Regime shift in aerosol ammonium between the rainy and dry season: Perspective from stable isotopes in bulk deposition

Abstract

Atmospheric ammonia (NH3) is a precursor of the ammonium (NH4+) in rainwater and aerosol. However, relations among them are largely unknown, which preventing our understanding from the sources and spatio-temporal variability of the NH4+ in rainwater and aerosol. Concentration and stable nitrogen isotope (δ15N) of NH4+ were measured in bulk deposition to reveal the relations between rainwater-NH4+ and aerosol-NH4+ and identify NH3 sources in a tropical coastal city, Zhanjiang. During the dry season, δ15N values of NH4+ in the aerosol (aerosol-δ15N–NH4+) were enriched with increased air temperature and consistently heavier than those in the rainwater (rainwater-δ15N–NH4+), suggesting that relatively high air temperature in the tropics were dominant regulator in the dry season. The negative correlation in the Rayleigh model also evidenced that air temperature caused a kinetic fractionation (fractionation effect: −4.8 ± 0.7‰) in aerosol. By contrast, rainfall was a dominant factor on aerosol and rainwater in the rainy season, as suggested by the progressively enriched aerosol-δ15N–NH4+ and depleted rainwater-δ15N–NH4+. These scenarios were ascribed to isotopic exchange (fractionation effect: +1.0 ± 0.4‰), indicating by positive coefficients in the Rayleigh model. After considering the isotopic fractionation in aerosol, the major sources of NH3 in Zhanjiang calculated by the SIAR mixing model were volatilization of livestock waste and fertilizer (>95% in both season), followed by fossil fuel combustion (2% in the rainy season and 3% in the dry season), and biomass burning (1% in the rainy season and 1% in the dry season).