Improving the understanding of the influence of emissions of biogenic volatile organic compounds on climate.
University of Cambridge
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Weber, J. (2021). Improving the understanding of the influence of emissions of biogenic volatile organic compounds on climate. (Doctoral thesis). https://doi.org/10.17863/CAM.81484
The feedback between the emissions of biogenic volatile organic compounds (BVOCs), atmospheric composition and climate is a key component of the Earth System. However, there remains uncertainty as to its magnitude and sign. An important factor is the gas phase chemistry of BVOCs since this influences atmospheric oxidising capacity and thus radiative forcing from CH4, O3, aerosol and clouds. Recent advances in the understanding of BVOC oxidation chemistry, which reduce the oxidant depletion typically simulated with enhanced BVOCs, are considered in very few climate models and their impact on the BVOC feedback has not been assessed. More attention has been paid to the impacts of BVOCs solely on organic aerosol which only leads to a partial description of BVOCs’ climatic impact. This thesis assesses the requirements for a BVOC chemical mechanism to be “state-of-the-art”, given the recent advances in chemical understanding. I further examine how updating the description of BVOC chemistry affects firstly simulated atmospheric composition and secondly the climatic impact of BVOCs. BVOC climatic impact and feedback is calculated from the climatic response to a doubling of BVOC emissions using the standard and state-of-the-art mechanisms in a pre-industrial atmosphere with a focus on the gas and aerosol phase changes. Updates to chemistry in the state-of-the-art mechanism result in greater terrestrial boundary layer OH and improved model performance against observations compared to standard mechanisms. The net radiative forcing and feedback from increasing BVOC emissions is positive in both mechanisms considered. Negative forcing from enhanced aerosol scattering is outweighed by positive forcings from O3 and CH4 increases and aerosol-cloud interactions, the latter contrary to past studies. However, the magnitude of the forcing (feedback) is 43% smaller when the state-of-the-art mechanism is used. This is attributed to the lower oxidant depletion which yields smaller increases in CH4 and smaller decreases in cloud droplet number concentration via the suppression of H2SO4 production from gas phase SO2 oxidation. Therefore, the pre-industrial climate is only about half as sensitive to a change in BVOC emissions with the state-of-the-art mechanism, highlighting the significant influence of simulated chemistry. The sensitivity to the chemical mechanism is comparable to the variation in BVOCs’ climatic impact across multiple climate models, suggesting inter-model variation in simulated chemistry, often overlooked, is important for understanding model uncertainty. Finally, a further development to the state-of-the-art mechanism describing the production of highly oxygenated organic molecules (HOMs) from α-pinene is presented, representing a new way for BVOCs to influence climate and the first semi-explicit HOM mechanism suitable for climate models. The α-pinene-HOM mechanism captures experimentally observed dependencies of HOMs on isoprene, NOx and temperature, while also agreeing with previous suggestions that nucleation from HOMs is likely to be more influential in a pre-industrial atmosphere. CRI-HOM provides a basis for further improving understanding of the BVOC-climate feedback. The model updates and analysis presented in this thesis will allow important policy-relevant questions, including those regarding climate change mitigation strategies, to be addressed in a more comprehensive manner.
Climate change, atmospheric chemistry, chemistry, BVOCs, isoprene
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This record's DOI: https://doi.org/10.17863/CAM.81484
Attribution 4.0 International (CC BY 4.0)
Licence URL: https://creativecommons.org/licenses/by/4.0/