Ozone Intercomparison Campaign Research Articles

Lidar measurements of stratospheric ozone during the STOIC campaign

The NASA Goddard Space Flight Center's Stratospheric Ozone Lidar Trailer Experiment participated in the Stratospheric Ozone Intercomparison Campaign (STOIC) at Table Mountain, California, from July 20 to August 2, 1989. From 20 to 39 km the average Goddard Space Flight Center (GSFC) profile agreed with the STOIC reference profile to better than 5%. The STOIC reference profile was generated by averaging all profiles from the eight participating instruments. In this same altitude region the GSFC and the Jet Propulsion Laboratory lidar average profiles also agreed to better than 5%. Similar results were found for an intercomparison of the lidar data with ozone data from ECC sondes launched from Table Mountain and from San Nicholas Island. The results of the intercomparison have led to a number of modifications to the GSFC lidar which have greatly improved the reliability of the ozone data, particularly at altitudes below 20 km and above 40 km.

Error analysis for the ground‐based microwave ozone measurements during STOIC

We present a formal error analysis and characterization of the microwave measurements made during the Stratospheric Ozone Intercomparison Campaign (STOIC). The most important error sources are found to be determination of the tropospheric opacity, the pressure‐broadening coefficient of the observed line, and systematic variations in instrument response as a function of frequency (“baseline”). Net precision is 4–6% between 55 and 0.2 mbar, while accuracy is 6–10%. Resolution is 8–10 km below 3 mbar and increases to 17 km at 0.2 mbar. We show the “blind” microwave measurements from STOIC and make limited comparisons to other measurements. We use the averaging kernels of the microwave measurement to eliminate resolution and a priori effects from a comparison to SAGE II. The STOIC results and comparisons are broadly consistent with the formal analysis.

Surface ozone levels at Table Mountain during STOIC 1989

As a part of the routine operations of the Jet Propulsion Laboratory atmospheric measurements program at the Table Mountain Facility, the surface ozone concentration is continuously monitored using a Dasibi photometer. The influence of the Los Angeles basin to the southwest of the facility and the height of the inversion layer cause large fluctuations in the ozone concentration. Peaks as high as 200 parts per billion by volume (ppbv) were observed during the Stratospheric Ozone Intercomparison Campaign (STOIC) compared to a normal background level near 50 ppbv. These measurements, made during STOIC, were important in assessing the impact of the surface ozone concentration on the various instruments participating in the campaign.

Comparison of STOIC 1989 ground‐based lidar, microwave spectrometer, and Dobson spectrophotometer Umkehr ozone profiles with ozone profiles from balloon‐borne electrochemical concentration cell ozonesondes

Ground‐based measurements of stratospheric ozone using a Jet Propulsion Laboratory (JPL) lidar, a NASA Goddard Space Flight Center (GSFC) lidar, a Millitech Corporation/NASA Langley Research Center (Millitech/LaRC) microwave spectrometer, and a NOAA Dobson ozone spectrophotometer were compared with in situ measurements made quasi‐simultaneously with balloon‐borne electrochemical concentration cell (ECC) ozonesondes during 10 days of the Stratospheric Ozone Intercomparison Campaign (STOIC). The campaign was conducted at Table Mountain Observatory, California, during the summer of 1989. ECC ozonesondes were flown by NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) personnel as well as by personnel from the NASA Wallops Island Flight Facility (WFF). Within the altitude range of 20–32 km, ozone measurement precisions were estimated to be ±0.6 to ±1.2% for the JPL lidar, ±0.7% for the GSFC lidar, ±4% for the microwave spectrometer, and ±3% for the NOAA ECC ozonesonde instruments. These precisions decreased in the 32 to 38.6‐km altitude range to ±1.3, ±1.5, and ±3% to ±10% for the JPL lidar, GSFC lidar, and the ECC sondes, respectively, but remained at ±4% for the microwave instrument. Ozone measurement accuracies in the 20 to 32 km altitude range were estimated to be ±1.2 to ±2.4% for the JPL lidar, ±1.4% for the GSFC lidar, ±6% for the microwave radiometer, and ±5% for the ECC ozonesondes. The accuracies decreased in the 32 to 38.6‐km altitude range to ±2.6, ±3.0, ±7, and 1 ± 4% to −4 ± 10% for the JPL lidar, the GSFC lidar, the microwave spectrometer, and the ECC ozonesondes, respectively. While accuracy estimates for the ECC sondes were obtained by combining random and estimated bias errors, the accuracies for the lidar instruments were obtained by doubling the measurement precision figures, with the assumption that such doubling accounts for systematic errors. Within the altitude range of 20–36 km the mean ozone profiles produced by the JPL, GSFC, and the Millitech/LaRC groups did not differ from the mean ECC sonde ozone profile by more than about 2, 4, and 5%, respectively. Six morning Dobson instrument Umkehr observations yielded mean ozone amounts in layers 3 and 5–7 that agreed with comparison ECC ozonesonde data to within ±4%. In layer 4 the difference was 7.8%. (Less favorable comparison data were obtained for six afternoon Umkehr observations made in highly polluted near‐surface air.) This good agreement in overall results obtained lends credence to the reliability of the ozone measurements made at Table Mountain Observatory during STOIC 1989.

