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TEMPERATURE INVERSIONS IN DEEP OPEN-CUT MINES? Mark F. Hibberd, Jim Hondros Centre for Australian Weather and Climate Research, CSIRO Marine & Atmospheric Research, Aspendale 3195 BHP Billiton, 55 Grenfell St, Adelaide 5000 Keywords: Stability, Froude number, Dispersion, Dust Do temperature inversions or strong stable conditions occur in deep open-cut mining pits? This question is important for modelling the dispersion of pollutants such as dust or radon in deep pits such as the proposed Olympic Dam mine in SA, which is planned to be almost 1 km deep. It is well known that strong nocturnal inversions occur in valleys and mountain basins on calm, clear nights (e.g. Hibberd 2003, Clements et al 2003) and that they can be caused by radiation cooling and/or cold air drainage, but there are almost no data on the nocturnal meteorology in deep open-cut mines. This study measured vertical temperature profiles in two 300 m deep mines in Western Australia on 25 nights during autumn and winter. Whiteman et al (2004) showed that in night-time stable conditions, measurements on the sidewall of a basin agree well with those from a tethersonde in the middle of the basin. Thus, in this study, data were collected using meteorological instruments mounted on a mine utility vehicle (Figure 1), which travelled on mine access roads between the top and bottom of the pit. A Vaisala PTU300 datalogger in the cab stored the data every 10 seconds. Height was derived from the ambient pressure measurements. typically constant thr ough the depth of the pit (e.g. Figure 2 ). Stable conditions were measured on 25% of the nights sampled. The uncertainty in the gradients was estimat ed at 0.002 K/m. There was no evidence of significant capping inversions in any of the measurements. Analysis in terms of Froude number Fr = U/(Nh) where U is pit-top wind speed, N is the BruntVäisälä frequency, and h is the depth of the pit, showed that stable conditions were only observed for Fr < 1, i.e. this criterion corresponded to poor pit ventilation. It is hoped that publication of this paper might encourage others to share any results they have that are relevant to dispersion in deep open-cut pits. 700 Average potential temperature gradient 1°C/100 m Mt Whaleback pit temperature profiles at time shown 23-24 July 2009 Descending Ascending 600 Reduced Level (m) 1. Introduction 500 400 1900 h 0400 h 0200 h 17 18 2230 h 2000 h 1800 h 19 20 21 Potential temperature (°C) 22 23 Figure 2. Examples of stable temperature profiles References Figure 1. Sensors on side of roof rack on mine utility vehicle. 2. Results A range of potential temperature gradients from neutral to a maximum of 0.01 K/m was observed, CASANZ 2011 Conference - Auckland - 31 July - 2 August 2011 Paper 214 Clements, C.B., Whiteman, C.D., and Horel, J.D. 2003. Cold-air-pool structure and evolution in a mountain basin: Peter Sinks, Utah. J. Appl. Meteorol. 42, 752-768. Hibberd, M.F. 2003. Nocturnal dispersion meteorology in an urban valley. Clean Air & Environ. Qual. 37 (4): 34-37. Whiteman, C.D., Eisenbach, S., Pospichal, B., and Steinhacker, R. 2004. Comparison of vertical soundings and sidewall air temperature measurements in a small alpine basin. J. Appl. Meteorol. 43, 1635-1647. Assistance from Dylan Bilske, Simon Hilder and Phillip Bryant is gratefully acknowledged. CASANZ 2011 Conference - Auckland - 31 July - 2 August 2011 Paper 214