Zenith-sky Observations Research Articles

A more representative “best representative value” for daily total column ozone reporting

Abstract. Long-term trends of total column ozone (TCO), assessments of stratospheric ozone recovery, and satellite validation are underpinned by a reliance on daily best representative values from Brewer spectrophotometers and other ground-based ozone instruments. In turn reporting of these daily total column ozone values to the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) has traditionally been predicated upon a simple choice between direct sun (DS) and zenith sky (ZS) observations. For mid- and high-latitude monitoring sites impacted by cloud cover we discuss the potential deficiencies of this approach in terms of its rejection of otherwise valid observations and capability to evenly sample throughout the day. A new methodology is proposed that makes full use of all valid direct sun and zenith sky observations, accounting for unevenly spaced observations and their relative uncertainty, to calculate an improved estimate of the daily mean total column ozone. It is demonstrated that this method can increase the number of contributing observations by a factor of 2.5, increases the sampled time span, and reduces the spread of the representative time by half. The largest improvements in the daily mean estimate are seen on days with the smallest number of contributing direct sun observations. No effect on longer-term trends is detected, though for the sample data analysed we observe a mean increase of 2.8 DU (0.82 %) with respect to the traditional direct sun vs. zenith sky average choice. To complement the new calculation of a best representative value of total column ozone and separate its uncertainty from the spread of observations, we also propose reporting its standard error rather than the standard deviation, together with measures of the full range of values observed.

Technical note: The US Dobson station network data record prior to 2015, re-evaluation of NDACC and WOUDC archived records with WinDobson processing software

Abstract. The United States government has operated Dobson ozone spectrophotometers at various sites, starting during the International Geophysical Year (1 July 1957 to 31 December 1958). A network of stations for long-term monitoring of the total column content (thickness of the ozone layer) of the atmosphere was established in the early 1960s and eventually grew to 16 stations, 14 of which are still operational and submit data to the United States of America's National Oceanic and Atmospheric Administration (NOAA). Seven of these sites are also part of the Network for the Detection of Atmospheric Composition Change (NDACC), an organization that maintains its own data archive. Due to recent changes in data processing software the entire dataset was re-evaluated for possible changes. To evaluate and minimize potential changes caused by the new processing software, the reprocessed data record was compared to the original data record archived in the World Ozone and UV Data Center (WOUDC) in Toronto, Canada. The history of the observations at the individual stations, the instruments used for the NOAA network monitoring at the station, the method for reducing zenith-sky observations to total ozone, and calibration procedures were re-evaluated using data quality control tools built into the new software. At the completion of the evaluation, the new datasets are to be published as an update to the WOUDC and NDACC archives, and the entire dataset is to be made available to the scientific community. The procedure for reprocessing Dobson data and the results of the reanalysis on the archived record are presented in this paper. A summary of historical changes to 14 station records is also provided.

Tropospheric nitrogen dioxide column retrieval from ground-based zenith–sky DOAS observations

Abstract. We present an algorithm for retrieving tropospheric nitrogen dioxide (NO2) vertical column densities (VCDs) from ground-based zenith–sky (ZS) measurements of scattered sunlight. The method is based on a four-step approach consisting of (1) the differential optical absorption spectroscopy (DOAS) analysis of ZS radiance spectra using a fixed reference spectrum corresponding to low NO2 absorption, (2) the determination of the residual amount in the reference spectrum using a Langley-plot-type method, (3) the removal of the stratospheric content from the daytime total measured slant column based on stratospheric VCDs measured at sunrise and sunset, and simulation of the rapid NO2 diurnal variation, (4) the retrieval of tropospheric VCDs by dividing the resulting tropospheric slant columns by appropriate air mass factors (AMFs). These steps are fully characterized and recommendations are given for each of them. The retrieval algorithm is applied on a ZS data set acquired with a multi-axis (MAX-) DOAS instrument during the Cabauw (51.97° N, 4.93° E, sea level) Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI) held from 10 June to 21 July 2009 in the Netherlands. A median value of 7.9 × 1015 molec cm−2 is found for the retrieved tropospheric NO2 VCDs, with maxima up to 6.0 × 1016 molec cm−2. The error budget assessment indicates that the overall error σTVCD on the column values is less than 28%. In the case of low tropospheric contribution, σTVCD is estimated to be around 39% and is dominated by uncertainties in the determination of the residual amount in the reference spectrum. For strong tropospheric pollution events, σTVCD drops to approximately 22% with the largest uncertainties on the determination of the stratospheric NO2 abundance and tropospheric AMFs. The tropospheric VCD amounts derived from ZS observations are compared to VCDs retrieved from off-axis and direct-sun measurements of the same MAX-DOAS instrument as well as to data from a co-located Système d'Analyse par Observations Zénithales (SAOZ) spectrometer. The retrieved tropospheric VCDs are in good agreement with the different data sets with correlation coefficients and slopes close to or larger than 0.9. The potential of the presented ZS retrieval algorithm is further demonstrated by its successful application on a 2-year data set, acquired at the NDACC (Network for the Detection of Atmospheric Composition Change) station Observatoire de Haute Provence (OHP; Southern France).

