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Atmospheric Chemistry in an Indian Megacity

Prof. James Lee (WACL, University of York), Dr. Andrew Rickard (WACL, University of York)

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The majority of the World’s population lives in heavily urbanized areas impacted by high levels of pollution, which can have severe detrimental effects on air quality and human health – reducing life expectancy by several months and costing society billions.  A recent study (WHO, 2014) identified Delhi as the most polluted city in the world (based on PM2.5 annual concentrations),  with considerable health impacts for ~19 million inhabitants, causing an estimated 20,000 premature deaths annually, mainly due to particulate matter (PM) and ozone (O3). This figure is anticipated to rise to ~30,000 by 2025 (Lelieveld et al., 2015).

The factors controlling Delhi’s air quality are complex; they include local and regional emissions, changes in synoptic meteorology as well as the detailed atmospheric chemistry controlling the composition of the urban atmosphere. Primary urban pollution emissions are dominated by particulate matter (PM), nitrogen oxides (NOx), carbon monoxide (CO) and an extensive reactive mix of volatile organic compounds (VOCs). Many of these species can react photochemically in the atmosphere to create secondary pollutants, such as ozone (O3), oxygenated VOCs (oVOCs), peroxy acyl nitrates (PANs) and condensed material in the form of secondary organic aerosol (SOA), which add to the overall PM load and contribute to photochemical “smog”.  Numerical models of atmospheric chemistry are therefore essential to understanding this complex chemical composition and are key to our ability to understand, predict and hence mitigate urban air quality problems. 

Aims and Objectives

The overall aim of this project is to take a unique and comprehensive observational dataset of primary and secondary pollutants and reactive intermediates as part of two intensive field campaigns (winter and summer) at two sites in the Delhi metropolitan area (see Figure 1).  These measurements will be made as part of a large consortium project involving 10 UK Universities and 4 Indian institutions, working together to look at key processes involved in gas and aerosol pollution formation, meteorological dynamics and the links between them in Delhi’s atmosphere. The measurement suite, as a whole will then be used to constrain photochemical models incorporating the highly detailed Master Chemical Mechanism (MCM). 

The MCM provides a highly comprehensive ‘state of the science” representation of atmospheric VOC degradation chemistry, which is extensively used by the atmospheric community in a wide variety of science and policy applications where chemical detail is required. The MCM is an internationally recognised resource, with registered users worldwide, and thus represents a highly regarded flagship facility for atmospheric science in the UK.  Much of its success stems from the availability of the MCM database on the web (, along with the provision of a range of tools to facilitate its use.

Figure 1: Left panel shows the location of the two measurement sites in Delhi (yellow markers), the right panel shows an example of severe air pollution around the Presidential Palace (red marker on the map).

Detailed photochemical box models will be constructed and used to not only try to understand the chemical factors controlling Delhi’s air quality but also to evaluate our fundamental understanding of urban atmospheric chemistry through model/measurement comparisons as well as comparisons to other, potentially quite different urban environments (in terms of geography and emissions) such as London, Beijing and Hanoi.

The project will involve the opportunity for the student to go to India during the two field campaigns to assist in making the measurements and also to enable collaborations between UK and Indian measurement and modelling scientists.

Specific areas of research are likely to centre around the following:

(1)  Understanding the OH reactivity of the Delhi atmosphere

Detailed speciated measurements of VOCs (Dunmore et al., 2015) will be used to constrain the MCM phortochemical box models in order to determine the important hydroxyl radical sinks in the Delhi atmosphere.  Through detailed speciation and model analysis we will able to determine which VOCs are significantly contributing to the hydroxyl radical (OH) reactivity, and hence are contributing to photochemical ozone formation – i.e. diesel vs. petrol sources, anthropogenic vs. natural biogenic contributions (see Figure 2a).

(2)  Determination of the radical sources of the Delhi atmosphere

Temporal sources of the OH radical sources will be investigated using the measurements made during the intensive field campaigns. Nitrous Acid (HONO) plays a critical role in the atmospheric chemistry discussed as it rapidly photolyses to produce OH, the primary tropospheric oxidant. Recent fieldwork (Lee et al., 2016) has shown that HONO photolysis can dominant urban OH production – up to 60% – controlling the oxidizing capacity of the atmosphere. Despite this the budget, source strengths and formation mechanisms remain unclear. In particular, a significant unknown daytime source of HONO exists, which has been strongly linked to photolysis/photo-enhanced homogeneous gas and heterogeneous surface chemical sources in the laboratory. New measurements of HONO made in Delhi, including its direct emission flux using the eddy covariance technique (Honrath et al., 2002) and gradient measurements (Villena et al., 2011) will be used to investigate the source strength of HONO, and whether its production is dominated by ground surface sources (vs. fast aerosol surface sources), hence to what extent HONO contributes to OH production throughout the urban boundary layer (see Figure 2b).


