1. Ionisation as a precursor to aerosol formation
Giles Harrison
Department of Meteorology
The University of Reading, UK

2. Overview Motivation for ion-aerosol research Ion-cloud microphysics
Particle conversion experiments
Project methodology and experimental plans

3. Motivation
The terrestrial radiation balance is affected by cloud, principally via changes in the albedo
Clouds are sensitive to background aerosol, both from changes in droplet concentrations (via Cloud Condensation Nuclei, CCN) and initiation of the ice phase (Ice Nuclei, IN)
Recent indications that the formation of molecular clusters, ubiquitous in the atmosphere from background radioactivity and cosmic rays, plays a role in ultrafine particle production
Changes in atmospheric ionisation from natural (e.g. solar modulation of cosmic rays) or artificial (e.g. from increased nuclear reprocessing) sources may therefore influence atmospheric aerosol production and, ultimately, climate

4. Atmospheric processes relevant to ion-aerosol-cloud problem

5. CCN changes and clouds Cloud albedo A depends on droplet number N
DA / A ~ (1 – A) (DN / N)
i.e. DA ~ 0.5%
for 1% change in N
(1 drop.cm-3 in 100 cm-3…)
(Kirkby J., Proceedings of workshop on ion-aerosol-cloud interactions, CERN )
Cloud cover
Cloud lifetime depends on precipitation rate
Precipitation rate depends on droplet number N

6. Radiolytic particle production
Aerosol concentrations cycled by the regular addition of -particles from Thoron

7. Low-dose radiolytic particle production
particle formation from radon, in artificial air in the presence of trace concentrations of sulphur dioxide, ozone and ethene,

8. Effect of charge on aerosol processes
Charge-mediated
nucleation
Charge-enhanced
coagulation
Increased CCN
concentration

9. How is new particle formation affected by ionisation?
Aerosol nucleation
How is new particle formation affected by ionisation?

10. Growth of ion-induced CN
…but can new CN grow into CCN, large enough to become cloud droplets?

11. Final step: CCN formation

12. Observations of ion growth
Charged particles growing from molecular clusters to “intermediate” ions (ultrafine particles)
From Horrak et al (1998) Bursts of intermediate ions in atmospheric air
J.Geophys. Res. 103(D12),

13. Apparent issues
Ion-mediated nucleation is the rate-determining step in CN formation only in the lower atmosphere, and then only sometimes (nucleation occurs at the kinetic limit in the upper troposphere)
Role of other condensable species ?
Competition with non-ion-induced nucleation ?
Frequency and location of occurrence/dominance ?

14. Ion-aerosol-cloud mechanisms
“Near-cloud mechanism”
ice nucleation (image charge) and particle size distribution changes
“Clear-air mechanism”
particle nucleation
Carslaw, Harrison and Kirkby Science 298, 5599, (2002)
…and forthcoming (2003) : Harrison and Carslaw, Rev. Geophys

15. Aerosol production from ionisation
Sources
Experimental and theoretical study
Cloud effects

16. Ion-aerosol interaction equation
(number concentrations n+ and n- of positive and negative ions)
Source term q, principally cosmic volumetric ion-pair production rate -?
Loss by ion-ion recombination, coefficient a
Loss by ion-aerosol attachment (monodisperse aerosol number concentration Z), coefficient b
NUCLEATION
TERM

17. Experimental investigation of nucleation term
Ion-aerosol equation tested experimentally in Reading air* using instruments to measure each term of the ion-aerosol equation:
q (ion production): Geiger tube including a response to a-radiation (1Hz data)
Z (condensation nuclei): Pollak counter (cutoff ~3nm, 2min sampling cycle)
n (small ions): programmable ion counter (10s data)
(cosmic ray showers identified by coincidence detection and Monte-Carlo simulation of random events. The background coincidence rate goes as nx, with x detectors).
*Harrison and Aplin (2001), J Atmos Solar Terr Phys 63, 17, 17

18. Particle increases during ionising events
Ion-aerosol theory predicts a decrease in ions with aerosol increase, the opposite of what was found
observations of increases in particles during an increase in ions and high energy “cosmic ray” ionisation events

19. Programme objectives
Ionisation as a precursor to aerosol formation
to investigate how ultrafine particle concentrations are linked with ionisation
to establish what fraction of atmospheric aerosol is produced by ionisation
to determine if the rate of ion-aerosol conversion changes with atmospheric composition
to quantify what fraction of surface ionisation is associated with extensive cosmic ray air showers

20. Outline methodology
Manufacture of 4 programmable ion mobility spectrometers, originally developed at Reading
Construction of a 3 component Geiger counter array, to monitor background ion production and cosmic ray air showers using coincidence detection (negligible background rate for 3 counters)
Deployment of ion counters and ion production sensors for long observations in Reading air, for simultaneous detection of ions in different polarity and mobility (size) categories (and meteorological parameters)
Analyse for frequency of ion growth events, the related conditions and estimate the ion-nucleation coefficient
Field experiments alongside TORCH with detailed aerosol size spectrometry and trace species analysis

21. A modern ion counter: the PIMS
Based on the classical Gerdien condenser
Use of a microcontroller provides programmable spectral capability: Programmable Ion Mobility Spectrometer (PIMS)
Multi-mode electrometer for current and voltage measurements
Connections to PC for logging and programming
DAC
Microcontroller and analogue to digital conversion
Rev Sci Inst, 72, (2001);
Rev. Sci. Inst, 71, 8, ;
Rev Sci Inst, 71, 12, (2000);
Rev. Sci. Inst 71, 8, (2000)
Cylindrical capacitor system

22. Project schedule

23. Collaborators / Staff
Dr George Marston (Reading, Chemistry) – surface measurements and ozone monitoring
Dr Ken Carslaw (Leeds) – ion-aerosol and aerosol-cloud modelling and theory
Dr Karen Aplin (RAL) – PIMS instrument operation
+ 1 tech (0.5) + 1 PDRA (to be appointed)
Since the original application, a US Government Laboratory has evaluated the PIMS instruments as anti-terrorist radioactivity detectors … instrument further improved
construction now well-underway to this new specification