Visibility as a proxy for air quality in East Africa

Ajit Singh,William R Avis,Francis D Pope

doi:10.1088/1748-9326/ab8b12

Abstract

Many urban areas in Africa do not have sufficient monitoring programs to understand their air quality. This study uses visibility as a proxy for PM pollution to provide insight into PM air pollution in three East African cities: Addis Ababa, Nairobi and Kampala, from 1974 to 2018. Overall, a significant loss in East African visibility was observed since the 1970s, where Nairobi shows the greatest loss (60%), as compared to Kampala (56%) and Addis Ababa (34%). These changes are likely due to increased anthropogenic PM emissions. Correspondingly, PM pollution levels, in Kampala, Nairobi and Addis Ababa, are estimated to have increased by 162, 182 and 62%, respectively, since the 1970s to the current period.Distinct variations in seasonal visibility are observed, which are largely explained by changing PM sources and sinks in rainy and dry seasons. Average PM hygroscopicity is investigated by comparing average visibilities under different RH conditions. It is observed that PM hygroscopicity has decreased over time in all three cities, which is consistent with increasing emissions of PM with hygroscopicity lower than the ambient background. A large urban increment in PM is observed, with poor visibility typically occurring when the wind brings air from densely populated urban areas.To investigate the intersection between increasing pollution, population and economic growth, changes in pollution are compared to available population growth and GDP statistics. Significant positive correlations between increasing PM and national GDP (and city population) were found for all three study cities. These cities have undergone rapid increases in population and national GDP growth (driven predominantly by study cities’ economies) during the study period. This has resulted in increased rates of citywide fuel use and motorization, which provides a direct link to increased PM emissions and thus visibility loss. The study suggests that socio-economic forecasts may enable future air quality projections.

Highlights

Ambient air pollution is a major environmental issue across the world (WHO 2016, HEI 2018, Landrigan et al 2018)
Clear downward trends in annual mean visibility is observed for all study sites, which is understood to be due to increasing concentrations of Particulate matter (PM)
The results suggest that the observed shape of the curves are consistent with Environmental Kuznets Curve (EKC) hypothesis with East African urban areas entering a phase of a weakening positive relationship between outdoor air quality and GDP

Summary

Introduction

Ambient air pollution is a major environmental issue across the world (WHO 2016, HEI 2018, Landrigan et al 2018). The main obstacle to measuring and monitoring the air pollutants in these countries is the high cost of air quality monitoring equipment including their appropriate calibration and certification (Crilley et al 2018, 2020, Pope et al 2018) To this end, there are increasing efforts to make air quality monitoring networks in the various African countries (Gaita et al 2014, Desouza et al 2017, Pope et al 2018, Raje et al 2018) but historical data is almost non-existent. There are increasing efforts to make air quality monitoring networks in the various African countries (Gaita et al 2014, Desouza et al 2017, Pope et al 2018, Raje et al 2018) but historical data is almost non-existent To fill this crucial data gap, visibility measurements that are recorded at major cities in East Africa can be used as a proxy for particulate matter (PM) air pollution (Singh et al 2017)

Methods

Results

Conclusion