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

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_<i>x</i>]), 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_3(g) concentration at which surface–air fluxes change direction. Ambient NH_3(g) was usually several orders of magnitude larger than both <i>χ</i>_ocean and <i>χ</i>_MP (&lt; 0.4–10 ng m³) indicating these surface pools are net sinks of NH₃. 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-SO₄²⁻) 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 NH₄⁺/nss-SO₄²⁻ 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-SO₄²⁻ 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 NH₃ emissions was in better agreement with observations for both NH_3(g) concentrations and nss-SO₄²⁻ neutralization. This is strong evidence that seabird colonies are significant sources of NH₃ 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 NH₃, 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