Robust Evidence of C-14, C-13, and N-15 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 C-14 fraction modern (f(M)) and delta C-13 of total carbon (TC) and delta N-15 of NH4+ in the PM2.5 collected in Seoul megacity during April 2018 to December 2019. The seasonal mean delta C-13 values were similar to -25.1% +/- 2.0% in warm and -24.2%+/- 0.82% in cold seasons. Mean delta N-15 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 fossilfuel sources on the basis of f(M) values, delta N-15 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.