Quantifying PM2.5 at Two Urban Sites in Akure, Nigeria Using Low-Cost Sensor Technology

doi:10.21203/rs.3.rs-4881818/v1

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Quantifying PM_2.5 at Two Urban Sites in Akure, Nigeria Using Low-Cost Sensor Technology

https://doi.org/10.21203/rs.3.rs-4881818/v1

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PM_2.5 has been a problem for inhabitants throughout the whole world. To reduce this, there is a need to know the amount that is present in the atmosphere. The quantification of PM_2.5 will help to determine the type of remedies for air quality. One of the methods used for quantification is the use of low-cost sensors which are easy to use, high temporal resolution, and small to handle. The aim of this study is to apply low-cost optical sensors to determine the PM_2.5 during the dry (Harmattan) season at two sites in Akure, Ondo State, Nigeria. The results showed that PM_2.5at a site (Site-B) ranged between 14.4 and 590.4 µg/m³, while that at another site (Site-C) ranged between 14.5 and 737.2 µg/m³. The mean values (75.7 and 87.5 µg/m³ at the Site-B and the Site-C, respectively) were significantly beyond the standard limits of WHO and NESREA. Shorter timescales peaks within a few hours were frequently observed especially at the Site C during daytime, likely due to the local emissions of PM_2.5 due to combustion activities. The higher-density multi-points observations are highly recommended to understand sources, behavior, and health effects of PM_2.5 in regional city in South-West Africa.

PM2.5

low cost optical PM2.5 sensor

Nigeria

Low air quality is a problem worldwide because of the impact on life due to health-related problems, even death [1, 2]. Many cities are facing the challenges of how to tackle air pollution. Even many European and African cities could not solve the menace because they could not meet the standards set by EEA [3]. Those residing in developing countries are most affected. According to WHO [4], the air quality of 97% of cities in low- and middle- income countries with 100,000 inhabitants is worse than WHO guideline), while this percentage decrease to 49% in developed countries.

To monitor the air quality, legislation, surveillance and scientific research must be put in place. One of the main requirements is the use of passive sampler for sample collection, which is expensive, bulky, and heavy [5], because of this, low-cost sensors were embarked upon. At present, many project results have produced sensors that are easy to use, low-cost, small, and many citizens have been able to monitor the air quality within their environs [5, 6].

The measurement of PM_2.5 in the urban environment is widely recognized in Nigeria and many studies have been performed in the metropolis of Lagos [7, 8, 9, 10, 11], with a strong focus on aerosols (PM_2.5 and PM_2.5−10. Obayan et al. (2018) reported on PM₁₀ and PM_2.5 on the island of Lagos. Owoade et al. [7] conducted a study on the association between concentrations of particulate matter and meteorological parameters around a smelting industry in Ile-Ife; however, it is noted that most of these researchers used passive samplers for their collections of samples.

One of the commonly used sensors employed is a low-cost palm-sized optical PM_2.5 sensor, which has low power consumption and high resolution [13, 14]. Researches on air quality with low-cost sensors are extensively on world-wide [15, 16], but only little has been done in Nigeria. To this end, a low-cost palm-sized optical PM_2.5 sensor designed and developed by a team from Panasonic Cooperation and Nagoya University in Japan [14] was used for the monitoring of PM_2.5 within the vicinities of two areas in Akure, Ondo State, Nigeria. Therefore the aim of the work is to monitor the PM_2.5 of two locations in Nigeria for a period of two months. The results are reported in this paper.

The two months (3rd December 2019–25th January 2020) observations of PM_2.5 mass concentration was conducted using two P-sensors (Shibata Cooperation, Japan) at two sites (Site-B – 7^o13’30.9’’ N 5^o10’20.1’’ E and Site-C − 7^o16’04.38’’ N, 5^o15’08.71’’E) Akure, Ondo State, Nigeria (Fig. 1). The sites are located in the urban area with a population of 666,000 in 2020 [17]. The Site B is located in residential, commercial, and medium traffic density areas (more urbanized and less residential activity area, whereas the Site C is located in sub-urban residential area with greater residential activities.

The P-Sensor used in this study is shown in Fig. 2. The P-sensor consists of a Panasonic-PM_2.5 sensor, a clock, and a data logger with a storage capacity of 37.7 days/1 min interval and 1 year/10 min interval, but no GPS sensor, and uses a PC downloading of stored data. It has a size of 52 mm x 45 mm x 22 mm and can be operated commercial electricity or mobile battery. The accuracy of the sensor is typically less than 23%, the response time is < 2 minutes, and the output range before multiplying a conversion factor (typically 1.3) is between 0 and 999 µg/m³. The full details of the sensor have been reported by Nakayama et al. [14].

The sensor was placed in an open space outside the building facing up (> 1.5 m height) where there was no obstruction in the study area. The cable from the commercial power supply was plugged in the P-Sensor through a USB cable after convert to DC 5V, as shown in Fig. 2. After switching on the sensor and warming up for 1 min. The PM_2.5 data were continuously stored at every 1-minute interval. To download data, the sensor was connected to a personal computer (PC – windows 10) using a USB cable. The measured data were retrieved on Excel showing the dates monitored, the output of PM_2.5, and the time of measurement. To obtain the PM_2.5 mass concentration, the sensor output was multiplied by a factor of 1.3 (determined from the average material density of ambient particles) [14].