Electrochemical concentration cell ozonesonde performance evaluation during STOIC 1989

Electrochemical concentration cell (ECC) ozonesondes flown by NOAA and NASA Wallops Flight Facility (WFF) personnel during the Stratospheric Ozone Intercomparison Campaign (STOIC) conducted at the Jet Propulsion Laboratory's Table Mountain Facility, Wrightwood, California, July 21 to August 1, 1989, exhibited highly similar ozone measurement precisions and accuracies even though considerably different methods were used by the two research groups in preparing the instruments for use and in calibrating the instruments. The Table Mountain data as well as data obtained in the past showed the precisions to range from about ±3 to ±12% in the troposphere, remain relatively constant at ±3% in the stratosphere to 10 mbar, then decrease to about ±10% at 4‐mbar pressure altitude. Corresponding ozone measurement accuracies for individual ozonesonde soundings were estimated to be about ±6% near the ground, decrease to −7 to 17% in the high troposphere where ozone concentrations are low, increase to about ±5% in the low stratosphere and remain so to an altitude of about 10 mbar (∼32 km), then decrease to −14 to 6% at 4 mbar (∼38 km) where ozone concentrations are again low. Stratospheric ozone measurements were also made during STOIC with ground‐based lidars and a microwave radiometer that will be used for ozone measurements in the future at sites of the Network for the Detection of Stratospheric Change (NDSC). The ECC ozonesonde observations provided useful comparison data for evaluating the performance of the lidar and microwave instruments.

Results from the Jet Propulsion Laboratory stratospheric ozone lidar during STOIC 1989

Stratospheric ozone concentration profiles measured by the Jet Propulsion Laboratory differential absorption lidar system during the Stratospheric Ozone Intercomparison Campaign in July/August 1989 are presented. These profiles are compared with the mean profiles based on all of the measurements made by the different participating instruments. The results from the blind intercomparison showed that the lidar results agreed with the overall Stratospheric Ozone Intercomparison Campaign average profile to better than 5% between 21 and 45 km altitude. At 20 km the difference was ∼10%, as it was also in the region from 47 to 50 km altitude. Some systematic features were observed in the comparison of the blind results and these were subsequently investigated. The results of this investigation allowed the analysis algorithm to be refined and improved. The changes made are discussed and the comparison of the refined results showed agreement with the STOIC average to better than 4% from 18 to 48 km altitude. For both cases the results above 45 km altitude are subject to the greatest uncertainty and error and are of questionable value even though they agree within 10% with the STOIC average. Examples of comparisons of individual lidar profiles with each of the other instruments are also presented.

