Implementation Actions

B3. Implementation of reactive chemical and NOx deposition rate in a CFD model

This action is developed by the Atmospheric Pollution and Ecotoxity Modelling Unit of the Environment Department of CIEMAT.

Through a Computational Fluid Dynamic (CFD) model, the dispersion of atmospheric reactant pollutants and the effect of the Nitrogen Oxide (NO) deposition on areas treated with photocatalytic materials are simulated, and lastly, preliminary simulations on the area of Alcobendas will be carried out, in order to optimise the location of monitoring points.

This B3 action has been divided into three phases:

  • Stage I: Implementation of a chemical mechanism defined by a set of chemical reactions and species in the CFD model
  • Stage II: Modelling of the photocatalytic effect
  • Phase III: Flow and pollutant reactants simulation in the urban area of Alcobendas

Stage I: Implementation of a chemical mechanism in the CFD model

The main objective in this first phase is the preparation of the CFD model, STARCCM+, used, in order to simulate the dispersion of atmospheric pollutant reactants. A fine-tuned of the CFD model in idealized urban scenarios is carried out in order to analyse with precision the dispersion of atmospheric pollutant reactants. To evaluate the effect of production and elimination due to the chemical reactions of the pollutants (NO and NO2 in this case), different estimates of the atmospheric chemistry are considered: (a) passive tracer gas (non-reactive), (b) photostationary chemical mechanism and (c) complex chemical mechanism (designed with CHEMATA (Kirchner 2005).

In this first fine-tuned phase with regards to the introduction of chemical reactions in the CFD model, the proposed objectives have been met successfully.

  • Development of a complex chemical mechanism capable of reproducing precisely the concentrations of NO and NO2 (CHEMATA, Kirchner).
  • Verification of results obtained from the implementation of the complex chemical mechanism in a CFD model through the comparison with results of a chemistry box model (Kirchner)
  • Through the sensitivity study certain criterias have been concluded which enables the identification of the most appropriate chemical mechanism to use in real, large urban geometries, taking into account the atmospheric conditions. On certain occasions a simple chemical mechanism can provide similar results to those of a more complex chemical diagram but with less calculation requirements. Furthermore this sensitivity study enables a better understanding of the processes which take place in the atmosphere and the simplification of the number of settings to simulate in the cases of real streets where the computational cost is higher.
Stage II: Photocatalytic effect modelling

The deposition effect on photocatalytic surfaces is modelled as a negative flow equal to the product of the atmospheric concentration around the surface due to a deposition rate (Vd) which depends on the type of photocatalytic material. This flow establishes a term which is added into the transport equation of NO (given that only NO is deposited) as a sink concentration around the surface ( Fdeposit=-[NO] Vd). The deposition rate used is calculated through the values measured in the tests carried out in the laboratory in controlled conditions. In the action B2 an experimental system was carried out to enable the evaluation of the photocatalytic effect in environmental conditions. Therefore, the objective of this phase is to model the dispersion and the deposition effect of NO on the photocatalytic surface of the experimental system of Action B2. The influence of the wind speed on the effect of NO deposition in photocatalytic material is also studied, as well as the difference between temperature reached by the surface and the temperature of the air.

The proposed objectives for this phase have been successfully achieved:

  • Implementation of the function of the NO deposition rate in the CFD model, reproducing with precision the effect of deposition around the photocatalytic surface.
  • Evaluation of the experimental and simulated results with a correlation higher than 0.6.

Stage III: Flow simulation and pollutant reactants in the urban scenario of Alcobendas

Once these two aspects have been evaluated, there was a preliminary simulation with the CFD model on the urban scenario of Alcobendas in order to choose suitable locations for the experimental measurements. This simulation has been carried out in Paseo de la Chopera and its surrounding area.

To this end, the detailed modelling of the buildings’ 3D geometry has taken place and the prevailing weather conditions (Southwest wind) have been simulated, taking similar intake concentration values of NO, NO2 y O3 to those measured in an episode in the pre-campaign (Action B4). For the calculation of the emissions, the road traffic statistics was available for only some streets in the area (Source: Local Authority from Alcobendas). As for the rest of the streets near to the area, it has been assumed that they are similar to the streets where the statistics are already known.

Due to the fact that the aim of this simulation is to choose the best measuring position for future campaigns, given the high uncertainty in the emission data and considering the previous results on the different chemical mechanisms, the photostationary mechanism was used as the initial approach to estimate the NO2 and NO distribution in the area of Paseo de la Chopera. These results provide additional useful information (areas of maximum and of minimum) in order to optimize the positioning of measurement devices during the future campaigns.

Ultimately, the area of Paseo de la Chopera (Alcobendas) has been simulated in atmospheric conditions and of prevailing concentrations according to the pre-campaign carried out (Action B4). And NO y NO2 distributions for these conditions have been obtained, which help to be able to optimize the location of monitoring points.

Real image of the field of study in Paseo de la Chopera for the development of phase III: Flow simulation and pollutant reactants in the urban scenario of Alcobendas NO concentration in ppb at 2.5m NO concentration in ppb at 2.5m