Measurements of Surface Ozone and its Precusrors with relevance to Urban Air Pollution; A Case of Nairobi County.

Abstract

Ozone is an important greenhouse gas and a key global air pollutant. It is known to be the most damaging air pollutant whose concentrations continue to rise globally to beyond tolerable levels, and attributed mainly to human activities. Nairobi city is important to Kenya’s economy and acts as regional headquarters for many organizations. The city faces many challenges associated with high population, which include lack of an effective public transport network resulting in frequent vehicular traffic jams leading to increased emissions of air pollutants. The dynamic changes in air quality, due to atmospheric pollutants and more specifically surface ozone is of interest in this work because it influences the quality of life in the city, therefore it needs to be systematically monitored and analyzed. This study focused on: The current levels of surface ozone over Nairobi taken from the continuous and mobile measurements; the types of the precursor gases of surface ozone over Nairobi; their current levels from various selected locations around the city and finally; the past trends and contributions of vertical profile ozone over Nairobi. The continuous surface ozone measurements were obtained from a stationary surface ozone analyzer Thermo scientific Model 49i and the mobile surface ozone alongside the precursor gasses taken from Ecotech Serinus analyzers for Ozone, NOx, NMHC, CO and CH 4 mounted on a mobile air pollution laboratory deployed to several sites around the city and the ozonesonde for the vertical profile ozone measurements. The main results show that the continuous surface ozone measurements have diurnal and seasonal variations being the same for the past four years. Day time ozone levels were seen to peak at mid-day hours, then the seasonal levels seen to have two prominent peaks twice a year in the months of March having hourly mean concentrations 32 +/- 5 ppb and August/September with 36 +/- 5 ppb. Mobile measurements were found to be within the WHO limits. Ozone diurnal variations were found to be within WHO limits of 50 ppb for 8 Hr. mean, with Valley road exhibiting the highest with an 8 Hr. mean of 14.9 ppb. The study concludes that, surface ozone over Nairobi is majorly contributed by vehicular sources as precursors such as NOx and NMHC were high in most mobile site measurements. Based on this work, it is recommended that more continuous and mobile measurements around the city be adopted to enable a diverse data base, which may assist in evaluating the impacts of surface ozone, more so with regards to health and agriculture, critical to inform policy makers and the general public of the trends and the risks posed by the increasing urban air pollution.