Ion-mediated nucleation in the atmosphere
Raffaella D'Auria and Richard P. Turco
Department of Atmospheric Sciences, University of California, Los Angeles
(dauria@atmos.ucla.edu)
Abstract: In situ formation of new particles in the troposphere is well-documented observationally [e.g., Clarke et al. 1998]. However, a theory
that successfully explains the process of particle formation from precursor vapors, is consistent with basic physical and chemical principles, and
explains existing data, is still unresolved [e.g., Clarke et al., 1998]. Recently, massive molecular ionic clusters have been detected in the upper
troposphere [Eichkorn et al., 2002; Heitmann and Arnold, 1983]. Similar observations have also been made in the lower troposphere [Eisele, 1988;
Tanner and Eisele, 1991], stratosphere and in the mesosphere [e.g.; Arnold et al., 1977, 1982]. Ionic clusters have the property of stabilizing the
formation of condensation nuclei through the strong interaction of a charge center with polar molecules at the atomic scale. Hence, ion production
in the atmosphere can influence the abundances of cloud condensation nuclei that ultimately impact Earth's climate. The primary ionization source
throughout the lower atmosphere (excluding the first few kilometers over land) are galactic cosmic rays (GCRs). Because the intensity of galactic
cosmic radiation penetrating into the middle and lower atmosphere is significantly modulated by solar activity, a potentially critical connection may
exist between solar activity and changes in climate forcing as mediated by ionic clusters. In this paper we present results from an ongoing research
effort to determine the atmospheric roles of ionic clusters with respect to heterogeneous chemistry and cloud microphysics. We focus on the
properties and growth of small molecular clusters that form on hydronium, nitrate and sulfate core ions with water, nitric acid, and sulfuric acid
ligands. We present results that describe the sensitivity on cluster properties and behavior to changes in ionization levels and ambient
environmental conditions.
Our Approach: Ion Cluster Nucleation Kinetics
kf,(n-1)
/k
1. Uses available experimental data on ion cluster equilibria:
kf,(n-1)
+
ligand
molecule
A Hybrid Approach to Determine the Ion Cluster Thermodynamics
logK(n-1),n = -DG(n-1),n
kr,n
r,n=
(
K(n-1),n = exp - DG(n-1,n)
(n-1),n
we have used density functional theory (DFT)
3. Uses the classical Thomson liquid droplet model for the larger
clusters (once the microscopic results have converged to the
macroscopic ones):
/RT)
Using thermodynamic data (DG0,n), and noticing that the forward rate
coefficient, kf,n, typically fall within a narrow range of value (~10-9 cm3/s),
the reverse rate coefficients, kr,n, can be estimated.
DG0,n = -nRTlnS + 4pr2sNA + (NAq2/8pe0)(1-1/er)(1/r - 1/r0)
A Comparison of the Different Approaches: Gibbs Free Energy
Change for Hydronium Ion Clusters, H3O+(H2O)n at 300 K
Nucleation kinetics
Taking into account ion sources and sinks, ion concentrations are
governed by:
Some example of ion cluster
structures obtained from the
quantum mechanical simulations:
Hydronium ion-water clusters
.
c0 = -kf,0 l c0 + kr,1 c1 - arec C c0 + Q0
...
.
cn = -kf,(n-1) l c(n-1) + kr,n cn - kf,n l cn - arec C cn + kr,(n+1) c(n+1)
...
{
(n-1),n
2. Uses quantum mechanical simulations to infer the structure and
the thermochemistry of ion clusters:
ion cluster,
n ligands
ion cluster,
(n-1) ligands
/RT = -DH /RT + DS /R
.
cn = -kf,(N-1) l c(N-1) + kr,N cN - arec C cN
where: c0 is the core ion, cn is the ion cluster containing n ligands,
C is the total concentration of the ion clusters of opposite
charge, and arec (~ 10-6 cm3s-1) is the ion recombination
coefficients and Q0 (~ 10 ion-pair cm-3) is the ionization rate
The general behavior of the Gibbs free
energy change associated with the formation
of a ionic cluster composed of n ligands, DG0,n
(Note that from n=1-10 the curves are built
from actual data or quantum mechanical
simulations as oppose of being derived from
the classic liquid drop Thomson model).
The number of ligands corresponding to the
minimum on the DG0,n is plotted as a function
of temperature, T, and water concentration,
[H2O] (cm-3). The parameters span the entire
range of the atmospheric conditions (i.e.,
troposphere, stratosphere and mesosphere).
Where are the ion clusters nucleating in the
atmosphere?
The following graphs shows in blue areas where
nucleation is not occurring, in green areas for
which there is a finite energy barrier to nucleation
and in red areas where ions nucleate freely.
The plot represents
100
H3O+(H2O)n clusters.
H O (H O)
190 K
Note that in most of
p = 100 mbar
50
the likely atmospheric
220 K
180 K
situations ions are
210 K
0
in the green area or
200 K
close to it. Upon
175 K
condensation of
50
170 K
other ligands (i.e.,
100
H2SO4, HNO3, NH3, etc.) the hydrated hydronium
10
10
10
10
10
n, # of ligands per cluster
ions are likely to grow and undergo ion-mediated
nucleation. Furthermore massive ion can act as
Changing the ionization source strength in response to solar variation (either in the form of
freezing nuclei thus modifying the particle
altering the GCRs flux or, in the mesosphere, by directly changing the ionization source)
microphysics.
changes the concentration of ion clusters throughout the atmosphere. Studies of such
variations together with studies of the effects of different ligands is under way.
300
280
+
3
2
260
n
240
T (K)
220
DG0,n [kcal/mol]
1
nmin
10
200
180
100
160
0
10
150
15
14
13
140
200
12
11
10
[H2O] (cm-3)
250
9
8
300
120
T (K)
100
7
8
9
10
11
12
13
14
15
16
[H2O] (cm-3)
0
0
1
Acknowledgments:
2
3
5
10
15
4
We thank Professor Kendall Houk for his support in the implementation of the quantum mechanical simulations.
This work has been funded by NASA under grant NAG1-1899. RD is also supported by the NASA Earth System Fellowship
ESS/00-0000-0080.