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 (fM) and 13C of total carbon (TC) and 15N of NH4+ in the PM2.5 collected in Seoul megacity during April 2018 to December 2019. The seasonal mean 13C values were similar to -25.1‰ ± 2.0‰ in warm and -24.2‰ ± 0.82‰ in cold seasons. Mean 15N values were higher in warm (16.4‰ ± 2.8‰) than in cold seasons (4.0‰ ± 6.1‰), highlighting the temperature effects on atmospheric NH3 levels and phase-equilibrium isotopic exchange during the conversion of NH3 to NH4+. While 37% ± 10% of TC was apportioned to fossil-fuel sources on the basis of fM values, 15N indicated a higher contribution of emissions from vehicle exhausts and electricity generating units (power-plant NH3 slip) 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 PM2.5 pollution episodes. These findings demonstrate the efficacy of a multiple-isotope approach in providing better insight into the major sources of PM2.5 in the urban atmosphere.