Ammonia in the summertime Arctic marine boundary layer: sources, sinks, and implications
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2016
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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.
| Reference Key |
wentworth2016ammoniaatmospheric
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| Authors | Wentworth, G. R.;Murphy, J. G.;Croft, B.;Martin, R. V.;Pierce, J. R.;Pierce, J. R.;Côté, J.-S.;Courchesne, I.;Tremblay, J.-É.;Gagnon, J.;Thomas, J. L.;Sharma, S.;Toom-Sauntry, D.;Chivulescu, A.;Levasseur, M.;Abbatt, J. P. D.; |
| Journal | atmospheric chemistry and physics |
| Year | 2016 |
| DOI |
DOI not found
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| URL | |
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