Direct measurement of aerosol acidity: recent advances, challenges, and opportunities.

Acidity is a critical property of atmospheric aerosols, dictating their behaviors and environmental, climatic, and health impacts. However, direct measurement of aerosol acidity remains a formidable challenge. This article reviews recent advances in direct measurement of aerosol acidity, critically assessing the strengths, limitations, and operational ranges of available methods, as well as their applications to laboratory and ambient aerosols. We also re-assess the widely used phase partitioning method, arguing that it shares fundamental principles with direct measurement methods. We further highlight key unresolved challenges in aerosol acidity measurement, and explore potential strategies for addressing them in future research. Ultimately, this work aims to foster further debates and inspire future advances in understanding aerosol acidity, and more broadly acidity within confined micro- and nanoscale spaces, which are ubiquitous in both natural and engineered systems.