Source and Pathway Sensitivity in Troposphere-to-Stratosphere Transport: An Adjoint-Based Approach

Ozone in the upper troposphere / lower stratosphere region of the atmosphere (8-20km) is an important component in Earth's radiative budget, and therefore has a role in determining climate. Ozone in this region is also known to be sensitive to chemical depletion by halogenated species. Since the Montreal protocol came in to force in 1989, emissions of long-lived halogenated species such as chloro-fluoro carbons (CFCs) have been in decline. Consequently, interest has focussed on replacement species with shorter chemical lifetimes, as well as natural halogenated species emitted from the oceans. Many of these species, known as `very short-lived species' (VSLS), have chemical lifetimes of six months or less. The relatively short lifetime means that the proportion of an emitted chemical that subsequently reaches the lower stratosphere, where it may cause ozone depletion, is uncertain. In particular, the proportion reaching the lower stratosphere may be highly sensitive to the geographical location where the chemical is emitted, due to differing transport effects associated with local weather patterns. The current proposal aims to quantify the relative importance of different geographical emission locations for the transport of VSLS to the lower stratosphere. Further, the ozone depletion potential of example VSLS emitted at different locations will be calculated in the form of a `sensitivity map'. The sensitivity maps will allow policymakers to straightforwardly assess the amount of ozone depletion that is likely to occur if a particular VSLS is emitted from a particular surface site. A novel approach to calculate the sensitivity maps, based on solving the `adjoint' to the chemistry-transport equation, is proposed. In addition to efficiently solving the VSLS problem the new method has the potential to be applied to many different problems in atmospheric chemistry and transport. A secondary aim of the proposal is therefore to promote the adjoint approach within the UK atmospheric science community.