New mass spectrometry techniques for studying physical chemistry of atmospheric heterogeneous processes

Ambient particles and droplets have a significant effect on climate, visibility and human health. Once formed, they undergo continuous transformations through condensation and evaporation of water, uptake of low-volatility organic molecules and photochemical reactions involving various gaseous and condensed-phase species in the atmosphere. These transformations determine the physical and chemical properties of airborne particles, such as their ability to absorb and scatter solar radiation and nucleate cloud droplets. The complexity, heterogeneity and size of ambient particles make it challenging to understand the kinetics and mechanisms of their formation and chemical transformations. Mass spectrometry (MS) is a powerful analytical technique that enables detailed chemical characterisation of both small and large molecules in complex matrices. This capability makes MS a promising tool for studying chemical transformations of particles and droplets in the atmosphere. This review is focused on new and emerging experimental MS-based approaches for understanding the kinetics and mechanisms of such transformations. Some of the techniques discussed herein are best suited for ambient samples, while the others work best in laboratory applications. However, in combination, they provide a comprehensive arsenal of methods for characterisation of particles and droplets. In addition, we emphasise the role of fundamental physical chemistry studies in the development of new methods for chemical analysis of ambient particles and droplets.