Arianna Alvarez Cruz

Particulate matter (PM) is widely recognized as a major air pollutant with significant impacts on human health, highlighting the need for accurate monitoring. In developing countries, low-cost sensors are crucial for accessible PM monitoring, but their accuracy and reliability must first be assessed. This study benchmarked the Alphasense OPC-N3 and Next PM sensors through laboratory and field evaluations. Laboratory tests were performed in controlled conditions with HEPA-filtered air at low humidity and varying concentrations of water droplets from nebulized deionized water. A 27-day field study in Cienfuegos, Cuba, provided additional insights into real-world performance. The OPC-N3 showed susceptibility to perturbations and was more affected by outliers (especially PM₁₀), relative humidity, and interference from aqueous aerosols. In contrast, the Next PM sensor demonstrated superior stability, lower noise levels, and consistent performance across different environmental conditions. Despite a substantial price difference, both sensors provided valid measurements. Additionally, both sensors produced lognormal PM concentration distributions during field campaigns. This feature could aid in addressing the calibration stability challenges commonly associated with low-cost sensors through in situ calibration methods. While the PM measurements by affordable sensors are not perfect, they are sufficiently reliable for supporting air quality assessments in resource-limited settings.

The study investigates methods to enhance the reliability of NO₂ monitoring using low-cost electrochemical sensors to measure gaseous pollutants in air by addressing the impacts of temperature and relative humidity. The temperature within a plastic container was controlled using an internal mica heater, an external hot air blower, or cooling packs, while relative humidity was adjusted using glycerine solutions. Findings indicated that the auxiliary electrode signal is susceptible to temperature and moderately affected by relative humidity. In contrast, the working electrode signal is less affected by temperature and relative humidity; however, adjustments are still required to determine gas concentrations accurately. Tests involving on/off cycles showed that the auxiliary electrode signal experiences exponential decay before stabilizing, requiring the exclusion of initial readings during monitoring activities. Additionally, calibration experiments in zero air allowed the determination of the compensation factor n_T across different temperatures and humidity levels. These results highlight the importance of compensating for temperature and humidity effects to improve the accuracy and reliability of NO₂ measurements using low-cost electrochemical sensors. This refinement makes the calibration applicable across a broader range of environmental conditions. However, the experiments also show a lack of repeatability in the zero air calibration.

The study investigates methods to enhance the reliability of NO2 monitoring using low-cost electrochemical sensors to measure gaseous pollutants in air by addressing the impacts of temperature and relative humidity. The temperature within a plastic container was controlled using an internal mica heater, an external hot air blower or cooling packs, while relative humidity was adjusted using glycerin solutions. Findings indicated that the Auxiliary Electrode (AE) signal is susceptible to temperature and moderately affected by relative humidity. In contrast, the Working Electrode (WE) signal is less affected by temperature and relative humidity; however, adjustments are still required to determine gas concentrations accurately. Tests involving on/off cycles showed that the AE signal experiences exponential decay before stabilizing, requiring the exclusion of initial readings during monitoring activities. Additionally, calibration experiments in zero air allowed the determination of the compensation factor nT across different temperatures and humidity levels. These results highlight the importance of compensating for temperature and humidity effects to improve the accuracy and reliability of NO2 measurements using low-cost electrochemical sensors. This refinement makes the calibration applicable across a broader range of environmental conditions. However, the experiments also show a lack of repeatability of the zero calibration.

