Rainwater is a fundamental part of the Earth’s hydrologic cycle, transferring water from the atmosphere back to the land and oceans. This process also serves a vital function in atmospheric cleansing, as precipitation washes particles and dissolved gases from the air. Although often perceived as pure, rainwater is never chemically neutral or pristine because it interacts with the atmosphere. It is a dilute solution containing various dissolved substances that establish its baseline chemical composition. The contents of rainwater range from naturally occurring compounds to human-made pollutants, reflecting the constantly changing chemistry of the air it passes through.
Natural Gaseous Components
The chemical composition of rainwater begins with the gases it dissolves while falling through the atmosphere. The two most abundant gases, nitrogen and oxygen, dissolve readily into water droplets, though they do not significantly alter the water’s acidity. The most influential natural gaseous component is carbon dioxide (CO₂), which is constantly present in the air.
When atmospheric carbon dioxide dissolves into water, it forms a weak acid known as carbonic acid (H₂CO₃). This natural process explains why pure water, with a neutral pH of 7.0, is rarely found in nature. The dissolution of carbon dioxide establishes the natural pH of rain, which is slightly acidic, typically measuring around 5.6 on the pH scale. This weak carbonic acid forms the baseline against which the more severe acidity caused by pollutants is measured.
Natural Particulate Matter
Beyond dissolved gases, rainwater also incorporates various solid and liquid particles, collectively known as aerosols, that are naturally suspended in the air. These particles are crucial components known as cloud condensation nuclei (CCN). Water vapor requires a non-gaseous surface to condense upon, and these tiny airborne specks provide that surface, making cloud and raindrop formation possible.
One significant natural source is sea salt spray, where the bursting of ocean bubbles injects sodium and chloride ions into the air, which are then incorporated into coastal and marine rainfall. Mineral dust, originating from arid regions and deserts, represents another source, introducing soil particles that can sometimes contain alkaline components like calcium and magnesium. These alkaline compounds can act as a natural buffer, slightly neutralizing the carbonic acid in the rain.
Volcanic eruptions also contribute natural aerosols, including ash and sulfur dioxide, which can temporarily affect the composition of precipitation. Additionally, biological matter, such as pollen, fungal spores, and bacteria, is regularly scavenged from the air by precipitation, adding organic components to the water.
Industrial and Vehicular Emissions
Human activities, particularly the burning of fossil fuels, introduce powerful acid precursors into the atmosphere that drastically alter the composition of rainwater. The two most significant pollutants in this category are sulfur dioxide (SO₂) and nitrogen oxides (NOₓ). These gases are primarily released from industrial facilities, such as coal-fired power plants, and from the combustion engines of motor vehicles.
In the atmosphere, these compounds undergo complex chemical reactions involving water, oxygen, and other chemicals to form strong inorganic acids. Sulfur dioxide is oxidized to sulfur trioxide (SO₃), which then reacts with water to form highly corrosive sulfuric acid (H₂SO₄). Similarly, nitrogen oxides react to produce nitric acid (HNO₃). The formation of these strong acids causes the pH of rain to drop significantly below the natural 5.6 baseline, resulting in what is commonly known as acid rain.
Acid-impacted rain typically exhibits a pH below 5.0, sometimes dropping as low as 2.4 in heavily polluted areas. This pollution-induced acidity leaches nutrients from soil, damages forests, and acidifies lakes and streams, harming aquatic life. Although regulatory efforts have significantly reduced the emissions of SO₂ and NOₓ in many developed nations, the problem persists globally, especially in regions relying on sulfur-containing coal for energy.
Modern Chemical Contaminants
While traditional acid rain has lessened in many areas, contemporary industrial and consumer practices have introduced a new class of persistent pollutants into the global water cycle. Rainwater acts as a carrier for these modern chemical contaminants that demonstrate high stability and widespread distribution. These substances are often referred to as “forever chemicals” because they degrade extremely slowly in the environment.
Per- and polyfluoroalkyl substances (PFAS), used in products like nonstick cookware, water-repellent fabrics, and firefighting foam, are now found ubiquitously in rainwater across the globe. Research indicates that these compounds have traveled through the atmosphere and are present even in remote locations like Antarctica and the Tibetan plateau, sometimes exceeding safe drinking water guidelines. These chemicals are concerning due to their link to various adverse health effects, including developmental issues and altered immune function.
Microplastics represent another widespread contaminant, originating from the breakdown of synthetic textiles, vehicle tires, and plastic waste. These tiny plastic fragments are swept up into the atmosphere and deposited back onto the land and sea by rain, indicating a pervasive global cycle of plastic pollution.
Rain also deposits heavy metals, such as mercury, lead, and cadmium, which are emitted from localized industrial processes and smelters. Persistent organic pollutants (POPs), like certain legacy pesticides and industrial chemicals, are semi-volatile, allowing them to travel vast distances through atmospheric transport before being washed out by precipitation.