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

R. Wagner, O. Möhler, H. Saathoff, M. Schnaiter, T. Leisner
2010 Atmospheric Chemistry and Physics Discussions  
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. Pre-20 sition and condensation mode. At times, the heterogeneous ice nucleation ability of oxalic acid dihydrate significantly changed when the particles had been processed
more » ... ad been processed in preceding cloud droplet activation steps. Such behaviour was also observed for the second investigated species, namely sodium oxalate. Our experiments address the atmospheric scenario that coating layers of oxalic acid or its salts may be formed by 25 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 11514 improve or reduce the ice nucleation ability of the seed aerosol particles. 20 oxalic acid, starting with the aerosol flow tube/FTIR study by Braban et al. (2003) . The authors have generated a polydisperse aerosol of anhydrous oxalic acid particles by spray-drying of an aqueous solution at a relative humidity (RH) of less than 5%. As apparent from the recorded infrared spectrum, the chain-like β form of anhydrous oxalic acid has formed where the molecules are linked together in pairs by cyclic hydrogen 25 bonds (Bellamy and Pace, 1963; Villepin and Novak, 1978a, b). Upon increasing relative humidity, two phase transitions were observed: from anhydrous β oxalic acid to 11515 oxalic acid dihydrate at 9 -21% RH (294-259 K) and the deliquescence transition to an aqueous solution at 97-99% RH (294-279 K). The structure of oxalic acid dihydrate is similar to that of the anhydrous β form, with two water molecules included between the carboxyl groups. Its infrared spectrum is characterised by a prominent doublet feature at around 3500 cm −1 due to the hydration water whereas aqueous oxalic acid solution 5 droplets reveal a broader and less-structured absorption band in the O-H stretching regime. Note that two polymorphs of oxalic acid dihydrate have only been observed for the completely deuterated form (Ebisuzaki and Angel, 1981) . According to Braban et al. (2003) , it was difficult to effloresce oxalic acid aerosol particles. In a number of experiments the aerosol water was not completely removed by the drier, leaving be-10 20 oxalic acid aerosol at ambient temperature, generated by nebulisation of an aqueous solution, drying at RH<5%, and size selecting the 100 nm dry particle diameter by a differential mobility analyser. A closer inspection of the growth curve, however, revealed some trends and small-scale fluctuations that are inconsistent with the assumption that liquid-like aerosol particles were present: (i) a decrease of the growth factor in the 25 range from 5 to 40% RH, (ii), a small, stepwise increase in the growth factor at about 43% RH, going along with a sudden broadening of the mode width of the measured size distribution, and, (iii), small-scale fluctuations in the growth curve until reaching a relative humidity of 80%. As an explanation, the authors proposed that spray-drying of 11516 the solution droplets had led to the formation of amorphous, highly porous oxalic acid particles with a gel-like structure. The discontinuities in the growth curve could then be attributed to microstructural rearrangements and transitions between collapsed and swollen gel structures. Only above a relative humidity of 80%, the close agreement between the mode widths of the particle size distribution in repetitive hydration and 5 dehydration experiments points to the presence of fully deliquesced liquid aqueous solution droplets. Complementing the contradictory results on the hygroscopic behaviour of oxalic acid particles, have detected an efflorescence transition in 10 -20 µm sized oxalic acid particles at 52-57% RH. The particles were levitated in an electrody-10 25 tinuous flow diffusion chamber (CFDC) to measure ice formation. As discussed above, these particles were supposedly either concentrated solution droplets, or, according to the interpretation from Mikhailov et al. (2009), gel-like amorphous particles. The threshold ice relative humidity to freeze 1% of those particles at temperatures between 11517 25 190 K. In contrast, Parsons et al. (2004) have reported that 2-40 µm sized particles of saturated C3-C6 dicarboxylic acids are not good ice nuclei in the deposition mode at temperatures above 243 K. Instead of heterogeneously nucleating ice at temperatures below the eutectic temperature, i.e., at ice supersaturated conditions, these particles 11518 Abstract ACPD Abstract ACPD Abstract ACPD Abstract 20 Experimental A schematic view of the AIDA aerosol and cloud chamber facility, featuring the relevant instrumentation for the oxalic acid and sodium oxalate crystallisation and expansion cooling experiments, is shown in Fig. 2 . The aerosol chamber, a 84.3 m 3 sized aluminium vessel of 4 m diameter, is located inside an isolating containment whose 25 interior can be cooled to temperatures as low as 183 K. The spatial temperature fluctuations throughout the whole chamber interior, as measured by arrays of horizontally 11522 Abstract ACPD Abstract 25 oxalic acid/air mixture was then directed via stainless steel connection tubes into the AIDA chamber that was held at 228 K, thereby inducing the binary nucleation of oxalic acid and water. A total injection time of about 2 h was needed to positively identify oxalic acid dihydrate as the major outcome of this procedure by FTIR extinction mea-11524 Abstract ACPD Abstract ACPD Abstract ACPD Abstract Heterogeneous ice nucleation potential of oxalic acid dihydrate R. Abstract ACPD Abstract 25 sition mode ice activity. During Exp. 6, where the oxalic acid dihydrate particles were formed from less supersaturated oxalic acid solution droplets at a higher temperature and were allowed to slowly grow by mass transfer from still unfrozen solution droplets over a very long time scale of about 12 h, see Appendix A in Zobrist et al. (2006), more 11548 Abstract ACPD Abstract ACPD Abstract ACPD Abstract ACPD Abstract Heterogeneous ice nucleation potential of oxalic acid dihydrate R. signed and constructed instrument for coupled infrared extinction and size distribution measurements of aerosols, Aerosol. Sci. Tech., 41, 701-710, 2007. Kanji, Z. A. and Abbatt, J. P. D., Laboratory studies of ice formation via deposition mode nucleation onto mineral dust and n-hexane soot samples,
doi:10.5194/acpd-10-11513-2010 fatcat:g54ck2knojhdhjo2pj4dlp5p7q