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 \cite{Zeng2012}. HCN is also a good indicator for tropospheric pollution and transport as it has an atmospheric lifetime of 2-4 months \cite{Rinsland2001,Li2000,Holzinger1999}.