Influence of ornamental trees species diversity, height and crown diameter on suspended particulate matter 2.5 levels in selected Kampala precincts

Abstract

Ninety seven percent (97%) of the world’s population living in cities dwell in air polluted cities and this is no expectation for Kampala, Uganda’s capital city. Air pollution is, however, ranked as the fifth highest mortality factor causing diseases such as cancer and diabetes. Of total pollution, particulate matter 2.5μm is considered the most dangerous. Particulate matter levels in Kampala, at 165μg/m3 and 75μg/m3 in September, 2018 and February 2021, respectively were six and three times higher than the recommended WHO global standard of 25g/m3. KCCA is emulating cities such as Beijing that have successfully employing phytoremediation to mitigate increasing particulate matter levels. The challenge was a lack of sufficient documentation on the phytoremediation capacity of plants species in Kampala. This study focused on quantifying particulate matter levels and establishing the relationship between tree species diversity, selected physical tree traits and particulate matter levels in Nsambya, Makerere, Civic center, Nakasero A and Nakasero B precincts of Kampala. PM2.5 levels for the five precincts were accessed from data collected from BAMs established by AIRQO and KCCA. Weekly and monthly trends of particulate matter was almost the same for the five precincts, with Tuesday being the most polluted day of the week and Thursday the least polluted. January was the most polluted month of the year in all apart from the Civic center where December was the most polluted month. May was the least polluted month of the year for all apart from Makerere that had November with the least particulate matter levels. Monthly and daily particulate matter levels seemed to follow expected levels of human activity. Overall PM2.5 levels were obtained by running a one-way ANOVA (p<0.01) and Nsambya had the highest levels (129.13μg/m3) followed by Makerere (53.68 μg/m3), Civic center (43.60 μg/m3), Nakasero B (43.03μg/m3) then Nakasero A (33.71 μg/m3). These values seemed to be influenced by location with respect to entry or exit points into and out of Kampala city. There is no association between particulate matter levels and tree crown diameter and tree species diversity influences particulate matter levels in areas with extremely high PM2.5. There was a very weak negative correlation between particulate matter and tree species number (r=-0.27) and average height (r=-0.11).PM 2.5 levels differ with location of the precinct in terms of heavy traffic and activities taking place the precinct. There is no relationship between tree species diversity and PM2.5 levels though there was a negative relationship between particulate matter and tree height and number. Faster growing tall tree should be planted in proximity to the PM2.5 source in order to enhance the tree phytoremediation abilities on PM2.5