Brewer Spectrophotometer Research Articles

Ozone recovery as seen in perspective of the Dobson spectrophotometer measurements at Belsk (52°N, 21°E) in the period 1963–2008

The total ozone monthly means derived from measurements by the Dobson spectrophotometer at Belsk (52°N, 21°E) and satellite observations over central and midlatitudinal Europe are analyzed for the long-term changes between 1995 and 2008. Standard explanatory variables representing physical and chemical processes known to influence the ozone distribution are considered. The potential proxies are: atmospheric loading by the ozone depleting substances characterized by the equivalent effective stratospheric chlorine (EESC) time series, various drivers of ozone dynamical variability including solar cycle, teleconnection patterns, temperature at 50 hPa, and pressure at the tropopause level. The multivariate adaptive regression splines (MARS) methodology is used to find optimal set of the explanatory variables and shape of the anthropogenic trend curve. Following options for the trend curve are examined: proportional to EESC, piecewise linear (with the turning points in 1980 and 1995), and selected from a smooth curve fit to the total ozone time series having “natural variations” removed. Statistical estimates and their uncertainties are calculated using block bootstrapping. The analyses indicate that ozone over Belsk, in central Europe, and in midlatitudinal Europe reaches at least first stage of recovery as defined by the World Meteorological Organization: a statistically significant reduction in the rate of decline. Model using the EESC time series as a proxy for the anthropogenic trend pattern yields even the second stage, i.e., a positive trend in time series that remains after removal dynamical signal from the analyzed data. Substantial seasonal dependent long-term ozone oscillations by the dynamical drivers are revealed causing estimation of the ozone recovery time even more uncertain.

A model study of the January 2006 low total ozone episode over Western Europe and comparison with ozone sonde data

Abstract. Total column and stratospheric ozone levels at mid-latitudes often reveal strong fluctuations on time scales of days caused by dynamic processes. In some cases the total ozone column is distinctly reduced below climatological values. Here, a very low total ozone episode around 19 January 2006 over Western Europe is investigated when the observed total ozone column over Uccle (BE), measured by a Brewer spectrophotometer, reached a daily minimum of 200 DU, the lowest recorded value at this station. In order to investigate the mechanisms leading to the ozone minimum, the present study used data from (i) six ozone sounding stations in Western and Middle Europe, (ii) ECMWF meteorological fields, (iii) a simulation of the CLaMS model for January 2006, (iv) a multi-year run of the chemistry transport model KASIMA, and (v) a six-year run of the climate chemistry model ECHAM5/MESSy1. The ozone decrease at different heights was quantified and it was determined to what extent different transport mechanisms, and instantaneous, in-situ chemical ozone depletion contributed to the event. All three models reproduced the evolution and formation of the event. The ozone column decrease between Θ=300 and 750 K was strongest at Uccle (BE) and De Bilt (NL) with 108 and 103 DU, respectively, and somewhat lower at Hohenpeissenberg (DE), Payerne (CH), Prague (CZ) and Lerwick (UK) with 85, 84, 83 and 74 DU, respectively. This analysis demonstrated that mainly the displacement of the ozone depleted polar vortex contributed to the ozone column decrease. Advection of ozone-poor low-latitude air masses was important in the UTLS region. The vertical displacement of isentropes connected with divergence of air out of the column was found to be of minor importance compared to the horizontal transport processes. Severe low total ozone episodes seem to occur when the mentioned mechanisms are superimposed. Instantaneous, in-situ chemical ozone depletion accounted for only 2±1% of the overall total ozone decrease at the sounding stations.

Seasonal start of the Antarctic ozone hole derived from observations with Dobson spectrophotometers

The Antarctic ozone hole appears in the Antarctic spring, just after the polar night period when sunlight is not available on the surface. Because it is not possible to have a large solar elevation angle in the Antarctic winter and spring, it is hard to make observations of total ozone amounts with TOMS or Dobson spectrophotometers using sunlight. Because of this there is not enough information about the total ozone amounts over the Antarctic just before and after the seasonal appearance of the Antarctic ozone hole. We have made an analysis of the total ozone amounts with a Dobson spectrophotometer, using moonlight as well as sunlight at the three Antarctic stations and Macquarie Island. The results derived from the analysis are as follows: (1) The decrease of the total ozone amounts leading to the Antarctic ozone hole started at the beginning of August at around 69° south and then took place at the higher latitudes later in the period from 1993 to 2005: (2) The total ozone amounts at the higher latitudes in the Antarctic show the smaller value as a latitudinal gradient of about 1.6 DU per degree in the period from 1993 to 2005. The author hopes that these results will be useful for better understanding of the Antarctic ozone hole.

