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 focuson lessons learned on how continuing abating pollution. Trafficflow decreased by up to 80% during the lockdownand remained relatively low during the full relaxation (June and July). After the lockdown a significant shift frompublic transport to private vehicles (+21% in Barcelona) persisted due to the pervasive fear that using publictransport might increase the risk of SARS-CoV-2 infection, which need to be reverted as soon as possible. NO2levels fell below 50% of the WHO annual air quality guidelines (WHOAQGs), but those of PM2.5were reducedless than expected due to the lower contributions from traffic, increased contributions from agricultural and do-mestic biomass burning, or meteorological conditions favoring high secondary aerosol formation yields. Evenduring the lockdown, the annual PM2.5WHOAQG was exceeded in cities within the NE and E regions withhigh NH3emissions from farming and agriculture. Decreases in PM10levels were greater than in PM2.5due to reduced emissions from road dust, vehicle wear, and construction/demolition. Averaged O3daily 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 O38hDM responses were heterogeneous, with increases or decreases depend-ing on the period and location. Thus, after canceling out the effect of meteorology, 5 out of 11 cities experiencedO3decreases during the lockdown, while the remaining 6 either did not experience relevant reductions or in-creased. During the relaxation period and coinciding with the growing O3season (June–July), most cities expe-rienced decreases. However, the O3WHOAQG was still exceeded during the lockdown and full relaxationperiods in several cities. For secondary pollutants, such as O3and PM2.5, further chemical and dispersion model-ing along with source apportionment techniques to identify major precursor reduction targets are required toevaluate their abatement potential