Ozone pollution during the COVID-19 lockdown in the spring of 2020 over Europe, analysed from satellite observations, in situ measurements, and models

Abstract

Abstract. We present a comprehensive study integrating satellite observations of ozone pollution, in situ measurements, and chemistry-transport model simulations for quantifying the role of anthropogenic emission reductions during the COVID-19 lockdown in spring 2020 over Europe. Satellite observations are derived from the IASI+GOME2 (Infrared Atmospheric Sounding Interferometer + Global Ozone Monitoring Experiment 2) multispectral synergism, which provides better sensitivity to near-surface ozone pollution. These observations are mainly analysed in terms of differences between the average on 1–15 April 2020, when the strictest lockdown restrictions took place, and the same period in 2019. They show clear enhancements of near-surface ozone in central Europe and northern Italy, as well as some other hotspots, which are typically characterized by volatile organic compound (VOC)-limited chemical regimes. An overall reduction of ozone is observed elsewhere, where ozone chemistry is limited by the abundance of NOx. The spatial distribution of positive and negative ozone concentration anomalies observed from space is in relatively good quantitative agreement with surface in situ measurements over the continent (a correlation coefficient of 0.55, a root-mean-squared difference of 11 ppb, and the same standard deviation and range of variability). An average difference of ∼ 8 ppb between the two observational datasets is observed, which can partly be explained by the fact the satellite approach retrieves partial columns of ozone with a peak sensitivity above the surface (near 2 km of altitude over land and averaging kernels reaching the middle troposphere over ocean). For assessing the impact of the reduction of anthropogenic emissions during the lockdown, we adjust the satellite and in situ surface observations for subtracting the influence of meteorological conditions in 2020 and 2019. This adjustment is derived from the chemistry-transport model simulations using the meteorological fields of each year and identical emission inventories. Using adjustments adapted for the altitude and sensitivity of each observation, both datasets show consistent estimates of the influence of lockdown emission reduction. They both show lockdown-associated ozone enhancements in hotspots over central Europe and northern Italy, with a reduced amplitude with respect to the total changes observed between the 2 years and an overall reduction elsewhere over Europe and the ocean. Satellite observations additionally provide the ozone anomalies in the regions remote from in situ sensors, an enhancement over the Mediterranean likely associated with maritime traffic emissions, and a marked large-scale reduction of ozone elsewhere over ocean (particularly over the North Sea), in consistency with previous assessments done with ozone sonde measurements in the free troposphere. These observational assessments are compared with model-only estimations, using the CHIMERE chemistry-transport model. Whereas a general qualitative consistency of positive and negative ozone anomalies is observed with respect to observational estimates, significant changes are seen in their amplitudes. Models underestimate the range of variability of the ozone changes by at least a factor 2 with respect to the two observational datasets, both for enhancements and decreases of ozone. Moreover, a significant ozone decrease observed at a large hemispheric scale is not simulated since the modelling domain is the European continent. As simulations only consider the troposphere, the influence from stratospheric ozone is also missing. Sensitivity analyses also show an important role of vertical mixing of atmospheric constituents, which depends on the meteorological fields used in the simulation and significantly modify the amplitude of the changes of ozone pollution during the lockdown.

Impact of different sources of precursors on a high-ozone event over Europe analysed with IASI+GOME2 multispectral satellite observations and model simulations

“…We consider afternoon averages (12:00-16:00 local time -LT) of these surface concentrations that are expected to be vertically mixed within the mixing boundary layer. This is likely more comparable to IASI+GOME2 retrievals, that mainly measure ozone concentrations at the residual atmospheric boundary layer, than morning surface concentrations that have not been mixed within the whole boundary layer (Cuesta et al, 2022).…”

“…Lower ozone abundances retrieved by the satellite approach may be induced by high ozone concentrations near the surface, as typically remarked during ozone outbreaks of anthropogenic origin. Previous comparisons of IASI+GOME2 data with surface in situ measurements showed relatively larger values of correlation coefficients, being ∼0.69 for a major ozone outbreak over Eastern Asia (11 surface stations over Japan, Cuesta et al, 2018) and ∼0.55 for differences of 15-day averages over Europe (Cuesta et al, 2022). In this last case, ozone satellite retrievals over Europe are also lower than surface concentrations (by -8.6 ppb over the whole continent).…”

A comprehensive assessment of yield loss in rice due to surface ozone pollution in India during 2005–2020: A great concern for food security

“…For instance, there is a good qualitative agreement between the near-surface ozone pollution outbreak across East Asia as shown by in situ measurements and IASI+GOME2 retrievals, but it is not captured by the IASI-only retrievals (Cuesta et al, 2018). These data are validated with global ozonesondes and surface in situ ozone measurements in East Asia (Cuesta et al, 2018) and Europe (Cuesta et al, 2022;Okamoto et al, 2023), and are suitable for lower tropospheric ozone studies.…”

Assessment of the sensitivity of model responses to urban emission changes in support of emission reduction strategies

“…The indicator of variability is generally higher for responses to emission reductions than for the variability of base case concentrations. It is particularly noteworthy for the ozone episodes (only 6% of variability in base case concentrations) because of the large influence of long-range transport, leaving little impact for the local reductions, as reported in Boleti et al (2019) and Bossioli et al (2007) and verified during the COVID-19 pandemic (Cuesta et al 2022;Menut et al 2020). However, even if local emission reductions have little effect, they can be very different from model to model leading to a high variability of small values.…”