Air Quality in the Italian Northwestern Alps during Year 2020: Assessment of the COVID-19 «Lockdown Effect» from Multi-Technique Observations and Models

Abstract

The effect of COVID-19 confinement regulations on air quality in the northwestern Alps is assessed here based on measurements at five valley sites in different environmental contexts. Surface concentrations of nitrogen oxides (NO and NO2), ozone (O3), particulate matter (PM2.5 and PM10), together with a thorough microphysical (size), chemical, and optical (light absorption) aerosol characterisation, complemented by observations along the vertical column are considered. Even in the relatively pristine environment of the Alps, the «lockdown effect» is well discernible, both in the early confinement phase and in late 2020. The variations observed during the first confinement period in the city of Aosta (−61% NO, −43% NO2, +5% O3, +9% PM2.5, −12% PM10, relative to average 2015–2019 conditions) are attributed to the competing effects of air pollution lockdown-induced changes (−74%, −52%, +18%, −13%, −27%, relative to the counterfactual scenario for 2020 provided by a predictive statistical model trained on past measurements) and meteorology (+52%, +18%, −11%, +25%, +20%, relative to average conditions). These changes agree well with the ones obtained from a chemical transport model with modified emissions according to the restrictions. With regard to column-integrated quantities and vertical profiles, the NO2 column density decreases by >20% due to the lockdown, whereas tropospheric aerosols are mainly influenced by large-scale dynamics (transport of secondary particles from the Po basin and mineral dust from the Sahara desert and the Caspian Sea), except a shallow layer about 500 m thick close to the surface, possibly sensitive to curtailed emissions (especially exhaust and non-exhaust particles from road traffic and fugitive emissions from the industry).

Vertical profile of the clear-sky aerosol direct radiative effect in an Alpine valley, by the synergy of ground-based measurements and radiative transfer simulations

“…In particular, employing a 1064-nm laser beam can raise possible concerns about the ALC efficiency in detecting fine mode particles, which are especially dominant in case study A. However, the reliability of the ALC in this situation can be justified by the following arguments: (a) as also stated in Section 2.3, other studies (Dionisi et al 2018;Barragan et al 2020;Diémoz et al 2021) have successfully employed such an instrument in similar contexts, showing good agreement with independent data sources;…”

“…These issues are analysed in more depth in Section 3.4. Previous studies (Dionisi et al 2018;Barragan et al 2020;Diémoz et al 2021), by the way, have already demonstrated the efficacy of this technique, by comparing its retrievals to independent observations from other instruments (such as sky radiometers or research lidars).…”

“…The valley lies in the W-E direction, and the site is surrounded on the northern and southern sides by mountains up to 3500 m high. The site area can be classified as semi-rural, partly influenced by anthropogenic aerosols locally emitted by human activities (such as vehicular traffic, domestic heating, steel production), and by Diémoz et al 2021); advections of Saharan dust from Northern Africa also occur occasionally (Diémoz et al 2014). The meteorological conditions are typical of a mountain valley, with frequent thermally driven or dry downslope (Foehn) winds.…”

Change Points Detection and Trend Analysis to Characterize Changes in Meteorologically Normalized Air Pollutant Concentrations

“…The procedure illustrated in the present work reveals promising in elucidating the changes in the air pollutant levels that are not easily identifiable in the original data and in identifying an atmospheric response after an intervention or an unplanned event [3]. Finally, it is expected that the proposed procedure can also be worthwhile applied to assess the atmospheric response to social and anthropic activities lockdown established in Italy in the first part of 2020 to fight the spreading of the coronavirus disease (COVID-19) [27,28].…”

Sensitivity of Pollutant Concentrations to the Turbulence Schemes of a Dispersion Modelling Chain over Complex Orography

“…This area is an ideal scenario for investigating how the orography impacts the development of pollution events: mechanically and thermally driven winds, Foehn and temperature inversions are frequent. In particular, plain-valley circulations strongly influence the long-term atmospheric composition [15,16] and can trigger severe pollution episodes [17].…”