Question

A friend of yours has seen each of the following items in newspaper articles and would like an explanation: (a) acid rain, (b) greenhouse gas, (c) photochemical smog, (d) ozone depletion. Give a brief explanation of each term and identify one or two of the chemicals associated with each.

Short answer

Acid rain is precipitation with a pH lower than 5.6 caused by the release of sulfur dioxide (SO2) and nitrogen oxides (NOx), forming sulfuric acid (H2SO4) and nitric acid (HNO3). Greenhouse gases, like carbon dioxide (CO2) and methane (CH4), trap heat in the Earth's atmosphere, leading to global warming. Photochemical smog is air pollution caused by reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in sunlight, resulting in pollutants like ozone (O3) and peroxyacetyl nitrate (PAN). Ozone depletion refers to the reduction of the Earth's ozone layer due to substances like chlorofluorocarbons (CFCs), which release chlorine and bromine atoms that destroy ozone molecules.

Step-by-step solution

(a) Acid Rain

Acid rain refers to precipitation with a pH of less than 5.6 (normal rainwater has a pH of around 5.6) due to atmospheric pollution. It is caused by the release of sulfur dioxide (SO2) and nitrogen oxides (NOx) into the atmosphere. These pollutants can be emitted from industrial processes, power plants, and vehicle exhausts. When they react with water, oxygen, and other substances in the atmosphere, they form sulfuric acid (H2SO4) and nitric acid (HNO3) which then fall to the ground as acid rain.

(b) Greenhouse Gas

A greenhouse gas is a gas that traps heat in the Earth's atmosphere, contributing to the greenhouse effect and global warming. These gases absorb and emit infrared radiation, which increases the temperature of the Earth's surface. Examples of greenhouse gases include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water vapor (H2O). Among these, carbon dioxide and methane are the most significant contributors to the greenhouse effect.

(c) Photochemical Smog

Photochemical smog is a type of air pollution resulting from chemical reactions between pollutants, mainly nitrogen oxides (NOx) and volatile organic compounds (VOCs), in the presence of sunlight. It is characterized by smoky, hazy conditions, which can reduce visibility and harm human health. The primary sources of nitrogen oxides and volatile organic compounds include automobile exhaust, industrial emissions, and chemical solvents. The most well-known pollutant associated with photochemical smog is ozone (O3), which is formed through the reaction of nitrogen oxides and volatile organic compounds in sunlight. Another pollutant is peroxyacetyl nitrate (PAN).

(d) Ozone Depletion

Ozone depletion refers to the reduction of the Earth's ozone layer, which is located in the stratosphere and serves as a protective barrier against the harmful ultraviolet (UV) radiation from the sun. Chlorofluorocarbons (CFCs) and other substances containing chlorine and bromine are the primary culprits of ozone depletion. These substances are released from human activities such as the use of refrigerants, aerosol propellants, and various industrial processes. Once in the stratosphere, CFCs and other ozone-depleting substances release chlorine and bromine atoms, which react with ozone (O3) molecules and destroy them, leading to a decrease in ozone concentration.

Key concepts

Acid Rain

When you hear of 'acid rain,' it's referring to rain that's more acidic than normal. Regular rainwater has a pH close to neutral, about 5.6, due to the natural carbon dioxide in the air, which forms a weak carbonic acid. Acid rain, however, has a pH of less than 5.6. It occurs when sulfur dioxide (SO₂) and nitrogen oxides (NO_x) are released into the atmosphere, often from human activities like burning fossil fuels in power plants, factories, and cars. These pollutants react with water vapor to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃), which then fall to the earth in precipitation. The repercussions of acid rain are significant, leading to damaged forests, deteriorated water quality in rivers and lakes, and harm to wildlife.

Moreover, acid rain is a global problem, affecting buildings and structures, particularly those constructed with materials susceptible to acid corrosion, such as limestone and marble. Reducing emissions of SO₂ and NO_x through regulations and the use of cleaner energy sources can mitigate the impact of acid rain.

Greenhouse Gas

The term 'greenhouse gas' (GHG) refers to gases in the Earth's atmosphere that trap heat, similar to how a greenhouse for plants keeps warmth inside. These gases absorb and emit infrared radiation, leading to the warming of Earth's surface. The most common greenhouse gases include carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and water vapor (H₂O). Each of these plays a distinct role in the greenhouse effect: CO₂, for instance, is emitted largely through the burning of fossil fuels and deforestation, whereas CH₄, a much potent GHG although less abundant, comes from sources such as agriculture (especially from the digestive systems of grazing animals) and from decomposing organic waste in landfills.

These gases are critical to life as we know it because they contribute to the Earth's stable climate. However, human activities have significantly increased the concentrations of these gases, particularly CO₂, propelling climate change and global warming. Mitigation efforts center around reducing emissions, transitioning to renewable energy sources, and improving energy efficiency.

Photochemical Smog

Photochemical smog is a type of air pollution that's formed when sunlight catalyzes chemical reactions between various pollutants in the atmosphere. It typically arises in urban areas under certain conditions, such as warm, sunny weather, and stagnant air. The primary ingredients for photochemical smog are nitrogen oxides (NO_x) and volatile organic compounds (VOCs), which can come from car exhaust, industrial emissions, and even some natural sources like plants.

Ozone (O₃) is a key component of photochemical smog, formed by reactions among NO_x, VOCs, and sunlight. While ozone high in the stratosphere protects us from ultraviolet light, at ground level, it's a harmful pollutant that can cause respiratory problems and harm vegetation. Peroxyacetyl nitrate (PAN) is another harmful substance found in photochemical smog. Mitigation strategies include driving less, using cleaner forms of transportation, and regulating industrial emissions.

Ozone Depletion

Ozone depletion is a critical environmental issue involving the loss of ozone in the stratosphere. The ozone layer, located about 10-30 kilometers above the Earth's surface, plays a crucial role in absorbing the majority of the sun's dangerous ultraviolet (UV) radiation. Without this protective layer, life on Earth would be exposed to higher levels of UV radiation, which can lead to skin cancer, cataracts, reduced agricultural productivity, and disruption of marine ecosystems.

The primary cause of ozone depletion is the release of chemicals known as chlorofluorocarbons (CFCs), as well as other related halogenated organic compounds such as brominated hydrocarbons. Once CFC molecules reach the stratosphere, they are broken down by UV light, releasing chlorine atoms that react with ozone molecules (O₃), thereby reducing the ozone concentration. The famous manifestation of this issue is the 'ozone hole' over Antarctica. To address this, the global community adopted the Montreal Protocol, which successfully reduced the production and release of CFCs and has led to signs of recovery of the ozone layer.