On-line analysis of free-tropospheric water-soluble acidic gases and particulate anions on the summit of Mt. Fuji, Japan

Atmospheric gaseous and particulate pollutants emitted from local or regional sources cause transboundary air pollution. To better understand this environmental issue, it is useful to monitor the free troposphere, reflecting the global air quality, with high temporal resolution. During the summer of 2016, we operated an original on-line monitor on the summit of Mt. Fuji, Japan (3776 m a.s.l.) to measure water-soluble acidic gases and particulate anions in the free troposphere. Mt. Fuji is a free-standing mountain and, therefore, hardly affected by air pollutants emitted from the local region. The lab-made gas/particle collector coupled ion chromatograph successfully provided meaningful data on an hourly basis. The average concentrations of HNO₃, SO₂, NO₃⁻, and SO₄²⁻ (n = 64 each) were, respectively 1.8 ± 0.9, 2.3 ± 2.5, 0.22 ± 0.16, and 3.9 ± 2.6 nmol m⁻³. The gaseous HNO₃ was the predominant form compared to particulate NO₃⁻. For the sulfur component, the percentage of particulate form was higher than that of the gaseous one. The high time-resolution monitoring enabled us to classify the observed data based on the air parcel inflow direction to the sampling point even when it changed dramatically. As a result, the oxidation of SO₂ to SO₄²⁻ in atmospheric transport can be discussed for each air parcel. It was suggested that the SO₂ oxidation was enhanced in the air parcels that passed over the Asian Continent.