Metrology meets Meteorology

CITAC: 2014 Best Paper award presentation for 

Applied Physics B 116:883–899 (2014) Absolute validation of a diode laser hygrometer via 

intercomparison with the German national primary water vapor standardby 

B.Buchholz1,2, N.Böse1, V.Ebert1,2

30th CITAC members meeting , 19  April 2015

1Physikalisch‐Technische Bundesanstalt Braunschweig, Germany 2Center of Smart Interfaces, Technische Universität Darmstadt, Germany 

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MOTIVATION

sufficient answer

scientific question ?

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MEASUREMENTSin atmospheric science

Remote sensing (ground /satellite) 

lab input (e.g. spectral ref data)

for  remote sensing

lab input (e.g. spectral data)andlab validationfor in‐situ instrument 

validationvia Airborne In situ measurements

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QUALITY of “field measurements” ?

Our focus:  Improving atmospheric water vapor measurements 

BUT:• even in static H2O lab  conditions ± 10% inconsistency = “state of the art”• in airborne campaigns: local, dynamic deviations up to 30% are not unusual

Water vapor in environmental sciences/meteorology :• very important for 

• atmospheric radiative balance (most important greenhouse gas)• Important correction factor for many other atmospheric sensing techniques (strongest 

atmospheric constituent with the highest variability)• very important simulation parameter for climate models• atmospheric chemistry ( OH)• Cloud + aerosol formation ( phase transition  water/ice) • Weather + agriculture industry  • …

Field instruments: Need for pragmatic solution  often far away from “metrology”• link to the SI units ?• traceability in general ?• common evaluation, calibration, interpretation strategies ? • well defined uncertainties calculations ?

WHY is there a need for improvement ?(in airborne hygrometry)

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(D. W. Fahey et al, AMTD, 2014) 

e.g. AquaVIT‐I in 2008(blind comparison of world best airborne hygrometer)

Inconsistency span: 20%

B. Buchholz

reanalysis + correction of huge data sets

(e.g. MOZAIC)

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OUR philosophy

MetrologyMeteorology HOW TO LINK ?

?

Direct operation of an metrological airborne transfer standard in the field , i.e. on board of an research aircraft

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STATE of development in 2011(awarded paper)

named “SEALDH – 0”

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0.0 0.5 1.0 1.50.00.51.01.52.0

0.0 0.4 0.8 1.2 1.6 2.00

50100

time (ms)

dete

ctor

cur

rent

(m

A)

relative wavenumber (1/cm)

lase

r cur

rent

(mA

)

1368 1370 137220406080

100

tran

smis

sion

(%)

wavelength (nm)

H2O HITRAN SIMULATION

TDLAS PRINCIPLE

RAWSIGNAL1370 nm H2O absorption line

BASELINE-FIT

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⋅⋅ ⋅ ⋅

∙ ∙ ∙ ∙ ∙Extended Lambert‐Beer‐law

with ideal gas law:measured parametersconstantsmolecular line data

E(t)

I

t

Tr(t)

calibration free! …no need for calibration procedures … just validation 

DIRECT TDLAS EVALUATION

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Hygrometers andsensors under testThermostatic bath

Saturatorpressure ps

Pressure in themeasuring cell pm

Pre-saturator

Saturator temperature TsSupply withcompressed air

Outlet

TDLAS-instrument (SEALDH-0)• 1.4 µm Absorption Spectroscopy• extractive, closed‐path• novel in‐house development • Absolute 1st principles instrument 

• No calibration needed !

Intercomparison SetupH2O-SOURCE (National Primary Standard)

11

Validation Procedure

here:  5 different H2O concentration levels during validation 

13.05.2015 B. Buchholz

13.05.2015 12B. Buchholz

RESULT

relative deviation from primary standard Sophisticated further 

explanation

deviations explainable as spectroscopic effectsee awarded paper

Applied Physics B 116:883–899 (2014)

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WHAT happened between 2011 und today ?

• Improvements of  concept

• Development of airborne instruments

• Extensive field applications 

14

2014 development status New family of calibration‐free, airborne H2O transfer standardsSEALDH‐0 =>   SEALDH‐I;    SEALDH‐II;    HAI‐Ia;    HAI‐Ib

SEALDH‐IISelective Extractive Airborne Laser Diode HygrometerNew, completely own development New autonomous , airborne, extractive, closed‐path,  

absolute  1.4 µm  FIELD TDL Hygrometer  1st principles = calibration‐free

Metrological uncertainty: 4.3% , ± 3 ppmv

< 3 % deviation to National Primary Standard  (over UTLS pressure range 200 ‐ 1000 hPa)

Extremely temperature stable < 0.02% / K Very long‐term stable   < 0.2%  in 18 month

= CITAC Award

15

CURRENT statusHAI: Hygrometer for Atmospheric Investigations 

new airborne multi‐channel TDL Hygrometer

highly accurate highly precise < 0.033 ppmv Very high speed: up to 120 Hz very large dynamic range 1‐ 40000 ppmv

1st airborne multi‐phase instrument: vapor , total and ice water 

self validating, 1st principles = calibration‐free Metrological uncertainty 

extractive in‐situ

16

Extensive Field Applications

…many laboratory and airborne comparisons & campaigns

EUFAR DENCHAR (FZJ)PHG‐Comparison (PTB)THG‐Comparison (PTB)AquaVIT‐II (KIT)

EUFAR DENCHAR IFIC (LearJet 35A)AIRTOSS‐ICE – I (LearJet 35A)AIRTOSS‐ICE – II (LearJet 35A)TACTS (Gulfstream G550)ESMval (Gulfstream G550)ML‐CIRRUS (Gulfstream G550)ACRIDICON (Gulfstream G550)

metrology meets meteorology requirement for field applicability 

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CONCLUSION

B. Buchholz

MetrologyMeteorology

Linking Metrology and Meteorology on the highest level …..while taking into account all typical airborne restrictions and requirements ‐ such as:Robustness, low maintenance, stand‐alone,  high reliability, …

highly accurate and fast measurements, …  airborne certification issues, ...

