Abstract
Active collection methods such as ChemComb Speciation Cartridge samplers (CCSCs) are considered more accurate for determining the isotopic composition of ammonia (NH3) and particulate ammonium (NH4+) (i.e., δ15N(NH3) and δ15N(NH4+)), especially in ambient air. However, the discrepancy in δ15N(NH4+) values are unknown between speciated particle samples collected using lab-verified CCSCs with different filter types (Teflon, cellulose, and glass fiber) and particles samples collected separately without gas-particle speciation and a backup/breakthrough filter. In this study, we deployed two CCSCs to collect speciated NH4+, and an EPA-approved Spiral Ambient Speciated Sampler (SASS) side-by-side to collect ambient NH4+ and to compare their δ15N(NH4+) values in urban Beijing during summer 2019. The initial δ15N(NH3) values were derived from δ15N(NH4+) values for speciated NH4+ based on theoretical estimation of isotopic fractionation that could occur during NH3 gas-to-particle conversion, or field-based isotopic fractionation, measured δ15N(NH3) values and observed NH4+ partitioning during NH3 and NH4+ speciation. The results show that the mean δ15N(NH4+) values retained in Teflon filters were 10.2 ± 8.4‰ (n=7) and 12.4 ± 4.1‰ (n=7) for CCSCs A and CCSCs B, respectively. In contrast, the measured δ15N(NH4+) values in SASS quartz filters were 15.9 ± 2.5‰ (n=7). The higher δ15N(NH4+) values observed in particle samples collected using SASS can be attributed to a positive bias resulting from NH4+ volatilization. Additionally, this study revealed that the initial δ15N(NH3) values, derived using theoretically derived isotopic fractionation, were 5.6‰ and 10.1‰ lower compared to the initial δ15N(NH3) values derived using field-based isotopic fractionation in CCSCs A and B, respectively. These findings underscore the significance of using acid-coated backup filters in active sampling systems to effectively capture volatilized NH3. Additionally, they highlight the necessity of utilizing field-based isotopic fractionation to accurately predict initial δ15N(NH3) values, thereby reducing uncertainties in source apportionment of NH4+ in ambient air. This approach is essential for obtaining reliable isotopic data for understanding the sources and processes contributing to NH4+ in the atmosphere.
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