The reasons for the collapse in air quality are both man-made and natural:

Natural Reasons:

• Change in Wind Direction: October usually marks the withdrawal of monsoons in Northwest India and during this time, the predominant direction of winds is northwesterly. The direction of the wind is northwesterly in summers as well, which brings the dust from northern Pakistan and Afghanistan.
• Dust Storms: Dust storms from Gulf countries enhance the already worse condition. Dry cold weather means dust is prevalent in the entire region, which does not see many rainy days between October and June. Dust pollution contributes to around 56% of PM10 and the PM2.5 load.

• Temperature Inversion: As temperature dips, the inversion height is lowered and the concentration of pollutants in the air increases when this happens. Inversion height is the layer beyond which pollutants cannot disperse into the upper layer of the atmosphere.

Man-made reasons

• Vehicular and industrial emissions that get trapped in the winter fog: It is one of the biggest causes of dipping air quality in Delhi in winters and around 20% of PM2.5 in winters comes from it.
• Fireworks in the run-up to Diwali: It may not be the top reason for air pollution, but it definitely contributed to its build-up.
• Construction Activities: Due to rising population leading to increased spread of Urbanisation, large-scale construction in Delhi-NCR is taking place. This is another culprit that is increasing dust and pollution in the air.
• Open Waste Burning: Delhi also has landfill sites for the dumping of waste and burning of waste in these sites also contributes to air pollution.
• High Population: Over-population adds up to the various types of pollution e.g. huge solid waste, water waste, construction activities emitting particulate pollution etc

• While civil society and media attention for Delhi’s air pollution problem and high PM2.5 concentrations peaked in the last decade, with annual episodes of pollution induced smog setting over the Capital in winter months, air pollution has been on the rise since the 1990s.
• In March 1995, the Supreme Court, while hearing a plea by environmentalist and lawyer M.C. Mehta about Delhi’s polluting industries, noted that Delhi was the world’s fourth most polluted city in terms of concentration of suspended particulate matter (SPM) in the ambient atmosphere as per the World Health Organisation’s 1989 report.
• The Court took note of two polluting factors— vehicles and industries, and in 1996 ordered the closure and relocation of over 1,300 highly-polluting industries from Delhi’s residential areas beyond the National Capital Region (NCR) in a phased manner.
  • Multiple brick kilns were also directed to be relocated outside city limits.
• In 1996, Mr. Mehta filed another public interest litigation alleging that vehicular emissions were leading to air pollution and posed a public health hazard.
  • Later that year, the Delhi government submitted an action plan.
• The Supreme Court, recognising the need for technical assistance and advice in decision-making and implementation of its orders, asked the Environment Ministry to establish an authority for Delhi, leading to the creation of the Environmental Pollution Control Authority of Delhi NCR (EPCA) in 1998.
• The EPCA submitted its report containing a two-year action plan in June 1998 and the Supreme Court subsequently ordered the conversion of the whole Delhi Trasport Corporation (DTC) bus fleet, taxis, and autos to Compressed Natural Gas (CNG), and the phasing out of all pre-1990 autos.
  • Other measures between the late 1990s and early 2000s included the complete removal of leaded petrol, removal of 15 and 17-year-old commercial vehicles and a cap of 55,000 on the number of two-stroke engine auto rickshaws (which reports at the time said were contributing to 80% of pollution in the city).
  • Coal-based power plants within Delhi were also later converted to gas-based ones.
• Around this time, the Centre also decided to revamp its monitoring programme and establish a network of monitoring stations under the National Air Quality Programme (NAMP) to measure key pollutants.
• Under the National Ambient Air Quality Standards (NAAQS) specified by the Central Pollution Control Board (CPCB), pollutants like PM10 (particulate matter with a diameter exceeding 10 microns), sulphur dioxide and nitrogen oxides were measured.

• The National Ambient Air Quality Standards were revised in 2009 to include 12 categories of pollutants including PM2.5 (diameter under 2.5 microns).
  • PM2.5 is a noxious pollutant which can penetrate deep into the lungs and even enter the bloodstream, resulting in cardiovascular and respiratory impacts, and potentially also affecting other organs.
  • Particulate Matter (PM) is primarily generated by fuel combustion in different sectors, including transport, energy, households, industry and agriculture.
• According to the revised NAAQS, the acceptable annual limit for PM2.5 is ​​40 micrograms per cubic metre (µg/m 3) and 60 µg/m 3 for PM10. The renewed WHO standards meanwhile, prescribe an accepted annual average of 5 µg/m 3 for PM2.5 and 15 µg/m 3 for PM10.
  • A study by the US-based Health Effects Institute released this year studied data between 2010 and 2019, finding Delhi to be the most polluted city in the world in terms of PM2.5 levels, reporting an average annual exposure (relative to population) of 110 µg/m3.
• In January and April 2016, the Delhi government tested the odd-even vehicular rationing rule in two fortnightly windows.
• In the winter of 2016, Delhi witnessed one of its worst incidents of pollution-induced smog, with PM2.5 and PM10 levels reaching a whopping 999 µg/m 3 in parts of Delhi on November 1, 2016.
• Subsequently the MoEFCC in early 2017 came out with the Graded Response Action Plan (GRAP), which involved coordination between multiple agencies in Delhi to activate reactive pollution control measures corresponding to increasing Air Quality Index (AQI) levels.

• Multiple researchers have noted that the policy approach and measures taken by Central and State authorities for specific polluting sectors over the years have been fragmented and often reactive.
• A study conducted by IIT Bombay noted that the 2002 public transport overhaul to CNG did not yield the desired results. While SPM and PM10 levels fell only marginally, carbon monoxide levels increased.
• Meanwhile, the Supreme Court’s 55,000 cap on two-stroke auto rickshaws in 1997 was not revised till 2011. Studies note that between 1997 and 2011, Delhi’s population grew by 45% and registered cars and two-wheelers grew by 250%, meaning the lower availability of autos could have likely contributed to increased private vehicle ownership.
• Regarding the odd-even rule applying to private vehicles, a study by IIT Delhi noted that although vehicular emissions contribute 25% to Delhi’s PM2.5 levels, passenger vehicles contribute just 8%, of which cars constitute 5%. This means that if all passenger vehicles within Delhi stopped operating, PM2.5 levels would reduce by an average of 8%.
  • However, the remaining 17% is contributed by heavy freight vehicles which are not covered under the odd-even rule
• Regarding coordinated waste management in Delhi, the daily waste generation rate is over 10,000 tons, the capacity of its already overflowing landfills to collect and manage garbage is under 6,000 tons. This leads to the practice of burning waste around residential areas.
• One argument for the failure to tackle Delhi’s pollution problems is that a large proportion of these polluting sources are present all year round and high pollution levels are witnessed in winter months due to unfavourable meteorological conditions.
• Therefore, stop-gap and seasonal measures often yield unsatisfactory outcomes
• As for the burning of farm residue or stubble in Delhi’s neighbouring States— Haryana, Punjab, Uttar Pradesh, and Rajasthan— researchers have emphasised the need for airshed management, along with improved machinery subsidies from the government and alternatives to crop burning.
  • An airshed is a common geographic area where pollutants get trapped. An airshed management approach would require coordinated responses from States.