A number of satellite and model products are available through NASA's
Giovanni Data and Information Services Center
(http://disc.sci.gsfc.nasa.gov/giovanni). We focus here on products
related to atmospheric particulates, presenting decadal averages (1 January
2000 to 31 December 2009 or some multi-year subset, depending on data
availability) of Aerosol Optical Depth (AOD or τ), Ångström
Exponent (α), Ultraviolet Aerosol Index (UVAI), and chemical component
contributions to AOD from The Goddard Chemistry Aerosol Radiation and
Transport (GOCART) model. Each of these parameters describes a unique aspect
of the aerosol column, and results will be presented as follows: (i) a brief
description of the parameter, (ii) general regional and seasonal trends, and
(iii) discussion of those trends, incorporating other data platforms to
support conclusions where appropriate. Main conclusions are synthesized in
Sect. , and satellite data are summarized in
Table .
Aerosol Optical Depth (AOD)
AOD550 from the MODIS (Aqua and Terra) and AOD555 MISR satellite
platforms, and total AOD predicted by GOCART are presented in
Fig. . AOD is a measure of the reduction of intensity of light
due to scattering and absorption of particles along path length, and is
defined for a specific height (z) and wavelength (λ) according to
the Beer–Lambert law:
F(z,λ)FTOA(λ)=exp(-τ(z,λ)),
where transmittance, or F(z,λ)FTOA(λ), is the
ratio of radiation intensity at height z to intensity at the top of
atmosphere (TOA), for a given wavelength λ. Here, z is the entire
atmosphere path length between the ground and TOA, and λ is 550 nm
for both MODIS platforms (Aqua and Terra) and 555 nm for MISR. For
simplicity, MISR AOD (555 nm) has been converted to equivalent AOD550
e.g., and simply referred to as AOD. AOD is a function
of particle extinction efficiency (Qext), which depends on both
particle size and absorption efficiency, and particle number concentration.
Low values of AOD (< 0.1) correspond to little light attenuation through
the atmosphere – meaning relatively clean conditions with low particle
concentrations, few large particles, and aerosol number distributions
comprised primarily of small, non-absorbing particles. High values of AOD
(> 0.3) correspond to significant light attenuation in the atmosphere
– meaning high concentrations of large particles such as dust, high
concentrations of absorbing aerosol such as black carbon at any size, or some
combination of the two. Typical values of AOD range from 0.05 in remote,
clean environments to 1.0 in areas of extreme particle concentrations
.
Monthly averaged FRP integrated over 0.5 × 0.5∘
boxes in the Southern Africa Domain over the period 1 January 2000 to 31 December 2009 from MODIS thermal anomaly retrieval.
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Discrepancies in AOD between satellite platforms are generally within
uncertainty, and arise due to differences in factors such as processing
methods , calibration and retrieval algorithms
, and even measurement time (MODIS-Terra and MISR pass over
South Africa between 09:00 and 11:00 LT, while MODIS-Aqua passes over between
13:00 and 15:00 LT). In this study, AOD from MODIS Aqua and Terra are
well-correlated with AOD from MISR, with Pearson's R correlation coefficients
(r)>0.7 (not shown), except in Cape Town and Bloemfontein (r=0.36 and
0.59, respectively). Total AOD modeled by GOCART correlates well with all
satellite platforms in all sites (r>0.7). In Cape Town and Durban, MISR
AOD appears significantly higher than either MODIS platform or GOCART. The
surface at both of these sites displays a great deal of heterogeneity, with
satellite grid boxes extending over both bright land and dark ocean surfaces.
While estimated biases between MISR and MODIS are expected to be low over
uniformly dark surfaces , assumptions about surface
reflectivity tend to introduce significant discrepancies with increased
surface heterogeneity . MISR is less sensitive to surface
characteristics, and so in regions with significant heterogeneity such as
Cape Town or Durban, one may expect AOD from MISR to be more accurate.
Considering Fig. , AOD is consistently lower in Cape Town than
other major metropolitan areas of South Africa, owing to frequent air-mass
origin over clean marine areas and minor influence by large or absorbing
aerosols. At continentally influenced sites (Bloemfontein, Gauteng, HVAPA,
and Durban), there is a maximum in AOD between 0.12 and 0.20 from late winter
to early spring. AOD is moderate through summer, averaging 0.08–0.17, and
reaches a minimum during late fall and early winter (MJJ), with AOD ranging
from 0.05–0.10. AOD is particularly high in magnitude and variability in
Durban between August and October, with MISR AOD showing monthly averages
approaching 0.3. The seasonal trends evident in Fig. can be
explained by considering annual variability in (1) biomass burning and (2)
dust generation. The impact of these parameters on AOD is explored below.
