Balloon Dual-beam UV in situ O₃ Photometer

Measurement Description: This instrument provides in situ measurements of ozone on balloon flights. It is virtually identical to the instrument that flies aboard the ER-2 (NOAA ozone). Ozone is measured by absorption of the 254 nm mercury line in a two channel instrument, one chamber containing unperturbed air (I), the other containing air scrubbed of ozone (Io). Every ten seconds the flows are switched. Air is drawn into the chambers by two identical gear pumps, at a flowrate sufficient to completely flush the chamber in less than 0.5 sec. I/Io is measured simultaneously and continuously, cancelling out lamp intensity fluctuations. This I/Io ratio, coupled with the very well known cross section (1%) and path length give the ozone density in the absorption cell. Knowledge of the cell temperature (controlled and easily measured) and pressure gives an ozone mixing ratio, which is invariant between the cell and the atmosphere. Pressure is directly measured with MKS Baratrons (10, 100, 1000 torr ranges) which have stated accuracies of 0.1%, but more realistically perform near the 1-3% level as demonstrated by before and after flight calibrations as well as comparisons between overlapping sensors during the flight. Vertical profiles are obtained during ascent and descent; ozone is also measured continuously at float. Ozone values are obtained every second. Altitude resolution is about 5 meters on ascent (1 sec time resolution), variable (worse) on parachute descent. Accuracy in the ozone mixing ratio is 3-5%, due mostly to uncertainty in the total pressure measurement. Instrument precision is about 1%; UV cross section uncertainty about 1%. Both are included in the 3-5% total accuracy.

In the past, the Balloon Ozone instrument has flown aboard gondolas carrying other instruments to provide data for the validation of the ozone measurements of several of several satellite instruments including UARS MLS, HALOE, CLAES and ADEOS ILAS. More recently, a suite of in situ instruments has been assembled and flown as the Observations of the Middle Stratosphere (OMS) gondola, providing high resolution tracer measurements to expand the database obtained with the ER-2 instruments. For SOLVE, we will again fly with these in situ instruments aboard the OMS gondola. Our objectives in participating in SOLVE are to provide correlative ozone data for SAGE III and, more importantly, to provide measurements of ozone that can be used, in conjunction with other tracers (e. g., N₂O), to better define the composition of the polar vortex early in the season (November) before any significant loss of ozone has occurred. Since air will descend throughout the winter, the air sampled in this early flight will be studied more intensively by the ER-2 throughout the winter. The O₃-N₂O relationship measured on the November balloon flight will serve as the starting point in analyzing the ER-2 data for a signature of ozone loss. The second OMS in situ balloon flight in late winter (February-March) will extend the measurements of ozone loss above the altitude of the ER-2.