Simulation and Evaluation of Regional Air Quality in the UK

Air quality is the largest environmental health risk in the UK, contributing to chronic illness. Long-term exposure to ambient PM2.5 results in more than 29,000 premature deaths each year and leads to a reduction in life-expectancy of 7-8 months. However, throughout the year air pollutant concentrations vary, controlled by the complex interaction between emissions, meteorology, chemistry and topography, leading to short-term high pollution events. The complexity of the relationships between these factors means that it is challenging to untangle the drivers of high air pollutant concentrations. An improved understanding of the drivers and impacts of high air pollution episodes is vital in informing policy to reduce the impact of short-term air pollutant exposure on population health. The aim of this thesis was to quantify the impact of different sources and processes on short-term changes in ambient PM2.5 across the UK as epidemiology studies have shown that short-term exposure to PM2.5 is associated with increased mortality and morbidity. Several studies had previously shown that the varied meteorology the UK experiences plays a large role in controlling the concentrations of nitrogen dioxide and ozone. Therefore, we extend this analysis to ambient PM2.5 concentrations. Additionally, wildfires are an emerging threat in the UK due to climate change. The impacts of UK wildfires on air quality and health had not previously been studied, since they had been so rare in the past. Quantifying the impacts of wildfires on air quality and health is particularly important as wildfires since projected to continue to occur more often in the future due to climate change. This is likely to result in pollution from wildfires representing a larger fraction of the population’s annual exposure to air pollution in the future. Therefore, it is important that the impacts are minimised through effective science-led policy. This thesis is split into two key themes as a result: the impact of synoptic weather on ambient PM2.5 concentrations and the impact of wildfires on air quality and health. Ground-based observations of PM2.5 concentrations, a back-trajectory model and output from an atmospheric chemistry transport model were used to investigate the impact of synoptic weather on ambient PM2.5 concentrations. This indicated that synoptic meteorology has a substantial influence on ambient PM2.5 across the UK. Easterly, south-easterly and southerly winds transport pollutants from continental Europe to the UK, increasing ambient concentrations observed. Alongside this, anticyclonic conditions lead to higher PM2.5 concentrations due to the build-up of local emissions under slack winds. This indicates that population exposure to ambient PM2.5 concentrations is closely linked to synoptic weather. Therefore, policies which only consider reductions in local emissions may not yield the greatest reductions in PM2.5 and international cooperation is also required. The Saddleworth Moor and Winter Hill fires in 2018 were one of the first large wildfires in the UK to occur close to a large urban population. They were used as a case-study for the potential impacts of a UK wildfire on ambient air pollutant concentrations and human health. A combination of observational data from satellites, ground-based monitoring, an aircraft flight and an atmospheric chemistry transport model (WRF-Chem) were used to investigate the impacts of the fires. Observations showed that concentrations of pollutants close to the fires were high but, in areas downwind (> 80 km away), concentrations were also enhanced above background values, exposing populations to high concentrations far from the fires. Alongside this, secondary pollutants, such as ozone, were formed in the downwind smoke plume. Modelling results indicated that a large proportion of the population in the region to the west of Saddleworth Moor and Winter Hill were exposed to PM2.5 concentrations above the WHO guideline limit and the moderate DAQI limit. The fires led to increases in the number of deaths brought forward due to exposure to PM2.5 compared to if there were no fires and as a result had a large economic impact. Finally, the impacts of the 2019/2020 Australian bushfires were estimated using WRF-Chem. Fire emissions from FINN indicated PM2.5 emissions from the fires were unprecedented. The WRF-Chem model was used to quantify the air quality and health impacts of PM2.5 from the fires. This indicated that large proportions of the population were exposed to dangerous (‘Poor’,’V. Poor’ and ‘Hazardous’) air quality levels between September 1st 2019 and January 31st 2020. The impacts of the bushfires on AQ were concentrated in the cities of Sydney, Newcastle-Maitland and Canberra-Queanbeyan and Melbourne, with Brisbane and Adelaide less severely affected by the fires. Exposure to PM2.5 from the fires led to an estimated 180 (95% CI: 74, 294) deaths being brought forward between October 1st and January 31st. The health impacts were largest in New South Wales, Queensland and Victoria. At a city-level the health impacts were concentrated in Sydney, Melbourne and Canberra since they have large populations that were exposed to high PM2.5 concentrations for prolonged periods during the fires. Overall, this thesis aimed to quantify the drivers and impacts of short-term air pollution episodes across the UK. Synoptic weather was shown to play and important role in ambient PM2.5 concentrations through the build-up of local emissions and long-range transport of PM2.5 to the UK under continental air masses. This highlights the need for continued cooperation to reduce emissions across Europe. The impact of short-term emerging threats, such as wildfires, was also quantified. This indicated that short-term high pollution events have the potential to have a substantial impact on air quality and health. With increased wildfire frequency projected in the future due to climate change, the results of this work highlight that more research is required to quantify the cost-benefits of public health interventions or changes in land-management practices that may reduce the risk of wildfires in the UK and Australia.