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Abstract. Airborne particles have been collected using a dichotomous virtual impactor at three different locations in the eastern part of Botswana: Serowe, Selibe-Phikwe, and Francistown. The particles were separated into two fractions (fine and coarse). Sampling at the three locations was done consecutively during the months of July and August, which are usually dry and stable. The sampling time for each sample was 12 hours during the day. For elemental composition, energy-dispersive x-ray fluorescence technique was used. Correlations and principal component analysis with varimax rotation were used to identify major sources of aerosol particles. In all the three places, soil was found to be the main source of aerosol particles. A copper-nickel mine and smelter at Selibe-Phikwe was found to be not only a source of copper and nickel particles in Selibe-Phikwe but also a source of these particles in far places like Serowe. In Selibe-Phikwe and Francistown, car exhaust was found to be the major source of fine particles of lead and bromine. In all the three sampling places the sampling instrument was placed -1 m above ground level. The sampling site in Serowe was a residential place inside a secondary school yard. The school is in the eastern part of the village and is built close to a small hill (<10 m high). In Selibe-Phikwe the sampling site was in a military camp -1 km from the city center. The site is approximately 300 m from a copper-nickel mine and smelter. In Francistown a residential place inside a secondary school yard was used for sampling. The school is -1 km north of the city center.The sampling instrument is composed of a pump and a dichotomous virtual impactor. The pump, which is fitted with a control module for the total and the coarse particle flow, draws the air through the impactor at a total flow rate of 1 m3/h. Two types of impactors were used in this work. The two impactors differed in both the upper cutoff aerodynamic diameters and the two size ranges into which the impactor segregates particles. The two size ranges will hereafter be referred to as the coarse (c) and fine (f) fraction/particles. In Serowe the size ranges were 3.5-18/•m and <3.5/•m, while in Selibe-Phikwe and Francistown the ranges were 2.5-10/•m and <2.5/•m for coarse and fine fractions, respectively. The two impactors are usually referred to as PM-18 and PM-10 to signify their upper cutoff points. As elaborated on by Foltescu and Selin [1993], this difference in cutoff points may be significant and needs to be taken into account when comparing data from two different 17,811

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Surface soil and airborne particles were sampled at two locations in Botswana, Serowe and Gaborone. EDXRF and TXRF were used to analyse the soil samples for elemental concentrations and the mobile (bio‐available) fractions of the soil. EDXRF was also used for analysis of the aerosol particles in two size fractions, PM2.5 and PM2.5–10. To extract the mobile fraction, the soil samples were leached in ammonium acetate solution at pH 7. Most of the elements in the soils were found to have very low mobility at the two locations. In order to identify elements for which the major sources in the fine and coarse airborne particles are anthropogenic, elemental concentrations in the soil were used to calculate enrichment factors of airborne particles with respect to titanium in soil. From the calculated enrichment factors, it appears that the elements attributed to bio‐mass burning and those attributed to car exhausts have strong anthropogenic sources in Serowe and Gaborone, respectively. Copyright © 2001 John Wiley & Sons, Ltd.

A significant number of fire-induced power disruptions are observed in several countries every year. The faults are normally phase-to-phase short circuiting or conductor-to-ground discharges at mid-span region of the high-voltage transmission system. In any case, the wildfire plumes provide a conductive path. The electrical conductivity is due to intense heat in combustion zone of the fire which creates ion and electrons from flame inherent particulates. Increase in the ion concentration increases the electrical conductivity of the fire plume. The main purpose of this study was to measure dielectric breakdown electric field for vegetation and hydrocarbon flames. The experimental data is needed for validation of simulation schemes which are necessary for evaluation of power grid systems reliability under extreme wildfire weather conditions. In this study, hydrocarbon and vegetation fuels were ignited in a cylindrically shaped steel burner which was fitted with type-K thermocouples to measure flame temperature. The fuels consisted of dried weeping wattle (Peltophorum africanum) litter, butane gas and candle wax. Two pinned copper electrodes supported by retort stands were mounted to the burner and energized to a high voltage. This generated a strong electric field sufficient to initiate dielectric breakdown in the flames. Breakdown electric field strength (E crit) obtained from the experiment decreased from 10.5 to 6.9 kV/cm for the flames with temperature range of 1003 to 1410 K, respectively.