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

[Image: see text] Carbon- and nitrogen-containing aerosols are ubiquitous in urban atmospheres and play important roles in air quality and climate change. We determined the (14)C fraction modern (f(M)) and δ(13)C of total carbon (TC) and δ(15)N of NH(4)(+) in the PM(2.5) collected in Seoul megacity during April 2018 to December 2019. The seasonal mean δ(13)C values were similar to −25.1‰ ± 2.0‰ in warm and −24.2‰ ± 0.82‰ in cold seasons. Mean δ(15)N values were higher in warm (16.4‰ ± 2.8‰) than in cold seasons (4.0‰ ± 6.1‰), highlighting the temperature effects on atmospheric NH(3) levels and phase-equilibrium isotopic exchange during the conversion of NH(3) to NH(4)(+). While 37% ± 10% of TC was apportioned to fossil-fuel sources on the basis of f(M) values, δ(15)N indicated a higher contribution of emissions from vehicle exhausts and electricity generating units (power-plant NH(3) slip) to NH(3): 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 NH(4)(+), in conjunction with TC, and an increased contribution from vehicle emissions during the severe PM(2.5) pollution episodes. These findings demonstrate the efficacy of a multiple-isotope approach in providing better insight into the major sources of PM(2.5) in the urban atmosphere.