Southern African Fire-Atmosphere Research Initiative Research Articles

A Method to Correct the Thermal Dome Effect of Pyranometers in Selected Historical Solar Irradiance Measurements

Abstract In using pyranometers to measure solar irradiance, it is important to know the magnitudes and the consequences of the thermal effect, which is introduced by the glass domes of the instruments. Historically, the thermal dome effect was not monitored on a regular basis. Case studies show that, due to the thermal dome effect, the output of the pyranometers altered from less than 5 W m−2 in the nighttime to over 20 W m−2 around noontime during the Aerosol Recirculation and Rainfall Experiment (ARREX) in 1999 and the Southern African Fire–Atmosphere Research Initiative (SAFARI) in 2000 field campaigns, depending on sky conditions. A calibration and data processing procedure with the thermal dome effect incorporated has been tested to resolve the issue. It is demonstrated that the intrinsic calibration constants of the pyranometers can be obtained if two pyranometers are used side by side, and the thermal dome effect may be inferred whenever a pyranometer and a pyrgeometer are collocated.

Assessment of the status of biogenic organic emissions and impacts on air quality in southern Africa

Air quality management in South Africa has traditionally focused on industrial, vehicular and domestic coal burning as emission sources of air pollution. Only recently through, research initiatives such as Southern African Fire-Atmosphere Research Initiative in 2000 (SAFARI 2000) and the Cross Border Air Pollution Impact Assessment Project, have the importance of biogenic volatile organic compound (BVOC) emissions been recognized in regional atmospheric chemistry and composition, particularly relating to surface ozone formation. These projects have identified significant gaps in the understanding of BVOC emissions in southern Africa.BVOC emissions are relatively well understood for southern African savanna and woodland plant species and landscapes. Considering that the region is home to more than 20 000 plant species, the most significant gap in the understanding of BVOC emissions is the acute lack of plant specific emission information. Equally important is an understanding of the unique local factors that control BVOC emissions.A project has been launched in South Africa to develop capacity and technologies to address the BVOC knowledge gaps. It focuses on measurement and modelling and aims to develop a cadre of skilled scientists in BVOC sampling techniques using leaf and branch enclosures, relaxed eddy accumulation and lidar techniques in conjunction with development and application of photochemical modelling.

Semivolatile particulate organic material in southern Africa during SAFARI 2000

During August and September 2000, the University of Washington's (UW) Cloud and Aerosol Research Group (CARG) with its Convair‐580 research aircraft participated in the Southern African Fire‐Atmosphere Research Initiative (SAFARI) 2000 field study in southern Africa. Aboard this aircraft was a Particle Concentrator‐Brigham Young University Organic Sampling System (PC‐BOSS), which was used to determine semivolatile particulate material with a diffusion denuder sampler. Denuded quartz filters and sorbent beds in series were used to measure nonvolatile and semivolatile materials, respectively. Results obtained with the PC‐BOSS are compared to those obtained with conventional quartz–quartz and Teflon–quartz filter pack samplers. Various 10–120 min integrated samples were collected during flights through the free troposphere, in the atmospheric boundary layer, and in plumes from savanna fires. Significant fine particulate semivolatile organic compounds (SVOC) were found in all samples. The SVOC was not collected by conventional filter pack samplers and therefore would not have been determined in previous studies that used only filter pack samplers. The SVOC averaged 24% of the fine particulate mass in emissions from the fires and 36% of the fine particulate mass in boundary layer samples heavily impacted by aged emissions from savanna fires. Concentrations of fine particulate material in the atmospheric mixed layer heavily impacted by aged savanna fire emissions averaged 130 μg m−3. This aerosol was 85% carbonaceous material.

