Abstract
Abstract. 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°13′ N, 84°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 (χ), 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 χocean and χMP (< 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 NH3(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 PM2.5 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 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.
Highlights
Ammonia (NH3(g)) is the dominant alkaline gas in the atmosphere and is an important component of the global nitrogen cycle
Wentworth et al.: Ammonia in the summertime Arctic marine boundary layer to partition to acidic fine particulate matter (PM2.5) to form particulate-phase ammonium (NH+4(p)), which alters various aerosol properties, such as scattering efficiency (Martin et al, 2004), hygroscopicity (Petters and Kreidenweis, 2007), ice nucleating ability (Abbatt et al, 2006), and heterogeneous chemistry occurring on surfaces (Fickert et al, 1999)
Measured values of NH3(g) range between 30–650 ng m−3 with the highest values occurring in Lancaster Sound as the ship was steaming eastward into Baffin Bay
Summary
Ammonia (NH3(g)) is the dominant alkaline gas in the atmosphere and is an important component of the global nitrogen cycle. Its transport and deposition can have harmful effects for N-sensitive ecosystems such as eutrophication, loss of biodiversity and soil acidification (Krupa, 2003). The presence of NH3(g) can impact climate by increasing rates of new particle formation via stabilization of sulfuric acid clusters (Kirkby et al, 2011). Wentworth et al.: Ammonia in the summertime Arctic marine boundary layer to partition to acidic fine particulate matter (PM2.5) to form particulate-phase ammonium (NH+4(p)), which alters various aerosol properties, such as scattering efficiency (Martin et al, 2004), hygroscopicity (Petters and Kreidenweis, 2007), ice nucleating ability (Abbatt et al, 2006), and heterogeneous chemistry occurring on surfaces (Fickert et al, 1999)
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