Exploring the Ionic Strength Effects in Atmospheric Photochemistry

Professor Sasho Gligorovski

The high concentrations of ions in solutions of high ionic strength such as sea surface, the sea salt aerosol particles and the urban aerosols can alter the absorption properties of the organic constituents. The ionic strength in atmospheric aerosol deliquescent particles can reach about 6 M in marine areas, 8-18 M in urban environments, and 2-17 M in continental regions.

Here, we demonstrate that the absorption spectrum of pyruvic acid (PA) exhibits both an increase of the absorption intensity and a red shift of 13 nm while going from a dilute aqueous phase to a solution containing the inert salt sodium perchlorate (5M NaClO4). If this phenomenon turns out to be more general, many compounds that do not absorb actinic light in clouds and fog could become light absorbers at elevated salt concentrations in aerosol deliquescent particles. Compared to the direct photolysis of PA in dilute aqueous solution, the photolysis rate is increased by three times at high ionic strength (5M NaClO4), with the potential to significantly affect aerosol composition and optical properties. This is the first report of a significant effect of the ionic strength on the rate of an atmospheric photochemical reaction.

Fig. 1. The absorption spectra of 5 mM PA in the presence of different ionic strengths, and the photon flux emitted by the Xe lamp (black solid line). The left axis corresponds to the absorption spectra of PA and the right axis corresponds to the emission spectrum of the Xe lamp.

Such a considerable enhancement can be rationalized in the framework of the Debye-McAulay approach for reactions of ionic + neutral (or neutral + neutral) species, considering that the PA direct photolysis likely involves interaction between its photogenerated triplet state and water. The phenomenon has important implications for the fate of PA and, potentially, of other organic compounds in atmospheric aerosol deliquescent particles.

The photochemical processes involving PA in the aqueous phase are an important topic in atmospheric chemistry, because they contribute to the formation of secondary organic aerosols in the atmosphere. For more details the readers are referred to our article in Atmospheric Environment: