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# 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 benchmarks for reporting. The NEPM covers six chemical groups including Ozone (O$$_3$$), and the benchmarks are shown in figure \ref{fig:nepm}.

The primary source of ozone in the lower troposphere is chemical formation following emissions of precursor gases, including VOCs, and NO$$_X$$. Globally the greatest sources of NO$$_X$$ include fossil fuel combustion ($$\sim$$50%), biomass burning ($$\sim$$20%), lightning, and microbial activity in soils (Delmas et al., 1997). Estimates using CHASER (a global Chemical Transport Model (CTM)) constrained by measurements from two satellites as well as the in-situ measurements taken through LIDAR and aircraft (INTEX-B) put global tropospheric NO$$_X$$ emissions at 45.4 TgN yr$$^{-1}$$ in 2005 (Miyazaki 2011).

The majority of this chemical formation is due to photochemical oxidation of carbon monoxide (CO), methane (CH$$_4$$), and other Volatile Organic Chemicals (VOCs) in the presence of nitrogen oxides (NO$$_X$$ $$\equiv$$ NO $$+$$ NO$$_2$$) (Stevenson 2006).

While photochemical production is the dominant source, stratosphere to troposphere transport (STT) of ozone is also important and climate change may drastically increase this quantity (Hegglin 2009). Ozone-rich air mixes irreversibly down from the stratosphere during certain meteorological conditions (Sprenger et al., 2003; Mihalikova et al., 2012). In the extra-tropics, STTs most commonly occur during synoptic-scale tropopause folds (Sprenger et al., 2003) and are characterised by tongues of high Potential Vorticity (PV) air descending to low altitudes. PV is a metric which can be used to determine whether a parcel of air is stratospheric, based on its local rotation and stratification. These tongues become elongated and filaments separate from the tongue which mix into tropospheric air. Stratospheric ozone brought deeper (lower) into the troposphere is more likely to affect the surface ozone budget and tropospheric chemistry (Zanis et al., 2003; Langford et al., 2009).

Photolysis of NO$$_2$$ forms NO + O(3P), which combines with O$$_2$$ to form O$$_3$$, leading to reaction with NO to form NO$$_2$$ + O$$_2$$. These reactions reach a steady state where O$$_3$$ is proportional to the ratio between NO$$_2$$ and NO (Sillman 2002). The following formulae show an example of this with CO, however, similar reactions occur for many VOCs: \begin{aligned} NO_2 + hv &\overset{k_1}{\rightarrow}& NO + O({}^3 P) \\ O({}^3 P) + O_2 &\overset{M}{\rightarrow}& O_3 \\ NO + O_3 &\overset{k_2}{\rightarrow}& NO_2 + O_2 \\ \left[O_3\right] &\sim& \frac{k_1}{k_2} \frac{\left[NO_2\right]}{\left[NO\right]} \\ OH + CO &{\rightarrow}& HOCO \\ HOCO + O_2 &{\rightarrow}& HO_2 + CO_2 \\ HO_2 + NO &{\rightarrow}& OH + NO_2 \\\end{aligned} where $$k_1$$ and $$k_2$$ are reaction rates, and hv represents photons. The balance of these reactions is: \begin{aligned} CO + 2O_2 + hv {\rightarrow} CO_2 + O_3 \end{aligned}

Isoprene (C$$_5$$H$$_8$$) is a precursor to ozone through radical oxidative chemistry. Isoprene in the atmosphere reacts rapidly with hydroxyl radicals (OH) and then O$$_2$$ to form peroxy radicals (RO$$_2$$). These react with nitrogen oxides and can lead to ground-level ozone formation similarly to the CO reaction listed prior.

Together formaldehyde (HCHO) and NO$$_2$$ regulate tropospheric oxidation capacity through O$$_3$$ production, as well as being health hazards. The HCHO/NO$$_2$$ ratio can be used to determine whether surface O$$_3$$ is NO$$_2$$ or VOC limited (Mahajan 2015). If O$$_3$$ is NO$$_2$$ limited then an increase in NO$$_2$$ will increase O$$_3$$ levels while an increase in HCHO will not, and vice versa when O$$_3$$ is HCHO limited. NO$$_2$$ is a common pollutant in populated areas, released primarily by combustion in power generation and transport. Outside of cities in Australia, VOCs and NOx are emitted from biogenic sources, although lightning, and biomass burning (most clearly in the Northern Territory) also play a role (Guenther 2006, VanDerA 2008).

The amounts of tropospheric ozone from STTs and photochemical production are estimated to be around 550 and 5100 Tg yr$$^{-1}$$, respectively. The main ozone removal processes are chemical destruction and dry deposition, respectively removing 4650, 1000 Tg yr$$^{-1}$$ from the the troposphere (Stevenson 2006).