Chemical Speciation of Particulate Matter Pollution in Urban Dhaka City

Dhaka, the capital and largest city of Bangladesh, lacks a regular air pollution monitoring network to measure and assess ambient air pollution level. Several academic and research projects attempted to measure a limited number of air pollutants near roadways in the central business district. These studies revealed that ambient concentrations of lead, benzene, particulate matter, carbon monoxide, oxides of sulfur, and nitrogen in the air of Dhaka have exceeded health based standards set by both the World Health Organization (WHO) and United States Environmental Protection Agency (US EPA). The measured peak concentrations for some pollutants were found to be several folds higher than the international standards. In one instance, a study conducted by the Bangladesh Atomic Energy Commission showed that Dhaka’s ambient lead level was highest in the world for a part of the year 1996. With the assistance of World Bank, Bangladesh Department of Environment (DOE) recently began an air quality management project in the Dhaka City. An international consultant helped the DOE personnel to setup an air-monitoring network in selected locations of Dhaka City to collect and measure particulate emissions. This paper presents results of the chemical analysis conducted on the ambient filters collected by the World Bank and DOE monitoring effort. Due to financial constraint, the World Bank and DOE limited their objective to only measurement of the level of ambient particulate concentrations. However, without a detailed chemical speciation of the collected filter, it is not possible to identify pollution sources and their relative contributions. Furthermore, in order to understand public health implications and asses exposure risks, chemical components of the particulate pollution must be known. In a joint effort, the Bangladesh Environment Network (BEN) and the international consultant for the Dhaka air quality management project arranged for the laboratory analysis of the collected filters. The world renowned Desert Research Institute (DRI), Reno, Nevada, conducted chemical analysis of the filters. Dr. Judith Chow of DRI, who pioneered many chemical techniques for analysis of particulate pollution, graciously offered the service to analyze these filters at no cost. Among the sampling sites, the Farmgate site has shown the maximum ambient level, as high as 526 microgram of PM2.5 per cubic meter of air volume. This level is more than eight folds higher than the US EPA standard of 65 microgram per cubic meter. The fine PM2.5 size pollutants are primarily emitted by combustion processes and automobile emissions. As Farmgate is considered one of the most congested traffic intersections in Dhaka, such an extremely high PM2.5 level is the direct result of vehicular emissions. The chemical analysis of the filters at DRI laboratory identified presence of the following chemical species in the Dhaka’s particulate air pollution: elements of sodium (Na) to lead (Pb), chloride (Cl), nitrite (NO2), nitrate (NO3), sulfate (SO4), ammonium (NH4), organic carbon (OC), elemental carbon (EC), etc. Very high concentrations of sulfate, organic carbon, led, nitrate, etc. suggest that particulate emissions are predominately formed by the combustion products of dirty fuels (i.e., high sulfur diesel and leaded petrol) used in automotive engines. Nature and Sources of the Particulate Matter Pollutant Particulate matter (PM) is the general term used for a mixture of solid particles and liquid droplets found in the ambient air. Some particles are large or dark enough to be seen as soot or smoke. Others are so small that they can be detected only with an electron microscope. These particles, which come in a wide range of sizes (“fine” particles are less than 2.5 micrometers in diameter and coarser size particles are lager than 2.5 micrometers), originate from many different stationary and mobile sources as well as from natural sources. Fine particles (PM2.5) result from fuel combustion from motor vehicles, power generation, and industrial facilities, as well as from residential fire-places and wood stoves. Coarse particles (PM10) are generally emitted from sources, such as, vehicles travelling on unpaved roads, materials handling, and crushing and grinding operations, as well as windblown dust. Some particles are emitted directly from their sources, such as smokestacks and cars. In other cases, gases such as sulfur oxides, SO2, NOx, and VOC interact with other compounds in the air to form fine particles, which are called secondary particulate matters. Their chemical and physical compositions vary depending on location, time of year, and weather. Health and Environmental Impacts Inhalable PM includes both fine and coarse particles. These particles can accumulate in the respiratory system and are associated with numerous health effects. Exposure to coarse particles is primarily associated with the aggravation of respiratory conditions, such as asthma. Fine particles are most closely associated with such health impacts as increased hospital admissions and emergency room visits for heart and lung disease, increased respiratory symptoms and disease, decreased lung function, and even premature death. Sensitive groups that appear to be at great risks to such effects include the elderly, individuals with cardiopulmonary disease, such as asthma in children. In addition to health problems, PM is major cause of reduced visibility and increased haze in many parts of the world. Airborne particles also can cause damage to paints and building materials. Sampling Location and Methodology Bangladesh DOE has measured ambient levels of suspended particulate matters in Dhaka in the past few years but few measurements of PM10 or PM2.5 have been taken by the agency. During the period of August 3 to October 3, 1998, DOE used Airmetrics Minivol Portable Samplers to measure PM10 and PM2.5 concentrations at three locations in Dhaka. These locations included the roof of the DOE office building, the roof of World Bank office in central Dhaka, and at street level near the police box in the Farmgate intersection of the city. Most samples were taken over six hour periods during mid-day when weather conditions were mostly windy, rainy, and cloudy. Two samplers were run concurrently: one equipped with a quartz fiber filter to be used for carbon analysis and the other with a stretched Teflon filter to be used for elemental analysis. The patented low-flow technology featured in Airmetrics Minivol Portable sampler was jointly developed by the US EPA and Lane Regional Air Pollution Authority, Oregon, USA to address the need for portable survey sampling. The US EPA owns nearly 200 Minivol samplers, which are housed at East and West Coast repositories for agency use. Compact, lightweight, and battery-operated, the minivol sampler is ideal for sampling at remote locations, at sites where no permanent monitoring site exists, or at areas without electrical power. The Airmetrics Minivol Portable Sampler is basically a pump with a programmable timer which can be set to make up to 6 sampling "runs" within a 24 hour period. The timer can start and stop the pump at pre-set times over a 7 day period. The pump draws air through a particle size separator and then through a filter medium. The 10-micron or 2.5 micron particle separation is achieved by impaction. The particulate sample can be caught on any standard 47mm filter medium, which enables gravimetric or chemical analysis. Inside the sampler, air is pulled through the inlet tube (downstream from the filter) to the twin cylinder diaphragm pump. From the pump, air is forced through a standard flow meter, where it is exhausted to the atmosphere inside the sampler body. The flow meter is used to indicate and adjust the flow rate.