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:
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.
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:
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.