Lessons from the COVID-19 air pollution decrease in Spain: Now what?

We offer an overview of the COVID-19 -driven air quality changes across 11 metropolises in Spain with the focus on lessons learned on how continuing abating pollution. Traffic flow decreased by up to 80% during the lockdown and remained relatively low during the full relaxation (June and July). After the lockdown a significant shift from public transport to private vehicles (+21% in Barcelona) persisted due to the pervasive fear that using public transport might increase the risk of SARS-CoV-2 infection, which need to be reverted as soon as possible. NO₂ levels fell below 50% of the WHO annual air quality guidelines (WHOAQGs), but those of PM_2.5 were reduced less than expected due to the lower contributions from traffic, increased contributions from agricultural and domestic biomass burning, or meteorological conditions favoring high secondary aerosol formation yields. Even during the lockdown, the annual PM_2.5 WHOAQG was exceeded in cities within the NE and E regions with high NH₃ emissions from farming and agriculture. Decreases in PM₁₀ levels were greater than in PM_2.5 due to reduced emissions from road dust, vehicle wear, and construction/demolition. Averaged O₃ daily maximum 8-h (8hDM) experienced a generalized decrease in the rural receptor sites in the relaxation (June-July) with -20% reduced mobility. For urban areas O₃ 8hDM responses were heterogeneous, with increases or decreases depending on the period and location. Thus, after canceling out the effect of meteorology, 5 out of 11 cities experienced O₃ decreases during the lockdown, while the remaining 6 either did not experience relevant reductions or increased. During the relaxation period and coinciding with the growing O₃ season (June-July), most cities experienced decreases. However, the O₃ WHOAQG was still exceeded during the lockdown and full relaxation periods in several cities. For secondary pollutants, such as O₃ and PM_2.5, further chemical and dispersion modeling along with source apportionment techniques to identify major precursor reduction targets are required to evaluate their abatement potential.