Experimental evidence for the role of ions in particle nucleation under atmospheric conditions
Henrik Svensmark1, *, Jens Olaf P. Pedersen1, Nigel D. Marsh1, Martin B. Enghoff1 & Ulrik I. Uggerhøj1, 2
1Centre for Sun-Climate Research, Danish National Space Centre, 2100 Copenhagen, Denmark hsv@spacecenter.dk
2Institute of Physics and Astronomy, University of Aarhus, 8000 Aarhus, Denmark
*Author for correspondence
hsv@spacecenter.dk
Received 16 February 2006; Accepted 17 August 2006
Abstract Experimental studies of aerosol nucleation in air, containing trace amounts of ozone, sulphur dioxide and water vapour at concentrations relevant for the Earth's atmosphere, are reported. The production of new aerosol particles is found to be proportional to the negative ion density and yields nucleation rates of the order of 0.1-1cm-3s-1. This suggests that the ions are active in generating an atmospheric reservoir of small thermodynamically stable clusters, which are important for nucleation processes in the atmosphere and ultimately for cloud formation.
Keywords: cosmic rays; aerosol nucleation; clouds
1. Introduction
The role of ions in the production of aerosols is among the least understood, but potentially is an important, process in the Earth's atmosphere. Atmospheric (
Hoppel et al. 1994;
Clarke et al. 1998;
Birmili et al. 2003;
Lee et al. 2003;
Kulmala et al. 2004) and experimental (
Berndt et al. 2005) observations have shown that the nucleation of aerosol particles can occur under conditions that cannot be explained by classical nucleation theory. Several ideas have been put forward to solve this nucleation problem, e.g. ion-induced nucleation (
Arnold 1980;
Raes et al. 1986;
Turco et al. 1998) and ternary nucleation (
Kulmala et al. 2000). However, experimental investigations exploring the role of ions in particle production are scarce and often at conditions far removed from those relevant for the lower part of the atmosphere (
Bricard et al. 1968;
Vohra et al. 1984;
Kim et al. 2002;
Nagato et al. 2005;
Wilhelm et al. 2003). Here, we show that the production of new aerosol particles is proportional to the negative ion density under experimental conditions similar to those found in the lower troposphere over oceans. Since production rates at 3nm were observed to be 1-10cm-3s-1, and numerical simulations yielded nucleation rates of initial stable clusters (approx. 1nm) around 0.1-1cm-3s-1, it is suggested that ions are active in continuously generating a reservoir of small thermodynamically stable clusters that can then rapidly grow in the presence of condensable vapours. These results demonstrate that ions probably play an important role in the production of new aerosol particles in the Earth's atmosphere.
2. Experimental procedure
The measurements were performed in a 7m3 reaction chamber, which was continuously flushed with purified air at 30.5lmin-1 at standard temperature and pressure (STP). Variable concentrations of water vapour (H2O), ozone (O3) and sulphur dioxide (SO2) could be added to the chamber, which is shown schematically in
figure 1. Five walls of the chamber were constructed from mylar with the final wall constructed from Teflon to allow for the transmission of UV light. Air was obtained via an inlet situated externally to the building housing the experiment. The air was compressed and dried with an oil-free compressor and then passed through a series of filters, including active charcoal, a molecular sieve, citric acid and both 10 and 3nm particle filters. The air finally entered the experiment via a tube inserted about half a meter into the chamber.
Figure 1
The reaction chamber. 1, the chamber; 2, UV array; 3, honeycomb collimator; 4, air inlet; 5, ozone line; 6, SO2 line; 7, gas and aerosol outlet; 8, electrodes.
