Retrieval of aerosol optical thickness and size distribution from the CIMEL Sun photometer over Inhaca Island, Mozambique

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. Uthui 5 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 Angström 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 mmatt a = 0.42 to 0.14 mm att a = 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 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 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 atmospheric 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 SAF 45-1

SAF 45-2 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 Angström 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 (<0.15), are investigated. For comparison, analyses of seasonal variability of aerosol optical thickness and Angström exponent from Mongu in Zambia and Bethlehem in South Africa are made. 2. Instrumentation and Methods [5] Aerosol optical thickness measurements were made at Inhaca Island Biological Station (26 02 0 S; 32 54 0 E; elevation 73 m), off the east coast of southern Africa, between April and November 2000. The site is located 32 km east of Maputo city, capital of Mozambique (approximately 1 million inhabitants). No other significant sources of pollution are present in the proximity of the site. Two other stations with identical instruments and data processing were considered in this paper: Mongu in Zambia (15 15 0 S; 23 09 0 E; elevation 1107 m) and Bethlehem in South Africa (28 14 0 S; 28 19 0 E; elevation 1709 m) (Figure 1). [6] Measurements were made with a CIMEL Sun photometer, which makes two important solar extinction measurements, direct sun and diffuse sky radiances. The direct Sun radiances are made every 15 min in eight spectral channels 340, 380, 440, 500, 675, 870, 940 and 1020 nm (nominal wavelengths). Sun radiances are acquired in approximately 10 s across the eight spectral bands. A sequence of three such eight band measurements are taken 30 s apart to yield triplet observations in each wavelength [Holben et al., 1998]. These solar extinction values are then further used to compute aerosol optical thickness at each wavelength. The uncertainties related to these measurements have been estimated to be approximately 0.01 0.02 [Eck et al., 1999]. The 940 nm channel is used to retrieve total precipitable water. Sky radiance almucantar measurements at 440, 675, 870 and 1020 nm in conjunction with the direct sun measured AOT at the same wavelengths were used to retrieve aerosol size distribution following the methodology of Dubovik and King [2000]. An almucantar is a series of measurements taken at the elevation angle of the Sun for specified azimuth angles relative to the position of the Sun [Holben et al., 1998]. The data presented here have been quality- and cloud-screened following the methodology of Smirnov et al. [2000]. The retrieved aerosol volume size distribution corresponding to days of high (>0.4) and low (<0.15) AOT have been analyzed in order to investigate the dominant particle sizes under both scenarios. Figure 1. Regional map of southern Africa showing the location of the three AERONET sites used in this study. [7] The Angström exponent a, an indirect measure of aerosol size, is calculated as ln t870 a t 440 a a ¼ ; ln 870 = 440 where t a is the aerosol optical thickness and the superscripts denote at 870 and 440 nm. 3. Summary of Results 3.1. Magnitude of the Aerosol Optical Thickness Over Inhaca Island [8] Daily average values of AOT at 500 nm between April and November 2000 range from 0.05 to 1.12 (Figure 2). The overall average for the entire period of observations is 0.26 with a standard deviation of ±0.19. A high degree of optical variability is evident over Inhaca Island. This variability is largely attributed to the diverse contributions of aerosol impacting the tropospheric column above the Island and are likely to include sources such as stationary and fresh sea-spray, local and transported wind-blown dust, and long-range transport of industrial and biomass burning emissions from the subcontinent. [9] AOT values show two distinct periods through the year, with low values recorded between April and July and high loadings from August to November. The average AOT value during the first period was 0.18 ± 0.11. AOT nearly doubles over Inhaca Island during the high value period of the year to an average of 0.34 ± 0.22. The increased frequency of high turbidity and data gaps associated with the presence of cloud contribute to the large standard deviation in the second period and also suggest a strong contribution of biomass burning to the atmospheric aerosol.

