Precipitation: the great atmospheric scrubber

Falling raindrops collide with airborne particles and either incorporate them (the particle dissolves or is absorbed into the droplet) or scavenge them mechanically (the particle sticks to the droplet surface). How much gets removed depends strongly on particle size. Andronache's calculations show that rain clears the smallest particles (under 0.01 µm) and coarse particles such as pollen and dust (over 2 µm) quickly, halving them in about an hour even in light rain of 1 mm/hr, while particles between 0.1 and 1 µm, which carry most PM_2.5 mass, are barely touched by rain falling through them. Rain also washes out soluble gases such as SO₂ and NH₃.

Snow scavenges particles too. A review of snow-scavenging research concludes that, for the same amount of water, snow is probably better at removing particles than rain, though estimates vary by more than tenfold and a firm conclusion needs more evidence. See snow depth and winter IAQ.

For Terrestream, this matters in three contexts. First, while it rains, Terrestream suggests keeping exposed windows closed. Second, after rain, check the outdoor readings before airing out: pollen and dust are often knocked down, but fine particles, including most wildfire smoke, wash out far less, so rain alone is not a reliable fix for a smoke event. Third, very high outdoor humidity after a storm raises indoor humidity through infiltration.

What rain does not do well: remove gas-phase ozone (sparingly soluble in water), CO₂ (in trace amounts), or NO₂ (partially). It clears coarse particles and the most-soluble acidic gases; it largely leaves photochemical pollutants alone. So a sunny afternoon two days after a rain can still have elevated ozone even when particle levels are low.