Increasing concentration of suspended particles, including pollutants, in the atmosphere is blocking sunlight over most Indian cities, affecting visibility, reveals data from NASA’s research satellites.
A two-member team from the Centre for Astroparticle Physics and Space Science (CAPSS), Bose Institute, Kolkata, found very high Aerosol Optical Depth (AOD), which signifies very high atmospheric aerosol loading – suspension of solid and liquid particles in the air – between 2004 and 2015 in 29 Indian cities.
Aerosols hinder the transmission of solar radiation through the atmosphere by absorbing or scattering incoming light.
They originate from manmade activities such as burning of agriculture waste and forests, industries, smoke and are also found in nature in the form of windblown dust, volcanic ash and sea salts.
Using measurements from the MODIS sensor on board NASA’s Terra and Aqua satellites, which map the amount of aerosol in the air, the team found average AOD in the range of 0.16 to 0.74 in these cities. An AOD of less than 0.1 indicates clear skies with maximum visibility, while 1 denotes dense aerosols with difficulty seeing the sun.
Of the four mega cities, Delhi showed the maximum average AOD levels at 0.74 (Terra) and 0.73 (Aqua), followed by Kolkata (0.62 and 0.65), Mumbai (0.42 and 0.40) and Chennai (0.41 and 0.38). Among the smaller cities, Lucknow scored 0.65 and 0.69, Patna (0.69 and 0.64), Varanasi (0.65 by both Terra and Aqua) and Kannur (0.68 and 0.41).
Globally studies are trying to understand the impact of aerosols on the environment. Researchers said certain aerosols are important, while some are bad for the climate and health.
“Manmade aerosols include carbon and sulphur compounds that are being emitted by industries and vehicles in large quantities are harmful and toxic,” said professor Sanjay K Ghosh, CAPSS. “On the other hand, if there is solar light coming in, but no aerosols in the atmosphere, human beings can’t exist owing to the heat and temperature generated. Natural occurring aerosols cool the earth.”
Aerosols also affect rainfall patterns. “Aerosols act as condensation nuclei centre on which clouds condense for precipitation or rainfall. But increase in the concentration of fine particle aerosols will result in a lot of cloud cover, but no rain,” said Subhasis Banerjee, CAPSS.
In a related study, the team analysed AOD over three cities – Mumbai, Chennai and Kolkata – to infer atmospheric pollution over four decades (1973 to 2015). Taking visibility data from these airports, the team estimated AOD values. They have also co-related the above calculated AOD with satellite AOD data for validation of their findings. They have found, from visibility data, an increasing pollution trend in Mumbai both from 1973 to 1993 and 1995 to 2015. While Chennai showed no significant increasing trend in the first two decades, it increased over the past two decades. Kolkata was an exception, with a decreasing trend between 1994 and 2015, but increasing in the first two decades of the study period.