# Background

## Natural gas and aerosol emissions in Australia

The Australian landscape is largely uninfluenced by human activity. These environments are sources of naturally released trace gases which make up less than 1% of earth’s atmosphere. Naturally occurring trace gases in the atmosphere can have a large impact on living conditions. They react in complex ways with other elements (anthropogenic and natural), as well as affecting various ecosystems upon which life depends. Natural emissions can drift over populous areas and influence local pollution levels in various ways. Ozone can be produced when some natural trace gases interact with pollutants from petrol combustion, and in the lower atmosphere ozone is a serious hazard that not only causes health problems (Hsieh 2013), and billions of dollars worth of damage to agricultural crops (Avnery 2011), but also increases the rate of climate warming (Myhre 2013). Particulate matter in the atmosphere is also a major problem, causing an estimated 2-3 million deaths annually (Hoek 2013, Krewski 2009, Silva 2013, Lelieveld 2015).

Natural emissions can also alter the radiative and particulate matter distribution of the atmosphere, complicating simulations and increasing uncertainty when not properly accounted for. The Australian outback includes large and diverse environments, which can have unique chemical sources. Much of the landscape outside of urban areas is undeveloped and sparsely inhabited. In Australia most long term air quality measurements are performed in or near large cities. However, estimates of atmospheric gas and particulate densities and distributions over much of the rest of the continent are uncertain and lack in-situ measurements.

One source of information which covers the entirety of Australia is remote sensing performed by instruments on satellites which overpass daily, recording reflected solar radiation (and emitted terrestrial radiation). These can be used to quantify the abundance of several chemical species as well as estimate their vertical distribution over the land. While satellite data is effective at covering huge areas, these data have their own limitations and uncertainties. Satellite data does not have high temporal resolution, is subject to cloud cover, and generally does not have fine horizontal or vertical resolution.

The existence of satellite data covering remote areas provides an opportunity to develop more robust models of global climate and chemistry. Natural emissions from areas with little anthropogenic influence and no ground based measurements characterise the majority of Australian land mass (VanDerA 2008). Understanding of emissions from these areas is necessary to inform national policy on air pollution levels. Satellite data allow us to verify large scale model estimates of natural emissions. These measurements can be used to improve models, inform national policy, and predict harmful events.

This thesis will combine satellite and ground based atmospheric measurements with chemical transport modelling to clarify the impact of Australian natural emissions on atmospheric composition and chemistry. In the following subsections the main atmospheric species to be analysed in this thesis are discussed. Then the satellite and modelling techniques to be used will be examined.

## Tropospheric ozone

Ozone is a toxic trace gas which increases mortality rates when populations are exposed for extended periods of time. The amount of global premature deaths per year due to atmospheric ozone exposure has recently been estimated at $$\sim$$150-470 thousand (Silva 2013, Lelieveld 2015). Long term effects of ozone overexposure increase the risk of respiratory disease and may also increase other cardiopulmonary risks (Jerrett 2009).

The Ambient Air Quality (AAQ) National Environment Protection Measure (NEPM), which is the Australian framework for air quality measurement and reporting aiming for “adequate protection of human health and well-being” has set national standards and be