Physicochemical properties of indoor and outdoor particulate matter in residential areas near a ferromanganese smelter

Abstract

Approximately 90% of the world’s population resides in areas where ambient air quality standards are exceeded. Particulate matter with an aerodynamic size of less than 2.5 µm (PM2.5) has been identified as the leading contributor to indoor and outdoor air pollution. PM2.5 can be released from natural and anthropogenic sources, however, anthropogenic sources such as ferromanganese smelters are among the major sources. Ferromanganese smelters tend to release Mn-bearing PM2.5. Owing to their size, Mn-bearing PM2.5 can remain suspended for longer periods and travel a greater distance from the emitting source and penetrate indoor environments. It is suggested that most people, especially vulnerable groups spend approximately 80%–90% of their time indoors. Chronic exposure to Mn-bearing PM2.5 is associated with neurological disorders. Most of the evidence on the causal relationship between Mn exposure and neurological disorder was drawn from epidemiological studies that lacked exposure assessments. Purpose To systematically characterise the similarities between indoor and outdoor PM2.5 airborne particles in residential areas within the vicinity of the smelter in Meyerton. Methods Indoor and outdoor airborne PM2.5 were collected concurrently from selected households using two identical active samplers. A gravimetric sampling technique was used to sample PM2.5 continuously over seven days from the 30 selected households. GilAir plus pumps were used to draw in air containing PM2.5 into the sampling media at a flow rate of 2.75 L/min. The sampling media comprised of a 37 mm cassette, which was housed in a 37 mm polycarbonate filter. A 1.5 m long Teflon tubing was used to connect the sampling media outlet to the inlet of the pump. A total of 60 samples were collected over three months (August–November 2019) and comprised 30 indoor and 30 outdoor samples. PM2.5 mass concentrations were obtained gravimetrically using a microbalance scale. The physicochemical properties were analysed using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX). Inductively coupled plasma-mass spectrometry (ICP-MS) was used to analyse the elemental composition of indoor and outdoor PM2.5. The difference between indoor and outdoor PM2.5 mass concentration was determined by calculating the indoor-outdoor ratio, where a ratio greater than 1 indicated that indoor PM2.5 is less than the outdoor. Furthermore, statistical analysis for indoor and outdoor PM2.5 was performed using an F-Test and a Student t-test. A P-value of <0.05 indicated a statistically significant difference between indoor and outdoor PM2.5. Results Indoor PM2.5 mass concentration ranged between 2.88 and 19.19 µg/m3 with an average of 10.99 ± 5.10 µg/m3 while outdoor concentration ranged between 11.68 and 40.44 µg/m3 with an average of 24.97 ± 6.77 µg/m3 . Outdoor PM2.5 mass concentration in Meyerton was 2.7 fold greater than the indoor. The I/O ratio of PM2.5 was 0.44 indicating that indoor PM2.5 was lower than the outdoor. The I/O ratio of less than 1 further indicated and that indoor PM2.5 was influenced by PM from the outdoor environment. A statistically significant difference was found (P=5.8×10-13) between indoor and outdoor PM2.5 mass concentrations. SEM images showed that indoor PM2.5 consisted of irregular and agglomerated particles ranging from 0.1 to 0.7 µm in diameter while PM2.5 outdoor also consisted of irregular but single spherical particles ranging from 0.1 to 1.3 µm in diameter. SEM images showed similarities between indoor and outdoor particles suggesting that they are from the same or similar source. ICP-MS results indicated an abundance of elements in decreasing order of Si > Fe > Zn > Mn both on indoor and outdoor PM2.5. However, outdoor PM2.5 had the highest concentration of Mn, Zn, Si, and Fe relative to indoor PM2.5. The average indoor and outdoor Mn concentration was 1.30 µg/m3 and 5.36 µg/m3 , respectively. Conclusion This study investigated the similarities between the physicochemical properties of indoor and outdoor PM2.5 sampled in residential areas near a ferromanganese smelter and found that most indoor PM2.5 originated from an outdoor source. Moreover, indoor and outdoor PM2.5 were enriched with Mn and other elements, which can lead to chronic health outcomes amongst vulnerable groups. Since Mn and the other elements found in this study are mainly released from ferromanganese smelters, the nearby smelter might be a major source of indoor and outdoor Mn-bearing PM2.5 in Meyerton. However, further studies using advanced source apportionment techniques are recommended to verify the findings. Lessons drawn from this study suggest the need for integrated town planning and development where ferromanganese smelter are not supposed to be built near residential areas. Measures such as tree plantation may be introduced in such residential areas to reduce or trap airborne PM