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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D13, 8509, doi:10.1029/2002JD002374, 2003

Retrieval of aerosol optical thickness and size distribution from the CIMEL Sun photometer over Inhaca Island, Mozambique Antonio J. Queface,1,2 Stuart J. Piketh,1 Harold J. Annegarn,3 Brent N. Holben,4 and Rogerio J. Uthui5 Received 26 March 2002; revised 12 September 2002; accepted 7 March 2003; published 15 July 2003.

[1] Characterization of aerosol optical properties over southern Africa is needed to better

understand the impact of aerosols on regional climate change. CIMEL Sun photometer measurements of aerosol optical thickness over Inhaca Island, Mozambique, between April and November 2000 are analyzed. Comparisons with two other sites, Mongu, Zambia, and Bethlehem, South Africa, are made. The aerosol optical thickness observed at Inhaca Island indicates high turbidity. In 50% of the measurements, aerosol optical thickness values are above 0.2, with an overall mean of 0.26 ± 0.19. The Angstro¨m exponent parameter has a wide range from 0.2 to 2, with a modal value of 1.6. This indicates a wide range in particle sizes and the dominance of fine mode aerosols at this site. Data from all three sites reveal seasonal variability, with a significant increase in aerosol content between August and October. This suggests a strong contribution of biomass burning to the atmospheric aerosols content during this time of year, which corresponds to the period of maximum burning in southern Africa. A north to south gradient in aerosol optical thickness is confirmed. The highest aerosol content is observed over Mongu, while Bethlehem has the lowest. The retrieved aerosol volume size distribution over Inhaca Island demonstrates that at high levels of aerosol optical thickness, accumulation mode aerosols dominate. In contrast, coarse mode aerosols dominate when aerosol optical thickness is very low. It is noted that there is a tendency for decreasing particle size as aerosol optical thickness increases, with the peak in distribution of the accumulation mode volume radius decreasing from 0.19 mm at ta = 0.42 to 0.14 mm at ta = 1.12. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; KEYWORDS: aerosol optical properties, SAFARI 2000, biomass burning, single scattering albedo Citation: Queface, A. J., S. J. Piketh, H. J. Annegarn, B. N. Holben, and R. J. Uthui, Retrieval of aerosol optical thickness and size distribution from the CIMEL Sun photometer over Inhaca Island, Mozambique, J. Geophys. Res., 108(D13), 8509, doi:10.1029/2002JD002374, 2003.

1. Introduction [2] The extent of local aerosol perturbations on a global scale is the subject of extensive ground level, airborne and satellite research [Kaufman et al., 1997; King et al., 1999]. Spatially and temporally resolved information on the atmo1 Climatology Research Group, University of the Witwatersrand, Johannesburg, South Africa. 2 Also at Department of Physics, Eduardo Mondlane University, Maputo, Mozambique. 3 Atmosphere and Energy Research Group, University of the Witwatersrand, Johannesburg, South Africa. 4 Biospheric Sciences Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. 5 Department of Physics, Eduardo Mondlane University, Maputo, Mozambique.

Copyright 2003 by the American Geophysical Union. 0148-0227/03/2002JD002374

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spheric burden and radiative properties of aerosol is needed to estimate radiative forcing [Intergovernmental Panel on Climate Change (IPCC), 2001]. Without a comprehensive assessment of present aerosol concentrations and optical properties, it is impossible to measure the change in the aerosol radiative forcing, and thus the impact on climate change [Smirnov et al., 2002]. Field experiments provide the most comprehensive analysis of aerosol properties. The aerosol optical thickness (AOT), which can be derived from measurements of attenuated direct solar radiation, the aerosol size distribution and the single-scattering albedo are the key parameters defining the optical state of the atmosphere [King et al., 1999; Kaufman et al., 1997]. [3] Sun photometer measurements of aerosol optical thickness from Aerosol Robotic Network (AERONET) instruments [Holben et al., 1998] have been made in southern Africa (Zambia, Mozambique, South Africa, Botswana and Namibia). Analyses of these data sets have been done only