Total ozone measurements made with the Brewer ozone spectrophotometer during STOIC 1989

The Canadian Atmospheric Environment Service participated in the Stratospheric Ozone Intercomparison Campaign (STOIC) by operating a Brewer ozone spectrophotometer at the Table Mountain Observatory from July 19 to August 2, 1989. Frequent direct Sun measurements of total column ozone were made throughout most days during the 2 weeks of the intercomparison. Results of the Brewer instrument total ozone measurements are given. They show that there is a diurnal variation of total ozone with values increasing on average by 6.6±2.4 Dobson units between early morning and late afternoon. Comparing the total ozone measurements with ozone concentration measurements made at ground level suggests that the daytime increase is a result of a buildup of surface ozone during the day. The total ozone measurements made by the Brewer instrument during STOIC are compared with total ozone measurements made by the National Oceanic and Atmospheric Administration (NOAA) Dobson instrument, the NASA/Wallops electrochemical concentration cell ozonesondes and the total ozone mapping spectrometer satellite instrument.

Electrochemical concentration cell ozone soundings at two sites during the Stratospheric Ozone Intercomparison Campaign

The Stratospheric Ozone Intercomparison Campaign (STOIC) was designed to compare proposed instruments for the Network for the Detection of Stratospheric Change (NDSC) with established measurement techniques. Ground‐based measurements were conducted at Table Mountain, California (34.4°N, 117.7°W), and rocket profiles were made at San Nicolas Island, California (33.3°N, 119.5°W). In an effort to estimate site‐to‐site differences during the intercomparison, daily soundings were made with balloon‐borne electrochemical concentration cell (ECC) ozonesondes at the two sites. Comparisons have been made of the mean values for the soundings at each site. The soundings showed small temperature differences between the two sites with the nighttime Table Mountain measurements up to 2°K cooler than the daytime San Nicolas Island temperatures. Day/night temperature corrections based on the emissivity and absorptivity of the thermistors in the balloon instruments can account for these apparent temperature differences. Ozone partial pressure profiles from the ECCs have been used to estimate atmospheric ozone variability during STOIC. Above 20 km, the lowest altitude for the STOIC comparisons, the average data sets from both sites showed standard deviations that were at or below the estimated 5–6% profile‐to‐profile repeatability for the ozonesondes. This indicates that atmospheric ozone variability was only of the order of a few percent above 20 km during the 2 weeks of the STOIC campaign. Above 20 km the ozone partial pressures over the Table Mountain site averaged 2.0% lower than over San Nicolas Island.

Stratospheric Ozone Intercomparison Campaign (STOIC) 1989: Overview

The NASA Upper Atmosphere Research Program organized a Stratospheric Ozone Intercomparison Campaign (STOIC) held in July–August 1989 at the Table Mountain Facility (TMF) of the Jet Propulsion Laboratory (JPL). The primary instruments participating in this campaign were several that had been developed by NASA for the Network for the Detection of Stratospheric Change: the JPL ozone lidar at TMF, the Goddard Space Flight Center trailer‐mounted ozone lidar which was moved to TMF for this comparison, and the Millitech/LaRC microwave radiometer. To assess the performance of these new instruments, a validation/intercomparison campaign was undertaken using established techniques: balloon ozonesondes launched by personnel from the Wallops Flight Facility and from NOAA Geophysical Monitoring for Climate Change (GMCC) (now Climate Monitoring and Diagnostics Laboratory), a NOAA GMCC Dobson spectrophotometer, and a Brewer spectrometer from the Atmospheric Environment Service of Canada, both being used for column as well as Umkehr profile retrievals. All of these instruments were located at TMF and measurements were made as close together in time as possible to minimize atmospheric variability as a factor in the comparisons. Daytime rocket measurements of ozone were made by Wallops Flight Facility personnel using ROCOZ‐A instruments launched from San Nicholas Island. The entire campaign was conducted as a blind intercomparison, with the investigators not seeing each others data until all data had been submitted to a referee and archived at the end of the 2‐week period (July 20 to August 2, 1989). Satellite data were also obtained from the Stratospheric Aerosol and Gas Experiment (SAGE II) aboard the Earth Radiation Budget Satellite and the total ozone mapping spectrometer (TOMS) aboard Nimbus 7. An examination of the data has found excellent agreement among the techniques, especially in the 20‐ to 40‐km range. As expected, there was little atmospheric variability during the intercomparison, allowing for detailed statistical comparisons at a high level of precision. This overview paper will summarize the campaign and provide a “road map” to subsequent papers in this issue by the individual instrument teams which will present more detailed analysis of the data and conclusions.