Comparison of the Athens Dobson spectrophotometer with World Standard Instruments

The results of the intercomparison between Dobson spectrophotometer No. 118 operating in Athens, Greece and the Reference Instruments Dobson No. 065 and No. 064 in the period 1990–2006 is presented. The Athens Dobson spectrophotometer participated in the Dobson intercomparison campaigns held at Arosa, Switzerland (1990), Kalavryta, Greece (1997) and Hohenpeißenberg, Germany (2006). The results obtained at Arosa showed a deviation of 0.03% in total ozone observations, whereas at Kalavryta and Hohenpeißenberg the highest deviations were –0.4% and 0.6%, respectively. Hence the Dobson spectrophotometer installed in Athens since 1989 can be regarded as a reliable ground-truth station that can be used for the validation of satellite ozone observations and the study of trends in total ozone content (TOC).

Measurement of internal stray light within Dobson ozone spectrophotometers

The Dobson ozone spectrophotometer is the primary ground-based instrument making measurements of total column ozone. Knowledge of the ozone layer prior to the satellite measurements is derived from these instruments' more than 80-year record. Internal stray light within the instrument limits its ability to make accurate measurements at high total ozone amounts and high solar zenith angles (SZAs). Additionally, a recent analysis of comparative Dobson observations of the Umkehr effect (an SZA-dependent series of measurements on the clear zenith that are used to produce an ozone vertical profile) has shown that internal stray light produces incompatible results between compared instruments. We present a method of making a measurement of this internal stray light, as well as other approaches to evaluating the approximate level of the internal stray light. These techniques can also be used to improve the results of direct Sun observations at high latitudes and high ozone.

Stratospheric ozone during the arctic winter: Brewer measurements in Ny-Ålesund

To restrict the human impact on the environment, many preventive actions have been adopted in recent decades e.g. the Vienna Convention (1985) and the Montreal Protocol (1987). So monitoring of the atmosphere assumes the important purpose to check the effectiveness of mitigation strategies. The studies on stratospheric ozone are relevant because of its filtering action for UV and its high sensitivity to CFC. During the past 20 years the trend of ozone depletion has been strongly negative, particularly in Polar Regions. Recently weak signals indicate that a recovery has started, even if to disguise anthropogenic impact from natural trend is difficult. So studies on stratospheric ozone during the polar night are important to understand the chemistry involved. This paper shows the results of a first polar winter campaign (September 2007 to March 2008) by Brewer spectrophotometer in Ny-Alesund, Norway, using solar UV reflected by the Moon. TOMS, MLS-AURA and SCIAMACHY data are also shown.

Changes observed in sun photometer derived total column ozone and possible implications on surface-reaching UV radiation over a tropical Indian station

Ozone depletion or unusual decreases in ozone content has now become a global issue because of the corresponding increases in surface reaching UV-B radiation. Sun photometer derived total column ozone along with that from Dobson spectrometer at a tropical urban Indian station, Pune (18°32′N, 73°51′E, 559 m Above Mean Sea Level) for the period 1998–2003 has been investigated. On a monthly mean scale the agreement between ozonometer and Dobson is within ± 3%. Ozone content at Pune during the winter months of January 1999 and 2002 were almost 6–7% lower than that observed during other years. This reduction in ozone content led to enhancement of erythemal dose at the surface during following summer months. Day-to-day variation on some occasions also showed ozone decreases accompanied by significant enhancement in surface reaching UV radiation. Optical remote sensing methods thus help to monitor unusual changes in ozone content.

