Ammonia in the summertime Arctic marine boundary layer: sources, sinks, and implications

• Published in: Atmospheric Chemistry and Physics
• Main Authors: G. R. Wentworth, J. G. Murphy, B. Croft, R. V. Martin, J. R. Pierce, J.-S. Côté, I. Courchesne, J.-É. Tremblay, J. Gagnon, J. L. Thomas, S. Sharma, D. Toom-Sauntry, A. Chivulescu, M. Levasseur, J. P. D. Abbatt
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2016
• Subjects: Physics; QC1-999; Chemistry; QD1-999; Arctic; Nunavut; Northwest Territories; Baffin Bay; Canadian Arctic Archipelago; Guano; Lancaster Sound; Arctic Archipelago; Baffin
• Online Access: https://doi.org/10.5194/acp-16-1937-2016; https://doaj.org/article/008a7a065f06430b99cc47d3c58a3fd4

Continuous hourly measurements of gas-phase ammonia (NH 3(g) ) were taken from 13 July to 7 August 2014 on a research cruise throughout Baffin Bay and the eastern Canadian Arctic Archipelago. Concentrations ranged from 30 to 650 ng m −3 (40–870 pptv) with the highest values recorded in Lancaster Sound (74°13′ N, 84°00′ W). Simultaneous measurements of total ammonium ([NH x ]), pH and temperature in the ocean and in melt ponds were used to compute the compensation point ( χ ), which is the ambient NH 3(g) concentration at which surface–air fluxes change direction. Ambient NH 3(g) was usually several orders of magnitude larger than both χ ocean and χ MP (< 0.4–10 ng m 3 ) indicating these surface pools are net sinks of NH 3 . Flux calculations estimate average net downward fluxes of 1.4 and 1.1 ng m −2 s −1 for the open ocean and melt ponds, respectively. Sufficient NH 3(g) was present to neutralize non-sea-salt sulfate (nss-SO 4 2− ) in the boundary layer during most of the study. This finding was corroborated with a historical data set of PM 2.5 composition from Alert, Nunavut (82°30′ N, 62°20′ W) wherein the median ratio of NH 4 + /nss-SO 4 2− equivalents was greater than 0.75 in June, July and August. The GEOS-Chem chemical transport model was employed to examine the impact of NH 3(g) emissions from seabird guano on boundary-layer composition and nss-SO 4 2− neutralization. A GEOS-Chem simulation without seabird emissions underestimated boundary layer NH 3(g) by several orders of magnitude and yielded highly acidic aerosol. A simulation that included seabird NH 3 emissions was in better agreement with observations for both NH 3(g) concentrations and nss-SO 4 2− neutralization. This is strong evidence that seabird colonies are significant sources of NH 3 in the summertime Arctic, and are ubiquitous enough to impact atmospheric composition across the entire Baffin Bay region. Large wildfires in the Northwest Territories were likely an important source of NH 3 , but their influence was probably limited to the ...