KONINKLIJK METEOROLOGISCH INSTITUUT VAN BELGIEINSTITUT ROYAL METEOROLOGIQUE DE BELGIQUE

Zenith observations of total ozone in Uccle

Hugo De Backer

Uitgegeven door het KONINKLIJK METEOROLOGISCH INSTITUUT VAN BELGIE

Ringlaan �� B����� Brussel

Verantwoordelijke uitgever� Dr� H� Malcorps

Edit�e par l�INSTITUT ROYAL METEOROLOGIQUE DE BELGIQUE

Avenue Circulaire �� B����� Bruxelles

Editeur responsable� Dr� H� Malcorps

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i

Abstract

In Uccle the total ozone content of the atmosphere is measured since ���� with a Dobsonspectrophotometer and since ���� with a Brewer spectrophotometer� For observations onthe direct sun� total ozone can be derived analytically� To handle the zenith observationsan empirical method has to be implemented for each instrument and each location� InUccle direct sun observations are often prevented by clouds� Therefore� special attentionmust be paid at the zenith sky observations� The zenith observations are separated intwo groups� zenith blue and zenith cloudy� For the zenith blue values a polynomial isdetermined by a least squares method� using the data of quasi�simultaneous direct sunand zenith observations� For the observations with clouds a correction� linear dependenton the total ozone content� is introduced� In this way the mean dierence between dailymeans based on direct sun and on zenith observations is �������� and ������ � forthe Dobson and Brewer instrument� respectively�

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Contents

� Introduction �

� Instrumentation and Data Processing �

��� Direct sun procedure � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � ������ The Dobson instrument � � � � � � � � � � � � � � � � � � � � � � � � ������ The Brewer instrument � � � � � � � � � � � � � � � � � � � � � � � � � �

��� Zenith sky procedure � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � ������ Zenith blue observations � � � � � � � � � � � � � � � � � � � � � � � � ������ Zenith cloudy observations � � � � � � � � � � � � � � � � � � � � � � � �

� Results and discussion �

��� Dobson no � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � ��� Brewer no �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �

� Conclusion ��

�

� Introduction

Ozone is a minor atmospheric constituent with particular physical and chemical proper�ties� Among these� the strong absorption of UVB �radiation with wavelengths between�� and �� nm� is very important for the biosphere� Indeed� ozone in the stratosphereis an eective protective shield against damaging radiation of the sun in this spectral re�gion� After the discovery of the potential in�uence of human activities on the ozone layer�eorts have been made to understand the dynamical and chemical properties of atmo�spheric ozone� The measurement of total ozone is an important tool for many studies inthis �eld� Total ozone �or equivalent thickness� is expressed as a vertical column of pureozone� The unit established for this quantity is the Dobson Unit �DU�� equal to ���m at oC and standard pressure ������ hPa�� The global average of total ozone is approxi�mately �DU� equivalent to a thickness of �mm� The Uccle data set �see e�g� De Backer������� has a mean value of about �� DU� with a clear annual cycle ������DU� showinga maximum during spring and a minimum during fall� Besides this annual variation thereis a large day�to�day variability �the standard deviations of the monthly means are typicalbetween � and �DU�� caused by the dynamics of the atmosphere� The time series alsoshows a downward long term trend of ���� per decade� probably due to chemistry�

Total ozone is measured world�wide� mainly with Dobson and Brewer spectropho�tometers from the ground� Additionally satellite based instruments improve the globalcoverage of the measurements� In Uccle total ozone is measured several times a day withDobson spectrophotometer no � since July ����� and with Brewer spectrophotometerno �� since July ����� The data are regularly sent to the World Ozone Data Centre inToronto� A comparative study of the measurements with both instruments by De Backerand De Muer ������ showed that with rigorous calibration procedures� very good agree�ment between both instruments on Direct Sun �DS� observations can be achieved� SinceDS measurements are often prevented by clouds� also the zenith sky �ZS� measurementswere considered� The ZS measurements of both instruments were analysed by using a so�called zenith sky chart for each instrument� as proposed by Komhyr ������� This methodis based on the manual graphical analysis of quasi�simultaneous DS and ZS observations�No distinction was made between zenith measurements on the blue sky �ZB� or on acloudy sky �ZC�� A method to take into account the dierence between ZB and ZC� to�gether with a numerical algorithm to create the zenith sky charts is presented in the nextsection� Subsequently the results of the application of the methods on the data of theDobson and Brewer spectrophotometers at Uccle are discussed�

