Ionisation as a precursor to aerosol formation

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 DA / A ~ (1 – A) (DN / N) i.e. DA ~ 0.5% for 1% change in N (1 drop.cm-3 in 100 cm-3…) Cloud albedoA depends on droplet number N (Kirkby J., Proceedings of workshop on ion-aerosol-cloud interactions, CERN-2001-007) • 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. 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), 13909-13915

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, 1732-1737 (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, coefficienta 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 Phys63, 17, 1811-1819. 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, 8 3467-3469 (2001); Rev. Sci. Inst, 71, 8, 3037-3041; Rev Sci Inst, 71, 12, 4683-4685 (2000); Rev. Sci. Inst 71, 8, 3231-3232 (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