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\subsection{Background}  \textbf{TODO: following 2 paragraphs to be combined or reworked}   Ozone Tropospheric ozone  is present in important for both air quality and climate change. Over  the troposphere due industrial period, tropospheric ozone, the third most potent greenhouse gas has been estimated  to exert  a variety radiative forcing equivalent to a quarter  ofdynamical and photochemical processes, including downward transport from the ozone-rich stratosphere and anthropogenic pollution. Ozone-rich air mixes irreversibly down from  the stratosphere during meteorologically conducive conditions \cite{Sprenger2003, Mihalikova2012}; these CO2 forcing.  The primary sources of tropospheric ozone  are referred chemical creation and stratospheric input, estimated using a model ensemble  to as Statosphere - Troposphere Transport events (STTs). In the extra-tropics, STTs most commonly occur during synoptic-scale tropopause folds \cite{Sprenger2003} be $5100\pm600 Tgyr^{-1}$  and $550\pm170 Tgyr^{-1}$, respectively.  The primary sinks  are characterised by tongues of high PV air descending to low altitudes. These tongues become elongated chemical destruction  and filaments separate from the tongue which mix into tropospheric air. Stratospheric ozone brought deeper (lower) into the troposphere is more likely dry deposition, estimated  to affect the surface ozone budget be $4700\pm700 Tgyr^{-1}$  and tropospheric chemistry \cite{Zanis2003}. $1000\pm200 Tgyr^{-1}$, respectively \cite{Stevenson_2006}.  Tropospheric ozone Ozone  is important for both air quality and climate change. Over the industrial period, tropospheric ozone, present in  the third most potent greenhouse gas has been estimated to exert a radiative forcing equivalent troposphere due  to a quarter variety  of dynamical and photochemical processes, including downward transport from  the CO2 forcing. There are two sources of tropospheric ozone: ozone-rich  stratospheretroposphere transport (STT)  and photochemical production, with estimated source fluxes of 550 Tgyr-1 anthropogenic pollution. Ozone-rich air mixes irreversibly down from the stratosphere during meteorologically conducive conditions \cite{Sprenger2003, Mihalikova2012}; these are referred to as Statosphere - Troposphere Transport events (STTs). In the extra-tropics, STTs most commonly occur during synoptic-scale tropopause folds \cite{Sprenger2003}  and5100 Tgyr-1, respectively \cite{Stevenson_2006}. Loss processes  are chemical destruction characterised by tongues of high Potential Vorticity (PV) air descending to low altitudes. 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 wet deposition. tropospheric chemistry \cite{Zanis2003, Zhang_2014}.  While the amount of tropospheric ozone is small compared with that found in the stratosphere, it is an important constituent. The relative contributions to the tropospheric ozone budget of photochemistry and STT (dynamical transport) is still uncertain \cite{Zanis2003}. \cite{Zanis2003, paper in open tab}.  A high correlation is found between lower stratospheric and tropospheric ozone \cite{Terao_2008} with the highest STT associated with the jet-streams over the oceans in winter.  Irreversible STT of ozone have been shown to be important for explaining tropospheric ozone variability \cite{Tang_2011}.