PM2.5 wintertime sensitivity to changes in NOx, SO2, and NH3 emissions in Lombardy Region

The Po Valley stands out as one of the most urbanized and industrialized regions in Europe. For decades, it has been grappling with the impact of human exposure to fine particulate matter (PM2.5). The Lombardy Region in northern Italy, a key area within the Po Valley, experiences a complex interplay of main emission precursors (NH3, NOx, and SOx), leading to the formation of secondary inorganic aerosols. In this paper, we investigate the effects of reducing NH3, NOx, and SOx emissions, individually and in combination, on PM2.5 concentrations in the Lombardy Region. Our analyses employ the operational Air Quality model, used daily by ARPA Lombardia. The focus of the study is wintertime period (1 January-31 March), recognized as the most challenging period of the year due to strong correlations between NH3, PM10, (NH4)NO3, and (NH4)(2)SO4 concentrations and peaking days of ammonia emissions from slurry spreading. Our results reveal that, during wintertime emission reductions, (a) PM2.5 concentration changes exhibit linearity within 25% reduction of precursors, with non-linearities increasing as precursors reductions surpass this threshold; (b) the NOx-sensitive areas extend spatially up to 50% reductions and localize in areas with higher NH3 emissions; (c) NH3-sensitive areas are concentrated in densely urbanized regions of the Lombardy Region; and (d) sensitive chemical regimes are mixed, and it is more effective to reduce both NH3 and NOx rather than SOx (emissions of which are not abundant). Final discussions, considering furtherly both summertime and yearly simulations, underscore the significance of focusing on wintertime. PM2.5 responses to precursors display significant spatial variations compared to summertime, where NOx-sensitive areas predominate. These findings provide valuable insights into reducing PM2.5 concentrations, assisting air quality policymakers in understanding the impacts of precursor emissions on PM2.5 levels during wintertime.