Temporal and spatial variability of tropospheric ozone in Nairobi City, Kenya

Abstract

In both developed and developing nations urban air pollution is increasingly being recognized as a major public health and environmental issue. Poor or deteriorating air quality in many cities results from high levels of energy consumption by industries, transport and domestic use. The nature of air pollution is dependent on the source profile of the city and the presence of sunlight to promote production of secondary pollutants, such as ozone, through photochemical reactions. The study sought to analyze the surface ozone over Nairobi city, and identify the source of the surface ozone. Nairobi city is one of the fastest growing industrial and economic hubs in East Africa. Increased population which results in increased production and transport activities is therefore expected to increase the surface Ozone which is likely to cause a lot of negative effects to both fauna and flora beings. Surface ozone data for Nairobi was obtained from Kenya meteorological department ranging from 2011 to 2014; another set of data was collected from four sampling sites to determine the special variability and source of the surface ozone over Nairobi area. Analysis meteorological field from National Centre for Environmental Prediction -National Centre for Atmospheric Research (NCEP- NCAR) was used in running Hybrid Single Particle Lagrangian Integrated (HYSPLIT) model. From the analysis it was found out that June July August experiences the highest ozone levels as compared to the other months of the year in both lower and upper levels. This is due to incursion from the south according to the backward trajectories from the HYSPLIT model, which has been proven to have high ozone concentration during this season due to high biomass burning. On the other hand, the diurnal variation of ozone in the four site: Industrial area, Nakumatt Junction, Landhies road and Pangani Round about showed low amount in the early morning and at night hours, with the peak realized during the day. The peak in midday is due to the fact that surface ozone is produced by photochemical oxidation of precursor gases that are produced by motor vehicle and industries. The highest eight-hour mean was 20.2 ppb from Industrial area site, which is below the WHO mean of 51 ppb. Therefore, no much health effects are expected due to the exposure to ozone. This study recommended that there should be a continuous monitoring of ozone and other gases that are harmful to human health for better understanding and advice to the citizen.