Size-dependent airborne metal solubility and associated analytical techniques at bulk and single particle levels: A review

Size-dependent airborne metal solubility in air and living organisms has been identified as a key parameter to understanding aerosol-cloud interactions, global biogeochemical cycles, indoor air quality, and aerosol toxicity. Emission sources play a dominant role in size-dependent airborne metal solubility, which could be enhanced by atmospheric aging. Studies show that toxic metals (e.g., heavy metals) concentrated in submicron particles tend to have a higher solubility compared to crustal metals and possess even higher solubility levels in biological fluids than those in water, which may trigger more adverse health effects. Factors related to metal solubility and particle size, such as liquid water concentrations and acidity levels, mixing states, and particle parameters of particles, were discussed. Analytical techniques on various scales play an essential role in accurately measuring size-dependent metal solubility, particularly for soluble fractions. Available detection techniques at bulk and single particle levels were examined, including offline, online, and mapping analysis. The purpose of this overview is to summarize the current understanding of size-dependent airborne metal solubility and related state-of-the-art analytical techniques. We also identified future challenges and provided perspectives for studies of size-dependent metal solubility and the development of associated analytical techniques.