Comparison of stratospheric ozone profiles retrieved from microwave-radiometer and Dobson-spectrometer data

Passive Microwave Remote Sensors (MRS) can provide information on the composition of the atmosphere by measuring the thermal radiation emitted from rotational transitions of atmospheric molecules Ozone profiles simultaneously obtained from an MRS and a Dobson instrument (‘Umkehr’ method) are compared over a time period of approximately four months. The microwave measurements yield ozone concentrations which are 20–30% higher than the ‘Umkehr’ values. A critical, though not well known, parameter for the microwave inversion procedure is the foreign gas pressure broadening parameter (C) for the observed 142GHz ozone resonance. Throughout the intercomparison we used a value of 3 MHz mb⁻¹. There is recent theoretical and experimental indication that C is more likel y to be as low as 2.5 MHz mb⁻¹. If we use this new value for C all microwave retrieved profiles decrease by 20–25%, thus leading to a far better agreement with the ‘Umkehr’ results. Our measurements therefore strongly support the proposed value of 2.5 MHz mb⁻¹. A final answer on MRS ‘Umkehr’ correlation accuracy cannot be given. We feel that comparison on a day-to-day basis may be rather meaningless and monthly mean values should be used. On the other hand, there was relatively little change in these mean values during the intercomparison period The MRS showed its potential to retrieve ozone profiles also under adverse meteorological conditions, such as cloud cover or fog.