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.
