Trends and variability of atmospheric PM_2.5 and PM_10–2.5 concentration in the Po Valley, Italy

Abstract. The Po Valley is one of the largest European regions with a remarkably high concentration level of atmospheric pollutants, both for particulate and gaseous compounds. In the last decade stringent regulations on air quality standards and on anthropogenic emissions have been set by the European Commission, including also for PM_2.5 and its main components since 2008. These regulations have led to an overall improvement in air quality across Europe, including the Po Valley and specifically PM₁₀, as shown in a previous study by Bigi and Ghermandi (2014). In order to assess the trend and variability in PM_2.5 in the Po Valley and its role in the decrease in PM₁₀, we analysed daily gravimetric equivalent concentration of PM_2.5 and of PM_10–2.5 at 44 and 15 sites respectively across the Po Valley. The duration of the times series investigated in this work ranges from 7 to 10 years. For both PM sizes, the trend in deseasonalized monthly means, annual quantiles and in monthly frequency distribution was estimated: this showed a significant decreasing trend at several sites for both size fractions and mostly occurring in winter. All series were tested for a significant weekly periodicity (a proxy to estimate the impact of primary anthropogenic emissions), yielding positive results for summer PM_2.5 and for summer and winter PM_10–2.5. Hierarchical cluster analysis showed moderate variability in PM_2.5 across the valley, with two to three main clusters, dividing the area in western, eastern and southern/Apennines foothill sectors. The trend in atmospheric concentration was compared with the time series of local emissions, vehicular fleet details and fuel sales, suggesting that the decrease in PM_2.5 and in PM₁₀ originates from a drop both in primary and in precursors of secondary inorganic aerosol emissions, largely ascribed to vehicular traffic. Potentially, the increase in biomass burning emissions in winter and the modest decrease in NH₃ weaken an otherwise even larger drop in atmospheric concentrations.