Concentration effects and ion properties controlling the fractionation of halides during aerosol formation

During the aerosolization process at the sea surface, halides are incorporated into aerosol droplets, where they may play an important role in tropospheric ozone chemistry. Although this process may significantly contribute to the formation of reactive gas phase molecular halogens, little is known about the environmental factors that control how halides selectively accumulate at the air-water interface. In this study, the production of sea spray aerosol is simulated using electrospray ionization (ESI) of 100 nM equimolar solutions of NaCl, NaBr, NaI, NaNO ₂, NaNO ₃, NaClO ₄, and NaIO ₄. The microdroplets generated are analyzed by mass spectrometry to study the comparative enrichment of anions (fX ^-) and their correlation with ion properties. Although no correlation exists between fX ^- and the limiting equivalent ionic conductivity, the correlation coefficient of the linear fit with the size of the anions RX ^-, dehydration free-energy ΔG _dehyd, and polarizability α, follows the order: RX ^--2 > RX ^--1 > RX ^- > ΔG _dehyd > α. The same pure physical process is observed in H ₂O and D ₂O. The factor fX ^- does not change with pH (6.8-8.6), counterion (Li ⁺, Na ⁺, K ⁺, and Cs ⁺) substitution effects, or solvent polarity changes in methanol- and ethanol-water mixtures (0 ≥ xH ₂O ≥ 1). Sodium polysorbate 20 surfactant is used to modify the structure of the interface. Despite the observed enrichment of I ^- on the air-water interface of equimolar solutions, our results of seawater mimic samples agree with a model in which the interfacial composition is increasingly enriched in I ^- < Br ^- < Cl ^- over the oceanic boundary layer due to concentration effects in sea spray aerosol formation.