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QUEFACE ET AL.: AOT OVER INHACA ISLAND, MOZAMBIQUE

for the highest biomass burning subregion in Zambia [Holben et al., 2001; Eck et al., 2001]. More recently, Eck et al. [2003] analyzed data obtained from 10 sun-sky radiometers distributed throughout southern Africa, during the Southern African Regional Science Initiative (SAFARI 2000) dry season campaign in August –September 2000 [Swap et al., 2002]. [4] In this paper, results of aerosol optical thickness data collected at Inhaca Island, Mozambique from April to November 2000 are presented. Data were obtained by a CIMEL Sun photometer as part of Aerosol Robotic Network (AERONET) of sun-sky radiometers [Holben et al., 1998] under the SAFARI 2000 program. Specifically, aerosol optical thickness, the Angstro¨m exponent a and the temporal variability of these parameters are analyzed. In addition, retrieved aerosol volume size distributions for different optical conditions, high AOT (>0.4) and low AOT (0.4) and low ( 0.4) and low (AOT < 0.15) were selected for size distribution analyses. The retrieved aerosol volume size distributions demonstrate that fine mode aerosols dominate the aerosol load during high AOT case studies (Figure 11). In contrast, coarse mode aerosols dominate the air mass during low AOT. [27] It is noteworthy that there is a tendency for particle size to decrease as aerosol optical thickness increases, with the peak in distribution of the accumulation mode volume radius decreasing from 0.19 mm at ta = 0.42 to 0.14 mm at ta = 1.12. Two of the three days representing high AOT and fine mode dominance occurred in the non-biomass burning season on 22 April and 25 May 2000. It is reasonable to suggest that the high AOT are linked with fine aerosol from urban/industrial emissions between April and June and a mixture of industrial and biomass burning aerosols between August and November. 3.4.3. Aerosol Optical Thickness and Water Vapor Content [28] The relationship between AOT and precipitable water vapor (PWV) at Inhaca Island over the entire period

Figure 10. Three-day backward trajectories to Inhaca Island, on 2 July (a) and 5 September (b).

QUEFACE ET AL.: AOT OVER INHACA ISLAND, MOZAMBIQUE

a

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fossil fuel combustion [Dubovik et al., 2002]. The absorption occurring on 25 May 2000, evident from the low wo values, is linked to urban pollution from Maputo. The source of the absorbing aerosols is low temperature combustion of wood and coal for domestic purposes.

4. Conclusions

b

Figure 12. Time series of daily averages of precipitable water vapor (PWV) and aerosol optical thickness at 440 nm at Inhaca Island, (a) during the non-biomass burning season and (b) during the biomass burning season. indicates little or no direct correlation (Figure 12). The best correlation between AOT and PWV was observed in the biomass burning season, although these were not significant (r2 = 0.36). For the rest of the monitoring period no relationship could be found between these two parameters (r2 = 0.02). In addition a lack of a relationship between PWV and particle size is observed (Figure 13). [29] The AOT and PWV relationship observed at Inhaca is quite different from results observed at Mongu Zambia, where a high correlation (r2 = 0.76) between AOT and PWV was found for predominantly biomass burning aerosols [Eck et al., 2001]. This may explain the higher correlation at Inhaca Island during the period affected by biomass burning aerosols. 3.4.4. Single Scattering Albedo [30] The retrieved spectral single scattering albedos (wo) during high AOT (=0.4 at 440 nm) episodes show high absorption with strong spectral dependence. The wo values ranges between [wo (440)  0.89 – 0.87] to [wo (1020)  0.82 – 0.77]. All selected case study days are associated with air originating over the subcontinent (Figure 14). [31] The observed high absorption at Inhaca Island for the case studies presented can be explained by the presence of biomass burning aerosols transported from south and central southern Africa (3 and 5 September 2000). During 2 – 6 September a huge plume of smoke was identified over Zambia [Eck et al., 2003], air being transported southeastward out over the Indian Ocean. Previous studies [Dubovik et al., 2002; Eck et al., 2001] revealed highest absorption for African savanna regions due to the presence of black carbon in the combustion products from biomass burning. On the other hand, strong absorption has also been reported for urban-industrial aerosols from low temperature

