Contrasting intra-urban variability of ultrafine particle number and fine particle mass concentrations in Dhaka, Bangladesh, and Pittsburgh, USA

Abstract

In this study, we investigated the intra-urban variability of fine particle mass (PM2.5) and ultrafine particle number concentration (PNC) in Dhaka (Bangladesh), and Pittsburgh (USA), two cities with vastly different pollution sources and levels. We collected measurements of PNC and PM2.5 at a wide range of sites spanning a variety of urban land use attributes (35 sites in Dhaka and 30 sites in Pittsburgh). We found that PNC levels exhibited a 3-4-fold variability between sites in each city, ranging from 20,000–100,000 # cm−3 in Dhaka and 7,000–28,000 # cm−3 in Pittsburgh. PM2.5 levels varied within 50% of the urban background level, ranging from 80 to 110 μg m−3 in Dhaka and 6–12 μg m−3 in Pittsburgh. We observed a moderate level of spatial correlation between PNC and PM2.5 measurements in both cities (R2 0.3 in Dhaka, and 0.4 in Pittsburgh), and consistent within-city spatial patterns in different meteorological seasons. Compared to the Pittsburgh levels, both PNC and PM2.5 levels in Dhaka were significantly higher, however, PM2.5 was disproportionately higher (10–12 times higher) than PNC (3–4 times higher). The slope of PNC versus PM2.5 varied significantly between the two cities, suggesting that the sources and size distribution of particles that make up the majority of the PM2.5 were different in the two cities. The results found in Dhaka indicate that aerosol particles are influenced by solid fuel combustion, including solid biomass burning, waste burning, and road dust, which have a greater impact on particle mass concentration due to their larger size. Contrary, in Pittsburgh, traffic (gasoline and diesel combustion) particles, relatively smaller in size compared to solid fuel combustion particles, play a dominant role in determining the within-city variability of PNC and PM2.5. Our findings suggest that while controlling traffic emissions can contribute to reducing PNC, substantial efforts will be required to mitigate accumulation mode particles from various region-specific sources in order to decrease PM2.5 mass concentrations in Dhaka.