Modeling the Impact of the Bidirectional Exchange of NH3 From the Great Lakes on a Regional and Local Scale Using GEM-MACH

Abstract

Ammonia emissions from oceans are recognized as one of the most significant natural sources of ammonia globally; however, freshwater sources are rarely considered significant. The Great Lakes region, containing the largest network of freshwater lakes in the world, and a significant urbanized population exceeding 20 million, provides a unique opportunity to evaluate the potential for lacustrine (lake-associated) surfaces to contribute to regional ammonia levels. In this work, we combine an analysis of 20 years of water quality data from the Great Lakes region with the GEM-MACH (Global Environmental Multiscale (GEM)-Modelling Air quality and CHemistry (MACH)) chemical transport model to examine the influence of the Great Lakes on atmospheric ammonia. This analysis demonstrates that while regional ammonia levels are largely controlled by known terrestrial anthropogenic sources, lacustrine surfaces with an emission potential of only 200 increase summertime (July–September) monthly average ammonia (NH₃) levels by 5%–8% over the largest regional urban centers, with daily increases of up to 10%–20%. Supplemental water measurements collected from within 1 km offshore of the Greater Toronto Area were found to have an emission potential of 2000, suggesting that lacustrine emissions offshore of large urban areas could be significantly larger than those predicted by GEM-MACH. Our findings reveal that the Great Lakes may represent a regionally significant natural source of ammonia to the atmosphere.