QUEFACE ET AL.: AOT OVER INHACA ISLAND, MOZAMBIQUE SAF 45-3 Table 1. Frequency Distribution of Aerosol Optical Thickness (500 nm) at Inhaca Island for the Entire Period of Observation, 195 Days Cumulative Frequency, % Frequency, % Bin, AOT 0.0 0.0 0.05 13.8 13.8 0.1 50.2 36.4 0.2 68.1 17.9 0.3 83.5 15.4 0.4 93.2 9.7 0.6 97.8 4.6 0.8 98.8 1.0 1 99.8 1.0 1.2 99.8 0.0 1.4 Biomass burning does not have a constant source strength or fixed spatial location. For example, Eck et al. [2003] have shown an episode of increased turbidity related to biomass burning from 2 6 September 2000 over the entire subcontinent as far south as about 27 S. The plume of smoke was advected from central Zambia southeast over southern Africa, exiting the subcontinent over Inhaca Island. [10] During the measurement period at Inhaca Island, AOT (500 nm) values ranged between 0.1 1.2, with a modal value of 0.2 (Table 1). Values greater than 0.4 occurred 16.3% of the time, while 33.3% of values fell between 0.2 0.4. For the most part aerosol loadings were equal to or below 0.2 (50.2%). The levels of aerosol optical thickness at Inhaca Island are frequently characteristic of a polluted atmosphere. When compared to measurements reported in the literature for areas other than Southern Africa, the overall average AOT (0.26) is significantly higher than 0.08 reported by Holben et al. [2001] for both the unpolluted marine air at the AERONET Island sites of Lanai, Hawaii (20 49 0 N; 156 59 0 W; elevation 80 m) and San Nicholas Island, California (33 15 0 N; 119 29 0 W; elevation 133 m). The aerosol load at Inhaca Island is comparable with other Island sites with influences of a variety of continental aerosol sources (Table 2) [Holben et al., 2001]. 3.2. Seasonal Variation of AOT Over Inhaca Island [11] Monthly averages of AOT (500 nm) over Inhaca Island range from 0.15 to 0.52 (Figure 3). These values are well above the aerosol background level, defined as less than 0.10 (AOT (500 nm)). The monthly averages show highest aerosol loadings during September and October of 0.42 and 0.52 respectively, followed by August and November with 0.25 each. This corresponds to the biomass burning season in southern Africa and confirms the impact of this Figure 2. Daily average time series of aerosol optical thickness (500 nm) at Inhaca Island during the period April November 2000. source on regional scale turbidity as described in the previous section. July represents the month with the lowest overall aerosol loading. This is not observed at Bethlehem, the site most closely related to Inhaca in terms of atmospheric circulation. It is unclear why this would occur at Inhaca during July. [12] AOT values do not vary significantly between April and June. These months represent the best time of the year to evaluate impacts at the site from non-biomass burning sources. AOT values range from 0.15 to 0.19 during this period. Studies on optical properties of the atmospheric aerosol at five other clean maritime environments by Smirnov et al. [2002] found the AOT values fall between 0.07 and 0.08. It seems reasonable to suggest therefore that Inhaca Island is influenced by additional continental aerosol sources. This will be investigated further later in this paper. 3.3. Comparison to Other Sites [13] Monthly averages of AOT (500 nm) at Mongu, Inhaca and Bethlehem demonstrate similar seasonal trends, with significant increases in tropospheric aerosol loading during the biomass burning season (Figure 4). The highest monthly averages were recorded in September for Mongu (t a = 0.89) and October for both Inhaca (t a = 0.52) and Bethlehem (t a = 0.31). The three sites had overall mean AOT values of 0.33 ± 0.32, 0.26 ± 0.18 and 0.14 ± 0.14 respectively. These results clearly indicate a north to south gradient in AOT. The slight delay in highest AOT values at Inhaca and Bethlehem probably relates to the southward migration of biomass burning toward the end of austral spring in southern Africa [Cahoon et al., 1992]. Scholes et al. [1996] and Justice et al. [1996] also reported a strong north-south gradient of the amount of biomass burned in southern Africa, primarily due to a north-south gradient in Table 2. Aerosol Optical Parameters for the Different Locations Site Location t a500,nm a Lanai, Hawaii 20 49 0 N; 156 59 0 W; elevation 80 m 0.08 0.71 San Nicolas, California 33 15 0 N; 119 29 0 W; elevation 133 m 0.08 1.13 Bermuda 32 22 0 N; 64 41 0 W; elevation 10 m 0.14 0.92 Dry Tortugas, Florida 24 36 0 N; 82 47 0 W; elevation 0 m 0.18 1.12 Kaashidoo, Maldives 04 57 0 N; 73 27 0 W; elevation 0 m 0.20 0.82 Inhaca, Maputo 26 02 0 S; 32 54 0 E; elevation 73 m 0.26 1.22 Bahrain 26 19 0 N; 50 30 0 W; elevation 0 m 0.32 0.95

