Size-dependent scavenging efficiencies of multicomponent atmospheric aerosols in clouds

Abstract

The size-dependent scavenging efficiencies of inorganic and carbonaceous fractions of aerosol have been investigated in clouds present at the Puy de Dôme (1465 m above sea level, France) in winter. In-cloud interstitial aerosols and residues of evaporated cloud droplets are separated by a round jet impactor (RJ1) collecting the interstitial phase and a counterflow virtual impactor (CVI) collecting cloud droplets hereinafter evaporated. In addition, interstitial and droplet phases are collected together in a whole air inlet. Particles (interstitial aerosol and evaporated cloud droplets) are subsequently sampled using low-pressure cascade impactors, weighted, and analyzed for major ions, organic (OC) and elemental (EC carbon. RJI and CVI data provide mass-scavenging efficiencies of the different aerosol components. The scavenged fraction of inorganic species ions (εinorg) is 0.76 ± 0.07 on a first accumulation mode (0.2 ± 0.1 μm geometric diameter (GMD) (Accl) and 0.93 ± 0.03 on a second (0.5 ± 0.2 μm GDM) accumulation mode. Instead, the scavenged fraction of organic carbon (εOC) is much lower with averaged values of 0.14 ± 0.07 on both modes. These differences are clearly indicative of a high degree of external mixing between inorganic and organic aerosols. Gravimetric data confirm these values Of εOC and εinorg with total mass scavenging efficiency of 0.41 ± 0.07 and 0.54 ± 0.07 for Accl and Acc2 modes, respectively. Higher mass scavenging efficiencies are obtained for EC than OC (εEC(Accl) = 0.33 ± 0.09 and εEC (Acc2)= 0.74 ± 0.09). This suggests either internal mixing between black carbon and more soluble species such as sulfate or nitrate or presence on the surface of EC particle of species containing more hygroscopic properties.