H2O was introduced by circulating deionized water through a GoreTex tube inserted into the air flow, which allowed the relative humidity to vary over the range 2-90%. SO2 was added as a 5p.p.m. mixture of SO2 in dry air (Air Liquide) and O3 was introduced by directing a fraction of the flow (0.5lmin-1 STP) through an ozone generator. Both the flows containing SO2 and O3 were passed through the 3nm particle filters before mixing with the main air flow at the entrance to the chamber. At all times during the experiments, the concentrations of H2O, SO2 and O3 were maintained at constant levels. Teflon tubing was used everywhere, except for a stainless steel section between the compressor and the chamber's gas system. The pressure in the chamber was held a few Pascals above the atmospheric pressure and a stable room temperature was maintained by the laboratory air-conditioning system (296
1K). All flow outlets, used for measurements of particles and gases extended into the chamber to avoid possible artefacts from wall effects.
UV radiation, primarily at 253.7nm, was provided by an array of seven mercury discharge lamps mounted so that they illuminated the chamber through the wall constructed from Teflon. The radiation was collimated via an 80mm thick, honeycomb wall (1/4in. pore size) mounted outside the chamber and coated with an UV-resistant matt black paint. The UV initiates a photochemical reaction where O3 is photolysed to produce OH-radicals and through a series of reactions, involving SO2, O2 and H2O, produces sulphuric acid (H2SO4). The resulting H2SO4 concentration was estimated using the empirical relation between aerosol growth rate and sulphuric acid concentration given by
Kulmala et al. (2001). The UV intensity was mapped out over approximately a quarter of the chamber with a total of
ca 21000 measurement points across the front, middle and back of the chamber with 1cm resolution. This ensured that the UV field was homogeneous.
Temperature, pressure and relative humidity were measured at the front wall of the chamber, the main inlet and the humidifier. Temperature was additionally measured at the top and bottom mid-points of the chamber. The O3 and SO2 concentrations were monitored using trace gas analysers, which were sensitive down to 0.1p.p.b. for O3 (Teledyne model 400A) and 0.05p.p.b. for SO2 (Thermo model 43 CTL). The rough composition of the gas mixture in the chamber could also be analysed using a Pfeiffer Quadrupole QMG 422 mass spectrometer.
The ions were produced in the chamber by the naturally occurring galactic cosmic radiation and the decay of the natural abundance of radon (monitored with a Durridge RAD7 instrument). In addition to this, the average production of ions could be enhanced to ionization levels relevant for the atmosphere (1-60 ion pairs cm-3s-1), with two 35 MBq Cs-137 gamma sources, placed on either side of the chamber at a distance that ensured the entire chamber was irradiated with the additional ionization. It was also possible to reduce the number of ions via an electric 'clearing' field (0-12000Vm-1). This was achieved by applying a voltage drop across two acid proof stainless steel plates (1.7
1.7m2) mounted within the chamber on the opposing walls. The field strength could be varied over a range of voltages corresponding to ion lifetimes of
ca 1-200s.
The ion current in the chamber was monitored using a Gerdien (
Gerdien 1905;
Aplin & Harrison 2000) type ion detector mounted on the front wall of the chamber. The detector consists of a cylindrical aluminium tube of length 25.8cm and inner radius 1.25cm (outer electrode), with a coaxial thin metal rod suspended at its centre (inner electrode). Air from the chamber flowed through the tube at a rate of
ca 9lmin-1. By applying a positive (negative) potential across the electrodes, negative (positive) ions were forced to move towards the central electrode causing a current to flow. This current was measured by a Keithley Model 6514 electrometer. No significant difference was found between the total concentrations of positive and negative ions with mobilities greater than 1cm2V-1 per second.
Varying the field strength within the tube controls, the minimum mobility of the ions contributing to the measured current and thus a spectrum of ion mobilities can be obtained. The spectra, obtained over the full range of experimental conditions reported here, indicate that the majority of small ions possess a mobility,
1cm2V-1 per second. The small ion density in the reaction chamber was then estimated from the current,
I, that was measured when the potential across the electrodes was fixed at 1.23V. This limits the small ions collected to those having a minimum mobility of approximately 1cm2V-1 per second. The mobility cut-off is not sharply defined since ions with lower mobilities will still be collected on the central electrode if they enter the Gerdien tube near this electrode. The contribution of these ions to the current measurement can, however, be estimated from the instrument response function. Using the results from
Tammet (1995), the minimum ion mobility was then converted into a maximum ion diameter of approximately 1nm. Finally, the ion density,
ne, was obtained from
ne(
>1cm2V-1 per second)=
I/(
ef), where
e is the charge per ion and
f is the rate of flow through the tube. A result of this procedure is that larger ions or ionized aerosols potentially contribute to the extracted ion density, but the mobility spectra suggest that their contribution is negligible.
