[Vertical Distributional Characteristics of Inorganic Ions of PM_2.5 at Mt. Huashan, Inland China]

PM_2.5 was simultaneously collected using a high-volume sampler at 4 h intervals from the mountainside and the foot of the mountain in the Mt. Huashan region, inland China, during the summer of 2016, and the samples were analyzed for inorganic ions, to investigate the chemical characteristics and vertical distribution of the fine particles in the region. The results showed that the concentrations of PM_2.5 were (46.9±38.2) μg·m^-3 and (76.0±44.3) μg·m^-3 on the mountainside and at the foot of Mt. Huashan, respectively. The concentrations of inorganic ions in PM_2.5 was higher at the foot of the mountain than on the mountainside, with the order of the mass concentrations of the major ionic species being SO₄^2- > NO₃^- > NH₄⁺ > Ca²⁺. Among all the inorganic ions, SO₄^2-, NO₃^-, and NH₄⁺ are the dominant species, accounting for 89% and 85% of the total, on the mountainside and at the foot, respectively. The fine particulate NH₄⁺ existed mostly in the forms of (NH₄)₂SO₄ and NH₄NO₃ on the mountainside, and in the forms of NH₄HSO₄ and NH₄NO₃ at the foot of the mountain. Mass concentrations of PM_2.5 and its major components on the mountainside showed clear diurnal variations, with maximums between 12:00-16:00, mainly due to the transport of the pollutants from the ground surface by the planetary boundary layer height variation and the valley breeze. In contrast, the diurnal variations of PM_2.5 and its major components at the foot are characterized with two maxima, peaking between 08:00-12:00 in the daytime and 00:00-04:00 in the nighttime, respectively, mainly due to the increased emissions from both the morning rush-hour traffic and the nighttime on-road heavy-duty vehicles. Acidity of the fine particles was estimated by using the equivalent ratio of anions to cations and the thermodynamic equilibrium model ISORROPIA Ⅱ. Both methods showed that the acidity of PM_2.5 at the ground surface site is stronger than that on the mountainside in the Mt. Huashan region.