Polluted atmospheric layer in the making


‘What is that layer?’, I exclaimed, when I photographed this on my journey to Uran a few months back. ‘That’s pollution’, I said as I gazed at the distant industries at the shore. It looks brown but is it the unsightly Asian brown cloud? It could be a photochemical smog. Its brown color reflects its NOx content. A process in which primary pollutants like NOx are converted to secondary pollutants like ozone in the presence of sunlight.

Nature 448, 575-578 (2 August 2007)
2002 Study : Nature 448, 575-578 (2 August 2007)

The Asian Brown Cloud was discovered in 1999 by an Indian scientist Veerabhadran Ramanathan and his team. It is not just dominant over Asia, but the name stuck. It is also called the Giant Brown Cloud or the Atmospheric Brown Cloud (ABC).

Common air pollutants:

  • Sulphur oxides (SOx)
  • Nitrogen oxides (NOx)
  • Carbon Monoxide (CO)
  • Volatile OrganiccCompounds (VOCs)
  • Ammonia (NH3)
  • Cloroflourocarbons (CFCs)
  • Ground level ozone
  • Soot

These are emitted from several processes: man-made and natural. Natural processes include volcanoes, thunderstorm, forest fire and dust storms. Man-made processes include air-conditioners, industrial processes, vehicle exhaust, refrigerators, aerosol sprays etc.

Aerosols and Particulate Matter:

Aerosols are suspensions of particulate matter in a gas/air. This particular matter can be a solid or a liquid. The mentioned gaseous pollutants eventually form particular matter through a process called particle conversion process. Particulates are classified as follows:

Classification based on diameter:

Inhalable coarse particles (larger than 2.5 micrometers and smaller than 10 micrometers in diameter). Popularly known as PM10 and are capable of causing severe health damage.

Fine particles (2.5 micrometers in diameter and smaller). These cause visibility problems. and mostly come out of diesel exhaust system of a car. Filters are put in place in the system to control their emission into the atmosphere. PM2.5 and PM1 are popular in this category.

Classification based on formation:

Primary: Emitted directly from sources like smoke stacks

Secondary: Created via a chemical reaction in the atmosphere

By observing the data by World Bank, you can see developing countries like Africa and India have high concentrations of PM10. The reasons for these are lack of advancements or implementation of pollution control technology and use of primitive ones such as dung-fueled cooking used in many Indian homes.

Aerosols and climate change:

Brown clouds have been known to disrupt existing weather patterns because these clouds flow distances. Soot, formed from incomplete combustion of hydrocarbons is a mixture of carbon particles called black carbon and organic acids. It absorbs sunlight because it is dark. Soot is believed to be the second largest contributor to global warming after greenhouse gas emissions. Aerosols such as nitrates and sulphates on the other hand have a cooling effect. Cool surfaces mean less evaporation of water from oceans. Less evaporation means less rain and then drought. Northern India is already seeing drought like conditions. This threatens us at so many levels: no water, no food, polluted air to breathe.

The Air (Prevention and Control of Pollution) Act in India was enacted in the year 1981 and amended 6 years later. After three decades, India’s air quality ranks among the lowest five countries in the world. Aerosols contributing industries include:

  • Base metal and iron ore mining
  • Cement manufacturing
  • Coal mining and production
  • Electricity generation
  • Foundries
  • Iron ore and steel melting
  • Lead and zinc smelting
  • Phosphate fertilizer plants

Water pollution has been taken up very seriously by the Indian Government, as is seen from the reports of the new budget. National Action Plan on Climate Change and the new ministry of Forest and Climate change will hopefully stop atmospheric pollution from getting worse. UN report says that over 2/3rd Indian population still rely on dung-based fuel. Providing a cleaner source of energy to such a large part of the country is a big challenge.

Kerosene is used for lighting mainly in villages and also as a fuel to run generators in small towns. The government’s inability to provide electricity to each and every household is compensated by giving huge subsidy to villagers on kerosene. If this subsidy is removed, it will put onus on the government to electrify all villages on an urgent basis. – Why petro products subsidies should be done away with

The following 13 mega-city ABC hotspots in Asia have been identified and reported by UNEP.


Further reading on ABCs:


A conversation with the scientist who discovered ABCs: In India, Battling Global Warming One Stove at a Time – PBS Newshour


13 thoughts on “Polluted atmospheric layer in the making

  1. Thank you, Anuja, for this interesting post.
    I do not think that I had heard of the term “Atmospheric Brown Clouds” previously.

    Although, I also grew up with them. Where I grew up (http://umwelt-ddr.argus-potsdam.de/index.php?luft) air pollution by soot, SOx and NOx were the norm. I remember many days seeing a brown cloud in the valley in which Jena is situated. And when I was young we also used solid fuel (wood and coal) for heating. We also had a wood/coal-fired stove for cooking from my grandparents’ time, but it was not used much anymore since we already had a liquified petroleum gas stove.

    Related to your post and the 2009 interview you linked, Nature recently published an update about the slow progress of replacing solid fuel stoves by LPG stoves in India.

    1. Thank you for sharing the Nature article. ‘Smoky stoves are responsible for 4 million deaths a year.’ Terrible. May be the government should also be ambitious about having a smoky stove-free country.

  2. My goodness, that’s horrifying — two million deaths from air pollution! I hope you’re right, and the government takes action. The fact that is has addressed water pollution is a good sign.

    1. Bikramjit, yes we should if we want to be a part of a healthy life supporting system. Thing is, it is how we are you know – we make stuff for our convenience and development of human species, we learn about our environment and build stuff by using it. We are that kind of species – ones with brains. I hope the same brain helps us realize that some changes are necessary now. If not, may be we all will meet on another planet. Or may be we could engineer ourselves. ;) http://www.bbc.com/future/story/20140716-the-most-extreme-way-to-be-green

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