Spatiotemporal variability of tropospheric NO2 over four megacities in Southern Africa: Implications for transboundary regional air pollution

Abstract Nitrogen dioxide is one of the atmospheric trace pollutants that is a product of anthropogenic activities and has been found to have negative environmental-human health outcomes. Within urban settings, the NO2 column levels are a good proxy indicator to show the quality of ambient air. The purpose of this study was to investigate the tropospheric nitrogen dioxide column levels during the year 2020 over four megacities in Southern Africa Region (Gaborone, Harare, Johannesburg and Maputo). Data were derived from AURA OMI complemented by the HYSPLIT Model-NCEP/ NCAR Reanalysis and GEOS-5 Model-MERRA 2. Findings were that the city of Johannesburg recorded the highest NO2 column levels during 2020 at 14.07 × 1015 molecules per cm2 recorded in the winter season while the city of Harare recorded the lowest NO2 column levels at 0.36 × 1015 molecules per cm2 which were recorded in spring. This present study also confirmed that 90 % of the year the cities were under the influence of long-range air masses originating from both the South Atlantic and the Indian Ocean however the city of Harare's tropospheric NO2 seemed more regionally determined than those of Johannesburg, Gaborone and Maputo. On the other hand, the city of Gaborone NO2 column levels was seemingly under the influence of the South African Highveld. Besides, its local emissions the city also received long-range transport air masses from the Indian Ocean thus getting influenced. The study highlighted the phenomenon of transboundary air pollution over Southern Africa cities and brought to foe the need to adopt a uniform Southern Africa policy and guidelines on air quality management.