Sulfur isotope-based source apportionment and control mechanisms of PM sulfate in Seoul, South Korea during winter and early spring (2017-2020).

High level of particulate matter (PM) concentrations are a major environmental concern in Seoul, South Korea, especially during winter and early spring. Sulfate is a major component of PM and induces severe environmental pollution, such as acid precipitation. Previous studies have used numerical models to constrain the relative contributions of domestic and trans-boundary sources to PM sulfate concentration in South Korea. Because of the scarce measurement result of δS for PM sulfate in South Korea, poorly defined δS value of domestic sulfur sources, and no application of sulfur isotope fractionation during sulfate formation in previous observation-based studies, source apportionment results conducted by model studies have not been corroborated from independent chemical observations. Here, we examined the δS of PM in Seoul and domestic sulfur sources, and considered the sulfur isotope fractionation for accurate source apportionment constraint. Accordingly, domestic and trans-boundary sulfur sources accounted for approximately (16-32) % and (68-84) % of the sulfate aerosols in Seoul, respectively, throughout the winter and early spring of 2017-2020. Air masses passing through north-eastern China had relatively low sulfate concentrations, enriched δS, and a low domestic source contribution. Those passing through south-eastern China had relatively a high sulfate concentrations, depleted δS, and high domestic source contribution. Furthermore, elevated PM sulfate concentrations (>10 μg m) were exclusively associated with a weak westerly wind speed of <3 m s. From December 2019 to March 2020, Seoul experienced relatively low levels of PM sulfate, which might be attributed to favorable weather conditions rather than the effects of COVID-19 containment measures. Our results demonstrate the potential use of δS for accurate source apportionment and for identifying the crucial role of regional air mass transport and meteorological conditions in PM sulfate concentration. Furthermore, the data provided can be essential for relevant studies and policy-making in East Asia.