National monitoring networks have reported an increase in ammonium (NH₄⁺) deposition, such that NH₄⁺ now dominates inorganic nitrogen deposition across most of the United States. Atmospheric deposition of fixed nitrogen has significant environmental consequences, including acidification and eutrophication. Thus, understanding source contributions is critical for formulating policies to mitigate the effects of excess nitrogen in sensitive ecosystems. Here, we investigated sources of wet-deposited NH₄⁺ (w-NH₄⁺) in Providence, RI, U.S.A., a mid-sized coastal city at the head of Narragansett Bay. We utilized concentration measurements, nitrogen stable isotopes (δ¹⁵N), and air mass back trajectory analysis for precipitation events and intra-event samples collected between January–November 2018. There was a general lack of seasonality in δ¹⁵N(w-NH₄⁺). Air mass origin had a strong influence on [w-NH₄⁺], but [w-NH₄⁺] was not related to δ¹⁵N(w-NH₄⁺), suggesting the potential dominance of a single NH₃ emission source type. An average δ¹⁵N(w-NH₄⁺) of −3.7 ± 3.5‰ (n = 42) was measured for daily-based precipitation. This value is consistent with previous measurements in the U.S. over the past 40 years, indicating a similar source of NH₃ across the U.S. likely derived from agricultural activities. Intra-event analysis from precipitation events collected throughout the year found frequent mid- and end-event [w-NH₄⁺] peaks that were suggested to be related to in-cloud [w-NH₄⁺] changes rather than changes in below-cloud scavenging due to a consistent corresponding shift in precipitation intensity. Large intra-event δ¹⁵N(w-NH₄⁺) variations as high as 15.9‰ were also observed. However, the cumulative mass-weighted δ¹⁵N(w-NH₄⁺) of the intra-event series tended to converge to a similar value (−4.8 ± 1.3‰; n = 6). Overall, our results indicate that long-range transport via in-cloud scavenging tended to play a stronger role in shaping w-NH₄⁺ patterns compared to local emissions at our study site.

国家监测网络报告了铵 (NH ₄ ⁺ ) 沉积的增加，因此 NH ₄ ⁺现在在美国大部分地区的无机氮沉积中占主导地位。固定氮的大气沉积具有显着的环境后果，包括酸化和富营养化。因此，了解来源的贡献对于制定政策以减轻敏感生态系统中过量氮的影响至关重要。在这里，我们调查了位于美国罗德岛州普罗维登斯的湿沉积 NH ₄ ⁺ (w-NH ₄ ⁺ ) 的来源，这是一个位于纳拉甘西特湾头的中型沿海城市。我们利用浓度测量、氮稳定同位素 (δ ¹⁵N)，以及对 2018 年 1 月至 11 月期间收集的降水事件和事件内样本的气团反向轨迹分析。δ ¹⁵ N(w-NH ₄ ⁺ )普遍缺乏季节性。气团起源对[w-NH ₄ ⁺ ]有很强的影响，但[w-NH ₄ ⁺ ]与δ ¹⁵ N(w-NH ₄ ⁺ )无关，表明单一NH ₃排放源的潜在优势类型。平均δ ¹⁵ N(w-NH ₄ ⁺ ) -3.7 ± 3.5‰ ( n = 42) 是针对每日降水量进行测量的。该值与美国过去 40 年的测量结果一致，表明美国各地的 NH ₃来源类似，可能来自农业活动。对全年收集的降水事件进行的事件内分析发现，频繁的事件中期和结束 [w-NH ₄ ⁺ ] 峰值被认为与云内 [w-NH ₄ ⁺ ] 变化有关，而不是以下变化- 由于降水强度的一致相应变化而导致的云清除。还观察到了高达 15.9‰ 的大事件内 δ ¹⁵ N(w-NH ₄ ⁺ ) 变化。然而，累积质量加权 δ ¹⁵事件内序列的N(w-NH ₄ ⁺ ) 趋于收敛到相似的值 (-4.8 ± 1.3‰; n  = 6)。总体而言，我们的结果表明，与我们研究地点的本地排放相比，通过云内清除的远程传输往往在塑造 w-NH ₄ ⁺模式方面发挥更大的作用。