Dual carbon and sulfur isotopes as tracers of PM1 pollution sources after COVID-19 confinement in Vilnius, Lithuania

Particulate matter levels can change dramatically in an urban environment as the contribution of anthropogenic sources varies due to pollution reduction strategies and pandemic restrictions. Concentrations and isotopic compositions of sulfur and total carbon (TC) were measured for PM1 samples collected during the COVID-19 quarantine (2020-11-11–2021-03-16) in Vilnius, Lithuania. In this study, dual carbon (14C/12C, 13C/12C) and sulfur isotope analysis (34S/32S) was applied to quantify the contribution of fossil and non-fossil emissions to carbonaceous and sulfur-containing PM1. The contribution of fossil and non-fossil total carbon sources was equal to 23 ± 6% and 77 ± 6%, respectively. TC concentrations before and during the pandemic were in the same range, but isotopic composition (δ13C) was affected. The contribution of fossil fuel sources was 1.3 times smaller than reported previously during the years 2014–2015. In current study, dominant sulfate sources were coal combustion (60 ± 10%) and biomass burning (40 ± 10%). However, biomass burning becomes the predominant local source (up to 60%) of PM1-related sulfate in Lithuania associated with increased wintertime household heating activities. The origin of air masses was additionally considered to interpret changes in isotopic composition and source contributions of PM1.