Aerosols, black carbon and particulate matter

Detail

1. Aerosols ("dust particles" in NCERT)

• Aerosols are tiny solid or liquid bits that float in the air. NCERT Class 11 calls them "dust particles".
• What they are made of:
• sea salt (from sea spray)
• fine soil and dust
• smoke-soot (from burning)
• ash (from volcanoes and fires)
• pollen (from plants)

• broken meteors (bits of meteors that burn up in the upper air)

• Height: most aerosols stay in the lower layers of the atmosphere, close to their sources on the ground.

• How some go higher:
• The sun heats the ground and the air above it.
• The warm air rises in convection currents (upward movement of heated air).
• These rising currents carry particles high into the atmosphere.

2. Where aerosols gather

Region	Amount of aerosols	Why
Subtropical and temperate regions	Highest	Dry winds lift and carry soil and dust; deserts lie here
Equatorial region	Lower	Frequent rain washes particles out of the air
Polar regions	Lower	Few local sources; land is under ice and snow

• Trap: aerosols are not highest over the equator. The equator is warm, but rain there keeps the air clean.

3. Why aerosols matter for weather

• Hygroscopic nuclei (specks that attract water, also called condensation nuclei):
• Dust and salt particles attract water vapour.
• Water vapour condenses on these specks and forms tiny droplets.
• Millions of these droplets together make a cloud.

• Without aerosols, clouds would hardly form, so there would be little rain, fog or mist. (The link to condensation and precipitation is covered under atmospheric moisture.)

• Effect on sunlight: aerosols scatter sunlight (bounce it in many directions) and absorb it (take it in as heat).

• Scattering by small particles helps explain the blue sky and red sunrises and sunsets.
• A large aerosol load can cause global dimming, which means less sunlight reaches the ground. (These points are covered under insolation and the heat budget.)

4. Black carbon

• Black carbon (BC) is soot. It forms when diesel, coal, wood and crop residue burn incompletely, without enough oxygen.
• It is a part of fine particulate matter (PM2.5), the air pollutant most closely linked to early death and disease [5].
• Global sources: open fires are the largest single source. Household fuel burning and transport together make up about 77% of black carbon released by human activity [5].
• Main sources in 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)

How black carbon 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 [6].

• 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 [6].

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 [5]. Carbon dioxide, by contrast, stays for centuries.
• Black carbon leaves the air within days to weeks, mostly because rain washes it out.
• Because it leaves the air so quickly, cutting emissions gives quick results for both climate and health.
• Strong cuts in SLCPs could slow warming by up to 0.6 °C over the next few decades [5].
• IPCC AR6 (2021) groups black carbon with methane, ozone and other aerosols as short-lived climate forcers [6].

5. Particulate matter (PM)

• Particulate matter is the mix of solid and liquid particles in the air. It is grouped by size:
• PM10: particles 10 micrometres or smaller. These are coarse particles, such as road dust, construction dust and soil. They lodge in the nose, throat and upper lungs.

• PM2.5: particles 2.5 micrometres or smaller. These are fine particles, such as smoke, soot and vehicle exhaust. They reach deep into the lungs and can enter the bloodstream, which damages the heart and blood vessels [4].

• Health burden: the combined effect of outdoor and household air pollution causes about 7 million early deaths each year, mostly from non-communicable diseases [4].

• In 2019, more than 90% of the world's people lived where PM2.5 levels were above even the older 2005 WHO guideline [4].

Standard (µg/m³)	PM2.5 annual	PM2.5 24-hour	PM10 annual	PM10 24-hour
India NAAQS (2009)	40	60	60	100
WHO Air Quality Guidelines (2021)	5	15	15 [4]	45 [4]

• NAAQS means the National Ambient Air Quality Standards. These are India's legal limits for outdoor air quality.
• India's annual PM2.5 limit (40) is 8 times WHO's 2021 guideline (5).
• WHO estimates that about 80% of deaths linked to PM2.5 could be avoided if countries met the annual guideline level [4].

