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The Angera stone is a sedimentary Triassic dolostone mainly composed of dolomite, which was widely employed in the Lombard architecture. In the present work, the study of the Angera stone of the 'Cortile del Richini', a masterpiece of the Lombard baroque, is discussed as an example of a particularly challenging conservation issue. The courtyard suffered from a particularly troubled conservative history and the highly polluted urban environment of Milan provided very favourable conditions for the decay of such a delicate stone type. The last extensive conservative intervention was performed during the nineties due to massive black crust formation and granular disintegration of the stone. Almost 20 years later, a thorough study of the stone has been aimed at the definition of a long-term maintenance project. The Angera stone has been investigated in order to define an updated version of its state of conservation, as well as to track any changes in the degradation mechanisms due to the environmental variations of the Milan city centre. In particular, the soluble salts' content of the stone substrate has been evaluated as an indicator of the damage level. The results highlighted that the stone is currently in critical condition: An extensive sulphation process is taking place, leading to the chemical transformation of the dolomitic substrate into gypsum. Moreover, the powder deposits affecting the stone surfaces have a similar composition to those generally found in the black crusts growing in urban environment. Therefore, they can be considered as precursor of this particularly dangerous decay mechanism. [ABSTRACT FROM AUTHOR]

The Milan metropolitan area in Northern Italy experiences historically severe particulate matter pollution episodes characterized by high organic aerosol (OA) concentrations. However, the main sources of OA, especially atmospherically formed secondary OA (SOA) are not well understood. Here, we investigated the emission sources contributing to the directly emitted OA (Primary – POA) and to the SOA in urban Milan, where such information is particularly lacking. We used advanced mass spectrometric analytical techniques for the characterization of archive samples, as well as statistical receptor modeling (positive matrix factorization, PMF) and air quality modeling, to establish a base case for investigating the time evolution of source contributions. We found that residential heating biomass burning POA (BBOA) were a major detrimental factor for air quality during the winter (37% of OA, under polluted conditions up to 56% of OA). Inefficient combustion conditions identified by high BBOA/K+ ratios contributed to the high relative contribution of BBOA to OA. Long-term tracer analyses suggest that BBOA concentrations remained approximately constant over the last decade (2012–2022), supporting the conclusion that emissions from biomass burning remained a major driver of winter-time OA pollution. Yet assessing changes in the contribution of other OA sources require future research. While POA emissions were the most important contributor to OA during winter (62% of OA), SOA dominated OA during summer (62% of OA). Our combined advanced mass spectral source apportionment and air quality modelling analyses indicated that winter-time SOA were mostly affected by biomass burning related precursor emissions, while summer-time SOA were linked to both the remaining anthropogenic emissions (industry, energy production, shipping, and traffic) and to biogenic emissions. Altogether, this study quantified the major emission sources of OA and thus provides crucial information about OA sources and a baseline for comparison to the present situation which is needed for tackling OA pollution in one of the major pollution hotspots in Europe. Overall, this study presents a transferable framework combining chemical source apportionment with bottom-up air quality OA source analyses in order to better understand the formation of SOA. • We identify the organic aerosol sources in Milan, a pollution hotspot in Europe. • Residential heating (wood) governed wintertime OA throughout the last decade. • In winter, secondary OA is mainly produced by residential wood burning emissions. • In summer, secondary OA is formed both from biogenic and anthropogenic emissions. [ABSTRACT FROM AUTHOR]