The number of aerosols present in the chamber was measured with a TSI Ultra Fine Condensation Particle Counter (model 3025A) capable of measuring particles in the range of 3-150nm.
The gas mixture in the reaction chamber during all of the experiments resembles those found in the lower troposphere over the oceans, i.e. O3 (approx. 25p.p.b.), H2O (approx. 35% relative humidity) and SO2 (80-230p.p.t.) (
Seinfeld & Pandis 1998, p. 61 & 96).
3. Burst and steady-state experiments
Two types of experiments were performed: (i) a 'burst' of H2SO4 was produced from a short exposure to the UV light, and (ii) steady-state conditions of H2SO4 were achieved under continuous exposure to the UV light.
In the burst experiment, the chamber was exposed to a 10min illumination of UV light resulting in the rapid production of sulphuric acid that reached a peak concentration around 4
107cm-3. This is well below the limit of homogeneous nucleation, which is around 109cm-3 (
Seinfeld & Pandis 1998, p. 578). The peak concentration lasted for a few minutes before decaying exponentially owing to losses to the walls of the reaction chamber.
Figure 2b shows the typical response of particles larger than 3nm, where an initial steep rise in aerosol number to a maximum is followed by a slow decay lagging the H2SO4 concentration. Clearly, the system possesses some very long transients requiring typically 4h before returning to initial conditions and allowing the cycle to be repeated with another burst of UV light. The average number of particles with diameter greater than 3nm captured by the TSI particle counter,
N(
d>3nm), over each 4h cycle was found to be proportional to the small ion (with
>1cm2V-1 per second) density in the chamber (
figure 2a).
Figure 2
(
a)
N(
d>3nm) averaged over the 4h cycle of each burst experiment versus ion density,
ne (red stars). The observed ion current,
I, is indicated on the upper horizontal scale. The model response for a range of nucleation rates,
S (black diamonds, right-hand scale), together with 1 sigma uncertainties (dotted) are also included. (
b) A typical 4h cycle, where
t=0 is the time when the UV lights were activated. Plotted are the 4h averages of
N(
d>3nm), from both experiment (black) and model (red) with parameters (see text)
S=0.44cm-3s-1, loss term
=55
10-4s-1 and
k0=5. The grey shading indicates the evolution of H2SO4 which reaches a maximum of 4
107cm-3 before decaying.
The aerosol response to different ion densities can also be characterized by the time it takes from the initial UV exposure until the first particles begin to appear at 3nm (
figure 3b). This time-interval is referred to here as the delay time,
(
figure 3b) and is the time it takes for the aerosol concentration greater than 3nm to reach a threshold of 10cm-3. The observed delay times also show a dependence on the ion density, with the lowest ion densities yielding a delay time of around 11min, which decreases to about 6min at higher ion densities (
figure 3b). Relaxation to steady state under continuous exposure to UV light typically took
ca 8h and resulted in a constant concentration of H2SO4 around 2
108cm-3.
Figure 4a shows the number of particles larger than 3nm obtained under steady-state conditions as a function of the observed ion-current. The typical response of the steady state
N(
d>3nm) under various ionization levels in the chamber can be seen in
figure 4b. These measurements further demonstrate that the particle production depends on the ion density.
Figure 3
(
a) Delay time,
, as a function of ion density,
ne. (
b) Magnification of
figure 2b (see figure 2 caption for details).
