Understanding the behavior of particulates in the atmosphere is important for modeling climate, weather and public health. They can be nucleation sites for rain, fog and smog, they can have thermal impact due to absorption of sunlight and possibly radiation in thermal infrared, and the smallest can deposit chemicals deep inside our lungs.
Their behavior can be characterized by their particulate aerodynamic diameter, one way to characterize their size according to their aerodynamic behavior.
The rate at which particulate matter deposited in the atmosphere settle to the ground is a strong function of size. Throw a handful of sand into the air and it returns to the ground within second, create soot with fire and it rises in the column of hot air produced and may take weeks, months or even years before it returns to Earth.
Of possible interest:
- High-flying bacteria spark interest in possible climate effects
- Microbes Survive, and Maybe Thrive, High in the Atmosphere
- Living Bacteria Are Riding Earth’s Air Currents
Is there any way to estimate at least approximately as a function of size the time it takes airborne particulates found high in the atmosphere to return to Earth, and which ones return due to gravity and which due to formation of precipitation? I know it's a complex topic and it may depend strongly on the altitude at which they start. Perhaps some rules-of-thumb or examples might be sufficient to get an idea of what's involved in such estimates. That may be helpful in order to formulate more specific follow-up questions.