Ammonia in the summertime Arctic marine boundary layer: sources, sinks, and implications
Item
Title (Dublin Core)
Ammonia in the summertime Arctic marine boundary layer: sources, sinks, and implications
Description (Dublin Core)
Continuous hourly measurements of gas-phase ammonia (NH<sub>3(g)</sub>) 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<sup>−3</sup> (40–870 pptv) with the highest values recorded in Lancaster Sound (74°13′ N, 84°00′ W). Simultaneous measurements of total ammonium ([NH<sub><i>x</i></sub>]), pH and temperature in the ocean and in melt ponds were used to compute the compensation point (<i>χ</i>), which is the ambient NH<sub>3(g)</sub> concentration at which surface–air fluxes change direction. Ambient NH<sub>3(g)</sub> was usually several orders of magnitude larger than both <i>χ</i><sub>ocean</sub> and <i>χ</i><sub>MP</sub> (< 0.4–10 ng m<sup>3</sup>) indicating these surface pools are net sinks of NH<sub>3</sub>. Flux calculations estimate average net downward fluxes of 1.4 and 1.1 ng m<sup>−2</sup> s<sup>−1</sup> for the open ocean and melt ponds, respectively. Sufficient NH<sub>3(g)</sub> was present to neutralize non-sea-salt sulfate (nss-SO<sub>4</sub><sup>2−</sup>) in the boundary layer during most of the study. This finding was corroborated with a historical data set of PM<sub>2.5</sub> composition from Alert, Nunavut (82°30′ N, 62°20′ W) wherein the median ratio of NH<sub>4</sub><sup>+</sup>/nss-SO<sub>4</sub><sup>2−</sup> 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<sub>3(g)</sub> emissions from seabird guano on boundary-layer composition and nss-SO<sub>4</sub><sup>2−</sup> neutralization. A GEOS-Chem simulation without seabird emissions underestimated boundary layer NH<sub>3(g)</sub> by several orders of magnitude and yielded highly acidic aerosol. A simulation that included seabird NH<sub>3</sub> emissions was in better agreement with observations for both NH<sub>3(g)</sub> concentrations and nss-SO<sub>4</sub><sup>2−</sup> neutralization. This is strong evidence that seabird colonies are significant sources of NH<sub>3</sub> 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<sub>3</sub>, 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.
Creator (Dublin Core)
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.
Date (Dublin Core)
2018-09-18
Type (Dublin Core)
Text
Format (Dublin Core)
application/pdf
Identifier (Dublin Core)
10.5194/acp-16-1937-2016
https://acp.copernicus.org/articles/16/1937/2016/
Source (Dublin Core)
eISSN: 1680-7324
Language (Dublin Core)
eng
