Delhi’s Hidden Air Pollution: Toxic Layer Above the City

For years, residents of Delhi have anticipated relief from the city’s notorious air pollution with the arrival of spring. Although, recent research utilizing drone technology reveals a persistent and previously underestimated layer of toxic air hovering above the Indian capital, challenging the perception of seasonal improvement. This elevated pollution poses a significant threat to public health and necessitates a reevaluation of current mitigation strategies.

A Blanket of Toxic Air

A study conducted by a team at the Indian Institute of Technology (IIT) Delhi, published in Nature, demonstrates that air quality 100 meters above the ground can be 60% more toxic than at street level. Researchers used a custom-designed drone platform equipped with a low-cost sensor to measure particulate matter (PM2.5) concentrations. The study, carried out between March 11-23, 2021, found that the volume of PM2.5 present above Delhi, considering a 30km by 40km area up to 1km altitude, is approximately 240,000 kg – equivalent to around 1,200 oil drums.

Unexpected Pollution in Favorable Conditions

The findings are particularly concerning as they occurred in mid-March, a period when climatic conditions – including rising temperatures – typically favor the dispersal of pollutants, unlike the stagnant air of winter. On one day during the study, PM2.5 concentrations reached 160 µg/m³ at an altitude of 100m, while ground monitors recorded only 100 µg/m³. The World Health Organization’s (WHO) daily safe limit for PM2.5 is 15 µg/m³.

The Planetary Boundary Layer and Pollution Trapping

The research highlights the role of the “planetary boundary layer” (PBL) in trapping pollutants. This layer, which can range from 50 meters to a couple of kilometers in height, acts as a ceiling, preventing pollutants from dispersing. During the early morning hours (5 am to 8 am), a suppressed PBL, characterized by cooler air, low wind speeds, and humidity, exacerbates the problem, trapping pollutants near the surface. As the sun rises and the ground warms, the PBL expands, allowing the toxic air to disperse to higher altitudes.

Sources of Delhi’s Pollution

The study identifies several key sources contributing to Delhi’s air pollution, including vehicular emissions (40-50% of PM2.5 during peak hours), industrial activities, waste burning, and residential cooking. The presence of chloride, often linked to the burning of plastic and electronic waste, spikes in the early morning, contributing to particle growth and haze thickening. Pollutants identified include chloride, black carbon, sulphur dioxide, nitrogen dioxide, and ozone.

Traffic’s Significant Contribution

Traffic is identified as a major contributor to the hovering haze, with emissions building up overnight due to the shallow boundary layer. Drones detected high levels of “equivalent black carbon” near major traffic corridors, indicating the significant impact of vehicular exhaust.

The Limitations of Current Mitigation Efforts

The findings raise questions about the effectiveness of current emergency measures, such as water sprinklers and smog towers, which are designed to address ground-level pollution. Researchers argue that a systemic, long-term approach focused on emission reduction is crucial, rather than relying on short-term fixes to filter the large volume of toxic air.

The Need for Expanded Drone Surveillance

The authors of the study call for more extensive drone-based surveillance, at higher altitudes and with improved equipment, to better understand the dynamics of air pollution above the city and to refine models for policy action. The initial study involved 40 drone flights near heavy traffic corridors, utilizing a drone weighing approximately 7.5 kg.

Health Impacts of Exposure

Exposure to pollutants, particularly fine particulate matter like black carbon, is linked to severe respiratory and cardiovascular diseases, including asthma, lung cancer, and strokes. Nitrogen dioxide and ground-level ozone can likewise reduce lung function and trigger chronic obstructive pulmonary disease (COPD) exacerbations.