The marine boundary layer is critical for our understanding of global atmospheric composition. Around half of all methane and a significant fraction of ozone is destroyed in this region. The chemistry of the marine boundary layer is often considered ‘simple’ and therefore uninteresting in comparison to other regions of the planet, however, there are still some mysteries associated with it.
Ship emissions of NOx constitute a significant and increasing fraction of anthropogenic NOx emissions. However, when included in global chemistry transport models they appear to cause the models to grossly overestimate O3 and NOx within the marine boundary layer. It has been suggested that the reason for this is the combination of coarse model resolution and the non-linear nature of the atmospheric chemistry. To test this, we have developed a ‘high’ (200m by 200m) resolution model of the marine boundary layer and have included a ship plume. We can systematically and accurately degrade the resolution of the model from the 200m by 200m resolution to a resolution of 10km by 10km. Changing the model resolution in this way changes the OH, NOx lifetime and Ozone production efficiency by 8%, 32% and 31% respectively. Interpolating to the resolution of the current generation of chemistry transport models suggests that they will overestimate there parameters by 15%, 55% and 59% respectively. This has implications for the compositional and climate impact of ship plumes.
The UK has recently established a long-term tropical composition monitoring station on one of the islands making up Cape Verde . We find that the diurnal cycle of O3 concentration is much larger than that predicted by either a global chemistry transport model or a constrained box model. We also find the NO to NO2 ratio is significantly different from those calculated. This suggests an extra sink for O3 within the tropical marine boundary layer. The inclusion of BrO and IO observations made at the site brings the box model into agreement with the observations. We make a series of plausibility arguments to suggest that halogen chemistry within the tropical marine boundary layer may be a pervasive influence on its composition and outline a field campaign to investigate this.
