Spatial-temporal levels, variations and sources of NO2/NOx in the sub-Saharan African city of Accra, Ghana

Abstract

Background. As the world’s fastest urbanizing region, rapid economic growth in Sub-Saharan Africa (SSA) is raising levels of urban air pollution from diverse local and regional sources. While measurement data are now emerging from SSA on particulate matter pollution, there is limited information on major combustion related gaseous pollutants. We examined the levels, spatial, seasonal, and socioeconomic patterns as well as sources of oxides of nitrogen (NOx: NO2/NO) in Accra, Ghana’s capital and one of SSA’s largest metropolitan cities.Methods. We collected weekly integrated ambient NO2 and NOx samples using OGAWA passive samplers at 150 sites, consisting of a year-long (n=10) and week-long (n=140) averages which included a range of land-uses and emissions sources (traffic, biomass).Results. Annual mean NO2 across all sites was 52 µg/m3. Both NO2 and NOx concentrations were highest between November and February, the ‘Harmattan’ period when local and regional meteorology is altered, with means of 73 and 218 µg/m3, respectively, comparing to 47 and 116 µg/m3 during non-Harmattan period. The mean NO2 and NOx levels were highest at roadside sites (82 and 285 µg/m3), followed by residential (46 and 102 µg/m3) and sub-urban background sites (27 and 67 µg/m3). There was no difference between high-density (i.e. low-income and high biomass use) and low-density (i.e. high-income and low biomass use) residential sites. The overall sample mean NO2/NOx ratio was 0.53, varying from 0.35 at roadside sites to 0.63 at residential sites.Conclusions. Traffic is the most important contribution for NOx in Accra, while contribution from biomass burning shows no difference at high- and low-income neighborhoods. NO2/NOx ratios are significantly lower than in North American, European and East Asian cities, indicating a less oxidative atmospheric condition in Accra, especially among traffic sites. “Harmattan” increases local pollution level significantly, possibly by altering local and regional meteorology conditions.