IAQ 2003, Presentation 25:
Modelling of the Time Dependent Deposition of O3, SO2 and NO2 to Material Surfaces
(Work in EU-Project IMPACT. NO EUK4-CT-2000-0031)
As a part of the EU project IMPACT measurements of the deposition velocities of O3, NO2 and SO2 to a number of typical indoors material surfaces were performed at NILU (Norwegian institute for Air Research). These measurements were made in chamber experiments where typical indoors surface material samples were hung up and exposed to the pollutant gas. The concentrations of the pollutant gases into and out of the chamber were measured continuously with time. The gas flux to the samples and the deposition velocity, equals gas flux/chamber concentration, were calculated after 48 h. run on each sample. These data were used together with collected literature data to calculate predicted I/O-ratios of the pollutant gases in the IMPACT model presented at the IAQ 2003 conference.
The experimentally monitored time dependent curves for the deposition velocity of the gases to concrete floor tile samples were modelled with a range of mathematical models to understand the chemical deposition mechanism of the gases. The models included Langmuir models assuming adsorption of the gas on the material surface and models assuming direct reaction on the material surface and diffusion of the gas into the material from the air. The best fit for O3 and NO2 at RH ranging from O % to 90 % was obtained with a simple two-parameter model assuming direct reaction of O3 and NO2 with adsorbed surface water and direct diffusion of O3 and NO2 into the material from the air. The best fit for SO2 at RH = 90 % was obtained with a Langmuir model assuming adsorption of SO2 on the surface before reaction on the surface or diffusion into the material. At lower air humidity the modelling was inconclusive for SO2. The different result for SO2, from that for O3 and NO2, could be explained with the much higher effective Henry constant for SO2 dissolution in surface water than that for O3 or NO2.
Models assuming an additional second order reaction of O3 with an adsorbed surface species with a given start mass, gave improved curve fit in the first 500 min. Modelling of the best fitting model for O3 to the experimental curves obtained over the whole humidity range, gave markedly lower equilibrium deposition velocities, for than those measured after 48 h. run. The modelling for O3 gave a deposition velocity minimum in the intermediate relative air-humidity range of 50-70 % in agreement with observations. The deposition velocity minimum seemed to be explained with a reduced constant for the reaction of O3 with water or OH- ions on the surface.
Key words: ozone deposition velocity, time dependence, modelling, surface reaction, diffusion, concrete.
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© November 2003