The first and most distinct annual feature across South African metropolitan
areas is the cycle in AOD, from minima during late spring and early winter
(May–July) to maxima from August to October. This maximum coincides with the
burning season in Southern Africa, which has been identified as major
regional source of transported aerosol in South Africa . Figure presents average monthly integrated
fire radiative power (FRP) per km2 from 2000–2010 in Sub-Saharan
Africa, and indicates that the most significant burning in Southern Africa
occurs between May and September and is located in the tropics (between 0 and
20∘ S). These fires are massive sources of biomass burning emissions
that are often transported in stratified layers aloft ,
and this transport significantly impacts aerosol optical properties in South
Africa .
Winkler et al. () demonstrated that transported
biomass burning emissions represent the dominant source of irradiation losses
during late winter and early spring in South Africa, with the majority of
scattering attributable to carbonaceous aerosols . Queface
et al. () indicated that transported fire emissions
represent the bulk of column aerosol loading between August and October.
Strong north-to-south gradients in AOD are observed during the burning
season, with significantly higher AOD near sources, where aerosols are small,
but highly absorbing and concentrated . Lower AOD is typically observed at downwind
receptor sites after plumes have evolved to comprise less absorbing and
predominantly Aitken and accumulation mode particles with sufficient
condensed water-soluble secondary aerosol species for the particles to serve
as cloud condensation nuclei (CCN) .
On the basis of strong N–S gradients in biomass-burning-related AOD, and the
finding that AOD is highest nearest sources (above), it follows that AOD in
South Africa is enhanced most significantly by close-proximity fires within
South African borders and in neighboring Mozambique and Zimbabwe. These
close-proximity fires reach their peak frequency in August, September, and
October and coincide with the maxima in AOD observed across all
continentally influenced sites in this study. Because AOD is highest in
closest proximity to fires, the influence of biomass burning on AOD is
expected to be strongest nearer sources in eastern South Africa. Proximity
explains the high magnitude and variability of AOD in Durban between August
and October, where very close proximity sugar cane field burning typically
occurs, releasing high and variable concentrations of absorbing biomass
burning particles . Periodically compounding the impact
of proximity in eastern South Africa is the so-called “river of smoke”
phenomenon , whereby synoptic air flows are occasionally
favorable for the transport of subtropical biomass burning into a narrow
latitudinal region over the northeastern part of the country – the area that
is also closest to high frequency and intensity of seasonal fires.
Monthly averaged values of RH, accumulated rainfall, wind speed, and
temperature for sites in each satellite study area for the period 1 January 2000
to 31 December 2009. Note different y scales for each site.
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In addition to coinciding with biomass burning, elevated AOD in August and
September is correlated with enhanced generation of windblown dust across
South Africa. Ground-based meteorology data from South African Weather
Service (SAWS) monitoring sites in each region are presented in
Fig. , and indicate that in all regions but Cape Town,
wind speeds increase in July and August before an enhancement in
precipitation in September and October. High winds combined with dry
conditions result in generation of wind-blown dust particles, which are both
large and strongly absorbing and thereby have strong bearing on AOD. Enhanced
AOD coincides with dust generation during August and September, and we
therefore suggest that dust may be an additional, previously underestimated
factor influencing AOD across South Africa.
Average monthly column water vapor (CWV, black line) for the period
1 January 2000 to 31 December 2009, with gray shading representing minimum and
maximum monthly CWV during that period.
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Column water vapor (CWV; Fig. ) derived from the Modern
Era-Retrospective Analysis for Research and Applications
MERRA; was analyzed for each site in order to
determine whether enhanced water vapor might be a factor contributing to
elevated AOD through a mechanism of hygroscopic growth of particles. No significant correlation was found between AOD and CWV at any site,
supporting findings in Kumar et al. (), which found no
relationship between AOD and CWV at a site in rural, northeastern South
Africa.
Overall, AOD values reported here are consistent with those reported by
Tesfaye et al. () based on broad, regionally averaged
10-year MISR AOD trends for northern, central, and southern regions of South
Africa. The observation of enhanced AOD associated with transported biomass
burning aerosol also agrees with similar trends for central and northern
South Africa in that work. But overall, AOD is lower than many
previously published values over Southern Africa. Queface et al. () reported long-term, monthly averaged AOD ranging
from 0.10 to over 0.60 at a site near dense biomass burning sources in Zambia,
and monthly averages between 0.17 and 0.30 at a site in northeastern South
Africa near the Mozambique border. A strong N–S gradient in AOD is typically
observed during biomass burning season in Southern Africa ,
with sites to the north experiencing significantly higher AOD than sites in
the south due to closer proximity to sources. This suggests that while
transported biomass burning aerosol has a clear impact on AOD across the
major metropolitan areas of South Africa (except Cape Town), this impact is
nonetheless much less pronounced than for sites farther to the north and
closer to denser burning sources.
