Temporal Variations in Ambient Carbon Monoxide Concentrations between Weekdays and Weekends in Akure Central Business District, South West Nigeria

摘要

The temporal variations in ambient carbon monoxide (CO) concentration has been investigated between weekdays and weekends through the analysis of a year-long measurement data between March 2009 - February 2010 taken at the central business district in Akure, Nigeria. This area has air pollution problems, and the study of the weekdays-weekends effects is used as a tool to plan and develop strategies for air pollution mitigation measures considering the on-going extremely rapid urban growth. CO was measured using portable Lascar EL-USB-CO sensor and logger mounted 3 meters above the ground on a lamp post with a sampling frequency of 30 seconds. 1 hour and 8 hour averages were then computed from the raw data using descriptive statistics: weekly evolution, weekdays-weekends differences and average daily difference between weekends minus workdays were computed and hypothesis on the difference in holiday and non-holiday carbon monoxide concentrations was tested. Based on daily parameters and weekend-weekday differences, all the measured air pollutants exhibited clearly reduced concentrations. Results show distinct weekdays and weekend variations with respect to traffic rush hours. Weekdays exhibited two peaks that coincide with morning rush hours to work and evening rush hours back home. Weekends had two and one peak related to traffic for morning church service and evening social outings respectively. The highest CO concentrations during the morning and evening rush hours for 1 hour averages were 12.57 ppm and 14.51 ppm for weekdays and 8.50 ppm and 14.62 ppm for weekends respectively. While the highest CO concentrations during the morning and evening rush hours for 8 hour averages were 9.57 ppm and 13.17 ppm for weekdays, it was 8.40 ppm and 11.72 ppm for Sunday morning and evening peaks respectively, for Saturday the value was 14.51 ppm. Weekdays CO concentration for daytime mean 1 h values of 8-20 ppm and daytime mean 8 h values of 4-12 ppm were observed while the weekends concentration exhibited a consistent similar cyclic diurnal pattern throughout the week, with daytime mean 1 h values of 2-10 ppm and day-time mean 8 h values of 4-8 ppm. The magnitude of the differences between mean weekdays and weekends CO concentration during December, January, February (DJF), was 45% compared to 21% between March, April, May (MAM). While June, July, August (JJA) was 18% compared to 16% between September, October, November (SON) in the central business district.