Linking Peroxy Radical Chemistry to Global Climate: The Common Representatives Intermediates Chemical Mechanism in the UK Earth System Model

Production of ozone and secondary organic aerosols (SOA) in the troposphere is driven by the photo-oxidation of volatile organic compounds (VOCs). Crucial intermediates in these oxidation steps are peroxy radicals, which enable ozone generation when reacting with NO. Recent pioneering studies have shown peroxy radical chemistry to have much broader impacts on the atmosphere, with many of these species undergoing autoxidation and forming highly oxidised organic molecules (HOMs), including accretion products, which can form new particles, contribute to SOA growth and influence global climate. However, explicitly simulating the full complexity of this chemistry is impractical due to the many thousands of VOC species in the atmosphere; techniques for reducing complexity are therefore necessary. The Master Chemical Mechanism (MCM) is a near-explicit scheme, with ~6,000 species and ~19,000 reactions, but is almost exclusively used in box-model applications due to its high cost. The Common Representative Intermediates (CRIv2-R5) mechanism is an effective compromise, preserving the ozone forming potential of the MCM from the emission and atmospheric degradation of isoprene, α/β-pinene, and 19 other primary VOC species whilst reducing the number of species and reactions to be feasible in a 3D model (approximately 240 species and 650 reactions, including 47 non-transported peroxy radical species and their associated reactions).

We have implemented CRIv2R5 into a global chemistry-climate model, the UK Earth System Model (UKESM1). We present results from a present-day emissions scenario for the Coupled Model Intercomparison Project (CMIP6) to enable a broad scope of model simulations with more basic chemistry and observations to evaluate the model changes against. We find significant differences to tropospheric ozone production and oxidative capacity of the atmosphere, with a strong sensitivity to magnitude and speciation of VOC emissions, highlighting the importance of accurately simulating VOC chemistry to understand trends in tropospheric ozone under changing emissions and climate.

Moving forward, having the comprehensive CRIv2R5 mechanism within UKESM1 provides the framework for investigating the impacts of recently discovered peroxy radical chemical processes on global climate. We present further work that has focused on expanding the CRI mechanism in box-model studies with a semi-explicit treatment key peroxy radical processes including (i) the autoxidation of peroxy radicals from the hydroxyl radical and ozone initiated reactions of α-pinene, (ii) the formation of multiple generations of peroxy radicals, (iii) formation of accretion products (dimers) and (iv) isoprene-driven suppression of accretion product formation as observed in experiments. This new CRI-HOM mechanism is now being implemented into the global UKESM1 model and coupled with its aerosol mechanism. This work will enable pioneering investigations linking best process-level understanding of gas-phase peroxy radical chemistry to SOA formation and thus improving our understanding of the relationship between biogenic VOC emissions and global climate.