Alain Sarkissian

Intercomparison of UV/visible spectrometers for measurements of stratospheric NO2 for the Network for the Detection of Stratospheric Change

During the period May 12–23, 1992, seven groups from seven countries met in Lauder, New Zealand, to intercompare their remote sensing instruments for the measurement of atmospheric column NO2 from the surface. The purpose of the intercomparison was to determine the degree of intercomparability and to qualify instruments for use in the Network for the Detection of Stratospheric Change (NDSC). Three of the instruments which took part in the intercomparison are slated for deployment at primary NDSC sites. All instruments were successful in obtaining slant column NO2 amounts at sunrise and sunset on most of the 12 days of the intercomparison. The group as a whole was able to make measurements of the 90° solar zenith angle slant path NO2 column amount that agreed to about ±10% most of the time; however, the sensitivity of the individual measurements varied considerably. Part of the sensitivity problem for these measurements is the result of instrumentation, and part is related to the data analysis algorithms used. All groups learned a great deal from the intercomparison and improved their results considerably as a result of this exercise.

Depletion of Column Ozone in the Arctic During the Winters of 1993-94 and 1994-95

The total ozone reduction in the Arctic during the winters of 1993/94 and 1994/95 has been evaluated using the ground-based total ozone measurements of five SAOZ spectrometers distributed in the Arctic and from number density profiles of a balloon-borne version of the instrument. The ozone change resulting from transport has been removed using a 3D Chemistry Transport Model (CTM) run without chemistry. A cumulative total ozone depletion at the end of winter in March of 18% ± 4% in 1994 and of 32% ± 4% in 1995 was observed within the polar vortex, and of 15% ± 4% in both years outside the vortex. This evaluation is not sensitive to the vertical transport in the model. The periods, locations and altitudes at which ozone loss occurred were tightly connected to temperatures lower than NAT condensation temperature. The maximum loss was observed at 50 hPa in 1994 and lower, 60-80 hPa, in 1995. Half of the depletion in 1994 and three quarters in 1995 occurred during the early winter, showing that a late final warming is not a prerequisite for large ozone destruction in the northern hemisphere. The timing, the geographical location and the altitude of the ozone losses are well captured by the 3D CTM photochemical model using current chemistry, but its amplitude at low sun during the early winter, is underestimated. The model simulations also capture the early season reductions observed outside the vortex. This suggests that the losses occurred in situ in the early winter, when low temperatures are frequent, and not later in March, when ozone is most reduced inside the vortex, which would be the case if leakage from the vortex was the cause of the depletion.

Investigation of Pole-to-Pole Performances of Spaceborne Atmospheric Chemistry Sensors with the NDSC

Abstract Spaceborne atmospheric chemistry sensors provide unique access to the distribution and variation of the concentration of many trace species on the global scale. However, since the measurements and the retrieval algorithms are sensitive to a variety of instrumental as well as atmospheric sources of error, they need to be validated carefully by correlative measurements. The quality control and validation of satellite measurements on the global scale, as well as in the long term, is one of the goals of the Network for the Detection of Stratospheric Change (NDSC). Started in 1991, at the present time the NDSC includes five primary and two dozen complementary stations distributed from the Arctic to the Antarctic, comprising a variety of instruments such as UV–visible spectrometers, Fourier transform infrared spectrometers, lidars, and millimeter-wave radiometers. After an overview of the main sources of uncertainty which could perturb the measurements from space, and of the ground-based data provided ...

Slant column measurements of O3 and NO2 during the NDSC intercomparison of zenith-sky UV-visible spectrometers in June 1996

In June 1996, 16 UV-visible sensors from 11 institutes measured spectra of the zenith sky for more than 10 days. Spectra were analysed in real-time to determine slant column amounts of O3 and NO2. Spectra of Hg lamps and lasers were measured, and the amount of NO2 in a cell was determined by each spectrometer. Some spectra were re-analysed after obvious errors were found. Slant columns were compared in two ways: by examining regression analyses against comparison instruments over the whole range of solar zenith angles; and by taking fractional differences from a comparison instrument at solar zenith angles between 85° and 91°. Regression identified which pairs of instruments were most consistent, and so which could be used as universal comparison instruments. For O3, regression slopes for the whole campaign agreed within 5% for most instruments despite the use of different cross-sections and wavelength intervals, whereas similar agreement was only achieved for NO2 when the same cross-sections and wavelength intervals were used and only one half-days data was analysed. Mean fractional differences in NO2 from a comparison instrument fall within ±7% (1-sigma) for most instruments, with standard deviations of the mean differences averaging 4.5%. Mean differences in O3 fall within ±2.5% (1- sigma) for most instruments, with standard deviations of the mean differences averaging 2%. Measurements of NO2 in the cell had similar agreement to measurements of NO2 in the atmosphere, but for some instruments measurements with cell and atmosphere relative to a comparison instrument disagreed by more than the error bars.

