Ozone Absorption Cross Section Research Articles

Stratospheric Aerosol and Gas Experiment II‐Umkehr ozone profile comparisons

This study compares 1789 pairs of ozone profiles derived from 1384 Umkehr observations at 14 different stations and 1163 Stratospheric Aerosol and Gas Experiment (SAGE) II profiles coincident within 1000 km and 14 hours between October 1984 and April 1989. The comparison indicates the following significant percentage differences (SAGE II‐Umkehr)/Umkehr with 2 × standard errors of the mean: Umkehr layer 4, (18.3±0.8)%; layer 5, (−1.6±0.4)%; layer 6, (−6.2±0.5)%; layer 7, (0.8±0.6)%; layer 8, (7.7±0.6)%; and layer 9, (12.0±1.1)%. Differences in layers 4 and 6 are due, at least in part, to inaccurate Umkehr climatologies. Average SAGE II/Umkehr differences in layers 5 through 9 at individual stations are generally less than 10%. While the Umkehr retrievals are known to be sensitive to aerosol interference, the mean layer 8 correction during the period of this study is estimated to be only 2% with large station‐to‐station variability. The correction in lower layers is smaller. We have chosen to ignore the small Umkehr aerosol correction in this study. The mean difference would decrease if Umkehr profiles were corrected for a priori profile effects calculated by DeLuisi et al. (1989a). However, using the newer Bass and Paur (1985) ozone absorption cross sections would tend to increase the differences at most levels. The profile of mean differences is similar to previously observed differences between Umkehr and solar backscattered ultraviolet (SBUV) observations. Comparing SAGE II/Umkehr differences to SAGE I/Umkehr differences at seven common stations shows a bias of −4% at the ozone peak (layer 4). This bias increases with altitude to 8% in layer 8 and 15% in layer 9, with SAGE II ozone partial pressures higher than or equal to those of SAGE I (version 6.1) relative to Umkehr in all layers above 4. A systematic upward reference‐altitude shift between 0.25 and 0.50 km for SAGE I, similar to the quoted uncertainty, would increase SAGE I ozone 4% to 8% in layer 8 and would result in similar SAGE and Umkehr ozone trends during the 1980s. Cross correlations of numerous variables associated with the Umkehr and SAGE II data sets show a minimum correlation between SAGE II and Umkehr ozone partial pressures in layers 5, 8, and 9. This correlation is a result similar to the one previously noted in other comparisons against Umkehr data. We discuss these minimum correlations in relationship to the seasonal cycle in ozone and synoptic scale variations at midlatitudes based on model results. Substantial differences between SAGE II and Umkehr exist in both the mean and the variability of ozone in layers 8 and 9. Substantial differences also exist in layer 6 where the Umkehr algorithm does not retrieve the low ozone values periodically observed by SAGE II during winter.

Ozone UV spectroscopy. II. Absorption cross-sections and temperature dependence

Absolute absorption cross-sections of ozone have been measured at five temperatures (218–295 K) in the UV wavelength region corresponding to the Hartley and Huggins bands (195–345 nm). The aim is to give to the users, a coherent set of data determined under high resolution. A comparison with previous work has been made.

O3 → O(¹D) photolysis frequencies determined from spectroradiometric measurements of solar actinic UV‐radiation: Comparison with chemical actinometer measurements

The solar actinic flux has been measured directly and spectrally resolved between 280 nm and 330 nm by means of a spectroradiometer. From these measurements the photolysis frequency J(O¹D) of ozone is calculated using ozone absorption cross‐sections and O(¹D) quantum yields from the literature. Different published wavelength dependencies of the O(¹D) quantum yield data are used to compare the spectroradiometric J(O¹D) values to J(O¹D) data measured simultaneously with a chemical actinometer. This field comparison supports laboratory studies that have measured O(¹D) quantum yields of about 0.2 to 0.3 between 315 nm and 320 nm. Implications for tropospheric photochemistry are discussed.

Ozone absorption cross section measurements in the Wulf bands

A tandem dual‐beam spectrometer has been developed to determine ozone absorption cross sections for 13 selected wavelengths between 750 and 975 nm at room temperature. The increasingly pronounced structure in this region may interfere with atmospheric trace gas transitions that are useful for remote sensing and complicate the measurement of aerosols. Ozone concentrations were determined by absorption at the common HeNe laser transition near 632.8 nm using the absolute cross section reported previously. The overall accuracy of these room temperature measurements is generally better than 2%. A synoptic near‐IR spectrum scaled to these measurements is employed for comparison with results of previous studies.

