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

Abstract

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 δ ¹³C of total carbon (TC) and δ ¹⁵N of NH ₄ ⁺ in the PM _2.5 collected in Seoul megacity during April 2018 to December 2019. The seasonal mean δ ¹³C values were similar to −25.1‰ ± 2.0‰ in warm and −24.2‰ ± 0.82‰ in cold seasons. Mean δ ¹⁵N 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 ¹⁵N 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.

Abstract

δ ¹⁵N of NH ₄ ⁺ and f _M and δ ¹³C of TC highlight the importance of vehicle emissions to the PM _2.5 mass increase in Seoul.