[32] Measurements of spectral optical thickness were made with a CIMEL Sun photometer at Inhaca Island off the east coast of southern Africa from April to November 2000 to characterize the aerosol optical properties over this region. The principal findings of the study are summarized as follows: [33] 1. The aerosol optical thickness at Inhaca Island shows day to day variability in aerosol loading, with 50% of total measurements (195 days) above 0.2, indicating high turbidity in the atmosphere at this location. [34] 2. The levels of aerosol optical thickness observed at Inhaca, with an overall mean of 0.26, are characteristic of a polluted marine atmosphere. Previous studies on aerosol optical properties found mean values of 0.07 – 0.08 for aerosol optical thickness in the clean marine environment. [35] 3. A seasonal variation in monthly average aerosol optical thickness is observed over Inhaca Island. There is a significant increase in aerosol loading during August – October. This suggests a strong contribution of biomass burning to the aerosol content, as August – October is biomass burning season in Southern Africa. [36] 4. Analysis of AOT from Sun photometers located in Mongu, Zambia; Inhaca, Mozambique and Bethlehem, South Africa confirm the north – south gradient in AOT. This is the result of the north to south gradient in vegetation due to higher rainfall in the north. [37] 5. All three sites demonstrate similar seasonal trends in AOT, with significant increases in aerosol content during the biomass burning season, although Bethlehem, the southernmost site, is least likely to be influenced by biomass burning. The highest monthly average AOT in 2000 was recorded in September for Mongu (0.89) and October for both Inhaca (0.52) and Bethlehem (0.31). [38] 6. The Angstro¨m exponent frequency distribution over Inhaca Island ranged between 0.2 and 2. This provides evidence that the tropospheric aerosol loading has a diverse number of contributing sources. For 70% of observations,

Figure 13. Scattergram of Angstro¨m exponent versus the precipitable water vapor for entire period of measurements at Inhaca Island.

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QUEFACE ET AL.: AOT OVER INHACA ISLAND, MOZAMBIQUE

Figure 14. Variation of spectral single scattering albedo retrievals (440 to 1020 nm) (center) and associated atmospheric transport for four case studies from Inhaca Island during 2000.

QUEFACE ET AL.: AOT OVER INHACA ISLAND, MOZAMBIQUE

the Angstro¨m exponent was above 1 over Inhaca Island, suggesting that fine mode aerosols dominate atmospheric turbidity at this site. [39] 7. Impacts of fine mode aerosols over Inhaca Island occur throughout the year. From April to June the source is not biomass burning and must therefore be related either to the urban complex of Maputo or to the long-range transport of industrial emissions from the South African Highveld. [40] Acknowledgments. This work was undertaken in collaboration with Climatology Research Group and Atmosphere and Energy Research Group from Wits University, Department of Physics of Eduardo Mondlane University (UEM) and AERONET in the framework of the SAFARI 2000 initiative. We thank the Sida-SAREC project from Sweden, for supporting longstanding research cooperation with Eduardo Mondlane University. We also thank the Mongu, Zambia site manager, Mukufute Mukulabai and the Bethlehem, South Africa site manager Roelof Burger. The AERONET sites for SAFARI 2000 were funded as part of the NASA NAG5-7266 grant. We would to thank the NOAA Air Resources Laboratory team for providing the HYSPLIT_4 trajectory model online.

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H. J. Annegarn, Atmosphere and Energy Research Group, University of the Witwatersrand, Private Bag 3, WITS 2050, Johannesburg, South Africa. ([email protected]) B. N. Holben, Biospheric Sciences Branch, NASA Goddard Space Flight Center, Code 923, Room G412, Greenbelt, MD 20771, USA. (brent@ spamer.gsfc.nasa.gov) S. J. Piketh and A. J. Queface, Climatology Research Group, University of the Witwatersrand, Private Bag 3, WITS 2050, Johannesburg, South Africa. ([email protected]; [email protected]) R. J. Uthui, Department of Physics, Eduardo Mondlane University, C.P. 257, Maputo, Mozambique. ([email protected])