� Instrumentation and Data Processing

A detailed description of the instruments can be found in De Backer ������� or in themanuals of the Dobson �Dobson� �� �a and �� �b� and the Brewer �SCI�TEC� �����spectrophotometers�

��� Direct sun procedure

The primary observations of these instruments are on the direct beam of the sun� If weassume that� apart from aerosols� ozone is the only �or at least the main� absorber inthe atmosphere with strong wavelength dependence in the considered wavelength range�

�

we can write �see e�g� Basher ������� the following basic equation for the signal S���received in a spectrophotometer measuring the intensity of the direct radiation of thesun at wavelength � at the Earth�s surface as a function of the intensity outside theatmosphere�

S��� � K��� I��� � K��� I���� exp�������X �m����p

p�� ���� sec�z�� ���

where�

K��� represents the proportionality factor of the instrument�s response to the in�cident radiation at wavelength ��

I��� irradiance at the Earth�s surface at wavelength ��

I���� irradiance outside the Earth�s atmosphere at wavelength ��

� relative optical airmass of the ozone layer a height of �� km�see Eq� ��� with �h���km��

���� absorption coe�cient of ozone at wavelength ��

X equivalent thickness of the ozone layer�

m relative optical airmass for the Rayleigh scattering of the at�mosphere at km �see Eq� ��� with �h� km��

���� Rayleigh scattering optical thickness of a vertical paththrough the atmosphere normalised to standard pressure p��

p air pressure at the station�

p� standard air pressure�

���� aerosol scattering optical thickness of a vertical path throughthe atmosphere�

z zenith angle of the sun�

If we write R for the mean radius of the Earth ���� km� we can calculate the relativeoptical airmass am��h� for a layer at a height �h above the Earth�s surface with thefollowing equation which can be derived from the geometry of the observation�

am��h� � sec�arcsin�R

R��hsin�z�� ���

In practice the irradiation is measured at dierent wavelengths� and logarithms of�weighted� ratios of the intensities are determined� If the wavelengths and the weightingfunctions are selected in such a way that the contribution of the aerosol term can beneglected� we can calculate from Eq� ��� the total ozone amount XDS from direct sunobservations� expressed in DU as follows�

XDS ��F� � C�� ��F � C� �m�� p

p��

������

�

where�

F represents the weighting function of the logarithms of the incident radiationintensities I����

F� weighting function of the logarithms of the extraterrestrialradiation intensities I�����

C weighting function of the logarithms of spectral responsitivi�ties K����

�� weighting function of the ozone absorption coe�cients�

�� weighting function of the Rayleigh scattering optical thicknessnormalised to standard air pressure p��

For both instruments F��C is the so�called extraterrestrial constant of the instrument�which has to be determined by a calibration procedure� The extraterrestrial constantmay be subject to changes with time to re�ect instrumental changes due to ageing ofcomponents� These shifts can be detected by the analysis of lamp test results� or byregular comparisons with other instruments� The calibration history is documented byDe Backer and De Muer ������ for the Brewer spectrophotometer and by De Muer andDe Backer ������ for the Dobson instrument in Uccle�

����� The Dobson instrument

During standard operation the Dobson instrument measures the ratios of the radiationintensities of two wavelength pairs� The A pair wavelength bands are centered at � � nm�A� and �� �� nm �A��� while the D pair wavelength bands are centered at ����� nm �D�and ����� nm �D��� Then� for the standard observing scheme �the AD combination on thedirect sun�� the weighting of the intensities is done by�

