Temporal variations, acidity, and transport patterns of PM2.5 ionic components at a background site in the Yellow River Delta, China

To better understand the pollution characteristics and potential sources of PM2.5 ionic components at the Yellow River Delta (YRD), a semicontinuous measurement was conducted to observe water-soluble ions in PM2.5 at a nature reserve in Dongying of Shandong province, China, in 2011. The results showed that SO4 (2-), NO3 (-), and NH4 (+) were the dominant ionic species (constituting 93 % of the total ionic mass) with their annual average concentrations of 22.48, 12.77, and 11.21 mu g/m(3), respectively. These three ion concentrations were generally lower than those observed in major cities in China but higher than those in other rural and nature reserve sites. Ion concentrations exhibited large seasonal variations, and maximum values were observed in summer. SO4 (2-) concentration presented a daytime peak in summer, autumn, and winter, while in spring, a relative flat diurnal cycle was observed. NO3 (-) concentration changed with that of SO4 (2-) during most of measurement period. Transport from surrounding areas contributed to the diurnal cycle of secondary ions. In addition, photochemical reaction and thermodynamic equilibrium played important roles on the diurnal variation of SO4 (2-) and NO3 (-), respectively. The aerosol at the YRD was weakly acidic, and it was most acidic in winter. A cluster analysis showed that fine particle pollution at the YRD was mainly affected by southwest local emissions and northern middle- to long-distance transport.