To interpret measurements of atmospheric species, it is often useful to model the chemistry these species may undergo, and compare the results from the models with observations in the field. This group is involved in modelling chemistry in the boundary layer, troposphere and the stratosphere both locally and and globally . We have several different tools developed in collaboration with other groups around the world and these have been described briefly below:
Box modelling
We use two different box models to study halogen chemistry and organic chemistry. The first model was originally developed by Plane and Nien (1991) to study NO3 chemistry and was extended to study the interactions between nitrogen and sulphur in the MBL (Yvon and Saltzman, 1993; Yvon et al., 1996). The model was later modified to study halogen chemistry at Mace Head, Ireland (McFiggans et al., 2000; Saiz-Lopez et al., 2006b). this model contains iodine, bromine, HOx and NOx photochemistry as well as some basic heterogeneous chemistry.
The second model was recently developed to study in detail organic chemistry and was first applied to interpret the observations of HCHO at Cape Verde (see figure). The model is fully coupled to the Master Chemical Mechanism (MCM) and contains a full description of halogen, HOx and NOx photochemistry and basic heterogenous reactions in addition to organic chemistry.
1-dimensional modelling
The group makes use of a one-dimensional (1-D) model, called the Tropospheric HAlogen chemistry MOdel (THAMO), which is a 1-D chemical and transport model which uses a multistep implicit-explicit (MIE) integration routine (Jacobson, 2005) coupled to a vertical diffusion routine described in Shimazaki (1985). The model is also coupled to a dynamic particle production and growth code, which can be used to model iodine particle formation.
The model has four main components: i) a chemistry module that includes photochemical, gas phase and uptake reactions using the MIE procedure; ii) a transport module that includes vertical eddy diffusion; iii) a radiation scheme that calculates the solar irradiance as a function of altitude, wavelength and solar zenith angle (SZA); and, iv) a particle formation and growth module.
This model has been devised and is used in collaboration with Prof. John Plane's group at Univsersity of Leeds.
3-dimensional global modelling
Our group uses the global climate-chemistry model CAM-Chem (Lamarque et al., 2011), from the National Center for Atmospheric Research (NCAR). CAM-Chem is based on the Community Atmosphere Model (CAM) (Collins et al., 2006; Gent et al., 2010), modified to include interactive chemistry, i.e. the coupling of chemistry, radiation and transport calculations in the atmosphere. This model contains a detailed description of both tropospheric and stratospheric chemistry as well as parameterisations for a number of aerosol species. At present time we use CAM-Chem with a horizontal resolution of 1.9° (latitude) x 2.5° (longitude) and 26 vertical levels from surface to around 40 km.
In collaboration with the Atmospheric Chemistry Division at NCAR we have implemented a tropospheric halogen chemistry scheme in CAM-Chem that includes oceanic emissions of very short-lived halocarbons (Ordóñez et al., 2012). For this purpose, the standard chemistry scheme has been extended to include the photochemical breakdown of these species in the atmosphere and the recycling of halogens on sea salt aerosol. The inclusion of these processes in the model leads to an average depletion of around 10% of the tropospheric ozone column over tropical areas, with largest effects in the middle to upper troposphere (see figure). The implications of this tropospheric ozone loss for the radiative balance of the Earth's atmosphere have been quantified by Saiz-Lopez et al. (2012).