During the first 24 months of activities within the SOFA project, we were quite successful after an initial period of preparing and setting up various instruments and protocols. We have now published manuscripts in high-ranking (hybrid) journals, namely PNAS, JACS, and ES&T. In these publications, we have already made significant progress beyond the initial state-of-the-art.
We presented experimental evidence that OH radicals are spontaneously produced at the air-water interface of aqueous droplets in the dark and in the absence of known precursors, possibly due to the strong electric field that forms at such interfaces (
https://doi.org/10.1073/pnas.2220228120(öffnet in neuem Fenster)). These results are linked to activities in WP1 of the project. The measured OH production rates in atmospherically relevant droplets are comparable to, or significantly higher than, those from known aqueous bulk sources, especially in the dark. Since aqueous droplets are ubiquitous in the troposphere, this interfacial source of OH radicals could significantly impact atmospheric multiphase oxidation chemistry, with substantial implications for air quality, climate, and health.
We also presented experimental evidence that atomic and molecular iodine (I and I2) are spontaneously produced in the dark at the air-water interface of iodide-containing droplets without any added catalysts, oxidants, or irradiation, corresponding to activities in WP3 (
https://doi.org/10.1021/acs.est.3c05777(öffnet in neuem Fenster)).
Following these initial observations, we investigated the effect of halide anions (Cl⁻, Br⁻, I⁻), which are abundant in marine aerosols, on H2O2 production (
https://pubs.acs.org/doi/10.1021/jacs.3c14040(öffnet in neuem Fenster)). Our results showed that only Br⁻ contributes to interfacial H2O2 formation by acting as an electron donor, while Na2SO₄ and NaCl stabilized the droplets by reducing their evaporation. TAOH was observed in the collected droplets and, for the first time, directly in the particle phase using online fluorescence spectroscopy, confirming interfacial OH production. A mechanistic study suggests that H2O2 is formed by OH and HO2 self-recombination, as well as HO2 reactions with H atoms. This work enhances our understanding of interfacial processes and their impact on climate, air quality, and health.
We also focused on the effect of acidity on spontaneous interfacial hydrogen peroxide formation in salt-containing droplets (
WP1 and WP3(öffnet in neuem Fenster). Na2SO₄, NaCl, and NaBr bulk solutions, at pH levels ranging from 4 to 9.5 were nebulized using ultra-high purity N2/O2 (80%/20%), and H2O2 was measured in the collected droplets. All experiments were conducted at T = 292 ± 1 K and humidity levels of 90 ± 2%. For Na2SO₄ and NaCl, H2O2 concentrations increased by ~40% under alkaline conditions, suggesting that OH⁻-enriched environments promote its production. When CO2 was added to the ultra-pure air, H2O2 levels were lower at higher pH, suggesting that dissolved CO2 can initiate reactions with OH radicals and electrons, affecting interfacial H2O2 production. H2O2 formation in NaBr droplets showed no dependence on pH or bath gas, indicating that secondary reactions occur at the interface with Br⁻, which serves as an efficient interfacial source of electrons.
