Spatial, seasonal trends and transboundary transport of PM_2.5 inorganic ions in the Veneto region (Northeastern Italy)

The Veneto region lies in the eastern part of the Po Valley (Italy). This is one of the hotspots in Europe for air quality, where efforts to meet the European standard for PM<inf>2.5</inf> according to current and future legislation have been generally unsuccessful. Recent data indicating that ammonium, nitrate and sulphate account for about one third of total PM<inf>2.5</inf> mass show that secondary inorganic aerosol (SIA) plays a key role in the exceedence of the standards. A sampling campaign for PM<inf>2.5</inf> was carried out simultaneously in six major cities (2012-2013). The water soluble inorganic ions were quantified and data processed to: (1) investigate the seasonal trends and the spatial variations of the ionic component of aerosol; (2) identify chemical characteristics at the regional-scale and (3) assess the potential effects of long-range transport using back-trajectory cluster analysis and concentration-weighted trajectory (CWT) models. Results indicated that PM<inf>2.5</inf> and SIA ions have an increasing gradient in concentrations from North (mountain) to South (lowland) and from East (coastal) to West (more continental), whereas K⁺ and Ca²⁺ levels are quite uniformly distributed. Similar seasonal trends in PM<inf>2.5</inf> and ions are seen across the region. Simultaneous daily changes were observed and interpreted as a consequence of similar emission sources, secondary pollutant generation and accumulation/removal processes. Sulphate and nitrate were not directly related to the concentrations of their precursor gases and were generally largely, but not completely, neutralised by ammonium. The clustering of back-trajectories and CWT demonstrate that the long-range movement of the air masses has a major impact upon PM<inf>2.5</inf> and ion concentrations: an area spreading from Eastern to Central Europe was identified as a main potential source for most ions. The valley sites are also heavily influenced by local emissions in slow moving northerly air masses. Finally, two episodes of high nitrate levels were investigated to explain why some sites are experiencing much higher concentrations than others. This study identifies some key features in the generation of SIA in the Po Valley, demonstrating that SIA generation is a regional pollution phenomenon and mitigation policies are required at regional, national and even European scales.