Solar radiative forcing by biomass burning aerosol particles during SAFARI 2000: A case study based on measured aerosol and cloud properties

This study investigates the top of atmosphere (TOA) solar radiative forcing by biomass burning aerosol from the African continent. Radiative transfer calculations are based on airborne aerosol and cloud measurements made on 7 September 2000, being representative for the aerosol optical properties and the relative location of aerosol and clouds during Southern African Fire‐Atmosphere Research Initiative (SAFARI) 2000. We focus on how the radiative effects of biomass aerosol are changed in the presence of clouds to the west of the southern African coastline. The typical scenario observed showed a clear separation between an elevated biomass aerosol layer (1.8–3.7 km altitude) and low‐level stratiform clouds (below 1 km, liquid water path 85 g m−2). The aerosol was characterized by particle concentrations of 1800 cm−3and a single scattering albedo at 550 nm wavelength of around 0.90. Presuming no effect by the biomass aerosol on the clouds themselves, the presence of clouds converted the negative TOA forcing by the biomass aerosol in clear skies into a positive one (−13.0 W m−2converted to +11.5 W m−2for average optical properties of the biomass aerosol and a solar zenith angle of 60°). As biomass aerosol was found above cloud thousands of kilometers away from the southern African coastline, positive direct TOA forcings can be expected in extended sea areas over the Namibian cloud sheet. These results suggest errors in global climate model (GCM) modeling assessments of the direct radiative forcing due to biomass aerosols. Additionally, we estimate the possibility that a potential indirect effect dominates the overall forcing, resulting generally in strong negative TOA forcings.

Horizontal and vertical transport of air over southern Africa

Tropospheric air trajectories that occurred during the Southern African Fire‐Atmosphere Research Initiative (SAFARI) in August–October 1992 are described in terms of a circulation classification scheme and the vertical stability of the atmosphere. Three major and frequently occurring stable discontinuities are found to control vertical transport of aerosols in the subtropical atmosphere at the end of the dry season. Of these, the main subsidence‐induced feature is a spatially ubiquitous and temporally persistent absolutely stable layer at an altitude of about 5 km (3.5 km above the interior plateau elevation). This effective obstacle to vertical mixing is observed to persist without break for up to 40 days. Below this feature an absolutely stable layer at 3 km (1.5 km above the surface) prevails on and off at the top of the surface mixing layer for up to 7 days at a time, being broken by the passage of regularly occurring westerly wave disturbances. Above the middle‐level discontinuity a further absolutely stable layer is frequently discerned at an altitude of about 8 km. It is shown that five basic modes can be used to describe horizontal aerosol transportation fields over southern Africa. Dominating these is the anticyclone mode which results in frequent recirculation at spatial scales varying from hundreds to thousands of kilometers. In exiting the anticyclonic circulation, transport on the northern periphery of the system is to the west over the Atlantic Ocean via a semistationary easterly wave over the western part of the subcontinent. On the southern periphery, wave perturbations in the westerly enhance transports which exit the subcontinent to the east into the Indian Ocean. Independently derived data suggest that during SAFARI only 4% of the total transport of air from three locations south of 18°S was into the Atlantic Ocean. Over 90% of the transport was into the Indian Ocean across 35°E. This result reflects circulation fields typical of the extremely dry conditions prevailing in 1992. The integrated effect of the control exerted by atmospheric stability on vertical mixing, on the one hand, and the nature of the horizontal circulation fields, on the other, is to produce a distinctive suite of transport patterns that go a long way to explain the observed high concentrations of tropospheric aerosols and trace gases observed over the subcontinent in winter and spring, as well as over the tropical South Atlantic and southwestern Indian Oceans.

Characteristics of the nocturnal boundary layer: Okaukuejo, Namibia, during SAFARI‐92

During the Southern African Fire‐Atmosphere Research Initiative (SAFARI) in September and October 1992, nocturnal tethersonde soundings were carried out at Okaukuejo, measuring dry‐bulb and dew‐point temperature, ozone concentration, wind speed, and wind direction. These showed the existence of a surface temperature inversion and a low‐level wind maximum on most nights. The inversion formed shortly after sunset, reached a maximum depth and strength at sunrise, and then gradually eroded from the surface upwards. The low‐level jet (LLJ) started developing after the onset of inversion formation. Speeds in excess of 11 m s−1 were not unusual. The height of the core of the jet ranged between 50 and 200 m above ground level and corresponded fairly closely with the level of the top of the inversion. While daytime thermal eddy convection prevailed, ozone in the boundary layer was well mixed, and no vertical structure to ozone concentration was evident. With the onset of the surface inversion ozone levels increased systematically with height from less than 10 ppbv near ground level to more than 30 ppbv at the LLJ core.

