Traffic can contribute up to 40% of total organic aerosols at urban roadside

A study published in the journal Nature Communications has investigated the sources and health impacts of PM2.5 in Northern India, particularly in the Indo-Gangetic Plain. The study has examined PM2.5 composition and oxidative potential, a key indicator of its health risks, using samples from five sites: urban and roadside locations in Delhi, rural and industrial peripheries, and a suburban site in Kanpur. Addressing local inefficient combustion processes can effectively mitigate particulate matter health exposure in northern India, the study finds.

The study found that although uniformly high particulate matter concentrations were recorded across the entire region, local emission sources and atmospheric processes dominate particulate matter pollution. “In Delhi, PM2.5 is dominated by ammonium chloride and organic aerosols from vehicular emissions, residential heating, and fossil fuel oxidation,” says Dr. Sachchida N. Tripathi, Professor at the Department of Civil Engineering & Department of Sustainable Energy Engineering, IIT Kanpur, and one of the corresponding authors. “Outside Delhi, ammonium sulfate, ammonium nitrate, and biomass-burning-derived organic aerosols are more prominent.” The study highlights that PM2.5 oxidative potential is primarily influenced by organic aerosols from incomplete combustion of biomass and fossil fuels, particularly from traffic and residential sources. This trend is observed across all locations, emphasizing that inefficient local combustion is a major contributor to PM2.5-related health risks.

Hydrocarbon-like organic aerosols originate from fresh vehicular tailpipe emissions. The study found that the highest average hydrocarbon-like organic aerosols concentrations (8 micrograms per metre cube) were recorded at the urban roadside site in Delhi. The hydrocarbon-like organic aerosol concentrations do not show great variations across seasons.

Consistent with previous studies, the current study found that hydrocarbon-like organic aerosols are primarily from traffic and contribute up to 20% of total organic aerosols mass with higher relative contributions in the warm season. From 20%, the contribution from traffic can increase to 40% at urban roadside. “In all, hydrocarbon-like organic aerosols constitute 50% of the total fossil (coal, petrol, diesel) organic aerosols,” says Dr. Tripathi.

Cow dung combustion during winter for heating and cooking contributes to cold-season primary organic aerosols. The cold-season primary organic aerosols are highly elevated during the night and exhibit spatially homogeneous contribution. Also, concentration of cold-season primary organic aerosols during cold weather are up to 10 times higher than during warmer weather. This is because of increased residential heating or cooking emissions and shallower boundary layer conditions.

Urban oxygenated organic aerosols are affected by both fossil emissions from vehicle exhausts and non-fossil emissions from cooking, and have similar concentration levels across seasons. While hydrocarbon-like organic aerosols and urban oxygenated organic aerosols are especially important inside Delhi, cold-season oxygenated organic aerosol forms outside Delhi, the authors write.

Comparative analysis shows that the oxidative potential of PM2.5 in Indian cities is among the highest globally, exceeding levels in Chinese and European cities by up to five times. “The study provides crucial insights for policymakers to design effective air quality control strategies focused on reducing primary emissions from incomplete combustion,” says Dr. Tripathi.