Chemical characteristics of PM 1 /PM 2.5 and influence on visual range at the summit of Mount Tai, North China.

Daytime and night-time PM1 and PM2.5 samples were simultaneously collected at the summit of Mount Tai during summer and autumn 2014. The mass concentrations and chemical compositions were analysed to determine the temporal variations of PM1 and PM2.5 and their contributions to visibility impairment. In summer, the average mass concentrations of PM1 and PM2.5 were 38.16μg/m3 and 53.33μg/m3 , respectively. In autumn, the values were 42.75μg/m3 and 59.16μg/m3 . Water-soluble inorganic ions were the most abundant species in both PM1 and PM2.5 , followed by organic mass (OM). Among the major water-soluble ions, SO4 2- and NH4 + had higher concentrations in summer than in autumn, whereas the concentration of NO3 - showed the opposite seasonal trend. Lower concentrations of organic carbon (OC), elemental carbon (EC) and water-soluble organic carbon (WSOC) were obtained in summer than in autumn. The water-soluble components (SO4 2- , NO3 - , NH4 + and WSOC) showed a clear diurnal variation due to the specific meteorological conditions of Mount Tai. The water content in PM1 and PM2.5 was enhanced by the water-soluble components, especially NH4 NO3 . The decreased visibility resulted from the combined influence of particulate matter and relative humidity (RH). The threshold PM2.5 concentration corresponding to a visibility of <10km was 56.60μg/m3 , which decreased with an increase in RH. A revised IMPROVE equation was applied to estimate the light-extinction coefficient bext , which was found to be lower for these chemicals in autumn (364Mm-1 ) than in summer (482Mm-1 ). (NH4 )2 SO4 made the largest contribution to bext in both summer and autumn, with an average rate of 56.97%. OM (17.32%) and NH4 NO3 (15.13%) were also important contributors, with similar contribution rates. The contribution of NH4 NO3 to bext was higher during summer, and OM contributions were higher during autumn.