Southern African Regional Science Initiative Research Articles

Micropulse lidar observations of tropospheric aerosols over northeastern South Africa during the ARREX and SAFARI 2000 dry season experiments

During the Aerosol Recirculation and Rainfall Experiment (ARREX 1999) and Southern African Regional Science Initiative (SAFARI 2000) dry season experiments, a micropulse lidar (523 nm) instrument was operated at the Skukuza Airport in northeastern South Africa. The lidar was colocated with a diverse array of passive radiometric equipment. For SAFARI 2000, a daytime time series of layer mean aerosol optical properties, including layer mean extinction‐to‐backscatter ratios and vertical extinction cross‐section profiles are derived from the synthesis of the lidar data and aerosol optical depths from available AERONET Sun photometer data. Combined with derived spectral Angstrom exponents, normalized broadband flux measurements, and calculated air mass back‐trajectories, the temporal evolution of the surface aerosol layer optical properties is analyzed for climatological trends. For dense biomass smoke events the extinction‐to‐backscatter ratio is between 50 and 90 sr, and corresponding spectral Angstrom exponent values are between 1.50 and 2.00. Observations of an advecting smoke event during SAFARI 2000 are shown. The smoke was embedded within two distinct stratified thermodynamic layers causing the particulate mass to advect over the instrument array in an incoherent manner on the afternoon of 1 September 2000. Significant surface broadband flux forcing of over −50 W/m2 was measured in this event. The evolution of the vertical aerosol extinction profile is profiled using the lidar data. Finally, observations of persistent elevated aerosol layers during ARREX 1999 are presented and discussed. Back‐trajectory analyses combined with lidar and Sun photometer measurements indicate the likelihood for these aerosols being the result of long‐range particulate transport from the southern and central South America.

Tropospheric carbon monoxide measurements from the Scanning High‐Resolution Interferometer Sounder on 7 September 2000 in southern Africa during SAFARI 2000

Retrieved tropospheric carbon monoxide (CO) column densities are presented for more than 9000 spectra obtained by the University of Wisconsin‐Madison (UWis) Scanning High‐Resolution Interferometer Sounder (SHIS) during a flight on the NASA ER‐2 on 7 September 2000 as part of the Southern African Regional Science Initiative (SAFARI 2000) dry season field campaign. Enhancements in tropospheric column CO were detected in the vicinity of a controlled biomass burn in the Timbavati Game Reserve in northeastern South Africa and over the edge of the river of smoke in south central Mozambique. Relatively clean air was observed over the far southern coast of Mozambique. Quantitative comparisons are presented with in situ measurements from five different instruments flying on two other aircraft: the University of Washington Convair‐580 (CV) and the South African Aerocommander JRB in the vicinity of the Timbavati fire. Measured tropospheric CO columns (extrapolated from 337 to 100 mb) of 2.1 × 1018 cm−2 in background air and up to 1.5 × 1019 cm−2 in the smoke plume agree well with SHIS retrieved tropospheric CO columns of (2.3 ± 0.25) × 1018 cm−2 over background air near the fire and (1.5 ± 0.35) × 1019 cm−2 over the smoke plume. Qualitative comparisons are presented with three other in situ CO profiles obtained by the South African JRA aircraft over Mozambique and northern South Africa showing the influence of the river of smoke.

Coordinated airborne, spaceborne, and ground‐based measurements of massive thick aerosol layers during the dry season in southern Africa

During the dry season airborne campaign of the Southern African Regional Science Initiative (SAFARI 2000), coordinated observations were made of massive thick aerosol layers. These layers were often dominated by aerosols from biomass burning. We report on airborne Sun photometer measurements of aerosol optical depth (λ = 0.354–1.557 μm), columnar water vapor, and vertical profiles of aerosol extinction and water vapor density that were obtained aboard the University of Washington's Convair‐580 research aircraft. We compare these with ground‐based AERONET Sun/sky radiometer results, with ground based lidar data (MPL‐Net), and with measurements from a downward pointing lidar aboard the high‐flying NASA ER‐2 aircraft. Finally, we show comparisons between aerosol optical depths from the Sun photometer and those retrieved over land and over water using four spaceborne sensors (TOMS, MODIS, MISR, and ATSR‐2).

