Implementation Actions

B2. Characterization of the decontaminating effect of the photocatalytic materials in the real environment

This Action is developed by the Atmospheric Pollution Characterization Unit and COPs and the Emissions Unit of CIEMAT's Department of the Environment.

The characterization of the decontaminating effect in the real environment of one of the materials selected during the course of Actions A2 and A3 has been performed. Its implementation has been carried out on the 700 m2 cover used in the experimental platform developed in Action A4 (photocatalytic island).

The experimental platform has been operating continuously from September 2014 to February 2015, and during this period of measurements three air pollution episodes have taken place per NOx.

The study of the decontaminating effect of NOx of the photocatalytic material used was performed by measuring the concentration of nitrogen oxides at three heights on the meteorological tower located in the centre of the photocatalytic island and at the reference level outside the island (Action A4). In addition, the meteorological variables were recorded: total radiation, relative humidity, temperature at two levels, wind speed and direction. In addition, nanosensors were installed that allowed the simultaneous measurement in the experimental platform of NOx ,O3 and CO.

The inherent difficulty in the need to be at the expense of meteorological and climatic conditions in order to produce episodic conditions (severe contamination by NOx) and the plume reaching the CIEMAT area of experimentation, have compounded the fact that the sink effect induced by the photocatalytic surface has been difficult to observe. In the experimental conditions of the measurement period, it has been possible to detect an NOx deposit flow and, therefore, a macroscopic effect of NOx removal, highly dependent on radiation, relative humidity and wind intensity.

In addition, CIEMAT has developed a parameterization, based on a resistance model, to calculate the NOx deposition rates derived from the presence of photocatalytic surfaces based on the measurements offered by the experimental platform. Thus, the proposed parameterization makes use of direct measurements of speed and wind direction and relative humidity and radiation at a height, as well as measures of temperature at two levels and of concentration of the gases being studied at the three levels mentioned.

In addition, the data obtained in the laboratory through the tests under ISO 22197-1:2007, during the development of Actions A3 and B1, allowed the calculation of NO surface deposition rates.

The NO deposition rates estimated using the NOx gradients measured on the CIEMAT experimental shelf are of the same order of magnitude as the estimated surface deposition rates when using the information given by the photo-reactor in the ISO tests. In the case of NO2, vertical positive gradients could only be observed for two days of said episodic period. However, average deposition rates could be estimated with values of an order of magnitude approximately lower than those estimated for NO.

Furthermore, the need to know the concentrations of VOCs that exists in the experimental area under episodic conditions, raised the need to fine-tune a measurement method that allowed the identification and quantification of the most determinant volatile organic compounds (VOCs) of the atmospheric chemistry of the area of experimentation and the carrying out of simulations committed to within the framework of Action B3.

Finally, the weather tower permanently installed in CIEMAT (which registers wind, temperature, radiation and humidity at different levels) has allowed the completion of the characterization of the existing flows in the area studied.

All these data have been processed and have allowed the CFD model to be fed, so that it has been possible to simulate the flows and dispersion of NOx and O3 in the experimental area for different episodic conditions and to validate the numerical prototype under the experimental conditions indicated, obtaining excellent concordance between the experimental data and the results of the simulations.

The development Action B2 has allowed the characterization of the macroscopic effect induced by the material arranged in the experimental zone on the vertical distribution of environmental NOx concentration and the quantification of this effect, having estimated the deposition rates of NOx on this photocatalytic surface. Finally, the validation of the implementation of the reactive chemistry and the deposition function in the chosen CFD model (Action B3) was carried out.

Experimental platform for the measurement of the NOx deposition rate on photocatalytic surfaces in the outdoor environment (CIEMAT facilities). Experimental platform details: a) mast with sensors and probes; b) control booth; c) data control, acquisition and processing system. High volume collector and Orbo 32 Small adsorption tubes used for VOC sampling.