By Andi Anderson
Wildfire smoke often raises concerns among farmers, outdoor workers, and the general public. A common question during smoke events is whether rain or irrigation can help remove smoke from the air. While water is effective at controlling dust, scientific research shows that it has only limited success in reducing wildfire smoke concentrations.
Wildfire smoke contains extremely small particles known as PM2.5. These fine particles measure 2.5 micrometers or less in diameter, with many smoke particles ranging between 0.1 and 0.5 micrometers. Because of their tiny size, they can remain suspended in the atmosphere for long periods and travel great distances from the source of a wildfire.
Scientists have studied how water removes airborne particles through a process known as washout. During rainfall or water spraying, particles can be captured by water droplets and brought to the ground. However, the efficiency of this process depends largely on particle size.
One mechanism involved in washout is called Brownian collision. This process is most effective for extremely small particles, generally those smaller than 0.1 micrometers. These particles move randomly in the air and are more likely to collide with water droplets.
Another mechanism is known as kinematic collision. This process works best for larger particles, usually those greater than 1 micrometer in size. Because they are larger and heavier, these particles are more likely to collide directly with falling droplets and be removed from the atmosphere.
The challenge is that most wildfire smoke particles fall between these two ranges. They are often too large for efficient Brownian collision and too small for effective kinematic collision. Scientists refer to this difficult-to-remove particle range as the Greenfield Gap. As a result, smoke particles within this size range resist washout and remain airborne even during significant rainfall.
For this reason, heavy rain or irrigation generally does not clear wildfire smoke effectively. Although rainfall may improve visibility and reduce larger airborne particles, many of the fine smoke particles responsible for poor air quality and health concerns can remain in the atmosphere.
Water can still provide benefits in certain situations. Spraying water on roads, fields, or construction areas can reduce dust and prevent larger particles from becoming airborne. However, smoke particles behave differently and are much harder to remove. Over time, these particles are gradually dispersed by wind, diluted in the atmosphere, and deposited onto the ground through natural processes.
Understanding this behavior is important during wildfire smoke events. Rather than relying on rain or irrigation, individuals should monitor local air quality reports and limit outdoor activities when smoke levels are high. Indoor air filtration systems can improve air quality inside buildings. Farmers, construction workers, and others who must work outdoors may benefit from taking breaks in cleaner environments and using properly fitted N95 respirators when conditions warrant.
Although rain may make the air appear cleaner, research shows that it is not a reliable solution for eliminating wildfire smoke. Staying informed and taking appropriate precautions remain the most effective ways to reduce exposure to harmful smoke particles.
Photo Credit: ontario-ministry-of-natural-resources
Categories: Michigan, Education