12 May 2022
12 May 2022
Status: this preprint is currently under review for the journal ACP.

Source Apportionment and Evolution of N-containing Aerosols at a Rural Cloud Forest in Taiwan by Isotope Analysis

Ting-Yu Chen1, Chia-Li Chen1, Yi-Chi Chen2, Charles C.-K. Chou3, Haojia Ren2, and Hui-Ming Hung1 Ting-Yu Chen et al.
  • 1Department of Atmospheric Sciences, National Taiwan University, Taipei, 10617 Taiwan
  • 2Department of Geosciences, National Taiwan University, Taipei, 10617 Taiwan
  • 3Research Center of Environmental Changes, Academia Sinica, Taipei, 11529 Taiwan

Abstract. Ammonium and nitrate are two major N-containing aerosol compositions. The deposition of N-containing aerosols has impacts on regional ecology and the biogeochemical cycle. In this study, aerosols in a rural cloud forest (Xitou in Taiwan) were studied using 15N and 18O isotope analysis to assess the sources and formation pathways of the local N-containing aerosols linking to a metropolitan. Aerosol samples were collected for different size ranges using a micro-orifice uniform deposit impactor (MOUDI) on a half-day basis in December 2018. The chemical functional groups were analyzed using a Fourier transformed infrared spectroscopy with attenuated total reflection technique (FTIR-ATR), while the isotope analysis was performed using a gas chromatography-isotope ratio mass spectrometer (GC-IRMS). The average measured aerosol concentration (PM10) was 0.98 (ranging from 0.15 to 3.31) and 0.25 (ranging from 0.00 to 1.51) μg/m3 for NH4+ and NO3-, respectively. In general, a higher functional group concentration was observed during the daytime by a factor of 1.5 to 6 than nighttime, likely due to the transportation of pollutants from upper stream urban and industrial regions through the local sea breeze combined with valley wind. The presence of fog can further elevate the concentration by a factor of 2 to 3, resulting from the stronger inversion and lower boundary layer height. The higher NH4+ concentration in fine particles under foggy conditions can further promote submicron-sized NO3- formation via aqueous phase dissolution with NH4+ neutralization. Furthermore, the higher RH during fog events shifted the mass distribution of aerosol functional groups to a larger mode size. By comparing the δ15N value directly or the analysis using a statistical isotope mixing model, MixSIAR, the NH4+ is probably originated from the industries, coal-fired power plants (CFPP), or fertilizer plants, while NO3- might be contributed from the CFPP, industrial or urban sources. The overall δ18O of NO3- is +72.66 ‰ ± 3.42 ‰, similar to that in other winter Asia studies, suggesting the major formation pathway via O3 oxidation (δ18O = +72.5 to 101.67 ‰). However, a lower δ18O (< +67 ‰) for particles less than 0.56 μm during foggy daytime suggests the local contribution via the peroxyl radical oxidation before partitioning into aerosol phase under foggy conditions. Overall, the δ15N and δ18O distribution profiles as a function of particle size in the studied rural forest site reveal the evolution of aerosol composition from remote coastal regions with chemical processes along the transport process, which can be further affected by weather conditions such as fog events.

Ting-Yu Chen et al.

Status: open (until 23 Jun 2022)

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Ting-Yu Chen et al.

Ting-Yu Chen et al.


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Short summary
The anthropogenic influence on aerosol composition in a downstream river-valley forest was investigated using FT-IR and isotope analysis. A higher N-containing species concentration during daytime fog events indicates that a stronger inversion leads to higher pollutant concentrations, and the fog enhances the aqueous-phase chemical processes. Moreover, the observed size-dependent oxygen isotope suggests the contribution of organic peroxyl radicals to local nitrate formation for small particles.