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The heterogeneous ice nucleation ability of oxalic acid in the immersionmode has been investigated by controlled expansion cooling runs withairborne, ternary solution droplets composed of, (i), sodium chloride,oxalic acid, and water (NaCl/OA/H2O) and, (ii), sulphuric acid, oxalicacid, and water (H2SO4/OA/H2O). Polydisperse aerosolpopulations with median diameters ranging from 0.5–0.7 μm andvarying solute concentrations were prepared. The expansion experiments wereconducted in the AIDA aerosol and cloud chamber of the Karlsruhe Instituteof Technology at initial temperatures of 244 and 235 K. In the ternaryNaCl/OA/H2O system, solid inclusions of oxalic acid, presumablynucleated as oxalic acid dihydrate, were formed by temporarily exposing theternary solution droplets to a relative humidity below the efflorescencepoint of NaCl. The matrix of the crystallised NaCl particulates triggeredthe precipitation of the organic crystals which later remained as solidinclusions in the solution droplets when the relative humidity wassubsequently raised above the deliquescence point of NaCl. The embeddedoxalic acid crystals reduced the critical ice saturation ratio required forthe homogeneous freezing of pure NaCl/H2O solution droplets at atemperature of around 231 K from 1.38 to about 1.32. Aqueous solutiondroplets with OA inclusions larger than about 0.27 μm in diameterefficiently nucleated ice by condensation freezing when they were activatedto micron-sized cloud droplets at 241 K, i.e., they froze well above thehomogeneous freezing temperature of pure water droplets of about 237 K. Ourresults on the immersion freezing potential of oxalic acid corroborate thefindings from a recent study with emulsified aqueous solutions containingcrystalline oxalic acid. In those experiments, the crystallisation of oxalicacid diyhdrate was triggered by a preceding homogeneous freezing cycle withthe emulsion samples. The expansion cooling cycles with ternaryH2SO4/OA/H2O solution droplets were aimed to analyse whetherthose findings can be transferred to ice nucleation experiments withairborne oxalic acid containing aerosol particles. Under our experimentalconditions, the efficiency by which the surface of homogeneously nucleatedice crystals triggered the precipitation of oxalic acid dihydrate was verylow, i.e., less than one out of a hundred ice crystals that were formed byhomogeneous freezing in a first expansion cooling cycle left behind anice-active organic crystal that acted as immersion freezing nucleus in asecond expansion cooling cycle

New cloud chamber experiments on the heterogeneous ice nucleation ability of oxalic acid in the immersion mode