Figure 3 depicted the PM_2.5 mass concentration (µg/m³) of the Site-B and the Site-C. The gaps of the data are due to the lack of the power supply. The similar day-to-day variations were observed at the two sites, suggesting that the PM_2.5 level at Akure is mainly controlled by the regional and national scale emission sources. The observation period is within dry season (Harmattan season) in Nigeria and a large amount of PM are emitted from biomass burning for agriculture and uncontrolled natural forest burning [18] In addition, dust particles can be transported from Saharan dessert to Nigeria [18]. Efficiency of transportation of these particles to Akure would change periodically depending on meteorological conditions (air circulation patterns and wind speed/direction), and therefore, may led to several-days to one-week scale variations of PM_2.5 concentrations observed from begging to end of January.

At shorter time scales, high peaks of PM_2.5 were observed at different timings at the two sites, suggesting existing of non-negligible contributions of local emission sources of PM_2.5. The data at Site-C in less urbanized area tended to have frequent spiky peak series of PM_2.5 compared to the Site-B in more urbanized area. Greater combustion activities for cooking and wastes treatments near the Site C are considered to cause these observed spiky peaks on PM_2.5 mass concentrations.

Figures 4 shows the histograms of the PM_2.5 mass concentration obtained in this work. At the Sites B and C, most frequent concentration range was 50–60 µg/m³ and 30–40 µg/m³, respectively. At both sites, second peak were observed at around 120–160 µg/m³. At the Site-C, the higher concentrations above 200 µg/m³ were also frequently observed. The PM_2.5 mass concentration ranged between 14.4 and 590.4 µg/m³ at the Site-B, while it did between 14.5 and 737.2 µg/m³at the Site-C. The mean (median) value at the Site-B were 75.7 (60.1) and were less than the value 87.5 (73.5) µg/m³ at the Site-C. The higher mean and maximum PM_2.5 mass concentrations at Site C were likely due to the difference in the residential activities around the two sites. The mean PM_2.5 mass concentrations obtained in this study at the two sites are significantly beyond the limits stipulated by WHO air quality guideline level (PM_2.5 not exceed 5 µg/m³ for annual mean, or 15 µg/m³ for daily mean) [19] and the National Environmental Standards and Regulations Enforcement Agency [20] (PM_2.5 not exceed 20 µg/m³ for annual mean, or 40 µg/m³ for daily mean).

Figure 5 shows the average diurnal variations of PM_2.5 mass concentrations. At the Site-B, slightly larger concentrations were observed in the morning at local time around 8–9 am and in the evening around 19:00–20:00, may be due to the vehicular usage or cooking activities around the site in the residential area. At the Site-C, large concentrations were observed during day time, presumably due to additional local sources including biomass burning for cooking, waste treatments, and/or generator and vehicular usage because of greater residential activities around the Site C.

The low-cost palm-sized optical PM_2.5 used in this study has the potential for monitoring of aerosol concentrations in a regional cities and rural areas in Nigeria, because it is cheap, robust, and good for outdoor use with a proper enclosure. The results from the two sites used in this study depicted high mass concentrations of PM_2.5 during the observation periods in dry (Harmattan) season. Frequency of shorter timescales peaks within a few hour and diurnal variation were different between two sites. Greater contributions of local emission sources of PM_2.5 at the Site-C, owing to the higher occurrences of combustion activities, were suggested. This work demonstrated that continuous and high temporal resolution monitoring of PM_2.5 using low-cost optical sensors enabled us to detect the enhancement of PM_2.5 mass concentrations due to local emissions. It is recommended to conduct higher-density multi-points observations for more detailed understandings of spatial-temporal variation of PM_2.5 in regional city in South-West Africa.

Acknowledgments

The authors express their appreciation to Prof. Y. Matsumi (Nagoya University) and Mr. K. Kanegae (Nagasaki University) for their supports on sensor preparation and data analyses.

Funding

This research was supported by the Leading Initiative for Excellent Young Researchers and the Grant-in-Aid for Scientific Research (KAKENHI, Grant number: JP21KK0187), MEXT, Japan, the Sumitomo Foundation for Environmental Research Project, and the Nagasaki University WISE Programme.

Author Contributions

For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used “Conceptualization, T.N. and F.O.A.; sensor preparation and calibration, T.N. and Y.M.; observation and data collection, F.O.A., M.M.N., A.A., S.D.O.; data curation and analysis, S.S. and T.N.; visualization, S.S.; discussion and writing, S.S. and T.N.; funding acquisition, T.N. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: Data available upon request

Conflicts of Interest: The authors declare no conflicts of interest.

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No competing interests reported.

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Quantifying PM_2.5 at Two Urban Sites in Akure, Nigeria Using Low-Cost Sensor Technology

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