Airborne lidar measurements of aerosol optical properties during SAFARI‐2000

The Cloud Physics Lidar (CPL) operated onboard the NASA ER‐2 high‐altitude aircraft during the Southern African Regional Science Initiative (SAFARI)‐2000 field campaign. The CPL provided high spatial resolution estimates of aerosol optical properties at both 1064 and 532 nm. We present here results of planetary boundary layer (PBL) aerosol optical depth analysis and profiles of aerosol extinction. Variation of optical depth and extinction are examined as a function of regional location. The wide‐scale aerosol mapping obtained by the CPL is a unique data set that will aid in future studies of aerosol transport. Comparisons between the airborne CPL and ground‐based Micro‐Pulse Lidar Network (MPL‐Net) sites are shown to have good agreement.

Clean air slots amid dense atmospheric pollution in southern Africa

During the flights of the University of Washington's Convair‐580 in the Southern African Regional Science Initiative (SAFARI 2000) in southern Africa, a phenomenon was observed that has not been reported previously. This was the occurrence of thin layers of remarkably clean air, sandwiched between heavily polluted air, which persisted for many hours during the day. Photographs are shown of these clean air slots (CAS), and particle concentrations and light scattering coefficients in and around such slot are presented. An explanation is proposed for the propensity of CAS to form in southern Africa during the dry season.

Inorganic and carbonaceous aerosols during the Southern African Regional Science Initiative (SAFARI 2000) experiment: Chemical characteristics, physical properties, and emission data for smoke from African biomass burning

We collected filter samples of the atmospheric aerosol during the Southern African Regional Science Initiative (SAFARI 2000) experiment onboard the UK Met Office C‐130 aircraft. The main operational area was the Atlantic Ocean offshore of Namibia and Angola, where biomass‐smoke haze at least 1–2 days old was widespread. The size‐fractionated aerosol samples were analyzed for the major inorganic ions, carbonaceous material (elemental and organic carbon), and elements with atomic numbers between 11 (Na) and 82 (Pb). The regional haze aerosol was composed mostly of carbonaceous aerosols (on the average, 81% of the submicron mass), with secondary inorganic aerosols (sulfate, ammonium, and nitrate) accounting for another 14%. K+ and Cl−, typical pyrogenic species, constituted only 2% of the mass. The aerosol chemical data were used to estimate mass emission fluxes for various aerosol components. For African savanna/grassland burning, the estimated emission flux of carbonaceous particles (particulate organic matter plus elemental carbon) is 14 ± 1 Tg yr−1, and that of the nitrogen species (nitrate and ammonium) is 2 ± 2 Tg yr−1. For the flight segments in regional haze, the mean particle scattering coefficient at 550 nm was σs = 101 ± 56 Mm−1 and the mean particle absorption coefficient σa at 565 nm averaged 8 ± 5 Mm−1 (mean single scattering albedo of 0.93 ± 0.06 at 550 nm). The dry mass scattering efficiency αs, calculated from the linear regression of the mean scattering versus the estimated submicron mass, is estimated to be between 4.2 ± and 4.6 ± 0.6 m2 g−1, depending on the assumptions made in calculating the aerosol mass. The dependence of the scattering enhancement ratios Δσs/ΔCO on the distance from the burning regions suggests that the evolution of particle size with time influences the light scattering efficiency. Fresh smoke was sampled during a dedicated flight in the proximity and within the plume of an active biomass burning fire. Here the enhancement ratio with respect to CO of particles in the Aitken‐size range (5–100 nm diameter) was ΔNAitken/ΔCO ∼25 cm−3 (STP) ppb−1. These particles were removed rapidly after emission, and they were not detectable in the regional haze. The enhancement ratio for accumulation mode particles (0.1–1 μm diameter) ΔNAcc/ΔCO was ∼26–30 cm−3 (STP) ppb−1 in young smoke, and 16 ± 3 cm−3 (STP) ppb−1 in aged haze, suggesting that the number concentration of accumulation mode particles was reduced by about 41% during aging.

