Aerosol optical depth climatology from the high–resolution MAIAC product over Europe: differences between major European cities and their surrounding environments

Abstract. The aerosol optical depth (AOD) is a derived measurement useful to investigate the aerosol load and its distribution at different spatio–temporal scales. In this work we use long–term (2000–2021) MAIAC (Multi–Angle Implementation of Atmospheric Correction) retrievals with 1 km resolution to investigate the climatological AOD variability and trends at different scales in Europe: a continental (30–60° N; 20° W–40° E), a regional (100x100 km²) and an urban local scale (3x3 km²). The AOD climatology at the continental scale shows the highest values during summer (JJA) and the lowest during winter (DJF) seasons. Regional and urban local scales are investigated for twenty–one cities in Europe including capitals and large urban agglomerations. Analyses show AOD average (550 nm) values between 0.06 and 0.16 at the urban local scale, while also displaying a strong north–south gradient. This gradient corresponds to a similar one in the European background, with higher AOD being located over the Po–Valley, the Mediterranean basin, and Eastern Europe. Average enhancements of the local with respect to regional AOD of 57 %, 55 %, 39 % and 32 % are found for large metropolitan centers such as Barcelona, Lisbon, Paris and Athens respectively, suggesting a non–negligible enhancement to the aerosol burden through local emissions. Negative average deviations are observed for other cities, such as Amsterdam (–17 %) and Brussels (–6 %) indicating higher regional background signal and suggesting a heterogeneous aerosol spatial distribution that conceals the urban local signal. Finally, negative statistically significant AOD trends for the entire European continent are observed. A stronger decrease rate at the regional scale with respect to the local scale one occurs for most of the cities under investigation.