Wintertime PM10 measurements and modelling in Alexandra and Mosgiel, Otago, New Zealand.

Abstract

Particulate air pollution from domestic sources adversely affects wintertime air quality in many small towns in New Zealand, particularly in the south. Low air temperatures warrant the use of domestic heating appliances, and during calm conditions, the emissions released by these appliances stagnates in the near surface atmosphere. This is exacerbated in towns that are topographically constrained, as sheltering from synoptic winds occurs and light drainage flows develop instead, leading to the development of temperature inversions, and high air pollution levels as a result. Alexandra, Central Otago, and Mosgiel, in eastern, coastal Otago are two such towns, and because of their air pollution history, and a relative paucity of knowledge of their air pollution meteorology, were the focus of this research. The research aims were to investigate the air pollution meteorology of each town through observational point source data, before numerical modelling was employed to gain further insight into these processes, both horizontally and vertically. The suitability of the current location of the Otago Regional Council (ORC) monitoring sites was reviewed, and the final aim was to determine the necessary percentage reduction in emissions for each town in order to reach the standards set be the National Environmental Standard (NES) for air quality. Three years of meteorological and PM10 data were analysed before selecting the winter of 2008 for modelling, using The Air Pollution Model (TAPM). TAPM was run from 1 May to 31 August for each town and was found to correctly predict daily PM10 concentrations 66% of the time in Alexandra and 71% of the time in Mosgiel, in terms of breaches and non-breaches of the NES. At a daily scale, TAPM was able to simulate diurnally switching thermal winds in the Alexandra basin, and reproduced both the location and magnitude of highest pollution concentrations over a 10 day case study. Drainage flows were also modelled well in Mosgiel, and temperature inversions were simulated in both towns, although with no vertical observational data to test the vertical modelled data against, these must be treated with some caution. TAPM also simulated in considerable detail the spatial variability of the wind regime, both horizontally and vertically. This research has shown that neither of the ORC monitoring sites is currently located in the area of worst air quality, which is a requirement of the NES. However, the percentage reductions in emissions required to meet the NES could not be calculated. This research has shown that TAPM is a useful tool for simulating meteorological and air pollution processes, and is potentially a valuable asset for air pollution management. Selecting the parameters in the model set-up that will reflect the conditions of the study site most closely will improve model performance, thus, providing a second and third dimension to existing point source datasets.