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Infra-red absorption by molecular cluster-ions formed by cosmic rays in the lower atmosphere

Dr Karen AplinDepartment of Physics

University of Oxford

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Overview

1. Introduction– Atmospheric radiation – Atmospheric cluster-ions– laboratory spectroscopy experiments

2. Detecting cluster-ion absorption in the atmosphere– Instrumentation: filter radiometer– Atmospheric testing of filter radiometer– Filter radiometer response to atmospheric cluster-ions

3. Atmospheric experiment with cosmic ray detector– Methodology – Instrumentation: cosmic ray detector– Overview of experiment– Data analysis– Response of filter radiometer to ionisation changes

4. Summary

Instrumentation: radiometer

http://www.globalwarmingart.com/wiki/File:Atmospheric_Absorption_Bands_png

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Atmospheric cluster-ions and IR radiation• Ionisation from cosmic rays and natural

radioactivity occurs continuously.• Atmospheric ions are 1-3nm clusters -

charged central molecule surrounded by hydrogen-bonded ligands e.g. X+(Y)n where Y is water, ammonia, pyridine etc

• Charged clusters absorb infra-red (IR) radiation, through stretching and bending of their hydrogen bonds.

• Dipole moment makes them efficient absorbers in comparison to neutrals.

• Protonated dimer absorption predicted and measured in the lab (Asmis et al, Science, 2003) – but IR calculations are computationally difficult for all but the simplest clusters (n~2).

• The need to quantify radiative effects of charged clusters, plus early measurements by Carlon, motivated lab spectroscopy experiments.

Typical geometries for X+(H2O)n clusters with 2≤n≤7 (Likholyot et al, Geochim. Cosmochim. Acta, 2007)

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Lab spectroscopy experiment

•Artificial positive ion source activated and de-activated, and spectra compared in an artificial atmosphere (surface temperature and pressure)• Positive ions chosen, as most work has been done on protonated clusters, though negative ions also expected to absorb in IR

•Two absorption bands of 1-3% detected at 12.3 and 9.15 μm at ion concentrations ~1013m-2

•Predicted positive ion column concentration in troposphere/stratosphere ~4x1014m-2

•Lab data suggests a direct absorption effect should be detectable in a cloud-free atmosphere

H H H

“Air”

“Air”+H2O

FTS

Corona ion source and fan Humidity

sensorOptical beam

Mirror

Ion detector sampling inlet and recirculation exit

Fourier transform infrared spectrometer 9m (optical path length 545m)

(artificial air used)

Aplin and McPheat, J. Atmos. Sol.-Terr. Phys. (2005)

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Detecting direct absorption of atmospheric ions

To detect IR absorption from cluster-ions in the atmosphere, need to measure:

1. cluster-ion absorption in bands identified in lab experiment: develop filter radiometer “tuned” to 9.15μm band

2. atmospheric ion concentration: • Either measure directly, using specialised

instrumentation (needs well-maintained scientific research site)

• Or measure ion production from cosmic rays with compact, low power detector (suitable for remote, long-term measurements).

Approach taken here: 1. Develop filter radiometer and show that it responds to ion concentration

measured at Reading University Atmospheric Observatory2. Deploy the filter radiometer as part of a long-term atmospheric

experiment where the ion production rate is monitored

Absorption region selected for filter radiometer

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Instrumentation: Filter radiometer

Modification to Swissteco net radiometer, replacing upper polythene dome with specialised filters. Filter radiometer is known as “Infrared Absorption Radiometer” (IAR)

Aplin and McPheat, Rev. Sci. Instrum. (2008)

Inlets for purging with N2

Upper (auxiliary) filter: Ge, outer face coated with diamond-like carbon, inner with anti-reflective coating

Lower (bandpass) filter: 9.15±0.07m centred, 5% bandwidth (selected to match observed cluster-ion absorption)

30mm

Anodized Al body

Spectral response

Laboratory calibration with black body source to determine instrument sensitivity: 29.3±0.1 µV/(Wm-2)

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Tfr i

Field mill (sensing aperture upwards)

Electrode for vertical current density

Td

Tw

Vfr

Jz PG IR

Low noise radiometer amplifier

psychrometer

ELECTRICAL RADIATIVE

Radiometer with narrowband filter

ions + + + + - -

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Atmospheric testing of filter radiometer

Filter radiometer at 9.15 m

Aplin, Space Sci. Rev. (2008)

Radiometer run concurrently with atmospheric electricity instruments at Reading University Atmospheric Observatory

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Filter radiometer calibration

Concentration of bipolar atmospheric ions n is given by:

PG: atmospheric Potential Gradient (typically 100 Vm-1) Jz : downwards conduction current of ions (measured at site to be 2pAm-2 (Bennett, PhD Thesis, 2007))

