Environmental Pollution - 4 | Environment for UPSC CSE PDF Download

What is Acid Rain?

Acid rain refers to any form of precipitation-rain, snow, fog, mist or dry particulate deposition-that has a higher than normal concentration of hydrogen ions (H+), making it unusually acidic. Acid deposition can damage plants, aquatic ecosystems, soil chemistry, built structures and human health by increasing acidity and mobilising toxic metals.

Types of Acid Deposition

Wet deposition

• Wet deposition occurs when acid gases and particles are scavenged by cloud droplets and fall to the surface as rain, snow, fog or mist.
• Precipitation removes gases and particles from the atmosphere by two main processes: rain-out, in which particles are incorporated into cloud drops that then fall, and wash-out, in which materials below the cloud are swept down by falling rain or snow.
• The ecological and chemical effects of wet deposition depend on the acidity of the precipitation, the buffering capacity and mineral composition of soils and bedrock, and the sensitivity of local biota (fish, trees, microbes).
• As acidic water flows over and through soil and bedrock, it can leach nutrients and mobilise toxic metals into waterways.

Dry deposition

• Dry deposition occurs when acid gases and particles settle out of the air in the absence of precipitation, adhering to surfaces such as soil, vegetation, buildings and vehicles.
• Dry deposited pollutants can later be washed off surfaces by rainstorms; the run-off can be highly acidic and contribute to surface and groundwater acidification.
• Approximately half of the acidity deposited from the atmosphere may occur by dry deposition, depending on local climate and emissions.

The pH Scale

• The pH scale measures the acidity or basicity (alkalinity) of an aqueous solution and is defined by the relation pH = -log10[H+], where [H+] is the hydrogen ion concentration in moles per litre.
• The conventional scale runs from 0 to 14, with pH 7 being neutral; values less than 7 are acidic and values greater than 7 are basic. Lower and higher values are theoretically possible.
• The scale is logarithmic: each integer change of one pH unit corresponds to a tenfold change in hydrogen ion concentration. For example, a solution of pH 4 is ten times more acidic (ten times higher [H+]) than pH 5 and one hundred times more acidic than pH 6.
• The pH concept was introduced in 1909 by Søren P. L. Sørensen and remains central for describing acidification of ecosystems and waters.

Sources of Compounds Causing Acid Rain

Sulphur compounds (primarily SO2)

• Natural sources: seas and oceans, volcanic eruptions and biological processes in soils (decomposition of organic matter).
• Anthropogenic sources: combustion of coal (contributing about 60% of atmospheric SO2 in the cited estimate) and petroleum products (about 30% in the cited estimate); smelting of metal sulphide ores; industrial processes that produce sulphuric acid in metallurgical, chemical and fertiliser industries.

Nitrogen compounds (NOx)

• Natural sources: lightning, volcanic eruptions and biological nitrogen cycling.
• Anthropogenic sources: combustion of oil, coal and gas in power stations, industry and vehicles; forest fires contribute episodically.

Organic acids and other acids

• Formic acid (HCOOH) is emitted during biomass burning (forest fires) and is also produced in the atmosphere from photo-oxidation of formaldehyde (HCHO).
• Other acids include hydrochloric acid, phosphoric acid and chlorine compounds emitted from some industrial sources and smokestacks.
• Carbon dioxide from vehicles and other combustion sources dissolves in water to form weak carbonic acid (H2CO3), which also contributes to acidity though it is a relatively minor component compared to sulphuric and nitric acids in typical acid-rain chemistry.

Transport of pollutants

• SOx and NOx emitted in one region can be transported long distances by winds, so acid deposition often affects areas remote from the emission sources. Consequently, environmental damage from acid deposition is spatially non-uniform and transboundary in nature.

Common Characteristics of Areas Prone to Acid Rain

• Most acid-rain problems concentrate in industrialised belts of the northern hemisphere where fossil-fuel combustion and industrial emissions are high.
• Upland and mountainous regions that receive high precipitation (rain and snow) are especially sensitive because runoff and surface waters can receive concentrated acidic deposition; these areas often contain lakes and streams that are vulnerable to acidification.
• Thin soils and glaciated bedrock in upland areas give low buffering capacity; these conditions accelerate acidification and nutrient leaching.

Global hot spots

• Significant regions affected include parts of Scandinavia, Canada (Nova Scotia, Southern Ontario and Quebec), the northeastern United States (Adirondacks, Great Smoky Mountains), parts of the US Midwest and mountain regions such as the Colorado Rockies and upland Britain and western Europe.

India

• The first reported instance of acid rain in India was from Bombay (Mumbai) in 1974.
• Instances of acidic precipitation have been reported from metropolitan and industrial regions. Annual SO2 emissions in India have risen substantially over recent decades because of increased fossil-fuel consumption; reported trends show a near doubling of SO2 emissions in the cited decade.
• Lowering of soil pH has been reported from north-eastern India, coastal Karnataka and Kerala, and parts of Odisha, West Bengal and Bihar.

