A reader of the blog took the initiative to establish a contact between PurpleAir and me, as a result the PurpleAir II outdoor Particulate Matter monitor is here for a review. Thank you Daniel.

The monitor is very compact and it can be easily distinguished from the rest because it has 2 Particulate Matter sensors. Both of them work together to establish better results and a coefficient of determination (R²).

Hardware

• Microcontroller ESP8266
• Built in WiFi for logging to “The Cloud” WiFi 802.11 b/g/n 2.4 GHz | WPA/WPA2
• 2 PMS5003 laser counters
• BME280 Temperature, Humidity and Pressure sensor
• Micro USB Power Supply
• *Optional SD Card Slot and Real Time Clock

The estimated total mass for PM1.0, PM2.5 and PM10 is averaged by the PurpleAir Internet of Things (IoT) control board. Readings are then uploaded to the cloud every 80 seconds or so where they are stored for download and display on the PurpleAir map.

A field evaluation has been conducted by the SCAQMD in which two Federal Equivalent Method were used (BAM & GRIMM) and they found the following results (full evaluation):

• PM1.0 sensor data correlated very well (R²> 0.96) with the corresponding values collected using a substantially more expensive particle instrument (GRIMM) and were quite accurate.
• PM2.5 sensor data correlated very well with the corresponding FEM GRIMM and FEM BAM values (R² > 0.93 and R² > 0.86) and were quite accurate.
• PM10 sensor measurements correlated well with the corresponding GRIMM and FEM BAM values (R² >0.68 and R² > 0.60) (1-hr average).

On a different technical evaluation, which was done by the San Joaquin Valley Air Pollution Control District, they found a correlation of R² ∼ 0.83 on a 24-hour average comparison (full evaluation).

These results are very good, a closer value to R²=1 means that the device is as good as a US$25,000 instrument (MetOne BAM FEM).

The BME280 is a small SMD sensor made by Bosch which can measure Temperature, Humidity and Pressure. The sensor is reporting conditions inside the sensor housing, not ambient conditions. As a result readings are affected by the electronics so temperature may be elevated and humidity under estimated.

Software

The ESP8266 has a limited internal memory typically of 4MB and the software of the device becomes very limited too, at least locally. This is the reason the device doesn’t offer a lot of configuration options rather than the Wi-Fi and there isn’t an option to change the Temperature from ºF Fahrenheit to ºC Celsius yet.

When you register the device for the first time there are a lot of options to consider, like if you want the data to stay private or public (please, allow others to see the air quality in your area), the location of the device, Data Processors such as the Weather Underground or your own server and some contact info.

The end-user has two ways to access the data. One way is to access the real-time data through a soft-AP IP address the router creates when the device connects to it and another way to access the data is through the online portal map at https://www.purpleair.com

The web site will provide a different version of the Map to the users depending the web browser you are using. For example, on Safari we can see on the left the map and on the right of the screen the Real-Time AQI, Trends, Temperature/Humidity, a lot of tabs to choose between and even a graph which compares the sensor A and B. Safari doesn’t support well WebGL API and the company has been forced to develop a different version of the map.

On the other hand on Firefox and Chrome the information is presented in more clean way and there is an option to download a PNG/JPG/PDF/SVG file of the graph something that it isn’t possible on Safari. However, you can’t see real-time Temperature and Humidity and PM2.5… values all together unless you choose different Map Data Layer each time. It supports the US EPA AQI standards and Canadian AQHI standards and raw PM1, PM2.5 and PM10 data in μg/m³ with conversions to AQandU and LRAPA. The conversions help accommodate different types of pollution with different particle densities and for specific airsheds. (An airshed is a part of the atmosphere that behaves in a coherent way with respect to the dispersion of emissions.)

Finally, users can download the measurements from their sensor and from other sensors as well in a .csv file format at https://www.purpleair.com/sensorlist

• Find and mark the sensor/s in the list you want to download – use CTRL + F to quickly search by sensor name.
• Return to the top of the page and enter the desired date range, then click Download Selected

Although PurpleAir currently does not have an app, there is a free iOS app called Local Haze which supports PurpleAir’s monitors. The app detects the nearest sensor to you and displays some basic data for the PM2.5 concentration in real-time.

Conclusion

I highly recommend you to use Firefox/Chrome as the browser of your choice to take full advantage of the PurpleAir’s website. I like the soft access point (soft-AP) which the device creates on the local Wi-Fi network, it is clean and very easy to understand, but it misses the visualization of the temperature in ºC Celsius simply because a lot of people understand temperature in Celsius and so do I (Keep in mind the temp/humidity data are not useful). The online portal offers a lot of data but a more simple and uncomplicated way to display and download data is needed. However, I can say with certainty that there is a scientific team behind the PurpleAir monitor which works hard to deliver a great product. The ability to download a .csv file with the measurements your device has taken is awesome. So, software-wise the device needs some work because it needs to be more approachable. Hardware-wise the device offers a concrete solution for those who want reliability thanks to the dual particulate matter sensors.

The device can be purchased online at https://www.purpleair.com/

Stay tuned because in the next article I will compare the PurpleAir II with the DIY Luftdaten PM2.5 sensor.