300 350 400 450 500

-0.1

5-0

.10

-0.0

50.0

00.0

5

Day 309 2007 (fog)

ions /cc

IAR

/W

m-2

1000 1200 1400

-0.1

5-0

.10

-0.0

50.0

00.0

5

Day 112 2007

ions /cc

IAR

/W

m-2

σ: Stefan’s constantG: gain of amplifier (500)τ: radiometer transmissivity (5.8%)K: radiometer sensitivity (29.3 µVWm-2)Tb: atmospheric brightness temperatureTr: radiometer body temperatureVr: measured voltage

Radiation in IAR absorption band Lλ, assuming the atmosphere is a black body, given by:

Radiative measurementsElectrical measurements

Results

Aplin, Space Sci. Rev. (2008)

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Filter radiometer response in different ion regimes

0 500 1000 1500

-0.1

5-0

.10

-0.0

50.0

00.0

5

Response of IAR to ion concentration

ions /cc

IAR

/W

m-2

Fog

Clear sky

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Atmospheric experiment with cosmic ray detector

Logger box containing cosmic ray telescope (11° FOV) – detects energetic particles > 400MeV

Radiometers mounted on building (180° FOV)

Cosmic ray atmospheric cascade produces tropospheric cluster-ions

+

++

++-

--

-

-+ -

-+--

IR radiation is emitted from the surface, absorbed and re-emitted by the atmosphere

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Instrumentation: Compact cosmic ray detectorGeiger counter “telescope” sensitive to particles >400MeV (mostly muons at surface)

Geiger count rate is a good proxy for ion concentration in clean air (Aplin and Harrison, Rev. Sci. Instrum. 2001)

Aplin and Harrison, Rev. Sci. Instrum. (2010)

Telescope A, used in experiments described here, was tested in a mountainous region, and the count rate increased as expected.

Telescopes A and B were also compared in an experiment at Reading University Observatory. Short term time series are not expected to agree, but hour to day timescale changes in muon production are modulated by the atmosphere and should show some agreement.

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Overview of experiment• Apparatus set up at semi-rural UK site

(Cheltenham), with IAR, CNR1 net radiometer (to measure downwelling solar and broadband longwave radiation) and cosmic ray telescope

• Cosmic ray telescope is triggered by energetic ionising particles, usually muons

• The individual Geiger counters in the cosmic ray telescope respond to local radioactivity (β, radiation)

• High-energy particle events, background radioactivity and radiative data are recorded on a Campbell CR3000 data logger

• 5 min averages are recorded, and conditional logging is also employed so that individual data points are stored 400s before and after each high-energy particle event (median separation 250s).

• Principle is to “trigger” on individual cosmic ray measurements and analyse composites of many events to remove background variability

120Ah battery

Data logger

Cosmic ray telescope

Solar panelRadiometers

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Filter radiometer response to high-energy particle events

• Experiment run from July 2008 – June 2009, ~32000 usable events recorded in total

• Events indicate ionisation in the column above the radiometer. Detector is relatively insensitive (~10 events/hr) but all event-causing particles pass directly above the radiometer (180° field of view)

• Data is sampled every 20s (determined by radiometer time response) around coincidence events, and a composite created by setting the event at 0s and normalising to the median value before the event

• Composite plots of response to all events shows substantial background variability:

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Response in clear sky

• Calculate median across all (8164) events and compare to the natural variability expected from data randomly selected from times before events (indicated by shading below)

• Clear sky, selected by downwelling longwave radiation < 295 Wm-2

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Results

• A small but statistically significant change of a few mWm-2 occurs in the IAR signal in response to ionisation events in the column above

• Effect is most apparent in clear sky conditions• Duration of effect could be linked to lifetime of cluster-ion• False positives are unlikely, as every cosmic ray event detected

will have created cluster-ions in the column above the radiometer

• Many events are needed to see the effect – possible reasons for this include

• atmospheric variability• differences in “view” of radiometer and cosmic ray detector• Is the effect caused by air showers?

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Summary • Molecular cluster-ions are constantly created in Earth’s atmosphere, and consist of a charged central atom surrounded by ligands (often water) •IR absorption from cluster-ions at 9.15 and 12.3 µm previously identified in the lab, and the signal expected to be detectable in cloud-free atmosphere.• A robust filter radiometer adaptor for a net radiometer: Infrared Absorption Radiometer (IAR) was developed to respond to IR radiation in a band centred on 9.15 µm ± 5%.• IAR responded positively to atmospheric ions in different ion concentration and humidity regimes.• Another experiment was carried out at semi rural site in west of UK (Cheltenham) from May 2008-June 2009

• Cosmic ray counter used to detect ion production in the column above the IAR• Analysis uses composites centred on each ionisation event detected

• A small change in the IAR signal of a few mWm-2 is robustly seen in response to ionisation events above the radiometer• This is consistent with a broad-band signal from ambient charged clusters, and provides a simple method for ionisation to affect the radiative balance.