Quantification of NOx emission from St Petersburg (Russia) using mobile DOAS measurements around the entire city

We present mobile differential optical absorption spectroscopy (DOAS) zenith-sky observations of tropospheric NO2 from field experiments carried out at St Petersburg (Russia) on a number of days during May–October of 2009–2012. We conducted a detailed analysis of our measurements on a closed route around the entire city that took place on 14 August 2012. We used the data from these circular observations to derive our top–down estimate of total NOx emission from St Petersburg on that day, i.e. (12.6 ± 2.4) ×1024 molecules s−1. This value, if assumed to be constant throughout the year, would correspond to a total emission rate of (31 ± 6) kt/year (calculated on condition that all NOx was NO2). Our estimate is half of that reported in a published official inventory of St Petersburg (63 kt/year). Possible reasons for that discrepancy and an outlook for further improvements are considered and discussed further.

New method for deriving total ozone from Brewer zenith sky observations

[1] The Brewer spectrophotometer derives total column ozone from UV measurements using two operational modes: observing either direct sunlight (DS) or zenith sky (ZS) scattered sunlight. While DS measurements are more accurate, ZS measurements are necessary for generating long-term ozone time series unbiased by meteorological conditions and for the validation of satellite algorithms for cloudy scenes. In the ZS mode, column ozone (X) is obtained using the slant path (μ) and a weighted sum of the logarithms of the ZS intensities (F). Prior to this, however, a set of nine empirical coefficients, specific to each instrument, must be derived that relate μ and X to F. This requires a large set of near-simultaneous ZS and DS measurements that span the operational range of X and μ, which is difficult to obtain. In this work, a modified algorithm is presented in which radiative transfer model simulations of ZS observations are used to derive generic coefficients that describe the sky response to different observing conditions. Instrument-specific coefficients are obtained through a simple scaling and offset of these generic values, derived through ZS-DS comparisons. These two parameters may be accurately determined in a relatively short period of time and using a much more limited data set, such as during regular instrument calibrations. Since the new algorithm provides nine coefficients, no changes to the existing Brewer software are required. It is demonstrated that the new algorithm is as useful as the standard Brewer ZS algorithm with nine empirically estimated parameters derived using a vastly more extensive data set.

Car MAX-DOAS measurements around entire cities: quantification of NO<sub>x</sub> emissions from the cities of Mannheim and Ludwigshafen (Germany)

Abstract. We present car Multi-Axis (MAX-) DOAS observations of tropospheric NO2 carried out on circles around the cities of Mannheim and Ludwigshafen (Germany) on 24 August 2006. Together with information on wind speed and direction, the total emissions of the encircled source(s) are quantified from these measurements. In contrast to recent similar studies based on zenith scattered sun light (elevation angle of 90°), we use a MAX-DOAS instrument mounted on a car, which observes scattered sun light under different elevation angles (here 45°, and 90°). Compared to simple zenith sky observations, MAX-DOAS observations have higher sensitivity and reduced uncertainty, and avoid systematic offsets in the determination of the vertically integrated trace gas concentration. The determination of the absolute value of the integrated tropospheric trace gas concentrations is especially important for the calculation of absolute trace gas fluxes through arbitrary transects. However, even if emission sources are completely surrounded, systematic offsets in the measured vertically integrated trace gas concentration can lead to errors in the determined total emissions, especially for observations around extended areas. In this study we discuss and quantify different error sources. In most cases, the largest error source is the variability and imperfect knowledge of the wind field. In addition – depending on the trace species observed - also chemical transformations between the emission sources and the measurement location have to be considered. For that purpose we use local observations within the encircled area to quantify and/or correct these errors. From our observations we derive a total NOx emission from the Mannheim/Ludwigshafen area of (7.4±1.8)×1024 molec/sec, which if assumed to be constant throughout the year would correspond to a total emission of 17 830±4340 t/yr (calculated with the mass of NO2 t/yr, consistent with existing emission estimates. From our observations it is also possible to separately determine the average influx into the Mannheim/Ludwigshafen area (5.4±0.9×1024 molec/sec or 13 010±2170 t/yr) and the average outflux (12.8±1.8×1024 molec/sec or 13 010±4340 t/yr).

