Quantifying the impact of on-road transport on fine particulate matter over Delhi megacity

Outdoor air pollution is an increasing public health burden. Fine particulate matter (PM2.5) is a pollutant of major concern for human health, and it also affects the climate and ecosystems. Understanding and quantifying emission sources and their impact on particulate air pollution is critical for improving global health and for informing climate action. Poor air quality across the globe disproportionately affects middle- and lower-income countries. Delhi, the capital of India, is one of the most populated and polluted megacities in the world, where in 2017 almost 12,000 premature deaths were attributed to outdoor air pollution. My thesis aims to advance our understanding of outdoor air pollution in Delhi megacity, with a focus on the impact of on-road transport emissions on surface levels of PM2.5 and its implications for air quality policymaking. To do this, I use a combination of a state of the art regional atmospheric chemistry transport model, recently developed local emissions inventories, and sensitivity analysis techniques. In the first research chapter I use the WRF-Chem atmospheric chemical transport model to understand the regional influence on air quality over Delhi. As part of this work, I characterise seasonal anthropogenic, pyrogenic, and biogenic influences on fine particulate matter and one of its main constituents, organic aerosol (OA), over the Indo-Gangetic Plain (IGP). My results show that anthropogenic emissions influence the magnitude and distribution of PM2.5 and OA throughout the year, especially over cities including Delhi, while pyrogenic emissions from crop residues burning result in localized contributions over the central and upper parts of IGP in all non-monsoonal seasons, with the highest impact during the post-monsoon season that correspond to the post-harvest season in the agricultural calendar. Biogenic emissions play an important role in the magnitude and distribution of PM2.5 and OA during the monsoon season, particularly over the lower IGP. In all seasons mean values of PM2.5 still exceed the recommended levels, indicating that air pollution is a year-round problem. In the second research chapter I develop the WRF-Chem model used in my first chapter to include local emission inventories, in order to quantify the contribution of the on-road transport sector to surface PM2.5 over Delhi during the highly polluted post-monsoon season. This contribution is compared to the contributions of other local (within Delhi) and regional (within the National Capital Region, NCR) anthropogenic sectors. My results show that emissions from the local transport sector contribute typically less than 10% to daily mean PM2.5 values over Delhi, rising to 17% when regional transport sources are included. The contribution from the local transport sector is largest (18%) during the evening traffic peak. The total transport impact is dominated by contributions from twoand three-wheelers (50%) and heavy-duty vehicles (30%). The largest individual contributions to daily mean PM2.5 values are found to be from regional power and industry (14%) and domestic (11%) sectors. In the third research chapter I drive the WRF-Chem model with future transport emissions scenarios to investigate the potential impact of electric and clean-fuel vehicles on surface PM2.5 and ozone (O3) over Delhi for two contrasting seasons, pre-monsoon and post-monsoon. My results show that the conversion of diesel vehicles to compressed natural gas (CNG) brings a greater reduction in PM2.5 concentrations than the full electrification of two- and three-wheelers. However, the maximum reduction of daily mean PM2.5 concentrations for all scenarios is within 5% compared to baseline values for both seasons. Electrification of two- and three-wheelers increases average 8-hour daily maximum (MDA8) ozone (1.3-3.5% in pre-monsoon 5-13% in post-monsoon) compared to baseline values. On the other hand, conversion of all diesel vehicles to CNG reduces MDA8 O3 in both seasons (by 2.3-5.3% in pre-monsoon and by 1-1.5% in post-monsoon) compared to baseline values. In conclusion, the findings of my thesis highlight different factors that can be relevant for designing effective policies to meet PM2.5 air quality standards over Delhi megacity, with a focus on mitigating the impact from the on-road transport sector. First, air quality over Delhi is strongly influenced by regional and seasonal pollution sources from the IGP. As such, effective mitigation of PM2.5 pollution over Delhi will require a range of regional and state-level policies. In particular, cooperative mitigation strategies between the Delhi megacity and the broader NCR is needed if PM2.5 pollution is to be reduced. Second, two-and three-wheelers and heavy-duty vehicles dominate on-road transport impact on PM2.5, thus emissions reductions from these vehicles should be given priority, both within Delhi and in the NCR. Third, cleaner mobility plans of electrification of two- and threewheelers should be accompanied by diesel vehicles conversion to compressed natural gas, to limit ozone pollution increase and further reduce PM2.5 concentrations. This also highlights the importance of coordinated control of PM2.5 and other pollutants such as O3 when considering emission control strategies for transport over Delhi.