For five days last week, a thick layer of dust hung over Delhi, keeping the air quality ‘severe’, the worst category in the pollution index. It seemed odd because it happened in the peak summer, which is generally considered the off-season for air pollution in Delhi.
But despite air pollution being more visible in the winter, the capital’s air is almost as toxic in the summer. A 2015 IIT Kanpur study found the summer average for PM10 to be over 500 µg/m — five times the national average. About 40% of PM10 particles — with diameter less than 10 micron — was dust. While the major air pollution threat in Delhi is from the tiny PM2.5 particles that get embedded in the lungs, during the summer, PM10 is the primary pollutant.
Last week’s phenomenon was triggered by a dust storm that began over Rajasthan and was carried by strong westerly winds, the IMD said. The dusty blanket that spread itself over Punjab, Haryana, Delhi and western Uttar Pradesh was kept close to the surface by the anticyclonic flow of winds that swirl clockwise in the northern hemisphere, pushing the local air down and preventing outside air from entering the region.
Besides soil, sand and rock particles, windblown dust also contains “re-suspended” dust kicked up by vehicles, digging or construction. The dust hosts toxic materials, including, an IIT Delhi study of the capital’s air found this year, heavy metals such as lead, chromium and nickel.
So was the dust haze a one-off, resulting from the desert storm? The ongoing desertification around Delhi, the uncontrolled urban development, and climate change could make such incidents more common, experts say.
“Delhi’s summer aandhis, like Kolkata’s kaalbaisakhis, are localised events. What we saw this time was different in scale and impact. All of North India was enveloped, and this is something we need to prepare for in the future,” Anumita Roychowdhury of the Centre for Science and Environment said.
Desertification is the process of relatively dry land becoming increasingly arid due to factors ranging from loss of vegetation and overexploitation of soil to climate change. In April, Environment Minister Harsh Vardhan put the rate of desertification in India at 23 hectares of dryland per minute. Nearly 70% of India’s area was dryland, he said, a third of which was affected by degradation and a quarter by desertification. Rajasthan and Delhi were among the worst affected.
What this means is that in the absence of a longterm action plan to stop or reverse the process, we will see more dust. A senior official in the Delhi Environment Ministry said, “These climatic conditions can’t any longer be seen in isolation, we need to start preparing for this to become the new normal.”
The word ‘desertification’ was coined in 1927 by the French colonial forester Louis Lavauden. One of the first to make the connection between desertification and human action was E P Stebbing, a British forester who worked for long in India. In 1935, Stebbing wrote that “misutilisation of soils” and “overutilisation of the vegetable covering of the soil” results in “reduction of water supplies” and “lowering of water table in the soil”.
Today, Australia and several countries in sub-Saharan Africa and West Asia carry out dustfall monitoring (measuring dust deposits in the air) alongside ambient air monitoring, and use the data to plan mitigation processes. The African Union-led “Great Green Wall for the Sahara and Sahel Initiative” aims to create a mosaic of trees stretching 6,500 km across North Africa, Sahel and the Horn. In Australia, where air quality standards for dust are tougher than in Europe, US and UK, vegetation buffers are positioned between residential areas and industrial areas or roads.
Delhi has historically had a barrier of trees in the form of the Delhi Ridge and the linked Aravalli range. But a survey by the Wildlife Institute of India in 2017 found 12 vegetative gaps in southern Haryana, increasing the probability of desertification. From 1999-2012, the forest cover in Haryana, UP and Rajasthan declined from 4.3% to 3.3%, found the National Capital Region Planning Board.
“We need to rethink our urban design. Greening has to be done intelligently. Roads need to be designed with tree cover. The Aravalli and the Ridge need to be protected. This in turn will protect the water table and benefit the city throughout the year,” Roychowdhury said.