# Decadal-scale trends and variability in Australasian atmospheric composition

AbstractThe abstract goes here.

# Introduction

## Trace Gases

CO is produced globally from four main sources; biomass burning, fossil fuel burning, oxidation of non-methane hydrocarbons (NMHC), and oxidation of methane (CH$$_{4}$$) (Seiler 1987, Novelli 1992). Biomass burning in particular is a major source year round in the tropics and during summer and autumn in the high northern latitudes (citation not found: Galanter2000). In the southern hemisphere, the distribution of CO is largely affected by biomass burning (Edwards 2006, Gloudemans 2006). The main sink of CO in the troposphere is through reaction with the hydroxyl radical (OH). With a relatively long lifetime ranging from 2 weeks to a few months, depending on the levels of OH, CO is it an appropriate indicator of tropospheric pollution and transport (Hough 1991, Khalil 1983, Holloway 2000, Novelli 1992). In the mid to high northern latitudes the seasonal cycle of CO is driven by the variation of OH. CO is at a minimum in summer when OH concentrations are high and CO is at a maximum at the end of winter after a gradual build up due to the low OH concentrations. In the tropics, the maximum CO concentration occurs in September-October, correlating to the end of the biomass burning season (Hough 1991, Novelli 1992). CO has a relatively long lifetime, ranging from two weeks to a few months, depending on the levels of OH, making it an appropriate indicator of tropospheric pollution and transport (Hough 1991, Khalil 1983, Holloway 2000, Novelli 1992).

HCN is produced mainly from biomass burning and its main sink is ocean uptake. It shows significant seasonality over Australasia, with a maximum at the start of January and a minimum at the start of July (Zeng 2012). HCN is also a good indicator for tropospheric pollution and transport as it has an atmospheric lifetime of 2-4 months (Rinsland 2001, Li 2000, Holzinger 1999).