1. UV measurements Henri Diémoz
Environmental Protection Agency of the Aosta Valley
Volodya Savastiouk
International Ozone Services Inc.
16th WMO-GAW BREWER OPERATOR COURSE ASIA/PACIFIC

2. Presentation outline
Characteristics of UV radiation and consequences for measurements
How does a Brewer work? How are data processed?
What are my duties as an operator? How can I keep my Brewer calibrated?
A quick look to more advanced topics

3. Several slides taken from
Kaisa Lakkala, Finnish Meteorological Institute
Julian Gröbner, PMOD/WRC

4. Introduction

7. Consequences on measurements (1)

8. Consequences on measurements (1)
Rayleigh scattering → diffuse component
We need global optics

9. Consequences on measurements (2)
Extra-terrestial irradiance
Surface irradiance
Very steep irradiance gradient
(in a short wavelength range)
→ i) Linearity
→ ii) Wavelength accuracy

10. Brewer measurement physical principles and basic data processing

13. 1 - Dark current correction and conversion to photons/sec
Fi = 2 (Ci-Csh) / (CY*IT)
Ci = Brewer raw counts
Csh = Brewer raw counts
CY = num of cycles
IT = integration time ( s)
Fi = countrates [photons/sec]
Measurement

14. 2 – Dead time compensation Fcorr ← F * exp(Fcorr * DT)
Linearity is achieved by choosing proper ND filter and correcting for photomultiplier (PMT) dead time.
Poisson statistics and iterative scheme.

15. No temperature compensation in standard algorithm
No explicit filter correction
(will be included in responsivity)

16. 3 – Spectral sensitivity
Isun [W m-2 nm-1] = Fsun [countrates] / responsivity

19. What do we need to make (good) UV measurements?
1a) Wavelength accuracy
(dispersion relation)
Step = f(wavelength)
Motor command
Desired
Polynomial

20. What do we need to make (good) UV measurements?
Dispersion test
Done during the audit. Emission lines used to fit the step-wavelength polynomial.
Who?

21. What do we need to make (good) UV measurements?
Dispersion test
Can be further improved by post-processing (cf. advanced topics) by the user
Done during the audit. Emission lines used to fit the step-wavelength polynomial.
Who?

22. What do we need to make (good) UV measurements?
1b) Wavelength accuracy
(positioning check)
i) hg test: use internal mercury lamp to correct for wavelength shift(s) by monochromator(s), e.g. due to instrument temperature variations and gradients
ii) hp test: use internal standard lamp to sync gratings (MkIII only)
User’s responsibility. Both must be included in the schedule!
Who?

23. What do we need to make (good) UV measurements?
1b) Wavelength accuracy
(positioning check)
iii) gs test: records scans, with the standard lamp on, at different step-numbers to collect the necessary data to determine the relationship between the two spectrometers (MkIII only)
Included in ED routine
(user should check regular execution).
Data are analysed during the calibration visit.
Who?

24. What do we need to make (good) UV measurements?
2) Brewer sensitivity
→ UV calibrations
User’s responsibility (cf. next section)
Who?

25. What do we need to make (good) UV measurements?
3) Linearity → dead time test
Who?
Set during the audit, but user must include dt test in schedule and regularly check the results

26. What do we need to make (good) UV measurements?
4) Temperature dependence
5) Angular response
6) Resolution
User’s responsibility
(only for advanced processing)
Who?

27. Calibration

28. Responsivity
Known radiation source: tungsten-halogen lamp with a calibration certificate
→ continuous spectrum
Assuming linearity (lamp countrates are much lower!),
Ilamp (certificate) [W m-2 nm-1] = Flamp / responsivity
responsivity = Flamp / Ilamp (certificate)

33. How to transfer the laboratory calibration to the outdoor measurement site?
Calibrated transfer standards (1000 W lamps, not shown) are used in a dark room laboratory in controlled ambient conditions (temperature, humidity, stray light).
200W (new) or 50W (old) stability kits can be employed
outdoor to check the Brewer stability (within the
uncertainty characteristic of the specific lamp system).

34. 1% current → 10% irradiance
Lamp Current
Spectral Irradiance standards operate at a stable nominal current. The voltage is monitored to check for drifts and changes of the lamp.
Rule of thumb:
1% current → 10% irradiance

35. Stability kit(s) vs laboratory calibration
Portable calibrator:
Allows frequent (e.g., 1/week) calibration in the field
Should be regularly calibrated relative to laboratory 1000 W systems
Short distance between source and optics
Uncertainty
Warms the diffuser (Fountoulakis et al 2017)
Low intensity
The kit should be only used to monitor the stability of your instrument (not to calibrate it)

36. From Kipp&Zonen Operators Manual
How it works
From Kipp&Zonen Operators Manual

37. How it works

38. How it works

39. How it works

40. How it works

41. How it works

42. Lamps set

43. Brewer UV commands

44. Brewer UV commands (“o3 mode” only)
uv: standard scan (forward/backward scan in UV-B spectral range)
ux: extended (286.5–363 nm, MkIII and MkIV-e only)
ua: timed UV scans (extended scan, start at next half hour)
uf: fast UV scan (ascending direction only)

45. Brewer UV commands (“o3 mode” only) ul: external UV lamp scan
xl: extended scan of UV lamp
(MkIII and MkIV-e only)
ql: quick scan of UV lamp (24/12 wavelengths)
→ provides corrected intensities!

46. Advanced topics

47. undesired wavelengths
Straylight OK
Single Brewers.
Light scattered and
detected from
undesired wavelengths
More effective
rejection by
MkIII Brewers

48. Straylight

49. Some advanced methods exist to partially correct for straylight.
Suggested reading: Fioletov et al., 2000, Correction of stray light for the Brewer single monochromator

52. Temperature dependence
To remove this effect, characterisation of temperature sensitivity from specialised laboratories is needed.
Temperature coefficients for ozone measurements cannot be used!
Suggested readings: Weatherhead et al., 2001, Temperature dependence of the Brewer ultraviolet data (J. Geophys. Res)
Fountoulakis et al., 2017, Temperature dependence of the Brewer global UV measurements (Atmos. Meas. Tech.)

56. Spectral resolution
Software for wavelength alignment can also normalise to standard (1 nm) bandwidth, making spectra from different instruments comparable

57. Angular response

58. Angular response
Special devices / laboratory needed to characterise your instrument
Correction is based on radiative transfer models
Suggested reading: Bais et al., 1998, Correcting global solar ultraviolet spectra recorded by a Brewer spectroradiometer for its angular response error (Appl. Opt.)

59. Angular response
TU test → check alignment of prism position to global port for UV measurements.
Sometimes useful to improve angular response and increase UV throughput.

60. Summary of all advanced
processing steps
Procedure to go from raw (counts) to calibrated (Wm-2nm-1) spectrum:
Raw spectrum S(λ) counts
Remove dark signal S(λ)-dark counts
Remove Straylight (Single Br) S(λ)-dark-S(<292nm) counts
Convert to photons/sec (S(λ)-dark-sr)*4/IT photons/sec
Correct for Linearity S'(λ) photons/sec
Apply Sensitivity E(λ)=S'(λ)/Sens(λ) Wm-2nm-1
Advanced
Apply Temp. Corr dT/K
Apply Wavelength shift d λ
Spike correction
Cosine correction

61. Conclusions Differences between UV and O3 measurements
Differences between UV and O3 processing
UV needs regular operations by the user
(e.g., calibration, tests in schedule, analysis of tests results)
Advanced processing is possible, but advanced characterisation is also needed in that case