Retrieval of stratospheric and tropospheric BrO profiles and columns using ground-based zenith-sky DOAS observations at Harestua, 60° N

Abstract. A profiling algorithm based on the optimal estimation method is applied to ground-based zenith-sky UV-visible measurements from Harestua, Southern Norway (60° N, 11° E) in order to retrieve BrO vertical profiles. The sensitivity of the zenith-sky observations to the tropospheric BrO detection is increased by using for the spectral analysis a fixed reference spectrum corresponding to clear-sky noon summer conditions. The information content and retrieval errors are characterized and it is shown that the retrieved stratospheric profiles and total columns are consistent with correlative balloon and satellite observations, respectively. Tropospheric BrO columns are derived from profiles retrieved at 80° solar zenith angle during sunrise and sunset for the 2000–2006 period. They show a marked seasonality with mean column value ranging from 1.52±0.62×1013 molec/cm² in late winter/early spring to 0.92±0.38×1013 molec/cm² in summer, which corresponds to 1.0±0.4 and 0.6±0.2 pptv, respectively, if we assume that BrO is uniformly mixed in the troposphere. These column values are also consistent with previous estimates made from balloon, satellite, and other ground-based observations. Daytime (10:30 LT) tropospheric BrO columns are compared to the p-TOMCAT 3-D tropospheric chemical transport model (CTM) for the 2002–2003 period. p-TOMCAT shows a good agreement with the retrieved columns except in late winter/early spring where an underestimation by the model is obtained. This finding could be explained by the non-inclusion of sea-ice bromine sources in the current version of p-TOMCAT. Therefore the model cannot reproduce the possible transport of air-masses with enhanced BrO concentration due to bromine explosion events from the polar region to Harestua. The daytime stratospheric BrO columns are compared to the SLIMCAT stratospheric 3-D-CTM. The model run used in this study, which assumes 21.2 pptv for the Bry loading (15 pptv for long-lived bromine species and additional 6 pptv for very short-lived species (VSLS) added by a scaling of CH3Br), significantly underestimates the retrieved BrO columns. A sensitivity study shows that a good agreement can only be obtained if 6 to 8 pptv accounting for VSLS are added directly (and not by a scaling of CH3Br) to the SLIMCAT long-lived bromine species profile. This contribution of the VSLS to the total bromine loading is also consistent with recently published studies.

Retrieval of stratospheric and tropospheric BrO columns from multi-axis DOAS measurements at Reunion Island (21° S, 56° E)

Abstract. Spectral measurements of BrO using zenith-sky and off-axis viewing geometries are combined in a linear multiple regression retrieval algorithm to provide stratospheric and tropospheric BrO vertical columns. One year of measurement data are investigated over Reunion-Island (20.9° S, 55.5° E), from August 2004 to June 2005. A comparison between the stratospheric columns retrieved at 45°, 80°, 85°, 87.5° and 92.5° solar zenith angles and photochemical simulations initialized by chemical fields from the 3-D-CTM SLIMCAT and further constrained by observed NO2 profiles shows a good agreement only by considering a contribution from the very short-lived organic bromine substances to the stratospheric inorganic bromine budget, of 6 to 8 pptv. Furthermore, stratospheric BrO profiles retrieved from late twilight zenith-sky observations are consistent with a total inorganic bromine (Bry) loading of approximately 23 pptv. This represents 6 to 7 pptv more than can be supplied by long-lived organic bromine sources, and therefore supports an added contribution from very short-lived organic bromine substances as recently suggested in several other studies. Moreover strong evidences are presented for the existence of a substantial amount of BrO in the tropical free-troposphere, around 6 km altitude, possibly supplied by the decomposition of short-lived biogenic bromine organic compounds. Tropospheric BrO vertical columns of 1.1±0.45×1013 molec/cm² are derived for the entire observation period. Comparisons between ground-based BrO vertical columns and total BrO columns derived from SCIAMACHY (onboard the ENVISAT satellite) nadir observations in a latitudinal band centered around 21° S present a good level of consistency, which further strengthens the conclusions of our study.

