Anthropogenic contribution to air pollution with background emissions; case of Nairobi, Mombasa and Kisumu

Abstract

This study aimed at assessing the contribution of human beings to pollution levels in Nairobi, Kisumu and Mombasa; and helping understand the pollution trends in changing climate. The study followed a Driver-Pressure-State-Impact and Response (DPSIR) framework where cause-effect-response model was adopted. Mt Kenya Global Atmosphere Watch (GAW) station was used as background station with the assumption of natural pollution occurrence. The pollutants that were studied include Particulate Matter (PM2.5), Sulphur dioxide (SO2) and Carbon dioxide (CO2). The pollution dataset was sourced from Modern-Era Retrospective Analysis for Research and Application (MERRA – 2) while wind data was sourced from Kenya meteorological department (KMD) from 2000 to 2016. Time series and correlation analysis were used to describe data characteristics while cross-sectional analysis was done to ascertain spatial behavior of pollutants. Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) and Wind Rose plotting (Wrplot) were used to determine wind trajectory and wind frequency The study found that PM2.5, SO2 and CO2pollution levels varied in both time and space based on the position of the Inter-Tropical Convergence Zone (ITCZ). The amounts of the pollutant however had close resemblance for seasonal cycles. The winds in all the three cities exhibited variable tendencies that were also based on the position of the sun. When the sun is in the northern hemisphere, the winds showed moderate to strong south-southerly in most stations and northeasterly to easterly during southern summer. Within the MAM and SON seasons dominant winds were light to moderate easterlies and sometimes southeasterly. However, the spatial and temporal interactions with different scales bring in a different flow characteristics of wind. This differential scale otherwise known as inhomogenity, is seen in most stations that have built canopy particularly in urban centers. Moreover, the densely populated built structures that alter the flow of pollutants complicate the nature of flow. Northeasterly and easterly winds correlated with higher levels of pollutants in all stations while southerly winds contributed to pollutants though the levels were low.CO2 did not show high variation except for Mombasa station while there were correlations of more pollutants during dry seasons of the year. The pollutants were dispersed beyond 50 km within short periods with least dispersion occurring during the long rain season. The findings of this study indicate that background emission is increasing at a lower rate compared to increases at station located in urban areas due to human activities. The main contributors to the burden of pollutants in the near surface atmosphere are thus human activities followed by wind and other natural factors. Therefore, the study recommends for a consultative planning process of the management of urban centers that accounts not only for the expected increased human activities but also for the observed wind characteristics and other natural factors over the towns.