Predictions of cloud droplet activation in the late summertime
(September) central Arctic Ocean are made using κ-Köhler theory with novel observations of the aerosol chemical
composition from a high-resolution time-of-flight chemical ionization
mass spectrometer with a filter inlet for gases and aerosols (FIGAERO-CIMS)
and an aerosol mass spectrometer (AMS), deployed during the Arctic Ocean 2018 expedition onboard the Swedish icebreaker Oden. We find that the hygroscopicity parameter κ of the total aerosol is 0.39 ± 0.19 (mean ±
std). The predicted activation diameter of ∼25 to 130 nm particles
is overestimated by 5%, leading to an underestimation of the cloud
condensation nuclei (CCN) number concentration by 4–8%. From
this, we conclude that the aerosol in the High Arctic late summer
is acidic and therefore highly cloud active, with a substantial CCN
contribution from Aitken mode particles. Variability in the predicted
activation diameter is addressed mainly as a result of uncertainties
in the aerosol size distribution measurements. The organic κ
was on average 0.13, close to the commonly assumed κ of 0.1, and therefore did not significantly influence the predictions.
These conclusions are supported by laboratory experiments of the activation
potential of seven organic compounds selected as representative of
the measured aerosol