It's raining hard yet the AQI is over 100 in my area

It’s raining hard here in the San Francisco Bay Area, when I checked the purpleair map I was surprised to see the AQI over 100 on average. I thought the rain would have the AQI closer to 5 or 10.

Can someone explain why I’m seeing these higher numbers? It’s raining hard in this entire area. Here’s a screen grab.

I’ve seen plenty of high numbers in my area when it rains. Often after the rain it will go down, if the rain was a bringing a front in, with cleaner air behind it - center of country, southerly winds bring bad air typically, while rain will often bring a cold front from Canada, with change to northerly winds and cleaner readings (often dramatically within in a short time after the rain stops).

I’m not sure if your assumption, that rain will wash the pollution out of the air is valid. Seems intuitive, but might be a more complicated scenario where particulate matter is concerned/remains.

India has a lot of monsoon rains, but their meters are always outrageously high, and unfortunate.


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Hi Robert, thank you for your reply. I don’t think the storm front brought in the higher pollutants. Looking at the map, there are a few AQI numbers showing 0 and those line up with our coastal mountain ridges, these are at 2000 foot height, I don’t think it was raining at that elevation.

I just checked and the humidity at the time was 87 where the over 100 AQI is being shown. I suspect the sensors are giving false information during a heavy rain storm. I will keep an eye on the relationship between humidity and AQI numbers and report back.

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I have the same suspicion – I’ve seen a huge AQI spike during heavy rain with high humidity on my sensor as well. AirNow for that same area showed great air quality.

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I saw this online article and thougt it could be of interest to your topic:
A more technical paper:
Also, an FYI: PurpleAir typically reports higher than federal regulatory monitors (FRMs) or federal equivalent monitors (FEMs). To better compare PurpleAir values on the PurpleAir map to the EPA AirNow map (which uses FRM/FEMs) one should apply the EPA correction factor in the drop down menu of the PurpleAir map.


Thank you, Jasmine. That first link explains:

Data shows that rain has a relatively small impact on reducing air pollutants.

Interesting insight. IDEA: I’m noticing that in the same area, today, there is a preponderance of high readings, because of wildfires, but also a handful of absurdly low readings. Having looked at a few, I’m guessing that’s because the outliers are mismarked as being outdoors. I propose that 1: some sort of deprecation and notification to users when their readings are consistently wildly off in the lower reading direction, from a sufficiently large number of sufficiently close meters would improve displayed data quality with a very low false positive rate. Without either the “in the lower reading direction” or “consistently” qualifiers, I’d expect a much higher false positive rate. (2: A second set of deprecations+notices with “in the higher reading direction” and “VERY consistently” qualifiers would also, I’d expect, also be useful with not quite as low a false positive rate, as a consistent point source of pollution could explain it.).

(PS I’m reviving this 9-month old topic because same area and also about false readings, and will thus notify folks I expect would likely be interested. A judgement call one could disagree with.)

PPS: It is documented in a comment today that sea salt aerosols due to high winds increase coastal particular matter readings: - Air quality on the PNW coast shows as poor, with seemingly no reason - #2 by Nalo. I suppose raindrops on a dirty surface could do the same.

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Hi Jasmine,
The site has been added to my favs. It is as suspected through my very unscientific approach; low level clouds and light wind would keep the pollution closer to the ground.