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.

Popular Abstract

According to guidelines set by the World Health Organisation, 9 out of 10 people are exposed to polluted air. One type of pollutant which has been linked to poor visibility, changes in the climate, and serious health issues is something called particulate matter. Also known as aerosol particles, particulate matter refers to particles of liquid or solid matter with varying size and chemical composition that are suspended in the air. Particles with a diameter less than 10 μm (PM10) and less than 2.5 μm (PM2.5) are regulated air pollutants in ambient air due to their negative impacts on both human health and the environment.
However, regulations and guidelines for indoor air quality are almost non-existent. Apart from outdoor air infiltrating our indoor spaces through windows, doors, and ventilation, there are many activities inside which can generate particulate matter themselves. Cooking, vacuuming, cosmetic aerosols, and burning candles are just some sources of different indoor aerosol particles. Considering the average person spends 85-90% of their time indoors, it is definitely important to properly understand and monitor the quality of the air they are breathing.
The standard reference method for measuring particulate matter uses instruments that collect the particles on special filters and weigh them with very sensitive scales. While this may be the most reliable method, these instruments are typically very expensive and require special know- how and many man-hours to obtain results, normally with only 24-hour resolution. An emerging field within aerosol science is that of low-cost sensors. These are smaller, more portable devices which can continuously measure air pollutants such as particulate matter at a fraction of the cost of laboratory-grade sensors. These features provide an opportunity to, among other things, provide new insights into the particulate matter found in our homes.
This study uses a low-cost optical particle sensor to explore how common indoor and outdoor activities affect the levels and trends of PM10, PM2.5, and additionally PM1 (less than 1 μm). The sensor determines the size of the particles by measuring how they cause light from a laser to scatter. This information is then used to calculate the total mass concentration of particles in the air flowing through the sensor. Since low-cost sensor technology is still evolving, there is often a trade-off between the price and useability and the quality of the data. For this reason, data from four co-located sensors is compared to assess their precision, and the average PM10 measurements from two sensors placed outdoors are compared with official data from the air quality monitoring station in Lund.
Results indicate that these low-cost sensors are indeed able to identify trends in particle mass concentration associated with residential activities both indoors and outdoors. While the outdoor concentration was generally more than 3 times more than indoors, there were significant peaks in indoor concentration matching periods of cooking and burning candles. Outdoors, the concentration spiked in accordance with vaping episodes. In comparison to the Lund measurements, the low-cost sensors actually performed relatively well, except for a period when the outdoor sensors unexplainably registered much higher values.

• author: Soroka, Ellen Grace
• supervisor: Erik Ahlberg
• organization: Nuclear physics; Department of Physics
• course: FYSK02 20201
• year: 2020
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• language: English
• id: 9017097
• date added to LUP: 2020-06-15 09:03:37
• date last changed: 2020-06-15 09:03:37