Explained: What does AQI mean and just how bad are Mumbai’s pollution levels?

Of the world's 30 most polluted cities, 22 are in India. Delhi, whose Air Quality Index regularly falls in the "poor" or "very poor" categories, ranks as the most polluted capital city worldwide. Although Mumbai did not figure in that list, released by the Swiss Organisation IQAir in March this year, there have been instances when South Mumbai has recorded similarly alarming AQI levels.

Recently, the Supreme Court of India asked the Centre and states in the National Capital Region to devise a proposal to switch all industrial units to piped natural gas or cleaner fuels in order to keep a check on Delhi's rising air pollution levels. The SC also approved setting up an Enforcement Task Force by the Centre's Commission for Air Quality Management to ensure strict action against those violating the Commission's directions.

Mid-day Online reached out to Dr B.S. Murthy, director of System of Air Quality and Weather Forecasting and Research (SAFAR) and scientist at the Indian Institute of Tropical Meteorology, to understand AQI and various factors affecting significant indicators of air pollution.

Could you please explain what ‘Air Quality Index' (AQI) is and why it is important?

Air Quality Index or AQI is an indicator of air quality; the lower the AQI, the better the air quality. It is calculated based on the 24-hour average concentration of important air pollutants like particulate matter or PM10, PM2.5, NO2 (Nitrogen Dioxide), O3 (Ground-level ozone), CO (Carbon Monoxide), SO2 (Sulfur Dioxide), NH3 (Ammonia) and Pb (Lead), which are harmful for human health if they are above the prescribed national ambient air quality standards.

AQI is grouped into 6 categories; AQI < 50 as ‘Good', between 51 and 100 as ‘Satisfactory', between 101 and 200 as ‘Moderate', between 201 and 300 as ‘Poor', between 301 and 400 as ‘Very Poor' and between 401 and 500 as ‘Severe'. AQI is calculated for each pollutant (called Sub index) and the final AQI is the highest of the sub-indexes. In other words, the highest sub-index signifies the leading pollutant determining the AQI. Mostly PM10 or PM2.5 is found to be the leading pollutant.

Other locations in Mumbai with 'very poor' air quality include Mazagaon at 331 and Malad at 327. Bandra-Kurla Complex's AQI was 331 on November 16. Pic/Mid-Day photographers

What are the factors affecting the AQI?

AQI depends mainly on concentration of PM10 and PM2.5 defined as particles with size less than or equal to 10 micrometres and < or = 2.5 micrometres respectively. PM10 or PM2.5 concentration depends on emissions of pollutants from various sources like vehicular exhaust, industries, road dust, wind-blown dust, coal and fossil fuel combustion, brick kilns, garbage incineration, etc. The emitted pollutants get redistributed in space and time horizontally and vertically, due to wind circulations and convection.

For a given pollutant concentration, stable atmospheric conditions (in winter or night time) degrade air quality whereas convective mixing (in summer or day time) dilutes and improves the air quality. An influx of relatively more polluted air mass increases pollution levels whereas that of pristine air mass reduces pollution levels due to the mixing of air masses.

Considering Delhi is enclosed by a hypothetical box, for a given background pollution level in the box, the fluctuation in concentration depends on the difference between the rates of inflow and outflow of air mass consisting of pollutants. That inflow depends on the magnitude of pollution level in the incoming flow and the wind speed and direction. The outflow from the box depends on vertical dispersion or ventilation and wind speed and direction. Atmospheric chemistry - involving chemical reactions among the pollutants and gases - generates secondary particles that add up to pollution level.

What levels of particulate matter (PM) are considered safe?

PM2.5 concentration (24-hour mean) of less than 30 microgram per cubic metre is considered ‘Good'. PM2.5 between 30 and 60 is ‘Satisfactory', between 60 and 90 is ‘Moderate', between 90 and 120 is ‘Poor', between 120 and 250 is ‘Very Poor' and more than 250 is ‘Severe'. These are the national ambient air quality standards applicable for any location in India.

There has been a lot of discussion about increasing the use of public transport. Could you please elaborate on the possible solutions, specific to Mumbai, to maintain the PM levels?

In most of the urban or metropolitan cities, transport (vehicles) is the major source of pollution. If public transport is made comfortable and available at regular time intervals across the city, then people would prefer to go by public transport rather than using private or personal vehicles. This would reduce the number of vehicles plying on the roads, thus controlling emissions partially.

Improving traffic management for a smooth flow of vehicles with a few signals would reduce the idling time of vehicles at road junctions leading to less fuel consumption and emissions. Other options are to use solar and wind energy or nuclear energy for power generation rather than coal in thermal power plants. Phasing out old vehicles and encouraging the purchase of electric vehicles by offering incentives is one strategy. Planting trees along both sides of roads and maintaining proper ventilation in residential and office buildings helps reduce power consumption.

Collection of garbage locally and recycling it (instead of dumping in landfills and burning) to produce biogas or compost at multiple locations is to be done. Implementation of these plans requires public participation voluntarily with awareness about the implications of air pollution for global warming and climate change.

Recently, the AQI of South Mumbai was reportedly poorer than that of Delhi (and this has been noted multiple times)? Could you please explain the major reasons for it?

Annual statistics of air quality categories in Mumbai indicate that ‘poor' or ‘very poor' occurs only 3-10 percent of the time in a year. During the remaining 90-97 percent, it is ‘Good', ‘Satisfactory' or ‘Moderate'. In comparison, Delhi experiences 'Poor' or 'Very poor' category for about 50-60 percent of the time in a year.

At times in some locations, the hourly air quality in Mumbai may witness 'severe' or 'very poor' levels, which could be due to either some sporadic emission source like biomass burning. It could also happen due to poor ventilation of air mass due to high rise buildings blocking the wind flow resulting in temporary stagnant conditions leading to accumulation of pollutants.

How do the weather and seasonal changes affect the AQI in Mumbai?

In winter and summer, PM2.5 is observed in the range 25-125 microgram per cubic metre whereas in the monsoon season it is 20-50 microgram per cubic metre. So in winter and summer, AQI indicates mostly 'Moderate' while in monsoon it is 'Good/Satisfactory'. In the monsoon season, frequent rains scavenge pollutants in the atmosphere through wet deposition, leading to better air quality.

In winter, stable or neutral atmospheric conditions reduce dispersion of pollutants thus helping accumulation near the ground. In summer, relatively higher air temperature increases convective mixing of pollutants thus diluting their concentration near the ground.

Does being surrounded by the sea help in maintaining the pollution levels of Mumbai? Is there a link and could you please explain the science behind it?

Daily mean observations indicate negative correlation between PM2.5 concentration and relative humidity (RH) indicating that the higher the RH, the lesser the PM2.5 and the better is the air quality.

Mumbai, being a coastal city, experiences an RH of 40-70 percent in summer, 60-80 percent in winter and 80-95 percent in monsoon. High moisture (high RH) availability in the atmosphere helps PM2.5 particles (which are hygroscopic with affinity towards water) absorb moisture, become bigger in size and heavier, leading to enhanced deposition on the ground thus reducing its concentration in the atmosphere.

Relatively wet ground prevents lifting of dust by wind and vehicles. Coastal cities also experience sea breeze that brings in pristine air mass (if marine air is less polluted relative to that over land) over land, leading to reduction in pollution over land. Land surface does not cool much during night time in coastal cities due to high RH that helps maintain neutral stability which is favourable for the dispersion of pollutants.

Also Read: Delhi's air quality still 'very poor' with AQI at 311