APES 7.5-7.6: Acid Rain and Noise Pollution

Acid rain

Precipitation with unusually high hydrogen-ion concentration and a pH below about 5. It commonly results from atmospheric sulfur dioxide and nitrogen oxides forming sulfuric and nitric acids.

Why is unpolluted rain naturally slightly acidic?

Atmospheric carbon dioxide dissolves in water and establishes the equilibria $CO_2 + H_2O \rightleftharpoons H_2CO_3$ and $H_2CO_3 + H_2O \rightleftharpoons HCO_3^- + H_3O^+$. Therefore, clean rain generally has a pH below 7, often above about 5.

How does pH relate quantitatively to the acidity of rainwater?

The pH is defined as $pH=-\log[H_3O^+]$. A decrease of one pH unit corresponds to a tenfold increase in hydronium-ion concentration.

Primary anthropogenic gases responsible for acid deposition

Sulfur dioxide, $SO_2$, and nitrogen oxides, collectively written $NO_x$, are the major human-produced precursor gases. They are emitted especially by fossil-fuel combustion, industrial processes, vehicles, and agriculture.

Which human activities are major sources of sulfur dioxide and nitrogen oxides?

Coal-fired power plants and other fuel-burning facilities are major sources; metal smelting, steel production, motor vehicles, fertilizers, and intensive animal agriculture also contribute. Power plants have historically accounted for a large fraction of atmospheric $SO_2$ and $NO_x$ emissions.

How do sulfur dioxide emissions become sulfuric acid in the atmosphere?

Sulfur dioxide is oxidized in the troposphere and reacts with water: $SO_2 + \frac{1}{2}O_2 + H_2O \rightarrow H_2SO_4$. Atmospheric oxidants such as ozone and hydrogen peroxide can participate in the oxidation process.

How is nitric acid formed from nitrogen dioxide?

Nitrogen dioxide reacts with hydroxyl radicals in the troposphere: $NO_2 + OH\cdot \rightarrow HNO_3$. Photochemical reactions and atmospheric oxidants help convert nitrogen oxides into acidic species.

Why do $SO_2$ and $NO_x$ emissions lower precipitation pH?

Their oxidation products, primarily $H_2SO_4$ and $HNO_3$, dissociate in atmospheric water to produce $H_3O^+$. Increasing $[H_3O^+]$ lowers pH according to $pH=-\log[H_3O^+]$.

Natural sources of acid-forming atmospheric gases

Volcanic eruptions release sulfur dioxide, lightning produces nitrogen oxides, and biological processes in wetlands and oceans release sulfur compounds such as dimethyl sulfide. These natural sources can contribute to precipitation acidity.

Why can lightning contribute to nitric acid in rain?

The high energy of an electrical discharge enables nitrogen and oxygen in air to form nitrogen oxides. Subsequent oxidation and reaction with water produce nitric acid, $HNO_3$.

Wet deposition

The removal of acidic gases, particles, or aerosols from the atmosphere by precipitation such as rain, snow, or fog. The acids may form within droplets or be scavenged from clouds and the air below them.

Dry deposition

The transfer of acidic gases and particles to Earth's surfaces in the absence of precipitation. These substances can adhere to soil, vegetation, buildings, and other surfaces and may account for a substantial fraction of total acid deposition.

Acid deposition

The combined delivery of acidic substances to Earth's surface through both wet deposition and dry deposition. It is broader than the term acid rain because it includes snow, fog, gases, and particles.

Why can acid rain affect regions far from the emission source?

Atmospheric circulation transports $SO_2$, $NO_x$, and their oxidation products over long distances. Tall smokestacks can reduce local pollutant concentrations while increasing regional transport and downwind deposition.

Why do mountainous regions often receive substantial acid deposition?

Mountains frequently receive more precipitation and can intercept pollutant-containing clouds. This increases contact between atmospheric acids and the Earth's surface.

How does the geology of a watershed affect its sensitivity to acid rain?

Watersheds containing carbonate minerals can neutralize added acid through reactions that consume $H^+$. Lakes over silicate bedrock generally have less buffering capacity and therefore acidify more readily.

Acid-neutralizing capacity

A water system's ability to resist pH change when acid is added. Carbonate-containing rocks and minerals provide buffering because carbonate species react with $H_3O^+$ and consume acid.

What happens when acid rain reacts with carbonate minerals?

Carbonate minerals consume hydronium ions and are dissolved, reducing the immediate pH change in the water. A simplified net ionic relationship is $CO_3^{2-}+2H_3O^+\rightarrow H_2CO_3+2H_2O$.

How does acid rain affect aquatic ecosystems?

It lowers surface-water pH, increases dissolved aluminum and nitrogen, and alters biogeochemical processes. These changes can damage plants, microbes, invertebrates, fish, and food webs, reducing biodiversity.

Why can acid rain harm fish even when the acid is not directly toxic at the measured concentration?

Acidification can reduce prey populations and mobilize toxic aluminum from soils into surface water. Thus, fish may decline indirectly through food-web disruption as well as through direct physiological effects.

How does low pH affect fish reproduction and survival?

At sufficiently low pH, fish eggs may fail to hatch, and still lower pH can kill adult fish. The exact tolerance varies among species, with some organisms more acid-tolerant than others.

Why does acid rain increase aluminum toxicity in aquatic systems?

