Insights into Aqueous Glyoxal Chemistry via Glass Transition Measurements
Authors: Peters Jan-Hendrik , Dette Hans Peter , Koop Thomas 
Published: February 2023

Atmospheric aerosols affect the Earth&#8217;s radiative budget directly by scattering, reflecting, and absorbing light and also indirectly by acting as nuclei for the formation of liquid water and ice clouds. Many of these effects are directly related to the chemical and physical properties of the aerosol particles, e.g. their phase state, which is unknown for many atmospherically relevant compounds.Glyoxal is one of the most abundant organic species in the atmosphere with a production rate of about 45 teragram per year.1 Despite being a simple gaseous dialdehyde in the absence of water, glyoxal exhibits a complex chemistry in aqueous solutions.2 Upon drying such solutions, glyoxal does not evaporate completely, but remains in the condensed phase due to the formation of water adducts and oligomeric species.3In our work, we present differential scanning calorimetry (DSC) experiments on dried aerosolized as well as bulk aqueous glyoxal solutions. We studied the effect of the drying rate, of the concentration of the initial glyoxal solution, of temperature, and of the addition of atmospherically relevant ammonium salts on the glass transition temperature (Tg) of the glyoxal solutions. During fast and very slow drying, highly viscous or even glassy phase states were detected via DSC measurements, and we report the corresponding glass transition temperatures of such systems. After diluting the aqueous solutions with water, mimicking atmospheric water uptake in the atmosphere, Tg of the dried solution varies with time until a new chemical equilibrium is established. Considering their temperature dependence, the time scale of these processes can range from hours to days. We use the measured time-dependent glass transition temperatures to infer dependencies of the aqueous phase equilibria between monomer, dimer, and trimer glyoxal species and their water adducts and support these by infrared spectroscopy.4 We show that glass transition measurements can be used to infer information on the aqueous chemistry of organic molecules in solution in slowly equilibrating systems.&#160;References:(1) Fu, T.-M.; Jacob, D. J.; Wittrock, F.; Burrows, J. P.; Vrekoussis, M.; Henze, D. K. Global budgets of atmospheric glyoxal and methylglyoxal, and implications for formation of secondary organic aerosols. J. Geophys. Res. 2008, 113 (D15). DOI: 10.1029/2007JD009505.(2) Ervens, B.; Volkamer, R. Glyoxal processing by aerosol multiphase chemistry: towards a kinetic modeling framework of secondary organic aerosol formation in aqueous particles. Atmos. Chem. Phys. 2010, 10 (17), 8219&#8211;8244. DOI: 10.5194/acp-10-8219-2010.(3) Loeffler, K. W.; Koehler, C. A.; Paul, N. M.; De Haan, D. O. Oligomer formation in evaporating aqueous glyoxal and methyl glyoxal solutions. Environmental Science & Technology 2006, 40 (20), 6318&#8211;6323. DOI: 10.1021/es060810w.(4) Peters, J.-H.; Dette, H. P.; Koop, T. Glyoxal as a Potential Source of Highly Viscous Aerosol Particles. ACS Earth Space Chem. 2021, 5 (12), 3324&#8211;3337. DOI: 10.1021/acsearthspacechem.1c00245.