Most people living in the Dhaka City are now aware of the severe air pollution in this city. What they are informed about are the levels of suspended particles in the air they breathe, the concentrations of lead, a very toxic metal, present in the ambient air and its consequent effect on human health. What they are not yet probably aware of the organic compounds present in vehicular emissions, ranging from benzene to any higher hydrocarbon, normally present in petrol, which are mostly aromatic in nature and carcinogenic in bioactivity. These compounds are grouped as volatile organic compounds (VOCs) present in high concentrations in the autorickshaw and automobile exhausts and are so considered as the most serious polluters of the ambient air in the Dhaka City. As has been measured in real time situation, these compounds have reached an alarming level. The results of volatile and semi-volatile organic compounds in ambient air as have been obtained in this work provide a reliable database information of primary nature to the policymakers in formulating appropriate mitigation measures for controlling air pollution in the Dhaka City. It should be noted that the measurements reported here are first of their kind in Bangladesh.
Ambient air samples were collected for 30-45 minutes by applying SPME technology at four locations on the Mirpur road near Shewrapara. More than 200 organic compounds were detected in the air; 35 of these compounds were identified with known chemical structures. These are normal hydrocarbons like pentane (n-C5H12) through nonacosane (n-C29H60); aromatic hydrocarbons: benzene, toluene. ethylbenzene, xylenes (BTEX mixture). 3-rnethoxy benzenethiol, l-isocyanato-3-methoxybenzene, n-propylbenzene, n-butylbenzene. 1,3,5-trimethylbenzene and 3,3'-dimethoxy-2-butanone as the ketone. Samples collected near an autorickshaw stand contained 783,000 -1,479,000 ìg/m3 (micrograms per cubic meters of air) of lacs. These values are 400% to 745% higher than accepted normal value of VOCs 193,000 ìg/m3. In particular, the concentration of toluene (a known carcinogen) was 50-100 times higher than the threshold limiting value of 2000 ìg/m3. Other samples collected under normal traffic conditions at the mentioned site showed 135,000 ìg/m3 to 180,000 ìg/m3 of total VOCs. The values are within the normal range of the accepted value. Samples analyzed by GC-MS show that significant portions of the total VOCs come from aromatic and polynuclear aromatic hydrocarbons (PAHs). It is now well established that many of these compounds are highly carcinogenic in nature.
For a comparative study and in order to understand the origin of VOCs, local fuel samples such as octane, petrol, diesel, and mobile were also analyzed by GC and GC-MS. Most VOCs were found to originate from low boiling octane and petrol. Significant portions of the semi-VOCs showed clear signature similar to that of unburned and weathered diesel and were present in all samples. Signature at concentrations of high boiling mobile fraction was also found.
This work was carried out in the Laboratories of the Intronics Technology Centre (ITC), Dhaka. For complete analysis of air quality, ITC has measuring facilities for S02, NOx, O3 and Pb.
Work is in progress to identify the chemical nature of aromatic hydrocarbons, PAHs, aldehyde-ketones, and other compounds in air samples. After the introduction of unleaded petrol in Bangladesh as higher aromatization of petrol is necessary to maintain the required octane vales. This would probably increase the aromatics in the air unless the mechanical conditions of automobiles are improved and better fuel combustion is ensured.