Some of us pay attention to all the messages WeatherFlow posts. And Field Testers are privileged to additional information.
how do the purple air units handle fog or mist conditions ? i use a IQAIR unit https://www.iqair.com/air-quality-monitors/airvisual-pro a bit on the expensive side but its smart in appearance but goes a bit nuts when combi boiler is on as its nearby but i guess thats expected .no idea on its accuracy as ive nothing to compare it with but its a nice addition to a weather station. i put mine in a small home made shield and have it on the balcony ā¦
not sure if these are available worldwide or outside of europe but you can build your own kit for less than 100 ukp https://weatherspares.co.uk/collections/air-quality-monitoring
Canāt answer that one without a reference unit in a similar location to compare it to. FWIW, all the PA units within a 5 mile radius seem to read similarly, but that of course doesnāt mean they are āaccurateā (or not).
Map centered on me is HERE. Currently it is very hazy/foggy and weāre all reporting 100 or so AQI, but the air quality āhasā been rather worse than usual for the last month or so around here.
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most of these particle sensors use lasers. They are influenced by fog, on the other hand I wouldnāt be surprised if the fog itself contained plenty of tiny particles.
AQI is not based on a single measurement but on a combination (in my case the various measurement (pm1, pm2.5, pm10, no2, voc) get their own weighting curve. The maximum of these weighted values is the AQI. But there are many different AQIās defined.
Keeping that in mind you might compare your values with https://air.plumelabs.com/en/ or http://aqicn.org/city/usa/washington/seattle/duwamish/
was just curious however what is accurate ? 
most serve as a good indicator of conditions localised ā¦ I generally see 100+ on foggy or mist periods . us urban city dwellers term it as smog during the rush hour
I frequently have no idea what your point is. This is one of those times.
Accurate, to me, would be a calibrated professional quality instrument sited in the same location as mine. The PA is not one of those instruments.
Best I can do is see what such instruments located close to me report at the same time, understanding terrain and prevailing wind effects.
Example - the measurements 5 miles or so ENE of me make little sense unless the wind is from the east, which is unusual. Our prevailing wind is usually from the SW. The sensor 3 miles SSW from me is a better reflection of reality, unless the industrial activity in the Port of Tacoma is particularly pollution-laden at that time. In āgeneralā, my unit reads the same as multiple PA at the same elevations nearby, but then weāre in the same fog layer.
Iād need a totally calm day or night with no āmarine layerā to try to estimate how accurate the PA are versus professional quality calibrated instruments from the local governments and weather services.
That said, I notice āthe air seems badā when the PA reads over 100, as long as itās not a foggy day.
(and yes, I know AQI is an hourly average not an instantaneous value, and how all that calculation works).
my plumelabs sensor is made to carry around and luckily we do have an official measuring station nearby. I will use that to check on accuracy. However I did return my original plumelabs flow and have to wait until the end of November for the flow2 to arrive. Iāll try a few times and hopefully also on a foggy day.
sorry i get under you skin like a rash but its not intent perhaps a simpler question US aqi or Chinese aqi as you can see the values below the 200 threshold are different . like many other metrics they are open to the accuracy or reliability or preferences question i.e Fahrenheit or celsius , hpa or inhg and so on ā¦meant no intent just a friendly discussionā¦
There are many more different AQI. Your little table suggests that you can calculate the AQI form a single measurement of something in ug/m3. You shouldnāt. Look at Wikipedia https://en.wikipedia.org/wiki/Air_Quality_Index . The AQI of USA for example is based on at least 6 different measurements. The AQI is the maximum of the weighted values of these measurements.
Of course many consumer versions cheat a bit. If they canāt measure for example SO2, they calculate AQI based on the other parameters and hope that the weighted SO2 value is lower that their calculated value. So you can get away with it most of the time/places but presenting AQI based on only one measurement is doing the general public a favor.
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Iām in the US, so Iām using the US formula for AQI, converting the average of the two pm2_5 sensors that the PA has. AQI in the US is generally based on an average of several hours of readings, however in my web pages I report a value for the last reading, as people typically understand the AQI green/yellow/red/etc. ranges commonly, and do not typically know which microgram/m3 values mean which color.