Sydney's Contribution to the Climate Balance

Cities are dirty places, contributing noxious gases and particulates to the local environment. Some of their effects are felt locally, while some may be detected at much greater distances. Of these emissions, particulates are now recognized as being able to exert a measurable influence on the flows of solar radiation through the atmosphere, flows which are the key to the Earth's climate. Indeed, near the more polluted parts of the globe, such as Europe, eastern North America and east Asia, the reflective properties of sulphate aerosol particles are actually greater than the heat trapping effects of greenhouse gases, leading to a local cooling.

So what is the situation here in Sydney? A team of students from the Atmospheric Physics group has been studying the physical, chemical and radiative properties of aerosols in Sydney for the past 5 years, and has recently released its first analysis of this data set. Our group has measured the aerosol optical thickness (a measure of the attenuation of sunlight through the atmosphere) with an instrument at UNSW, and from this inferred the size distribution of the aerosols. Analysis of the chemistry of aerosol particles by collaborators at ANSTO (and more recently by us), has yielded their refractive index. Finally a combination of chemical and radiation measurements has provided information on aerosol absorption, one of the key variables.

The first message is good: particulate levels in the atmosphere above Sydney are certainly low by world standards. Sydney aerosols are, however, quite "dirty", with a soot component of roughly one quarter. This has a major influence on their radiative properties. Without the soot, we would be reflecting 2 to 3 Watts per square metre of sunlight back to space. However, absorption by soot prevents this. The final upshot is that more energy is absorbed in the atmosphere, and less at the ground, which may or may not have effects on Sydney's weather.