Cloud condensation nuclei closure study on summer arctic aerosol
Date
2011
Authors
Martin, M.
Chang, R. Y-W
Sierau, B.
Sjogren, S.
Swietlicki, E.
Abbatt, J. P. D.
Leck, C.
Lohmann, U.
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Abstract
We present an aerosol - cloud condensation nuclei (CCN) closure study on summer high
Arctic aerosol based on measurements that were carried out in 2008 during the Arctic
Summer Cloud Ocean Study (ASCOS) on board the Swedish ice breaker Oden. The data
presented here were collected during a three-week time period in the pack ice (>
85 degrees N) when the icebreaker Oden was moored to an ice floe and drifted passively
during the most biological active period into autumn freeze up conditions. CCN number
concentrations were obtained using two CCN counters measuring at different
supersaturations. The directly measured CCN number concentration was then compared with
a CCN number concentration calculated using both bulk aerosol mass composition data from
an aerosol mass spectrometer (AMS) and aerosol number size distributions obtained from a
differential mobility particle sizer, assuming kappa-Kohler theory, surface tension of
water and an internally mixed aerosol. The last assumption was supported by measurements
made with a hygroscopic tandem differential mobility analyzer (HTDMA) for particles
> 70 nm. For the two highest measured supersaturations, 0.73 and 0.41 %, closure
could not be achieved with the investigated settings concerning hygroscopicity and
density. The calculated CCN number concentration was always higher than the measured one
for those two supersaturations. This might be caused by a relative larger insoluble
organic mass fraction of the smaller particles that activate at these supersaturations,
which are thus less good CCN than the larger particles. On average, 36% of the mass
measured with the AMS was organic mass. At 0.20, 0.15 and 0.10% supersaturation, closure
could be achieved with different combinations of hygroscopic parameters and densities
within the uncertainty range of the fit. The best agreement of the calculated CCN number
concentration with the observed one was achieved when the organic fraction of the
aerosol was treated as nearly water insoluble (kappa(org) = 0.02), leading to a mean
total kappa, kappa(tot), of 0.33 +/- 0.13. However, several settings led to closure and
kappa(org) = 0.2 is found to be an upper limit at 0.1% supersaturation. kappa(org)
<= 0.2 leads to a kappa(tot) range of 0.33 +/- 013 to 0.50 +/- 0.11. Thus, the
organic material ranges from being sparingly soluble to effectively insoluble. These
results suggest that an increase in organic mass fraction in particles of a certain size
would lead to a suppression of the Arctic CCN activity.
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Citation
Martin, M., R. Y-W Chang, B. Sierau, S. Sjogren, et al. 2011. "Cloud condensation nuclei closure study on summer arctic aerosol." Atmospheric Chemistry and Physics 11(22): 11335-11350.