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

1. 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

2. Overview Introduction Atmospheric radiation Atmospheric cluster-ions laboratory spectroscopy experiments Detecting cluster-ion absorption in the atmosphere Instrumentation: filter radiometer Atmospheric testing of filter radiometer Filter radiometer response to atmospheric cluster-ions Atmospheric experiment with cosmic ray detector Methodology Instrumentation: cosmic ray detector Overview of experiment Data analysis Response of filter radiometer to ionisation changes Summary 2

3. Instrumentation: radiometer http://www.globalwarmingart.com/wiki/File:Atmospheric_Absorption_Bands_png

4. 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. 4 Typical geometries for X+(H2O)n clusters with 2≤n≤7 (Likholyot et al, Geochim. Cosmochim. Acta, 2007)

5. Lab spectroscopy experiment Ion detector sampling inlet and recirculation exit “Air” (artificial air used) FTS “Air” +H2O Fourier transform infrared spectrometer Corona ion source and fan H H H Mirror Optical beam 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 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 Aplin and McPheat, J. Atmos. Sol.-Terr. Phys. (2005) 5

6. Detecting direct absorption of atmospheric ions To detect IR absorption from cluster-ions in the atmosphere, need to measure: • cluster-ion absorption in bands identified in lab experiment: develop filter radiometer “tuned” to 9.15μm band • 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: • Develop filter radiometer and show that it responds to ion concentration measured at Reading University Atmospheric Observatory • Deploy the filter radiometer as part of a long-term atmospheric experiment where the ion production rate is monitored 6

7. 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 30mm Lower (bandpass) filter: 9.15±0.07mm centred, 5% bandwidth (selected to match observed cluster-ion absorption) Spectral response Laboratory calibration with black body source to determine instrument sensitivity: 29.3±0.1 µV/(Wm-2) 7 Aplin and McPheat, Rev. Sci. Instrum. (2008)

8. Atmospheric testing of filter radiometer Radiometer run concurrently with atmospheric electricity instruments at Reading University Atmospheric Observatory Filter radiometer at 9.15 mm 8 Aplin, Space Sci. Rev. (2008)

9. Filter radiometer calibration Radiative measurements Electrical measurements Radiation in IAR absorption band Lλ, 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) τ: radiometer transmissivity (5.8%) K: radiometer sensitivity (29.3 µVWm-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)) Results Aplin, Space Sci. Rev. (2008) 9

10. Filter radiometer response in different ion regimes Fog Clear sky 10

11. 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 (180° FOV) - + - - + - Logger box containing cosmic ray telescope (11° FOV) – detects energetic particles > 400MeV 11

12. 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. 12 Aplin and Harrison, Rev. Sci. Instrum. (2010)

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 (β, 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 Overview of experiment Data logger 120Ah battery Cosmic ray telescope Radiometers Solar panel 13

14. 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: 14

15. 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

16. 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? 16

17. 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. 17