Ambient air pollution and assessment of ozone creation potential for reactive volatile organic compounds in urban atmosphere of southwestern, Nigeria

Abstract

Volatile organic compound (VOC) species react at different rate and exhibit differences in reactivity with respect to ozone formation in polluted urban atmosphere. To assess this, the variations pattern, reactivity relative to OH radical and ozone creation potential of ambient VOCs were investigated in field studies at designated sites in urban centers of Akure and Ado-Ekiti, Southwestern, Nigeria, between June, 2009 and May, 2010. The ambient air samples were drawn into a tube using active sampling method with the low volume sampling pump (Acuro, Drager, Lubeck, Germany). The tube contains Chromosorb 106 as adsorbent. Air samples were collected four times per month at the height of 1.5 m from the ground surface so as to reflect the human breathing zone. On each sampling day, air samples were collected three times between 6.00 a.m and 7.00 p.m to reflect morning, afternoon and evening period. After sampling, the adsorbed VOCs were desorbed with carbon disulphide (CS₂) and the solution analyzed using gas chromatography coupled with mass spectrophotometer. Analysis of the VOCs data revealed that four major classes of abundant VOC groups were captured in the ambient air of the two studied centers. Among the four classes of abundant VOC groups, aromatic dominated with 61 to 77 %, followed by aliphatics (7 to 20%), halogenated (5 to 21%). The results reveal that the total VOCs at various sites showed significant (p<0.05) diurnal pattern with bimodal peaks that followed the cycles of economic activities and traffic loads in the areas. The total ozone formation by abundant nonmethane hydrocarbons in this study were 52.54 and 71.53μg/m3 at Akure and Ado-Ekiti, respectively with m,p-xylene, ethylbenzene being the lead contributors at both centres. The abundant alkane in the atmosphere calls for monitoring as they can be move farther distance from their sources and produce a higher concentration of ozone even though the local concentrations are very low.

Key words: Volatile organic compounds, photochemical reactive hydrocarbons, ozone formation potential.