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.

Identification of polar stratospheric clouds from the ground by visible spectrometry

When sighted from the ground in clear weather, stratospheric clouds make large changes in the sky color during twilight. Spectrometric measurements performed from the ground during CHEOPS show that the color changes can be either large reddenings or blueings. A radiative transfer model demonstrates that the first are caused by thin hazes above 22 km while the second are related to thick clouds below, and that the color change during twilight is little sensitive to tropospheric clouds. Statistics of presence of PSCs above Kiruna during CHEOPS which shows that reddenings are correlated with temperature below NAT condensation at high altitude (30 hPa, 21.5 km) and blueings at low altitude (50 hPa, 18.5km), support this interpretation, as also the consistency with PSC measurements with in-situ aerosol counters, balloon radiometer and from satellite observations. Spectrometric observations of the zenith sky at twilight is therefore thought to be a powerful method for identifying the presence and the altitude of PSCs above a station in all tropospheric weather conditions.

Combined characterisation of GOME and TOMS total ozone measurements from space using ground-based observations from the NDSC

Several years of total ozone measured from space by the ERS-2 GOME, the Earth Probe TOMS, and the ADEOS TOMS, are compared with high-quality ground-based observations associated with the Network for the Detection of Stratospheric Change (NDSC), over an extended latitude range and a variety of geophysical conditions. The comparisons with each spaceborne sensor are combined altogether for investigating their respective solar zenith angle (SZA) dependence, dispersion, and difference of sensitivity. The space- and ground-based data are found to agree within a few percent on average. However, the analysis highlights for both GOME and TOMS several sources of discrepancies: (i) a SZA dependence with TOMS beyond 80° SZA; (ii) a seasonal SZA dependence with GOME beyond 70° SZA; (iii) a difference of sensitivity with GOME at high latitudes; (iv) a difference of sensitivity to low ozone values between satellite and SAOZ sensors around the southern tropics; (v) a north/south difference of TOMS with the ground-based observations; and (vi) internal inconsistencies in GOME total ozone.

Total nitrogen dioxide at the Arctic Polar Circle since 1990

Daily ground-based NO2 column measurements have been conducted with UV-visible SAOZ diode-array spectrometers at Sodankyla in northern Finland since January 1990 and at 3 additional stations during EASOE. Inspection of the data indicates: a total NO2 reduction by 30% by the Pinatubo aerosol in spring and summer 1992 compared to the previous years; a poor correlation in winter with potential vorticity indicative of motion of the vortex and of advection of air from mid-latitude, frequent during EASOE; an absence of significant global denitrification by sedimentation of nitric acid on PSC particles inside the vortex; and a high correlation between total NO2 and 30–50 hPa temperature. The latter is unexpected in winter 1991–92, since most of NOx (NO + NO2 + ClONO2 + 2N2O5) at the altitude of the aerosol layer should have been converted into liquid nitric acid. This is expected to revert only slowly to NOx at high latitude in winter. The correlation suggests a temperature dependent saturation of the aerosol water / sulfuric acid droplets and / or a temperature dependent mechanism of restitution of NOx to the gas phase.

Intercomparison of the influence of tropospheric clouds on UV‐visible absorptions Detected during the NDSC Intercomparison Campaign at OHP in June 1996

The influence of tropospheric clouds on zenith sky light (or brief ZSL-DOAS) measurements of stratospheric gases is investigated. From a large set of intercomparison studies including six simultaneously operated UV/visible spectrometers, the zenith sky absorptions of O3, O4, NO2, and H2O are found to increase considerably under the investigated Cumulonimbus (Cb) cloud. The accuracy of the inferred visible O3 absorptions, however, are affected by interfering H2O absorptions. The increased cloudy sky absorptions are attributed to increased pathlengths due to multiple Mie scattering and hence interstitial gaseous absorptions inside the cloud. The absorptions detected for chemically inert gases like O4 (and H2O) are found to be inconsistent with those detected for NO2 and O3. This finding indicates that O3 and NO2 are modified by cloud related transport or chemical processes.

PSC and volcanic aerosol observations during EASOE by UV-visible ground-based spectrometry

Twilight sky colour measurements were made using ground-based UV-visible SAOZ spectrometers at four stations along the Arctic circle during EASOE. The results show that volcanic aerosol from the Pinatubo eruption of June 1991 first appeared above the Arctic in September 1991. The aerosol layer thickened progressively during the autumn and had spread to all four stations, outside in the very low stratosphere, as well as inside the polar vortex, by mid-January. For the rest of the campaign, to mid-March, little further change was seen. The aerosol would have masked any PSCs that may have occurred at altitudes less than 22 km. However, PSCs at higher altitudes should have been detected, had their optical thickness exceeded 0.01. Only one was recorded during EASOE: on 28–29 December 1991, in contrast to their frequent occurrence in January and early February 1990.