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

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

Several slides taken from Kaisa Lakkala, Finnish Meteorological Institute http://www.eubrewnet.org/cost1207/2016/06/13/eubrewnet-wmo-gaw-brewer-operator-course-in-edinburgh/ Julian Gröbner, PMOD/WRC http://kippzonen-brewer.com/wp-content/uploads/2014/10/BrewerUV-meas-and-cal_Grobner_17032014.pdf

Introduction

Consequences on measurements (1)

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

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

Brewer measurement physical principles and basic data processing

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 (0.1147 s) Fi = countrates [photons/sec] Measurement

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.

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

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

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

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?

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?

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?

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?

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

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

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?

Calibration

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)

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).

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

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)

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

How it works

How it works

How it works

How it works

How it works

Lamps set

Brewer UV commands

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)

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!

Advanced topics

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

Straylight

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

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.)

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

Angular response

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.)

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.

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

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