Investigating the sensitivity of modelled nitrogen inputs in the Mediterranean to dry deposition parameters

There are several nitrogen-sensitive areas in Europe, some more sensitive than others, and the Mediterranean climate zone is where many of the highly sensitive areas are. One of these areas is Portugal, where very few studies focus on this issue. The lack of infrastructure to monitor nitrogen concentrations and deposition in the country, as well as policies to enforce nitrogen emission reductions, poses a challenge, as ammonia is the most critical pollutant to fulfil Portugal's future emission goals.
Currently, modelling is the only cost-effective option to effectively study nitrogen deposition in the Mediterranean. Due to the extent of agriculture in these areas, nitrogen (N) deposition assessments have been conducted for many years in other countries, such as the Netherlands and Germany, using Chemistry Transport Models (CTMs) to assess the dry deposition of reduced and oxidised N. Among these CTMs, LOTOS-EUROS is regarded as one of the most advanced models that includes a compensation point parametrization for ammonia. However, applying this model to a Mediterranean area requires some adaptation since its deposition parameters are mostly based on studies from North-Western Europe. Since vegetation parameters influence the surface resistance of gas-phase deposition, and this resistance is crucial for gas deposition, using these models in climates different from where they were initially developed will likely lead to inaccurate results. Due to this, there is a need for a CTM that better represents different climate zones. 
Here, we use a new version of the LOTOS-EUROS model incorporating a three-tiered vegetation approach. The three tiers considered are Tier 1—climate zones; Tier 2—land use classes; and Tier 3—vegetation type. This method incorporates 140 combinations of land use and vegetation types, allowing us to differentiate the Mediterranean from the standard temperate climate by changing vegetation parameters.
With this study, we aim to adapt and enhance the dry deposition module of LOTOS-EUROS by including specifications for the Mediterranean climate and vegetation. To achieve this goal, sensitivity runs were performed for multiple vegetation and climate-specific parameters to assess which are the most influential variables for the study region. Then, the most sensitive parameters were analysed to understand the variations.
This work found that adapting the maximum stomatal conductance is highly prone to introduce changes in the modelled deposition fluxes and concentrations of oxidised and reduced nitrogen and ozone. Maximum and minimum vapour pressure deficit and maximum, optimal and minimum temperature were also among the most susceptible to cause impacts in the model results over the Mediterranean. Also, the start and end of the growing season greatly impacted the modelled deposition fluxes since the growing season starts earlier and finishes later in the Mediterranean. Hence, adapting the deposition parameters to the Mediterranean climate and vegetation significantly impacts the modelled concentration and deposition fluxes of oxidised and reduced nitrogen compounds and ozone.