Year

2014

Degree Name

Doctor of Philosophy

Department

School of Chemistry

Abstract

There remains much uncertainty in processes driving atmospheric composition, particularly in relation to the Southern Hemisphere. This thesis examines contemporary atmospheric composition in the poorly characterised Southern Hemisphere, focusing on the Australasian region. A combination of measurements and modelling has been used to investigate atmospheric composition over a range of time and spatial scales.

The frst semi-continuous record of in situ measurements of CO, CO2, CH4, N2O and δ13C from the University of Wollongong (UOW), Australia, was developed using FTIR measurements from 2011 through to 2013. This record was used to understand the boundary layer atmospheric signature of the UOW site. Clean air was found to arrive at UOW in approximately 10% of air masses. Whilst the dataset represented a short record, preliminary trends were determined for background air for CO (-3.8 [95th: -4.76, -3.08] nmol mol-1 per year), CO2 (2.05 [95th: 1.19, 2.66] μmol mol-1 per year) and CH4 (3.5 [95th: 0.24, 5.34] nmol mol-1 per year). Annual cycles in background data re ect the widespread in uence from Southern Hemispheric biomass burning during Austral spring. Daily cycles, cluster analysis and O3-CO correlation analysis were performed, revealing an atmosphere strongly in uenced by urban and industrial sources.

Total column CO measurements from the ground-based solar Fourier-Transform infrared Spectrometer (FTS) at UOW were compared with satellite-based measurements from the Measurements Of Pollution In The Troposphere (MOPITT) instrument. Direct comparison of vertical column amounts from these instruments allowed distinction between local and transported pollution to the site. Anomalous CO events were identied in the 2000-2010 record of each instrument and back trajectory analysis was used to identify likely contributing causes. High anomalous events in both instruments resulted from large-scale transported biomass burning sources from Africa, South America or northern Australia. Two large-scale biomass burning events within the east Australian region were also measured by both instruments, Sydney bushfires in December 2001-January 2002 and Canberra bushfres in January 2003. Anomalously low column amounts corresponded with transported air-masses from the northern latitudes surrounding Antarctica. Anomalies found only in the FTS measurements re ected urban out ow, local biomass burning, or local industrial activity and were accompanied by slow moving air masses.

Two global models have been used in this study, the Global Chemical Transport Model, GEOS-Chem and the Australian Community Climate Earth System Simulator (ACCESS). Diferences between measurements and the GEOS-Chem model were investigated for CO, HCHO and C2H6. GEOS-Chem simulations of daily averaged trace gas amounts were compared with ground-based FTS measurements for three stations in the Australasian region: Wollongong (34.4oS, 150.9oE), Darwin (12.4oS, 130.9oE) and Lauder (45.0oS, 169.7oE). GEOS-Chem performed well in matching column measurements of CO and HCHO. Ethane was biased high in the model, which was later found to be due to emission inventory biases. Analysis of CO was extended to cover the entire region by comparing month averaged GEOS-Chem results with MOPITT measurements over Australasia, and was found to generally reproduce spatial patterns. However, a low bias in GEOS-Chem was found over the whole region during September to November, and was concluded to be due to underestimates of emissions in the GFED2 biomass burning inventory.

Evaluation was performed for one of the frst simulations of the Earth System Model ACCESS, in Chapter 8. Average annual cycles of total column values from ACCESS were compared with both FTS measurements and the GEOS-Chem model. As a result of the ACCESS evaluation, several recommended improvements were made for the model. In particular, it was recommended to implement a more recent emission inventory due to annual cycle mis-matches in CO and C2H6. Also, inclusion of the updated isoprene chemical mechanism would improve the magnitude of HCHO and C2H6. Finally, NO2 stratospheric boundary conditions corrections are required to accurately represent Southern Hemisphere atmospheric composition.

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