Fine particulate matter (PM2.5) concentrations at the Middle Adriatic coastal site of Croatia were affected by different air-mass inflows and/or local sources and meteorological conditions, and peaked in summer. More polluted continental air-mass inflows mostly affected the area in the winter period, while southern marine pathways had higher impact in spring and summer. Chemical characterization of the water-soluble inorganic and organic ionic constituents is discussed with respect to seasonal trends, possible sources, and air-mass inputs. The largest contributors to the PM2.5 mass were sea salts modified by the presence of secondary sulfate-rich aerosols indicated also by principal component analysis. SO42- was the prevailing anion, while the anthropogenic SO42- (anth-nssSO42-) dominantly constituted the major non-sea-salt SO42- (nssSO42-) fraction. Being influenced by the marine origin, its biogenic fraction (bio-nssSO42-) increased particularly in the spring. During the investigated period, aerosols were generally acidic. High Cl- deficit was observed at Middle Adriatic location for which the acid displacement is primarily responsible. With nssSO42- being dominant in Cl- depletion, sulfur-containing species from anthropogenic pollution emissions may have profound impact on atmospheric composition through altering chlorine chemistry in this region. However, when accounting for the neutralization of H2SO4 by NH3, the potential of HNO3 and organic acids to considerably influence Cl- depletion is shown to increase. Intensive open-fire events substantially increased the PM2.5 concentrations and changed the water-soluble ion composition and aerosol acidity in summer of 2015. To our knowledge, this work presents the first time-resolved data evaluating the seasonal composition of water-soluble ions and their possible sources in PM2.5 at the Middle Adriatic area. This study contributes towards a better understanding of atmospheric composition in the coastal Adriatic area and serves as a basis for the comparison with future studies related to the air quality at the coastal Adriatic and/or Mediterranean regions.
Keywords: Adriatic Sea; Aerosol acidity; Cl− depletion; PM2.5; Sea salt; Secondary sulfate; Water-soluble ions.