Today in Vienna VA we have rain. And forest-fire smog.
This seems unfair, somehow.
This post asks two things.
Does rain clear the air? That is, remove dust, pollen, and fine particulates? And if it does, then why is my AQI in the 80s for fine particulates, after it’s been raining overnight.
On the flip side, does forest fire smoke generate rain? That is, does the smoke in the air increase the chance of rain, as cloud seeding does?
Bottom line: Rain clears the air. But only a bit. A typical value for PM 2.5 reduction by moderate rain might be 25%. Forest fire smoke doesn’t appear to make it rain. But if the atmosphere is already primed for rain, there is some evidence that a high load of smoke in the air will intensify thunderstorms.
Does rain clear the air?
The answer is an unambiguous yes. You can numerous credible reference saying that, including university researchers who have quantified that in the lab (reference).
But how much and how consistently does rain clear the air. Well, that’s a different question, and there’s much less to go on.
These researchers looked at 18 rain and snow events in China, and concluded that a) precipitation usually (but not always) reduces particulates in the air, but b) not by a whole lot. Summarizing their detailed results, on average, rain reduced PM 2.5 by about 25% while it was raining, and by about 40% following the end of the rain.
This one-off study suggests about a 30% PM 2.5 reduction for moderate to heavy rain.
But my go-to site in air filtration ,Smart Air, disagrees. They cite research saying that you might get up to a 30% reduction in total air pollution during a heavy rain. But that the effect on PM 2.5 is under 10%, on average.
That’s enough for me to get the gist of it. Rain typically does clear the air, but only a bit. The effect varies, both by the intensity of the rain, and the size of the particulates. You get more clearance with heavy rain and larger particles. But in terms of fine particulates (PM 2.5), you can’t expect it to do miracles. Probably can’t count on it to reduce PM 2.5 by more than 25%.
Does forest fire smoke make it rain?
Spoiler alert: Cloud formation is complex. It’s clouds’ illusions I recall. I really don’t know clouds … at all.
Best I can tell, the simple answer is no.
Near to the fire, smoke provides nucleation sites for water droplets to form. And they do form, but they remain too small to coalesce into raindrops. With a large load of particulates over a forest fire, you get a large number of useless tiny droplets. You may get extra clouds, but you don’t get extra rain. That’s my paraphrase of this research.
This study of fires in the Amazon basin says the same thing. There, smoke from wildfires reduced rainfall. The mechanism is the same as above. Smoke leads to formation of large numbers of tiny water droplets that rise and disperse before they can coalesce into raindrops.
However, if it’s already going to rain — if thunderstorms have already formed — the collision of a forest fire smoke plume and a storm front creates significantly more intense thunderstorms. Even great distances from the forest fire. Per this write up.
Was the recent once-in-a-century rainstorms in Vermont and adjacent areas might have been made worse by Canadian smoke. No. The intense rain fell in Vermont mostly on Monday 7/10/2023. At that point, air quality in that are was quite good, per this website. AQI for particulates was low.
Conclusion
Rain does, in fact, clear some of the smoke out of the air. Best guess, typical case, 25% of it. But smoke does not seem to create rain unless it happens to run into atmospheric conditions that are already ripe for rain. Then, if a smoke plume hits a thunderstorm, the likely result is a somewhat more violent thunderstorm.
