PM2.5 samples were collected at a background site of the Yangtze River Delta in China, during summertime, of which the carbonaceous components, i.e., OC and EC, and water-soluble inorganic ions, including sulfate, nitrate, and ammonium, were quantified. The average concentrations of OC and EC in PM2.5 during summer at Lin'an were (14.3±3.95) μg·m-3 and (3.33±1.47) μg·m-3, respectively. Compared to an urban site, the correlation between OC and EC during summertime in Lin'an, which was a rural site, was relatively weak (R2=0.31, P<0.01). The concentrations of secondary inorganic aerosols (sulfate as SO42-, nitrate as NO3-, and ammonium as NH4+) were much higher than those of other water-soluble inorganic ions, with average values of (8.70±5.66), (2.04±1.07), and (3.25±2.29) μg·m-3, respectively. Based on long-term intensive observations, combined with the analysis of back trajectory and fire spots observations, it was revealed that regional transport and stable synoptic conditions both play important roles in controlling the variations in aerosol chemical components. During the transition from clean to hazy days, the EC and POC fractions showed no obvious variation; in contrast, the SOC fraction decreased obviously. Unlike for SOAs, relative contributions of sulfate and ammonia to PM2.5 increased during hazy days, suggesting enhanced chemical production via other pathways for these two secondary inorganic ions, probably through heterogeneous reactions.
Keywords: Lin'an; PM2.5; haze; organic aerosol; water-soluble inorganic ions.