Variability in sun photometer derived summertime total column ozone over the Indian station Maitri in the Antarctic region

Extensive observations of total column ozone (TCO) made over the Antarctic region at the Indian station ‘Maitri’ (70.76oS, 11.74oE) during two summer periods of 2004–2005 and 2006–2007 using a ground-based optical remote sensing instrument (ozonometer) have been examined to study the short-term variations. Mean TCO during 2004–05 summer was 288.7 DU with a variability of 8% and the corresponding values during 2006–2007 summer were 280.4 DU and 11%. Both ozonometer and Dobson spectrophotometer measurements at this Antarctic station during January 2005 showed a short-term decrease in TCO by about 70 DU before regaining original value. Simultaneous changes in precipitable water and surface temperature points to a possible interplay of chemistry and meteorological conditions. Occurrence of an intense solar proton event in January 2005 also suggests the possible role of precipitating charged particles in causing short-lived ozone decrease through production of OH radical.

A study of the total atmospheric sulfur dioxide load using ground-based measurements and the satellite derived Sulfur Dioxide Index

We present characteristics of the sulfur dioxide (SO 2) loading over Thessaloniki, Greece, and seven other selected sites around the world using SO 2 total column measurements from Brewer spectrophotometers together with satellite estimates of the Version 8 TOMS Sulfur Dioxide Index (SOI) over the same locations, retrieved from Nimbus 7 TOMS (1979–1993), Earth Probe TOMS (1996–2003) and OMI/Aura (2004–2006). Traditionally, the SOI has been used to quantify the SO 2 quantities emitted during great volcanic eruptions. Here, we investigate whether the SOI can give an indication of the total SO 2 load for areas and periods away from eruptive volcanic activity by studying its relative changes as a correlative measure to the SO 2 total column. We examined time series from Thessaloniki and another seven urban and non-urban stations, five in the European Union (Arosa, De Bilt, Hohenpeissenberg, Madrid, Rome) and two in India (Kodaikanal, New Delhi). Based on the Brewer data, Thessaloniki shows high SO 2 total columns for a European Union city but values are still low if compared to highly affected regions like those in India. For the time period 1983–2006 the SO 2 levels above Thessaloniki have generally decreased with a rate of 0.028 Dobson Units (DU) per annum, presumably due to the European Union's strict sulfur control policies. The seasonal variability of the SO 2 total column exhibits a double peak structure with two maxima, one during winter and the second during summer. The winter peak can be attributed to central heating while the summer peak is due to synoptic transport from sources west of the city and sources in the north of Greece. A moderate correlation was found between the seasonal levels of Brewer total SO 2 and SOI for Thessaloniki, Greece ( R = 0.710–0.763) and Madrid, Spain ( R = 0.691) which shows that under specific conditions the SOI might act as an indicator of the SO 2 total load.

On the relationship between erythemal and vitamin D action spectrum weighted ultraviolet radiation

Erythemally weighted solar ultraviolet (UV) radiation is often used to characterize the production of vitamin D in a human body. However, the vitamin D production action spectrum is different than that for erythemal UV. The vitamin D action spectrum weighted UV is more sensitive to UV-B, while the erythemal UV action spectrum has higher weighting coefficients than the vitamin D action spectrum in the UV-A part of the spectrum. Therefore, by using the erythemal UV as an estimate for the vitamin D action spectrum weighted UV can give results that differ by up to a factor of 5. This study examines the relationship between erythemal and vitamin D action spectrum weighted UV radiation using measurements of spectral UV at the surface by Brewer spectrophotometers that are part of the US and Canadian observational networks. It is shown that the ratio of vitamin D action spectrum weighted UV to erythemal UV is nearly constant for high levels of UV (UV Index greater than 5.5) and therefore vitamin D action spectrum weighted UV can be described in terms of erythemal UV. For lower levels of UV though this relationship should not be used. A simple formula that calculates vitamin D action spectrum weighted UV from UV Index is developed. An empirical formula that expresses the ratio of vitamin D action spectrum weighted UV to erythemal UV as a function of the solar zenith angle and column ozone is also suggested. The geographical distributions of vitamin D action spectrum weighted UV in the US and Canada are discussed.

