Chemistry of Rain

After a rainstorm the air always smells cleaner. Rain is a welcome site for me, being a person that suffers from allergies. The rain tends to wash many of these allergens out of the air, but I was curious about what else is in the rainwater.

Water droplets don't form on their own, making just a drop of distilled water. Typically in the atmosphere water droplets form around pollen, aerosols and other chemicals in the air. Once formed, they continue absorbing these chemicals until the drop gets heavy enough to fall to the earth.

Chemicals in rainfall include sodium, potassium, calcium, chloride, sulfate, nitrates, and ammonia. The quantities of these chemicals varies from storm to storm, location to location, and from season to season. For instance, sodium is introduced into our atmosphere from the salt water in the oceans. If you live near the coast, your rain will have more sodium. The direction of the storm also plays a role in that even if you live on the coast but the storm comes from over land, there will be less sodium in the rainwater than from a storm that moves in from over the ocean.

Your location plays a role away from the coast based on the chemistry of the dust particles that have been blown into the air. If the dust particles in the air near your location has a higher composition of any of the chemicals noted above, the rain water will reflect this chemical composition.

If you notice, the chemicals included in rain are often used in fertilizers. Rainfall works to fertilize the soil and thus growing plants. Rainfall would also tend to re-distribute the chemicals, spreading them out over large areas. Areas with high concentrations of these chemicals would over hundreds of years have those chemicals reduced. The chemicals being blown by wind into the atmosphere represents what is commonly referred to as wind erosion.

Remember the acid rain scare back in the 1970s and 1980s? We were warned about the rain killing vegetation and eating the paint off our cars and houses. The acid level of a substance is measured by its pH which varies on a scale from 0 to 14 with 7.0 being neutral (neither acidic or alkalinic). A pH lower than 7.0 indicates a substance is acidic and above 7.0 means it is alkalinic. It turns out that the pH of naturally occurring rainwater, although it varies, is around 5.7. So all rain is acid rain.

So when it rains, we aren't only getting the water we need to survive but cleaner air and fertilization for the plants around us. The acid in the rain is never going to hurt you, so you can still enjoy that little dance in the rain if you want.

Sources:
Rainwater as a Chemical Agent of Geologic Processes - A Review, Dorothy Carrol.
Rainwater Chemistry Across the United States, Ellen Root, Whit Jones, Bessie Schwarz, Jack Gibbons