Solar spectral radiative forcing during the Southern African Regional Science Initiative

During the dry season component of the Southern African Regional Science Initiative (SAFARI) in late winter 2000, the net solar spectral irradiance was measured at flight levels throughout biomass burning haze layers. From these measurements, the flux divergence, fractional absorption, instantaneous heating rate, and absorption efficiency were derived. Two cases are examined: on 24 August 2000 off the coast of Mozambique in the vicinity of Inhaca Island and on 6 September 2000 in a very thick continental haze layer over Mongu, Zambia. The measured absolute absorption was substantially higher for the case over Mongu where the measured midvisible optical depth exceeded unity. Instantaneous heating from aerosol absorption was 4 K d−1 over Mongu, Zambia and 1.5 K d−1 near Inhaca Island, Mozambique. However, the spectral absorption efficiency was nearly identical for both cases. Although the observations over Inhaca Island preceded the “river of smoke” from the southern African continent by nearly 2 weeks, the evidence here suggests a continental influence in the lower tropospheric aerosol far from source regions of burning.

Measurements of biomass burning influences in the troposphere over southeast Australia during the SAFARI 2000 dry season campaign

Several studies have observed midtropospheric atmospheric composition anomalies and suggested a link to tropical biomass burning. Such anomalies complicate the use of trace gas profiles in remote regions to infer their surface sources/sinks based on the vertical gradients. The Southern African Regional Science Initiative (SAFARI 2000) campaign in Africa and coordinated downwind measurements in Australia provided an opportunity to confirm this link and elucidate the specific surface and atmospheric processes. Five aircraft missions were conducted by Commonwealth Scientific and Industrial Research Organisation (CSIRO) Atmospheric Research during the campaign. They were scheduled after African outflows of polluted air were observed in satellite images over the Indian Ocean flowing east toward Australia. Air samples collected from near the surface to 7 km were analyzed for a suite of trace gases (12CO2, CH4, CO, H2, N2O, and C2 and C3 hydrocarbons) and one isotopomer (13CO2) to provide vertical composition profiles. Ozone was monitored continuously during flight while a ground‐based lidar was employed in the Melbourne region to detect aerosol layers. A preliminary statistical analysis on the Australian data confirms covarying midtroposphere enhancements in the biomass burning products. Making rudimentary corrections for photochemical evolution during transit, the trace gas enhancement ratios in affected air samples are comparable to emission ratios in fresh biomass burning plumes. The 13CO2/12CO2 ratios are also consistent with a source from terrestrial plants. Back‐trajectory analysis for strongly enhanced samples suggests long‐range transport from tropical regions in Africa or South America, the proof of which requires a follow‐on analysis with a global chemistry transport model.

Spatial and seasonal variations in CCN distribution and the aerosol‐CCN relationship over southern Africa

The sources and transport of cloud condensation nuclei (CCN) over southern Africa have been investigated using in situ measurements collected during the Aerosol Recirculation and Rainfall Experiment (ARREX) wet season projects and during the Southern African Regional Science Initiative (SAFARI‐2000) intensive dry season campaign. CCN concentrations over the subcontinent are generally higher in the dry season than in the wet season and exceed 2000 cm−3in highly polluted air masses. In the late dry season, CCN concentrations are highest in the northern regions of the subcontinent due to the burning of savanna biomass. Emissions from industries and power plants on the South African Highveld are a prolific year‐round source of CCN and are sufficient to account for CCN levels south of 20°S throughout the year. Most CCN are contained within the mixing layer, which extends to an altitude of 3000–4000 m over the plateau and is capped by a temperature inversion. Multiple inversions and absolutely stable layers control the stratification of aerosols and CCN. Biomass burning particles are efficient CCN, and the median diameter of the accumulation mode is large (up to 0.19 μm). Recently emitted industrial aerosols are less soluble and have a smaller median diameter (0.11 μm). Twice as many aerosols act as CCN in the dry season (68%) than in the wet season (34%). The fraction is highest in the dry season over the tropical regions (>80%), where smoke aerosol predominates. Elevated aerosol and CCN concentrations are expected to have implications for direct radiative forcing, especially in the dry season, and for indirect forcing in the wet season.

