Abstract. MICROTOPS II is a five-channel, handheld Sun photometer that can be configured to measure total ozone, total water vapor, or aerosol optical thickness at various wavelengths. The instrument measures 10 x 20 x 4.3 cm and weighs 600 g. A principal design goal was the measurement of total ozone to within 1% of ozone measurements made by much larger, heavier, and more expensive Dobson and Brewer spectrophotometers. This goal has been met for a maximum air mass of up to -2.5, as demonstrated by comparisons of MICROTOPS II and its immediate predecessor, Supertops, with Dobson and Brewer instruments at various locations. Conventional interference filters are subject to gradual and unpredictable degradation. MICROTOPS II avoids these problems by using highly stable ultraviolet filters manufactured with an ion deposition process. The 2.4 nm (FWHM) band pass of the UV filters was selected to balance noise and ozone measurement performance. The optical collimators and electronics of the instrument were carefully designed to optimize pointing accuracy, stray light rejection, thermal and long-term stability, signal-to-noise ratio, and data analysis. An internal microcomputer automatically calculates the total ozone column based on measurements at three UV wavelengths, the site's geographic coordinates, and universal time, altitude, and pressure. The coordinates can be entered manually or by a Global Positioning System (GPS) receiver. A built-in pressure transducer automatically measures pressure. MICROTOPS II saves in nonvolatile memory up to 800 scans of the raw and calculated data. Measurements can be read from a liquid crystal display or transferred to an external computer. IntroductionAerosols and ozone within the atmosphere modulate the intensity of ultraviolet radiation at the surface of the Earth. Since ozone absorbs shorter wavelengths more effectively than longer wavelengths, the ratio of the intensity of direct sunlight at two wavelengths within the range of 300-320 nm is related to the total abundance of ozone in a column through the atmosphere. This forms the basic operating principle for a variety of instruments that measure the ozone layer. The bestknown ground-based ozone-monitoring instruments are the Dobson and Brewer spectrophotometers. Both these instruments divide sunlight into its constituent wavelengths by means of a spectrometer. The dispersing element is a quartz prism in the Dobson and a diffraction grating in the Brewer.While the Dobson and the Brewer are universally accepted instruments for measuring column ozone, these instruments are expensive, heavy, and large. There has long been a need for 14,573
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