Scavenging efficiency of 'aerosol carbon' and sulfate in supercooled clouds at Mt. Sonnblick (3106 m a.s.l., Austria)

Cloud water and interstitial aerosol samples collected at Mt. Sonnblick (SBO) were analyzed for sulfate and 'aerosol carbon' to calculate in-cloud scavenging efficiencies. Scavenging efficiencies for sulfate (epsilon(SO4)) ranged from 0.52 to 0.99 with an average of 0.80. 'Aerosol carbon' was scavenged less efficiently with an average value (epsilon(AC)) of 0.45 and minimum and maximum values of 0.14 and 0.81, respectively. Both epsilon(SO) and epsilon(AC) showed a marked, but slightly different, dependence on the liquid water content (LWC) of the cloud. At low LWC, epsilon(SO) increased with rising LWC until it reached a relatively constant value of 0.83 above an LWC of approximate to 0.3 g/m(3). In the case of 'aerosol carbon', we obtained a more gradual increase of epsilon(AC) up to an LWC of approximate to 0.5 g/m(3). At higher LWCs, epsilon(AC) remained relatively constant at 0.60. As the differences between epsilon(SO) and epsilon(A) varied across the LWC range observed at SBO, we assume that part of the `aerosol carbon' was incorporated into the cloud droplets independently from sulfate. This hypothesis is supported by size classified aerosol measurements. The differences in the size distributions of sulfate and total carbon point to a partially external mixture. Thus, the different chemical nature and the differences in the size and mixing state of the aerosol particles are the most likely candidates for the differences in the scavenging behavior.