Abstract

Particulate nitrogen has far-reaching negative effects on human health and the environment, and effective strategies for reducing it require understanding its sources and formation processes. To learn about these factors, we recorded size-resolved nitrogen isotope ratios (δ¹⁵N) of total particulate N at an urban site in northwest Germany during a four-week measuring campaign. We observed a steady decrease in δ¹⁵N when going from fine to coarse particles, with values between +18 ‰ and −2 ‰. This difference based on particle size is caused by different isotope fractionation processes during particle formation: The fine particles contain ammonium nitrate, which is formed in an equilibrium process, leading to an enrichment of ¹⁵N. Moreover, fine particles are more reactive due to their larger surface areas and relatively long residence times in the atmosphere, which leads to an additional enrichment of ¹⁵N; a key step of this process likely occurs when the ammonium particles interact with ammonia from agricultural sources. In contrast to fine particles, coarse particles are formed by direct absorption of HNO₃ on preexisting particles; the HNO₃ stems from traffic emissions of NO_x and subsequent oxidation in the atmospheric gas phase. Because only a small amount of isotope fractionation is associated with non-equilibrium processes during phase transitions, there is less ¹⁵N enrichment in the coarse particles. Overall, nitrogen isotopes clearly reflect the different formation processes of fine and coarse aerosol particles.

摘要

颗粒氮对人类健康和环境具有深远的负面影响，有效的减少它的策略需要了解它的来源和形成过程。为了了解这些因素，我们在为期 4 周的测量活动中记录了德国西北部城市站点的总颗粒 N 的尺寸分辨氮同位素比 (δ ¹⁵ N)。当从细颗粒到粗颗粒时，我们观察到 δ ¹⁵ N稳步下降，值在 +18 ‰ 和 -2 ‰ 之间。这种基于粒径的差异是由颗粒形成过程中不同同位素分馏过程引起的：细颗粒含有硝酸铵，硝酸铵在平衡过程中形成，导致富集¹⁵N。此外，细颗粒由于其较大的表面积和在大气中相对较长的停留时间而更具反应性，这导致额外的¹⁵ N富集；当铵颗粒与农业来源的氨相互作用时，该过程的关键步骤可能发生。与细颗粒​​相反，粗颗粒是由 HNO ₃直接吸附在预先存在的颗粒上形成的；HNO ₃源于交通排放的 NO _x和随后在大气气相中的氧化。因为只有少量的同位素分馏与相变过程中的非平衡过程有关，所以只有¹⁵粗颗粒中的 N 富集。总体而言，氮同位素清楚地反映了细颗粒和粗颗粒气溶胶颗粒的不同形成过程。