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The following claim on a German weather forecasting site caught my eye:

Der Himmel ist wieder einmal blitzblank geputzt, die Sonne scheint von früh bis spät, die Luft ist angenehm warm – da sollten die Alarmglocken schrillen: Sonnenbrandgefahr! Was in diesem Frühjahr noch hinzu kommt: Durch die Corona-Krise ist die Luft besonders sauber und lässt die gefährlichen UV-Strahlen ungehindert auf unsere Haut treffen. Sofern wir sie nicht schützen.

My translation:

The sky is clear again, the sun shines all day and the air is comfortably warm. That should ring alarm bells: Watch out for sunburns! This spring, the air is particularly clean due to the corona crisis and allows the dangerous UV rays to reach our skin unhindered. Unless we protect them.

The same claim appears elsewhere, but none that I've seen cite any convincing sources.

Does air quality really have a non-negligible impact on the atmospheric transmission or absorption rate of UV radiation?

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    This also coincides with the arctic ozone hole being over nothern Europe. In NASA Ozone Watch, compare e.g. March vs. April 2020. Of course this happens every now and then and has nothing to do with the virus situation, but combined with the reduced pollution the UV levels could be quite high compared to typical spring. – jpa Apr 20 at 16:06
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    If you would like to do some research of your own regarding air pollution around the world and how the COVID-19 pandemic affects it: aqicn.org/map/world. You can look up pollution levels in real-time for thousands of locations around the world and see daily historic values for several pollution types reaching back several years. – Philipp Apr 20 at 16:49
  • From personal experience, Delhi is heavily air polluted, you can't even see the sun. I didn't use sun scream. In nearby Rajasthan, with clean air, I would burn quickly without high factor sun cream. – paj28 Apr 21 at 7:44
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Yes.

Based on [Relationship between surface UV radiation and air pollution in Beijing] from 2008 by An JL1, Wang YS, Li X, Sun Y, Shen SH.

This study also shows that a substantial reduction (up to 50%) in the UV radiation on days with high levels of air pollution. Larger fluctuations are found in UV radiation in the summer. The effects of clouds and air pollution on UV are higher than on total solar radiation, and the reduction in UV is about twice as large as the total solar radiation values

From www.cpc.ncep.noaa.gov

Dust/Haze: These two conditions act on UV radiation the same way. They both scatter UV radiation. Enough UV radiation is scattered that on hazy or dusty days there is less UV radiation reaching the surface than would otherwise be there on a clear day.
Air Pollution/Smog: This encompasses many greenhouse gases. Emissions from traffic and manufacturing plants form smog as UV radiation and heat cause the necessary chemical reactions to take place. As a result, the amounts of UV radiation reaching the surface is smaller under these conditions.

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  • This seems to not consider contrails, which should also add to the effect. – gerrit Apr 20 at 9:10
  • Thanks. You didn't finish the link tag for the 2008 paper but I found it here. – TypeIA Apr 20 at 9:36
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    It might be worth pointing out that the air pollution level in Beijing is a lot more severe than in most places of Germany. The answer could also be improved by quantifying how the corona pandemic affects air pollution in Germany and how much effect that has on UV radiation. – Philipp Apr 20 at 15:43
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    @Philipp The question was whether clean air is more transmissive, not whether or by how much the air is in fact cleaner than usual due to the corona crisis; but that would be very interesting information too! Might be worth a follow-up question. – TypeIA Apr 20 at 16:18
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    @gerrit Contrails are mostly water. Water doesn't absorb UV very well - plenty of people get sunburned when it's overcast, because the sun doesn't feel warm anymore (visible and infrared light is scattered and absorbed well), but even for pretty heavy cloud cover, the UV transmission only drops by about half. – Luaan Apr 21 at 11:05

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