ROCOZ‐A ozone measurements during the Stratospheric Ozone Intercomparison Campaign (STOIC)

We present a set of ROCOZ‐A (rocket ozonesonde) ozone measurements during the October/November 1988 (pre‐STOIC) and the July/August 1989 Stratospheric Ozone Intercomparison Campaign (STOIC) in southern California. ROCOZ‐A and its associated electrochemical concentration cell (ECC) ozonesondes participated in the comparisons as established techniques for the validation of lidar and microwave instruments that have been proposed for the Network for the Detection of Stratospheric Change (NDSC). For the proposed network instruments, STOIC has provided a picture of their performance characteristics in 1989 and has given an estimate of their future performance in the NDSC. For ROCOZ‐A, STOIC has added new information on its accuracy and precision. It is this continuing characterization that gives ROCOZ‐A its value in comparisons. The STOIC comparisons show a shift of 5–6% in ROCOZ‐A ozone densities (ROCOZ‐A higher) from October/November 1988 to July/August 1989. This shift has been seen in comparisons with the Stratospheric Aerosol and Gas Experiment II (SAGE II), ECC ozonesondes, and the Jet Propulsion Laboratory (JPL) lidar. The source of this shift has not been determined. Until this new error source is resolved, we recommend that the previously quoted accuracy estimate for ROCOZ‐A ozone measurements be increased from 5–7% to 8–10%. About 2% of the difference between ROCOZ‐A ozone measurements and those from the proposed network instruments in 1989 appears to be due to differences in atmospheric ozone between the two STOIC sites. A correction for these site‐to‐site differences brings the ROCOZ‐A ozone measurements within 10% of all of the other STOIC instruments, and the average agreement (ROCOZ‐A 6% higher) becomes consistent with the historical set of ROCOZ‐A comparisons. The STOIC comparisons have shown structures in stratospheric ozone that cannot be resolved by ROCOZ‐A with its 4‐km vertical resolution. In addition, comparisons with nighttime measurements from the Millitech microwave above 45 km show a divergence from the daytime ROCOZ‐A values that agrees with the general characteristics of the diurnal cycle in upper stratospheric ozone that is predicted by photochemical models. Evidence of this ozone cycle is also seen in ROCOZ‐A comparisons with the SAGE II zonal mean above 45 km but not in comparisons with the nighttime measurements from the JPL lidar. The effects of diurnal ozone change in the upper stratosphere and mesosphere will be an important consideration in future comparisons of NDSC instruments.

Lidar measurements of stratospheric temperature during STOIC

Measurements of stratospheric temperature and density were acquired by the NASA/GSFC lidar during the Stratospheric Ozone Intercomparison Campaign (STOIC) experiment at the Jet Propulsion Laboratory Table Mountain Facility (TMF) (34.4°N, 117.7°W) in July and August 1989. Lidar temperatures, obtained on 21 nights preceding and during this experiment, are compared with temperatures derived by radiosondes, datasondes, Stratospheric Aerosol and Gas Experiment (SAGE II) satellite experiment, and National Meteorological Center (NMC) analyses. Radiosondes were flown from the TMF site as well as from San Nicholas Island (33.2°N, 119.5°W) located about 225 km southwest of TMF. Datasondes were deployed from Super‐Loki rockets also launched at San Nicholas Island. SAGE II satellite temperature measurements were made within 1000 km of the Table Mountain site. NMC temperature analyses derived from the NOAA satellite measurements were interpolated to coincide in space and time with the lidar measurements. The lidar temperatures, which were derived for altitudes between 30 and 65 km, were within 2–3 K of the temperatures measured by the other sensors in the altitude range 30–45 km. Between 30 and 35 km, lidar temperatures were about 2 K cooler than those obtained from the datasondes and the NMC analyses but were about 1–2 K warmer than those obtained from the radiosonde. These differences may be due to the time difference between the measurements as well as possible nonnegligible aerosol scattering near 30 km. Near and above the stratopause the temperature differences increased to 3–8 K. Lidar temperature profiles also show small‐scale variations possibly caused by wave activity.

