Experimental measurements of the washout radio of the aerosols by the rain: comparison models/measurements
Congress title :CFA 2007/2008 - 23ème Congrès Français sur les Aérosols
Congress town :Paris
Congress date :16/01/2008
The removal of atmospheric aerosols by raindrops plays an outstanding role on air quality. The falling raindrops collide with aerosols and collect them following three mechanisms: Brownian diffusion, interception and inertial impaction. The efficiency which particles are collected is given by the scavenging coefficient: Λ (s-1). This one depends of a lot of external factors like aerosol size distribution, raindrop size distribution and chemical properties of the particles. The study of the washout coefficient of the aerosols by the raindrops is a domain of research which has been few studied directly in the environment.
For operational needs of safety, the IRSN (Institut de Radioprotection et de Sûreté Nucléaire) uses a constant scavenging coefficient of 10-4 s-1 in their models of calculation of atmospheric transfers of pollutant. Moreover, a lot of models, existing in the literature, are used to calculate this scavenging coefficient.
The goal of this study is to evaluate the existing uncertainties on the washout coefficient in comparing models all together, then with the results of in situ experimental measurements of Λ.
The experimental protocol can be sum up by the use in parallel of an ELPI (Electrical Low Pressure Impactor) and a pluviometer. The EPLI gives the particles concentration by range of particles size and its coupling with the pluviometer gives the evolution of the number of particles during a rain event. Thanks to these measurements, it's possible to calculate the scavenging coefficient function of the size of the particles and function of the rain intensity.
Campaigns of measurements have been done in the Laboratoire de Radioécologie de Cherbourg-Octeveille (IRSN/LRC) between January 2007 and May 2007. The scavenging coefficients obtained are between 10-4 and 10-3 s-1 for rain intensities in the range of 0.3 to 12 mm/h and for submicronic particles.
The comparison between these results descended from measurements and models shows a consistent evolution of calculating models of the scavenging coefficient with the rain intensity and the particles size distribution. But, studied models underestimate measurements with two decades and the value of 10-4 s-1 used for the studies of safety might be lightly underestimated for strong rain.
Those results have been obtained for one campaign of experiences done in Cherbourg-Octeville, it is necessary, in the future, to develop other campaigns of measurements in different places and with different environmental conditions so as to enlarge the field of investigation.