Many people notice a sense of freshness in the air after rainfall, leading to a common belief that rain effectively purifies the atmosphere. This article explores the scientific reality behind this widespread feeling, delving into how rain interacts with airborne pollutants.
How Rain Cleans the Air
Rain cleans the air primarily through processes collectively known as wet deposition. As raindrops form and fall, they act as natural scrubbers, capturing various airborne particles and gases. This cleansing action occurs through two main mechanisms: rainout and washout.
Rainout refers to in-cloud scavenging, where pollutants become incorporated into cloud droplets before precipitation begins. Atmospheric particles can serve as condensation nuclei, allowing water vapor to condense around them and form cloud droplets. These droplets then grow and fall as rain, effectively bringing the trapped pollutants to the ground.
Washout, or below-cloud scavenging, involves falling raindrops colliding with and absorbing pollutants present beneath the cloud layer. As raindrops descend, they collect particulate matter such as dust, pollen, and soot, as well as water-soluble gases like sulfur dioxide and nitrogen oxides. This interaction effectively cleanses the atmosphere by depositing these contaminants onto surfaces.
What Rain Cannot Clear
While rain effectively removes many pollutants, it does not clear all atmospheric contaminants. Some pollutants are not water-soluble or are too small to be efficiently captured by raindrops. For instance, certain volatile organic compounds and very fine nanoparticles are less effectively scavenged by rain.
Gases like carbon monoxide and ozone do not readily dissolve in water, meaning rain has little impact on their atmospheric concentrations. Rain can also have unintended consequences, such as re-emitting pollutants from contaminated surfaces or transporting them from one area to another.
Factors Influencing Rain’s Effectiveness
The effectiveness of rain in clearing air pollutants depends on several atmospheric and rainfall characteristics. Rain intensity plays a significant role; heavy downpours generally remove pollutants more effectively than light drizzles. For example, rainfall exceeding 10mm/h can achieve a clearance rate above 35% for certain pollutants.
The duration of rainfall also influences its cleansing power. Prolonged rain can lead to increased reductions in particulate matter. Additionally, the size of raindrops affects scavenging efficiency, with larger droplets often having a greater capacity to collect impurities. The initial concentration of pollutants in the atmosphere also influences the observed improvement, as higher initial concentrations often result in a more noticeable cleansing effect.
Perceived Versus Actual Air Quality After Rain
The feeling of fresh, clean air after a rain shower is a common human experience. This perception is largely due to rain’s effectiveness in removing larger particulate matter like dust, pollen, and visible airborne particles, which can obscure visibility. The washing away of certain odors also contributes to this subjective sense of purity.
While the air may not be entirely “pure” after rain, measurable improvements in the levels of many common pollutants occur. For instance, particulate matter levels, particularly PM10 (particles less than 10 micrometers in diameter), often decrease significantly following a storm. The psychological effect of rain, often associated with a sense of renewal and cleanliness, further enhances the perception of improved air quality. Rain therefore contributes to both tangible reductions in specific pollutants and a subjective sense of atmospheric freshness.