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Author Topic: Purple Air  (Read 489 times)

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Offline ALITTLEweird1

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Purple Air
« on: August 10, 2018, 06:00:36 PM »
Just got this email from our local clean air agency.

Quote
Hi Mark,

 

Im happy to see more people getting into air monitoring locally!

 

I am ccing Graeme who has done most of this work (although he is out today), but we found that the Purple Airs are reporting nearly double the actual values unfortunately. 

 

Weve done some collocation studies, where we put the purple air monitors next to our regulatory monitors and found that the Purple Airs are reading higher than reality.

 

These low-cost sensors have a few issues.  Namely, they are very humidity dependent and particle-type dependent.  That is, wood smoke or wildfire smoke would scatter light differently than diesel exhaust, sea salt particles, etc.  Even aged versus fresh wood smoke can have different scattering properties.  Also, at higher pollution concentrations, the purple air may not always be linear.  In other words, it may become more or less linked to the actual values depending on how high the pollution gets.  So in summary, the purple airs could certainly be better calibrated to our local conditions, and Graeme has demonstrated that with his analysis, as well.

 

If you really want to geek out more on data, feel free to reach out to Graeme or me.

 

Thanks again for your interest Mark, Erik



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Offline niko

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Re: Purple Air
« Reply #1 on: August 10, 2018, 06:06:31 PM »
Very interesting, thanks for posting that.

Offline bitsostring

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Re: Purple Air
« Reply #2 on: August 10, 2018, 06:35:28 PM »
Yes, thanks indeed. Maybe someone should tell WUnderground? I seem to recall they were promoting Purple Air. . .

It's meant to be fun. . .

Offline ALITTLEweird1

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Re: Purple Air
« Reply #3 on: August 10, 2018, 06:43:32 PM »
Now im not sure if i regret buying one or not now.  :roll:
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Offline Harry

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Re: Purple Air
« Reply #4 on: August 10, 2018, 07:40:25 PM »
Thanks a lot for posting that. Has Purple Air been made aware of that finding?  I'd like to know what their response is. Actually, mine doesn't seem to be that far off compared to what AirNow shows for Fremont, CA.  Of course, I have no idea exactly where their sensor is.  Like right now, my sensor shows 54 AQI and Fremont is showing 59 AQI so, I am under what they are showing.

Offline niko

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Re: Purple Air
« Reply #5 on: August 10, 2018, 07:42:39 PM »
I'll try to find it again but I have seen a study comparing the Plantower sensor (as used in the Purpleair) against big bucks pro sensors, and it came out very favorably. Purpleair uses two Plantowers and either averages or votes to improve the quality of the output so I'm kind of surprised that your guys got such a poor result  :?
« Last Edit: August 10, 2018, 07:44:51 PM by niko »

Offline wvdkuil

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Re: Purple Air
« Reply #6 on: August 10, 2018, 07:49:47 PM »
I'll try to find it again but I have seen a study comparing the Plantower sensor (as used in the Purpleair) against big bucks pro sensors, and it came out very favorably. Purpleair uses two Plantowers and either averages or votes to improve the quality of the output so I'm kind of surprised that your guys got such a poor result  :?
This topic refers to a more in-depth report, I think
http://www.wxforum.net/index.php?topic=34897.msg356051#msg356051
Wim
« Last Edit: August 10, 2018, 08:09:26 PM by wvdkuil »
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Offline ALITTLEweird1

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Re: Purple Air
« Reply #7 on: August 10, 2018, 07:57:45 PM »
Harry,  I've not forwarded that email to purple rain yet. Im not sure if i should or not.
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Offline niko

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Re: Purple Air
« Reply #8 on: August 10, 2018, 08:11:36 PM »
Harry, yup, we went around this block a year ago...

Here's a report from the LA Air Quality folks actually comparing the Purpleair performance http://www.aqmd.gov/docs/default-source/aq-spec/summary/purpleair-pa-ii---summary-report.pdf?sfvrsn=4


Offline ALITTLEweird1

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Re: Purple Air
« Reply #9 on: August 10, 2018, 08:17:10 PM »
Hmmmmm.... strange how the people here had bad results.
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Offline ALITTLEweird1

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Re: Purple Air
« Reply #10 on: August 10, 2018, 08:31:18 PM »
Im going to reply to that email and attach those pdf files and then ask them if they have any data to show their findings with the sensor.
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Offline Harry

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Re: Purple Air
« Reply #11 on: August 10, 2018, 09:26:50 PM »
Harry, yup, we went around this block a year ago...

Here's a report from the LA Air Quality folks actually comparing the Purpleair performance http://www.aqmd.gov/docs/default-source/aq-spec/summary/purpleair-pa-ii---summary-report.pdf?sfvrsn=4

Ah, yes.  Thanks, Niko.  I guess my memory banks have overflowed or maybe it's just old age.   :(

Offline niko

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Re: Purple Air
« Reply #12 on: August 10, 2018, 09:42:42 PM »
Ah, yes.  Thanks, Niko.  I guess my memory banks have overflowed or maybe it's just old age.   :(

Yeah, I know just what you mean. I'm selective about what I choose to remember these days, at least, that's what I tell my wife  :lol:

Offline niko

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Re: Purple Air
« Reply #13 on: August 11, 2018, 02:10:32 AM »
@Mark: Don't forget that the PA's AQI is only based on particulates, but your local WA air quality station uses Ozone too. As I understand AQI it will be driven by whichever of the two is higher. When I looked his afternoon the dominant pollutant in your area was ozone.

Offline ALITTLEweird1

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Re: Purple Air
« Reply #14 on: August 11, 2018, 02:23:31 AM »
@Mark: Don't forget that the PA's AQI is only based on particulates, but your local WA air quality station uses Ozone too. As I understand AQI it will be driven by whichever of the two is higher. When I looked his afternoon the dominant pollutant in your area was ozone.

That's cool. But they "our local agency" shouldn't be writing off a sensor that does actually work. They seem just to be comparing it to theirs and if this sensor doesn't match their readings, they seem to think its junk.
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