Building an evidence base for future air quality policy through global modelling and urban data analysis

Abstract

This thesis aims to develop an evidence base for future air quality policy by combining global modelling and urban measurement data analysis. The studies contained within utilise well-established global models and existing urban air quality monitoring data to develop a comprehensive understanding of air quality at the global and local scales. A range of notable pollutants have been considered throughout the course of this PhD, from ozone depleting substances causing global concern to locally emitted volatile organic compounds (VOCs) with regional impacts.
Global modelling (STOCHEM-CRI) is used to consider inadvertent effects of the evolution away from the initial replacements of ozone depleting substances (HFCs, which have significant GWPs) to alternatives (HFOs) as directed by amendments to the Montreal protocol. Specifically, the accelerated and localised production of TFA, an environmental contaminant produced by HFOs, is modelled and atmospheric concentrations are shown to undergo a thirty-three-fold increase globally if current plans for replacement of HFCs with HFOs proceed. This thesis also contains addition of the ‘Criegee Field’ to account for additional oxidation routes. Whilst this has been shown to be a significant loss process globally for other organic acids, the same is not seen for this contaminant. However, there is a significant local impact over the terrestrial tropics whereby the lifetime of TFA is significantly reduced (by up to 5 days) upon addition of the Criegee Field.
Moving forwards, the focus shifts towards local pollution strategies and their effects on urban atmospheric chemistry and pollutant concentrations. Measurement data from a range of urban sites (measured by governments as part of national monitoring efforts) were utilised to consider: anthropogenic influence on oxidant chemistry; speciated VOC contribution to tropospheric ozone formation; and health impacts of common toxic pollutants in urban centres. Given the nature of the sites and the focus of policy initiatives, most conclusions relate to abatement of traffic related emissions. Abatement of NO2 emissions from traffic were shown to decrease the oxidant concentration at an urban traffic site in the UK (measurement data collected by Department of Environment, Food and Rural Affairs, DEFRA). The same trend was seen at comparable sites in Bangkok (measurement data collected by the Pollution Control Department, PCD). Additionally, most VOC concentrations were shown to have significant decreasing trends at urban sites in the UK; these are also concluded to be as a result of traffic-related emission abatement. The formation of ozone by these pollutants shows a comparable decrease but highlights the emergence of previously neglected VOCs as dominant contributors to ozone formation in urban centres. Namely, ethane and propane are revealed as being increasingly significant contributors. Special attention was given to VOCs with known carcinogenic character, benzene and 1,3-butadiene, and potential health impacts were considered. Cancer impact indices were utilised allowing estimation of cancer impacts across the past 20 years for different site types in the UK. Rural areas were shown to have significantly lower cancer incidence as a result of benzene and 1,3-butadiene pollution than urban areas. However, urban concentrations appear to be converging to rural levels suggesting current legislation ignores alternative sources that are active outside an urban environment. As no safe level of these pollutants can be defined, further legislative intervention is required to lessen the health impact on the nation.
The findings of this study have important implications for policymakers, providing valuable insights into the real-world effects of air pollution mitigation strategies. For example, results presented could aid decision-making regarding future VOC control via consideration of their impact on urban ozone pollution. The study also highlights the need for continued efforts to monitor and reduce air pollution, both globally and locally. Overall, this thesis offers new insights into the complex relationships between air pollution, human health, and policy interventions.

Date of Award	20 Jun 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Dudley E Shallcross (Supervisor) & John Crosby (Supervisor)