Ž .
Atmospheric Research 55 2000 103–113 www.elsevier.comrlocateratmos
Heterogeneous nucleation on rough surfaces: implications to atmospheric aerosols
Mihalis Lazaridis
a
, Øystein Hov
a
, Kostas Eleftheriadis
a,b,
a
Norwegian Institute for Air Research NILU , Postoboks 100, N-2007 Kjeller, Norway
b
N.C.S.R. Demokritos, 15310 Ag. Paraskeui, Attiki, Greece Received 23 August 1999; accepted 9 June 2000
Abstract
The effect of the surface roughness of solid atmospheric aerosol particles on their heteroge- neous nucleation capability has been examined using the concept of Aself-affineB rough surfaces.
The surface roughness has a great influence on the contact angle between the particle surface and the nucleating liquid droplets. Roughness enhances wetting and the rate of heterogeneous
nucleation from the vapor to the liquid phase. The paper furthermore discusses the considerable influence of the surface roughness on the physico-chemical characteristics of atmospheric insolu-
ble aerosol particles. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Surface roughness; Heterogeneous nucleation; Aerosol; Wetting
1. Introduction
Ž Aerosol particles are important for weather and climate Charlson et al., 1992; Covert
. et al., 1992 . Aerosols determine the formation of Type I Polar Stratospheric clouds
Ž .
Ž .
PSCs , water condensation, ice, Cloud Condensation Nuclei CCN and contribute to Ž
the radiation forcing of the atmosphere Charlson et al., 1992; Clarke, 1992; Luo et al., .
Ž .
1994 . The activation processes of atmospheric aerosols CCN
have considerable consequences on the optical properties of clouds and eventually on earth’s climate.
Ž .
Corresponding author. Norwegian Institute for Air Research NILU , Postoboks 100, 2007 Kjeller, Norway.
0169-8095r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž
. PII: S 0 1 6 9 - 8 0 9 5 0 0 0 0 0 5 9 - 4
Atmospheric aerosols consist of soluble and insoluble particles. In reality, atmo- spheric aerosols consist of complex surfaces with soluble and insoluble parts and details
Ž of their aerosol dynamics history may further influence their surface energetics Knight,
. 1971; Lazaridis et al., 1991 . The activation process of soluble aerosols due to nitric acid
Ž .
vapor has been studied in a recent paper Kulmala et al., 1993 . Modification of the Kohler equilibrium curves of soluble particles due to the presence of insoluble material
¨
Ž .
in the aqueous solution drop has been also studied Seinfeld and Pandis, 1998 .
However, the assumption of no interaction between the insoluble portion and the liquid part was used. In the present work we examine the effect of surface roughness on the
heterogenous nucleation process of insoluble and solid particles where there is an interaction between the solid surface and the liquid solution of the nucleating embryo.
One reason for the complex geometry of atmospheric particles is the coagulation process itself which leads to the appearance of complex surfaces with varying geometric
orientations. The effect of aerosol shape and rigidity upon particle activation is very Ž
. important and complicated Knight, 1971; Lin et al., 1993; Lazaridis et al., 1992 .
Ž .
Fletcher 1969 has treated the case of a distribution of conical pits in the surfaces of Ž
. Ž
aerosols, Huh and Mason 1977 considered surfaces with periodicity cross, hexagonal .
Ž .
and radial grooves , Lin et al. 1993 considered concentric rings of cones and Lazaridis Ž
. et al. 1992 studied the effect of active sites on the process of heterogeneous nucleation,
but it is obvious that all possible shapes are not excluded in determining the particle Ž
. roughness. Knight
1971 referred to similar approaches as Aacademic exercisesB.
However, the effect of surface roughness on the physical characteristics of atmospheric particles is an important issue which needs to be eventually resolved since in the
majority of current modeling studies aerosols have been treated as smooth and spherical Ž
and their surfaces energetically uniform Luo et al., 1994; Charlson et al., 1992;
. Lazaridis et al., 1991 . For a detailed discussion on the activation processes of
Ž .
atmospheric aerosols we refer to Pruppacher and Klett 1997 . Here we examine the effect of surface inhomogeneities on the nucleating effective-
ness of atmospheric aerosol particles. The approach we follow is based on fluctuations Ž
and noise correlation of the quenched disorder on self-affine rough surfaces Barabasi .
and Stanley, 1994 . The paper follows the approaches adopted by the pioneering work of Ž
. Ž
. De Gennes 1985 on wetting phenomena and recent studies by Chow 1997, 1998 on
the wetting of rough surfaces. Here the focus is on the nucleation of atmospheric vapors Ž
. on pre-existing insoluble aerosol particles heterogeneous nucleation . In the current
work, the effect of rough surfaces on wetting is analyzed using the theory of the Ž
quenched disorder with the basis of stochastic differential and integral equations Chow, .
Ž 1997 . In Section 2.1, we consider the effect of weak fluctuations chemical contamina-
. tion effects
on the nucleation process. In Section 2.2, we consider the effect of Ž
. correlated noise of the quenched disorder on self-affine rough surfaces on the contact
angle between the nucleating embryo and the surface of the aerosol particles. Finally, a discussion of the effect of surface roughness on the nucleation characteristics of
insoluble aerosols in the atmosphere is presented. The classical theory of nucleation is used for the evaluation of the effect of contact angle modifications to the nucleation
flux.
2. Surface roughness
