Ozone Profiles Using Electrochemical
Concentration Cell (ECC) Ozonesondes
Ozone vertical profiles
are obtained using electrochemical concentration cell ozonesondes
(ECC), a standard technique for such measurements. The heart of
the instrument is the electrochemical cell that consists
of a cathode cell containing 3 ml of dilute KI solution, and an
anode cell containing 1.5 ml saturated KI (Figure 1). A Teflon
piston pump forces ambient air, containing ozone, into the cathode
cell solution, which immediately produces iodine. Current flows
through the cell via an external circuit, reducing the iodine back
to iodide. The cell current is theoretically directly proportional
to the amount of ozone (one ozone molecule results in the flow
of 2 electrons). Ozone partial pressure is then computed by the
following equation:
Рozone = 4.307x10-4 (I IBG) Tpump T100 PCF .
where
Рozone= Partial pressure
of ozone (millipascals).
4.307x10-4 = Constant
to convert current to ozone concentration.
I = Cell
current (microamps) ~
5 μamps @ O3 peak in
stratosphere.
IBG = Cell background
current ~ < 0.03 μamps on zero O3 filter.
Tpump = Temperature of sonde pump (K).
T100 = Pump flow rate: seconds
per 100 cc.
PCF = Pump flow rate efficiency correction factor.
Several
recipes for the sensing solution are currently in use at various
stations. In addition there are also several pump efficiency
correction factors (PCF) used. The NOAA/CMDL ozonesondes (Trinidad
Head, CA; Hilo, HI; and Cheju Island, Korea) all use a 2% unbuffered
KI solution and a PCF as described in Johnson et al. (2001).
The PCF is of importance for the stratospheric portion of the
profile or when integrating the profile to obtain a total column
ozone amount. Accuracy and precision of ±5% are achieved
for ozone measurements in the troposphere and stratosphere using
the protocols described here.
The cell is mated
through an electronics interface board to a standard meteorological
radiosonde that telemeters data to a ground receiving station (Figure
1). The accompanying Vaisala radiosonde also provides temperature
and tropospheric humidity information along with the ozone data.
The instrument package weighing approximately 1 kg is flown on
a meteorological balloon. The balloon size, designated by its weight,
is usually 1200 or 1500 grams. The balloon normally achieves an
altitude of 32 km and ascends at a nominal rate of 5 m/s. Depending
on the system being used the data are obtained at a rate between
1 8 seconds. In TRACE-P data are obtained at 1-second intervals
and for archival purposes are averaged into altitude increments
of 100 meters.
Reference:
Johnson, B.J., S.J.
Oltmans, H. Vφmel, T. Deshler, and C. Kroger, ECC ozonesonde pump
efficiency measurements and tests on the sensitivity to ozone of
buffered and unbuffered ECC sensor cathode solutions, J. Geophys.
Res., accepted, 2001.
Figure 1: Schematic drawing showing the ozonesonde
pump, electrochemical cell, and interfacing to the radiosonde.
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