NEW DELHI – For years, residents of the Indian capital have looked to the skies in March with a sense of relief, believing that the notorious winter pollution had finally retreated. 

However, a drone armed with an air quality monitor has shattered this perception, revealing a massive blanket of toxic air hovering just above the city’s skyline. This vertical pollution acts like a pollution lid over the capital. It also contains unexpectedly high concentrations of dangerous particles that ground-level monitors often fail to capture.

Dark surprise in the spring sky

The study indicates the vast scale of Delhi’s pollution. One of the authors, Prof Sagnik Dey of  theIndian Institute of Technology (ITT) Delhi, estimates this to be roughly 240,000 kgs, equivalent to some 1,200 oil drums.

“Taking Delhi, 30 km by 40 km, and considering PM2.5 up to 1 km, we are talking 1200 km3 volume of air,” Dey told Health Policy Watch. 

“When PM2.5 is 200 ug/m3 on a day, we are talking about 2,40,000 kg of particles to be cleaned out.”

Another surprise is that this smog blanket occurred in mid-March when climatic conditions, including and temperature, favour the dispersal of pollution, unlike in winter.

The study was conducted by a team at IIT Delhi, from its South Delhi campus between 11-23 March, 2021. 

The research, published in Nature this month, shows that while the air at street level might seem manageable, the atmosphere 100 metres above the ground was 60% more toxic. 

The new findings raise questions about how much of Delhi’s air crisis is driven by local sources and why current emergency measures are failing to clear the sky.

There is a lot of complex chemistry at work, churning out the lethal cocktail of pollutants. The ingredients and level of pollution change depending on whether it is morning, noon, evening or night. 

The authors – numbering 20 from institutions in nine countries ranging from the UK to China – say that the study provides enough impetus to study the pollution above the city more deeply, using drones, to improve models and policy action to reduce air pollution. 

Toxic mornings 

The most dangerous window for residents is between 5am and 8am, when a “suppressed planetary boundary layer” acts like a physical ceiling below 200 metres altitude, trapping pollutants near the surface thanks to factors like cooler air, low/no wind, and humidity levels. 

The planetary boundary layer (PBL) can be as low as 50m and as high as a couple of thousand, depending on the temperature, time of day or year, and other factors. 

Usually, the higher the PBL, the lower the concentration of pollutants at ground level, which is why in summer the level of particulate matter tends to be lower in hot places like Delhi. 

On one day, the study found PM2.5 concentrations reached a staggering 160 µg/m³ (micrograms/cubic metre) at an altitude of 100m, typically the height of a 30-storey residential building. Ground monitors recorded only 100 µg/m³; WHO’s daily safe limit is 15 µg/m³. 

The altitude of the toxic haze shows the strength of local emissions and regional transport, Dey says. 

“This calls for prioritising emission reduction as the main mitigation strategy through systemic long-term measures rather than short-term emergency measures trying to filter this large volume of toxic air,” he added. 

The Delhi government has resorted to using hundreds of water sprinklers on trucks and buildings to subdue dust. In the past, it built ‘smog towers’, essentially outdoor air purifiers, before conceding these are ineffective. 

As the sun rises and the ground warms, this “lid” expands, allowing the toxic air to disperse into higher altitudes. By noon and into the early evening, the air becomes significantly clearer, with temperatures reaching 30-35°C and humidity dropping to 40-50%.

What causes Delhi’s pollution peaks?

The report breaks down a list of pollutants, secondary pollutants formed as a result of primary pollutants and other factors like temperature and chemical reactions. 

The chemicals produced include chloride, black carbon or soot, sulphur dioxide to some extent from coal combustion, nitrogen dioxide and ozone, especially during traffic rush hour. These are emitted by burning biomass, solid fuels, waste, and from industries. 

An increase in the levels of chloride and black carbon are linked to the burning of biomass and solid fuel, which could be in waste as well. Tackling waste burning has often been identified by experts as a top priority to reduce the region’s air pollution. 

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

Traffic is a prime suspect

A significant cause for this hovering haze is the city’s roads. The report highlights the staggering impact of vehicular pollution, noting that traffic contributes 40-50% of PM2.5 during peak hours. In areas near major traffic corridors, the drones detected high levels of “equivalent black carbon,” a particularly soot-heavy component of exhaust.

This traffic-led pollution does not simply blow away. Instead, emissions from the millions of vehicles on Delhi’s roads build up overnight, trapped by the shallow boundary layer. This nocturnal build-up is further complicated by other local sources, including waste burning, industrial activities, and residential cooking. The study found that chloride, often linked to the burning of plastic and electronic waste, spiked in the early hours of the morning, acting as a chemical trigger that causes particles to grow and thicken the haze.

Call for more drones 

To capture this data, researchers utilised a custom-designed drone platform built by an Indian start-up, BotLab Dynamics. The drone, weighing roughly 7.5 kg, was equipped with a modified low-cost sensor (LCS).

The study involved 40 separate drone flights near heavy traffic corridors. However, back in March 2021, time was tight due to Covid restrictions. Now, the authors have called for more drone-based surveillance, at higher altitudes, and with better equipment. 

In the right conditions, tons of these pollutants can descend and threaten public health. 

Image Credits: Gustaf von Zeipel/ Unsplash, Chetan Bhattacharji.