Determination of ultraviolet cosine-corrected irradiances and aerosol optical thickness by combined measurements with a Brewer spectrophotometer and a multifilter rotating shadowband radiometer

Combined measurements of diffuse-to-global radiation ratio and global spectral irradiances in the UV are used to derive cosine-corrected UV irradiances and aerosol optical depth (AOD). The diffuse-to-global radiation ratio is used first in the cosine correction of the global irradiance, then to calculate absolutely calibrated direct irradiances. The Beer-Lambert law is applied to derive the UV AOD using independent measurements of the extraterrestrial solar flux. The AOD can be derived with an uncertainty of about 0.03 at 60 degrees solar zenith angle. The method was applied to measurements obtained with two UV multifilter rotating shadowband radiometers (UV-MFRSRs) and a MK III Brewer spectrophotometer on the Island of Lampedusa in the Central Mediterranean during two periods of 2002 and 2004. The derived AOD at 318 and 332 nm was compared with UV AOD measured at 318, 320, and 368 nm with different techniques. The retrieved AOD, combining MFRSR and Brewer measurements, is in good agreement with the optical depth derived with the other methods.

Performance of the ground‐based total ozone network assessed using satellite data

Dobson and Brewer spectrophotometer and filter ozonometer data available from the World Ozone and Ultraviolet Data Centre (WOUDC) were compared with satellite total ozone measurements from TOMS (onboard Nimbus 7, Meteor 3, and Earth Probe satellites), OMI (AURA satellite) and GOME (ERS‐2 satellite) instruments. Five characteristics of the difference with satellite data were calculated for each site and instrument type: the mean difference, the standard deviation of daily differences, the standard deviation of monthly differences, the amplitude of the seasonal component of the difference, and the range of annual values. All these characteristics were calculated for five 5‐year‐long bins and for each site separately for direct sun (DS) and zenith sky (ZS) ozone measurements. The main percentiles were estimated for the five characteristics of the difference and then used to establish criteria for “suspect” or “outlier” sites for each characteristic. About 61% of Dobson, 46% of Brewer, and 28% of filter stations located between 60°S and 60°N have no “suspect” or “outlier” characteristics. In nearly 90% of all cases, Dobson and Brewer sites demonstrated 5‐year mean differences with satellites to be within ±3% (for DS observations). The seasonal median difference between all Brewer DS measurements at 25°–60°N and GOME and OMI overpasses remained within ±0.5% over a period of more than 10 years. The satellite instrument performance was also analyzed to determine typical measurement uncertainties. It is demonstrated that systematic differences between the analyzed satellite instruments are typically within ±2% and very rarely are they outside the ±3% envelope. As the satellite instrument measurements appear to be better than ±3%, ground‐based instruments with precision values worse than ±3% are not particularly useful for the analyses of long‐term changes and comparison with numerical simulations.

Quality assurance of the Brewer spectral UV measurements in Finland

Abstract. The quality assurance of the two Brewer spectrophotometers of the Finnish Meteorological Institute is discussed in this paper. The complete data processing chain from raw signal to high quality spectra is presented. The quality assurance includes daily maintenance, laboratory characterizations, calculation of long-term spectral responsivity, data processing and quality assessment. The cosine correction of the measurements is based on a new method, and is included in the data processing software. The results showed that the actual cosine correction factor of the two Finnish Brewers can vary between 1.08–1.13 and 1.08–1.12, respectively, depending on the sky radiance distribution and wavelength. The temperature characterization showed a linear temperature dependence between the instruments' internal temperature and the photon counts per cycle, and a temperature correction was used for correcting the measurements. The long-term spectral responsivity was calculated using the time series of several lamps using two slightly different methods. The long-term spectral responsivity was scaled to the irradiance scale of the Helsinki University of Technology (HUT) for the whole of the measurement time-periods 1990–2006 and 1995–2006 for Sodankylä and Jokioinen, respectively. Both Brewers have participated in many international spectroradiometer comparisons, and have shown good stability. The differences between the Brewers and the portable reference spectroradiometer QASUME have been within 5% during 2002–2007.

