Suppression in Droplet Growth Kinetics by the Addition of Organics to Sulfate Particles

20 Aerosol-cloud interactions are affected by the rate at which water vapour condenses onto 21 particles during cloud droplet growth. Changes in droplet growth rates can impact cloud droplet 22 number and size distribution. The current study investigated droplet growth kinetics of acidic and 23 neutral sulfate particles which contained various amounts and types of organic compounds, from 24 model compounds (carbonyls) to complex mixtures (α-pinene SOA and diesel engine exhaust). 25 In most cases, the formed droplet size distributions were shifted to smaller sizes relative to 26 control experiments (pure sulfate particles), due to suppression in droplet growth rates in the 27 cloud condensation nuclei counter. The shift to smaller droplets correlated with increasing 28 amounts of organic material, with the largest effect observed for acidic seed particles at low 29 relative humidity. For all organics incorporated onto acidic particles, formation of high 30 molecular weight compounds was observed, probably by acid-catalyzed Aldol condensation 31 reactions in the case of carbonyls. To test the reversibility of this process, carbonyl experiments 32 were conducted with acidic particles exposed to higher relative humidity. High molecular weight 33 compounds were not measured in this case and no shift in droplet sizes was observed, suggesting 34 that high molecular weightcompounds are the species affecting the rate of water uptake. While 35 these results provide laboratory evidence that organic compounds can slow droplet growth rates, 36 the modeled mass accommodation coefficient of water on these particles (α > 0.1) indicates that 37 this effect is unlikely to significantly affect cloud properties, consistent with infrequent field 38 observations of slower droplet growth rates. 39