greater or lesser degree. Therefore, pollution levels vary from day to day. Due to variability in the weather, it has not been possible to establish “benchmark values” which the observations can be compared against in order to quantify the effects of the restriction measures.
However, modelling may provide these answers. In order to do this, Ineris has conducted analysis based on the simulation tools in PREV’air’s national air quality forecasting system (www.prevair.org). This analysis has found that concentrations of nitrogen dioxide, predominately emitted by road traffic and industrial operations, were approximately 50% lower on average than their expected levels in France’s largest cities.
How has the numerical simulation led to this conclusion?
The analysis has involved comparing air quality model forecasts based on the database of average pollutant emissions (which therefore does not take into account the decreases caused by the lockdown measures), with adjusted simulations carried out a posteriori which incorporate the observation data approved by the Official Air Quality Monitoring Associations (AASQA).
The curve below displays this comparison, established as an average across France’s 100 largest cities since March 2020. It shows that the forecasts and the adjusted simulations up to 12 March 2020 strongly match. Beyond that date, the curves diverge, showing that the emissions data used in the forecasting model no longer reflect the actual situation. Based on the findings, levels of background concentrations of nitrogen dioxide (NO2) reached a maximum of 25 µg/m3 on 18 March 2020, while a level of 50 µg/m3 was forecasted by the model (not taking into account the impact of the lockdown).
In short, the lockdown has therefore resulted in nitrogen dioxide pollution in France’s 100 largest cities halving.