Measurements of absolute absorption cross sections of ozone in the 185‐to 254‐nm wavelength region and the temperature dependence

Laboratory measurements of the relative absorption cross sections of ozone at temperatures 195, 228 and 295 K have been made throughout the 185 to 254 nm wavelength region. The absolute absorption cross sections at the same temperatures have been measured at several discrete wavelengths in the 185 to 250 nm region. The absolute cross sections of ozone have been used to put the relative cross sections on a firm absolute basis throughout the 185 to 255 nm region. These recalibrated cross sections are slightly lower than those of L. T. Molina and M.J. Molina (Journal of Geophysical Research 91, 1450, 1986), but the differences are within a few percent and would not be significant in atmospheric applications.

Ozone UV spectroscopy I: Absorption cross-sections at room temperature

Because of the importance of the absorption cross-sections of ozone in the atmosphere, we give a new set of data at ambient temperature for the whole spectral region within the 195–345 nm range. Two new methods have been investigated for the experiments to measure the ozone pressures. Our results are compared with all of the previous data. The agreement between the more recent works is relatively good and can be improved by making some corrections in the WMO set. The new reference value at 254 nm proposed by Mauersberger et al. is confirmed and we suggest taking it as a new standard.

Laser measurements of ozone absorption cross sections in the Chappuis Band

We have developed a sensitive spectrometer which exploits several He‐Ne laser transitions in the visible for precise, high resolution measurements of Chappuis band ozone absorption cross sections at room temperature. An overall, uncertainty of better than 1% has been achieved through a combination of transducer calibrations and an experimental procedure which unambiguously determines the impurity content of each ozone sample. The cross section near 602.5 nm is (5.16±0.03) × 10−21 cm2/molecule. Our results compare favorably with those from most previous studies in the visible range, although some are clearly lower than this consensus probably due to errors in ozone density measurements.

Absorption cross sections of ozone in the 590‐ to 610‐nm region at T = 230 K and T = 299 K

Measurements of the ozone absorption cross section in the 590‐ to 610‐nm region of the Chappuis bands have been carried out at the temperatures T = 230 K and T = 299 K. Ozone is produced with an electrical discharge and stored cryogenically. Differential absorption measurements are obtained in a slowly evolving mixture of ozone and molecular oxygen. Previous results by other authors indicate a certain disagreement on the temperature dependence of the absorption cross section. Our data at each wavelength show no significant dependence of the absorption coefficient on the temperature, within experimental errors. The statistical consistency of the results is discussed.

Uncertainties in total ozone amounts inferred from zenith sky observations: Implications for ozone trend analyses

In an effort to determine ozone measurement uncertainties associated with zenith sky radiation observations, a preliminary study involving radiative transfer calculations has been conducted to simulate ozone inference from Dobson spectrophotometer observations. By using a comprehensive radiative transfer algorithm, a model atmosphere, and a suitable set of ozone absorption cross sections, we have simulated, for a specific ozone profile and an ozone amount of about 350 Dobson units, the effects of cloud optical depth, cloud altitude, vertical distribution of ozone, temperature profile, and surface albedo on the total amount of ozone inferred by the Dobson procedure from zenith sky observations. Our simulations indicate that significant uncertainties may arise in determining ozone amounts from measurements of zenith sky intensity. The implications of these uncertainties for ozone trend analyses are described, showing, for example, that an appreciable error could arise in deduced ozone trend should tropospheric ozone double in the foreseeable future but not be taken into account in total ozone data processing. The radiative transfer algorithm may, furthermore, be employed to construct charts for use at each Dobson instrument site in processing zenith sky ozone observations. Such charts are currently derived empirically by comparing direct Sun and zenith sky observations. The theoretically derived charts would, however, need on‐site correcting to account for instrumental effects. Construction and use of synthetic correction tables for any particular station may alleviate difficulties encountered in the creation of empirical correction tables, since it avoids problems related to (1) time lapse between measurements and (2) diffuse radiation from forward scattering in clouds influencing the direct Sun measurements.

