Results from a pilot-scale air quality study in Addis Ababa, Ethiopia

摘要

Twenty-one samples were collected during the dry season (26 January-28 February 2004) at 12 sites in and around Addis Ababa, Ethiopia and analyzed for particulate matter with aerodynamic diameter <10 μm (PM_(10)) mass and composition. Teflon-membrane filters were analyzed for PM_(10) mass and concentrations of 40 elements. Quartz-fiber filters were analyzed for chloride, sulfate, nitrate, and ammonium ions as well as elemental carbon (EC) and organic carbon (OC) content. Measured 24-h PM_(10) mass concentrations were < 100 and 40 μg m~(-3) at urban and suburban sites, respectively. PM_(10) lead concentrations were < 0.1 μg m~(-3) for all samples collected, an important finding because the government of Ethiopia had stopped the distribution of leaded gasoline a few months prior to this study. Mass concentrations reconstructed from chemical composition indicated that 34-66% of the PM_(10) mass was due to geologically derived material, probably owing to the widespread presence of unpaved roads and road shoulders. At urban sites, EC and OC compounds contributed between 31% and 60% of the measured PM_(10) while at suburban sites carbon compounds contributed between 24% and 26%. Secondary sulfate aerosols were responsible for < 10% of the reconstructed mass in urban areas but as much as 15% in suburban sites, where PM_(10) mass concentrations were lower. Non-volatile particulate nitrate, a lower limit for atmospheric nitrate, constituted < 5% and 7% of PM_(10) at the urban and suburban sites, respectively. At seven of the 12 sites, real-time PM_(10) mass, real-time carbon monoxide (CO), and instantaneous ozone (O_3) concentrations were measured with portable nephelometers, electrochemical analyzers, and indicator test sticks, respectively. Both PM_(10) and CO concentrations exhibited daily maxima around 7:00 and secondary peaks in the late afternoon and evening, suggesting that those pollutants were emitted during periods associated with motor-vehicle traffic, food preparation, and heating of homes. The morning concentration maxima were likely accentuated by stable atmospheric conditions associated with overnight surface temperature inversions. Ozone concentrations were measured near mid-day on filter sample collection days and were in all cases < 45 parts per billion.