Comparison of total ozone and erythemal UV data from OMI with ground-based measurements at Rome station

Abstract. Ground-based total ozone and surface UV irradiance measurements have been collected since 1992 using Brewer spectrophotometer at Rome station. Erythemal Dose Rates (EDRs) have been also determined by a broad-band radiometer (model YES UVB-1) operational since 2000. The methodology to retrieve the EDR and the Erythemal Daily Dose (EDD) from the radiometer observations is described. Ground-based measurements were compared with satellite-derived total ozone and UV data from the Ozone Monitoring Instrument (OMI). OMI, onboard the NASA EOS Aura spacecraft, is a nadir viewing spectrometer that provides total ozone and surface UV retrievals. The results of the validation exercise showed satisfactory agreement between OMI and Brewer total ozone data, for both OMI-TOMS and OMI-DOAS ozone algorithms (biases of −1.8% and −0.7%, respectively). Regarding UV data, OMI data overestimate ground based erythemally weighted UV irradiances retrieved from both Brewer and YES Radiometer (biases about 20%). The effect of aerosols on UV comparisons was investigated in terms of Aerosol Optical Depth (AOD), showing medium-large correlation at SZA larger than 55°. Further sources of uncertainty, such as the difference in the atmospheric conditions between local noon and OMI overpass time and the OMI spatial resolution, were also discussed.

Comparison of ozone profiles by the Umkehr measurements taken at Belsk (52°N, 21°E) with the Aura Microwave Limb Sounder overpasses: 2004–2006

A comparison of ozone profile measurements by the Umkehr observations taken at Belsk (52°N, 21°E) by the Dobson spectrophotometer and collocated Earth Observing System (EOS) Microwave Limb Sounder (MLS) aboard the Aura platform during 2004–2006 site overpasses is presented. The MLS ozone vertical profiles from 1.5 and 2.2 retrieval algorithms and various Umkehr retrievals are considered. There is a good agreement between the ozone content in Umkehr layers. The mean difference ∼ ±5%, standard deviation of the relative differences < 10%, and the correlation coefficient > 0.5 are found in the midstratosphere (Umkehr layers 5–7) and in the upper stratosphere (combined layers 8, 9, and 10). In the lower stratosphere (combined layers 2 and 3 and layer 4) the mean difference and/or standard deviation are larger but the correlation coefficient is still high (∼0.8). Ozone content in Umkehr layers by integration of MLS version 1.5 and 2.2 ozone mixing ratio profiles behaves very similarly in the comparisons with the ground‐based data.

Comparison of ground‐based Dobson and satellite EP‐TOMS total ozone measurements over Vernadsky station, Antarctica, 1996–2005

Total ozone content derived from version 8 of the Earth Probe Total Ozone Mapping Spectrometer (EP‐TOMS) satellite data was compared with that from ground‐based data obtained with the Dobson spectrophotometer no. 031 at the Ukrainian Antarctic Vernadsky station. The period of comparison is 1996–2005. The statistics for cloudy and clear sky observations are presented separately, in order to assess the influence of cloudiness on the difference between the satellite and ground‐based measurements. The main findings obtained from the TOMS–Dobson difference analysis were: (i) a significant disagreement between clear and cloudy sky differences of about 9%, which is almost the same value as in version 7; and (ii) the difference for all data equals −2.0% (2625 days), whereas in cloudless and cloudy conditions the difference is 4.1% (137 days) and −4.5% (1048 days), respectively. We conclude that the TOMS (Dobson) measurements are mostly responsible for the data disagreement in clear (cloudy) sky conditions.

Comparison of GOME total ozone data with ground data from the Spanish Brewer spectroradiometers

Abstract. This paper compares total ozone measurements from five Brewer spectroradiometers located at the Iberian Peninsula with satellite observations given by the GOME (Global Ozone Monitoring Experiment) sensor. The analyzed period covers simultaneous ozone values from July 1995 until December 2004. The regression analysis shows an excellent agreement between Brewer-GOME values in the five locations; the coefficient of correlation is always higher than 0.92 and the root mean square error is about 3%. Moreover, the comparison shows that the satellite retrieval accuracy is within the uncertainty of current ground-based instruments. In addition, the effects of several variables, such as cloudiness, solar zenith angle (SZA), effective temperature and total ozone values in Brewer-GOME differences are analyzed. The results indicate that clouds induce a minor dependence of GOME values on the SZA. For example, during heavy cloudy conditions in Madrid station, GOME observations overestimate ground-based Brewer data for low AMF (low SZA values) by 2% while for high AMF (high SZA values) the satellite underestimates ground-based ozone values by 1%. Moreover, the dependence of Brewer-GOME differences with respect to SZA for cloud-free conditions may be due to the variability of effective temperature. This fact could indicate that the effective temperature estimated by GOME does not fully reflect the actual atmospheric temperature variability. Finally, GOME ozone observations slightly underestimate the highest values measured by the Brewer spectrophotometers and overestimates the lowest ground-based measurements.

