Day-night variability of water-soluble ions in PM₁₀ samples collected at a traffic site in southeastern Spain

The present work reports diurnal and nocturnal concentrations of water-soluble ions associated to PM₁₀ samples collected during the warm and cold seasons in the urban center of Elche (Southeastern Spain). Statistical differences between daytime and nighttime levels of PM₁₀ were only observed during winter. The lower concentrations during the night were most likely the result of a reduction in traffic-induced road dust resuspension, since nocturnal concentrations of calcium also exhibited a significant decrease compared to daytime levels. During the warm season, nitrate was the only component that showed a statistically significant increase from day to night. The lower nocturnal temperatures that prevent the thermal decomposition of ammonium nitrate and the formation of nitric acid favored by the higher relative humidity at night are the most probable reasons for this variation. The close relationship between nitrate formation and relative humidity during nighttime was supported by the results of the correlation analysis. The reaction of sulfuric and nitric acids with CaCO₃ occurred to a greater extent during daytime in summer.