D. J. Fish

Rotational Raman scattering and the ring effect in zenith‐sky spectra

The technique of zenith-sky spectroscopy is widely used to measure the vertical columns of O3, NO2, OClO and BrO in the atmosphere. In this paper, a model to simulate the effect of rotational Raman scattering by O2 and N2 on zenith-sky spectra is presented. The model is used to calculate the Raman-scattering cross-section for zenith-sky measurements and this cross-section is shown to correspond closely to the measured Ring cross-section, supporting the case that Raman scattering is the major cause of the Ring effect. Raman scattering is also shown to reduce the depths of structured molecular absorptions in scattered light spectra, leading to a general underestimation of the slant columns of molecules measured by zenith-sky spectroscopy which can be significant in some cases. This effect varies with solar zenith angle, so will affect particularly attempts to retrieve the vertical profile of an absorber from the variation of slant column with zenith angle. The calculated Ring cross-section is used to infer the proportion of multiply-scattered light which enters a zenith-sky spectrometer at twilight, and thus to estimate the magnitude of the corresponding underestimation of measured slant columns.

Ozone and NO2 air-mass factors for zenith-sky spectrometers: intercomparison of calculations with different radiative transfer models

Calculations of air-mass factors (AMFs) for ground-based zenith-sky UV-visible spectrometers are now well developed in laboratories where stratospheric constituents are measured with this technique. An intercomparison between results from the different radiative transfer models used to calculate AMFs at twilight is presented here. The comparison was made for ozone AMFs at 510 nm and for NO2 AMFs at 440 nm. Vertical profiles were specified. Results are presented firstly for calculations in a pure Rayleigh atmosphere, then including background aerosols. Relative differences between calculated AMFs from different models cause relative errors in vertical columns of ozone and NO2 measured by zenith-sky spectrometers. For commonly used averages over solar zenith angles, these relative errors are ±2.3% in the vertical column of ozone and ±1.1% in the vertical column of NO2. Refinements to the calculations, suggested by the intercomparison, should reduce these errors to ±1.0% for ozone and ±0.5% for NO2.

Interpolation errors in UV–visible spectroscopy for stratospheric sensing: implications for sensitivity, spectral resolution, and spectral range

UV-visible measurements of stratospheric constituents require the ratio of a pair of spectra to be determined. If their wavelength calibrations differ and if an array detector is used, at least one spectrum must be interpolated. This introduces error if the spectrum is undersampled; the error is smaller if wavelength stability is good. Increasing the sampling ratio by making the spectral resolution poorer reduces the optical depths of absorption by constituents. Exact values of interpolation errors from real spectra are a difficult topic, but with a theoretical study with a simulated spectrum we show that the sampling ratio should exceed ~4.5 pixels/FWHM but need not exceed 6.5 pixels/FWHM. To avoid significant reduction in the optical depth of NO(2), the resolution should be smaller than ~1.0 nm FWHM. Hence a spectrometer system that measures both OClO and NO(3) by observing one order from one stationary grating should have more than ~1500 pixels, more than many currently available array detectors.

Ozone measurements by zenith-sky spectrometers: An evaluation of errors in air-mass factors calculated by radiative transfer models

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.

Using stars for remote sensing of the Earth’s stratosphere

A new UV-visible spectrometer system that measures the absorption of light from stars and planets by constituents in the Earths atmosphere is described. Because it can be used to make measurements at night, the system has a significant advantage for measuring polar constituents in winter, when conditions that might give rise to ozone loss are initiated. Other advantages arise from the use of a cooled two-dimensional CCD array as the detector: an array detector avoids spectral noise resulting from scintillation of stars or from clouds passing overhead and allows for the possibility of measuring several constituents simultaneously; its second dimension permits auroral light from the atmosphere adjacent to the star to be measured simultaneously and subtracted from the stellar light, and a modern low-noise CCD allows us to use a telescope of modest diameter. The few previous measurements of constituents made by the use of stellar absorption did not have these advantages. The instrument was configured for simplicity and ease of use in field measurements and was deployed outside in winter in Northern Sweden in 1991. Examples of ozone measurements are shown.

