The main objective of this Ph.D. thesis is the implementation and evaluation of the tro- pospheric oxidant gas-phase chemistry within the online multiscale atmospheric model NMMB/BSC Chemical Transport Model (NMMB/BSC-CTM) to provide a basis for com- puter modelling studies of O3, PM, visibility, acid deposition and air toxics and further
extend the range of applicability to study interactions between the meteorological and the chemical processes occurring in the atmosphere. This main goal is divided into three main specific objectives:
1. Contribute to the development of a new state-of-the-art online model implementing
1.5. OBJECTIVES OF THE THESIS
the oxidant gas-phase chemistry of the troposphere.
2. Design the configuration of the modelling system and prepare the required input information to run the model at regional and global scales.
3. Evaluate the tropospheric gas-phase chemistry using a wide range of observational datasets for a regional and a global model scenarios.
1.5.1 Model development
This Ph.D. thesis has contributed to the development of the new online global/regional atmo- spheric model NMMB/BSC-CTM. The model developments are focused on the processes oc- curring in the troposphere that involve the oxidant gas-phase chemistry.
One of the major aims is to review and implement the main physical and chemical processes that affect pollutants in the atmosphere into the meteorological core, Non-hydrostatic Multiscale Model on the B grid (NMMB), using a modular approach. The strategy followed is:
1. Review several state-of-the-art photochemical mechanisms (e.g., CB04, CB05, RADM, RADM2, SAPRC07) that describe the chemical reactions, product yields and kinetics data and select a computational efficient scheme to be implemented into the model. 2. Review the most used and extended state-of-the-art photolysis schemes (e.g., Fast-J, Fast-
JX, FAST-TUV, Madronich) and check which improvements can be introduced in Fast-J, already implemented in the NMMB/BSC-CTM model. We focus in implement further CB05 photolysis reactions and update their different cross sections and quantum yield tables.
3. Extend the Wesely dry deposition scheme to all the gas-species considered by the CB05 chemical mechanism. The complex part of this scheme is to compute the canopy resis- tance, Rc. However, other parameters used in these parameterizations need to be updated
(i.e., Effective Henry’s law coefficient, reactivity factors, diffusion coefficients). These parameters describe soil type, vegetation and friction gas resistances.
4. Evaluate the cloud chemistry scheme, already implemented within the NMMB/BSC- CTM.
5. Implement a simplified linear stratospheric ozone scheme to model the Stratosphere- Troposphere O3 exchange properly with low computational cost. Since we are focused
in the tropospheric chemistry, we are not interested in including a detailed stratospheric chemistry. Hence, simple parameterizations of stratospheric O3are evaluated as an upper
boundary condition to model the O3Stratosphere-Troposphere Exchange (STE).
The importance of lateral Boundary Conditions (BCs) in the regional AQMs is well recognized by in the atmospheric community. For that reason, one of the specific technical implementations related to the BC is to design an algorithm that read, adapt and prepare the BCs for the regional configuration using the output data from a global or regional model.
1.5. OBJECTIVES OF THE THESIS
1.5.2 Experiment design
A second major aim of this Ph.D. thesis is the definition of the model configuration for runs at different scales. Hence, it is important to set up the domain, the horizontal and vertical resolu- tions, the meteorological model parameterizations, and all the required input data to execute the model properly. In this sense, two main domains of study are defined:
1. Global annual simulation with a horizontal grid spacing of 1.4◦x1◦and 64 vertical layers up to 1 hPa (Global run).
2. Regional annual simulation over Europe with a horizontal resolution of 0.2◦x0.2◦. To test the sensitivity of the results to the vertical grid discretization, two different vertical configurations are defined: 1) 24 vertical layers up to 50 hPa, and 2) 48 vertical layers up to 50 hPa (Regional run).
A critical part in an atmospheric chemistry model simulation is the required model inputs (i.e., meteorological initial and boundary conditions, geomorphological characterisation, chemistry emissions). The main critial inputs to computed are the emission fluxes of the primary gases under study. For that, emission inventories available in the community are used. Different emission sources (e.g. human activity, sea, soil, vegetation, biomass burning, etc.) have a relative contribution to the modelled final concentrations of the relevant pollutants. For that reason, two main tasks are done:
1. Review and select different emissions inventories currently implemented in the AQMs (ACCMIP and TNO).
2. Prepare emission inventory data as input to the regional and global modelling system (conservative remapping to the model grid, temporal disaggregation, speciation into the CB05 chemical mechanism, and pre-process the files into the model format).
1.5.3 Model evaluation
The third objective of this Ph.D. thesis is to perform a reference model evaluation in terms of the relevant air pollutants in order to determine and quantify the model’s performance capabilities and weaknesses. Therefore, the specific goals are:
1. Review the present air quality modelling evaluation studies in order to decide which sta- tistical metrics and observational data are used for gas-phase species and which relevant air pollutants are analysed.
2. Perform a complete model evaluation over a full year period for both Global and Re- gional runs. This evaluation consists of several qualitative and quantitative comparisons of the model results with the observational dataset selected (surface air quality monitoring stations, ozonesondes, satellite, climatology studies, and aircraft campaigns).