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This study investigates the concentration, cloud condensation nuclei (CCN)activity and hygroscopic properties of particles influenced by biomassburning in the eastern Mediterranean and their impacts on cloud dropletformation. Air masses sampled were subject to a range of atmosphericprocessing (several hours up to 3 days). Values of the hygroscopicityparameter, <i>κ</i>, were derived from CCN measurements and a HygroscopicTandem Differential Mobility Analyzer (HTDMA). An Aerosol ChemicalSpeciation Monitor (ACSM) was also used to determine the chemicalcomposition and mass concentration of non-refractory components of thesubmicron aerosol fraction. During fire events, the increased organiccontent (and lower inorganic fraction) of the aerosol decreases the valuesof <i>κ</i>, for all particle sizes. Particle sizes smaller than 80 nmexhibited considerable chemical dispersion (where hygroscopicity varied upto 100 % for particles of same size); larger particles, however, exhibitedconsiderably less dispersion owing to the effects of condensational growthand cloud processing. ACSM measurements indicate that the bulk compositionreflects the hygroscopicity and chemical nature of the largest particles(having a diameter of  ∼  100 nm at dry conditions) sampled.Based on positive matrix factorization (PMF) analysis of the organic ACSMspectra, CCN concentrations follow a similar trend as the biomass-burningorganic aerosol (BBOA) component, with the former being enhanced between 65and 150 % (for supersaturations ranging between 0.2 and 0.7 %) with thearrival of the smoke plumes. Using multilinear regression of the PMF factors(BBOA, OOA-BB and OOA) and the observed hygroscopicity parameter, theinferred hygroscopicity of the oxygenated organic aerosol components isdetermined. We find that the transformation of freshly emitted biomassburning (BBOA) to more oxidized organic aerosol (OOA-BB) can result in a2-fold increase of the inferred organic hygroscopicity; about 10 % of thetotal aerosol hygroscopicity is related to the two biomass-burningcomponents (BBOA and OOA-BB), which in turn contribute almost 35 % to thefine-particle organic water of the aerosol. Observation-derived calculationsof the cloud droplet concentrations that develop for typical boundary layercloud conditions suggest that biomass burning increases droplet number, onaverage by 8.5 %. The strongly sublinear response of clouds to biomass-burning (BB) influences is a result of strong competition of CCN for watervapor, which results in very low maximum supersaturation (0.08 % onaverage). Attributing droplet number variations to the total aerosol numberand the chemical composition variations shows that the importance ofchemical composition increases with distance, contributing up to 25 % ofthe total droplet variability. Therefore, although BB may stronglyelevate CCN numbers, the impact on droplet number is limited by water vaporavailability and depends on the aerosol particle concentration levelsassociated with the background

Biomass-burning impact on CCN number, hygroscopicity and cloud formation during summertime in the eastern Mediterranean