Traffic in Delhi contributes to highest share of toxic VOC emissions

Traffic emissions and the burning of solid fuel contribute to the highest share (80%) of volatile organic compounds (VOCs) in Delhi’s air, the national capital’s first-ever analysis of such compounds has revealed.

VOCs are gases emitted from certain solids and liquids, affecting local and regional air quality, human health and climate, both directly and indirectly. They are a group of pollutants that can cause a range of health issues, including injury to the central nervous system and even certain cancers, according to the US Environment Protection Agency.

A team of 14 researchers from the Indian Institute of Technology (IIT)-Kanpur, IIT-Delhi, Laboratory of Atmospheric Chemistry (LAC), Switzerland, Space and Atmospheric Sciences Division and the Geosciences Division (Ahmedabad) conducted the study of the source apportionment of VOCs in Delhi’s air. They collected high-frequency data during January -March 2018.

They collected the samples at two locations—IIT-Delhi campus (an urban area surrounded by residential and commercial areas) and Manav Rachna International University (MRIU), Faridabad (a suburban site in a relatively open area and small parks in the vicinity).

The study is part of the Central Pollution Control Board’s (CPCB) ongoing projects on real-time source apportionment of air pollution as well as the Department of Biotechnology’s on the impact of air pollution on health.

“During the analysis, we observed a high concentration of VOCs and trace gases. We also saw strong day-night variations in the concentrations at the two sites. We identified six major contributing factors to VOCs at the sites, including two traffic-emission related factors, two solid-fuel combustion factors and two secondary factors,” SN Tripathi, head, civil engineering department, IIT-Kanpur, and one of the corresponding authors of the study, said.

At the urban site, traffic-related emissions contributed 56.6% to the total VOC concentration compared to 36.0% at the suburban site. The researchers found a higher concentration at the urban site, mainly because of heavy vehicular movement during the day and the plying of heavy-duty vehicles during the night as compared to the suburban site.

Solid fuel combustion contributed 27.5% to emissions at the urban IIT-D site, and 30.4% to the emissions at the suburban MRIU site.

“Emissions from the burning of solid fuels (such as wood, charcoal, coal, biomass, leaves and waste), particularly in the night, were identified as a significant source contributing to a high concentration of VOCs at both the sites,” the study read.

According to the study, secondary factors were the most important source of VOCs during the daytime, accounting for 15.9% of the total VOC concentration at the urban site and 33.6% at the suburban site. At the suburban site, the factors were dominated by oxygenated VOCs such as benzene, toluene, xylene, which are mainly emitted from industries and the burning of biomass. These substances are carcinogenic and prolonged exposure to them can adversely impact human health.

“The study highlights the crucial role that anthropogenic sources (man-made emissions) play in pollution levels in Delhi. We urgently need measure to reduce such emissions from traffic exhaust and solid fuel combustion to mitigate the severe pollution and environmental impact of VOCs as well as aerosols in Delhi-NCR,” Tripathi said.

Interventions such as transitioning to cleaner fuels and ultimately to electric vehicles are required to cut down on these emissions and improve the ambient air quality in the region, he added.

“It is crucial to study VOCs to get a complete picture of air pollution and its sources. For instance, when Delhi switched to CNG (lesser polluting fuel) from diesel, it was thought that, for the time being, it would reduce pollution. But CNG emits more VOCs, and eventually, cities like Delhi will have to convert to cleaner fuels to actually control high levels of pollution,” Dilip Ganguly, associate professor, Centre for Atmospheric Sciences, IIT-Delhi, and one of the study’s corresponding authors, said.

An air quality analysis conducted by the Centre for Science and Environment (CSE) during January 1-May 31 this year also revealed high levels of ozone (a product of VOCs and NO2) pollution in Delhi-NCR. The analysis underlined vehicular and industrial emissions as the major reasons behind it.

Anumita Roychowdhury, the executive director of research and advocacy, CSE, said, “So far air pollution has been dealt with only in terms of particulate matter (PM), which is not enough, given the complex nature of pollution in Delhi-NCR. Gases such as VOCs are major components of air pollution. They are toxic and harmful and contribute to the formation of ozone and secondary particulates in the atmosphere. We require a holistic approach to address the issue of air pollution. Cleaner action plans, therefore, need to address particles and gases together.”