In case of release of pollutant or radionuclides into the
atmosphere, estimate of below-cloud scavenging of aerosol particles by
precipitation (or washout) is an essential data to evaluate contamination of the
biosphere. Many studies have already shown an interest to this wet deposition
process, but most of them are theoretical or have been conducted in laboratories
conditions. This study in situ conditions aims to improve knowledge of
below-cloud scavenging of aerosol particles by precipitation. For several
months, three sites with separate environments in terms of climate and ambient
dust have been instrumented to have such a varied palette of precipitation/dust
conditions as possible. A laser disdrometer and a granulomètre (electrical
and/or optical counter) measure respectively precipitations characteristics and
particles concentrations with a high temporal resolution (one minute). The use
of this original instrumental coupling has allowed determining washout
potentials for the nanometric size range of particles aerosol to the
supermicronique size range and for different types of precipitation (snowfalls
and rainfalls with specifics hyetograms).Initially, below-cloud scavenging
coefficients Λ (parameter describing kinetic of this process) were calculated
considering the gobal effet of a precipitation. This “macroscopic” approach is
limited by the influence of “concurrent” processes, as advection or local
emissions of aerosol particles close to the measurements sites. To minimise
effect of these processes on our results, a second methodology based on the high
temporal resolution of the instrumentation used was defined. With this
“intraevent” approach, washout coefficients are calculated on short time scales,
allowing study of impact of the variability of aerosol size and precipitations
characteristics on these coefficients.Results obtained with the two approaches
highlighted the need of considering particles diameter and characteristics of
precipitation to model accurately below-cloud scavenging of aerosol particles.
Comparing results for both type of precipitation, predomination of below-cloud
scavenging by snowfalls compared with below-cloud scavenging by rainfalls was
shown. The importance of the scavenged aerosol diameter was demonstrated. From
the “ultrafine” size range to coarse mode of particles, below-cloud scavenging
coefficient varies by an order of magnitude (from 2.10-3 to 2.10-4 s-1). Minimum
potential is obtained for particles of about 100 nm, which is consistent with
theory of “Greenfield gap” (from 0.1 to 1 μm). For modeling needs, a robust
parametrization between washout coefficient Λ and aerosol particles diameter
(from 10 nm to 10 μm) has been established. Also some relationships between
coefficient Λ and rainfall intensity are proposed for different particles size
range and compared in particular with values implemented in model ldX currently
used at the French Institute of Radioprotection and Nuclear Safety.