National Air Quality Index (AQI)

  • The introduction of the National Air Quality Index (NAQI) in New Delhi in September 2014 was part of the Swachh Bharat Abhiyan initiative, aimed at monitoring India’s air quality index.
  • Governmental organizations are in charge of informing the public about health risks and how to prevent them.
  • The Central Pollution Control Board (CPCB) has established a committee of specialists consisting of SPCBs, academicians, medical professionals, and experts in air quality.
  • IIT Kanpur was given a technical study as well. IIT Kanpur and the expert panel recommended an AQI program for India in 2014.
  • A real-time data tracking system for continuous monitoring is in place in six Indian cities: Ahmedabad, New Delhi, Kolkata, Mumbai, and Pune.
  • The AQI transforms complex air quality data of various pollutants into a single number (index value), nomenclature and colour.
  • The colour-coded AQI index helps the public and the government understand the condition of the air and what subsequent measures are to be taken to combat the situation, based on its severity.
  • The widely-used National Air Quality Index (NAQI) given by the Central Pollution Control Board is a 24-hour average. Its unit is micrograms per cubic meter.

National Air Quality Index (AQI): Pollutants

  • The following eight significant air pollutants in the atmosphere are monitored by the Air Quality Index (AQI):
    1. Particulate Matter (PM10)
    2. Particulate Matter (PM2.5)
    3. Nitrogen Dioxide (NO2)
    4. Sulphur Dioxide (SO2)
    5. Carbon Monoxide (CO)
    6. Ozone (O3)
    7. Ammonia (NH3)
    8. Lead (Pb)

Particulate Matter or PM: A Significant Air Pollutant

  • Particulate matter encompasses a mixture of solid and liquid droplets present in the air. While some PM, like dust and soot, is visible to the naked eye, others require an electron microscope for observation.
  • PM 2.5 and PM 10 stand out as the most common air quality metrics, measured in micrograms per cubic meter. 
  • PM 2.5 refers to microparticles with a diameter less than 2.5 microns, while PM 10 includes particles with a diameter less than 10 microns. 
  • These parameters are universally utilized by countries worldwide to evaluate atmospheric quality.
Sources of PM:
  • PM can come from a variety of places, including fields, fires, construction sites, and unpaved roads.
  • The majority of the particles are produced by complicated chemical reactions involving substances like sulfur dioxide and nitrogen oxides.
  • These are the pollutants that come from things like power plants, industry, and cars.
  • The following are some examples of natural sources of PM: sea salt, dust (also known as crustal debris), secondary sulphate, pollen, black carbon from wildfires, and volcanic ash.

Additional Pollutants in Focus: Lead and Ammonia

  • In addition to PM, India also monitors lead and ammonia as pollutants.
  • An AQI value below 50 is considered safe.

National Air Quality Index (AQI): Scores

  • The National Air Quality Index comprises six AQI categories: Good, Satisfactory, Moderately Polluted, Poor, Very Poor, and Severe.
  • Calculation of the index:
    • There are six or eight pollutants in the affected air and each of these pollutants is given a weight based on a formula. That weight depends on the kind of impact it has on human health.
      • Each pollutants quantity in the air is adjusted to a common scale (say, 0 to 500) that works for all pollutants.
    • The worst of these weights is given as composite air quality, so instead of giving six different numbers, and six different colours, it throws up one single colour, one single number to denote the overall impact. Monitoring stations across the country assess these levels.
Air Quality Index (AQI)
Air Quality IndexScoreHealth Impacts
0-50Good/ SafeLess Impact.
51-100SatisfactoryPeople who are sensitive may have slight breathing difficulty.
100-200Moderately PollutedPatients with lung diseases, heart conditions, children, and elderly people may find it difficult to breathe.
201-300PoorLong-term exposure might cause breathing pain as well as discomfort for people with heart disease.
301-400Very PoorLong-term exposure could cause respiratory conditions.

Severely impacts those who have lung and heart conditions.
401-500SevereWhile healthy individuals may encounter respiratory issues, persons with lung or heart illness may experience serious health effects.

People may experience negative effects on their health even when engaging in light exercise.

Graded Response Action Plan (GRAP)

  • GRAP is a set of emergency measures that kick in to prevent further deterioration of air quality once it reaches a certain threshold in the Delhi-NCR region.
  • It was approved by the Supreme Court in 2016 after the Supreme Court’s order in the matter of M. C. Mehta vs. Union of India (2016) and notified in 2017.
  • The GRAP is a set of anti-air pollution measures followed in the capital and its vicinity according to the severity of the situation.
  • It classifies the air quality in the Delhi-NCR under four different stages: 
    • Stage 1 – “poor” (AQI 201-300)
    • Stage 2 – “very poor” (AQI 301-400)
    • Stage 3 – “severe” (AQI 401-450) 
    • Stage 4 – “severe plus” (AQI above 450).
  • Implementation:
    • From 2021 onwards, the GRAP is being implemented by the CAQM.
      • Till 2020, the Supreme Court-appointed Environment Pollution (Prevention & Control) Authority (EPCA) used to order States to implement GRAP measures.
    • The EPCA was dissolved and replaced by the Commission for Air Quality Management (CAQM) in 2020.
    • CAQM relies on air quality and meteorological forecasts by the Indian Institute of Tropical Meteorology (IITM) and the India Meteorological Department (IMD).

National AQI: Calculation

  • The Sub-indices for individual pollutants at a monitoring location are calculated using its 24 – hourly average concentration value (8-hourly in case of CO and O3) and health breakpoint concentration range. The worst sub-index is the AQI for that location.
  • All the eight pollutants may not be monitored at all the locations. Overall AQI is calculated only if data are available for minimum three pollutants out of which one should necessarily be either PM2.5 or PM10. Else, data are considered insufficient for calculating AQI. Similarly, a minimum of 16 hours’ data is considered necessary for calculating subindex.
  • The sub-indices for monitored pollutants are calculated and disseminated, even if data are inadequate for determining AQI. The Individual pollutant-wise sub-index will provide air quality status for that pollutant.
  • The web-based system is designed to provide AQI on real time basis. It is an automated system that captures data from continuous monitoring stations without human intervention, and displays AQI based on running average values (e.g. AQI at 6am on a day will incorporate data from 6am on previous day to the current day).
  • For manual monitoring stations, an AQI calculator is developed wherein data can be fed manually to get AQI value.
National Air Quality Index

Limitations of National Air Quality Index

  • Not for Statutory Purposes: Since the data for real-time AQI is fed straight from the analyzers without being checked, it might not be for statutory purposes.
  • Technical Complexity: The operation of sensors and analyzers, their calibration, data capture at a local server, transmission to a central database utilizing the Internet, and other processes are all part of the monitoring and dissemination of the AQI.
  • Power And Maintenance Issues: Monitoring station operations may also be impacted by a variety of technical and operational factors, such as prolonged power outages and maintenance issues.
  • Interruption in Continuous Data Flow: Due to these restrictions, it’s likely that continuous data flow and dissemination will be interrupted in some way.
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