The formation and transport of nitrogen-containing species in aerosols over central mountain area of Taiwan using isotope analysis

Particulate matter (PM) is one major air pollutant that affects human health and the radiation balance of the earth. Thus, it is essential to identify the sources of air pollutants to provide feasible control strategies. In this study, we investigated the size-dependent ¹⁵N and ¹⁸O isotope ratio of N-containing species in aerosols to specify their sources, transport, and formation processes. Aerosol samples of different size ranges were collected using a micro-orifice uniform deposit impactor (MOUDI) on a half-day basis over Xitou Experimental Forest of National Taiwan University (23.40°N, 120.47°E, 1178 m a.s.l.) site at the valley southwest to the central Metropolitan of Taiwan in April 2021. Due to its location and topography, Xitou is downstream of the local circulation, which is dominated by the land-sea breeze and mountain-valley wind and brings the pollutants from the coastal industrial and agricultural activities to the forest during the daytime. Therefore, the samples collected at Xitou are a mixture of complex information. Chemical functional groups measurement was performed using Fourier-transform infrared spectroscopy with attenuated total reflection (FTIR-ATR) technique beforehand to provide a grasp of the concentration-size distribution for both nitrate and ammonium as a reference to ensure sufficient nitrogen requirement for further isotope analysis at gas chromatography–isotope ratio mass spectrometer (GC-IRMS). The daily average concentration is 3.78±1.82 and 2.47±2.47 ug/m³ for ammonium (NH₄⁺) and nitrate (NO₃^‑), respectively. The concentration during daytime is higher than at nighttime by a factor of 1.3-1.8. The result suggests that pollutants brought by the sea breeze windward contribute to nitrogen-containing aerosols. During a persistent 24-hour weak wind fog event, a significant concentration decreases for both substances (NH₄⁺: 5.34 to 2.12 ug/m³ and NO₃^‑: 4.62 to 0.56 ug/m³) in PM10, likely due to sedimentation. The observed δ¹⁵N in NO₃^‑ increasing with diameter suggests NO₃^‑ at larger particles formed at the upper stream and NO₃^‑ at finer particles formed locally. On the other hand, δ¹⁸O in nitrate shows a similar trend which might be the contribution of RO₂ as the oxidant locally. As NH₄⁺ in aerosols is contributed by ammonia partitioning, δ¹⁵N-NH₄⁺ only reflects the fractionation process during phase change and initial emission. The size-dependent trend of δ¹⁵N-NH₄⁺ shows similar behavior to our previous study in December 2018 and reflects the time points of partitioning. Furthermore, the quantitative analysis of the transport and formation processes based on the size-dependent isotope will be deconvoluted to understand the partitioning of N-containing species in aerosols, which would be necessary for the pollution control strategy and their impact evaluation.