Submitted as: model evaluation paper 14 Apr 2021

Submitted as: model evaluation paper | 14 Apr 2021

Review status: this preprint is currently under review for the journal GMD.

Improvements to the representation of BVOC chemistry-climate interactions in UKCA (vn11.5) with the CRI-Strat 2 mechanism: Incorporation and Evaluation 

James Weber1, Scott Archer-Nicholls1, Nathan Luke Abraham1,2, Youngsub Matthew Shin1, Thomas J. Bannan3, Carl J. Percival4, Asan Bacak5, Paulo Artaxo6, Michael Jenkin7, M. Anwar H. Khan8, Dudley E. Shallcross8, Rebecca H. Schwantes9,10, Jonathan Williams11,12, and Alex T. Archibald1,2 James Weber et al.
  • 1Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, UK
  • 2National Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, CB2 1EW, UK
  • 3School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
  • 4NASA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
  • 5Turkish Accelerator & Radiation Laboratory, Ankara University Institute of Accelerator Technologies, Gölbaşi Campus, 06830 Gölbaşi, Ankara, Turkey
  • 6Physics Institute, University of São Paulo, Rua do Matão 1371, CEP 05351-015, São Paulo, Brazil
  • 7Atmospheric Chemistry Services, Okehampton, Devon, EX20 4BQ, UK
  • 8Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, UK
  • 9Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA
  • 10Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, 80309, USA
  • 11Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, 55128, Germany
  • 12Energy, Environment and Water Research Center, The Cyprus Institute, Nicosia, Cyprus

Abstract. We present the first incorporation of the Common Representative Intermediates version 2.2 tropospheric chemistry mechanism, CRI v2.2, combined with stratospheric chemistry, into the global chemistry-climate United Kingdom Chemistry and Aerosols (UKCA) model to give the CRI-Strat 2 mechanism. A rigorous comparison of CRI-Strat 2 with the earlier version, CRI-Strat, is performed in UKCA in addition to an evaluation of three mechanisms, CRI-Strat 2, CRI-Strat and the standard UKCA chemical mechanism, StratTrop vn1.0, against a wide array of surface and airborne chemical data.

CRI-Strat 2 comprises a state-of-the-art isoprene scheme, optimised against the MCM v3.3.1, which includes isoprene peroxy radical isomerisation, HOx-recycling through the addition of photolabile hydroperoxy aldehydes (HPALDs) and IEPOX formation. CRI-Strat 2 also features updates to several rate constants for the inorganic chemistry including the reactions of inorganic nitrogen and O(1D).

The update to the isoprene chemistry in CRI-Strat 2 increases OH over the lowest 500 m in tropical forested regions by 30–50 %, relative to CRI-Strat, leading to an improvement in model-observation comparisons for surface OH and isoprene relative to CRI-Strat and StratTrop. Enhanced oxidants also cause a 25 % reduction in isoprene burden and an increase in oxidation fluxes of isoprene and other biogenic volatile organic compounds (BVOCs) at low altitudes with likely impacts on subsequent atmospheric lifetime, aerosol formation and climate.

By contrast, updates to the rate constants of O(1D) with its main reactants relative to CRI-Strat reduces OH in much of the free troposphere, producing a 2 % increase in the methane lifetime, and increases the tropospheric ozone burden by 8 %, primarily from reduced loss via O(1D) + H2O. The changes to inorganic nitrogen reaction rate constants increase the NOx burden by 4 % and shift the distribution of nitrated species closer to that simulated by StratTrop.

CRI-Strat 2 is suitable for multi-decadal model integrations and the improved representation of isoprene chemistry provides an opportunity to explore the consequences of HOx-recycling in the United Kingdom Earth System Model (UKESM1). This new mechanism will enable a re-evaluation of the impact of BVOCs on the chemical composition of the atmosphere and probe further the feedback between the biosphere and the climate.

James Weber et al.

Status: open (until 09 Jun 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

James Weber et al.

James Weber et al.


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Short summary
The new mechanism CRI-Strat 2 features state-of-the-art isoprene chemistry, not previously available in UKCA, and improves UKCA’s ability to reproduce observed concentrations of isoprene, monoterpenes and OH in tropical regions. The enhanced ability to model isoprene, the most widely emitted non-methane volatile organic compound, will facilitate improved understanding into how isoprene and other BVOCs affect atmospheric composition and, through biosphere-atmosphere feedbacks, climate change.