Citation:

Walker, J., G. Beachley, L. Zhang, K. Benedict, B. Sive, AND D. Schwede. A review of measurements of air-surface exchange of reactive nitrogen in natural ecosystems across North America. SCIENCE OF THE TOTAL ENVIRONMENT. Elsevier BV, AMSTERDAM, Netherlands, 698:133975, (2020).

Impact/Purpose:

Total reactive nitrogen (Nr) deposition budgets rely heavily on estimates of speciated fluxes derived from site specific inferential models, gridded chemical transport models, and hybrid approaches that fuse measurements and models. New measurements of dry deposition and bidirectional air-surface exchange are needed to develop more complete and accurate budgets of Nr deposition in sensitive ecosystems and to improve deposition models. In this paper we summarize the state of the science of measurements of dry deposition of Nr compounds, beginning with a review of the current understanding of the importance of dry deposition to total deposition at the continental scale, followed by a review of the micrometeorological and analytical methods used for direct measurements of air-surface exchange, and then summarizing the existing measurements of Nr air-surface exchange in natural and semi-natural ecosystems in North America. We then put forth recommendations for 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 to improve models used for deposition assessments.

Description:

Our review of existing North American flux datasets points to several data and knowledge gaps related to air-surface exchange processes that must be addressed to advance model algorithms. For example, recent studies of particulate (NO3-) and gas phase (NOx, HONO, peroxy nitrates) oxidized N fluxes challenge the fundamental conceptual framework of unidirectional flux from the atmosphere to the surface employed in field-scale and gridded chemical transport models. Measurements in several 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 to the atmosphere. 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., NOy or HNO3-NH3-NH4NO3) such that models can evolve to incorporate the most important underlying biological, chemical, and physical processes. With respect to specific ecosystems and geographical locations, additional flux measurements are needed in forest ecosystems, particularly deciduous forests, as well as high elevation and alpine environments, urban-to-rural gradients, agricultural regions, coastal zones, and arid ecosystems. Finally, while advances in analytical techniques have facilitated measurements of gas phase peroxy and alkyl nitrate fluxes in a few North American ecosystems, the contribution of organic N dry deposition to total N deposition remains poorly constrained.

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