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

Continuous hourly measurements of gas-phase ammonia (NH3(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 degrees 13'N, 84 degrees 00'W). Simultaneous measurements of total ammonium ([NHx]), pH and temperature in the ocean and in melt ponds were used to compute the compensation point (chi), which is the ambient NH3(g) concentration at which surface-air fluxes change direction. Ambient (NH3(g)) was usually several orders of magnitude larger than both chi(ocean) and chi(MP) (< 0.4-10 ngm(3)) indicating these surface pools are net sinks of NH3. Flux calculations estimate average net downward fluxes of 1.4 and 1.1 ngm(-2) s(-1) for the open ocean and melt ponds, respectively. Sufficient (NH3(g)) was present to neutralize non-seasalt sulfate (nss-SO42-) in the boundary layer during most of the study. This finding was corroborated with a historical data set of PM2.5 composition from Alert, Nunavut (82 degrees 30'N, 62 degrees 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 (NH3(g)) emissions from seabird guano on boundary-layer composition and nss-SO42- neutralization. A GEOS-Chem simulation without seabird emissions underestimated boundary layer (NH3(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 (NH3(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.