Application of airshed modelling to the implementation of the New Zealand National Environmental Standards for air quality

Christchurch is one of New Zealand's cities that suffers from high levels of particulate pollution due to the use of wood-burning heaters in the winter months. Frequent exceedences of the National Environmental Standards (NES) for ambient air quality of 50 μg m−3 occur for daily PM10 (particulate matter with an aerodynamic diameter of 10 μm or less). The main objective of this study is to assess the efficacy of the emission reduction strategy implemented by Environment Canterbury (the local air quality management authority) for the city of Christchurch using an air pollution model called TAPM. This model is used with and without data assimilation to simulate the meteorology and particulate matter dispersion for 1–9 July, 2001, when a stagnant anticyclone dominated the region and led to a prolonged period of poor air quality (the NES was exceeded on every day). This period is again tested with the projected 2013 emission inventory to investigate the reduction in PM10 concentrations due to predicted emission reductions. The last surveyed emission inventory for Christchurch was conducted in 2001, when particulate emissions for a ‘typical winter day’ were estimated to be 7145, 974, and 1001 tonnes per day from domestic, industrial, and transport sources, respectively. Emission inventories for 2013 – also for a ‘typical winter day’ – are based on policy implementation resulting in predicted 2013 particulate emission strengths of 998 (domestic), 1164 (industry), and 421 (transport) tonnes per day. Results show that the 2013 emission inventory will cause no exceedences at the location of the monitoring station in Christchurch. However, due to the complex wind regime and topography of the area, it is highly likely that the NES will be exceeded at locations away from the monitoring site, making Christchurch non-compliant with its objectives. This is particularly significant as the NES apply to the maximum concentration occurring anywhere in the polluted airshed.