F � log���IA

IA�

�� log���ID

ID�

� ���

The corresponding values of �� and �� are �� and ����� cm��� respectively�

����� The Brewer instrument

The Brewer spectrophotometer observes the intensities of the direct sun radiation atwavelength bands �� �� �� �� and centered at ���� nm� ���� nm� ���� nm� ����� nmand ���� nm� respectively� For the calculation of total ozone� four of them are usedthrough the following weighting�

F � log���I�

I��� � log

���I�

I��� ��� log

���I�

I�� � �

Then �� equals �� while �� diers from one instrument to another to account forsmall dierences in wavelength settings and slit functions� For Brewer no ��� located inUccle� �� equals �� � cm���

�

��� Zenith sky procedure

When DS observations are prevented by the presence of clouds� the zenithal radiationcan be measured instead� The simple attenuation model of section ��� is not valid then�The radiation seen by the spectrophotometer is now the result of multiple scattering atdierent heights� To avoid the inversion of the complicated multiple scattering probleman empirical method is used� For the Dobson instruments a method to construct so�called zenith�sky charts was suggested by Komhyr ������� In the software for the Brewerinstrument �SCI�TEC� ����� the use of a polynomial is proposed� The latter will beadopted here for the Brewer as well as for the Dobson instrument� This method worksreasonably well because� �rstly� the radiation received from the zenith sky is scatteredmostly well below the bulk of the ozone layer� and therefore experiences an optical pathsimilar to that of the direct beam� and� secondly� the scattering by cloud water dropletshas only a small wavelength dependence� However� a small systematic dierence betweenzenith ozone observations with clouds and direct sun ozone observations shows up� whichis corrected by the procedure explained in section ������

����� Zenith blue observations

The algorithm is based on the assumption that the �weighted� ratios in Eqs� ��� and � �of the zenithal radiation intensities can be approximated by a polynomial of the secondorder in � and X�

F� � F ��Xi��

�Xj��

ai�j �iXj ���

where the ai�j are regression coe�cients� which can be determined by the least squaresmethod from a series of simultaneous observations of F� � F �from ZS observation�� �and X �from DS observation� using Eq� ����� The graphical representation of Eq� ��� isequivalent to the so�called zenith�sky chart�Let us de�ne�

Aj ��Xi��

ai�j �i ���

Then� we can calculate the total ozone amountXZB� corresponding with the measurementof F� � F on the zenith at relative optical pathlength � as the physical meaningful rootof Eq� ��� �

XZB ��A� �

qA�

�� � �A� � �F� � F ��A�

�A�

���

����� Zenith cloudy observations

For observations on the cloudy zenith sky� Eq� ��� may be applied but a correction de�pending on the total ozone amount is required to account for the eect of the clouds�

XZC � XZB � �ZC ���

Then �ZC is approximated by�

�ZC ��Xi��

ciXiZB ���

The coe�cients ci are estimated by a least squares regression analysis on pairs of ozoneobservations directly on the sun and on cloudy zenith observations treated with Eq� ����Also a ��dependent correction could be introduced� This� however� did not improve theagreement between ZS and DS signi�cantly�

Figure �� Zenith sky chart for Dobson no �� showing the relationship between total ozone�X� in DU� � and F� � F from a series of quasi simultaneous DS and ZB observations�

� Results and discussion

In this section the results of the application of the methods explained above to the datasets of Dobson no � and Brewer no �� located in Uccle are discussed�

��� Dobson no ��

Total ozone measurements are taken manually with this instrument� A normal observingcycle consists of a DS �if direct sun radiation is available� and a ZS observation �if notprevented by precipitation or very thick clouds�� This measurement cycle is repeated �� times a day �with an interval of more than � hour�� depending on the elevation of the sun�To construct the zenith sky chart the data are treated as follows� All pairs of DS and ZSdata are considered as ZB� if the time dierence between the zenith measurement and thenearest DS is less than � minutes� Of course this includes also the cases where the DSmeasurement was taken on a small break in the cloud cover� In general this will howevercorrespond with smaller �thinner� clouds� aecting less the measurement� For the Dobson

