Note: trouble with registration? See this for more info.
0 Members and 1 Guest are viewing this topic.
Hi Mark, I’m happy to see more people getting into air monitoring locally! I am cc’ing 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. We’ve 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
Just got this email from our local clean air agency. QuoteHi Mark,I’m happy to see more people getting into air monitoring locally!I am cc’ing 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. We’ve 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
Hi Mark,I’m happy to see more people getting into air monitoring locally!I am cc’ing 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. We’ve 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
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