An overview of regional and local characteristics of aerosols in South Africa using satellite, ground, and modeling data

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 PM 10 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.

The GAW‐PFR aerosol optical depth network: The 2008–2013 time series at Cape Point Station, South Africa

“…They observed an average of 0.08 ( λ = 550 nm) over the Greater Cape Town area, which is ~0.090 at λ = 500 nm (using α 470–660 = 1.18). This value is somewhat more consistent with our value for continental air masses than Hersey et al 's [] study, although a precise comparison is hampered by different temporal and spatial scales. More interestingly, Kumar et al [] observed that the average AOD ( λ = 550 nm) for continental regions was fairly uniform at 0.07–0.08 except for most of the eastern half of South Africa and the Kalahari Desert, reaching ~0.12.…”

“…Monthly AOD averages ranged from~0.15 (λ = 558 nm) in September to~0.07 in June with an annual average~0.10. Hersey et al [2014] examined AOD from 2000 to 2009 at five major metropolitan regions of South Africa, including the Greater Cape Town area. The annual cycle in AOD determined from MODIS and MISR had identical features to the cycle at CPT in Figure 6.…”

“…AOD in the southern part of South Africa was found to strongly depend on air masses from the surrounding marine and neighboring continental regions. A more recent study investigated satellite AOD and ground‐based particulate mass concentrations in five major metropolitan areas of South Africa [ Hersey et al , ]. However, the correlation between both parameters was found to be poor.…”

Analysis of remotely sensed and surface data of aerosols and meteorology for the Mexico Megalopolis Area between 2003 and 2015

“…It is pertinent to first note that the intercomparison between columnar spaceborne aerosol data and surface in situ measurements requires caution due to the different spatial fingerprints of the two types of data. When assuming that the intercomparison is fair to do, it was shown for the greater Tehran area [ Crosbie et al ., ] and South Africa [ Hersey et al ., ] that differences in temporal profiles between AOD and surface PM are due to lower PBLHs concentrating PM in a smaller volume of air leading to higher surface PM, whereas columnar AOD is immune to this issue. In our study region when PBLH is lowest (September–December), the surface PM concentrations exhibit the full range of levels including the minimum (September) and maximum (December) mean value for the entire year.…”

Reconciling satellite aerosol optical thickness and surface fine particle mass through aerosol liquid water

“…Discrepancies between total column AOT and surface estimates of particles (dry mass + ALW) are noted in the literature and are often consequences of complex aerosol vertical structure [ Hersey et al ., ]. For example, particle plumes aloft may enhance AOT but not impact surface measurements [ Campbell et al ., ], as suggested by Ford and Heald, while high PM concentrations in a shallow boundary layer with particle‐free air aloft can have a “low” AOT retrieval [ Crosbie et al ., ].…”