Combining surface spectroscopy and optical microscopy can provide evidence of phase separation induced by photochemical aging of organic aerosol

Organic aerosols make up a considerable mass fraction of atmospheric particulate matter, and impact air quality and climate. In the atmosphere, organic aerosols are exposed to different relative humidities (RH), often ranging between 20% to 100% RH. Gas-particle partitioning of water equilibrates the aerosol particles with the ambient RH, forming aqueous organic aerosols. When exposed to solar radiation, photochemical reactions can occur within the aqueous organic aerosol particles. Such photochemical interactions are often enhanced at the interface formed between the aqueous organic phase and the surrounding air. Depending on the changes in composition these photochemical reactions can induce phase transitions of the particles, including liquid-liquid phase separation, resulting in aqueous organic aerosols with multiple condensed phases. Understanding of the interfacial photochemical reaction and impacts on the number of phases in aqueous organic aerosols remains poor but is critical to assess the impacts of aqueous organic aerosols on air pollution and climate. For example, the number of phases in aqueous organic aerosol particles impacts their reactivity and cloud formation potential, with important implications for air quality and climate.

Here, we propose how the combination of spectroscopy and microscopy tools can be exploited to address this issue: Sum-frequency generation, a surface-sensitive, nonlinear optical spectroscopy method, is used to investigate bulk laboratory proxies of atmospheric aqueous organic aerosols and study changes in their chemical surface composition, as a function of solar irradiation. In addition, we use optical microscopy, to directly study the number of condensed phases in individual particles of the same aerosol system. The combined methods provide microscopic- and molecular-level insights how photochemical reactions impact the phase behavior of aqueous organic aerosols.