SEALDH‐0SEALDH‐ISEALDH‐II HAI‐IaHAI‐Ib

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OUTLOOK (unpublished further validation)

All SEALDH‐II data are evaluated without any calibration !

long‐term accuracy validation at mid humidityagain at primary standards 18 month validation in three sessions 

0 200 400 600 800 1000

0

1

2

3

4

5

6SEALDH-II

rela

tive

devi

atio

n (%

)(S

EA

LDH

-II -

PHG

) / P

HG

gas pressure (hPa)

600 ppmv april 2012 2500 ppmv april 2012 8000 ppmv april 2012

4 month of permanent lab-operation,

1 airborne campaign (2h), transportations, etc.

600 ppmv september 20122500 ppmv september 20128000 ppmv september 2012

23 days of permanent operation at THG

10 days operation in temperature chamber

2 airborne campaign (25h), Airtoss-I and II

12 days permanent operation at AquaVIT-II

600 ppmv november 2013

18 month time span of validationat national primary humidity standard

0 200 400 600 800 1000-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5SEALDH-II

standard deviation 0.31%

gas pressure (hPa)

600 ppmv april 2012 2500 ppmv april 2012 8000 ppmv april 2012 600 ppmv september 2012 2500 ppmv september 2012 8000 ppmv september 2012 600 ppmv november 2013

rela

tive

devi

atio

n (%

)(S

EA

LDH

-II -

PH

G) /

PH

Gus

ing

the

600

ppm

v ru

ns a

s ca

libra

tion

over all average - 0.17 %

18 month long‐term stability:(600‐8000 ppmv, 65‐1000hPa)µ = ‐0.17% and σ = 0.31%

5/13/2015 19Bernhard Buchholz

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at traceable low H2O Standard

100 ‐ 1200  ppmv  20 ‐ 100  ppmv 

0 200 400 600 800 1000

-2

0

2

4

6

SEALDH-II

rela

tive

devi

atio

n (%

)(S

EA

LDH

-II -

THG

) / T

HG

gas pressure (hPa)

September 2013 at THGH2O concentration

600 ppmv 100 ppmv 75 ppmv 50 ppmv 35 ppmv 20 ppmv

0 200 400 600 800 1000-1

0

1

2

3

4

5

SEALDH-II

rela

tive

devi

atio

n (%

)(S

EA

LDH

-II -

THG

) / T

HG

gas pressure (hPa)

September 2013 at THGH2O concentration

1200 ppmv 600 ppmv 500 ppmv 400 ppmv 300 ppmv 200 ppmv 100 ppmv 600 ppmv all at PHG

difference: = 0.35 %

OUTLOOK (unpublished further results)

How to guarantee the accuracy under flight conditions ?

13.05.2015 21B. Buchholz

• online supervision of instrument status • pro active inclusion

of field testing and validation schemes

Full Instrument control

Full Validation

• robust metrological analysis • field equivalent lab tests

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HOW does the data look like in detail ?airborne operation of hygrometer

WVSS‐II (PI: H. G. J. Smit, FZJ )  FISH (PI: M. Krämer, FZJ)  

“uncertainties”  &  “errors” SEALDH‐II (4.3% ± 3 ppmv) Range 0‐40000 ppmv 0‐1200hPa

FISH (6% ± 0.15 ppmv) Range 0‐1000 ppmv 0‐500hPaWVSS‐II (5% ± 50 ppmv) Range 50‐40000 ppmv 200‐500hPa

100

1000

10000

-10

0

10

20

13:30 14:00 14:30 15:00 15:30 16:00 16:30-100

-50

0

50

100

SEALDH-II (to 1 Hz averaged) FISH (1 Hz)

abso

lute

dev

iatio

n(p

pmv)

rela

tive

devi

atio

n(%

)

2013-05-07

H2O

con

cent

ratio

n(p

pmv)

ice clouds

=2%=5%

(SEALDH-II - FISH) / FISH

(SEALDH-II - FISH)

=9 ppmv =5 ppmv

time (UTC)

100

1000

10000

-10

0

10

13:30 14:30 15:30 16:30-100

-50

0

50

100 (SEALDH-II - WVSS-II)

(SEALDH-II - WVSS-II) / WVSS-II

SEALDH-II (7 Hz) WVSS-II (0.5 Hz)

abso

lute

dev

iatio

n(p

pmv)

rela

tive

devi

atio

n(%

)

2013-05-07

H2O

con

cent

ratio

n(p

pmv)

5%

time (UTC)

=70 ppmv

SEALDH‐II  is our new, airborne, calibration‐free, wide range TDLAS hygrometer(TDLAS =  tunable diode laser absorption spectroscopy)