Laboratory and Field Evaluations of the GeoAir2 Air Quality Monitor for Indoor Exposure

There has been increasing demand for low-cost particulate matter sensors in the market. To meet this demand, new air quality monitors are being developed and released to the public. Low-cost sensors open routes to citizen science and community air quality monitoring, which can prove a great boon to public health and public health education. Optical particle counters are of interest in this regard. However, these lower-cost sensors are generally expected to lack the quality paid for in higher-cost sensors, thus leaving the question of their efficacy, especially at different concentrations and humidities, and for different aerosol types. The purpose of this study was to determine the accuracy and bias of 40 GeoAir2 devices, the effect of changes in relative humidity on the device, and the performance of these devices in field settings compared to reference instruments. A large number of devices were used to collect a more significant amount of data for better calibration. This study focused on the SPS30, an optical particle counter built into the GeoAir2 air quality monitoring platform. This platform was designed to measure particulate matter, carbon dioxide (CO2) on an H2-based measurement, and volatile organic compound (VOC) personal exposure while providing GPS location for geospatial monitoring and recording time, temperature, and humidity data. In order to test its correlation with related sensors in detecting PM2.5, a reference spectrometer (MiniWRAS by GRIMM Aerosol) and another common low-cost optical particle counter (OPC-N3 by Alphasense) were used. First, the devices were tested with two different aerosols, salt and Arizona road dust, with steady states measured by a medium-cost pDR-1500 aerosol sensor. The pDR-1500 is accompanied by a filter to provide gravimetric analysis for mass correction. For these experiments, 40 GeoAir2 devices were used, while 32 reported data across all experiments. The mass concentrations for salt and dust aerosol measured by the SPS30 (r=0.99) and OPC-N3 (r=0.99) were highly correlated with the MiniWRAS reference instrument. Second, the effect of humidity on the SPS30 sensor was tested against the devices mentioned above at 30, 50, 70, 80, and 90 percent relative humidity. Reported PM2.5 mass concentrations under different humidity values increased between 181% and 425% for the OPC-N3, while the GeoAir2 reported an increase between 100% and 137%, suggesting a lesser effect of humidity on the SPS30. The findings of this study suggest that the SPS30 built into the GeoAir2 monitoring platform is a competitive option compared to similar low-cost optical particle counters in dry conditions and is affected less by changes in relative humidity. The GeoAir2 device provides a new option for low-cost optical particle counters highly correlated with reference devices and can perform in changing humidity conditions