In Situ Surface Tension Measurements of Hanging Droplet Methylglyoxal/Ammonium Sulfate Aerosol Mimics under Photooxidative Conditions

Dark-chemistry products of ammonium sulfate (AS) and methylglyoxal (MG) have been observed to be surface-active, resulting in changes to solution surface tension with relatively small amounts of organic loading. These same solutions have been observed to be unstable under exposure to ultraviolet (UV) radiation, although the surfactant properties of these products have not been characterized. Here, small (10 μL) droplets of aged MG/AS solutions of varying organic concentration are suspended under dark and irradiated conditions and measured via hanging drop profile tensiometry. Irradiated droplets were exposed to broadband UV radiation (λmax = 254 nm) for durations up to 3 h. Solution surface tension was observed to depress upon exposure to radiation, dependent upon magnitude and duration of radiation. Solution organic loading was observed to not statistically affect the extent of surface tension depression. Bulk solution UV/visible absorbance measurements indicated a slight increase in the 280 nm absorption after exposure, contrary to similar bulk solution studies performed on similar systems. Electrospray ionization mass spectrometry analysis indicates an increase of signal in masses consistent with previously observed chromophoric MG/AS products after irradiation, suggesting that the sustained gas–droplet interface in which condensed-phase photochemistry takes place may have an effect on the prevailing aqueous secondary organic aerosol products that form.