Aircraft section measurements of meteorology and ozone in northern Namibia during SAFARI‐92

Ozone and meteorological data obtained via instrumented aircraft surveys over tropical southwestern Africa in the period October 8–12, 1992, are analyzed as part of the international Southern African Fire‐Atmosphere Research Initiative (SAFARI) program. Twice‐daily aircraft flights recorded measurements of wind, turbulence, temperature, dewpoint, and ozone at 1 Hz from 1–12 km in a series of successive profiles extending from 12° to 24°E along 18°S latitude. Wind patterns derived from aircraft and the European Centre for Medium‐Range Weather Forecasts (ECMWF) highlight a quasi‐permanent anticyclone in the midtroposphere over central southern Africa. Light meridional flow prevailed in the 3–7 km layer. Ozone values were in the range 40–60 parts per billion by volume (ppbv) from 1 to 4 km across the aircraft section and <20 ppbv in the surface marine layer off the west coast. In the continental air, ozone increased with height to 90 ppbv above 7 km. As a dry trough passed eastward, ozone values declined 30% in the layer above 4 km. Aircraft results indicate the presence of a potential recirculation mechanism around the midtropospheric anticyclone. Recirculation would facilitate photochemical reduction to ozone of regional pyrogenic emissions, following vertical mixing by turbulent diurnal heating. A persistent dry stable layer from 6 to 9 km may accumulate pollutants over tropical southern Africa, while upper westerly troughs could eject some of the continental air eastward.

Surface temperature and wind fields over the Skeleton Coast (Namibia) and adjacent interior during SAFARI‐92

As part of the meteorological component of the Southern African Fire‐Atmosphere Research Initiative (SAFARI), the University of Pretoria erected three automatic weather stations along the Skeleton Coast (Namibia) and adjacent interior. This paper presents the first study of the surface wind regimes and temperature fields north of Walvis Bay. The local circulation patterns in this region are primarily thermotopographically induced, with both the local wind regimes and temperature fields displaying clear diurnal signals. The presence of the Benguela current resulted in marked differences between the temperatures and temperature ranges observed at the coast and further inland. A nocturnal inversion developed regularly at Orupembi, while by day the temperature profile in the lower 10 m was superadiabatic at all three stations. Along the coast the wind speed and direction displayed limited diurnal variation, while further inland a marked diurnal variation was evident. The local circulation along the coast was dominated by the sea breeze. As the sea breeze developed, the wind veered to be southwesterly to westerly at sea breeze maximum. At Orupembi the local circulation was dominated by the draining of cool air from the mountain slopes and valleys by night and by a southerly flow by day, interrupted for a short time in the afternoon by the inland penetration of the sea breeze.

The Southern Tropical Atlantic Region Experiment (STARE): Transport and Atmospheric Chemistry near the Equator‐Atlantic (TRACE A) and Southern African Fire‐Atmosphere Research Initiative (SAFARI): An introduction

In November 1988 some 50 atmospheric scientists met at Dookie College, a small campus in the agricultural lands of Victoria, Australia, to map out the scientific goals of the International Global Atmospheric Chemistry (IGAC) Program, which was to become one of the first operational Core Projects of the International Geosphere‐Biosphere Program (IGBP). They identified the tropical regions as one of the priority areas for future international, coordinated research in atmospheric chemistry because of the vast biological activity in the tropics, with a correspondingly large potential for biogenic emissions, and the rapidly growing human populations and resulting land use change in these regions. In view of the prominent role that biomass burning plays in the tropics as a source of atmospheric pollutants and of the important ecological functions of vegetation fires in the tropics, the scientists at Dookie created the Biomass Burning Experiment (BIBEX) with the goals of characterizing the fluxes of gases and aerosols from biomass burning to the global atmosphere and assessing the consequences of pyrogenic emissions on chemical and physical climate.The southern tropical Atlantic region, defined here as the region containing the Amazon basin, the tropical South Atlantic, and southern Africa, was the obvious first focus of research for this project. Large tropical forest and savanna fires had been known to occur here every year. In addition, observations from satellites and from the space shuttle had shown high levels of tropospheric ozone and carbon monoxide to be present over this region every year in the August‐to‐October period. Results from previous campaigns (ABLE 2A, CITE 3, DECAFE 88) also suggested a widespread impact of vegetation fires on both continents on the trace gas and aerosol content of the troposphere in this region.