Indicators of acid deposition

• Lichens are sensitive bio-indicators of air quality and acid deposition; changes in lichen communities are used to assess atmospheric pollution.
• Declines in acid-sensitive aquatic invertebrates-freshwater shrimp, crayfish, snails and some mussels-signal acidification of lakes and streams and can precede declines in fish populations.

Chemistry and Formation of Acid Rain

Six basic steps in the atmospheric formation and deposition of acids:

• Oxides of sulphur (SO₂, SO₃) and nitrogen (NO, NO₂, collectively NOx) are released to the atmosphere from natural and anthropogenic sources.
• Some of these oxides are returned to the surface by dry deposition near or downwind of their source.
• Sunlight drives photochemical reactions that generate reactive oxidants (for example, ozone, OH radicals).
• Oxidation of SO₂ and NOx by atmospheric oxidants produces strong acids such as sulphuric acid (H₂SO₄) and nitric acid (HNO₃).
• These acids exist in the atmosphere in gaseous and particulate forms and are subject to transport and chemical transformation (including interactions with ammonia and other gases).
• The acids are removed by wet deposition (dissolved in precipitation) or by dry deposition of acidic gases and particles.

Key reactions (representative):

• Oxidation of sulfur dioxide to sulfuric acid (simplified): SO₂ + OH → HOSO₂; further reactions lead to H₂SO₄ formation.
• Oxidation of nitrogen oxides forms nitric acid: NO₂ + OH → HNO₃.

Difference between naturally acidified and anthropogenically acidified lakes

Impacts of Acid Rain

Soil

• Increased hydrogen ions in soil exchange with nutrient cations (potassium, calcium, magnesium), causing leaching of these essential nutrients and reducing soil fertility.
• Soil acidification can reduce microbial respiration and alter microbial community composition, slowing decomposition and nutrient cycling.
• Increased soil acidity can raise levels of ammonium relative to other nutrients, altering decomposition rates.
• The impact of acid rain on soils is moderated where soils are alkaline or possess good buffering capacity; many Indian soils are alkaline and so show comparatively greater resilience.

Vegetation

Acid deposition affects forest health and crop productivity. Typical symptoms and consequences include:

• Discolouration and loss of foliar biomass, visible damage to leaves and needles.
• Reduction of feeder-root biomass, especially in conifers, leading to impaired water and nutrient uptake.
• Premature senescence of older foliage.
• Increased susceptibility to secondary stressors such as pathogens, pests, drought and frost.
• Death of herbaceous vegetation beneath affected trees and, in severe cases, tree mortality.
• Changes in epiphytic communities such as increased or decreased lichen growth depending on species and local chemistry.

Microorganisms

• Soil and aquatic microbial communities are pH-sensitive. Most bacteria and protozoa prefer near-neutral pH; many fungi tolerate or prefer acidic conditions; cyanobacteria (blue-green algae) often favour alkaline conditions.
• Long-term acidification can shift microbial communities from bacterial dominance to fungal dominance, slowing decomposition of organic matter and altering nutrient availability.
• These shifts can increase fungal disease incidence in forests and affect nutrient cycling in aquatic systems.

Wildlife and Aquatic Life

• Acid rain can directly affect eggs, larvae and juvenile stages of amphibians and fish that breed in small forest ponds and streams; acidified waters can be lethal or impair development.
• Acidification mobilises aluminium and other metals from soils and sediments; these metals can be toxic to aquatic organisms and travel up the food chain, affecting birds and mammals that feed on aquatic prey.
• Loss and alteration of food and habitat resources from acidification have indirect negative effects on wildlife populations and biodiversity.

Humans

• Acid deposition indirectly affects human health by reducing air quality (irritation of skin, eyes and the respiratory tract), lowering visibility and contributing to respiratory diseases.
• Chronic exposure to air pollutants that cause acid rain (SO2, NOx, particulate matter) is associated with bronchitis, emphysema and other pulmonary illnesses.
• Acidification of drinking water supplies can increase the mobility of heavy metals (aluminium, manganese, cadmium, copper) and create public-health risks including food and water contamination.

Damage to Materials and Infrastructure

Acid rain accelerates corrosion and decay of building materials and cultural monuments, particularly calcareous stones (limestone, marble), metals and painted surfaces.

Socio-economic impacts

• Acid rain reduces agricultural yields, fisheries productivity and forest resources, which in turn can lower economic returns (GNP, per capita income) and affect livelihoods-especially in economies with large rural and resource-dependent sectors.

Trigger Effects of Acid Rain on Other Pollutants

Lower pH in soils and waters can mobilise or increase the bioavailability of several toxic elements and compounds. Important examples:

Mercury

• Methylmercury and related organic mercury compounds are highly toxic and bioaccumulate in fish tissue, posing health risks to humans and wildlife that consume contaminated fish.
• Acid deposition may not directly increase methylation rates everywhere, but increased acidity can enhance the partitioning and mobility of methylmercury into the water column, increasing uptake by aquatic organisms.
• Liming of acidified waters and catchments can reduce mercury availability in fish in some cases.

