Impacts of COVID19 Lockdown on Lowermost Tropospheric Ozone Pollution over Europe derived from Multispectral Synergism of IASI and GOME2 Satellite Measurements

The lockdown for limiting COVID-19 transmission drastically reduced human activities during springtime of 2020 in most European countries. This was reflected in significant reductions on the emissions of several air pollutants into the atmosphere. As shown by multiple observations, the abundance of short-lived primary pollutants such as nitrogen dioxide clearly diminished, particularly in major source regions. However, the evolution of secondary air pollutants such as tropospheric ozone does not have a univocal relationship with that of their precursors. Indeed, tropospheric ozone concentrations may enhance or reduce depending on photochemical processes for production and sinks, which depend on the abundance of other atmospheric pollutants. The unprecedented occurrence of a lockdown provides a unique in nature experience to analyze the link between these processes and the evolution of tropospheric ozone.

In order to investigate the evolution of lowermost tropospheric ozone over Europe during the COVID-19 lockdown, we have used innovative multispectral satellite observations of ozone derived from the synergism of IASI and GOME­2 measurements respectively at the infrared and ultraviolet (Cuesta et al., 2013; 2018). This novel method called IASI+GOME2 retrieves ozone at the lowermost troposphere, thus below 3 km of altitude, with sensitivity peaking down to 2 to 2.5 km over land during summer. It derives lowermost ozone abundances with a low mean bias, a linear correlation of 0.86 and a mean precision of 16 %, as compared to ozone sondes. IASI+GOME2 offers a unique capacity for observing near-surface ozone during pollution outbreaks and the seasonal evolution in agreement with respect to in situ measurements of ozone at the surface, over East Asia and Europe.

For assessing the impact of lockdown over Europe, we analyze the difference between IASI+GOME2 observations from springtime 2020 with respect to those from springtime 2019. Spatial patterns of enhancements and reductions of lowermost tropospheric ozone amounts show a fairly good agreement with those derived from the chemistry-transport model CHIMERE (using standard vs "lockdown-customized" emissions). A synergetic analysis of IASI+GOME2 and CHIMERE datasets allows us to investigate the origin of the increments and diminutions of ozone pollution.