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11_Aplin_IR_absorption_by_cluster-ions

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Infra-red absorption by molecular cluster-ions
formed by cosmic rays in the lower atmosphere
Dr Karen Aplin
Department of Physics
University of Oxford
1
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
2
2
Instrumentation: radiometer
http://www.globalwarmingart.com/wiki/File:Atmospheric_Absorption_Bands_png
Atmospheric cluster-ions and IR radiation
•
•
•
•
•
4
Typical geometries for X+(H2O)n clusters with 2≤n≤7
(Likholyot et al, Geochim. Cosmochim. Acta, 2007)
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.
Lab spectroscopy experiment
Ion detector sampling inlet and recirculation exit
Fourier
transform
infrared
spectrometer
H
H
FTS
Mirror
Optical
beam
Corona ion
source and fan
H
“Air
”
“Air”
+H2O
(artificial
air used)
Humidity
sensor
9m (optical path length 545m)
•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
Ојm at ion concentrations ~1013m-2
•Predicted positive ion column concentration in
troposphere/stratosphere ~4x1014m-2
•Lab data suggests a direct absorption effect should be
Aplin and McPheat, J. Atmos. Sol.-Terr. detectable in a cloud-free atmosphere
5 (2005)
Phys.
5
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).
Absorption region selected
for filter radiometer
Approach taken here:
1. Develop filter radiometer and show that it responds to ion concentration
measured at Reading University Atmospheric Observatory
2. Deploy the filter radiometer as part of a long-term atmospheric
experiment where the ion production rate is monitored
6
Instrumentation: Filter radiometer
Modification to Swissteco net radiometer, replacing upper polythene dome with specialised
filters. Filter radiometer is known as “Infrared Absorption Radiometer” (IAR)
Upper (auxiliary) filter: Ge, outer face
coated with diamond-like carbon, inner
with anti-reflective coating
Anodized
Al body
Inlets for
purging with
N2
Lower (bandpass) filter:
9.15В±0.07mm centred,
5% bandwidth (selected
to match observed
cluster-ion absorption)
30mm
Spectral
response
Laboratory calibration with black body source to determine instrument sensitivity:
29.3В±0.1 ВµV/(Wm-2)
Aplin
and McPheat, Rev. Sci. Instrum. (2008)
7
Atmospheric testing of filter radiometer
Radiometer run concurrently with atmospheric electricity instruments at Reading University
Atmospheric Observatory
ELECTRICAL
RADIATIVE
+
Jz
-
+
PG
ions
+
-
+
-
Filter radiometer at
9.15 mm
IR
Td
Tw
psychrometer
Vfr
Tfr
i
Electrode for
vertical current
density
Field mill (sensing
aperture upwards)
Aplin, Space Sci. Rev. (2008)
8
Radiometer
with
narrowband
filter
Low noise
radiometer amplifier
Filter radiometer calibration
Radiative measurements
Electrical measurements
Radiation in IAR absorption band LО»,
assuming the atmosphere is a black body,
given by:
Concentration of bipolar atmospheric ions
n is given by:
σ: Stefan’s constant
G: gain of amplifier (500)
П„: radiometer transmissivity (5.8%)
K: radiometer sensitivity (29.3 ВµVWm-2)
Tb: atmospheric brightness temperature
Tr: radiometer body temperature
Vr: measured voltage
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))
0.05
-0.15 -0.10 -0.05 0.00
IAR /Wm-2
0.05
IAR /Wm-2
9
Day 112 2007
-0.15 -0.10 -0.05 0.00
Day 309 2007 (fog)
Results
300
350
400
ions /cc
450
500
1000
Aplin, Space Sci.
Rev. (2008)
1200
ions /cc
1400
Filter radiometer response in different ion regimes
0.05
Response of IAR to ion concentration
-0.05
-0.10
IAR /Wm-2
0.00
Fog
-0.15
Clear
sky
0
500
1000
ions /cc
10
1500
Atmospheric experiment with cosmic ray detector
IR radiation is
emitted from the
surface, absorbed
and re-emitted by
the atmosphere
Cosmic ray atmospheric
cascade produces
tropospheric cluster-ions
+
-
-
-
+
+
+
+
-
-
+
+
-
Radiometers
mounted on building
(180В° FOV)
Logger box containing
cosmic ray telescope
(11° FOV) – detects
energetic particles >
400MeV
11
11
Instrumentation: Compact cosmic ray detector
Geiger 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)
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.
Aplin12and Harrison, Rev. Sci. Instrum. (2010)
Overview of experiment
•
Data logger
120Ah battery
•
•
•
Cosmic ray
telescope
•
•
Radiometers
13
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 (ОІ, g-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
Solar panel
13
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 (180В° 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:
14
14
Response in clear sky
• Clear sky, selected by downwelling longwave radiation < 295 Wm-2
• 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)
15
15
Results
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
•A
• 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?
16
16
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 Вµm В± 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.
17
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