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Calculations of air-mass factors (AMFs) for ground-based zenith-sky UV-visible spectrometers are presented and discussed. Causes and size of errors in AMFs of ozone in the visible are evaluated. Errors can be caused by approximations in the calculation (intensity-weight approximation, ignoring the finite field of view of the instrument); by approximation in the scheme of the calculation (single scattering, ignoring refraction); or by variable geophysical parameters (vertical profile of constituents). These relative errors in AMF cause identical relative errors in vertical columns of ozone deduced from measurements by zenith-sky spectrometers. The mean of the relative errors of ozone AMFs due to using one set of AMFs for all seasons and locations is ±2.4% when averaged over the commonly used range of solar zenith angles.

Part 1 Historical frontiers: vibrational spectroscopic contributions to chemisorption and catalysis. Part 2 Instrumentation: recent advances in kinetic infrared spectroscopy; spectroscopic concentration determination without calibration; scope and limitations of concentration-modulated absorption spectroscopy; electrochemically modulated infrared spectroscopy using a step-scanning FTIR spectrometer; application to the Fe(CN)6 3-/Fe(CN)6 4- couple; applications of Raman microscopy and Raman imaging; an error model for near infrared spectroscopic instruments; the new possibilities of luminescence spectroscopy of microscopic matter; characterization of single hydrocarbon fluid inclusions by fluorescence excitation-emission micro-spectroscopy; spectroscopic imaging of polyethylene; ozone measurements with star-pointing spectrometers. Part 3 Nuclear magnetic resonance: recent advances in high-resolution chemical NMR spectroscopy of solids; SFC/NMR on-line coupling; structural determinations of organic compounds in the environment by NMR spectroscopy and mass spectrometry; solid state proton NMR and dynamic mechanical analysis studies of polymer latex blends. Part 4 Biological applications: speciation of selenium in human serum by size exclusion chromatography and inductively coupled plasma mass spectrometry; the investigation of varietal differences among sorghum crop residues using near infrared reflectance spectroscopy; the influence of energy migration on fluorescence kinetics in photosynthetic systems; flow injection procedures with spectrophotometric detection for the determination nitrate and nitrite in riverine, estuarine and coastal waters; quantitative determination of chlorophyll. Part 5 Spectrometric determinations. Part 6 Chemometrics. (Part contents).

Visible spectra of the zenith-sky were recorded at Faraday in Antartica during 1990. Line-of-sight NO2 amounts were deduced by fitting laboratory cross-sections to the spectra. A new procedure was used to determine the offset to the zero of the measurement from the measurement series itself. This offset includes the effect of the reference spectrum—a single reference was used for the whole data set—plus any instrumental or analytic artefacts. It is important to determine this offset to the zero because it affects the absolute calibration of the zero of the vertical amount of NO2. In the new procedure, offsets were determined from fits to plots of the measured line-of-sight NO2 vs calculated air-mass factors, whose curvature had been precomputed from model predictions rather than the straight-line fits of Langley plots. The resulting vertical NO2 amounts were significantly lower than those deduced using a more subjective determination of offset, particularly in the early spring when denitrification might be expected.

Measurements of stratospheric NO2 by ground-based visible spectrometers rely on laboratory measurements of absorption cross-sections. We review low-temperature laboratory measurements, which disagree by amounts claimed to be significant. Our recalculation of their errors shows that in general disagreements are not significant and that errors in the ratios of cross-sections at low to room temperature are between ±3% and ±8.8%. Of these errors, up to ±3.5% was contributed by errors in the equilibrium constant,Kp, in those measurements where the pressure was above 0.1 mbar. We review measurements and calculations ofKp, which were accurate to ±5% from 300 to 233 K. Each method was potentially flawed. For example, infrared measurements of the partial pressure of NO2 ignored the dependence of absorption on total pressure. From thermodynamic theory, formulae forKpcan be derived from expressions for the variation of heat capacity with temperature. Contrary to common belief, coefficients in the formulae used by spectroscopists were not derived from the thermodynamic quantities. Rather, they were fitted to measurements or to calculations. Hence, they are empirical and it is dangerous to extrapolate below 233 K, the lowest temperature of the measurements. There are no measurements of NO2 cross-sections below 230 K. Extrapolation of these cross-sections to analysis of measurements of NO2 at the low temperatures of the Arctic and Antarctic stratosphere is also dangerous. For satisfactory analysis of polar spectra, the NO2 cross-sections should be measured at temperatures down to 190 K with a relative accuracy of ±1%. This difficult experiment would need a cell of minimum length 32 m whose length can be adjusted. Because their effects are circular, many errors cannot be removed simply. Although circular errors also arise in the measurements ofKpand of the infrared spectrum, their weights differ from those in the visible spectrum. The optimum experiment might therefore simultaneou