Rainfall refers to the amount of precipitation in the form of rain that falls over a specific period of time, typically measured in millimetres (mm). It is an important environmental parameter that influences air quality, atmospheric conditions, and the distribution of pollutants.
In outdoor environments, rainfall plays a significant role in improving air quality. Rain can help cleanse the atmosphere by removing pollutants through a process known as wet deposition. During this process, raindrops capture airborne particles and soluble gases, such as particulate matter (PM), nitrogen dioxide (NO₂), and sulfur dioxide (SO₂), and carry them to the ground. This often results in temporarily cleaner air following rainfall events.
However, precipitation can also have negative environmental effects under certain conditions. In areas with high levels of air pollution, rain can contribute to the formation of acid rain. This occurs when pollutants such as sulfur dioxide and nitrogen oxides react with water vapour in the atmosphere, forming acidic compounds that fall to the ground and can harm ecosystems, soil, water sources, and infrastructure.
Rainfall also affects the dispersion and resuspension of pollutants. While it can reduce airborne particle concentrations, it may lead to increased surface pollution, which can later be reintroduced into the air once surfaces dry and particles are disturbed again.
Although rainfall is not a pollutant itself, monitoring it alongside other environmental parameters provides valuable context for understanding changes in air quality. It can help explain short-term improvements in pollutant levels and support more accurate interpretation of air quality data.
Rainfall cannot be controlled, but understanding its impact can help in assessing environmental conditions and planning activities, especially in areas affected by varying pollution levels.