As AOD is frequently used as a proxy for megacity anthropogenic particulate
pollution, it is important to view results from South Africa within the
context of worldwide megacities. Considering the major metropolitan and
industrial areas of Gauteng and HVAPA, seasonally averaged AOD ranged from a
minimum of 0.12 in winter to a maximum of 0.17 in spring – values that are a
factor of 3–6 lower than those typical of megacities and major industrial
areas worldwide. Annual averages of AOD in industrialized and urban areas of
Hong Kong and China such as Nanjing, the North China Plain, and Yangtze
River are on the order of 0.6–0.8 . AOD in Mexico City ranges from a seasonally averaged minimum of
0.2 to a maximum of 0.6 , and in the Dheli region of India
AOD ranges from a seasonal minimum of 0.4 to a maximum during monsoon season
of 1.4. So while the Gauteng area and HVAPA areas are characterized as
a megacity on the basis of remotely sensed column NOx
, AOD is significantly lower than typical
megacity areas.
Ångström exponent (<inline-formula><mml:math display="inline"><mml:mi mathvariant="italic">α</mml:mi></mml:math></inline-formula>)
Monthly averaged Ångström exponent (α) from the MODIS Aqua and
Terra platforms is presented in Fig. . α captures
the spectral dependence of aerosol extinction based on measurements of AOD at
two different wavelengths:
α=-dlogτdlogλ=-log(τ1τ2)log(λ1λ2),
where τ1,2 are AODs at wavelengths λ1,2. For
small particles in the Rayleigh scattering regime (Dp<0.1 µm), AOD
scales with wavelength as (1λ3 to 1λ4), and α>2
where fine particles dominate. Large particles, on the other hand, are more
spectrally neutral and the lack of wavelength dependence of scattering and
absorption results in 0<α<1 where coarse particles dominate.
α is reported for both MODIS platforms, with Aqua representing
spectral dependence of AOD between λ=550 and 865 nm and Terra
between λ=470 and 660 nm.
Monthly averaged values of Ångström Exponent (α) for each region for the period 1 January 2000 to 31 December 2009.
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α is quantized in such a way that it cannot be used as a quantitative
metric for aerosol size integrated through the column . Instead, we interpret α results as being qualitatively
indicative of the aerosol type present in the column – either fine (values
close to 2) or coarse (values between 0 and 1). Due to the qualitative nature
of the variable, combined with the fact that α550/865 and
α470/660 are highly correlated (r>0.8) at all sites, we will
refer to α without specifying wavelength.
Considering Fig. , α shows trends similar to AOD
(Fig. ). Generally there is a maximum in α of 1.0–1.8
during spring and through summer, gradually decreasing through fall to reach
a minimum of 0.6–1.0 during winter. This suggests that fine particles are
most abundant in the column in spring when biomass burning impacts South
Africa, agreeing with previous results suggesting that aerosol plumes
transported to South Africa tend to be characterized by mainly Aitken mode
(0.01–0.1 µm) aerosol . α remains high through the hot, photochemically intense
summer months, when significant fine, secondary sulfate, nitrate, and organic
aerosol is produced from abundant industrial emissions of SO2,
NOx, and volatile organic compounds (VOCs), respectively , and when as many as 86 % of days in industrial areas display
new particle formation . This is in contrast to the dry
winter months, when α is low and it appears as though particles above
the major metropolitan areas of South Africa tend to be larger – suggesting
a greater contribution from dust or large soot particles. α is
relatively constant throughout the year in Cape Town, indicative of a
consistent aerosol size distribution resulting from clean, marine air-mass
origin and fairly constant local generation of marine aerosol. At the other
coastal site (Durban), α is similarly high throughout the year, but
displays a strong seasonal biomass burning signal in spring owing to its
location near major sources of small biomass burning particles in local
sugar cane fields and in Zimbabwe and Mozambique.
In addition to a clear impact on α by transported biomass burning
emissions, maxima and minima in α are highly correlated with CWV in
all regions except Cape Town (Fig. ; 0.64<r<0.98).
This result agrees with Kumar et al. (), who found that
CWV correlated well with α, but not AOD. The relationship between CWV
and α is unlikely to be causal, but rather coincidental since wet
summer months tend to have suppressed dust generation (fewer large particles and
higher α) and enhanced secondary photochemical aerosol production
(more small particles and higher α). Overall, α for inland
sites (Bloemfontein, Gauteng, HVAPA) is significantly lower on average than
for coastal sites (Cape Town and Durban), with α<1 for much of the
year. This supports previous findings that coarse particles such as dust form
an important contribution to column aerosol loading, and appear to dominate
the aerosol size distribution at inland sites .
Values of α reported here are lower than previously reported in
Southern Africa, primarily because major studies of aerosol optical
properties in the region have focused on intensive periods of biomass
burning. Eck et al. () found high α near biomass
burning sources in Zambia (1.8–1.9) during biomass burning season from
August-October, while Robles-Gonzalez, et al. () measured α as high as 2.1 in burning
regions. Kumar et al. () found strong seasonal
variability at a site in northeastern South Africa, with high α (up to
2.9) during burning season, but much lower α (0.5) during the rest of
the year. Ranges of α similar to those reported here have been
observed in urban and industrial sites in China (ranging from 0.5–1.6 in Qi
et al. (), with an annual average of 1.25 in Zhuang et al. ().
Monthly averaged values of UVAI for each region for the period 1 January 2000 to 31 December 2009.
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