An Observational Search for Cosmic Light

Cosmic light search by midlatitude zenith sky observations to extract diffuse light from totality of extragalactic light sources

Ozone column retrieval from solar UV measurements at ground level: Effects of clouds and results from six European sites

A differential absorption method, developed by the Laboratoire d'Optique Atmosphérique (LOA), is applied to retrieve the total ozone column from UV global irradiance spectra under clear and cloudy sky conditions. Contrarily to clear sky, cloudy sky generates a high uncertainty in the retrieved ozone column. This study shows that under cloudy conditions the daily ozone mean is a rather good estimation of the true value. The standard deviation allows us to estimate the relative uncertainty of this mean value, i.e., about 7%. Results for all conditions from 3 years at Villeneuve d'Ascq are presented, as well as a comparison with TOMS (Total Ozone Mapping Spectrometer) data. The results are very similar to those obtained for clear days: In 75% of cases both values agree to within 5%; however, some cases present larger relative differences. Over the 3 years considered, there is a bias less than 3% (LOA > TOMS). This method is applied to five other European sites involved in the EDUCE (European Database for UV Climatology and Evaluation) project. Three of them are Brewer stations. The results of the comparison between the LOA‐retrieved ozone and TOMS data show that a relative difference smaller than 5% is obtained in 63–80% of the cases, depending on site and year. Biases smaller than 3% are observed (LOA > TOMS). The comparison between the LOA‐retrieved ozone and total ozone data from direct sun and zenith sky observations of the Brewer instruments shows better agreement. For more than about 80% of cases, except at one site, the relative difference is smaller than 5%, and the biases are smaller than 1%. The LOA method allows us to obtain a complementary data set and hence to provide time series of reliable measurements of total ozone column under all sky conditions.

Applications of ultraviolet absolute radiometry in satellite and surface-based remote sensing of atmospheric ozone

Side-by-side comparisons of Langley-type Dobson AD double wavelength pair direct sun observations between SBUV/2, SSBUV flight models, and the NOAA world primary standard Dobson spectrophotometer 83 and 61 show that the SBUV/2-type instruments yield column ozone amounts that are 2% higher than the NOAA Dobson spectrometers. Similar results have been obtained with a radiometrically stable multifilter spectroradiometer (MFS) under less than ideal conditions. A new approach based on a modeled table look-up method for using zenith sky radiances to derive total column ozone from zenith clear sky conditions has been tested using SBUV/2, SSBUV flight models and the MFS equipped with narrowband interference filters at the Dobson AD wavelength pairs. These clear zenith sky observations yield total column ozone that is in good agreement with ozone from the NOAA Dobson spectrophotometer direct sun observations. New model calculations on the relationship between satellite nadir radiances and surface-based zenith clear sky radiances suggest that the combination of a very radiometrically stable surface-based spectroradiometer combined with a compact high performance double monochromator could be used to derive a common radiometric scale among satellite-borne ozone monitoring instruments.

Zenith-sky observations of stratospheric gases: the sensitivity of air mass factors to geophysical parameters and the influence of tropospheric clouds

The retrieval of accurate vertical column amounts of stratospheric constituents from zenith-sky spectroscopy is dependent on accurately modeling the transfer of radiation through the atmosphere and calculating suitable air mass factors (AMFs). Using a vector radiative transfer (RT) model we evaluate potential error sources in the AMF calculation, arising from incorrect geophysical parameters and computational approximations. Ozone and NO 2 AMFs were calculated using 19 different parameterizations, whereupon each was used to retrieve vertical column information from zenith-sky spectra recorded at a mid-latitude location. When the model was run with single-order scattering only, the derived AMFs were between 2 and 5% lower than those calculated using a multiple scattering scheme. Multiple scattering was also found to act in tandem with other parameters investigated, for example volcanic sulfate aerosols and surface albedo, and as such, we conclude that it is important to include multiple scattering in any RT model. Significant errors were also introduced by using a standard ozone profile rather than one derived from ozonesonde data. Substantially amplified AMFs were obtained from model runs that contained tropospheric clouds. Zenith-sky measurements recorded during cloudy conditions demonstrated that AMFs calculated from an RT model that neglects scattering by tropospheric clouds are unsuitable for analysis of spectra from overcast days. The feasibility of calculating AMFs tailored to account for the presence of clouds is also discussed.

