This paper is written by experts in the field and addresses new tropospheric chemical mechanism developments. It shows how new analytical methods, instrumentation and cheminformatics methods have led to the identification of key processes and knowledge gaps in our understanding of chemical mechanisms, both in the gas phase and condensed phases. The opinion paper presents a perspective on these developments in the present and future, and outlines new directions. This opinion will be a very valuable contribution for the community and I am convinced it will be frequently read and cited.

Tropospheric OH plays a central role in the chemical degradation and formation of air pollutants and climate-relevant trace gases. Traditional direct measurements of the extremely short-lived OH radicals provide localised information on the OH budget. Due to the high technical complexity and scarcity of such measurements, they are practically unsuitable for mapping global spatio-temporal OH distributions. Budget analyses of long-lived tracers (e.g. methyl chloroform) that react exclusively with OH provide estimates of the global mean OH and its temporal variability, but lack spatial resolution. This forward-looking opinion paper discusses new ideas and proposals to constrain global chemical sources and sinks of tropospheric OH using satellite and suborbital observations. The proposed approaches promise a better understanding of the global OH budget at higher spatial and temporal resolution than has been possible to date.

While it has been established for decades that particulate matter, especially the PM2.5 fraction, can be quantitatively linked to a variety of negative health outcomes from both long- and short-term exposure, our ability to conclusively link this to specific particle components or types has proved stubbornly elusive. If particular sources or chemical components were found to be the driving factors, these could lead to more effective interventions and regulations. Because of recent advances in our observational capabilities and fundamental understanding of particle composition and toxicity, this opinion piece argues that we are at a pivotal moment in this ongoing area of study, and summarises some recent advances in this area.