Black carbon

Also called: Soot · Topic: Atmosphere: Composition, Structure and Elements of Weather · NCERT: Beyond NCERT

Meaning

Black carbon (BC), also called soot, is made of fine dark particles that form when diesel, coal, wood and crop residue burn incompletely, without enough oxygen. It is part of fine particulate matter (PM2.5), the air pollutant most closely linked to early death and disease [4]. It matters for two reasons. It warms the air and speeds up the melting of Himalayan snow and glaciers. It also harms health. Because it stays in the air for only a short time, cutting it gives quick gains for both climate and health.

Explanation

How black carbon forms

• Incomplete burning (cause):
• Fuel burns without enough oxygen.

• The carbon in it does not fully turn into gas, so it is left behind as tiny black specks.

• Common fuels: diesel, coal, wood and crop residue.

• Where it fits:
• Black carbon is one kind of aerosol (a tiny solid or liquid bit floating in the air).
• NCERT lists "smoke-soot" among the "dust particles" in the atmosphere.
• By size, it falls in PM2.5 (particles 2.5 micrometres or smaller). These go deep into the lungs and can enter the bloodstream [3].

How it warms the Earth

• In the air:
• Black carbon is dark, so it absorbs sunlight.
• It absorbs both incoming sunlight and the heat going back up from the Earth [5].

• This warms the layer of air it sits in.

• On snow and ice:

• Soot settles on Himalayan snow and glaciers.
• The white surface turns darker, so its albedo (the share of sunlight a surface reflects) falls.
• The darker surface absorbs more heat, and the snow and ice melt faster.
• IPCC AR6 (2021) says with high confidence that black carbon and other dark particles on snow speed up snowmelt [5].

A short-lived climate pollutant (SLCP)

• An SLCP is a warming agent that stays in the air for a short time, from a few days to about a decade [4].
• Black carbon leaves the air within days to weeks, mostly because rain washes it out. Carbon dioxide, by contrast, stays for centuries.
• Why this matters:
• Black carbon does not build up in the air for long.
• So when emissions fall, its amount in the air falls quickly too.

• Strong cuts in SLCPs could slow warming by up to 0.6 °C over the next few decades [4].

• IPCC AR6 (2021) groups black carbon with methane, ozone and other aerosols as short-lived climate forcers [5].

Sources

• World: open fires are the largest single source. Household fuel burning and transport together make up about 77% of the black carbon released by human activity [4].
• India:
• household biomass stoves (cooking with wood, dung cakes and crop waste)
• brick kilns
• diesel vehicles
• stubble burning (setting fire to crop stalks left after harvest)

Where it occurs

• Punjab and Haryana: stubble burning after the paddy harvest (October–November) sends smoke and soot over North India.
• Indo-Gangetic Plain, including Delhi-NCR:
• Soot from stoves, brick kilns, diesel vehicles and stubble fires collects here.
• In winter, a temperature inversion (a warm air layer sitting on top of cold ground air, acting like a lid) and calm winds trap it near the ground.

• The result is thick smog (smoke plus fog) and very high PM2.5 levels.

• Himalayan snow and glaciers:

• Soot from Indo-Gangetic sources settles here.

• It darkens the snow and ice and speeds up melting [5].

• Rivers at risk: the Ganga, Indus and Brahmaputra, which are fed by Himalayan snow and glaciers.

• Across India: wherever households cook on biomass stoves, and around brick kilns and busy diesel traffic.
• Worldwide: open fires are the largest single source. Household fuel and transport together make up about 77% of black carbon from human activity [4].

Don't confuse with

• Aerosols: this is the whole family of floating particles, such as sea salt, dust, ash, pollen and soot. Black carbon is only the dark soot part of it. Dust and sea salt are the main hygroscopic nuclei (specks that attract water vapour and help form clouds), so black carbon is not the "cloud-making" aerosol tested in NCERT.
• PM2.5: this is a size class (particles 2.5 µm or smaller), not a substance. Black carbon is one part of PM2.5.
• Carbon dioxide (CO₂): it is a long-lived gas that stays in the air for centuries. Black carbon is a short-lived particle that lasts days to weeks.
• Methane: it is also a short-lived climate forcer [5], but it is a gas. Black carbon is a particle that also darkens snow and lowers its albedo.

Prelims Hooks

• Black carbon is a short-lived climate pollutant that stays in the air for days to weeks, mainly because rain washes it out. It is not long-lived like CO₂ [4].
• It warms the Earth in two ways:
• it absorbs sunlight and outgoing heat in the air [5]

• it lowers the albedo of snow and ice, so they melt faster [5]

• Open fires are the largest single source. Household fuel and transport together make up about 77% of black carbon from human activity [4].

• India's main sources are household biomass stoves, brick kilns, diesel vehicles and stubble burning.
• Black carbon is a part of PM2.5. IPCC AR6 (2021) groups it with methane, ozone and other aerosols as short-lived climate forcers [5].
• Strong cuts in SLCPs could slow warming by up to 0.6 °C over the next few decades [4].

Mains Points

• Black carbon and the Himalayan cryosphere (GS-III):
• Soot from the Indo-Gangetic Plain settles on Himalayan snow and glaciers, darkens them and speeds up melting [5].

• This threatens the long-term flow of the Ganga, Indus and Brahmaputra, and so the water, farming and hydropower that depend on them.

• Quick wins for climate and health:

• Black carbon is short-lived, so cutting emissions brings fast results [4].

• Useful steps include:

  • cleaner cooking fuel (LPG) in place of biomass stoves
  • modern brick kiln technology
  • cleaner diesel engines
  • crop-residue management in Punjab and Haryana in place of stubble burning

• Policy gaps:

• The National Clean Air Programme (NCAP) targets PM10, not the more harmful PM2.5, which contains black carbon [1][2].
• India's NAAQS (2009) are much weaker than the WHO 2021 guidelines.
• Winter smog builds up over the whole Indo-Gangetic airshed (the region that shares the same air), so the states there need to act together, along with city action plans.

Related concepts

• Aerosols
• Particulate matter

Read more

• Aerosols, black carbon and particulate matter