Monitoring NOX and O3 in Urban Atmosphere: A Colorimetric Approach in Ilorin, Nigeria

Abstract

This study presents an assessment of oxides of nitrogen (NOX) and total oxidants as ozone (O3) over the airshed of a typical average tropical equatorial city using a colorimetric approach. The concentrations of NOX and O3 were monitored at three urban site classes and a control site using a high volume sampler for three years. The mean concentrations for all sites during the wet season (May - October) were 3.31 ± 0.40 ppb for NOX and 21.86 ± 2.47 ppb for O3, while the dry season (November - April) mean concentration values were 2.03 ± 0.23 ppb for NOX and 32.44 ± 5.13 ppb for O3. The mean value at the control site was an order of magnitude lower than the urban sites during the two seasons, except for the concentration of O3, which was higher during the dry season. Traffic density and trace gas concentrations showed a good correlation of 0.98 (P<0.05). NOX was confirmed to be a strong precursor for O3 formation. The influence of Harmattan dust in impeding the photochemistry of O3 formation from November to January (NDJ) months was observed. The overall concentration levels of NOX and O3 for the airshed were 2.67 ± 0.32 and 27.15 ± 3.8, respectively, which are within the limit of United States Environmental Protection Agency (USEPA) guidelines for air quality standards.