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

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Wentworth, Gregory R., Murphy, Jennifer G., Croft, Betty, Martin, Randall V., Pierce, Jeffrey R., Côté, Jean-Sébastien, Courchesne, Isabelle, Tremblay, Jean-Éric, Gagnon, Jonathan, Thomas, Jennie L., Sharma, Sangeeta, Toom-Sauntry, Desiree, Chivulescu, Alina, Levasseur, Maurice, Abbatt, Jonathan P. D.
• Format: Text
• Language: English
• Published: 2018
• Subjects: Arctic; Baffin Bay; Canadian Arctic Archipelago; Guano; Lancaster Sound; Northwest Territories; Nunavut; Arctic Archipelago; Baffin
• Online Access: https://doi.org/10.5194/acp-16-1937-2016; https://www.atmos-chem-phys.net/16/1937/2016/

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 Central Canadian Arctic. Implications of seabird-derived N-deposition to terrestrial and aquatic ecosystems are also discussed.