Stable Nitrogen Isotopic Signatures Reveal The Nh4+ Evolution Processes In Pollution Episodes In Urban Southwestern China

In most of the urban atmosphere, the formation of ammonium (NH4+) is mainly controlled by the initial isotope value of ammonia (delta N-15-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 delta N-15-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 mu g.m(-3), average 3.9 +/- 2.8 mu g.m(-3)) and their delta N-15 values (-4.0 parts per thousand to 29.2 parts per thousand, average 20.6 parts per thousand +/- 6.1 parts per thousand) in PM2.5 were determined in the urban area of Nanning from September 1 to December 31, 2017. Based on the measured delta N-15-NH4+ values and the isotope fractionation that occurred during gas-to-particle partitioning, we estimated that the initial delta N-15-NH3 values were -22.8 parts per thousand to 3.1 parts per thousand (average -8.8 parts per thousand +/- 5.5 parts per thousand). These initial delta N-15-NH3 values were influenced by agricultural emission sources (-31.0 parts per thousand to -4.4 parts per thousand), fossil fuel-related NH3 emission sources (-14.6 parts per thousand to 10.1 parts per thousand) and biomass combustion sources (12 parts per thousand and 23 parts per thousand). 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 815N-NH3 and delta N-15-NH4+ values were observed, and there was an apparent positive linear correlation between these values (R-2 = 0.70, p 0.01). This suggested that the delta N-15-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 delta N-15-NH4+ values were consistent with the NH4+ concentrations and the NH4+/(NH4+ + NH3) values, and there were negative linear correlation between these values (R-2 > 0.6, p < 0.01). This suggested that the delta N-15-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).