Estimating emissions of a range of trace gases from a large city (Melbourne, Australia) by analysing and modeling measurements made about 250 km downwind at the Cape Grim Baseline Air Pollution Station

In this study we have analysed trace gas measurements made during the years 1995 to 1998 at the Cape Grim Baseline Air Pollution Station in northwest Tasmania. The trace gas measurements were generated using a coupled gas chromatograph system operated continuously at Cape Grim as part of the Advanced Global Atmospheric Gases Experiment (AGAGE). The ten trace gases measured simultaneously every forty minutes are: CCl₃F (CFC-11), CCl₂F₂ (CFC-12), CCl₂FCClF₂ (CFC-113), CH₄ (methane), N₂O (nitrous oxide), CO (carbon monoxide), H₂ (hydrogen), CHCl₃ (chloroform), CH₃CCl₃ (methyl chloroform) and CCl₄ (carbon tetrachloride).

The time series of these gases reveal regular enhancements above baseline known as 'pollution episodes'. These episodes are the focus of this study, and analysis of the time series was performed to investigate relationships between species in polluted air. These relationships are used to determine relative sources of urban constituents. Using carbon monoxide as a reference compound and assuming that Cape Grim reliably samples these polluted air masses from Melbourne, (population approx. 3.5 million) we estimate emissions of the other species for Melbourne.

The emission and transport of trace gases from the Melbourne region to Cape Grim is investigated using a regional atmospheric transport model with high spatial resolution (5C5 km horizontal grid, and 20 vertical grid levels). The model is a PC-based three-dimensional prognostic model which predicts meteorological and pollution parameters at the mesoscale level. Emissions inventory data in the form of spatially distributed source functions for species such as carbon monoxide have been released into the model to study the transport of trace gases from Melbourne to Cape Grim.