Trace gas chemistry in a young biomass burning plume over Namibia: Observations and model simulations

We investigated trace gas emissions from a biomass fire near Otavi, northern Namibia, on 13 September 2000 as part of the Southern African Regional Science Initiative 2000 (SAFARI 2000). Observations included fast measurements of carbon monoxide (CO), ozone (O3), acetone (propanone, CH3COCH3), and acetonitrile (ethanenitrile or methyl cyanide, CH3CN). Additionally, flask samples were taken and analyzed for nonmethane hydrocarbons (NMHCs). Measurements close to the fire were used to quantify the emissions of the different compounds. Several transects were flown through the plume downwind to study the chemical changes in the plume during the first 2 hours after the emission. Fast production of O3 was observed, with the O3/CO molar enhancement ratio ER(O3/CO) reaching ∼0.10 in the plume after 2 hours. Acetone molar enhancement ratios (ER(acetone/CO) were ∼5 × 10−3 in the very young plume but ∼9 × 10−3 after 1–2 hours of aging, pointing to a substantial fast secondary acetone formation. To understand the chemical processes occurring in the plume, we simulated the plume chemistry with a dilution box model. The fast O3 production is well reproduced, whereas acetone mixing ratios in the aging plume are underestimated by the model. This points to additional yet unidentified acetone precursors. The model suggests an overall molar enhancement ratio of these compounds with respect to CO of 7.5 × 10−3 and a reaction rate coefficient of 6 × 10−11 cm3 s−1 for the reaction with HO radicals. The secondary acetone production may cause the net acetone source from biomass burning to be underestimated when only data from observations immediately near the fires are considered. The atmospheric implications of these findings are discussed.

Variability of biomass burning aerosol optical characteristics in southern Africa during the SAFARI 2000 dry season campaign and a comparison of single scattering albedo estimates from radiometric measurements

Measurements of the column‐integrated aerosol optical properties in the southern African region were made by Aerosol Robotic Network (AERONET) Sun‐sky radiometers at several sites in August–September 2000 as a part of the Southern African Regional Science Initiative (SAFARI) 2000 dry season field campaign. Fine mode biomass burning aerosols dominated in the northern part of the study region (Zambia), which is an active burning region, and other aerosols including fossil fuel burning, industrial, and aeolian coarse mode types also contributed to the aerosol mixture in other regions (South Africa and Mozambique), which were not as strongly dominated by local burning. The large amount of smoke produced in the north lead to a north‐south gradient in aerosol optical depth (τa) in September, with biomass burning aerosol concentrations reduced by dispersion and deposition during transport. Large average diurnal variations of τa(typical diurnal range of 25%) were observed at all sites in Zambia as a result of large diurnal trends in fire counts in that region that peak in midafternoon. However, for all sites located downwind to the south, there was relatively little (∼5–10%) average diurnal trend observed as the aerosol transport is not strongly influenced by diurnal cycles. AERONET radiometer retrievals of aerosol single scattering albedo (ω0) in Zambia showed relatively constant values as a function of τafor τa440ranging from 0.4 to ∼2.5. The wavelength dependence of ω0varied significantly over the region, with greater decreases for increasing wavelength at smoke‐dominated sites than for sites influenced by a significant coarse mode aerosol component. Retrievals of midvisible ω0based on the fitting of Photosynthetically Active Radiation (PAR; 400–700 nm) flux measurements to modeled fluxes for smoke in Mongu, Zambia yielded an average value of 0.84. This is in close agreement with the estimated average of 0.85 derived from interpolation of the AERONET retrievals made at 440 and 675 nm for August–September 2000. The spectral dependence of ω0independently retrieved with the AERONET measurements and with diffuse fraction measurements in Mongu, Zambia was similar for both techniques, as a result of both methods retrieving the imaginary index of refraction (∼0.030–0.035 on one day) with very little wavelength dependence.