In this research, several SO2 removal alternatives were evaluated for the treatment of gaseous effluents generated by the "Carlos Manuel de Céspedes" power plant in Cienfuegos to be use in gaseous stream to Ulva lettuce intensive culture. A low-cost sensor was used to monitor the gas concentration during the proposed methods. The sensor was previously calibrated and a linear regression model with a coefficient of determination of 0,9926 was obtained. The alternatives evaluated were adsorption with activated carbon, absorption with calcium hydroxide and absorption with seawater. The removal was evaluated at three levels SO2 concentration. In addition, the parameters pH, salinity and alkalinity were determined to a seawater sample from the Bay of Cienfuegos and with which the experiments were carried out. The values obtained were 8,17±0,003; 26,50±0,10 ups and 171,67±1,03 mg/L respectively, which confirms its good performance for gas removal. The three methods studied showed satisfactory results for SO2 removal. The seawater absorption experiment was the most efficient alternative, achieving the lowest final concentrations in the shortest time. En esta investigación se evaluaron varias alternativas de remoción de SO2 para el tratamiento de los efluentes gaseosos generados por la central termoeléctrica «Carlos Manuel de Céspedes» de Cienfuegos para su uso en corriente gaseosa al cultivo intensivo de la lechuga Ulva. Se utilizó un sensor de bajo costo para monitorear la concentración de gases durante los métodos propuestos. El sensor fue previamente calibrado y se obtuvo un modelo de regresión lineal con un coeficiente de determinación de 0,9926. Las alternativas evaluadas fueron adsorción con carbón activado, absorción con hidróxido de calcio y absorción con agua de mar. La eliminación se evaluó a tres niveles de concentración de SO2. Además, se determinaron los parámetros pH, salinidad y alcalinidad a una muestra de agua de mar procedente de la Bahía de Cienfuegos y con la cual se realizaron los experimentos. Los valores obtenidos fueron 8,17±0,003; 26,50±0,10 ups y 171,67±1,03 mg/L respectivamente, lo que confirma su buen desempeño para la remoción de gases. Los tres métodos estudiados mostraron resultados satisfactorios para la eliminación de SO2. El experimento de absorción en agua de mar fue la alternativa más eficiente, alcanzando las concentraciones finales más bajas en el menor tiempo. Nesta investigação, foram avaliadas várias alternativas de remoção de SO2 para o tratamento de efluentes gasosos gerados pela central eléctrica “Carlos Manuel de Céspedes”, em Cienfuegos, para serem utilizados em fluxo gasoso para a cultura intensiva de alface Ulva. Foi utilizado um sensor de baixo custo para monitorizar a concentração de gás durante os métodos propostos. O sensor foi previamente calibrado e obteve-se um modelo de regressão linear com um coeficiente de determinação de 0,9926. As alternativas avaliadas foram adsorção com carvão ativado, absorção com hidróxido de cálcio e absorção com água do mar. A remoção foi avaliada em três níveis de concentração de SO2. Além disso, foram determinados os parâmetros pH, salinidade e alcalinidade a uma amostra de água do mar da Baía de Cienfuegos com a qual se realizaram as experiências. Os valores obtidos foram de 8,17±0,003; 26,50±0,10 ups e 171,67±1,03 mg/L respetivamente, o que confirma o seu bom desempenho na remoção de gases. Os três métodos estudados apresentaram resultados satisfatórios para a remoção de SO2. O experimento de absorção com água do mar foi a alternativa mais eficiente, alcançando as menores concentrações finais no menor tempo.

In the Western world, the SO₂ concentration in ambient air dropped to low levels, but some emission sources (e.g., merchant ships) and some regions (e.g., low-income countries) still emit substantial amounts of SO₂. At those locations, SO₂ monitoring is critical. However, low-income countries do not have much access to expensive reference instruments. Low-cost gas sensors might be an alternative, but it is unclear how reliable such measurements are. To evaluate the performance of the low-cost alternative, the same SO₂ gas sensor has been subjected to three different calibration methods: (1) low-cost calibration performed in the tropical climate of Cuba; (2) high-end calibration performed in Belgium; (3) a field calibration at an air quality measuring station in Belgium. The first two methods showed similar trends, suggesting that the gas sensor can be calibrated with a low-cost method. The field calibration was hampered by the low SO₂ concentrations. For the monitoring campaign in Cienfuegos, Cuba, the low-cost SO₂ sensor calibrated by the low-cost method appeared to be sufficiently reliable. The reliability of the sensor increases with the increase in SO₂ concentration, so it can be used in Cuba instead of Belgium.