DOAS Zenith Sky Observations: 2. Seasonal Variation of BrO Over Bremen (53°N) 1994-1995

Zenith sky observations of BrO over Bremen (53°N) are reported for the period of September 1994 to January 1996. BrO differential slant columns between 90° and 80° solar zenith angle showed a strong seasonal variation between a winter maximum of 1.9·1014 molec/cm2 and a summer minimum of 0.6·1014 molec/cm2. The seasonal variation in BrO twilight values is shown to be inversely correlated with NO2 columns in agreement with current knowledge of gas phase chemistry of bromine. In contrast to model predictions, no significant difference between morning and evening BrO measurements was observed. During a 6 day polar vortex excursion to mid-latitudes OClO could be measured above Bremen indicating chlorine activation in the vortex air. No significant increase in BrO differential slant columns was detected during this time.

Optical path modifications due to tropospheric clouds: Implications for zenith sky measurements of stratospheric gases

Potential influences of tropospheric clouds on zenith scattered light differential optical absorption spectroscopy (ZSL‐DOAS) of stratospheric gases, such as O3, NO2, OClO, BrO, and other species are discussed. It is argued that compared to clear sky, clouds can occasionally considerably change the geometrical path lengths in the troposphere, and hence change the relative contribution of the troposphere to the total atmospheric trace gas absorption. Major reasons for the geometrical path enhancements due to clouds are seen in the photon diffusion in optically thick clouds and in the multiple reflections of light between layers and patches of clouds. In addition, optically thin clouds in the upper troposphere (e.g., As and Ci clouds) may decrease the apparent absorption due to stratospheric species, whereby the magnitude of that effect largely depends on the solar zenith angle and wavelength. This is explained by the quite different air mass factors for the zenith scattered light observed under clear sky and cloudy sky conditions. Although opposite in sign, these “cloud effects” can combine and cause significant errors in ground‐based zenith sky observations of stratospheric gases. It is demonstrated that the proposed mechanisms can explain the recently reported cloudy sky absorption changes detected for O3, O4, NO2, BrO, and H2O.

Cloudy sky optical paths as derived from differential optical absorption spectroscopy observations

Recently, the influence of tropospheric clouds on zenith sky observations of atmospheric gases (O3, NO2, O4, H2O, and others) has been reported [Van Roozendael et al., 1994; Erle et al., 1995]. For measurements under heavy cloud cover, Erle et al. [1995] determined from simultaneously measured absorptions of different tropospheric species an average optical path for photons reaching the instrument of >100 km. This path length was much larger than expected from previous reports [Feigelson, 1981]. Thus an important question was whether the magnitude of the observation was a typical situation rather than a rare exception. Here we add a large set of new “cloud event” observations evaluated from data which were recorded in the Arctic, at midlatitudes, and the tropics. It is shown that the observed absorption enhancements of O3, H2O, and O4 perfectly coincide with indirect cloud indicators, i.e., changes in the average intensity and the color index as well as direct observations of cloud passages detected by satellite. In particular, we demonstrate that the observed optical paths have the same magnitude as the path lengths reported by Erle et al [1995], and thus optical paths under cloudy skies may indeed become much larger than previously reported. Besides these observations made during specific meteorological conditions we also studied optical paths continuously for several weeks in Arctic winter/spring conditions. We derived a frequency distribution of path lengths for cloudy skies with an average mean optical path of 29 km.

Groundbased DOAS UV/visible measurements at Kiruna (Sweden) during the sesame winters 1993/94 and 1994/95.

Zenith sky observations of O3, NO2, OClO and BrO are reported, which were performed at Kiruna (67.9°N, 21.1°E) within the SESAME winters 1993/1994 and 1994/95. For both winters large total amounts of OClO were observed inside the polar vortex at twilight, indicating the degree and the temporal variation of the halogen activation of the polar stratosphere. Occasionally OClO could also be observed outside the polar vortex, most likely due to export of halogen activated vortex air masses into the ambient stratosphere. BrO could also be detected in winter 1994/95, with the largest slant column amounts (5·1014/cm2) occuring in the polar vortex in mid-winter. Similar abundances of stratospheric BrO were observed at dusk and dawn, for both, air masses inside and outside the vortex. This observation is in reasonable agreement with previous studies on stratospheric BrO (observations and models) of Wahner et al. (1992), Arpag et al. (1994), Krug et al. (1996), and Lary et al. (1996a,b), but partly in disagreement with those of Solomon et al. (1989), Fish et al. (1995), and Sessler et al. (1996).