�

Figure �� Percentage dierences between ZB and DS observations of total ozone withDobson no � �with a maximum time dierence of �min�� The lines are the result of�ltering the data with a �lter with �� of � days �solid line� and � year �dashed line��

instrument this selection reveals ��� data points �each consisting of F� � F and � ofthe zenith measurement and a corresponding XDS�� With this data set the coe�cients ofEq� ��� are determined by the least squares regression analysis� The resulting zenith skychart is shown in Fig� ��

When we apply this sky chart to the data we can make an intercomparison betweenquasi�simultaneous DS and ZB measurements with the Dobson instrument� Figure �shows the result� The mean dierence is then ������� The mean is necessarily lowbecause of the least squares procedure� The standard deviation is nearly the same as theone found by De Backer and De Muer �������

The solid line in Fig� � shows that there is only a small seasonal variation� within therange ���� The only exceptions are the winters of ����� and ������ showing low ZBvalues �see also the corresponding discussion on the Brewer results in section ����� Themidwinter data should be treated with caution� since they are based on a smaller numberof comparisons�

The dashed line in Fig� � represents the long�term variation of the dierences� Theyare small ����� and show no systematic drift with time�

To make a correction for the cloudy cases we proceed as follows� All ZS observationswith a concurrent DS observation not closer than � min are considered as ZC observa�tions� To limit the in�uence of ozone changes with time� the maximum time dierenceis limited to � hours� This reveals ���� cases� displayed as a function of total ozone inFig� �� The solid line in Fig� � is the result of the regression in Eqs� ��� and ����

The remaining dierences between XZC and XDS are plotted as function of time inFig� �� The mean dierence here is �������� As expected the standard deviation ishigher than with ZB� Also the long�term variations are larger� but no systematic drift is

�

Figure �� Dierences between total ozone amounts obtained from quasi simultaneous �seetext for criteria� DS measurements �XDS� and measurements on a cloudy sky �calculatedwith Eq� ���� as a function of total ozone �XDS� for Dobson no �� The solid line is theresult of the regression in Eqs� ��� and ����

discernible �dashed line in Fig� ���

As a quality check� Fig� shows the dierences between the daily mean values obtainedwith Dobson no � on ZS �i�e� ZB and ZC� and DS values� For a total of ���� days we

Figure �� As Fig� � but for ZC observations �with a maximum time dierence of � hours��

�

Figure � Percentage dierences of daily mean ZS and DS ozone values obtained withDobson no ��

�nd a mean of ��������� Together with the results of the individual ZS�DS and ZC�DScomparisons� this shows that with the procedure described above� we get good agreementbetween direct and zenithal measurements�

From the standard deviations between ZS and DS observations we can conclude that�at the � � level� the uncertainty of the ZS measurements is about �� higher than forthe DS measurements�

Figure �� As Fig� � but for Brewer no ���

�

��� Brewer no ��

The same procedure is also applied to the data set of Brewer no ��� This instrument wasautomated in March ����� resulting in an increased number of observations� Before theinstrument was automated� the same operational cycles as for the Dobson instrumentwere used� Since March ����� each hour a DS and ZS measurement is attempted� DuringDS measurements the absolute intensities are tested to suppress observations when thesun is obscured� Moreover� only data obtained under stable conditions are accepted bythe analysing software �see the Brewer manual �SCI�TEC� ����� for details��

Figure �� As Fig� � but for Brewer no ���

For the construction of the sky chart in Fig� � we had ���� cases of quasi simultaneousZB and DS measurements� The mean of the dierences between ZB and DS observations�shown in Fig� �� is ���� �� Also for this instrument the results compare very well tothose mentioned in De Backer and De Muer �������

