Sensor Technology
The SCAQMD AQ-SPEC
program evaluates
the performance of low cost commercially available sensors. The sensors selected
for the star grant program are the model
PA-II
sensors manufactured by PurpleAir. Each PA-II sensor includes two
Plantower PMS5003
laser particle scanners (two scanners for redundancy and confidence calculations)
a Bosch BME280
Tempearture, humidty and pressure sensor along with
power supply, ESP8266
microprocessor and Wi-Fi components. The sensor is packaged in a PVC cap and is
about the size of a baseball. The retail price of the PA-II is approximately $230.
The sensors are mounted outside of each home and connect to the internet via Wi-Fi.
The particle scanner has a small fan that draws air through a small (0.1 liter)
measuring chamber. Particles intersect a laser beam that is focused on a photo detector.
The wavelength of laser light is approximately the same as the diameter of the particles of
interest and through a process known as
Mie scattering
particle mass concentration is measured.
The sensor is capable of measuring particle concentrations of 0 - 500 µg/m3 with a
maximum range for PM2.5 of 1,000 µg/m3 at a resolution of 1 µg/m3 and with a maximum error of
+/- 10% @100 - 500 µg/m3 or +/- 10 µg/m3 @ 0 - 100 µg/m3. Other types of pollutants such as ozone,
co, NO2 and NOx are not measured. For the smallest measurable particle size of 0.3 µm the counting
efficiency is 50% but it is 98% for particles greater than 0.5 µm.
Data Collection and Retrieval
Particle concentrations are binned in ranges of 0.3-1.0 µm, 1.0-2.5 µg/m and 2.5-10 µm. Data are uploaded by the sensors to ThingSpeak servers every 120 seconds. Particle data are displayed as various Air Quality Indices (AQI) or raw data on a real time map. The historical raw data for each sensor is available for download in CSV format. Real time sensor data may also be downloaded as JSON formatted data via REST API calls or via ThingSpeak. See Using PurpleAir data for more details.