Nitrogen oxides (NO and NO2) pollution in the Accra metropolis: Spatiotemporal patterns and the role of meteorology

Jiayuan Wang,Abosede Sarah Alli,Sierra Clark,Allison Hughes,Majid Ezzati,Andrew Beddows,Jose Vallarino,James Nimo,Josephine Bedford-Moses,Solomon Baah,George Owusu,Ernest Agyemang,Frank Kelly,Benjamin Barratt,Sean Beevers,Samuel Agyei-Mensah,Jill Baumgartner,Michael Brauer,Raphael E Arku

doi:10.1016/j.scitotenv.2021.149931

Abstract

Economic and urban development in sub-Saharan Africa (SSA) may be shifting the dominant air pollution sources in cities from biomass to road traffic. Considered as a marker for traffic-related air pollution in cities, we conducted a city-wide measurement of NOx levels in the Accra Metropolis and examined their spatiotemporal patterns in relation to land use and meteorological factors. Between April 2019 to June 2020, we collected weekly integrated NOx (n = 428) and NO2 (n = 472) samples at 10 fixed (year-long) and 124 rotating (week-long) sites. Data from the same time of year were compared to a previous study (2006) to assess changes in NO2 concentrations. NO and NO2 concentrations were highest in commercial/business/industrial (66 and 76 μg/m3, respectively) and high-density residential areas (47 and 59 μg/m3, respectively), compared with peri-urban locations. We observed annual means of 68 and 70 μg/m3 for NO and NO2, and a clear seasonal variation, with the mean NO2 of 63 μg/m3 (non-Harmattan) increased by 25–56% to 87 μg/m3 (Harmattan) across different site types. The NO2/NOx ratio was also elevated by 19–28%. Both NO and NO2 levels were associated with indicators of road traffic emissions (e.g. distance to major roads), but not with community biomass use (e.g. wood and charcoal). We found strong correlations between both NO2 and NO2/NOx and mixing layer depth, incident solar radiation and water vapor mixing ratio. These findings represent an increase of 25–180% when compared to a small study conducted in two high-density residential neighborhoods in Accra in 2006. Road traffic may be replacing community biomass use (major source of fine particulate matter) as the prominent source of air pollution in Accra, with policy implication for growing cities in SSA.

Highlights

Air pollution is a major environmental health threat globally, and both the amount and impact are estimated to be highest in Asia and Africa.[1]
Levels were highest in commercial/ business/industrial (CBI) areas, which are dominated by heavy vehicular traffic, followed by sites in high-density residential (HD) residential neighborhoods with relatively less traffic (Figure 2)
Concentrations in low-density residential (LD) residential neighborhoods were lower compared to HD and CBI areas, but were significantly higher than urban background (UB) locations, which registered the lowest values (Figure 2B)

Summary

Introduction

Air pollution is a major environmental health threat globally, and both the amount and impact are estimated to be highest in Asia and Africa.[1]. Similar to developed countries, road traffic emissions may be the dominant source of urban air pollution in SSA cities amid the increasing expansion, motorization, and downward trend in primary biomass use 10,11,13

Methods

Results

Discussion

Conclusion