Evaluating The Impact of Increased Heavy Oil Consumption on Urban Pollution Levels through Isotope (δ¹³C, δ³⁴S, ¹⁴C) Composition

The impact of heavy fuel oil (HFO) on the chemical and isotopic composition of submicron particulate matter (PM₁) was investigated. For this purpose, we conducted an analysis of water-soluble inorganic ions (WSIIs) and multiple isotopes (δ³⁴S, δ¹³C, ¹⁴C) of PM₁ and SO₂ collected during two heating periods: before (2021–2022) and during the use of HFO (2022–2023) in Vilnius, Lithuania. The results showed that the combustion of HFO increased the concentrations of SO₂ (by 94%) and PM₁-related sulfate (by 30%). It also altered the chemical composition of PM₁, with sulfate becoming the predominant component (~40%) of WSIIs. The stable sulfur isotope ratios of SO₂ (δ³⁴S_SO2) and sulfate (δ³⁴S_PM1) shifted significantly to more negative values (δ³⁴S_SO2 = 0.4‰, δ³⁴S_PM1 = −0.3‰) compared to the previous heating period. Anticorrelation between δ¹³C and δ³⁴S values indicated increased contributions of ¹³C-enriched fossil fuel sources (coal and HFO) in EC, although the share of fossil fuels in elemental carbon (EC) slightly decreased during the HFO period. The combustion of HFO affected the concentrations of PM₁ chemical components and substantially impacted the isotopic composition and source contributions of sulfate and EC.