Optimum spectral resolution for computing atmospheric heating and photodissociation rates

Rapid, reliable and accurate computations of atmospheric heating rates are needed in climate models aimed at predicting the impact of greenhouse gases on the surface temperature. Photolysis rates play a major role in photochemical models used to assess potential changes in atmospheric ozone abundance due to man's release of chlorofluorocarbons. Both rates depend directly on the amount of solar radiation available at any level in the atmosphere. We present a very efficient method of computing these rates in which integration over the solar spectrum is reduced to a minimum number of monochromatic (or pseudogray) problems by appealing to the continuum features of the ozone absorption cross-sections. To explore the resolutions needed to obtain adequate results we have divided the spectral range between 175 and 700 nm into four regions. Within each of these regions we may vary the resolution as we wish. Accurate results are obtained for very coarse spectral resolution provided all cross-sections are averaged by weighting them with the solar flux across any bin. By using this procedure we find that heating rate errors are less than 20% for all altitudes when only four spectral bands across the entire wavelength region from 175 to 700 nm are used to compute the heating rate profile. Similarly, we find that the error in the photodissociation of ozone is less than a few percent when 10 nm resolution is used in the Hartley and Huggins bands (below 330 nm), while an average over the entire wavelength region from 400 to 700 nm yields similar accuracy for the Chappuis band. For integrated u.v. dose estimates a resolution slightly better than 10 nm is required in the u.v.B region (290–315 nm) to yield an accuracy better than 10%, but we may treat the u.v.A region (315–400 nm) as a single band and yet have an accuracy better than 2%.

Temperature dependence of the absorption cross-section of ozone at 254 nm

A new technique for measuring the variation of the absorption cross-section of ozone with temperature has been developed. This technique, based on the application of Clapeyron's law, is particularly precise. It has been tested in the Hartley band at the wavelength of the mercury line (λ=253.65 nm) where the absolute values of the cross-sections are used for calibration of relative data. The variation from 295 to 229 K determined in this experiment as Δσ=(−1.3±0.6)×10 −19 cm 2 is in good agreement with other data.

Absolute absorption cross-section measurements of ozone in the wavelength region 238–335 nm and the temperature dependence

Laboratory measurements of the absolute absorption cross-section of ozone at the temperatures 195, 228 and 295 K have been made at several discrete wavelengths in the region 238–335 nm. Our results for ozone at 295 K are in excellent agreement with those of Hearn (1961, Proc. phys. Soc. Land. 78, 932), who used a different technique. Our absolute cross-section measurements of ozone at 195 K have been used to put our recent relative cross-section measurements at that temperature (Freeman et al., 1984, Planet. Space Sci. 32, 239) on a firm absolute basis throughout the region 240–335 nm; these recalibrated cross-sections are available as numerical compilations on magnetic tape from the National Space Science Data Center, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A.

Temperature dependence of the ozone absorption cross section at the 253.7‐nm mercury line

The temperature dependence of the ozone absorption cross section at 253.7 nm has been measured between 195 and 351 K. The experimental technique employed circumvents the necessity to determine the absolute ozone concentration for each temperature measurement. Below 273 K the cross section increases approximately 0.6%, while toward higher temperatures the cross section decreases rapidly. In a comparison, good agreement with other recently made measurements is shown.

Absolute absorption cross sections of ozone in the 185‐ to 350‐nm wavelength range

The absorption cross sections of ozone have been measured in the wavelength range 185–350 nm and in the temperature range 225–298 K. The absolute ozone concentrations were established by measuring the pressure of pure gaseous samples in the 0.08‐ to 300‐torr range, and the UV spectra were recorded under conditions where less than 1% of the sample decomposed. The temperature dependence is significant for wavelengths longer than ∼280 nm. The absorption cross‐section values around 210 nm were found to be about 10% larger than the previously accepted values.

Seven years of total ozone from the TOMS instrument‐A report on data quality

Seven years of TOMS total ozone data are currency available. Recently a new ozone retrieval algorithm based on improved ozone absorption cross section was implemented that has introduced 6‐7% discontinuity in the archived data sets at the end of the 5th year of instrument operation. Until all data are reprocessed with the new algorithm users can use a table of correction factors given in this paper to make the data set internally consistent.This paper also presents a comparison of TOMS results with Amundsen Scott Dobson station. Though the agreement in most years is good, in 1983‐84 the Dobson station reported unusually high values of ozone while TOMS saw the very low ozone values associated with the Antarctica ozone hole.