Atmospheric comparison of electrochemical cell ozonesondes from different manufacturers, and with different cathode solution strengths: The Balloon Experiment on Standards for Ozonesondes

A balloon flight to compare 18 ozonesondes with an ozone photometer and with ozone column measurements from Dobson and Brewer spectrophotometers was completed in April 2004. The core experiment consisted of 12 electrochemical concentration cell ozonesondes, 6 from Science Pump Corporation (SP) and 6 from ENSCI Corporation (ES), prepared with cathode solution concentrations of 0.5% KI (half buffer) and 1.0% KI (full buffer). Auxiliary ozonesondes consisted of two electrochemical concentration cell sondes with 2.0% KI (no buffer), two reconditioned sondes, and two Japanese‐KC96 sondes. Precision of each group of similarly prepared ozonesondes was <2–3%. The six ozonesondes prepared according to the manufacturer's recommendations (SP, 1.0% KI, ES 0.5% KI) overestimated the photometer measurements by 5–10% in the stratosphere, but provided ozone columns in good agreement with the ground‐based spectrophotometer measurements. This is consistent with the difference (∼5%) in ozone photometer and column measurements observed during the experiment. Using cathode cell concentrations of 1.0% KI for ES sondes caused overestimates of the photometer by 10–15% and of ozone column by 5–10%. In contrast, 0.5% KI in SP sondes led to good agreement with the photometer, but underestimates of ozone column. The KC96 sondes underestimated the photometer measurements by about 5–15% at air pressures above 30 hPa. Agreement was within 5% at lower pressures. Diluting the solution concentration and the buffers from 1.0% to 0.5% KI causes an approximately linear pressure‐dependent decrease in ozone for both SP and ES sondes, ratio (0.5 KI/1.0 KI) = 0.9 + 0.024 * log10(Pressure).

An alternative method for aerosol optical thickness retrieval in the UV range

Atmospheric aerosol particle size distribution data derived from almucantar scans performed by the CIMEL sunphotometer at Belsk, Poland, in 2005 are used for the estimation of aerosol optical thickness (AOT) in the UV range by applying the Mie theory. The results obtained are compared with the direct Sun measurement data from the CIMEL sunphotometer and collocated Brewer spectrophotometer No. 064, as well as with AOT obtained by Angstrom extrapolation from the direct Sun measurements performed by the CIMEL sunphotometer in the visible range of wavelengths. Mean differences between calculated and measured values of AOT are up to about 8% in the UV-B range, which is close to the measurement uncertainty of the Brewer spectrophotometer and much less than that obtained by means of Angstrom extrapolation (over 24% in the UV-B range).

Validation of Ozone Monitoring Instrument total ozone column measurements using Brewer and Dobson spectrophotometer ground‐based observations

In this paper we present validation results of the total ozone column data products of the Ozone Monitoring Instrument (OMI) on board the NASA EOS‐AURA satellite through comparisons with ground‐based observations by Dobson and Brewer spectrophotometer instruments. Quality‐controlled and archived total ozone column data from these ground‐based instruments located at stations worldwide have been used to validate more than 2 a of total ozone column observations from OMI. There are two operationally available satellite total ozone column data products, based on the OMI‐TOMS and the OMI‐DOAS retrieval algorithms, respectively. Validation with ground‐based data focused on global comparisons and seasonal dependence and the possible dependence on latitude and solar zenith angle. Our results show a globally averaged agreement of better than 1% for OMI‐TOMS data and better than 2% for OMI‐DOAS data with the ground‐based observations. The OMI‐TOMS data product is shown to be of high overall quality with no significant dependence on solar zenith angle or latitude. The OMI‐DOAS data product shows no significant dependence on latitude except for the high latitudes of the Southern Hemisphere where it systematically overestimates the total ozone value. In addition a significant dependence on solar zenith angle is found between OMI‐DOAS and ground‐based data. Comparisons of satellite and ground‐based data tend to show a marginal seasonal dependence even though it remains unclear whether this dependence originates from the ground‐based or spaceborne observations.