Long-Term Particulate Matter Measurements in Southern Sweden: A Comparative Study of Low-Cost Optical Sensor to Reference Instrument

Abstract

The UN-affiliated World Health Organization (WHO) estimated that in 2019, more than 90 % of people living on Earth were constantly exposed to concentrations of ambient outdoor pollutants higher than the air quality guideline limits. One of the most notorious air pollutants is particulate matter, consisting of fine natural or anthropogenic particles suspended in the air. Recent studies have shown that portable, commercially available low-cost sensors (LCS) can provide an excellent opportunity to improve regional and temporal data coverage of PM concentration. This thesis investigates the long-term measurements of PM10, PM2.5, and PM1 mass concentrations in ambient air using a low-cost optical particle counter (OPC), the Alphasense OPC-N3, across a two-year period in the environmental research stations of Hallahus and Hyltemossa in Southern Sweden. The performance of this low-cost sensor is evaluated against a co-located reference instrument, the Palas Fidas 200 S, highlighting the challenges and reliability of LCSs in harsh outdoor conditions, particularly under high relative humidity conditions. A correction factor based on κ-K¨ohler theory has been tested and proved to work as a functional, a priori, first-order approximation solution in outdoor environments. This corrective factor, based on previous literature, has shown different implementation weaknesses in this work, and more research is needed for improvement. Finally, this study has also highlighted the impact of seasonal variations and long-term degradation effects on sensor accuracy, offering insights into the complexities of particulate matter measurement with low-cost devices in real-world scenarios.

Popular Abstract

A thick layer of smog over a densely inhabited urban area, dusty desert storms, the haze over a busy street intersection: air pollution impacts our everyday life. The UN-affiliated World Health Organization (WHO) estimated that in 2019, more than 90 % of people living on Earth were constantly exposed to concentrations of ambient outdoor pollutants higher than the air quality guideline limit. One of the most notorious air pollutants is particulate matter (usually abbreviated as PM), consisting of fine solid or liquid particles suspended in the air. These particles, coming in different shapes and compositions, originate from natural sources (e.g., dust or pollen) and various human activities (e.g., the smoke from a car exhaust). Finer PM, which may be down to a few billionths of a meter in size, can go unnoticed and deposited into the lungs, causing breathing problems, reduced life expectancy, and a massive economic burden on public welfare. Therefore, measuring PM levels in cities is the first necessary step for any measure against air pollution. Traditional monitoring stations that provide accurate data are expensive and usually restricted to denser urban areas. Furthermore, for most people, especially in low- and middle-income countries, access to information about local air quality is often heavily limited, if not absent. Recent studies have shown that portable, commercially available low-cost sensors (LCS) can provide an excellent opportunity to improve regional and temporal data coverage of PM concentrations. The growth in the popularity of such devices is observed worldwide, thanks to their
user-friendliness and small size. For these reasons, they are good candidates for creating widely dispersed sensor networks. However, these sensors might not be precise and sensitive enough to meet regulatory standards. Weather conditions and physical factors can affect their performance in real-world settings. To address this issue, ongoing research is focused on improving the reliability of LCS by calibrating them. The calibration process involves comparing their readings with more standardized measuring techniques in order to account for seasonal variations, different locations, and various meteorological conditions. This thesis project focuses on a two-year-long measurement of PM number concentrations in ambient air using a low-cost optical particle counter (OPC), the Alphasense OPC-N3. The data were collected in the environmental research stations of Hallahus and Hyltemossa (Southern Sweden) from June 2021 to November 2023, a period longer than most current studies, which typically last only up to a year. The measurements are compared with those from the Palas Fidas 200 S (a certified PM reference instrument approved by the Swedish environmental agency), which is costlier but more precise. The final goal is to analyze the performance of cheap sensors for prolonged outdoor exposure. This study also seeks to develop a correction for the Alphasense OPC-N3 measurements when used in high external relative humidity environments (as on a typical rainy day in Southern Sweden), which may compromise the real-time air quality readings. This objective is pursued through a calibration process using data from the reference instrument (Palas Fidas 200 S), ensuring that the Alphasense OPC-N3’s measurements are accurate and reliable over the extensive period of outdoor deployment. Although these detectors are far from impeccable, this thesis wants to improve future air quality monitoring sensors by keeping them cheap and adaptable to any environment and meteorological condition.

• author: Sarno, Flavio
• supervisor: Erik Swietlicki; Erik Ahlberg
• organization: Department of Physics; Combustion Physics
• course: FYSK04 20241
• year: 2024
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• language: English
• id: 9169836
• date added to LUP: 2024-07-02 14:47:53
• date last changed: 2024-07-02 14:47:53