Figure 2a: Average diurnal profile of measured OH reactivity in London and a breakdown of modelled reactivity when the model is constrained to the measured VOCs. Figure 2b: Average relative diurnal primary OH radical production rates calculated from measurements taken during the summer 2012 in London.

(3)  Evaluation of reduced chemical schemes against the benchmark MCM

Constrained box models incorporating the Common Reactive Intermediate (CRI) v.2 chemistry scheme (Jenkin et al., 2008), a reduced form of the MCM, as well as a range of other simplified chemical mechanisms regularly used in policy orientated climate and air quality models will be evaluated against the full benchmark MCM simulations and the measurements in order to compare performance and aid optimization.  Such reduced chemical schemes are needed in chemical transport models in order to simulate atmospheric chemistry on regional and global scales, where the essential features of the chemistry represented in the MCM need to be retained whilst removing most of the complexity (Emmerson and Evans 2009).  New versions of automatically generated versions of the MCM will also be evaluated. The different models will be used to determine the relative importance of different sources of OH radicals in Delhi and to investigate the factors that drive in-situ ozone production.

Potential for high impact outcome

As well as identifiable significant scientific outcomes of the project, there are potentially wider societal impacts. The research will directly benefit policymakers in Delhi and the rest of India, including the Indian Ministry of Earth Sciences (ESSO-MoES) and Department for Biotechnology (DBT) who are responsible for policy-making in air quality in Delhi and the surrounding region. It also has a potentially large impact for the general public. The public profile of air pollution is currently very high, both in India and the UK, and this project provides opportunities for potential engagement with the public on the underlying science, and also potentially with a range of other organizations, for example non-government organizations (NGOs), for which air pollution may be one of many environmental and health issues of interest.


The student will work under the supervision of both Prof. James Lee ( and Dr. Andrew Rickard ( (University of York) and will be based at the Wolfson Atmospheric Chemistry Laboratories (WACL), part of the Department of Chemistry, University of York. Here, the student will develop transferrable skills in making atmospheric measurements (e.g. of VOCs and HONO), chemical mechanism development and evaluation, numerical and data skills associated with model/measurement inter-comparison and model analysis.  The student will also have the opportunity to take part in the field campaigns at the IAP tower site in India, collaborating with UK and Indian scientists.

The University of York and the wider NERC SPHERES DTP provide comprehensive training programmes for PhD students with a range of courses on both hard and soft skills. Prof. Lee and Dr. Rickard both work for the National Centre for Atmospheric Science (NCAS), and thus the student will have access to the wider resources that NCAS provides. The student will also have access to training provided by NCAS such as the Arran instrumental Summer School, the Earth System Science Summer School (ES4), and future further developments in computations and data analysis.

The student will work in the Wolfson Atmospheric Chemistry Laboratories, part of the department of Chemistry, University of York.  These were established in 2013 and comprise a state of the art 800 m2 dedicated research building, the first of its kind in the UK. Supported by a large award from the Wolfson Foundation and a private donor, the Laboratories enable experimental and theoretical studies relating to the science of local and global air pollution, stratospheric ozone depletion and climate change. The Laboratories are operated as collaborative venture between the University of York and the National Centre for Atmospheric Science (NCAS), co-locating around 40 researchers from seven academic groups and from NCAS. The Laboratories are also home to independent research fellows, postdoctoral researchers, PhD students and final year undergraduate research projects.

The student will have the opportunity to present their work to the scientific community at national and international meetings and conferences. They will also be encouraged to take part in outreach events organised by both WACL and NCAS in order to disseminate the research beyond the immediate scientific community (e.g. to policymakers and the general public).

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 very well supported with experienced scientists and training in these new techniques and disciplines is all part of the PhD.

Useful References

  • Dunmore et al., (2015): Diesel-related hydrocarbons can dominate gas phase reactive carbon in megacities; Atmos. Chem. Phys., 15, 9983–9996.

  • Emmerson and Evans, (2009):  Comparison of tropospheric gas-phase chemistry schemes for use within global models; Atmos. Chem. Phys., 9, 1831–1845.

  • Honrath et al., (2002): Vertical fluxes of NOx, HONO and HNO3 above the snowpack at Summit, Greenland, Atmos. Env., 36, 2629-2640.

  • Lee et al., (2016): Detailed budget analysis of HONO in central London reveals a missing daytime source; Atmos. Chem. Phys., 16, 2747-2794.  

  • Lelieveld et al., (2015): The contribution of outdoor air pollution sources to premature mortality on a global scale; Nature, 525, 367-371.

  • Jenkin et al., (2008):  A Common Representative Intermediates (CRI) mechanism for VOC degradation. Part 1: Gas phase mechanism development; Atmos. Env., 42, 7185–7195.

  • Villena et al., (2011): Vertical Gradients of HONO, NOx and O3 in Santiago de Chile; Atmos. Env., 45, 3867-3873.

  • WHO, (2014).

Related undergraduate subjects:

  • Chemistry
  • Environmental science
  • Mathematics
  • Physics