Accurate derivation of total and stratospheric vertical columns of NO2 from ground-based zenith-sky measurements

A new method for retrieving the vertical profile of NO2 from ground-based measurements is applied to four months of measurements made at Aberdeen (57°N) during part of SESAME from November 1994 to April 1995. The retrieval method is shown to be an invaluable tool both for deriving the true NO2 vertical column and for removing the tropospheric contribution to the vertical column. This dramatically reduces the effects of tropospheric pollution in the observations and enables a more appropriate comparison with stratospheric 3-D model results. The comparison confirms the accuracy of the models transport and its reactive nitrogen photochemistry, although there are some detailed discrepancies.

Five years of NO2 vertical column measurements at Faraday (65°S): Evidence for the hydrolysis of BrONO2 on Pinatubo aerosols

Summertime measurements Of NO2 vertical column amounts over a 5 year period from May 1990 until February 1995 from Faraday Base, Antarctica, show a marked reduction following the arrival of the Mount Pinatubo volcanic aerosol in December 1991. Model calculations show that this reduction can be explained by BrONO2 and N2O5 hydrolysis on the volcanically enhanced aerosol, with the former dominating. Given the measurement and model uncertainties and lack of any treatment the effects of the quasi-biennial oscillation, the reduction in NO2 is consistent with a BrONO2 sticking coefficient gamma of 0.4. However, the best agreement between the model and the measurements occurs using a gamma of 0.2. Over the time span of the measurements the known increases in chlorine and bromine loadings have an effect of less than 2% on midsummer NO2 columns. With background aerosols, summertime ozone catalytic losses are dominated by the HOx cycle between 12 and 18 km and by the NOx cycle at greater altitudes. With heavy aerosol loading, HOx is the primary loss cycle from 12 to 22 km. The total ozone loss increases by 38% at 16 km as a result of heavy aerosol loading.

Total ozone measured during EASOE by a UV-visible spectrometer which observes stars

Total ozone was measured from Abisko, Sweden (68.4°N, 18.8°E) from January to early March 1992, by a new instrument which uses stars and the Moon as sources of UV-visible light for absorption spectroscopy. In addition, some zenith-sky observations were made. Ozone measurements obtained using both techniques are presented and compared with those from other instruments. Good agreement with simultaneous ozonesonde measurements is observed, but the stellar measurements appear systematically higher than total ozone measured by both SAOZ and TOMS.

Automated ground-based star-pointing UV–visible spectrometer for stratospheric measurements

A novel automated ground-based star-pointing spectrometer system has been constructed for long-term deployment in Antarctica. Similar to our earlier stellar system, a two-dimensional detector array measures the spectra of the star and the adjacent sky, so that auroral emission from the sky can be subtracted from the stellar signal. Some new features are an altitude -azimuth pointing mirror, so that the spectrometer does not move; slip rings to provide its power thereby avoiding flexing of cables and restriction of all-around viewing; and a glazed enclosure around the mirror to ensure protection from rain and snow, made from flat plates to avoid changing the focal length of the telescope. The optical system can also view sunlight scattered from the zenith sky. The system automatically points and tracks selected stars and switches to other views on command. The system is now installed at Halley in Antarctica, and some preliminary measurements of ozone from Antarctica are shown.

A star-pointing UV-visible spectrometer for remote sensing of the stratosphere

Significant ozone loss due to reactive chlorine from man-made chemicals has occurred near the poles in the last decade. In this paper, we describe a novel star-pointing UV-visible spectrometer to measure amounts of some reactive gases in the ozone layer and discuss its advantages. The instrument has the capability of measuring stratospheric amounts of O3, NO2, NO3 and OClO at night. By using the most modern cooled array detectors, good signal-to-noise ratios can be obtained with a modest telescope and a short integration time. By using a two-dimensional array, light from the atmosphere adjacent to the star is measured simultaneously and subtracted from the stellar light. As with measurements using the sun as a source of light, before spectral analysis the observed spectrum at low elevation must be divided by the spectrum of the same star measured at higher elevation. This removes absorption features due to gases in the atmosphere of the star itself. The amount of absorbing constituent in the earths atmosphere is proportional to the ratio of the slant path to the vertical path through the atmosphere. This air-mass factor is maximized, and the random error in the measurement minimized, at elevation angles close to the horizon. The instrument was deployed at Abisko in Northern Sweden during the 1991/92 European Arctic Stratospheric Ozone Expedition. Despite unusually cloudy conditions, many spectra were recorded.