Figure 4
(
a)
N(
d>3nm) as a function of ion density,
ne, under steady state with environmental conditions corresponding to SO2 (155p.p.t.), H2O (RH 35%) and H2SO4 (2
108cm-3). The model response for a range of nucleation rates,
S (black solid, right-hand scale), together with 1 sigma uncertainties (dotted) are also included. (
b) The evolution of ion current (blue) and
N(
d>3nm) (red) over a 2 day steady-state experiment.
The experimental results were tested with a mixture of argon (79%) and oxygen (21%). No significant differences were observed from those described above, which suggests that the presence of nitrogen is not crucial. This indicates that particle formation involving ions most probably occurs via negative ion chemistry, since positive ion chemistry requires the presence of nitrogen (
Smith & Spanel 1996).
To exclude the possibility that the experimental results obtained were due to an impurity in the compressed air, the experiments were repeated using synthetic, dry and radon free air (Synthetic Air 5.0, Strandmøllen). No discernible differences were observed. The experiments were also performed with the purified air excluding O3 and SO2 alternatively. The lack of particles detected at 3nm in both the instances showed that these trace gases were essential for the observed production, although this does not rule out that additional gases are involved in the nucleation.
4. Reducing ion lifetime with the electric field
Both the 'burst' and steady-state experiments indicate that the production of new aerosol particles is proportional to the negative ion density. At these experimental conditions, it can be concluded that ions are key to producing stable clusters. Any other mechanism which could produce particles without the involvement of ions can only have made a minor contribution to the particle production. To further investigate the time-scale to produce a charged critical cluster, an electric 'clearing' field has been used. Applying an electric field of, for example, 600Vm-1 reduces the small ion life time to seconds, thus decreasing the time available for the ions to participate in nucleation. However, this did not have any effect on the nucleation process, and only at much larger electric field strengths did a significant response become apparent. For field strengths up to 6000Vm-1, the particle production was reduced by
ca 20% and at larger fields up to the current maximum of 12000Vm-1, the particle production was reduced by
ca 50%. Since, as stated earlier, ions are involved in almost all particles produced, this indicates that the characteristic time for producing a stable cluster is very short, i.e. 2s or less. The observation also indicates that the clusters become neutralized in the same time-scale as they would otherwise have been extracted from the chamber by the electric field.
In the experimental conditions reported here, classical theory suggests that the critical cluster size is of the order of 3nm. This is based on the parametrization in
Vehkamäki et al. (2002) and an estimate of the mole fraction of H2SO4 to be 0.2 (
Seinfeld & Pandis 1998, p. 527). It would be surprising if the subcritical clusters could stabilize at just under 3nm in less than 2s and then take a further
ca 10min to grow through the 3nm detection limit. More likely the clusters have stabilized at significantly smaller diameters and then, through coagulation, generated a distribution of thermodynamically stable clusters below the 3nm detection limit that can rapidly grow in the presence of condensable vapours. The existence of a sub-3nm reservoir of thermodynamically stable clusters was initially proposed by
Kulmala et al. (2000). However, they suggested ternary nucleation as the source for the reservoir rather than ions as is proposed here.
5. Numerical simulations
To investigate the size of the critical cluster
k0 and the production rate of stable clusters
S under the experimental conditions, numerical simulations of particle coagulation and condensation have been compared with the experimental results. The evolution of the cluster distribution is given by (
Seinfeld & Pandis 1998, p. 682)
(5.1)
Equation
(5.1) describes the number density,
Nk, of clusters with
k sulphuric acid molecules, where the equilibrium concentration of water in each cluster is reached instantaneously. The coagulation coefficient,
, where
is the mean velocity of clusters with
i H2SO4 molecules;
T, temperature;
kB, Boltzmann constant; and
Ri, the radius of a cluster with
i H2SO4 molecules. The rate of sticking to the walls,
, is approximated with a constant for all clusters and given by the observed loss rate of
N(
d>3nm) during the burst experiments. The
k-term describes the condensation of H2SO4 molecules in the gas phase to the
kth cluster and is given by
N1
Kk,1 (
Seinfeld & Pandis 1998, p. 550).
