Indoor and Outdoor Measurements of Particulate Matter with the Low-Cost Optical Sensor OPC-N3

Abstract: This paper evaluates the low-cost optical sensor OPC-N3 from Alphasense as a suitable scientific instrument for measuring particulate matter levels in air, both outdoors and in- doors. The time-keeping and data-retrieval was additionally improved by connecting the OPC-N3 to an Arduino microcontroller equipped with a real-time clock (RTC) module and an external SD card. Inter-comparison tests demonstrated good agreement between four co-located sensors, with approximately 15% relative standard deviation (RSD) before separation for indoor and outdoor measurements, and 13% after. The RSD was observed to increase during periods of very high and very low concentration. Indoor and outdoor measurements were run over 10 days, during which the times of relevant activities were logged. The median ratio of indoor to outdoor concentration was found to be 0.37 for PM1, 0.44 for PM2.5, and 0.53 for PM10. Time series of PM1, PM2.5, and PM10 mass con- centrations displayed considerably clear trends in accordance with activities, most notably cooking, burning candles, and vaping outdoors. While the indoor particulate matter con- centration was generally lower than outdoors, indoor activities appeared to have a more significant contribution to the concentration of particulate matter than recorded outdoor sources. The accuracy of the sensors was additionally evaluated through comparison of PM10 values from the outdoor sensors and from Lund Municipality’s TEOM instrument, which resulted in a reasonable correlation (median ratio of 1.15) despite the instruments not being co-located. The OPC-N3 sensor was deemed to be a sufficiently precise sensor for detecting trends in particulate matter concentrations, and a good tool for extending the spatial and temporal resolution of air quality monitoring networks. The results of this paper prompt the need for further research to give a more comprehensive evaluation of the OPC-N3, namely, longer measurement periods to evaluate seasonal variation and possible accumulation of errors, a more controlled analysis of sensor response to individual indoor sources, and co-location of the OPC-N3 with reference particulate matter measurement instruments.