Altitudinal effect to the size distribution of water soluble inorganic ions in PM at Huangshan, China

To investigate the vertical variation of water soluble inorganic ions (WSI) in aerosols at a regional background mountainous site, nine size fractions of particles (10.0-9.0, 9.0-5.8, 5.8-4.7, 4.7-3.3, 3.3-2.1, 2.1-1.1, 1.1-0.65, 0.65-0.43 and <0.43 μm) were collected at two different altitudes simultaneously at Huangshan in southeast China, from 14 September to 26 October of 2012. The mass concentrations of PM1.1, PM2.1 and PM10 were 17.07, 21.28 and 39.25 μg/m3 at the summit (SM, 1840 m), respectively and were 24.79, 29.02 and 42.39 μg/m3 at a lower height site (LL, 869 m). The average mass concentrations of total WSI for PM1.1, PM2.1 and PM10 were 9.59, 11.73 and 17.16 μg/m3 at SM, and were 16.88, 19.38 and 27.61 μg/m3 at LL. The concentrations of particulates and WSI both decreased with altitude increasing from 869 m to 1840 m. SO42− and NH4+ exhibited peak values 0.43-0.65 μm at SM, whereas maintained peak values 0.65-1.1 μm at LL. NO3− were mostly concentrated in fine mode for SM but in coarse mode for LL. Further analyses showed that at LL, the heterogeneous reaction on coarse particles containing more calcium and magnesium may explain the higher concentrations of NO3− in coarse mode and also the higher temperature may reduce the concentrations of NO3− in fine mode. Na+, Cl− and K+ exhibited bimodal size distributions; Ca2+ and Mg2+ showed maximum values in coarse mode. Aerosol acidity analysis showed a higher acidity of aerosol particles at LL when compared with those at SM. The average concentration of [H+] was relatively low when compared with those observed at two other mountains in China. This corresponds with the relatively low concentrations of SO42− and HSO4− and lower water content at Huangshan.