U.S. EPA, Office of Research and Development, Durham, NC, USA.
U.S. EPA, Office of Air and Radiation, Washington, DC, USA.
Sci Total Environ. 2020 Jan 1;698:133975. doi: 10.1016/j.scitotenv.2019.133975. Epub 2019 Aug 21.
This review summarizes the state of the science of measurements of dry deposition of reactive nitrogen (Nr) compounds in North America, beginning with current understanding of the importance of dry deposition at the U.S. continental scale followed by a review of micrometeorological flux measurement methods. Measurements of Nr air-surface exchange in natural ecosystems of North America are then summarized, focusing on the U.S. and Canada. Drawing on this synthesis, research needed to address the incompleteness of dry deposition budgets, more fully characterize temporal and geographical variability of fluxes, and better understand air-surface exchange processes is identified. Our assessment points to several data and knowledge gaps that must be addressed to advance dry deposition budgets and air-surface exchange modeling for North American ecosystems. For example, recent studies of particulate (NO) and gaseous (NO, HONO, peroxy nitrates) oxidized N fluxes challenge the fundamental framework of unidirectional flux from the atmosphere to the surface employed in most deposition models. Measurements in forest ecosystems document the importance of in-canopy chemical processes in regulating the net flux between the atmosphere and biosphere, which can result in net loss from the canopy. These results emphasize the need for studies to quantify within- and near-canopy sources and sinks of the full suite of components of the Nr chemical system under study (e.g., NO or HNO-NH-NHNO). With respect to specific ecosystems and geographical locations, additional flux measurements are needed particularly in agricultural regions (NH), coastal zones (NO and organic N), and arid ecosystems and along urban to rural gradients (NO). Measurements that investigate non-stomatal exchange processes (e.g., deposition to wet surfaces) and the biogeochemical drivers of bidirectional exchange (e.g., NH) are considered high priority. Establishment of long-term sites for process level measurements of reactive chemical fluxes should be viewed as a high priority long-term endeavor of the atmospheric chemistry and ecological communities.
本综述总结了北美的活性氮（Nr）化合物干沉降测量科学的现状，首先介绍了美国大陆尺度上干沉降的重要性，然后回顾了微气象通量测量方法。然后总结了北美的自然生态系统中 Nr 空气-表面交换的测量，重点是美国和加拿大。借鉴这一综合研究，确定了为解决干沉降预算不完整、更全面地描述通量的时间和地理变异性以及更好地了解空气-表面交换过程而需要开展的研究。我们的评估指出了几个数据和知识差距，必须解决这些差距才能推进北美的生态系统的干沉降预算和空气-表面交换模型。例如，最近关于颗粒态（NO）和气态（NO、HONO、过氧硝酸盐）氧化态 N 通量的研究挑战了大多数沉降模型中使用的从大气到表面的单向通量的基本框架。森林生态系统的测量记录了树冠内化学过程在调节大气和生物圈之间净通量方面的重要性，这可能导致净损失从树冠。这些结果强调需要进行研究以量化研究的 Nr 化学体系的完整成分（例如，NO 或 HNO3-NH-NHNO3）的冠层内和冠层附近的源和汇。关于特定的生态系统和地理位置，需要进行更多的通量测量，特别是在农业区（NH）、沿海地区（NO 和有机 N）以及干旱生态系统以及城市到农村梯度（NO）。考虑到非气孔交换过程（例如，沉积到湿表面）和双向交换的生物地球化学驱动因素（例如，NH）的测量被认为是高度优先事项。建立用于反应性化学通量过程水平测量的长期站点应被视为大气化学和生态社区的高度优先的长期努力。
