A one-year, on-line, multi-site observational study on water-soluble inorganic ions in PM2.5 over the Pearl River Delta region, China.

As the significant components of PM2.5, almost all of previous studies on water-soluble inorganic ions (WSIIs) have been limited by the use of single sampling station, short sampling times or low temporal resolution. This paper focuses on analysing one-year (2012) observations of WSIIs at a regional central (RCEN) site, a coastal urban (CURB) site and a coastal rural (CRUR) site in the Pearl River Delta region. On average, secondary inorganic aerosols (SIA) were the most abundant component and accounted for over 80% of the total WSIIs. The ratio among sulfate, nitrate and ammonium mass concentrations was close to 2:1:1 (5:2:1) at the RCEN and CURB sites (CRUR site). Most components (except Na+) showed higher concentrations in the dry season. The diurnal variations of different ions showed obvious differences, which were partially controlled by photochemical reactions and diffusion conditions in the boundary layer. Ionic formation patterns were different among the three sites. Secondary inorganic pollution was much more serious in the northwestern PRD, and it had a significant effect on pollution in the coastal areas. High SO42- concentrations at the CRUR site may be associated with local emissions, such as dimethysulfide (DMS). Long-range transport along the southeastern coastline also played an important role in SO42- pollution over the PRD region. Sea salt aerosols were an important source in coastal regions; they contributed large amounts of Cl-, Na+, Mg2+ at the CRUR site and large amounts of Na+, Mg2+ at the CURB site. The case studies found that sea salt aerosols concentrations increased obviously during the heavy precipitation period of typhoon. The presence of warm-wet air masses before continuous moist weather (CMW) was favourable for the formation of SIA. On the other hand, during CMW periods, SIA concentrations decreased rapidly.