Abstract
We present a comprehensive overview of particulate air quality across the five major metropolitan areas of South Africa (Cape Town, Bloemfontein, Johannesburg and Tshwane (Gauteng Province), the Industrial Highveld Air Quality Priority Area (HVAPA), and Durban), based on a decadal (1 January 2000 to 31 December 2009) aerosol climatology from multiple satellite platforms and detailed analysis of ground-based data from 19 sites throughout Gauteng Province. Satellite analysis was based on aerosol optical depth (AOD) from MODIS Aqua and Terra (550 nm) and MISR (555 nm) platforms, Ångström Exponent (α) from MODIS Aqua (550/865 nm) and Terra (470/660 nm), ultraviolet aerosol index (UVAI) from TOMS, and results from the Goddard Ozone Chemistry Aerosol Radiation and Transport (GOCART) model. At continentally influenced sites, AOD, α, and UVAI reach maxima (0.12-0.20, 1.0-1.8, and 1.0-1.2, respectively) during austral spring (September-October), coinciding with a period of enhanced dust generation and the maximum integrated intensity of close-proximity and subtropical fires identified by MODIS Fire Information for Resource Management System (FIRMS). Minima in AOD, α, and UVAI occur during winter. Results from ground monitoring indicate that low-income township sites experience by far the worst particulate air quality in South Africa, with seasonally averaged PM10 concentrations as much as 136 % higher in townships that in industrial areas. We report poor agreement between satellite and ground aerosol measurements, with maximum surface aerosol concentrations coinciding with minima in AOD, α, and UVAI. This result suggests that remotely sensed data are not an appropriate surrogate for ground air quality in metropolitan South Africa.
Highlights
The impacts of aerosols on human health, visibility, and climate are well documented
First we present remotely sensed and modeled column aerosol data related to total light extinction (AOD), particle size (Ångström Exponent, α), total light absorption, and aerosol optical depth (AOD) contribution from individual chemical components (Goddard Ozone Chemistry Aerosol Radiation and Transport, GOCART)
Rather than define study areas based on large geographical regions that require significant data averaging (e.g., Tesfaye et al, 2011), we focused our study of remotely sensed data on five smaller study areas that comprise the five major metropolitan and centers of population density in South Africa (Cape Town, Bloemfontein, Gauteng Province, Industrial Highveld Air Quality Priority Area or HVAPA, and Durban; Fig. 1)
Summary
The impacts of aerosols on human health, visibility, and climate are well documented. There are a number of natural sources of dust, sea salt, sulfate, and organic aerosol, and anthropogenic aerosol primarily derives from fuel combustion. Combustion aerosol can be directly emitted as soot or formed in the atmosphere through photochemical reactions that produce low volatility, secondary sulfate, nitrate, and organic species that either nucleate to form new aerosol or condense onto existing particles. Exposure to particulate pollution is typically worst in urban areas, where anthropogenic emissions are collocated with high population density. The lowest-income fraction of the population tends to experience the worst air quality because the least expensive property (or land available for informal settlements) is often close in proximity to major air pollutant sources like highways, power generation facilities, or industry.
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