The Arctic region is vulnerable to climate change and able to affect global climate. The summertime Arctic atmosphere is pristine and strongly influenced by natural regional emissions, which have poorly understood climate impacts related to atmospheric particles and clouds. Here we show that ammonia from seabird-colony guano is a key factor contributing to bursts of newly formed particles, which are observed every summer in the near-surface atmosphere at Alert, Nunavut, Canada. Our chemical-transport model simulations indicate that the pan-Arctic seabird-influenced particles can grow by sulfuric acid and organic vapour condensation to diameters sufficiently large to promote pan-Arctic cloud-droplet formation in the clean Arctic summertime. We calculate that the resultant cooling tendencies could be large (about −0.5 W m−2 pan-Arctic-mean cooling), exceeding −1 W m−2 near the largest seabird colonies due to the effects of seabird-influenced particles on cloud albedo. These coupled ecological–chemical processes may be susceptible to Arctic warming and industrialization.
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 30–650 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(g). 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 sulphate (nss-SO42-) in the boundary layer during most of the study. This finding was corroborated with a historical dataset of PM2.5 composition from Alert, NU (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(g) 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(g), 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.
Abstract. Measurements at high-Arctic sites show a strong seasonal cycle in aerosol number and size. The number of aerosols with diameters larger than 20 nm exhibits a maximum in late spring associated with a dominant accumulation mode (0.1 to 1 μm in diameter), and a second maximum in the summer associated with a dominant Aitken mode (10 to 100 nm in diameter). Seasonal-mean aerosol effective diameter ranges from about 180 nm in summer to 260 nm in winter. This study interprets these seasonal cycles with the GEOS-Chem-TOMAS global aerosol microphysics model. We find improved agreement with in-situ measurements of aerosol size at both Alert, Nunavut, and Mt. Zeppelin, Svalbard following model developments that: (1) increase the efficiency of wet scavenging in the Arctic summer and (2) represent coagulation between interstitial aerosols and aerosols activated to form cloud droplets. Our simulations indicate that the dominant summertime Aitken mode is associated with increased efficiency of wet removal, which limits the number of larger aerosols and promotes local new-particle formation. We also find an important role of interstitial coagulation in clouds in the Arctic, which limits the number of Aitken-mode aerosols in the non-summer seasons when direct wet removal of these aerosols is inefficient. Total aerosol number reaches a minimum in October at both Alert and Mt. Zeppelin. Our simulations indicate that this October minimum can be explained by diminishing local new-particle formation, limited transport of pollution from lower latitudes, and efficient wet removal. We recommend that the key processes of aerosol wet removal, interstitial coagulation and new-particle formation be carefully considered in size-resolved aerosol simulations of the Arctic. Uncertainties about these processes, which strongly control the seasonal cycle of aerosol number and size, limit confidence in estimates of aerosol radiative effects on the Arctic climate.
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