Stable nitrogen isotopic signatures reveal the NH4+ evolution processes in pollution episodes in urban southwestern China

Abstract

In most of the urban atmosphere, the formation of ammonium (NH4+) is mainly controlled by the initial isotope value of ammonia (δ15N-NH3 values) and the corresponding ammonia (NH3) conversion ratio (NH4+/(NH4+ + NH3)), as N isotope fractionation may occur under an NH3-sufficient environment. The significant influence of δ15N-NH3 values on the formation of NH4+ in the urban atmosphere has been demonstrated by many previous studies; however, exploration of the effect of the NH3 conversion ratios on NH4+ formation has been limited, especially during pollution episodes. To better understand the NH3 sources and the effect of the NH3 conversion ratios on the formation of NH4+ in the urban environment, NH4+ concentrations (0.2 to 13.4 μg·m−3, average 3.9 ± 2.8 μg·m−3) and their δ15N values (−4.0‰ to 29.2‰, average 20.6‰ ± 6.1‰) in PM2.5 were determined in the urban area of Nanning from September 1 to December 31, 2017. Based on the measured δ15N-NH4+ values and the isotope fractionation that occurred during gas-to-particle partitioning, we estimated that the initial δ15N-NH3 values were − 22.8‰ to 3.1‰ (average − 8.8‰ ± 5.5‰). These initial δ15N-NH3 values were influenced by agricultural emission sources (−31.0‰ to −4.4‰), fossil fuel-related NH3 emission sources (−14.6‰ to 10.1‰) and biomass combustion sources (12‰ and 23‰). Source apportionment indicated that these three potential NH3 emission sources contributed 43%, 33% and 24% to the ambient NH3, respectively. On clean days, similar trends in initial δ15N-NH3 and δ15N-NH4+ values were observed, and there was an apparent positive linear correlation between these values (R2 = 0.70, p < 0.01). This suggested that the δ15N-NH4+ signatures recorded changes in the NH3 sources and that NH4+ formation was controlled by the NH3 sources on clean days. However, during different pollution episodes the variations in δ15N-NH4+ values were consistent with the NH4+ concentrations and the NH4+/(NH4+ + NH3) values, and there were negative linear correlation between these values (R2 > 0.6, p < 0.01). This suggested that the δ15N-NH4+ signatures recorded NH4+ evolution processes and that the increased NH3 conversion ratios was the main reason for the development of NH4+ and PM2.5 in pollution episodes. We inferred that the decreased air temperature in cold months may stimulate the increased NH3 conversion ratios, based on the negative correlation of NH4+/(NH4+ + NH3) values with air temperature (r = −0.63, p < 0.01).