Aerosol size distribution and metal constituents in three sites in Bangkok

Airborne particulate matter is known to be deleterious to human health and exceeds exposure limits in many large cities. Some heavy metals and metalloids are known carcinogens and have been measured as constituents of PM in Bangkok air. There is growing interest in the sub-micron and ultrafine (< 100 nm) fractions due to their deeper penetration in the lung. Identifying distribution of metals over the size range can provide information on the metals source as well as providing information on the likely exposure to those particles.

Three sites, owned and managed by the Thailand Pollution Control Department, were identified to provide contrasting particulate samples in a measurement campaign during 2018. The Ayutthaya site was located within the grounds of a school, 80 km to the north of Bangkok. The site was chosen as concentrations due to city traffic would be lower and could be considered a reference site. The Bang Phli site was situated in an industrial area 50 km to the south-east of Bangkok, in an area near industry. The Chok Chai site was in central Bangkok near to a busy road.

At each site, three 3-day weekend and 3-day weekday gravimetric samples of size differentiated mass were drawn using an Electrical Low Pressure Impactor (ELPI; Dekati, Finland) over 12 size fractions in six different study visits. These were chosen to enable three measurements over the rainy season and three in the dry season. Each size fraction was weighed and then analysed by inductively coupled plasma mass spectroscopy to find the concentration of 17 elements (Mg, Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Mo, Cd, Sb, Ba and Pb). The ELPI also measured particle number concentration at 1 Hz. 

The number concentration of aerosol was highest in the Chok Chai roadside site, and lowest in the Ayutthaya background site. Al concentration was the highest in all three locations, with an average concentration over all measurements of 1909, 1012 and 1576 ng m^-3 in Ayutthaya, Bang Phli and Chok Chai respectively. Concentrations of Al, Cr, Mg and Fe were typically higher than 100 ng m^-3 in all sites, Cu and Zn higher than 10 ng m^-3 and the rest lower.

The shape of the metal distributions was consistent across all three sites for specific metals. Mg, Al, Cr, Mo, Ni and Cu could be described as having a flat distribution across all measured size distributions V, Mn, Cd, Sb, Pb, Zn As and Se had a peak in the sub-micron range, while Fe, Ba and Co peaked above 1 µm.

Some seasonal effects could be seen across all three locations, these included an increase in Al, Cr and Fe during four measurements in dry season (November, 2018). This was particularly observed at Ayutthaya, where total measurements of Al were between 4862 and 5961 ng m^-3, when all other measurements were between 98 and 264 ng m^-3, suggesting a strong local source.