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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
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
spectroscopy instruments
FTIR (fourier transform infrered spectroscopy)
fixed mirror
moving mirror
beam splitter
detector
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
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
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.
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/