Improved stratospheric ozone lidar

The NASA Goddard Space Flight Center's stratospheric ozone lidar has undergone several modifications and improvements since it participated in the Stratospheric Ozone Intercomparison (STOIC) campaign at Table Mountain, CA, in July 1989. Changes have been made in both the transmitter and receiver. The transmitter has been changed to include a XeF laser to generate the off-line wavelength; the detector has been changed to include mechanical choppers for each of the high-sensitivity channels, and the number of channels has been increased to six. In addition to the four elastic-scattering channels previously described, detectors at the N2 Raman-scattered wavelengths for each of the transmitted wavelengths have also been added. The vertical resolution of the acquisition has been improved from 300 to 150 m, with the capability to record data with 75-m resolution. The Raman channels permit the measurement of ozone in air parcels with heavy loadings of aerosols, and give additional information about the aerosols themselves. Data from recent campaigns are presented to illustrate these points.

Comparisons of satellite ozone data in the lower stratosphere for 1978/1979

Lower stratospheric zonal mean ozone distributions are compared in both hemispheres from the Nimbus 7 Solar Backscattered Ultraviolet (SBUV) (version 5) and Limb Infrared Monitor of the Stratosphere (LIMS) data sets. Generally, there is very good agreement with regard to individual profile shape and magnitude and with their longitudinal variability about the zonal mean. When the data are considered in terms of Umkehr layer amounts, there are subtle but significant differences in the respective meridional ozone gradients and their variations with time. These differences do not seem to be related to SBUV diffuser plate drift effects because differences are not apparent in the upper stratosphere and because they are pronounced in late spring to summer of both hemispheres of the lower stratosphere. It appears more likely that the differences are related to a changing ozone optical depth (LIMS) and constraints to an assumed profile climatology in the retrieval (SBUV). Stratospheric Aerosol and Gas Experiment (SAGE) satellite ozone results are also presented for April 1979 at 10, 16, and 30 mbar, and they tend to split the differences between SBUV and LIMS. Additional comparisons with electrochemical concentration cell (ECC) sonde and Mastsonde ozone profiles are presented in an attempt to sort out the more noticeable differences. At tropical latitudes, LIMS ozone is too large in layer 4; no measurements were available for layer 3. SBUV is too high in layer 3. Layer 5 results are not conclusive, but LIMS appears high by about 10% while SBUV appears low by a similar percentage. At higher latitudes (38°–55°N) of the northern hemisphere, ECC and Mastsonde comparison results are not accurate enough to differentiate between SBUV and LIMS in layer 5, even though there are persistent differences in the satellite data of the order of 10–15%. LIMS ozone is about 10% larger than balloon values in layer 4 during northern hemisphere spring. Both SBUV and LIMS are too high in layer 3. The sign of the SBUV/LIMS satellite differences changes with latitude in layers 4 and 5, and the differences in layer 4 approach 20% at mid‐latitudes during late spring and summer. The May SBUV/LIMS differences at 30°N are similar to the SBUV/ECC differences found at Palestine, Texas (32°N) in the June 1983 Balloon Ozone Intercomparison Campaign (BOIC) I ozone intercomparison. All of this means that annual variations in the SBUV and LIMS satellite ozone data and their meridional gradients in the middle to lower stratosphere must be judged as only qualitative at this time.