In the simulation, the resulting particle distribution after a number of hours reaches a steady state between particle losses to the walls and growth by coagulation, producing a reservoir of particles with sizes below the 3nm experimental detection limit. Following a simulated burst of UV light, this reservoir of particles begins to grow via condensation and appear above 3nm at the rates of 1-10cm-3s-1. The resulting evolution of simulated particles with
d>3nm for a single burst experiment compares favourably with the experimental data (
figures 2b and 3b). By varying the free parameters
k0 and
S in the model, the cumulative number of particles with
d>3nm is found to be dependent on both
k0 and
S (
figure 5a), while the time delay is strongly dependent on
k0 and only weakly dependent on
S (
figure 5b). To satisfy the constraint that
>6min, requires
k0
10 H2SO4 molecules (
figure 5b). Limiting
k0=5 allows for an estimate of
S which is found to be of the order 0.1-1cm-3s-1 (
figure 5a, and right-hand axis
figure 2a). The simulations then reproduce a delay time similar to that observed in the experiments (
figure 3a, black diamonds), and the averaged
N(
d>3nm) over each 4h cycle replicates the experimental data very well (
figure 2a, black diamonds). The simulations were also performed for the steady-state experiments (
figure 4a, solid line) and these too compared favourably with the experimental data.
Figure 5
Numerical simulations of the burst experiment indicating how the number of H2SO4 molecules in a critical cluster,
k0, varies with (
a) 4h integrated
N(
d>3nm) and (
b) delay time,
. The colours in both plots refer to the production rate
S, where 0.5, 1.0, 1.5 and 2.0cm-3s-1 are represented by red, green, blue and light blue, respectively. Grey shading indicates the range of experimental data in
figures 2 and 3.
Recent calculations of particle production owing to ion-induced nucleation at conditions found in the lower troposphere over the oceans find much smaller production rates at 3nm, e.g. less than 100 particles cm-3 in 12h (
Lovejoy et al. 2004). In contrast, models by
Yu & Turco (2001),
Yu (2006) and
Laakso et al. (2004) indicate much higher nucleation rates in closer agreement with our results. These uncertainties in simulated nucleation rates are likely to be due to differences in the treatment of cluster thermodynamics in each model.
6. Discussion
The above experiments and simulations suggest that a mechanism is active involving ions, the trace gases SO2, O3 and H2O, all at typical levels found in the atmosphere (
Seinfeld & Pandis 1998, p. 61 & 96). With these gases present, the mechanism apparently produces a reservoir of stable clusters, most probably containing H2SO4 molecules, in the absence of UV light. The observation that the negative ion chemistry seems to be important for the nucleation process hints at the presence of an electron as necessary for stabilizing the subcritical cluster during the initial nucleation phase. To illustrate this idea, a possible pathway is presented below.
When energetic ionizing particles interact with the gas in the chamber, free electrons are produced that attach to an O2 molecule within 10-9 s. The
molecule attracts a small number of water molecules,
Wn, and owing to the high concentration of O3, ligand switching with O2 is highly probable (
Fahey et al. 1982). This results in a negative O3/water cluster,
(6.1)
with a reaction constant of 7
10-10cm3s-1. The above cluster initiates the oxidation of SO2 and a corresponding charge transfer is possible (
Fehsenfeld & Ferguson 1974),
(6.2)
(6.3)
where the reaction constant for reaction
(6.3) is 1.7
10-9cm3s-1. If the cluster contains a sufficient number of water molecules, i.e. between 4 and 11, then
will react with the attached water molecules and form sulphuric acid (
Larson et al. 2000),
(6.4)
The electron has now been involved in the stabilization and formation of a cluster with a single sulphuric acid molecule, however, a stable cluster has to contain several sulphuric acid molecules. The above reactions are ion reactions and the reaction constants are close to the time-scales of gas kinetic collision, thus the time-scale for generating a sulphuric acid molecule is
ca 0.1s for 200p.p.t. SO2. This process continues to form H2SO4 molecules with the electron acting as a catalyst and stabilizing the cluster while it is smaller than the critical size.