The seasonal �solid line in Fig� �� and the long�term �dashed line in Fig� �� variationsare lower �about ��� than for the Dobson instrument� It is interesting to see that alsohere the winter of ����� shows the largest deviations� Perhaps this is linked to the aerosolload of the atmosphere after the eruption of the Pinatubo in June ����� The additionalscattering caused by the aerosol� may introduce additional uncertainty in the standardZS procedure� The eect of the increased aerosol load after major volcanic eruptions hasalso been noticed in direct beam measurements in Uccle �Jouko and Tempels� ������

Figure � shows the ��� ZC�DS data pairs that were found in the data set� usingthe same criteria as for the Dobson instrument� After application of the correspondingcorrection the remaining dierences between ZC and DS measurements are shown inFig� �� The mean dierence is ������� The variance is slightly higher than for theDobson instrument� but it must be considered that the Brewer spectrophotometer isweatherproof and completely automated� Therefore it also makes measurements undervery poor observational conditions� such as precipitation and fog� Taking this into account

�

Figure �� As Fig� � but for Brewer no ���

it is encouraging to see that the seasonal and long�term variations in Fig� � are small �� ��and � ��� respectively� and show no systematic drift�

Finally Fig� � shows the dierences between daily mean values obtained with Brewerno �� with ZS and DS values� The mean result for ���� days is ������ ��

The additional uncertainty for the ZS with respect to the DS measurements can beestimated� as for the Dobson instrument to be �� at the � � level�

Figure �� As Fig� � but for Brewer no ���

��

Figure �� As Fig� but for Brewer no ���

� Conclusion

A method to calculate total ozone from zenith sky observations �without and with clouds�with Dobson and Brewer spectrophotometers is developed� This method is easy to imple�ment and allows regular updating of the zenith sky charts of the instrument when moredata become available� It was shown that the uncertainty of the ZS observations is notmore than �� higher than for the DS observations for both instruments� Evaluationof the dierences between ZS and DS data as a function of time shows no systematicseasonal or long�term drift� Only the presence of high aerosol concentrations after majorvolcanic eruptions� like the one of the Mount Pinatubo in ����� may have decreased theagreement between ZS and DS observations�

Acknowledgements� The author is very grateful to Mr JC Grymonpont� Mr L Lebrun

and Mr A Massy of the technical sta� of the Royal Meteorological Institute of Belgium for the

numerous measurements with the spectrophotometers Thanks are also due to D Crommelynck�

D De Muer and A Jouko� for the constructive reviews of the manuscript

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References

Basher� R�E�� Review of the Dobson spectrophotometer and is accuracy� WMO Global

Ozone Research and Monitoring Project� Report No ��� Geneva� �����

De Backer H� and D� De Muer� Intercomparison of total ozone data measured with Dobsonand Brewer ozone spectrophotometers at Uccle �Belgium� from January ���� to March����� including zenith sky observations� J� Geoph� Res�� ��� ���������� �����

De Backer H�� Analysis and interpretation of ozone observations at Uccle ������������Ph� D� Thesis� Vrije Universiteit Brussel� �����

De Muer� D� and H� De Backer� Revision of � years of Dobson total ozone data atUccle �Belgium�� Fictitious Dobson total ozone trends induced by sulfur dioxide trends�J� Geoph� Res�� ��� ���� ���� �����

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Dobson� G�M�B�� Observer�s handbook for the ozone spectrophotometer� Ann� Int� geo�phys� Year� � ������ �� �b�

Jouko� A� and J� Tempels� Atmospheric turbidity and volcanic activity� Davos Interna�tional Pyrheliometer Comparison IPC VIII� �� Sep� �� Oct ����� Results and symposium�Swiss Meteorological Institute� Working Report No ���� Davos�Z�urich� ���� � �����

Komhyr� W�D�� Measurements of atmospheric ozone at Moosonee Canada � July �� �to July ��� ���� Can� Meteorol� Mem�� �� Meteorol� Brach� Department of Transport�Toronto� Ontario� Canada� �����

SCI�TEC� Brewer ozone spectrophotometer� Acceptance Manual� SCI�TEC InstrumentsInc�� Document number AM�BA�C �Rev C� �����