Winter chill and fog bring along an unexpected guest for millions of residents of Delhi-NCR: poor air.
A measure called the Air Quality Index (AQI) gauges just how bad the air has become in a particular place. Readings of which usually serve as a warning to people with respiratory conditions.
The air quality has recently plummeted to hazardous levels, with AQI readings soaring as high as 999 in Indirapuram, according to data from the Delhi Pollution Control Committee in the national capital.
For other parts of Delhi, the 24-hour average remained at 453 in the “severe+” category for the first time. Out of 36 monitoring stations, 35 stations remained in the 400+ AQI category.
So, “What exactly is the Air Quality Index? How do we interpret it? Where does the data come from to calculate it?” – such questions are on the minds of many across the country.
In the simplest terms, AQI is the measure of harmful pollutants in the air like carbon monoxide, sulphur oxide, ozone, nitrogen dioxide, and particulate matter (PM2.5 and PM10 particles) on a index of 0 to 500 – the higher the number, the worse the air.
HOW IS AQI CALCULATED?
The calculation involves converting pollutant concentration levels in the air into index values by a formula. The formula gives an index value for each pollutant, allowing each pollutant’s effect on air quality to be understood independently. The highest of these values becomes the AQI, meaning that the pollutant with the highest value determines the final AQI.
PM2.5 and PM10 particles are the main pollutant particles in most parts of the national capital. They measure 2.5 micrometres across or more – roughly 30 times smaller than the width of a human hair.
HOW IS IT MONITORED?
The monitoring activity of air quality is conducted region wise by the governing bodies. In India, there are three major governing bodies responsible for the data calculation and processing – Central Pollution Control Board (CPCB), Delhi Pollution Control Committee (DPCC), and researchers at IITM (Indian Institute of Tropical Meteorology).
The air monitoring process remains largely the same for all these institutes.
Data on concentration levels of pollutants are collected from monitoring stations. Currently, India has two types of ground based monitoring stations – manual monitoring stations and continuous ambient air quality monitoring stations (CAAQMS).
The major difference between them is the time period. While the turn-around time of AQI calculated via manual monitoring stations is around 8 hours and mostly used for chemical analysis, the continuous ambient air quality monitoring stations provide real time AQI used across the country for public information.
WHERE ARE THESE MONITORING STATIONS?
The country has 1,296 air quality monitoring stations, including 401 manual and 895 CAAQMS stations which use ground-based electronic sensors to measure air pollutants via a process called Beta Attenuation Technology.
Many experts say that India doesn’t have an adequate number of monitoring stations to accurately gauge the quality of air.
“The number of CAAQMS in Delhi is optimal. However, there is a requirement to install more stations across the country,” Mohan George, former Additional Director of Delhi Pollution Control Board, tells India Today.
Data from the Centre for Science and Environment (CSE) reveals that the air quality monitoring grid covers only 12 per cent of the 4,041 cities and towns across the country.
WHAT IS THE PROCESS TO CALCULATE POLLUTANT CONCENTRATION?
Monitoring stations on the ground with electronic equipment use a technique called Beta Attenuation Technology to measure air pollutants.
Here’s how it works in simple terms:
Beta rays on hitting a particle can be absorbed, reflected, or pass straight through.
When these rays pass through polluted air columns, some of their energy is absorbed by the particles. The more pollution there is, the more energy is absorbed. By measuring this energy loss, the system can determine how much pollution is in the air.
Several international space agencies like NASA and ESA have a fleet of earth-observing satellites with instruments which observe our planet’s oceans, biosphere, and atmosphere including air pollutants around the world.
However, the Central Pollution Control Board (CPCB) does not use satellite data for monitoring air quality at present but it has sponsored a research project in IIT Delhi to carry out studies for correlating the estimation of PM2.5 from satellite-based Aerosol Optical Depth (AOD).
DO ALL APPS SHOW THE SAME AIR QUALITY DATA?
Most AQI applications and services show the data sourced from the CPCB. Government-owned web-based applications like the SAFAR app provide real-time AQI readings, forecasts, and health advisories based on CPCB data. Third party apps like AirVisual, WAQI and Plume also use the government data.
However, smart devices like indoor air purifiers and air quality monitors have built-in low-cost sensors that provide localized AQI readings based on the immediate room environment, though these sensors are often less precise.