Physico-chemical properties of cloud drop residual and interstitial particles sampled inside hill capped clouds during a field experiment in Central Europe

Aerosol-cloud interaction, particle activation and phase partitioning of ambient aerosol particles between the liquid and interstitial phase in continental boundary layer clouds were investigated during the Hill Cap Cloud Thuringia field experiment in autumn 2010 (HCCT 2010). An interstitial inlet (INT) and two counterflow virtual impactors (CVI) were operated inside cloud in order to separate and collect non-activated interstitial particles (IP) and cloud drops, respectively, on the mountain site Schmücke (938 m asl) in Central Europe. Both inlet types were designed for a separation diameter of 5 μm. Inside the CVI systems the collected drops are evaporated releasing dry cloud drop residues (CDR), which are closely related to the original cloud condensation nuclei forming the cloud. By two sets of instruments that measured the same parameter connected to each inlet type, the CDR and IP were micro-physically and chemically characterized simultaneously. Number concentration and size distribution were determined by a condensation particle counter (CPC) and scanning mobility particle sizer (SMPS) + optical particle counter (OPC) at INT and CVI. The chemical composition of CDR and IP was inferred in various ways. The amount of black carbon in each reservoir was measured by two particle soot absorption photometers (PSAP) supplemented by a multiangle absorption photometer (MAAP) at the interstitial inlet only. The content of non-refractory chemical compounds was analyzed by a compact time-of-flight aerosol mass spectrometer (C-ToF-AMS) and a high-resolution (HR)-ToF- AMS for the CDR and IP, respectively. Filter and adsorption cartridge samples were taken at both inlet types to determine selected inorganic and organic species. The mixing state of single cloud drop residues was derived from the aircraft-based laser ablation aerosol mass spectrometer ALABAMA. During the analyzed cloud events, the cloud microphysical parameters measured by a particle volume monitor (PVM) and forward scattering spectrometer probe (FSSP), like liquid water content, effective drop diameter and drop concentration varied between 0.2 - 0.6 g m-3, 4 - 10 μm and 200 - 500 cm-3, respectively. First results from the AMS phase partitioning comparison shows that the activated mass fraction of the inorganic ions (about 0.9) is significantly higher than that of the organic mass (about 0.8). This finding will be discussed in combination with other chemical composition measurements mentioned above, the analysis of cloud water samples, and the microphysical properties of CDR and IP from which the activation diameter in the different cloud events will be inferred. BC phase partitioning was found to be about 0.5. At south-westerly winds a connected flow between a further cloud-free upwind and downwind measurement site is expected (with the mountain station in between), so that in this special meteorological situation the in-cloud aerosol characterization will be correlated with the physic-chemical properties of the ambient aerosol in the inflow and outflow of the clouds.