High variability of the heterogeneous ice nucleation potential of oxalic acid dihydrate and sodium oxalate

摘要

The heterogeneous ice nucleation potential of airborne oxalic acid dihydrateand sodium oxalate particles in the deposition and condensation mode hasbeen investigated by controlled expansion cooling cycles in the AIDA aerosoland cloud chamber of the Karlsruhe Institute of Technology at temperaturesbetween 244 and 228 K. Previous laboratory studies have highlighted theparticular role of oxalic acid dihydrate as the only species amongst avariety of other investigated dicarboxylic acids to be capable of acting asa heterogeneous ice nucleus in both the deposition and immersion mode. Wecould confirm a high deposition mode ice activity for 0.03 to 0.8 μmsized oxalic acid dihydrate particles that were either formed by nucleationfrom a gaseous oxalic acid/air mixture or by rapid crystallisation of highlysupersaturated aqueous oxalic acid solution droplets. The criticalsaturation ratio with respect to ice required for deposition nucleation wasfound to be less than 1.1 and the size-dependent ice-active fraction of theaerosol population was in the range from 0.1 to 22%. In contrast, oxalicacid dihydrate particles that had crystallised from less supersaturatedsolution droplets and had been allowed to slowly grow in a supersaturatedenvironment from still unfrozen oxalic acid solution droplets over a timeperiod of several hours were found to be much poorer heterogeneous icenuclei. We speculate that under these conditions a crystal surface structurewith less-active sites for the initiation of ice nucleation was generated.Such particles partially proved to be almost ice-inactive in both thedeposition and condensation mode. At times, the heterogeneous ice nucleationability of oxalic acid dihydrate significantly changed when the particleshad been processed in preceding cloud droplet activation steps. Suchbehaviour was also observed for the second investigated species, namelysodium oxalate. Our experiments address the atmospheric scenario thatcoating layers of oxalic acid or its salts may be formed by physical andchemical processing on pre-existing particulates such as mineral dust andsoot. Given the broad diversity of the observed heterogeneous icenucleability of the oxalate species, it is not straightforward to predictwhether an oxalate coating layer will improve or reduce the ice nucleationability of the seed aerosol particles.