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Ionisation as a precursor to aerosol formation

Giles Harrison

Department of Meteorology

The University of Reading, UK

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Overview

•Motivation for ion-aerosol research

•Ion-cloud microphysics

•Particle conversion experiments

•Project methodology and experimental plans

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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

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Atmospheric processes relevant to ion-aerosol-cloud problem

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CCN changes and cloudsCloud albedo A depends on droplet number N

Cloud coverCloud lifetime depends on precipitation ratePrecipitation rate depends on droplet number N

A / A ~ (1 – A) (N / N)

i.e.A ~ 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-2001-007)

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Radiolytic particle production

•Aerosol concentrations cycled by the regular addition of -particles from Thoron

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Low-dose radiolytic particle production

particle formation from radon, in artificial air in the presence of trace concentrations of sulphur dioxide, ozone and ethene,

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Charge-mediated nucleation

Charge-enhancedcoagulation

Increased CCNconcentration

Effect of charge on aerosol processes

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How is new particle formation affected by ionisation?

Aerosol nucleation

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…but can new CN grow into CCN, large enough to become cloud droplets?

Growth of ion-induced CN

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Final step: CCN formation

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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

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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 ?

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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

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Aerosol production from ionisationCOSMICRAYS

IONISATION

ULTRAFINE PARTICLEAND CONDENSATION

NUCLEI FORMATION

ICENUCLEI

CLOUD

CONDENSATION

NUCLEI

CLOUD and CLIMATE

RADIOACTIVITY NUCLEARINDUSTRY

Urbanair

troposphericair

Experimental and theoretical study

Sources

Cloud effects

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Ion-aerosol interaction equation

Znnnqdt

dn

Source term q, principally cosmic volumetric ion-pair production rate

Loss by ion-ion recombination, coefficient

(number concentrations n+ and n- of positive and negative ions)

Loss by ion-aerosol attachment (monodisperse aerosol number concentration Z), coefficient

-?

NUCLEATION

TERM

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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 -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).

17*Harrison and Aplin (2001), J Atmos Solar Terr Phys 63, 17, 1811-1819.

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Particle increases during ionising eventsCN increases and conductivity

(1400-1500, 25.2.2000)

y = 0.1402x + 1.2095

R2 = 0.2977

0

5

10

0 10 20 30 40negative conductivity (fS/m)

dC

Nd

t/(c

m-3

.s-1

)

•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

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Programme objectives

1. to investigate how ultrafine particle concentrations are linked with ionisation

2. to establish what fraction of atmospheric aerosol is produced by ionisation

3. to determine if the rate of ion-aerosol conversion changes with atmospheric composition

4. to quantify what fraction of surface ionisation is associated with extensive cosmic ray air showers

Ionisation as a precursor to aerosol formation

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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

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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

Microcontroller and analogue to digital conversion

Cylindrical capacitor system

DAC

Connections to PC for logging and programming

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)

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Project schedule Month 1-2 3-4 5-6 7-8 9-10 11-12

Literature review of ion counters and ion-aerosol physics

Atmospheric Measurements 1.

Atmospheric Measurements 2.

Analysis of experimental data

Experimental modifications

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PIMS instruments fabrication and calibration

Analysis of experimental data

Paris ICAE Conference

Field Experiment 1. (TORCH London field experiment)

Development of ion-aerosol model

Atmospheric Measurements 3.

Analysis of experimental data

archiving of experimental data

Development of ion-aerosol model

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Analysis of experimental data

Aerosol Society Conference

Analysis of experimental data

Field Experiment 2. (TORCH Weyborne field experiment.)

reporting of experimental data

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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