By Sarah Zeiler and Dr. Xufei Yang
When wildfire smoke fills the air, many people wonder if spraying irrigation water or waiting for rain will help clear the smoke away. It seems like a simple solution. After all, water can wash dust and dirt out of the air. However, scientists have found that rain and water spraying are usually not very effective at removing the tiny particles found in wildfire smoke. The reason has to do with the size of smoke particles.
Wildfire smoke is made up of extremely small particles, many of which fall within the PM2.5 size range. PM2.5 refers to fine particulate matter that is 2.5 micrometers in diameter or smaller and is one of the criteria air pollutants regulated by the EPA. In fact, most wildfire smoke particles are much smaller, only about 0.1 to 0.5 micrometers wide. To put this into perspective, a micrometer is so small that thousands of them could fit across the width of a penny. Because these particles are tiny, they can stay suspended in the air for long periods of time and travel many miles from the source of a fire.
Scientists have studied how water droplets remove particles from the air for decades. When raindrops fall or water is sprayed into the air, they can collect and carry some particles to the ground. This process is called washout. However, not all particles are captured equally well.
There are two main ways water droplets collect particles. The first is called Brownian collision. This process works best for the smallest particles, especially those smaller than about 0.1 micrometers. These tiny particles move around randomly and are more likely to bump into water droplets, increasing the likelihood of washout.
Source : msu.edu