Aluminium

• Acidified waters leach aluminium from soils and sediments into lakes and streams; soluble aluminium is toxic to fish and can impair gill function.
• Aluminium exposure has been implicated in neurological concerns in susceptible human populations; aluminium mobilisation is therefore a public-health concern where water treatment and intake are affected.

Cadmium

• Lower water pH increases the solubility and mobility of cadmium. Corrosion of metallic plumbing and solder in distribution systems can release cadmium into drinking water.
• A decrease in water pH from about 6.5 to 4.5 can greatly increase the concentration of dissolved cadmium and raise the risk of renal tubular damage from cadmium exposure.

Lead

• Foetuses and infants are especially susceptible to lead exposure from contaminated drinking water and food; elevated lead levels in young children are associated with biochemical and neurophysiological dysfunction and with impaired cognitive and behavioural development.

Asbestos

• Acidic waters and acids in the environment can cause weathering of natural rock and release asbestos fibres where asbestos minerals are present; mobilisation increases potential exposure.

Control Measures and Mitigation

Effective strategies combine source reduction, end-of-pipe controls, ecosystem remediation and policy measures:

• Emission reductions: reduce combustion of high-sulphur fuels by switching to low-sulphur coal or oil, natural gas and renewables (wind, solar, hydro, tidal) and by improving energy efficiency.
• Flue-gas desulphurisation (FGD): wet and dry scrubbing technologies remove SO₂ from power-station exhausts; captured sulphur can be converted to sulphuric acid or elemental sulphur for industrial use.
• NOx control: low-NOx burners, selective catalytic reduction (SCR) and modification of combustion conditions in engines and boilers reduce NOx emissions.
• Vehicle controls: catalytic converters, fuel quality improvements and tighter vehicle-emission standards lower roadside NOx and hydrocarbon emissions.
• Fuel switching and renewables: replacing coal and heavy oil with natural gas and renewable energy reduces precursor emissions.
• Buffering (liming): addition of neutralising agents such as lime (calcium oxide, CaO) or ground limestone (calcium carbonate, CaCO₃) to acidified lakes and catchments raises pH and mitigates biological impacts.
• Chemical neutralisation examples: addition of CaCO₃ neutralises acids according to the overall reaction H₂SO₄ + CaCO₃ → CaSO₄ + H₂O + CO₂; slaked lime (Ca(OH)₂ formed from CaO + H₂O) neutralises acidity and precipitates some dissolved metals.
• Process changes and material recycling: converting sulphur by-products to useful chemicals, and industrial process modifications, can reduce net emissions.
• Monitoring and ecosystem restoration: systematic monitoring of precipitation chemistry, surface waters and biological indicators (lichen, benthic invertebrates, fish) supports targeted remediation such as liming and habitat restoration.

Categorisation of Industrial Sectors (MoEFCC)

• The Ministry of Environment, Forest and Climate Change (MoEFCC) in India developed criteria to categorise industrial sectors into Red, Orange, Green and White categories based on a Pollution Index (PI). The PI is a numerical score (0-100) that reflects air emissions, effluent discharges, hazardous waste generation and resource consumption; higher PI denotes higher pollution load from that sector.
• Re-categorisation based on pollution load is intended to provide a more accurate, science-based classification of industries so that regulatory requirements and access to finance are better aligned with actual environmental impact.
• The White category denotes industries with negligible pollution potential; these units may be exempted from some clearances. Red category industries are those with high pollution potential and are normally not permitted in ecologically fragile or protected areas.

Additional Notes and Interesting Facts

• Many trees help reduce atmospheric carbon oxides, can enhance local air quality and in some cases contribute to nitrogen fixation and nutrient cycling.
• Seed and fruit-development phenomena such as agamospermy (formation of seeds without fertilisation) and parthenocarpy (fruit formation without seed) are biological curiosities sometimes introduced in general biology notes; they are not direct features of acid-rain science but illustrate complexity of plant reproduction.
• Horticultural practices such as bonsai development or bamboo cultivation are culturally and botanically interesting but peripheral to acid-rain impacts; they appear in general pedagogic side-notes in some texts.

Summary

Acid rain results primarily from atmospheric oxidation of sulphur and nitrogen oxides to form sulphuric and nitric acids. Its effects are widespread-affecting soils, forests, freshwaters, wildlife, materials and human health-and are often felt far from emission sources because of atmospheric transport. Effective mitigation requires source controls (fuel switching, emissions technologies, regulatory measures), local remediation (liming) and sustained monitoring. Policy tools such as industry categorisation and cross-border agreements are important for managing transboundary acid deposition and its socio-economic consequences.