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

Daytime and night-time PM and PM 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 PM and PM and their contributions to visibility impairment. In summer, the average mass concentrations of PM and PM were 38.16μg/m and 53.33μg/m, respectively. In autumn, the values were 42.75μg/m and 59.16μg/m. Water-soluble inorganic ions were the most abundant species in both PM and PM, followed by organic mass (OM). Among the major water-soluble ions, SO and NH had higher concentrations in summer than in autumn, whereas the concentration of NO 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 (SO, NO, NH and WSOC) showed a clear diurnal variation due to the specific meteorological conditions of Mount Tai. The water content in PM and PM was enhanced by the water-soluble components, especially NHNO. The decreased visibility resulted from the combined influence of particulate matter and relative humidity (RH). The threshold PM concentration corresponding to a visibility of <10km was 56.60μg/m, which decreased with an increase in RH. A revised IMPROVE equation was applied to estimate the light-extinction coefficient b, which was found to be lower for these chemicals in autumn (364Mm) than in summer (482Mm). (NH)SO made the largest contribution to b in both summer and autumn, with an average rate of 56.97%. OM (17.32%) and NHNO (15.13%) were also important contributors, with similar contribution rates. The contribution of NHNO to b was higher during summer, and OM contributions were higher during autumn.