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.

Ship tracks are the phenomenon of lines of enhanced cloud reflectivity visible from space that are caused by the particulate emissions of shipping vessels. The emissions cause cloud droplets to become smaller, with a more contentious possibility that they increase cloud water by shutting down precipitation, both effects having a cooling effect on the local climate. This study applies a clever new methodology to show that past studies that measured an increase of cloud water were likely erroneous due to a sampling artifact: they assumed ship tracks were randomly oriented where in fact ship tracks align with the winds. This orientation bias was cleverly ascertained by "sailing" ships through the winds and clouds of a different year than the one in which the ship tracks were measured. A climate cooling effect from increased cloud water was still obtained, but one much smaller than described previously. The result implies that ships may not be as effective a geoengineering tool as has previously been assumed.

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.