Spatially dense measurements of fine and ultra-fine particles in Dhaka city

Abstract

Exposure to airborne fine (PM2.5; particles less than 2.5 µm) and ultrafine (UFP; particles less than 100 nm) particulate matter causes adverse health effects. While ultrafine particles are a sub-component of PM2.5, the sources and characteristics and impacts of ultrafine particles are vastly different than PM2.5. Due to high spatial variability, a spatially dense measurements are needed to characterize the variability of ultrafine particles. The primary objective of this study is to characterize the intra-urban spatial variability of PM2.5 and ultrafine particles in Dhaka city through high-spatial resolution measurements.

To characterize intra-urban spatial variability, measurements were collected at 35 locations across the Dhaka city. Measurement locations cover a diverse range of land-use settings, including urban background, residential and commercial locations. UFP concentrations were measured as total particle number concentrations (PNC) using a condensation particle counter (CPC). PM2.5 measurements were collected using a light scattering-based (Purple Air) sensor. Repeated short-terms measurements (20~15 minutes per site per visit) were collected at selected 35 sites. Measurements included 8-10 days of short-term sampling per site for PM2.5 and 2-3 days of short-term sampling per site for PNC. Samplings at a particular site were distributed at different parts of the day (morning, afternoon, evening) and seasons (summer and winter).

Measurements show a factor of 2-3 intra-urban spatial variability of PNC. PNC levels are 100-200% higher in commercial and semi-commercial (residential with some commercial activities) locations compared to urban background levels. Intra-urban spatial variability of PM2.5 is moderate. PM2.5 concentrations in commercial and semi-commercial locations are about 15-20% higher relative to the urban background. Spatial correlation between site-average PNC and PM2.5 was relatively low (R2 ~ 0.25). This low spatial correlation indicates that the different process and sources drive the spatial variability of PNC and PM2.5. Substantially higher spatial variability of PNC indicates the influence of local sources (traffic and other urban combustion sources) on PNC exposure, while PM2.5 exposures mostly dominated by regional background air mass.