A Concurrent-Flow Cloud Chamber Study of Incorporation of Black Carbon into Droplets

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Abstract
A concurrent-flow cloud chamber (16 cm i.d., 1.5 m high) is described in which cold particle-free and humidified particle-laden air streams were mixed to condense water droplets onto combustion-generated soot particles. Total and interstitial black carbon (BC) concentrations in the cloud were measured as a function of input [BC] and the sample air stream dew point in real time, using a two-channel aethalometer. For black carbon particles produced by the combustion of propane, as much as 90% (by mass) of the black carbon when [BC] < 5 μg/m3 was incorporated; this decreased to ∼ 20% when [BC] > 20 μg/m3. It was observed that as sample dew point passed through a maximum, simultaneous maxima were observed in the fraction of black carbon incorporated into droplets and in the amount of black carbon associated with large droplets that settled out of the air stream. The effective supersaturation in the chamber is a function of particle concentration. For black carbon particle concentrations above ∼ 20 μg/m3, the estimated supersaturation is of the order of several percent for the experimental conditions used.