India's capital New Delhi is preparing a new weapon in the fight against deadly air pollution: artificial rainfall. The experiment, which could take place as early as next week, will introduce chemicals such as silver iodide into cloudy skies to create rain in the hope of washing away particulate matter hovering over one of the world's largest cities.
The need is urgent, and Delhi has tried traffic restrictions, multi-million-dollar air filtration towers and a fleet of water-spraying trucks to dissolve particulate matter in the air, but to no avail.
If artificial rainfall technology is used, it will be controversial. "It's not a good use of resources at all because it's not a solution, it's just a temporary relief," said Avikal Somvanshi, a researcher at the Center for Science and Environment in New Delhi.
Environmentalists and scientists worry that much of the government's response has focused on mitigating pollution rather than trying to cut off its sources. "There's no political intent to solve this problem, and that's one of the biggest problems," said Bhavreen Kandhari, activist and co-founder of Warrior Moms, a network of mothers demanding clean air.
The air is so bad that schools in and around Delhi have announced closures and offices have allowed employees to work from home. The government recommends that children, the elderly and people with chronic diseases stay indoors as much as possible. Diesel trucks are no longer allowed into the city, except for those transporting essential goods. A few spells of rain last week purified the air, but it was short-lived as the air quality deteriorated again due to firecrackers being set off during Diwali celebrations over the weekend.
Now, Delhi officials are seeking permission from India's federal agency to try artificial rainfall. This technique uses aircraft to spray salts such as silver iodide or potassium iodide or solid carbon dioxide (also known as dry ice) into clouds to induce precipitation. Chemical molecules attach to the moisture already in the clouds, forming larger water droplets that then form rainfall. China has used artificial rainfall in the past to tackle air pollution problems - but for cloud seeding to work properly, it requires large amounts of cloud cover with a reasonable moisture content, which Delhi typically lacks in winter. If weather conditions are favourable, scientists leading the project from the Indian Institute of Technology Kanpur plan to carry out artificial rainfall around November 20.
At least until then, Delhi will remain shrouded in the thick haze that has become a toxic ritual of winter. Smog, a dangerous mixture of particulate matter and toxic gases, is the result of a series of unfortunate events that occur at the start of winter.
In late October, farmers in northern India, especially wheat growers in the states of Punjab and Haryana northwest of Delhi, use a cheap and easy way to clear rice fields for reseeding - lighting fires to burn the stalks left after harvesting. In doing so, they inadvertently release smoke into the air. Authorities have tried to persuade farmers to switch to using machines to remove crop residue rather than burning it, but farmers have not always been able to afford this method. Some small startups turn crop residue into pulp, which is then used to make cardboard products. State and federal governments are also looking into paying farmers not to burn their fields.
Even on the worst days, smoke from crop burning accounts for only about a third of Delhi's pollution, Somvanshi said. Stopping farmland burning will not make the smog go away, as there is no shortage of polluting activities in and around Delhi - coal-fired power plants, brick kilns, garbage burning, household use of wood and charcoal for cooking, etc.
If external sources of pollution weren't enough, fine particulate matter is also formed through chemical reactions. When the conditions are right - a little moisture in the air, cooler temperatures - all gaseous pollutants, especially nitrogen oxides, sulfur oxides, volatile organic compounds and ammonia, start to react to form secondary particulate matter. Nearly 25% of Delhi's fine particulate matter (air pollutants, PM2.5 for short) is caused by this reaction. The formation of secondary particulate matter is also a risk of artificial rainfall, said Gufran Beig, a meteorologist and air pollution expert at the Indian Institute of Tropical Meteorology. A significant improvement in air quality would require sustained heavy rainfall. If cloud seeding produces only a drizzle, humidity will rise, likely stimulating the formation of secondary aerosols, he said.
Delhi's winter smog is also partly caused by an atmospheric phenomenon called inversion. In summer, the air temperature near the ground is higher, while the air temperature aloft is cooler. Therefore, any gases emitted from the ground will naturally rise and disperse with hot air, Somvanshi said: "When winter comes, the situation is completely reversed. As the ground temperature drops, the air close to the ground becomes colder, and as the temperature rises, the air becomes hotter. A thick layer of cold air forms in northern India, preventing any vertical movement. As a landlocked city, Delhi does not have sea breezes to move pollution horizontally. Therefore, the pollutants have nowhere to go."
At dusk, the haze gives the city a natural sepia filter. Across Delhi, electronic screens display the Air Quality Index (AQI), a measure of the concentration of pollutants in the air, in neon lights. The World Health Organization recommends that the average air quality index over a 24-hour period should be below 15. Data from the Central Pollution Control Board shows that the average air quality index across Delhi has been above 350 for most of this month. One block had a record high of 999, and several blocks exceeded 500.
The most harmful components of this deadly gas mixture are PM2.5 and PM10 - air particles or aerosols just a few microns in diameter - which enter the body when breathing, become lodged in the lungs and sometimes even enter the bloodstream. In the short term, they can cause difficulty breathing, leaving you with a prolonged headache, itchy eyes, or a persistent dry cough. Long-term exposure to fine particulate matter has been linked to heart and lung disease, respiratory infections, adverse birth outcomes and more. According to World Bank statistics, in South Asia alone, more than 2 million people die prematurely every year due to air pollution.
Throughout the pollution season, Delhi restricts construction activity to reduce the amount of dust in the air and uses water trucks to try to suppress the remaining dust. The city also has two "smog towers" - air purification systems with giant fans that can filter 1,000 cubic meters of air per second, costing more than $2 million each. But research shows that they can only purify the air within a few hundred meters around them, and are basically ineffective.
One measure that directly targets exhaust emissions is the "odd-even policy", which means that vehicles with odd number plates can be on the streets on odd-numbered days, and vehicles with even number plates can be on the streets on even-numbered days. But the rule is rarely enforced in Delhi and only applies to a small subset of vehicles. "It's like pulling a bucket of water out of the ocean, it doesn't really mean anything. To really work, rules like odd and even numbers should apply all year round. For this, Delhi needs a better public transport system," he said.
The silver lining is that Delhi's annual average PM2.5 levels have been falling since 2015, albeit gradually, new research shows. He said the improvement is more noticeable during the summer and monsoon seasons, when weather conditions are more favorable.
Khandhari said her biggest worry is not seeing a smog-free Delhi in her lifetime. Surprisingly, despite the seasonal clamor, Delhi's bad air has not become an electoral issue. So ahead of next year's election, Khandhari has an important request to her fellow Indians: When politicians ask you for your votes, ask them where clean air ranks among their priorities.