Real-time measurements of PM_2.5 water-soluble inorganic ions at a high-altitude mountain site in the western North Pacific: Impact of upslope wind and long-range transported biomass-burning smoke

Real-time measurement of PM_2.5 (particulate matter with aerodynamic diameter ≤ 2.5 μm) water-soluble inorganic ions (WSIIs) was conducted at Lulin Atmospheric Background Station (LABS; 2862 m above mean sea level) in the western North Pacific (WNP) during the autumn 2015 and spring 2016. During autumn, the mountain-valley (M-V) circulation is effective in increasing the mass concentrations of PM_2.5, WSIIs, and carbon monoxide (CO) at LABS from 12:00 to 18:00 local time every day. In sharp contrast to autumn, five high aerosol-loading events were recorded during the spring, with each event persisting for a few days. These events were harmonized with the long-range transport of biomass-burning (BB) smoke emissions from northern peninsular Southeast Asia (PSEA), as disclosed from the fire count map and backward trajectories. The plumes appear to mask their characteristic diurnal features driven by the local M-V circulation. During both seasons, the prominent pathway of SO₂ to SO₄²⁻ conversion was aqueous phase oxidation as revealed from the relationship between sulfur oxidation ratio (SOR) and aerosol liquid-water content. Furthermore, NO₃⁻ was found to be produced primarily via homogeneous reactions during the spring season. The optical measurements were also made along with the semi-continuous measurements of WSIIs in this study. The magnitude of aerosol scattering and absorption coefficients exhibited high values when LABS was either under the influence of M-V circulation or long-range BB transport, although the latter event was apparently greater. Large surface cooling (up to −30.8 W m⁻²) and atmospheric warming (up to +21.3 W m⁻²) at LABS attributed to absorption enhancement by BB-induced aerosols can potentially influence the regional radiation budget.