Abstract
Abstract. Cloud condensation nuclei (CCN) size distributions and numbers were measured for the first time at Puy-de-Dôme high altitude (1465 m a.s.l) site in Central France. Majority of the measurements were done at constant supersaturation (SS) of 0.24%, which was also deduced to be representative of the typical in-cloud SS at the site. CCN numbers during summer ranged from about 200 up to 2000 cm−3 and during winter from 50 up to 3000 cm−3. Variability of CCN number was explained by both particle chemistry and size distribution variability. The higher CCN concentrations were measured in continental, in contrast to marine, air masses. Aerosol CCN activity was described with a single hygroscopicity parameter κ. Range of this parameter was 0.29 ± 0.13 in summer and 0.43 ± 0.19 in winter. When calculated using SS of 0.51% during summer, κ of 0.22 ± 0.07 was obtained. The decrease with increasing SS is likely explained by the particle size dependent chemistry with smaller particles containing higher amounts of freshly emitted organic species. Higher κ values during winter were for the most part explained by the observed aged organics (analysed from organic m/z 44 ratio) rather than from aerosol organic to inorganic volume fraction. The obtained κ values also fit well within the range of previously proposed global continental κ of 0.27 ± 0.21. During winter, the smallest κ values and the highest organic fractions were measured in marine air masses. CCN closure using bulk AMS chemistry led to positive bias of 5% and 2% in winter and summer, respectively. This is suspected to stem from size dependent aerosol organic fraction, which is underestimated by using AMS bulk mass composition. Finally, the results were combined with size distributions measured from interstitial and whole air inlets to obtain activated droplet size distributions. Cloud droplet number concentrations were shown to increase with accumulation mode particle number, while the real in-cloud SS correspondingly decreased. These results provide evidence on the effects of aerosol particles on maximum cloud supersaturations. Further work with detailed characterisation of cloud properties is proposed in order to provide more quantitative estimates on aerosol effects on clouds.
Highlights
The global climate system is currently undergoing major changes due to the impacts of human actions (IPCC, 2007)
We examine three principal questions: (1) what is the range of measured cloud condensation nuclei (CCN) numbers during summer and winter seasons, (2) how do the particle cloud activation properties change with changing air masses, air chemistry, and environmental conditions, and (3) are the CCN numbers measured representative for observed particle in-cloud activation and cloud droplet number concentrations? Within these themes, our specific goals are to obtain a closure between measured particle chemistry and CCN numbers as well as to indirectly determine the representative cloud supersaturations in each of the cloud events
General overviews of summer and winter campaign meteorological conditions along with the measured CN and CCN concentrations are presented in Figs. 2 and 3
Summary
The global climate system is currently undergoing major changes due to the impacts of human actions (IPCC, 2007). The role of anthropogenic aerosols in this climate system is known to be important, though it remains inadequately defined. The aerosol interactions with clouds inflict several complex climatic modifications. Better understanding of the aerosol effects on clouds and of the aerosol–cloud interaction processes would largely improve the future climate predictions. The ability of an aerosol particle to act as a cloud condensation nuclei (CCN) depends largely on its size and chemical composition (or water solubility). One of the ways to describe the size independent ability of aerosols to interact with water
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