SAF 45-4 QUEFACE ET AL.: AOT OVER INHACA ISLAND, MOZAMBIQUE Figure 3. Monthly average of aerosol optical thickness at Inhaca Island during the period April November 2000. rainfall and thus a north-south gradient in vegetation production [Eck et al., 2001]. [14] From April to June the AOT values over Inhaca Island are well above values at the other two sites, where background aerosols were registered (t a <0.1). This adds credence to the idea that other continental aerosol sources impact over Inhaca Island. [15] Statistical analysis of the frequency histogram of AOT gave a similar modal value of about 0.2 for Mongu and Inhaca, while Bethlehem had a narrow range with a modal value of about 0.1 (Figure 5). The highest aerosol content of the sites considered occurred in Mongu where 27% of observations over 223 days were above 0.4, followed by Inhaca with 16% and Bethlehem (5.6%). In fact, Bethlehem is very clean compared with the other two sites. 55% of observations (193 days) at Bethlehem had AOT equal or less than 0.1. 3.4. Aerosol Size Distribution Characteristics 3.4.1. Analysis of Angström Exponent Over Inhaca Island [16] The Angström exponent parameter (a) provides some basic information about the aerosol size distribution [Eck et al., 1999; O Neill et al., 2001]. At Inhaca Island, daily averaged a (440 870 nm) ranged from 0.12 (2 July) to 1.96 (1 October), with an average of 1.22 ± 0.48 (Figure 6). It is noted that the Angström exponent computed over a long wavelength interval is sensitive to the relative Figure 5. Frequency of occurrence histogram of aerosol optical thickness (500 nm) at (a) Inhaca, (b) Mongu, (c) Bethlehem for entire data record during 2000. contribution of fine to coarse mode aerosol. If the Angström exponent is calculated over a short wavelength interval, for example 340 440 or 380 500 nm, it will only be sensitive to the influence on atmospheric optical properties of the fine mode particles [Eck et al., 1999]. In general when the a is less than 1, coarse mode particles dominate and for an a greater than 1, accumulation mode particles exert the greatest influence. [17] In this study it was found that for approximately 71% of all observations, the Angström exponent was above 1 over Inhaca Island. In addition, the Angstrom exponent frequency distribution (Table 3) for the entire period of observation ranged between 0.2 and 2. This shows that the tropospheric aerosol loading had a diverse number of contributing sources. The dominant range is 1.4 1.8 with a modal value of 1.6, indicating that the aerosol size distributions are for the most part, dominated by fine particles. [18] Monthly average Angström exponent values at Mongu and Bethlehem are well above those recorded at Figure 4. Monthly averages of aerosol optical thickness at Inhaca, Mongu and Bethlehem during the period April November 2000. Figure 6. Daily average time series of Angström exponent at Inhaca Island during the period April November 2000.

QUEFACE ET AL.: AOT OVER INHACA ISLAND, MOZAMBIQUE SAF 45-5 Table 3. Frequency Distribution of Angström Exponent at Inhaca Island for the Entire Data Record, 195 Days Cumulative Frequency, % Frequency, % Bin, AOT 0.0 0.0 0.1 1.5 1.5 0.2 7.1 5.6 0.4 12.7 5.6 0.5 14.8 2.1 0.6 21.5 6.7 0.8 28.7 7.2 1 37.9 9.2 1.2 55.3 17.4 1.4 75.3 20.0 1.6 93.2 17.9 1.8 99.9 6.7 2 99.9 0.0 2.2 Inhaca Island (Figure 7). The relatively low values over Inhaca (0.91 in July to 1.43 in October) can be associated with the influence of coarse mode maritime particles and the aging process of aerosols being transported from different continental sources. Eck et al. [2003], studying the optical characteristics of biomass burning during August September 2000 across southern Africa, also reported the lowest Angström exponent for Inhaca Island. This lower Angström exponent may have resulted from the additional contribution of aeolian dust to the aerosol mixture and also of sea salt at Inhaca Island. [19] The Angström exponent at Mongu varied considerably less than values derived for both Inhaca Island and Bethlehem. This is indicative that tropospheric aerosols at Mongu are similar throughout the year. This is not an unreasonable argument as the most important source of domestic energy production is derived through burning of wood. The emissions from domestic biofuel usage and biomass burning will probably not vary significantly and therefore the aerosols do not change in nature. [20] High values of a at Bethlehem (1.61 ± 0.32) indicate that the aerosol loading, despite being relatively low, is influenced by emissions from combustion processes. Accumulation mode aerosol sources in the region include the transport of industrial emissions southward to Bethlehem from the South African Highveld. It is likely that a mixed dust/industrial aerosol dominates the atmospheric load at Bethlehem. Uniquely coarse mode aerosols are more likely to impact at both Mongu and Inhaca Island (Figure 8). The source of these aerosols is likely to be wind blown dust from Namibia and sea salt aerosols, respectively. [21] The obvious influence of fine aerosols at Inhaca Island (a > 1) indicates that the sea salt is also mixed with combustion aerosols. The impact of these fine mode aerosols occurs 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 city or long-range transport of industrial emissions from the South African Highveld. Freiman and Piketh [2003] have shown that emissions from the industrialized Highveld are transported directly toward the Indian Ocean over Inhaca Island approximately 40% of the time. An alternative but unquantified source of fine aerosols at Inhaca may be emissions of dimethyl sulphide (DMS) from the surrounding ocean. [22] Fine aerosols are the most dominant at Mongu, where the Angström exponent had a modal value of about 2.0 (Figure 8b). This result is characteristic of small particles from biomass burning and confirms previous studies [Scholes et al., 1996; Herman et al., 1997; Eck et al., 2001; Holben et al., 2001; Eck et al., 2003] that the center of biomass burning activity is north of Zambia. 3.4.2. Aerosol Size Distribution Over Inhaca Island [23] The retrieved aerosol volume size distributions for different high and low aerosol optical conditions differ markedly, especially in the fine mode (Figure 9). The distribution on 2 July 2000, with smaller Angström exponent (a = 0.12) and low aerosol optical thickness (t a = 0.16), indicates the dominance of coarse mode particles with the maximum at about 5 mm. This would be associated with coarse mode maritime (salt) particles. In contrast, the distribution on 5 September 2000, when the highest aerosol optical thickness (t a = 1.12) was recorded at this site and the Figure 7. Monthly averages of Angstrom exponent at Inhaca, Mongu and Bethlehem during the period April November 2000. Figure 8. Frequency of occurrence histogram of Angstrom exponent at (a) Inhaca, (b) Mongu, (c) Bethlehem for the entire period of measurements during 2000.

