Investigating the impact of urban greening on urban air quality

Poor air quality is the biggest environmental factor contributing to premature mortality globally. As the Earth’s population has grown, the number of people living in urban areas has increased rapidly from 751 million in 1950 to 4.2 billion in 2018. By 2030, the UN estimates there will be 43 megacities (> 10 million inhabitants), with most of them located in developing countries in Africa, Asia and Latin America. In the recent update to the global burden of disease, long-term exposures to fine particulate matter (PM2.5) was estimated to contribute to 4.9 million premature deaths annually, with respiratory and cardiovascular diseases, cancer, diabetes and links to dementia being the main contributors. A large fraction of PM2.5 in cities is organic matter formed in the atmosphere, known as secondary organic aerosol (SOA), but based on current methods, we lack the ability to characterise whether the biogenic or anthropogenic sources are dominant.

 

Most cities have a high percentage of urban green space and plants, grasses and trees can lead to emissions of a complex mixture of biogenic volatile organic compounds (BVOC) including isoprene, monoterpenes, sesquiterpenes and green leaf volatiles. As the atmospheric makeup of cities changes due to (a) increased electrification of the vehicle fleet, (b) increased global temperatures and (c) pollutant mitigation strategies promoting urban greening, the reactive mix of VOCs will also change and BVOCs will become increasingly important contributors to particle formation.

Figure 1: Megacities can have large amounts of green space. This map of London shows that around 60 % of land is either open green space or domestic gardens.

Photochemical oxidation of highly reactive BVOC in the presence of anthropogenic pollutants, in particular nitrogen oxides (NOx) and sulfur dioxide (SO2), can lead to significantly enhanced SOA production, but the impact of this in urban areas is still unknown. A potentially large and unquantified B-A interaction route is the formation of organosulfates (OS) from the interaction of oxidation products of VOCs with sulfate aerosols. Hundreds of different OS have now been found in particles in a range of locations (urban, suburban and rural) and from a wide array of precursors, both biogenic and anthropogenic. Mixed nitroxyorganosulfates (NOS), containing both sulfate and nitrate functionalities, have been found to be very important sources of SOA and are dominated by BVOC precursors, even in cities.

 

Figure 2: Examples of organosulfate formation from the interaction of isoprene, emitted from plants, with combustion sources.

A recent review on OS indicates the formation routes for OS are uncertain and for NOS are completely unknown. Previous indirect measurements indicate that the sum of all OS can contribute 5-30 % of organic aerosol mass, but these are prone to high levels of uncertainty. Currently, there are very few compositional measurements of OS and NOS formed from B-A interaction in urban areas. We have shown that current methods for OS quantification can suffer from high level of uncertainty and the lack of a suitable analytical technique makes quantification of the complete OS content in PM currently impossible. Additionally, although OS and NOS are predicted to make up a significant fraction of ambient PM their toxicity has not been investigated. OS and NOS have both a water-soluble head and a tail that can be lipid soluble, suggesting they may both interact with and cross cellular and intracellular membrane bound compartments, with impacts on cell viability and function.

 

The aim of this PhD studentship is to develop new methodologies to determine the role and magnitude of biogenic-anthropogenic interactions in forming organosulfates in urban particulate matter and develop collaborations to determine their impact on particle toxicity. Non-targeted high resolution mass spectrometry will be combined with machine learning approaches to identify the sources of organosulfates in urban areas, including Beijing, Guangzhou, Delhi and Manchester and compare this to samples in forested regions with limited air pollution collected as part of this studentship. This work will lead to transformative understanding of the sources of organic aerosol, critically needed in order to create effective policies to improve urban air quality and health.

 

The student will work under the supervision of Professors Jacqui Hamilton and James Lee.  The student will be based in the Wolfson Atmospheric Chemistry Laboratory, part of the Department of Chemistry at the University of York. These were established in 2013 and comprise a state-of-the-art dedicated research building, the first of its kind in the UK.

 

The studentship is offered as part of the NERC PANORAMA Doctoral Training Programme that will provide training in addition to that offered by the department.  Through both the departmental and PANORAMA training, there are a wide range of training activities, including courses aimed at specific science objectives, at improving your transferable skills and putting your work into a wider scientific context.

 

You will have a strong background in the physical sciences (good degree in chemistry, physics or similar science), a keen interest in environmental issues, and an aptitude and enthusiasm for experimental work.

 

We appreciate that this PhD project encompasses several different science and technology areas, and we don’t expect applicants to have experience in many of these fields. The project is well supported with experienced scientists and training in these new techniques and disciplines.