Ryan's research explores variability and long-term changes to the chemical composition of Earth’s atmosphere and how such affect climate, the ozone layer and air quality. This involves development and application of numerical models, run on supercomputers, which simulate the past, present and future atmosphere and climate.

Ryan is an elected member of the International Ozone Commission and is co-chair of the Royal Meteorological Society’s Atmospheric Chemistry Special Interest Group (ACSG). He also leads the international APARC model intercomparison project, VSLS-MIP1.

His current research group comprises of 4 PhD students and 2 Post-Doctoral research fellows. Recent work has focussed on:

• Ozone-Depleting Substances (ODSs) and Ozone Layer Recovery. Despite most long-lived ODSs (e.g. CFCs) being phased-out by the Montreal Protocol, ozone depletion remains a persistent environmental issue. Our research examines trends in the atmospheric abundance and impacts of halogenated Very Short-Lived Substances (VSLS) – a class of ozone-depleting gases not controlled by the Protocol whose emissions are increasing. Our recent studies highlight the increasing threat to stratospheric ozone from industrial VSLS emissions, particularly from Asia, and that these gases are offsetting some of the Montreal Protocol’s benefits. See also our recent Nature reviews on ODSs and ozone layer recovery.

• Tropospheric Halogens, Atmospheric Oxidising Power and Climate. Halogen chemistry is infamous for its role in stratospheric ozone layer depletion. However, there is growing evidence that reactive halogens (iodine, bromine, chlorine) also exert significant influence on tropospheric composition, thereby affecting climate. For example, chlorine atoms are highly reactive and may influence the lifetime of methane and other important greenhouse gases, such as tropospheric ozone. The tropospheric abundance and trends of reactive halogens remains poorly known. By building models, our research seeks to understand the natural processes that govern the release and recycling of halogens from the biosphere and to quantify their direct and indirect climate effects.
• Environmental Impacts of F-Gases. Fluorinated gases (F-gases), such as hydrofluorocarbons (HFCs), are widely used in a range of industrial applications, including refrigeration and air conditioning units. Owing to the Montreal Protocol's Kigali Amendment and other F-gas regulations, production and use of high-Global Warming Potential (GWP) HFCs are being phased-down in favour of low-GWP alternatives (e.g. HFOs). Notably, the atmospheric breakdown of various F-gases may result in the formation of ultra-short chain per- and polyfluorinated alkyl substances (PFAS), including highly persistent trifluoracetic acid (TFA, CF3COOH). As scientific understanding of the health effects of TFA and other ‘forever chemicals’ evolves, our work aims to support policy decisions by quantifying trends in their production and deposition to the environment.

• UK Air Quality in a Changing Climate. Elevated ground-level ozone during recent hot summers and heatwaves has raised concern that climate change could exacerbate ozone air pollution in the UK. My group are building statistical models and applying downscaling techniques to better understand the relationship between ozone and meteorological covariates and to provide more skilful forecasts of extreme ozone events.

• Data-Driven Clustering Methods for Climate Science. This work is supported by the EPSRC to explore the use of clustering in the analysis of large climate model datasets from model intercomparison projects. Our recent work has shown that clustering can potentially provide powerful insight into such data, and may help characterise areas of uncertainty across models.