Acidic water promotes the dissolution and mobilization of aluminum-containing minerals in soils. The resulting dissolved aluminum can damage fish and other aquatic organisms, especially at low pH.

How does acid rain alter soil chemistry?

Acid deposition can leach essential nutrient cations such as $Ca^{2+}$ and $Mg^{2+}$ from soil. It can also increase mineral weathering and mobilize aluminum, reducing soil quality and stressing plants.

Why are calcium and magnesium losses important for plants?

Calcium and magnesium are essential plant nutrients and contribute to healthy soil chemistry. Their removal can impair plant growth and increase vulnerability to environmental stress.

How can acid rain weaken forests and other vegetation?

Nutrient loss, soil acidification, aluminum mobilization, and damage to tree bark can reduce plant health. Stressed trees become more susceptible to drought, fire, frost, intense sunlight, and pests.

How does acid rain damage limestone, marble, metals, and painted surfaces?

Acidic solutions react with carbonate stone, accelerating weathering of limestone and marble, and can corrode metals such as steel. Acid deposition can also promote paint peeling and deterioration of buildings, monuments, and statues.

What is the relationship between acid rain and ocean acidification?

Both involve increased acidity, but ocean acidification is primarily driven by absorption of atmospheric $CO_2$ and formation of carbonic acid. Acid rain is mainly associated with atmospheric $SO_2$ and $NO_x$ chemistry and localized deposition.

Why does sulfuric acid generally contribute more acid per mole than nitric acid?

Sulfuric acid is diprotic and can donate two protons, whereas nitric acid is monoprotic and donates one. The actual pH impact also depends on concentration, dissociation, dilution, and the buffering capacity of the receiving environment.

How can acid rain affect human health?

Acid rain itself is usually too dilute to cause burns like concentrated laboratory acids, but the air pollutants that produce it—especially sulfur oxides, nitrogen oxides, and associated particles—can worsen respiratory and cardiovascular health. Acid deposition also affects human health indirectly through contaminated ecosystems and degraded air quality.

Why can nitric acid deposition be both harmful and beneficial in ecosystems?

Excess nitric acid contributes to acidification and nitrogen enrichment, which can disrupt ecosystems. However, nitrate formed from nitric acid is also a source of fixed nitrogen that plants can use as a nutrient.

How can reducing sulfur dioxide and nitrogen oxide emissions prevent acid rain?

Reducing precursor emissions decreases the amount of sulfuric and nitric acids formed in the atmosphere. Strategies include cleaner fuels, emissions-control technologies, renewable energy, and regulations on power plants, vehicles, and industry.

Flue-gas desulfurization

A pollution-control process that removes sulfur dioxide from combustion exhaust, often by reacting the gas with a basic material such as limestone or lime. It reduces formation of atmospheric sulfuric acid.

How does a cap-and-trade system reduce acid-rain pollution?

A regulatory program sets an overall emissions cap and allows regulated sources to trade permits. Sources that can reduce emissions cheaply may sell permits, creating an economic incentive to lower total $SO_2$ and $NO_x$ emissions.

Why did acid-rain control programs generally reduce environmental acidity?

Limits on sulfur dioxide and nitrogen oxide emissions reduced the atmospheric production and deposition of sulfuric and nitric acids. As deposition declined, many affected water bodies and ecosystems showed chemical or biological recovery, although recovery can be incomplete and depends on local geology.

What is an important limitation when evaluating the effects of acid rain on a lake?

The lake's watershed geology, soil composition, existing acidity, and species composition influence its response. Therefore, the same amount of acid deposition can produce different ecological effects in different lakes.

How can acid rain cause a reduction in aquatic biodiversity?

Acidification and dissolved aluminum can eliminate acid-sensitive species and simplify food webs. The remaining tolerant organisms may become community dominants, decreasing species diversity.

Why can ecosystems recover after sulfuric acid deposition decreases, but not always immediately?

Lower acid input allows water chemistry and biological populations to move toward previous conditions. However, depleted soil nutrients, altered food webs, persistent chemical changes, and slow recolonization can delay or prevent complete recovery.

What is noise pollution?

Unwanted or excessive sound that interferes with normal activities or harms organisms. Common sources include transportation, construction, industrial activity, and urban development.

How is noise pollution measured?

Sound level is measured in decibels, abbreviated dB. The decibel scale is logarithmic, so a 10-dB increase represents a tenfold increase in sound intensity.

What are major effects of excessive noise on humans?

Excessive or chronic noise can cause hearing damage, sleep disruption, stress, difficulty concentrating, and other health problems. Risk generally increases with both sound intensity and duration of exposure.

Why is prolonged exposure to loud noise especially harmful?

Repeated exposure can damage sensory cells in the inner ear, causing temporary or permanent hearing loss and tinnitus. Limiting exposure time and using hearing protection reduce the risk.

How can noise pollution affect wildlife?

Noise can mask communication, interfere with mating and feeding, alter migration or habitat use, and increase stress. Underwater noise, including sonar, can particularly disrupt marine mammals that depend on sound.

How can communities reduce noise pollution?

Noise can be reduced with quieter machinery and vehicles, zoning and operational limits, sound barriers, building insulation or soundproofing, and increased distance between noise sources and people or wildlife.

Why are noise barriers and soundproofing different control strategies?

Noise barriers block or redirect sound between a source and a receiver, often outdoors near roads. Soundproofing reduces the transmission of sound into or between buildings by using insulation, sealing openings, and sound-absorbing materials.