SAF 45-6 QUEFACE ET AL.: AOT OVER INHACA ISLAND, MOZAMBIQUE Figure 9. Aerosol volume size distribution at Inhaca Island in 2 July and 5 September 2000. Angstrom exponent was high (a = 1.61), indicates a very high concentration (dv/dlnr = 0.13) of fine particles with a peak radius at 0.14 mm and a relatively low concentration of coarse mode particles with a peak radius of 3 mm. [24] The two size distributions observed clearly indicate the presence of both fine and coarse mode aerosols over Inhaca Island. This result leads to further investigation of possible sources for the fine mode aerosol. Two hypotheses will be considered. The first hypothesis is that low AOT may be related to clean marine air with mainly coarse maritime particles. The second hypothesis is that high AOT values are linked with fine particles from urban/ industrial sources and biomass burning aerosols. These assumptions are tested by examining the main pathways of air arriving at Inhaca Island for selected cases. [25] The first case considered is 2 July 2000 when AOT was low and coarse mode aerosols dominated. Three day backward trajectories (Figure 10a) demonstrate a maritime flow at all three levels considered (850, 700 and 500 hpa). These results support the first hypothesis. In contrast backward trajectories on 5 September 2000 (Figure 10b) show continental flow in the middle troposphere (700 hpa and 500 hpa), while maritime flow was observed at 850 hpa. The size distribution data has two modes: one in the fine fraction, which can be linked with the continental flow, and one in the coarse fraction, which can be associated with the low level maritime flow. The continental flow shows that the trajectories passed directly over the industrialized Highveld. It therefore seems that the continental aerosols would Figure 11. Aerosol volume size distribution at Inhaca Island for different aerosol optical thickness at 500 nm. mainly have been derived from power plants and industries. Biomass burning may have contributed as well. The possibility exists that both source categories contributed to the observed high AOT in the fine mode and the second hypothesis can be verified. [26] Once the hypotheses were verified for the two extreme cases, six additional days with high (AOT > 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 att a = 0.42 to 0.14 mm at t a = 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 SAF 45-7 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 (r 2 = 0.36). For the rest of the monitoring period no relationship could be found between these two parameters (r 2 = 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 (r 2 = 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 (w o ) during high AOT (=0.4 at 440 nm) episodes show high absorption with strong spectral dependence. The w o values ranges between [w o (440) 0.89 0.87] to [w o (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 a b fossil fuel combustion [Dubovik et al., 2002]. The absorption occurring on 25 May 2000, evident from the low w o 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 [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 Angström 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 Angström exponent versus the precipitable water vapor for entire period of measurements at Inhaca Island.

SAF 45-8 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 SAF 45-9 the Angström 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. References Cahoon, R. C., B. J. Stocks, J. S. Levine, W. R. Cofer, and K. P. O Neil, Seasonal distribution of African savanna fires, Nature, 359, 812 815, 1992. Dubovik, O., and M. D. King, A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements, J. Geophys. Res., 105, 20,673 20,696, 2000. Dubovik, O., B. N. Holben, T. F. Eck, A. Simirnov, Y. J. Kaufman, M. D. King, D. Tanré, and I. Slutsker, Variability of absorption and optical properties of key aerosol types observed in worldwide locations, J. Atmos. Sci., 59, 590 608, 2002. Eck, T. F., B. N. Holben, J. S. Reid, O. Dubovik, A. Smirnov, N. T. O Neill, I. 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