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Continuous hourly measurements of gas-phase ammonia (NH3(g)) were takenfrom 13 July to 7 August 2014 on a research cruise throughout Baffin Bay andthe eastern Canadian Arctic Archipelago. Concentrations ranged from 30 to 650 ng m−3 (40–870 pptv) with the highest values recorded in LancasterSound (74°13′ N, 84°00′ W). Simultaneous measurementsof total ammonium ([NHx]), pH and temperature in the ocean and in meltponds were used to compute the compensation point (χ), which is theambient NH3(g) concentration at which surface–air fluxes changedirection. Ambient NH3(g) was usually several orders of magnitudelarger than both χocean and χMP (< 0.4–10 ng m3) indicating these surface pools are net sinks of NH3. Fluxcalculations estimate average net downward fluxes of 1.4 and1.1 ng m−2 s−1 for the open ocean and melt ponds, respectively. SufficientNH3(g) was present to neutralize non-sea-salt sulfate(nss-SO42−) in the boundary layer during most of the study. Thisfinding was corroborated with a historical data set of PM2.5 compositionfrom Alert, Nunavut (82°30′ N, 62°20′ W) wherein the medianratio of NH4+/nss-SO42− equivalents was greater than0.75 in June, July and August. The GEOS-Chem chemical transport model wasemployed to examine the impact of NH3(g) emissions from seabird guanoon boundary-layer composition and nss-SO42− neutralization. AGEOS-Chem simulation without seabird emissions underestimated boundary layerNH3(g) by several orders of magnitude and yielded highly acidicaerosol. A simulation that included seabird NH3 emissions was in betteragreement with observations for both NH3(g) concentrations andnss-SO42− neutralization. This is strong evidence that seabirdcolonies are significant sources of NH3 in the summertime Arctic, andare ubiquitous enough to impact atmospheric composition across the entireBaffin Bay region. Large wildfires in the Northwest Territories were likelyan important source of NH3, but their influence was probably limited tothe Central Canadian Arctic. Implications of seabird-derived N-deposition toterrestrial and aquatic ecosystems are also discussed

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