Spontaneous interfacial oxidant formation as a key driver for aerosol oxidation

Project Information

SOFA

Grant agreement ID: 101052601

DOI

10.3030/101052601

EC signature date 9 June 2022

Start date 1 October 2022

End date 30 September 2027

Funded under

• European Research Council (ERC)

Total cost

€ 2 726 588,00

EU contribution

€ 2 726 588,00

Coordinated by

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE CNRS
France

Periodic Reporting for period 1 - SOFA (Spontaneous interfacial oxidant formation as a key driver for aerosol oxidation)

Reporting period: 2022-10-01 to 2025-03-31

Summary of the context and overall objectives of the project

Aerosols and clouds are key players in tropospheric chemistry. These tiny particles suspended in the air, with a radius ranging from a few nanometres to tens of micrometres, impact atmospheric composition, represent one of the largest uncertainties in climatic projections and cause millions of deaths worldwide every year. Hence, they have enormous societal and economic consequences. Nonetheless, there is still a knowledge gap preventing us from describing the chemical evolution of aerosols and clouds during their atmospheric lifetime. Supported by preliminary experiments, I therefore propose to unravel the impact of the spontaneous oxidant formation at the air/liquid interface as a key driver for multiphase oxidation processes.
Water molecules in bulk liquid are stable and inert under ambient conditions. In sharp contrast, it was very recently shown that the local orientation of water molecules at an air/water interface induces an electric field that generates spontaneous radicals in micron-sized droplets. This production does not involve any catalysts such as light or heat. It is an intrinsic property of the air/water interface, and therefore potentially ubiquitous in the troposphere.
This spontaneous interfacial oxidant formation has never been explored for its atmospheric significance. Therefore, the SOFA project aims to unravel the atmospheric importance of this interfacial (dark) chemistry. If oxidants (including OH radicals) are in fact spontaneously produced at the air-water interface, under atmospherically relevant concentrations, this would profoundly challenge our understanding and description of atmospheric multiphase chemistry.
SOFA will develop a novel strategy, scaling up from laboratory-based measurements to fieldwork and modelling to assess the importance of this interfacial chemistry. SOFA will advance an entirely new perspective on how to address the multiphase oxidation capacity of the troposphere, and will therefore have a wide impact.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

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). 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).

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). 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. 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.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The discovery and description of the spontaneous oxidant production at interfaces where water is present, i.e. most environmental surfaces, will certainly be a major breakthrough as it has the potential to profoundly impact our description of the multiphase oxidation potential of the troposphere. This will have major influence on our ability to describe air quality, associated health impacts and climate change.
The developments arising from these activities will have tremendous ramifications.
• We will train an international team of young scientists in new and emerging ideas with cutting-edge instrumentation that will have major impact on their careers. In fact, this project addresses a subject of great interest and importance for science and society at large, namely air pollution and climate change. In this context, this project will combine research stemming from chemistry, physics, surface science and climate sciences. Both the science and technical skills will come together to provide young researchers with a broad scientific perspective - a characteristic that is increasingly rare among today's scientists.
• The expected discoveries from this project will significantly impact our understanding of the chemical processes involving air/water interfaces, as we will produce research that is currently beyond reach due to the lack of appropriate tools.
• This new understanding will improve on our ability to describe atmospheric multiphase chemistry that has strong impact on air quality, health (due to the inhalation of particles) as well as climate change by providing new input for global climate models.
This pioneering project will open new and important technological and scientific horizons and will place the team at Lyon at the forefront of this research field worldwide, which will be highly beneficial for the host institution and frontier research in Europe.

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