Author Topic: Purple Air  (Read 403 times)

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Offline ALITTLEweird1

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Purple Air
« on: August 10, 2018, 02:02:19 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 Toxic

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Re: Purple Air
« Reply #1 on: August 10, 2018, 03:07:59 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

Hmm I would have thought he could have made an effort in producing statistics/analisys to stand by his statement.

something like this for instance.

http://www.aqmd.gov/docs/default-source/aq-spec/field-evaluations/purpleair---field-evaluation.pdf

https://www.wunderground.com/cat6/purple-airs-250-air-pollution-monitor-gives-government-equipment-run-money
« Last Edit: August 10, 2018, 03:11:14 PM by Toxic »

Offline ALITTLEweird1

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Re: Purple Air
« Reply #2 on: August 10, 2018, 04:45:21 PM »
Im going to reply to them with a couple pdf files showing purple air matched up pretty good to the other big boy sensors, and then ask them to provide some data in their testing. When they reply back, ill post it.
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Offline ALITTLEweird1

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Re: Purple Air
« Reply #3 on: August 13, 2018, 09:47:20 PM »
Here's the response I got today from our local clean air agency regarding the purple air sensor and how it stood up against other sensors. I tried posting the PDF file that they sent showing their report of the purple air sensor, but the file is too big so I upload the file to my site and the link is below.

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Hey Mark,

   Thanks for contacting us.  Just got back to the office today and am happy to share with you what we have found regarding the Purple Air monitors.  I have attached a report that summarizes our experience with Purple Air monitors at three of our sites this past winter/spring.  Purple Air monitors use two laser particle counters, these sensors shine a laser and read the amount of light scattered by particles this provides the number of particles per unit volume.  Most reference air monitors measure particle mass directly and report in mass of particles per unit volume.  Because these instruments are measuring different properties of particles they may not always agree.  Specifically, as the type of particles changes the relationship between the amount of light they scatter and their mass changes.  Furthermore, environment effects such as relative humidity can change this relationship as well.  The WA Dept of Ecology and PSCAA also use nephelometers, which are light scattering instruments.  However, they have already been calibrated to mass based instruments and so may also read different from an uncalibrated Purple Air.

Therefore, when interpreting data from these sensors it is important to calibrate them first.  The raw data from the Purple Airs, as shown by the AQ-SPEC field tests, is generally 1.3-1.6x higher in California.  To get this number calculate 1/slope using the slope in the linear regression equations.  We have found that here in Washington the Purple Airs are closer to 1.9x higher.  This value will change based on location since the particle type is different between California and Washington.  That said, the Purple Air monitors all agree very well with each other, which is very important for sensors.  This means that a calibration equation for a certain location should hold for all sensors.

In order to get accurate data from air sensors they should first be calibrated to regulatory monitors.  The factory calibration applied to sensors is done in a controlled environment using one type of particle usually something that is not a good representation of the ambient outdoor air.  However, once calibrated, Purple Airs and other sensors can provide good data and are much cheaper allowing monitoring in more locations.

Does this help answer your question?

-Graeme


Link to their data... https://www.northbendweather.com/Purple_Air_PS.pdf
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Offline Toxic

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Re: Purple Air
« Reply #4 on: August 14, 2018, 02:13:22 AM »
No mention of price difference, and online tools like mapping or wether the two other devices can be polled via an API.

I would think the other two devices maybe better however price will be different. Also the amount of data logged to a single website and mapped like Purple air is probably not an option.

I'd rather have a device reading higher position levels than lower. Perhaps this report can be sent to Adrian.

Offline Tarma

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Re: Purple Air
« Reply #5 on: August 14, 2018, 10:14:43 AM »
This is an old story (2016) local news story, but I found it interesting.

https://www.good4utah.com/news/local-news/dirty-air-monitors-are-they-telling-us-the-whole-truth/382744964

 

anything