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The NASA DC-8 aircraft characterized the aerosol properties, chemicalcomposition, and cloud condensation nuclei (CCN) concentrations of thesummertime Arctic during the 2008 NASA Arctic Research of the Composition ofthe Troposphere from Aircraft and Satellites (ARCTAS) campaign. Air massescharacteristic of fresh and aged biomass burning, boreal forest, Arcticbackground, and anthropogenic industrial pollution were sampled. Observationswere spatially extensive (50–85° N and 40–130° W) andexhibit significant variability in aerosol and CCN concentrations. Thechemical composition was dominated by highly oxidized organics (66–94% byvolume), with a water-soluble mass fraction of more than 50%. The aerosolhygroscopicity parameter, κ, ranged between κ = 0.08–0.32for all air mass types. Industrial pollution had the lowest κ of0.08 ± 0.01, while the Arctic background had the highest and mostvariable κ of 0.32 ± 0.21, resulting from a lower and morevariable organic fraction. Both fresh and aged (long-range transported)biomass burning air masses exhibited remarkably similar κ(0.18 ± 0.13), consistent with observed rapid chemical and physicalaging of smoke emissions in the atmosphere, even in the vicinity of freshfires. The organic hygroscopicity (κorg) was parameterizedby the volume fraction of water-soluble organic matter(εWSOM), with a κ = 0.12, such thatκorg = 0.12εWSOM. Assuming bulk(size-independent) composition and including the κorgparameterization enabled CCN predictions to within 30% accuracy for nearlyall environments sampled. The only exception was for industrial pollutionfrom Canadian oil sands exploration, where an external mixture andsize-dependent composition was required. Aerosol mixing state assumptions(internal vs. external) in all other environments did not significantlyaffect CCN predictions; however, the external mixing assumption provided thebest results, even though the available observations could not determine thetrue degree of external mixing and therefore may not always be representativeof the environments sampled. No correlation was observed betweenκorg and O : C. A novel correction of the CCN instrumentsupersaturation for water vapor depletion, resulting from high concentrationsof CCN, was also employed. This correction was especially important for freshbiomass burning plumes where concentrations exceeded1.5×104 cm−3 and introduced supersaturation depletions of≥25%. Not accounting for supersaturation depletion in these highconcentration environments would therefore bias CCN closure up to 25% andinferred κ by up to 50%

Analysis of CCN activity of Arctic aerosol and Canadian biomass burning during summer 2008