Global and regional modelling of trace gases and oxygenated volatile organic compounds

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

The chemical transport model, STOCHEM-CRI was used to investigate the global representation of atmospheric ethanol. Photochemical production (via the peroxy radical reactions) was best represented by STOCHEM-CRI, showing full representation of ethyl peroxy radical (C2H5O2) chemistry, thus providing a reasonable estimation. OH oxidation dominated the global sink, with wet depositional loss reported by STOCHEM-CRI being much larger than those observed in other studies. STOCHEM-CRI was also used to constrain global sources of ethanol using observational data from various locations, with underestimations at urban and suburban sites due to its coarse resolution. Three measurement locations provided a good basis for constraining global ethanol emissions using STOCHEM-CRI, though more long-term in situ measurements are required globally to improve this. Model bias analysis highlighted consistent underestimations of measured data.
The WRF-Chem-CRI model was used to predict air quality at sites in London and across the U.K. Absolute concentrations and diurnal variations were reproduced by the model, with particularly good representations of high O3 events. Notable discrepancies in the NOx data were observed at sites influenced by traffic emissions, highlighting scope for the refinement of the treatment of traffic-sourced NOx emissions in the model. The study also indicated an under-representation of VOCs in the model. Nested simulations at a higher spatial resolution were run, enhancing the amount of atmospheric structure captured but having minimal impact on model accuracy.
WRF-Chem-CRI was also used to investigate nitrate chemistry in London. The model’s chemical mechanism and the Master Chemical Mechanism (MCM) were used to deduce the production pathways of modelled organonitrates. This showed that all organonitrates in WRF-Chem-CRI are formed exclusively from the reaction of RO2 with NO, or the reaction of NO3 with alkenes. Temporal analysis highlighted a significant contribution of NO3-sourced organonitrates, even during daylight hours. Lifetime calculations showed that the NO3-sourced organonitrates in WRF-Chem-CRI act as NOx reservoirs, with particularly short-lived species impacting on air quality as contributors to downwind O3 formation.
STOCHEM-CRI was used to study the global impacts associated with biofuel-sourced butanol emissions. Simulations using current biofuel emission estimates (1.8 Tg/year) resulted in a 1.5% increase in upper tropospheric O3, highlighting a potential climatic impact. Sensitivity simulations showed that end-product branching ratios had similar impacts on O3. Increasing biofuel-sourced butanol emissions ten-fold resulted in noticeable levels of surface O3 across the Northern Hemisphere, with increases of up to 13 ppb in east Asia. This study has raised the potential environmental and epidemiological issues of biofuel use, thus bringing their suitability as fossil fuel alternatives into question.
Date of Award26 Nov 2020
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorDudley E Shallcross (Supervisor) & Mark H Lowenberg (Supervisor)

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