Analysis of the temporal evolution of total column Nitrogen Dioxide and ozone over Nairobi, Kenya using daily OMI measurements

Abstract

Concurrent measurement and analysis of Nitrogen dioxide (NO2) and Ozone (O3) are essential for improved understanding of ozone distribution. This study sought to analyse the temporal evolution of total column NO2 and O3 over Nairobi using satellite-derived daily data between 2009 and 2013. Seasonality is observed in O3 distribution with minimum and maximum occurring during the dry and wet seasons, respectively. Additionally, a lag of about a month or two occurs between the onset of a season and corresponding minimum or maximum NO2 and O3 concentration. The established association between monthly NO2 and O3 is such that, above average concentration of NO2 is likely to lead to above average levels of O3 during the same month (r=0.79) and below average levels about 5 months later (r=0.39). The Quasi Biennial Oscillation (QBO) is the main phenomenon behind the oscillating biennial feature exhibited by NO2 and O3 interannual trend. The study shows that NO2 and O3 are increasing at annual average rates of about 0.27% and 0.46% per year compared to mean values, respectively. Daily variation of both NO2 and O3 depicts stagnating trends over the entire period of study. This difference is attributed to the fact that, whereas daily NO2 and O3 are influenced by mechanisms that control the slow shift between the dry and wet periods within the course of a year, interannual variability is driven by the differences in each year’s general weather conditions.