Remote and ground-based measurements of ozone profiles during the MAP/GLOBUS 1983 campaign

Ozone observations were carried out during the MAP/GLOBUS campaign in September 1983 by five ground-based instruments located either at the Observatoire de Haute-Provence: the Dobson spectrophotometer, the i.r. SISAM spectrometer, the u.v. Lidar, or at the Bordeaux Observatory: a microwave spectrometer, and at the Biscarosse station there was another Dobson spectrophotometer. A balloon-borne microwave spectrometer was flown from Aire-sur-l'Adour on 28 September. A brief description of the instruments is given, which emphasizes the progress made since the preceding ozone intercomparison campaign in 1981. Results obtained by the different instruments are also given. Their comparison is done first with respect to the ozone content over the whole month of September, then within layers 4, 5 and part of 6 for which Brewer-Mast sonde measurements were included, and finally for vertical profiles at altitudes higher than 30 km. Most comparisons show a good agreement, in general within 5% on the average.

Results from the balloon ozone intercomparison campaign (BOIC)

The purpose of the Balloon Ozone Intercomparison Campaign (BOIC) was to assess the accuracy and precision of various ozone measurement systems under development and those flown operationally. Ozone observations made by in situ UV absorption photometers from four groups, two solar UV absorption photometers, three varieties of electrochemical sondes, and a mass spectrometer were intercompared in three flight missions, each involving several balloon flights. Concurrent Umkehr and satellite observations were also intercompared. The National Bureau of Standards (NBS)provided a reference ozone source for intercomparing several of the in situ instruments at ground pressure. Harvard University and the University of Minnesota each developed a calibration facility after the BOIC flights which enabled a comparison of the flight in situ photometers under simulated stratospheric pressure and ozone concentrations. The following is a summary of results. The standard deviation of the sensitivities among 18 instruments tested about the NBS reference was 11%. These differences appear in flight at the lower altitudes, but they change at higher altitudes, indicating height dependent errors, particularly for the electrochemical sondes. Comparisons at the two laboratory facilities simulating stratospheric conditions indicated differences of about 2% at 3 mbar for two of the flight in situ photometers. Ozone values among four in situ UV photometers flown together had a standard deviation about the mean value of about ±3% from the tropopause to about 41 km. This is very close to the expected accuracy of these measurements and therefore is an excellent result. However, during float at 42 km the difference nearly doubled. Experiments performed during flight demonstrated that even near 40 km, wall losses in three UV photometers were less than 5%, which is consistent with the laboratory simulations. A comparison of a complement of electrochemical sondes yielded results that agreed with the mean of the in situ UV photometers to within 0–20%, depending on the sonde type and altitude. The electrochemical sondes gave systematically lower ozone values at pressures lower than 10 mbar (above 31 km). A comparison between an in situ UV and a solar absorption photometer indicated a 10% difference in the stratosphere, where the in situ measurement was lower. Comparisons of electrochemical sondes and Umkehr and Solar backscattered ultraviolet (SBUV) satellite observations showed agreement to within their error bars (stated to be about 5–10%). In two comparisons between in situ UV photometers and satellite measurements, a consistent difference occurred between 10 and 3 mbar (28–40 km), implying a possible bias between the measurement types. Intercomparisons among all the instruments in the troposphere showed 20–30% differences from the mean. A comparison of pressure measurements performed by several experimenters resulted in differences as high as ±15% from the average measurement.

NASA-JSC measurements during “la campagne d'intercomparaison d'ozonometres”, Gap, France, June 1981

Measurements made by the NASA-JSC ozone instrument during the ozone intercomparison campaign from Gap, France during June 1981 are reported. Two flights were made on board the large balloon platform with other instruments using different techniques. The NASA-JSC instrument employs u.v. absorption photometry to obtain in situ results. Concentration (molecules cm −3) and mixing ratio (pp mV) profiles are given for altitudes from 16 km to float altitudes of 32 and 39 km, respectively for the two flights. A measure of the total column content of ozone was obtained by integrating the NASA-JSC results from 16 km to float altitude and combining them with results from other techniques below 16 km and above float altitudes. Comparisons with results from other instruments are reported elsewhere in this publication.

The ozone intercomparison campaign, 1981: Concluding remarks

Some of the more significant results of the international ozone comparison campaign held in France in June 1981 are elucidated and compared with results of preceeding intercomparisons.