Measurement techniques and data analysis

Instrument descriptions

Space instruments

What does a data set tell us?

Ozone instruments

Chemical cell:

2H+ + 2I- + O3 I2 + O2 + H2O

Variant: ECC ozone sondes

Electromagnetic force derived from KI solution in two different concentrations (0.06 Mol/l and >8 Mol/K). Ozone flows through cell with lower conc. and releases free iodine according to 2KI + O3 + H2O I2 + O2 + 2KOH. The iodine is converted to 2I- at the Pt cathode (and 2I- are converted to I2 at the anode) producing an electrical current, which is then measured.

Ozone instruments

O3 absorption cross section ~ 1.2·10-18 cm2 at 258 nm

UV absorption

Ozone instruments

chemoluminescence spectrum

O3 + C2H4 HCHO* + other

HCHO* HCHO + h

Chemoluminescence

NOx instruments

Chemoluminescence

NO + O3 NO2* + O2 + other

NO2* NO2 + h

Photolytic converterIn order to measure NO2, a photolytic converter is used in front of the CLD to convert NO2 to NO

HOx instruments

LIF (laser induced fluorescence)

Radical converterIn order to measure HO2, a gas flow of NO is added to the sample so that HO2+NO OH+NO2

Hydrocarbon instruments

Gas chromatography

For most gases, a cryogenic preconcentration is required

Sample gas chromatogram

CO, CO2 instruments

Gas correlation radiometer

c proportional to ln(I/I0)

spectroscopy instruments

DOAS (differential optical absorption spectroscopy)

1 2

I1

I2

spectroscopy instruments

FTIR (fourier transform infrered spectroscopy)

fixed mirror

moving mirror

beam splitter

detector

FTIR spectrum

spectroscopy instruments

TDLS (tuneable diode laser spectroscopy)

space instruments

nadir view limb view

(solar) occultation

GOME and SCIAMACHY

space borne DOAS

1E+08

1E+10

1E+12

1E+14

1E+16

200 300 400 500 600 700 800Wavelength [nm]

Inte

nsity [a

rbitra

ry u

nits]

O3 UV

O3 vis

HCHO

OClO

O4

O2

H2O

SO2 NO2

BrO

Satellite group: http://giger.iup.uni-heidelberg.de/

courtesy T. Wagner, IUP Heidelberg

Pixel resolution of tropospheric satellite measurements

GOME

SCIA

OMI

1996-2003

2002-

2005-

courtesy T. Wagner, IUP Heidelberg

Dependence of GOME measurements on zenith angle and surface albedo

2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0SZA [°]

0

4

8

12

AM

F

a lbedo 0.8

albedo 0.0

stratospheric AM F

tropospheric AM F

geom etric AM F

courtesy T. Wagner, IUP Heidelberg

Tropospheric NO2 retrieval from GOME

courtesy T. Wagner, IUP Heidelberg

Tropospheric NO2 retrieval from GOME

courtesy T. Wagner, IUP Heidelberg

Weekly NO2 cycle

courtesy S. Beirle, IUP Heidelberg

Mon Tue Wed Thu Fri Sat Sun

1.2x1015

1.4x1015

1.6x1015

1.8x1015

2.0x1015

2.2x1015USA

NO

2*

LA Chicago Detroit NY Eastcoast

Mon Tue Wed Thu Fri Sat Sun

6.0x1014

8.0x1014

1.0x1015

1.2x1015

1.4x1015

1.6x1015

1.8x1015

2.0x1015

London Milan Paris Rome Athens Berlin

Europe

NO

2*

Mon Tue Wed Thu Fri Sat Sun

9x1014

1x1015

1x1015

Tel Aviv Jerusalem

Israel

NO

2*

distinction between uncertainty (aka „accuracy“) and random error (aka „precision“)

calibration bias

digitization noise, counting statistics

error propagation

Measurement uncertainty

Uncertainty and random error

measured distributiontrue value

bias

precision

Calibration bias

Frequent sources of error:

offset problems

uncertainty of reference value

non-linearity of response curve

conditions differing from ambient measurement

instrument drift (e.g. temperature shifts)

Counting statisticsSeveral instruments detect their signal by counting photons (e.g. chemoluminescence detector). Obviously, the precision of such a measurement becomes better if the number of photons (the statistical sample) increases. The population standard deviation is given by:

2

1

1

2

1

1

N

ii xx

Ns

One effect of this is the lower limit of detection (LOD) achieved by averaging signals over longer time scales.

Error Propagation

...23

22

21 eeee

To keep it simple:

BibliographyMaterial for this lecture comes mostly from

Brasseur, G.P., Orlando, J.J., and Tyndall, G.S., Atmospheric Chemkistry and Global Change, Oxford University Press, Oxford, New York, 1999.

Finnlayson-Pitts and Pitts, 1986.

Chemical ozone cell description fromhttp://www.fz-juelich.de/icg/icg-ii/josie/ozone_sondes/

Error analysis: NCAR Advanced study programme course 1992 http://www.asp.ucar.edu/colloquium/1992/notes/