Regional fuel load for two climatically contrasting years in southern Africa

Available fuel loads for burning in savanna ecosystems in the southern African region have been estimated using a new Fuel load‐Net Primary Production model based on ecophysiological processes such as respiration and Potential Evapotranspiration. The model outputs 15‐day standing available fuel load layers for an entire year (a total of 24 layers). Published data from the Southern African Fire‐Atmosphere Research Initiative (SAFARI‐92) project and from the Southern African Regional Science Initiative (SAFARI 2000) field campaigns were generally in agreement with the estimations. Consistently with previous studies, precipitation was recognized to be the major climatic driver for fuel production. As a consequence, even though there was a regional increase in precipitation in 1999–2000 as compared to the 1991–1992 periods, the temporal and spatial variability in precipitation at fine scales (site level) was important enough to restrict generalities over the entire region for fuel load production. Four areas of interest, Etosha National Park (Namibia), Mongu (Zambia), Kasama (Zambia), and Kruger National Park (South Africa), were selected to reconstruct an aridity gradient and analyze their fuel load variability over the two years. These areas presented contrasting fuel load distributions for the two very different studied periods with arid areas producing heavier fuel loads in 1999–2000, and the more humid areas producing heavier fuel loads in 1991–1992. The consequences of such fuel load variability and the use of such results are discussed.

The mean physical and optical properties of regional haze dominated by biomass burning aerosol measured from the C‐130 aircraft during SAFARI 2000

Instrumentation on the Met Office C‐130 aircraft measured aerosol physical and optical properties during the Southern African Regional Science Initiative (SAFARI 2000) in September 2002 while flying from Windhoek, Namibia. Filter measurements of aged regional haze suggest a ratio of apparent elemental carbon (ECa) to organic carbon (OC) of 0.12 ± 0.02 and mass fractions of 5% ECa, 25% inorganic compounds, and 70% organic matter (OC plus associated elements). The submicron size distribution of aged regional haze may be fitted with three lognormal distributions with geometric mean radii (rn) of 0.12 ± 0.01, 0.26 ± 0.01, and 0.80 ± 0.01 μm and geometric standard deviations (σ) of 1.3 ± 0.1, 1.5 ± 0.1, and 1.9 ± 0.4. Measurements over 2500 km from the emission region show similar rn and σ for the smallest two modes, while the third mode is absent presumably as a result of sedimentation. At a wavelength (λ) of 0.55 μm, effective medium approximations suggest a refractive index of 1.54 − 0.018i for aged regional haze aerosol. The single scattering albedo (ωoλ) derived using this refractive index and measured size distributions are consistent with those from the nephelometer and Particle Soot Absorption Photometer (PSAP). The optical parameters for aged regional haze a few days old are specific extinction coefficient (keλ=0.55) of 5.0 ± 0.8 m2 g−1, asymmetry factor (gλ=0.55) of 0.59 ± 0.02, and ωoλ=0.55 of 0.91 ± 0.04. Measurements of fresh biomass burning aerosol a few minutes old show smaller more absorbing particles. Vertical profiles of carbon monoxide, aerosol concentration, and aerosol scattering show a good correlation. Over land, aerosols become well mixed in the vertical from the surface to approximately 500 hPa. Over ocean, the aerosols can be separated from underlying stratocumulus cloud by a clear gap and a strong inversion, which may limit the indirect effect.

Release of gaseous and particulate carbonaceous compounds from biomass burning during the SAFARI 2000 dry season field campaign

Source strengths to the atmosphere for different forms of carbon (CO2, CO, CH4, nonmethane hydrocarbons (NMHC), and PM2.5) produced by biomass burning were calculated during the Southern African Regional Science Initiative (SAFARI 2000) intensive study period (August and September 2000) for Africa south of the equator. For a given pixel burned, these calculations are a product of the aboveground biomass before a fire, the proportion of biomass that is burned by the fire, and the emission factor for a given carbon compound based on the combustion efficiency. The total emission for the region is the summation of such calculations for all the burned pixels. Products used for these calculations are a prototype satellite‐based burned area based on the SPOT‐VGT‐S1 satellite product, a fuel load map produced by a simulation model developed and applied to southern Africa at 1 km2 resolution, information from fire experiments from our own research in southern Africa to compute combustion completeness, and emission factors published for the region. Over August and September 2000, 31,067 fires detected by the SPOT satellite are calculated to have emitted 96.9 × 1012 g CO2, 4.6 × 1012 g CO, and lesser amounts of CH4, NMHC, and PM2.5. These calculations are in the range of previous estimates of the emissions of these compounds for southern Africa. Along with documenting the estimates of the emissions during the SAFARI 2000 campaign, regional emission estimates are very strongly controlled by the burned area of fires due to a correlation between the cumulative distribution of percent burned area and the cumulative distribution of percent emissions.

