Ozone Instrument

Our buoy-mounted ozone instrument was built by Physical Sciences Inc. (Andover, MA) to collect data accurately, reliably, and autonomously. This type of instrument was first used for ozone detection from aircraft and balloons, but with a few modifications it is now ready for use in the marine boundary layer.

The instrument's design offers sensitive and accurate ozone measurements, while still being compact and durable. Its waterproof casing is 55 x 34 x 22 cm, and the entire package weighs about 19 kg. The instrument is able to operate with good thermal stability, low power consumption, and low flow rates - attributes necessary for battery-powered data collection in areas of variable climate. With the instrument running autonomously for several months at a time, power consumption and reliability are very important.

Diagram of a 3-meter dia. buoy with ozone detector and
meteorological instruments in place.

Ozone instrument pictured from above.

The instrument currently requires approximately 55 watts during operation. However, the instrument's computer consumes more than half of this power. In the next phase, the current data system will be replaced with one that is more efficient, considerably decreasing the instrument's overall power consumption.

In order to protect the instrument in rough conditions (i.e. heavy rain or high winds) it has a controller that is attached to several meteorological instruments. At the onset of rain, high winds, or instrument malfunction, the controller shuts down the instrument automatically. When the rain or high winds diminish, the instrument is turned back on and continues to collect data. (Click here for a diagram of the buoy configuration)

The instrument can be thought of as having three major elements. (Click here for a flow diagram of the instrument) First, it has a computer that records the data. Second, it has pumps, tubing, and valves to bring ambient air in and out of the detector. The final and most important element is the ozone detector itself.

The detector consists of a two-cell photometer with a mercury lamp and a beamsplitter to direct 254-nm UV radiation down both absorption cells. The radiation is incident on photodiodes at the end of each cell, which record the radiation intensity. At any given time, one cell will contain ambient air with some unknown quantity of ozone, while the other cell will contain air from which all the ozone has been removed with a catalytic scrubber. Every ten seconds, the flows to each cell switch (Click here for a diagram of flow through the cells). Based on the intensities reported by the photodiodes, the length of the cells (20 cm), and the absorption cross section of ozone, the ambient concentration of ozone can be calculated by direct absorption (Beer's Law). Measuring temperature and pressure allows the concentration to be converted to a mixing ratio in units of parts per billion.