Nitrogen Isotope Fractionation from Ammonia Gas to Ammonium in Particulate Ammonium Chloride.

The nitrogen stable isotope ratios of NH3 gas (δ15N-NH3(g)) and particulate NH4+ (δ15N-NH4+(p)) have been used to determine the sources and elucidate the environmental dynamics of NH3(g) and NH4+(p). Some studies have determined the sources from δ15N-NH4+(p) in the ambient environment by using the nitrogen isotope fractionation value (Δδ15N). Studies of Δδ15N are limited and estimated the Δδ15N by theoretical calculation or laboratory experiments in closed systems. Here, we produced actual submicron-sized particles and exposed them to NH3(g) in a dynamic chamber under various experimental conditions of temperature and NH3(g) turnover rate. Δδ15N was lower when temperature was higher. For low turnover rates (about 0.9 per day), the Δδ15N was 31.6 ± 2.0‰ (mean ± 1SD), almost equal to the Δδ15N from theoretical calculations and laboratory tests in closed systems. Also, the relationship between Δδ15N, the temperature, and the turnover rate can be described in the equation form. This relationship can help to explain the discrepancies between Δδ15N values in closed and open systems. Our results could help to clarify the sources and mechanisms of particle formation.