Comparison of aerosol size distributions, radiative properties, and optical depths determined by aircraft observations and Sun photometers during SAFARI 2000

The Meteorological Office C‐130 aircraft performed a dedicated flight over the Etosha Pan surface‐based Aerosol Robotic Network (AERONET) Sun photometer site on 13 September 2000 during the Southern African Aerosol Regional Science Initiative (SAFARI 2000) intensive measurement campaign. Aerosol optical depths at different wavelengths, τaerλ, are derived from in situ measurements of the scattering and absorption coefficients and from various radiometric measurements and compared to those derived from the Sun photometer site. The estimates of τaerλ from the various measurements are shown to be in good agreement. The exception to this is when τaerλ is derived from the Passive Cavity Aerosol Spectrometer Probe (PCASP), as this method is shown to be extremely sensitive to the pitch angle of the aircraft; therefore, τaerλ differs for profile ascents and profile descents. However, the aerosol size distribution measured by the PCASP and derived from the AERONET site are in excellent agreement over the 0.05–1.0 μm radius range, which contains the majority of the optically active particles. C‐130‐derived refractive indices and single scattering albedos are also shown to be in excellent agreement with those derived from the AERONET site. The consistency between in situ and remotely sensed data suggests that, for aerosol well mixed in the vertical, data from AERONET may be used with confidence in validating satellite measurements and modeling studies of the radiative properties and effects of aerosols.

Micrometeorological and leaf‐level measurements of isoprene emissions from a southern African savanna

In February 2001, as part of the Southern African Regional Science Initiative (SAFARI 2000), isoprene fluxes were measured for 8 days using the relaxed eddy accumulation technique from a 21‐m tower in a Combretum‐Acacia savanna in Kruger National Park, 13 km from Skukuza, RSA. Despite warm and sunny conditions, midday isoprene concentrations were low, averaging 0.39 nL/L. Fluxes of isoprene increased through the morning hours, with midday fluxes averaging 0.34 mg m−2 h−1 and a maximum measured flux of approximately 1.0 mg m−2 h−1. Consistent with these low fluxes, leaf enclosure measurements of woody species within the tower footprint determined that only one isoprene‐emitting species, Acacia nigrescens, was present in significant numbers, comprising less than 10% of the woody biomass. Combining enclosure data with species composition and leaf area index data from the site, we estimated that the isoprene emission capacity of the vegetation within the vicinity of the tower was very low, approximately 0.47 mg m−2 h−1, and patchy. Under these circumstances, low and variable fluxes are expected. Additional leaf enclosure measurements, for a total of 121 species, were made at other locations, and approximately 35% of the species was found to emit significant amounts of isoprene. Important isoprene emitting plant families included Caesalpinaceae, Mimosaceae, Papilionaceae, Euphorbiaceae, Moraceae, and Myrtaceae. Twelve members of the important savanna genus Acacia were measured, of which five species, all belonging in Subgenus Aculeiferum, Section Aculeiferum, were found to emit significant amounts of isoprene. In contrast, the plant family, Combretaceae, dominant in many savanna ecosystems, was found to contain no species which emit isoprene.

Cloud physics lidar: instrument description and initial measurement results.

The new Cloud Physics Lidar (CPL) has been built for use on the NASA ER-2 high-altitude aircraft. The purpose of the CPL is to provide multiwavelength measurements of cirrus, subvisual cirrus, and aerosols with high temporal and spatial resolution. The CPL utilizes state-of-the-art technology with a high repetition rate, a low-pulse-energy laser, and photon-counting detection. The first deployment for the CPL was the Southern African Regional Science Initiative's 2000 field campaign during August and September 2000. We provide here an overview of the instrument and initial data results to illustrate the measurement capability of the CPL.