6. North India's winter smog

Cause → process → result

• Pollution builds up:
• Stubble smoke drifts in from Punjab and Haryana after the paddy harvest (October–November).

• Vehicle exhaust, factory emissions, brick kiln smoke and dust are added to it.

• The air gets trapped:

• On long winter nights the ground loses heat quickly, so the air touching it becomes cold.
• Warmer air lies above this cold layer. This is a temperature inversion (temperature rises with height instead of falling).

• The warm layer acts like a lid. Cold, heavy air cannot rise, and calm winds do not spread the pollution sideways.

• Result:

• PM2.5 levels shoot up across the Indo-Gangetic Plain, with Delhi-NCR among the worst-hit areas.
• Thick smog (smoke plus fog) forms and stays for days.
• (Inversion is covered under insolation and the heat budget; the policy side is covered under geographical issues.)

7. National Clean Air Programme (NCAP)

• Launched: January 2019 by the Ministry of Environment, Forest and Climate Change (MoEFCC) [2][3].
• Coverage: 131 cities across 24 States/UTs. These are non-attainment cities (cities that did not meet NAAQS for five years in a row) and million-plus cities [2][3].
• Target: up to a 40% cut in PM10, or meeting the national PM10 standard (60 µg/m³), by 2025-26 [2].

• (Revised target: an earlier version aimed for a 20–30% cut by 2024.)

• City targets: each city has a target to cut its annual PM10 by 4–15%, carried out through city action plans [3].

• Good days: the 49 million-plus cities/urban agglomerations must raise the number of "Good Days" (Air Quality Index below 200) by 15% each year [3].
• Funding: ₹11,211 crore has been given to these cities to meet the targets [2].

Prelims Hooks

• Hygroscopic nuclei come from dust and sea salt. Without them, clouds would hardly form.
• Aerosols are most concentrated in subtropical and temperate regions (dry winds), not in equatorial or polar regions.
• PM2.5 = 2.5 µm or smaller and can enter the bloodstream. PM10 = 10 µm or smaller.
• India's NAAQS (2009): PM2.5 40 annual / 60 for 24 hours. PM10 60 annual / 100 for 24 hours.
• WHO AQG (2021): PM2.5 5 annual / 15 for 24 hours. PM10 15 annual / 45 for 24 hours [4].
• Black carbon is a short-lived climate pollutant. It stays in the air for days to weeks, warms the air and lowers the albedo of snow and ice [5][6].
• Household fuel and transport make up about 77% of black carbon from human activity. Open fires are the largest single source [5].
• NCAP: launched in 2019 by MoEFCC, covers 131 cities, and targets up to 40% PM10 reduction by 2025-26. Trap: the target is for PM10, not PM2.5 [2][3].
• Winter smog over the Indo-Gangetic Plain comes from emissions plus a temperature inversion and calm winds.

Mains Points

• Aerosols as a two-sided agent (GS-I): they make clouds and rain possible by acting as condensation nuclei. In excess, they scatter and absorb sunlight, cause global dimming, change local heating and degrade air quality.
• Black carbon and the Himalayan cryosphere (GS-III): soot from Indo-Gangetic sources darkens snow and glaciers and speeds up melting [6]. This threatens the long-term flow of the Ganga, Indus and Brahmaputra. Because black carbon is short-lived, cleaner cooking fuel (LPG), modern brick kiln technology, cleaner diesel engines and an end to stubble burning bring quick gains for both climate and health [5].
• North India's winter smog is both a meteorological and a policy problem: the stubble-burning season comes at the same time as winter inversions and calm winds. Solutions need action across states, such as crop-residue management in Punjab and Haryana, along with city action plans.
• NCAP: gaps and way forward: it targets PM10, not the more harmful PM2.5. India's NAAQS (2009) are much weaker than the WHO 2021 guidelines, and the standards need revision. Funding of ₹11,211 crore for 131 cities is a start [2], but results depend on coordination across the whole airshed (the region that shares the same air).