Abstract
Abstract. We present a comprehensive characterization of cloud condensation nuclei (CCN) sampled in the Alaskan Arctic during the 2008 Aerosol, Radiation, and Cloud Processes affecting Arctic Climate (ARCPAC) project, a component of the POLARCAT and International Polar Year (IPY) initiatives. Four distinct air mass types were sampled including a cleaner Arctic background and a relatively pristine sea ice boundary layer as well as biomass burning and anthropogenic pollution plumes. Despite differences in chemical composition, inferred aerosol hygroscopicities were fairly invariant and ranged from κ = 0.1–0.3 over the atmospherically-relevant range of water vapor supersaturations studied. Organic aerosols sampled were found to be well-oxygenated, consistent with long-range transport and aerosol aging processes. However, inferred hygroscopicities are less than would be predicted based on previous parameterizations of biogenic oxygenated organic aerosol, suggesting an upper limit on organic aerosol hygroscopicity above which κ is less sensitive to the O:C ratio. Most Arctic aerosols act as CCN above 0.1 % supersaturation, although the data suggest the presence of an externally-mixed, non-CCN-active mode comprising approximately 0–20% of the aerosol number. CCN closure was assessed using measured size distributions, bulk chemical composition, and assumed aerosol mixing states; CCN predictions tended toward overprediction, with the best agreement (±0–20 %) obtained by assuming the aerosol to be externally-mixed with soluble organics. Closure also varied with CCN concentration, and the best agreement was found for CCN concentrations above 100 cm−3 with a 1.5- to 3-fold overprediction at lower concentrations.
Highlights
The Arctic is sensitive to climatic changes because of the complex feedbacks between surface temperature and surface albedo, among other factors
While Ng et al (2010) define fX as the ratio of m/z X to only the oxygenated organic aerosol mass and this study uses the total organic mass measured by the C-ToF-AMS, direct comparison is reasonable because of the expected dominance of OOA in the aged air masses sampled during ARCPAC
We find that size effects dominate the cloud condensation nuclei (CCN) activity because of the accumulation mode size distribution, and that most particles act as CCN above 0.1 % supersaturation
Summary
The Arctic is sensitive to climatic changes because of the complex feedbacks between surface temperature and surface albedo, among other factors. Yum and Hudson (2001) measured the vertical distribution of CCN concentrations (0.04–0.8 % superaturation) approximately 500 km north of the Alaskan coast as part of the Arctic Clouds Experiment (ACE) and Surface Heat Budget of the Arctic Ocean (SHEBA) projects in May, 1998 They found that both CCN and total particle concentrations increased with altitude, on average, from less than 100 cm−3 in the boundary layer to 150–200 cm−3 aloft. Average CCN concentrations at all altitudes increased from roughly 10 cm−3 at 0.02 % supersaturation to 100 cm−3 at 0.1 % supersaturation and 200–300 cm−3 at 0.8 % supersaturation, and the authors attributed the relatively high CCN-active fraction of 0.63 and the slope of the CCN-supersaturation spectrum as indicative of an aged aerosol with few small particles (Yum and Hudson, 2001), and whose variability was likely due to cloud scavenging of higher-altitude aerosol layers that descended into the boundary layer (Wylie and Hudson, 2002) This is in contrast to the prevalent small particles and low CCN activation ratios observed by Hegg et al (1995).
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