Characterization of aerosol size distributions and chemical compositions under strong wind and stagnant conditions during haze episodes in Lin'an, China

A long-lasting haze episode which included a strong wind pollution day (SWPD) and stable pollution days (SPD) occurred in Lin'an from January 21 to 26, 2015. On Jan. 21, the Lin'an atmospheric background station experienced a north wind at ground level, which resulted in short-term northern input-based pollution. During January 23 to 26, stable atmospheric conditions led to a local accumulation of pollutants. The average number concentration, surface area concentration, volume concentration, and concentration of PM2.5 were 14,183 +/- 7121 cm(-3), 576 +/- 154 mu m(2) cm(-3), 1009 +/- 151 mu m(3) cm(-3), and 145 +/- 41 mu g/m(3) in SWPD; 14,497 +/- 7418 cm(-3), 1033 +/- 241 mu m(2) cm(-3), 1435 +/- 540 mu m(3) cm(-3), and 205 +/- 44 mu g/m(3) in SPD. The extinction coefficients calculated in SWPD and SPD were 523.7 +/- 281.7 Mm(-1) and 918.0 +/- 416.9 Mm(-1). The number concentration spectra in SWPD were a bimodal distribution, which exhibited peak values at 38 nm and 88 nm (11,422 cm(-3) nm(-1) and 11,043 cm(-3) nm(-1)). The number concentration spectra in SPD were a unimodal distribution with a peak at 96 nm (15,375 cm(-3)nm(-1)). Surface concentration spectra in SWPD and SPD were three-peak distributions with maximum concentration peaks at 230 nm and 575 nm (808 mu m(2) cm(-3) nm(-1) and 1087 mu m(2) cm(-3) nm(-1)), respectively. The volume concentration spectra of SWPD had a four-peak distribution with peaks at 38 nm, 210 nm, 575 nm, and 1.3 mu m. Appropriate relative humidity (RH) and particles in the 0.5-1-mu m range were the main reasons for lower visibility for SPD. The contributions of (NH4NO3), sulfate ((NH4)(2)SO4), and organic matter (OM) to the extinction coefficients were 30.3%, 30.1%, and 19.7%, respectively, for SWPD; and 32.1%, 26.3%, and 23.4%, respectively, for SPD. The particles in SWPD came from the long-range transport and conversion of gas to particles, and SO42- and NO3- for SPD were mainly generated by heterogeneous reactions and homogeneous reactions, respectively. It was also determined that the enhancement of atmospheric oxidation resulted in greater secondary organic carbon (SOC) generation, and a reduction in RH led to a decrease in the main water-soluble ions.