Mid‐latitude observations of the seasonal variation of BrO 1. Zenith‐sky measurements

Zenith‐sky observations of the seasonal evolution of BrO slant columns above Aberdeen (57°N) between November 1994 and April 1995 are presented. The measurements show maximum differential BrO slant columns (90°–81°) of 2.4 × 1014 molecules cm−2 in January, decreasing to 1 × 1014 in spring. On a seasonal timescale, BrO slant columns are anticorrelated with NO2 slant columns, which is qualitatively consistent with BrONO2 being the main nighttime bromine reservoir. There is some evidence for a small (5 × 1013 molecules cm−2) increase in sunrise BrO slant columns inside the polar vortex, following chlorine activation on PSCs, as shown by measurements of large OClO slant columns.

Total ozone changes over Eurasia since 1973 based on reevaluated filter ozonometer data

Since the early 1960s, on the vast territory of the former USSR, 45 stations have been in continuous operation, utilizing the broadband filter M‐83 ozonometer. The quality of the ozone data during the first decade was unsatisfactory. After 1972 an improved version of the ozonometer was introduced together with improved quality control practices, including methodology of observations. The more reliable data of 1973 through March 1994 have been rigorously reexamined by applying variability analysis, comparison with lower‐stratosphere temperatures and/or nearby Dobson stations, and overpassing TOMS for identifying concurrence or discrepancies. These control procedures together with the information on instrument relocation and calibrations made it possible to reevaluate the record of all 45 stations. The accuracy of the improved ozonometer data is about 3% for direct Sun measurements and ∼5% for zenith sky observations; although not so good as that of the Dobson, in the long run it provides consistent ozone data sets. This data set is now made available to the World Ozone Data Center (WO3DC), Toronto. Thus for the first time, based on a 21‐year long record, information is deduced on the differences in the ozone annual cycle between Eastern Siberia and the European part, on the strong appearance of quasi‐biennial oscillation (QBO) signals especially pronounced as ozone deficiency during the western phase of the QBO, on the ozone variability, and on the long‐term changes over the huge territory from Central Europe to the Far East. The specifics of the ozone changes considered in concurrence with the prevailing general stratospheric circulation conditions permitted us to distinguish four broad regions with consistent ozone regimes. The appearance of the strongest northern hemisphere ozone maximum and a monthly mean of ∼470 matm cm over Siberia during winter‐spring, compared with ∼400 matm cm over Europe, the occurrence of the ozone annual minimums as early as August over Eastern Siberia, compared with October over the European part, was established and is probably related to the specifics of the atmospheric circulation patterns. The long‐term ozone changes are considered in relation to the stratospheric temperature at 100‐hPa. For each deviation from the monthly normal temperature by 1°C, there is a corresponding change by the same sign 5–6 matm cm in the monthly ozone deviations. The calculated long‐term ozone trends for 1973 through March 1994 (given in percent per decade ±2σ) are as follows: European part, −3.6±0.8; Central Asia, −2.0±0.6; Western Siberia, −3.5±0.8; and Eastern Siberia ‐ Far East, −3.2±0.8. They show a steady ozone decline similar to that deduced from the Dobson stations at the same latitudes. The decline during the past 15 years is stronger and, in general, concurs with Total Ozone Mapping Spectrometer (TOMS) trends which are, however, slightly more negative.

Intercomparison of total ozone data measured with Dobson and Brewer ozone spectrophotometers at Uccle (Belgium) from January 1984 to March 1991, including zenith sky observations

More than 7 years of quasi‐simultaneous observations of the total ozone amount measured with Dobson spectrophotometer 40 and Brewer spectrophotometer 16, both located in Uccle (50°48′N, 4°21′E), are compared. Direct Sun observations with both instruments show a significant drift relative to each other of 0.1% per year. On the other hand, the Brewer instrument measurements indicate that the SO2 amount at Uccle has had a strong downward trend of 8.9% per year over the same time period. This is confirmed by in situ measurements of SO2 density near the ground. If the SO2 trend is taken into account in the comparison between Brewer and Dobson instrument total ozone values, the two total ozone data sets show no drift with respect to each other. The question of SO2 tendency therefore deserves investigation in any trend analysis of Dobson instrument total ozone data. Quasi‐simultaneous zenith sky observations made with both instruments are also intercompared. Results indicate that the coherence between direct Sun and zenith sky data of the Brewer instrument is slightly better than that of the Dobson instrument.

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.