Year

2002

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

Observations of ten trace gases have been utilised to estimate emissions of trace gases from the Melbourne region. The ten trace gases measured by the AGAGE program at the Cape Grim Baseline Air Pollution Station in Tasmania are: CCI3F (CFC-11), CCI2F2 (CFC-12), CCI2FCCIF2 (CFC-113), methane (CH4), nitrous oxide (N2O), carbon monoxide (CO), hydrogen (H2), chloroform (CHCI3), methyl chloroform (CH3CCI3) and carbon tetrachloride (CCI4). The process of estimating emissions involved first identifying pollution episodes in the Cape Grim record that were a result of air masses that passed over the Melbourne region before travelling across Bass Strait to Cape Grim. To provide some confirmation that selected pollution episodes were associated with Melbourne emissions, meteorological back trajectory information for air parcels arriving at Cape Grim were examined. Correlations between the trace species and an estimate of carbon monoxide emissions from Melbourne were used to deduce the mass of Melbourne emissions. Where possible, comparisons of these estimates have been made with estimates from established inventory techniques. In particular, comparisons have been made with the National Greenhouse Gases Inventory. Results from this study showed emission estimates of CFC-12 and CFC-11 ranging from 30 - 54% and 30 - 55%, respectively, of the inventory estimates. These results indicate a more rapid phase out of these CFCs than industrial inventory models predicted. The emission estimates of CH4 for 1999 and 2000 compare well with inventory estimates at 89% and 95% of inventory estimates, respectively. Emission estimates of N2O ranged from 200% to 275% of inventory estimates over the years 1995 - 1999.

The emission and transport of trace gases in the atmosphere was also examined using a regional air pollution transport model, with the aim of simulating the transport of trace gases from the Melbourne region to Cape Grim. The model of choice for this study was The Air Pollution Model (TAPM) designed by CSIRO Atmospheric Research for air pollution studies. Gridded emissions of CO from Melbourne inventories were used as input to the model. TAPM successfully simulated the emission and transport of the trace gas, carbon monoxide, from Melbourne to Cape Grim. Pollution episodes at the CG grid point in the model were reproduced. The timing of the simulated episodes compared well with the timing of the observed pollution episodes. The simulated enhancements (both peak and average values) of CO concentrations during pollution episodes were estimated well with ratios for both the peak and average enhancements (simulated/observed) close to 1.0. The duration of simulated episodes was underestimated by a factor of 2.0 and the time-integrated enhancements of CO were underestimated by a factor of 2.5. The discrepancies were attributed to inaccuracies in the inventory.

Findings from comparisons of results between the tracer-ratio technique and independent inventories highlighted the possibility of underestimated or unknown sources of N2O in the Port Phillip region. A flask survey was undertaken to investigate possible sources of N2O in the Melbourne region. Emissions from wastewater treatment plants and disposal of treated wastewater into receiving waters provided the basis for this study. Enhancements of N2O above background of up to 20 ppb were observed, suggesting that wastewater is an important source of N2O in the Melbourne region. Results also indicated that aquatic emissions may contribute substantially more to the N2O budget than emissions from wastewater treatment plants.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.