Tiny Smoke Particles Resist Removal by Rain and Irrigation
Wildfire smoke can create unhealthy air conditions, leading many people to wonder whether rainfall or irrigation water can help remove smoke from the atmosphere. While water is effective at controlling dust, scientific research shows that it is generally not a reliable solution for clearing wildfire smoke.
Wildfire smoke contains extremely small particles known as PM2.5, which are among the air pollutants regulated by environmental agencies. These particles are often much smaller than ordinary dust and can remain suspended in the atmosphere for extended periods. Because of their tiny size, they can travel long distances from the source of a wildfire and continue affecting air quality far away from the fire itself.
Scientists have long studied how water removes airborne particles through a process called washout. During rainfall or water spraying, droplets can capture particles and carry them to the ground. However, the effectiveness of this process depends heavily on particle size.
One particle-removal method, known as Brownian collision, works best on extremely small particles. Another process, called kinematic collision, is more effective at removing larger particles. Unfortunately, most wildfire smoke particles fall between these two ranges, making them difficult to capture by either mechanism.
This challenging size range is known in atmospheric science as the Greenfield Gap. Particles in the Greenfield Gap are highly resistant to removal by precipitation. As a result, even heavy rainfall may only slightly reduce smoke concentrations in the air. Similarly, spraying irrigation water on farms, fields, or outdoor areas is unlikely to significantly improve smoky conditions.
Although water is not effective at removing wildfire smoke, it can successfully reduce larger airborne particles such as dust.
Watering roads, construction sites, or dry fields helps prevent dust from becoming airborne again, improving local air quality. Smoke particles, however, behave differently and are removed more slowly through natural processes such as wind dispersion and deposition onto surfaces.
Many people notice better visibility after rain and assume the smoke has disappeared. While rainfall can temporarily improve air appearance, dangerous fine smoke particles may still remain present and continue affecting human health.
During smoky conditions, experts recommend monitoring local air quality reports, limiting outdoor activities when pollution levels are high, and using indoor air filtration systems whenever possible.
Outdoor workers, including farmers and construction workers, may benefit from additional protection such as taking breaks indoors and using properly fitted N95 respirators when available.
Understanding the behavior of wildfire smoke helps people make informed decisions. Research shows that while water remains useful for dust control, it is generally not an effective method for removing wildfire smoke because the particles are trapped within the Greenfield Gap and resist washout by rain or irrigation.
Photo Credit: ontario-ministry-of-natural-resources