Atmospheric and Environmental Chemistry — Notes

Acid rain is rain that is more acidic than normal rain (normal rain is slightly acidic because of dissolved carbon dioxide). It forms when oxides of sulphur and nitrogen dissolve in the water droplets of clouds.

Sources. Sulphur dioxide: burning fossil fuels (coal, oil) that contain sulphur; roasting sulphide ores; volcanoes. Oxides of nitrogen: the very high temperature inside petrol and diesel engines, furnaces and power stations, where nitrogen and oxygen of the air combine; also lightning.

Equations (oxides of sulphur):

• $\ce{S + O2 -> SO2}$
• $\ce{SO2 + H2O -> H2SO3}$ (sulphurous acid)
• $\ce{2SO2 + O2 -> 2SO3}$ (oxidation in air, helped by dust and other pollutants)
• $\ce{SO3 + H2O -> H2SO4}$ (sulphuric acid)

Equations (oxides of nitrogen):

• $\ce{N2 + O2 ->[\text{very high temperature}] 2NO}$
• $\ce{2NO + O2 -> 2NO2}$ (brown gas)
• $\ce{4NO2 + O2 + 2H2O -> 4HNO3}$ (nitric acid). Some books write $\ce{3NO2 + H2O -> 2HNO3 + NO}$ instead; both are accepted.

Effects. Soil: washes out nutrients such as calcium and magnesium, harms plants and soil organisms. Water bodies: lakes and rivers become too acidic for fish eggs, fish and many water plants. Buildings: marble and limestone are attacked, for example $\ce{CaCO3 + H2SO4 -> CaSO4 + H2O + CO2}$.

Reducing acid rain: fuels with less sulphur (as of 2026, Indian petrol and diesel meet the very low sulphur limits of Bharat Stage VI), removing sulphur dioxide from chimney gases, catalytic converters in vehicles, and cleaner energy.

The greenhouse effect: sunlight passes through the atmosphere and warms the ground. The ground gives out heat as infrared radiation. Greenhouse gases absorb part of it and send some back to the ground, so the Earth is warmer than it would be without them. This natural effect makes life possible. Global warming is the extra warming caused by human activities that increase these gases.

Burning carbon in fuel: $\ce{C + O2 -> CO2}$. Making cement from limestone: $\ce{CaCO3 ->[\Delta] CaO + CO2}$.

Measures to reduce emissions: save energy; use solar, wind and hydro power; use public transport and share journeys; plant trees and stop clearing forests; reduce waste and manage fertiliser and manure use.

Time-sensitive points (as of 2026): the carbon dioxide level in the air is above 420 parts per million, compared with about 280 parts per million before the Industrial Revolution; the average surface temperature of the Earth has risen by roughly 1.2 °C or more since the late 1800s. Figures change every year; check the latest scientific reports.

Ozone ($\ce{O3}$) is a form of oxygen with three atoms in each molecule. Most of it lies in a band inside the stratosphere.

Formation (in the upper atmosphere, with ultraviolet radiation):

• $\ce{O2 ->[UV] 2O}$ (oxygen molecules are split into atoms)
• $\ce{O + O2 -> O3}$ (an atom joins an oxygen molecule)
• overall: $\ce{3O2 -> 2O3}$

Function: it absorbs the harmful ultraviolet radiation of the Sun, so less reaches the ground. Without it there would be more skin cancer and eye cataracts, damage to crops, and harm to plankton.

Depletion by named chemicals (only the names are required; their reactions are not part of this course): chlorofluorocarbons (CFCs) from old refrigerators, air conditioners, aerosol sprays and foams; halons from fire extinguishers; carbon tetrachloride; methyl bromide. The "ozone hole" is a large seasonal thinning of the layer over Antarctica.

Time-sensitive point (as of 2026): under the Montreal Protocol (1987) these chemicals have been phased out and the ozone layer is slowly recovering; full recovery is expected only around the middle of the century. Do not confuse the ozone hole (ultraviolet radiation) with global warming (infrared radiation).

Natural water carries dissolved substances that living things need.

Dissolved oxygen falls as water gets warmer, because gases are less soluble in warm water. Approximate textbook values for fresh water in contact with air at sea-level pressure: 14.6 mg per litre at 0 °C; 11.3 mg per litre at 10 °C; 9.1 mg per litre at 20 °C; 7.6 mg per litre at 30 °C; 6.5 mg per litre at 40 °C. Many fish need at least about 5 mg per litre (the value depends on the species). Dissolved oxygen is also reduced when bacteria decay large amounts of waste.

Books differ slightly in the solubility values; the trend (a fall with temperature) is what is examined.

Toxic heavy metals — Minamata disease. In the 1950s, a factory at Minamata Bay in Japan discharged waste containing mercury into the sea. Microbes changed it into methylmercury, which is absorbed by plankton and small fish and builds up along the food chain, reaching high amounts in large fish and shellfish. People who ate them developed damage to the nervous system: numbness, loss of coordination, and trouble with speech, sight and hearing; in severe cases paralysis, and harm to babies before birth. Other toxic heavy metals include lead, cadmium and arsenic. As of 2026, an international treaty (the Minamata Convention on Mercury, in force since 2017) controls mercury.

Plastics. They do not rot, so they stay for hundreds of years; they block drains, are swallowed by or tangle round fish, turtles and birds, and break into tiny pieces. As of 2026, India bans a list of single-use plastic items (in force since 2022).

Sewage. Untreated sewage carries disease germs and adds nitrates and phosphates. These make algae multiply (algal bloom). When the algae die, bacteria decay them and use up the dissolved oxygen, so fish and other animals die.

Pesticides. Washed from fields, they poison fish and other animals that were not the target, and may build up along food chains.

Exact figures for fish and mercury limits differ between countries; items use imagined figures.