Secondary organic aerosols (SOA) can be formed through aqueous-phase reactions in aerosol deliquescent particles, clouds and fogs, which has been widely observed in both ambient studies and lab simulations. However, the aqueous-phase formation of SOA and its impacts on oxidation degree are far from being clearly understood, particularly in heavily polluted atmospheres. For example, in conditions of severe Beijing Haze with PM2.5 levels higher than 300 μg m-3 the ionic strength in the wet particles varies between 13 and 43 M. This high ionic strength can affect the kinetics and products distribution within the aerosol deliquescent particles and, thus, it can affect aerosol composition and optical properties. However, the ionic strength effects on aqueous phase reactions of atmospheric relevance have been barely studied in the past.
In this work, Gligorovski and his-coworkers advanced the knowledge on ionic strength effects on the absorption properties of acetosyringone (AcS), a typical lignin derived compound and on the photoinduced degradation of AcS itself.
Surprisingly, it can be observed a bathochromic (red) shift of the n→π* absorption band of AcS, from λmax = 297 nm in the dilute aqueous phase to λmax = 355 nm at in the presence of 6 mol L-1 NaClO4. Also the π → π* band underwent a red shift, from λmax = 214 nm to λmax = 247 nm. They are also showing that the photodegradation of AcS is six times faster in the presence of elevated ionic strengths, demonstrating that photochemical processes in aerosol deliquescent particles may result in different kinetics than in the analogous diluted aqueous-phase chemistry, with the potential to significantly affect aerosol composition and optical properties.
Finally, the light-induced transformation of AcS leads to a pH decrease, which in the atmosphere would translate into a light-induced acidification of the aerosol particles due to liberation of protons.
For more information, see the paper recently published in Atmospheric Environment at: