Millimeter-wave spectrometers for remote sensing of ozone and other minor gases of the atmosphere

Abstract

Ozone plays very important role in the atmosphere because it protects life at the Earth's surface against the harmful solar UV-B radiation [1]. In the stratosphere, ozone critically controls the energy budget because it absorbs both solar UV and terrestrial IR radiation [1]. Spatial distribution and variations of the ozone content determine quantity and variations of the absorbed energy. Existing satellite instruments such as [2] can't provide both high resolution in horizontal plane and sufficiently dense data sets on concentration of ozone and ozonerelated species (in the first place, chlorine and nitrogen oxides) over selected regions. So reliable ground-based instruments for monitoring of ozone and other minor atmospheric gases are necessary in addition to the satellites.Ground-based millimeter-wave (MM-wave, microwave) instruments for monitoring of ozone and other molecules in stratosphere and mesosphere has a number of advantages over traditional optical instruments (UV spectrometers and lidars) and ozone sondes. The advantages are low dependence on weather conditions, broad altitude region, and possibility of continuous day-and-night measurements.Ozone observations in Moscow region are performed at P.N.Lebedev Physical Institute (LPI) of the Russian Academy of Sciences since 1987. At present, the upgraded version [3] of the spectrometer operating since 1996 is used for regular ozone observations. Vertical distribution of ozone at altitudes 12-75 km (up to 95 km at night) is retrieved from contour of the collisionally broadened 142.175 GHz ozone emission line. The instrument consists of low-noise heterodyne receiver based on Schottky-diode mixer, filter-bank spectrum analyzer and computer with special interfaces and software. The receiver SSB noise temperature is of about 1500 K at room temperature operation and of about 700 K under liquid nitrogen cooling. A back-wave oscillator is used as the first local oscillator of 138.5 GHz frequency. 96-channel spectrum analyzer has total bandwidth of 475 MHz. Its channel widths increase from 0.1 MHz at the ozone line center through steps of 0.2, 0.5, 2 and 5 MHz to 20 MHz at the line wings [3]. The set of channel widths was optimized to get both minimal distortions of the ozone line contour and approximately equal signal-to-noise ratio values in the channels. These provide the best accuracy of the ozone vertical profile retrieval. 96-channel ADC of DAQ-2208 type is used to digitize signals at the spectrum analyzer outputs. General view of the spectrometer is given in Fig. 1.A new transportable 142 GHz ozone spectrometer (ozonometer) is being created now at LPI. Its roomtemperature heterodyne receiver operating in amplitude-switching mode is based on planar Schottky diode mixer [4]. Both low-loss input Gaussian optics and solid-state Gunn-diode local oscillator are used in the receiver [5]. Optical block of the receiver includes antenna, chopper wheel, room-temperature (hot) calibration load, reference load with brightness temperature varying within 100-300 K, SSB fi...