As a chronic sufferer of allergies, I’ve always seen a silver lining in every dark cloud: At least rain cleans pollen and dust out of the air, and other particulate pollutants.
But is that really true? I read in one of Isaac Asimov’s trivia books that this is a myth – rain has no effect on any kind of air pollution.
Well, if rain doesn’t remove any pollutants from the air, why is there acid rain?
I believe it does have an effect, mainly to carry airborne particulates to the ground. But the effect is surely short-lived. As soon as trees are dry, the pollen goes up again, and cars dont stop polluting when it rains. So basically as soon as the suns comes back out, the pollution will be right back where it started, AFAIK.
And why is my previously clean car dirty after it rains?
Rain drops form around nucleates, mainly small dirt particles floating in the air, although any piece of particulate matter will do. When the rain falls, a good many of these particulates are no longer in the air, but on the ground, or on cars. Then the wind comes along and lofts them back into the air in a perpetual cycle.
Particulates are one thing that rain helps to remove, simply because rain requires particulates as a nucleus for condensation. Water-soluble pollutants (including acids) can also be removed from the air by rain; the term for this is ‘wet deposition’, if you want to search for more on it. Sometimes, a substance that is not water-soluble as emitted into the atmosphere – a hydrocarbon, for example – is oxidized by the hydroxyl radical into an alcohol or acid that is water-soluble and can be removed from the atmosphere by wet deposition. In some cases, wet deposition removes a harmful chemical from the air only to make it an environmental hazard on the ground; acid rain is a good example of this.
Somewhat related, but what about lightning? The air never smells cleaner than after a good lightning storm. Is it my imagination, or is there some basis in fact?
This is because the energy from the lightning creates ozone. Just like the electricity from computers does. Low concentrations of ozone do give air a “fresh” smell, but I don’t think large quantities of ozone are particularly good for you.