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

Abstract

Abstract. The heterogeneous ice nucleation potential of airborne oxalic acid dihydrate and sodium oxalate particles in the deposition and condensation mode has been investigated by controlled expansion cooling cycles in the AIDA aerosol and cloud chamber of the Karlsruhe Institute of Technology at temperatures between 244 and 228 K. Previous laboratory studies have highlighted the particular role of oxalic acid dihydrate as the only species amongst a variety of other investigated dicarboxylic acids to be capable of acting as a heterogeneous ice nucleus in both the deposition and immersion mode. We could confirm a high deposition mode ice activity for 0.03 to 0.8 µm sized oxalic acid dihydrate particles that were either formed by nucleation from a gaseous oxalic acid/air mixture or by rapid crystallisation of highly supersaturated aqueous oxalic acid solution droplets. The critical saturation ratio with respect to ice required for deposition nucleation was found to be less than 1.1 and the size-dependent ice-active fraction of the aerosol population was in the range from 0.1 to 22%. In contrast, oxalic acid dihydrate particles that had crystallised from less supersaturated solution droplets and had been allowed to slowly grow in a supersaturated environment from still unfrozen oxalic acid solution droplets over a time period of several hours were found to be much poorer heterogeneous ice nuclei. We speculate that under these conditions a crystal surface structure with less-active sites for the initiation of ice nucleation was generated. Such particles partially proved to be almost ice-inactive in both the deposition and condensation mode. At times, the heterogeneous ice nucleation ability of oxalic acid dihydrate significantly changed when the particles had been processed in preceding cloud droplet activation steps. Such behaviour was also observed for the second investiCorrespondence to: R. Wagner (robert.wagner2@kit.edu) gated species, namely sodium oxalate. Our experiments address the atmospheric scenario that coating layers of oxalic acid or its salts may be formed by physical and chemical processing on pre-existing particulates such as mineral dust and soot. Given the broad diversity of the observed heterogeneous ice nucleability of the oxalate species, it is not straightforward to predict whether an oxalate coating layer will improve or reduce the ice nucleation ability of the seed aerosol particles.

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

“…In case of lower heterogeneous ice nucleability, the aerosol particles will again be activated to a droplet cloud at point A2. Our previous experiments have shown that potentially immersed oxalic acid dihydrate crystals might then not get instantly and completely dissolved on the typical timescale of an expansion cooling experiment (Wagner et al, 2010). The embedded crystals therefore still can act as condensation mode ice nuclei in this experiment and could provoke the glaciation of the droplet cloud before the homogeneous freezing temperature of pure water droplets has been reached at point A3.…”

“…We have recently observed a similar discrepancy for the efflorescence behaviour of oxalic acid. Whereas several previous studies have reported on the difficulty to effloresce oxalic acid particles even at very low relative humidities of less than 5% (Braban et al, 2003;Mikhailov et al, 2009;Prenni et al, 2001), supercooled aqueous oxalic acid solution droplets readily crystallised in AIDA at 244 K and 70% RH on a time scale of about one hour (Wagner et al, 2010). As outlined in our recent work, we will attempt to quantitatively analyse the crystallisation process in future chamber studies by exploiting the potential of a novel light scattering instrument (Small Ice Detector probe SID3, University of Hertfordshire) to determine the crystallisation rate.…”

“…2). A detailed instrument description is given in the recent review by Wagner et al (2009) and in our companion paper on the ice nucleability of oxalic acid dihydrate (Wagner et al, 2010). In the following, we therefore only give a brief summary of the key components of the AIDA instrumentation.…”

“…In a recent cloud chamber study we have shown that also the ice nucleation ability of oxalic acid dihydrate particles is highly variable and depends on their formation mechanism (Wagner et al, 2010). Oxalic acid dihydrate crystals that were formed by nucleation from a gaseous oxalic acid/air mixture or by rapid crystallisation of highly supersaturated aqueous solution droplets at temperatures ≤244 K proved to be even more ice active in the deposition mode than the particles probed by Kanji et al (2008), showing ice formation at threshold ice saturation ratios lower than 1.10.…”

Influence of particle size and shape on the backscattering linear depolarisation ratio of small ice crystals – cloud chamber measurements in the context of contrail and cirrus microphysics

“…A detailed description of the chamber preparation, the chamber instrumentation, and the expansion cooling technique for ice cloud studies is given in the recent papers by Wagner et al (2010Wagner et al ( , 2011. In the following, we therefore only give a brief introduction to the chamber and its intrumentation.…”