Air Quality Index

What the Air Quality Index (AQI)?

AQI is a number used to communicate to the public how polluted the air currently is or how polluted it is forecasted to become.
As AQI increases, an increasingly large percentage of the population is likely to experience increasingly adverse health effects.
Different countries have their own air quality indexes, corresponding to different national air quality standards.
The AQI is most commonly used to describe ground-level ozone levels.
However, the AQI can be used to represent five pollutants that pose a threat to human health.
These pollutants are:

How it is calculated?

That weight depends on the kind of impact it has on human health, each of the pollutants is given a weight.
The worst of these weights is given as a composite air quality.
So instead of giving six different numbers, six different colours, it throws up one single colour, one single number.
The index will throw up one number which will be given to the public.
People will know the health of their air quality based on this number and one associated colour code.

Particulate Matter:

PM is the term for a mixture of solid particles and liquid droplets found in the air. Some particles, such as dust, dirt, soot, or smoke, are large or dark enough to be seen with the naked eye.
Others are so small they can only be detected using an electron microscope.
Particle pollution includes:
PM₁₀: inhalable particles, with diameters that are generally 10 micrometres and smaller; and
PM_2.5: fine inhalable particles, with diameters that are generally 2.5 micrometres and smaller.

Sources of PM

These particles come in many sizes and shapes and can be made up of hundreds of different chemicals.
Some are emitted directly from a source, such as construction sites, unpaved roads, fields, smokestacks or fires.
Most particles form in the atmosphere as a result of complex reactions of chemicals such as sulfur dioxide and nitrogen oxides, which are pollutants emitted from power plants, industries and automobiles.

Harmful effects of PM

Particulate matter contains microscopic solids or liquid droplets that are so small that they can be inhaled and cause serious health problems.
Some particles less than 10 micrometres in diameter can get deep into your lungs and some may even get into your bloodstream
Of these, particles less than 2.5 micrometres in diameter, also known as fine particles or PM2.5, pose the greatest risk to health.

Indoor emissions affect air-quality standards

Household emissions remained one of the major culprits behind PM 2.5 air pollution in India.

Household emission in India

A recent study has pointed out that the use of firewood, kerosene and coal in the households contributed to about 40% of the PM 2.5 pollution in the Gangetic basin districts.
The results showed that by eliminating household emissions the average outdoor air pollution levels could be reduced and brought within the national ambient air quality standards.
The paper published in the Proceedings of the National Academy of Science notes that if all households transitioned to clean fuels, about 13% of premature mortality in India could be averted.
At the national scale, mitigating household emissions is also expected to bring large health benefits.
In many villages, they still use firewood for room heating and water heating. People prefer cheap wood fuel despite LPG being provided to many households.

Using Satellite data

Using satellite data and chemical transport model simulations, the researchers pointed out that complete mitigation would bring down the country’s average annual PM 2.5 air pollution to 38 microgram/cubic metre.
Surprisingly, this is below India’s national ambient air quality standard of 40 microgram/cubic metre and slightly above the World Health Organization (interim target 1) standards of 35 microgram/cubic metre.

Need for a multipronged approach

But India’s pollution problem is much bigger than often perceived.
The study has demonstrated that mitigating at a household level is the easiest and more practical way out for the government to reduce not only the household pollution but also outdoor air pollution at the national scale.

Satellites to Assess Pollution Status

ISRO’s INSAT-3D & 3DR satellites are being used for assessment of air pollution.

Aerosol Optical Depth (AOD)

Aerosol Optical Depth (AOD) is a quantitative estimate of the amount of aerosol present in the atmosphere, and it can be used as a proxy for surface Particulate Matter PM2.5 (particles smaller than 2.5 µm median diameter).
AOD measures the extinction of a ray of light as it passes through the atmosphere.

How is AOD calculated?

Using medium resolution Indian Remote Sensing (IRS) satellite data, stubble burned area maps are generated at the end of stubble burning activity in Kharif season.
ISRO has been carrying out monitoring of stubble burning since 2015. The products generated are comparable to the NASA products.
The Imager payload on-board ISRO’s INSAT-3D & 3DR satellites are used to monitor Aerosol Optical Depth (AOD).

Significance of AOD

It is found that AOD, PM2.5 and PM10 concentrations are higher over Indo-Gangetic Plain covering parts of Delhi, Uttar Pradesh and Bihar during October and November.
High concentration of these pollutants is seen originating from parts of Punjab and Haryana during stubble burning.
Climatological study of satellite based fire occurrences and associated pollutant parameters reveal that fire occurrences increased by 4% over Punjab and Haryana region during Oct-Nov between 2003 and 2017.
The model based analysis suggests that there is a high probability of transportation of smoke aerosols from Punjab & Haryana, towards down-wind regions of Delhi, Uttar Pradesh and Bihar.