Evaluation of a heated inlet to reduce humidity induced error in low-cost particulate matter sensors

Abstract: The number of low-cost particulate matter (PM) sensors available on the market has increased in recent years. They are affordable, compact and require no maintenance which means they easily can be deployed and utilized for real-time air monitoring. However, the accuracy of low-cost PM sensors has been reported to vary depending on several factors. This thesis aims to investigate and reduce errors induced by relative humidity. Several hardware methods to reduce the relative humidity of air were considered. These include heating, diffusion, dilution and membrane drying. Heating was chosen as the most suitable method to use with low-cost PM sensors because of low maintenance, small size and low price. The chosen concept was developed into two identical prototypes consisting of a heated pipe that was attached to two Alphasense OPC-N2 sensors. To evaluate how the sensors behaved with varying humidity with and without the prototype, tests were performed in an urban environment (Malmö, Sweden). Tests in a controlled aerosol laboratory were performed as well (LTH, Lund, Sweden). The results of the urban field test show that, when compared to the reference instrument, the native OPC-N2 sensors are susceptible to relative humidity errors especially when the RH goes above 75%. Moreover, constant offset between the two identical sensors was observed. Attachment of the heated inlet prototype improved the correlation between the OPC-N2 sensors and the reference instrument. The results of the laboratory tests were inconclusive, possibly because of the majority of the generated particles being outside the specified detection range for the low-cost PM sensors.