Application of CALMET/CALGRID to Auckland, New Zealand

Although air quality in New Zealand is generally very good, the city of Auckland has recently experienced guideline exceedences of nitrogen dioxide and carbon monoxide. Also, there is much interest in secondary particulates and their affects on health and visibility, more so than in the potential for ozone events. A basic step towards the understanding of these urban air quality issues is to model the photochemistry of the city. This has not previously been attempted in New Zealand, and the present research applies the CALMET/CALGRID system to Auckland.

In Auckland, CO, NO_X and VOCs are predominantly emitted by motor vehicles, with domestic/commercial sources secondary in magnitude. The contribution from industry to these chemical species is small. Also, Auckland’s isolation from urban areas means that incoming air is relatively clean. Hence the applicability of CALMET/CALGRID to New Zealand conditions must first be ascertained. However, the success of urban airshed models in Australia indicates that they should be suitable for Auckland, whose emissions profile and air pollution meteorology are similar to Australian cities.

The complex coastline around Auckland often leads to the convergence of opposing sea breezes. Air quality under such conditions is potentially poor, due to the recirculation of pollutants. Thus it is important that a good representation of the local meteorology is obtained, and the CALMET diagnostic model is appropriate here, which is driven solely by local meteorological data.

The models simulate photochemistry over the Auckland region for January and March, 1999. Horizontal resolution is 3 km on a grid of 27x24 points. Meteorological data sources include eight surface observing stations and twice-daily vertical profiles from one station. Emissions data for vehicles and commercial sources are taken from the recent air emissions inventory for Auckland. Summer emission rates are input, varying from hour to hour, and between weekday and weekend. VOCs are classed according to the Carbon-Bond-IV chemical mechanism.

Ambient concentrations from CALGRID are compared with observations at air quality sites in and around the city. Due to the dominance of air pollutants from vehicles it is not appropriate to compare results from a grid-based model with point measurements when the site is at the roadside. For urban sites a few tens of metres away from primary sources, and the surrounding ozone monitoring sites, model results compare well with observations. These findings are very encouraging, as they indicate that transport of primary pollutants to sites downwind of the city, and the chemical reactions leading to the formation of ozone downwind of Auckland are simulated well by the photochemical model. However, exceedences of NO_X and CO are not captured, as they occur at sites close to city streets. Here the model concentrations are an order of magnitude too small.

The success of these initial numerical experiments shows that the CALMET/CALGRID system may be used with some confidence as a basis for addressing applied air quality problems in New Zealand.