Robust Evidence of ¹⁴C, ¹³C, and ¹⁵N Analyses Indicating Fossil Fuel Sources for Total Carbon and Ammonium in Fine Aerosols in Seoul Megacity

Carbon- and nitrogen-containing aerosols are ubiquitous in urban atmospheres and play important roles in air quality and climate change. We determined the ¹⁴C fraction modern (f_M) and δ¹³C of total carbon (TC) and δ¹⁵N of NH₄⁺in the PM_2.5collected in Seoul megacity during April 2018 to December 2019. The seasonal mean δ¹³C values were similar to -25.1‰ ± 2.0‰ in warm and -24.2‰ ± 0.82‰ in cold seasons. Mean δ¹⁵N values were higher in warm (16.4‰ ± 2.8‰) than in cold seasons (4.0‰ ± 6.1‰), highlighting the temperature effects on atmospheric NH₃levels and phase-equilibrium isotopic exchange during the conversion of NH₃to NH₄⁺. While 37% ± 10% of TC was apportioned to fossil-fuel sources on the basis of f_Mvalues, δ¹⁵N indicated a higher contribution of emissions from vehicle exhausts and electricity generating units (power-plant NH₃slip) to NH₃: 60% ± 26% in warm season and 66% ± 22% in cold season, based on a Bayesian isotope-mixing model. The collective evidence of multiple isotope analysis reasonably supports the major contribution of fossil-fuel-combustion sources to NH₄⁺, in conjunction with TC, and an increased contribution from vehicle emissions during the severe PM_2.5pollution episodes. These findings demonstrate the efficacy of a multiple-isotope approach in providing better insight into the major sources of PM_2.5in the urban atmosphere.