Assessing air pollution potential at a large industrial site – a climatology of deep inversions, mixing heights and atmospheric stability at Aramoana
Temperature stratification of the atmosphere at Aramoana, Otago Harbour, has been studied in detail during a one year period from April 1981 to March 1982. Information from thermographs, an acoustic sounder, airsonde and tethersonde ascents was used to describe the frequency and nature of deep ground-based and elevated inversions, mixing heights, and stagnation episodes. A method that requires wind speed and net radiation data was used to assess atmospheric stability. Ground-based inversions extending up to 80 m and 230 m occurred for 25% and 13% of the time, respectively, but were mostly weak and short lived. Elevated inversions were frequent at all times of the year but again were generally weak with gradients of less than 2º C/100 m. Average hourly mixing heights varied between 554 m (at 0200 hours) and 657 m (at noon). There was little seasonal, but large day to day variation. Over the measurement year mixing heights fell to below 200 m and 300 m for 14% and 23% of the time respectively, but these low values did not often combine with up-harbour winds. Stagnation episodes were rare, with only three occurrences during the measurement year. Atmospheric stability would not restrict dispersion for 69% of the time, but would do so 18% of the time.