New in‐situ measurements of the absorption cross‐sections of O2 in the Herzberg Continuum

The absorption cross‐sections of both O2, from 200 to 220 nm, and O3 from 220 to 235 nm have been deduced from measurements of the atmospheric absorption performed at night aboard a stratospheric balloon launched from Aire sur 1'Adour (France)The ozone column content was previously determined using the absorption measured from 278 to 289 nm, assuming that the ozone absorption cross sections are well known in this wavelength interval. The O2 and O3 cross sections have been found to meet most of the laboratory measurements. From 200 to 210 nm the results concerning the O2 cross‐sections do not confirm the low values measured recently in the stratosphere and in the laboratory.

Intercomparison of NIMBUS 7 solar backscattered ultraviolet ozone profiles with rocket, balloon, and Umkehr profiles

A global ozone data set covering the two years from November 1978 until October 1980, with an average of 1200 profiles per day, has been produced and archived from the solar backscattered ultraviolet (SBUV) instrument flown on the NIMBUS 7 spacecraft. The SBUV ozone profiles are compared with measurements from chemical and optical sensors launched on balloons and rockets and with ozone profiles obtained from the ground‐based Dobson spectrophotometers using the Umkehr method. The biases between the SBUV results and the balloon and Umkehr results are generally less than 10%. These biases are functions of layer height and latitude and are believed to be largely due to inconsistencies in the ozone absorption cross sections used for the various measurement systems. The precision of the SBUV measurements is found to be better than 8% for pressures between 1 and 64 mbar and better than 15% from 64 to 253 mbar.

Intercomparison of the NIMBUS 7 SBUV/TOMS total ozone data sets with Dobson and M83 results

Total ozone measurements made by the SBUV and TOMS instruments on the NIMBUS 7 spacecraft are compared with measurements from 62 Dobson and 18 M83 stations. On the average, TOMS ozone values are 6.6% smaller than Dobson and 9.1% smaller than M83; corresponding SBUV biases are 8.3% and 11.3%, respectively. Use of SBUV or TOMS as a transfer standard reveals an apparent bias between the Dobson and M83 networks of 3.0 or 2.5%respectively. Major portion of the bias between the space and ground measurements is attributed to uncertainties in the ozone absorption coefficients used in processing the measurements. Precision of total ozone retrieved from either the SBUV or the TOMS instrument is shown to be better than 2%, which is comparable to that of a well run Dobson station. Precision of a typical Dobson measurement is estimated at about 2% and that of an M83 measurement is estimated to be 4%. Apparent station to station biases of up to 11.2% for Dobson and 15.5% for M83 are shown. Daily and seasonal variations of ozone measured by the satellite over selected Dobson stations are found to be in excellent agreement with the ground observations. An instrumental drift is found in the SBUV/TOMS total ozone measurements that is essentially linear with time and has a rate of 0.5% per year for the first year of data. A better understanding of instrument changes is expected to help reduce any further drift.

New measurements of the absolute absorption cross-sections of ozone at 294 and 223 K in the 310-350 nm spectral range

Absolute measurements of the absorption cross-sections of ozone in the 310-350 nm range (Huggin's system) were carried out at 294 and 223 K with high resolution. Our results ― lower than all results known to date ― are, within a coefficient, in very close agreement with the (relative) values obtained recently by A.M. Bass and R.J. Paur with a resolution similar to that used in this work Determination de ces sections efficaces a haute resolution. Bon accord avec les donnees anterieures, en particulier avec les valeurs relatives de Bass et al.

Fast-response dual-beam UV-absorption ozone photometer suitable for use on stratospheric balloons

Various types of ozone detectors are currently in use, each with different advantages and compromises in response time, portability, sensitivity, accuracy, need for repeated calibration, and expense. We describe here a new dual-beam UV-absorption instrument for balloon-borne measurements of atmospheric ozone. It has two identical absorption chambers, each alternating between reference mode (ozone free) and sample mode by means of a four-port valve and ozone scrubber. The ratio of the absorption signals, along with the known lengths and ozone absorption cross section, yield the ozone concentration. The dual-beam feature cancels the effects of lamp intensity fluctuations, while the mode alternation compensates for mechanical changes and also provides continuous measurements. The absorption measurement requires no calibration and, hence, is independent of gas flow rate. The response time is 1 s and, for this measurement duration, the minimum ozone concentration detectable by this instrument (one standard deviation) is 1.5×1010 molecules/cm3 (0.6 ppbv at STP). The overall uncertainty of a 1-s measurement at the ozone maximum (22 km) is 3.6%, where 2% of this is the accuracy of the ozone cross section. The size and weight are suitable for launch by small balloons, but the cost of the instrument precludes its use as a disposable unit.