3 -- IN-SITU MEASUREMENTS

3.1 Ozonesondes

An example of an in-situ measurement technique used to measure ozone vertical profiles involves an ozonesonde, which is an instrument carried on a weather balloon. Ozonesondes are composed of an ozone sensor, a battery, a small gas pump, and some electronic circuit boards. The ozone sensor, whose workings are explained in Section 3.2, is connected to a meteorological radiosonde. This radiosonde transmits values of air temperature, air pressure, relative humidity, detector current, detector temperature, and pump speed to a ground receiving station. The air containing the ozone sample is pumped through a solution which is oxidized by this ozone producing an electrical current. The electrical current is proportional to the flow of ozone. By knowing the flow (pump speed), the ozone concentration can be deduced. The result is an ozone partial pressure vertical profile. More details on ozonesonde chemistry and calculations are presented below in Section 3.2.

3.2 Ozonesonde Ozone Measurement Calculations

The ozone sensor consists of two small chambers containing solutions of potassium iodide (KI). Each chamber has a platinum electrode at its base and the electrodes are connected by an ion bridge (a wire). Air is pumped into one chamber and the ozone reacts with the potassium iodide which produces iodine (I2) by the equation

When the iodide changes to iodine, the two cells are no longer in electrical equilibrium. Electrons flow from one cell to another in order to reestablish equilibrium. As the amount of ozone in the air increases, the faster the iodide is changed to iodine and the more electrons flow between the cells (current). The amount of current that flows between the chambers is measured and sent to the ground receiving station. The ozone amount can be derived by the simple equation

where Pozone = ozone partial pressure (in nanobars);

C= constant

i = current

Tp = pump temperature

t = amount of time to force 100 milliliters of air through the system.

Data from ozonesondes are used to generate ozone profiles, that is, ozone partial pressure profiles like those seen in Chapter 3. The integrated profile amount gives us the total column amount. The advantage of in-situ measurements is that they are usually very specific and accurate and do not require a precise understanding of the radiative properties of the atmosphere, such as are needed for remote sensing. An important concern of in-situ measurements is insuring the sample is not altered as it is being brought into the measuring device.

Both balloon ozonesondes, and as we shall see, the ground-based Dobson and Brewer photospectrometers (see Sections 4.2.1 and 4.2.2), have played a major role in validating satellite ozone measurement data.