As indicated by the experimental results when using an electric field, the residence time of an electron in the cluster is very short, i.e. 2s or less. The electron can become detached owing to excitation by a thermal photon or the release of chemical energy within the cluster. The detached electron might stabilize a new cluster, but there is a high probability that the electron is lost through (i) attachment to larger stable clusters, and/or (ii) recombination with a positive ion. In both the cases, the electron cannot form additional sulphuric acid molecules. The number of stable clusters nucleated is then determined by the typical residence time of an electron in a subcritical cluster and its attachment/recombination probability once detached. The studies involving negatively charged water clusters (
Beyer et al. 2001) suggest that the electron detachment process can take between 100 and 0.02s. The characteristic time identified above for producing a stable cluster (2s or less) is well within this limit, a feature not reproduced by the existing ion-induced nucleation models (
Yu & Turco 2001;
Laakso et al. 2004;
Lovejoy et al. 2004).
Finally, it should be stressed that the presence of trace amounts of other gases in the chamber cannot be excluded. It is possible that some trace gas may not have been absorbed by the filters, and might have contributed to the nucleation process. One candidate could be NH3 (see
Kulmala et al. 2000). However, the presence of other condensable gases does not affect the main conclusion of this work, namely that the nucleation process under atmospheric conditions is proportional to the ion density.
The experimental data for both the burst and the steady-state experiments (
figures 2a-4a) indicate that
N(
d>3nm) is proportional to the small ion density,
ne, such that
. The numerical simulations indicate that
, which leads to the semi-empirical relation between the production rate of stable clusters,
S and the ion density,
ne, viz.
S=
ane. Under the conditions of low background aerosol, the ion density is proportional to the square root of the ion production rate,
q. Note that this empirical relationship holds for both the burst and the steady-state experimental procedures. This allows for two independent estimates of the constant: for the burst experiment,
a=2.4
10-4
0.4
10-4s-1; and for the steady-state experiment,
a=2.3
10-4
0.3
10-4s-1. It is remarkable that the two different experimental procedures give the same nucleation constant. The observation that the particle production rates scale with the negative ion density suggests that it is the individual negative ions that are important, and not a collective effect between several ions, e.g. coagulation or recombination.
The experiment indicates that ions play a role in nucleating new particles in the atmosphere and that the rate of production is sensitive to the ion density. If this sensitivity is still relevant at the size of cloud condensation nuclei, one might expect to find a relationship between ionization and cloud properties (
Svensmark & Friis-Christensen 1997;
Svensmark 1998;
Shaviv 2002).
Marsh & Svensmark (2000) found that the correlation between cosmic ray ionization and clouds is mainly in low-level clouds and not as might have been expected, in high clouds where ionization variations are large.
This feature seems to be consistent with the present work. In the lower atmosphere, the limiting factor is the density of ions and, since the ion density (under conditions of low background aerosol) is proportional to
, the sensitivity of ion density to variations in the production rate increases for decreasing values of
q, i.e.
. In contrast, at higher altitudes in the atmosphere, the ion production rate can be 10 times larger than at the surface. In these regions, it has been suggested that the role of ions saturates and the nucleation process is no longer sensitive to changes in ionization (
Yu & Turco 2001). A response limited to regions where low-level clouds form is perhaps not surprising, especially when considering that high clouds usually consist of ice-particles, which involve nucleation processes not covered by the present work.
In summary, the present work indicates that (i) stable clusters are formed from SO2, O3 and H2O in the presence of ions, (ii) the experiments and simulations suggest that an initial distribution of sub-3nm stable clusters is formed containing H2SO4, (iii) the nucleation rate is proportional to the ion density, (iv) the negative ions are important, (v) the characteristic time for producing a stable cluster is very short 2s or less, and (vi) the charge (electron) only stays on the small cluster for a short time 2s or less.
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