Comunicación presentada en: EUBREWNET Training School and Brewer Ozone Spectrophotometer Open Congress junto con el 14th Biennial WMO-GAW Brewer Users Group Meeting celebrado del 24 al 28 de marzo de 2014 en Santa Cruz de Tenerife

Rodríguez Franco, Juan José

English

Agencia Estatal de Meteorología

Brewer AOD Calibration and Retrieval in theUV-BJ. J. Rodrguez-FrancoShort Term Scientic Mission COST-ES1207-140215-055087April 22, 2015IntroductionMotivationOutlineAerosol Optical Depth AlgorithmAlgorithm DescriptionRBCC-E Langley MethodologyWORCC { RBCC-E AlgorithmsPolarization SensitivityMethodologyPolarization ResultsCimel ComparisonAOD Calibration TransferConclusionsIntroduction 2 / 28Underlying GoalA Short Term Scientic Mission (STSM) was conducted at the PhysicalMeteorological Observatory / World Radiation Centre (PMOD/WRC),Davos, Switzerland, from 15th to 22th February, 2015, with the goals:Continue and complete the development of astandard methodology to transfer optical depth(AOD) calibration factors between BrewerspectrophotometersThe establishment of a common algorithmapplicable to the EUBREWNET network tocompute AODThis will contribute to standardize the Brewer aerosols measurementswithin the scope of the Eubrewnet network.Introduction 3 / 28OutlineAerosol Optical Depth algorithmAlgorithm descriptionRBCC-E LangleyComparing World Optical Depth Research and Calibration Center(WORCC { PMOD/WRC) and Regional Brewer Calibration Center forEurope (RBCC-E { AEMet) AOD algorithmsSolar zenith angle (sza) dependence. Quartz Window (QW)Brewer's polarization eectComparing Brewer AOD with co-located Cimel dataAOD Calibration Transfer: rst resultsIntroduction 4 / 28IntroductionMotivationOutlineAerosol Optical Depth AlgorithmAlgorithm DescriptionRBCC-E Langley MethodologyWORCC { RBCC-E AlgorithmsPolarization SensitivityMethodologyPolarization ResultsCimel ComparisonAOD Calibration TransferConclusionsAerosol Optical Depth Algorithm 5 / 28AlgorithmFi ← Fi(cy, it)Count RatesFi ← logFi(dt)× 104Photons per secondFi ← Fi + (PC + TCi)× TFi ← Fi +AFp,iWorking with individual DS measurementsTemp. Corr.ND Corr.1. Standard Data Reduction2. ND lters correction: attenuationscalculated from the  routine3. Remove the Earth{Sun distance annualcycle (eccentricity)4. Airmass factor5. Calculate the atmospheric extinctiondue to Ozone6. Calculate the atmospheric extinctiondue to Rayleigh scattering7. Outliers rejection: standard deviationfor the ve ozone and AOD values arebelow 2.5 DU and 0.02, respectivelyAerosol Optical Depth Algorithm 6 / 28AlgorithmFi ← Fi(cy, it)Count RatesFi ← logFi(dt)× 104Photons per secondFi ← Fi + (PC + TCi)× TFi ← Fi +AFp,iWorking with individual DS measurementsTemp. Corr.ND Corr.1. Standard Data Reduction2. ND lters correction: attenuationscalculated from the  routine3. Remove the Earth{Sun distance annualcycle (eccentricity)4. Airmass factor5. Calculate the atmospheric extinctiondue to Ozone6. Calculate the atmospheric extinctiondue to Rayleigh scattering7. Outliers rejection: standard deviationfor the ve ozone and AOD values arebelow 2.5 DU and 0.02, respectivelyAerosol Optical Depth Algorithm 6 / 28AlgorithmD =1.000110 + 0.034221 cos(T )+ 0.001280 sin(T )+ 0.000719 cos(2T )+ 0.000077 sin(2T )1. Standard Data Reduction2. ND lters correction: attenuationscalculated from the  routine3. Remove the Earth{Sun distance annualcycle (eccentricity)4. Airmass factor5. Calculate the atmospheric extinctiondue to Ozone6. Calculate the atmospheric extinctiondue to Rayleigh scattering7. Outliers rejection: standard deviationfor the ve ozone and AOD values arebelow 2.5 DU and 0.02, respectivelyAerosol Optical Depth Algorithm 6 / 28Algorithmµ(Θ) = 1√1− (R+r)2 sin2(Θ)(R+h)2Air Mass factorh = 22 km h = 5 kmOzone Rayleigh1. Standard Data Reduction2. ND lters correction: attenuationscalculated from the  routine3. Remove the Earth{Sun distance annualcycle (eccentricity)4. Airmass factor5. Calculate the atmospheric extinctiondue to Ozone6. Calculate the atmospheric extinctiondue to Rayleigh scattering7. Outliers rejection: standard deviationfor the ve ozone and AOD values arebelow 2.5 DU and 0.02, respectivelyAerosol Optical Depth Algorithm 6 / 28AlgorithmO3(λ)Ext = O3 ×O3(λ)XSec × µO3(Θ)1. Standard Data Reduction2. ND lters correction: attenuationscalculated from the  routine3. Remove the Earth{Sun distance annualcycle (eccentricity)4. Airmass factor5. Calculate the atmospheric extinctiondue to Ozone6. Calculate the atmospheric extinctiondue to Rayleigh scattering7. Outliers rejection: standard deviationfor the ve ozone and AOD values arebelow 2.5 DU and 0.02, respectivelyAerosol Optical Depth Algorithm 6 / 28AlgorithmR(λ)scatt = R(λ)coeff × p1013.25 × µR(Θ)1. Standard Data Reduction2. ND lters correction: attenuationscalculated from the  routine3. Remove the Earth{Sun distance annualcycle (eccentricity)4. Airmass factor5. Calculate the atmospheric extinctiondue to Ozone6. Calculate the atmospheric extinctiondue to Rayleigh scattering7. Outliers rejection: standard deviationfor the ve ozone and AOD values arebelow 2.5 DU and 0.02, respectivelyAerosol Optical Depth Algorithm 6 / 28Algorithm1. Standard Data Reduction2. ND lters correction: attenuationscalculated from the  routine3. Remove the Earth{Sun distance annualcycle (eccentricity)4. Airmass factor5. Calculate the atmospheric extinctiondue to Ozone6. Calculate the atmospheric extinctiondue to Rayleigh scattering7. Outliers rejection: standard deviationfor the ve ozone and AOD values arebelow 2.5 DU and 0.02, respectivelyAerosol Optical Depth Algorithm 6 / 28AlgorithmFinally, we compute the Aerosol Optical Depth as follows (we are nottaking into account the atmospheric extinction due to SO2. As well, weassume that aod = R):Aerosol Optical Depth Equationaod() =1aodf[log(I0())  log(I())]   O3()Ext1000 log(10)  R()scatt  log(10)gAerosol Optical Depth Algorithm 7 / 28Langley MethodSeveral steps are taken to reduce known variations from the Langley data:The ozone column and standard deviation are computed on groups ofve individual DS measurements. Data is accepted if the standarddeviation is lower than 2.5 DU (Brewer cloud-screening method).The number of such individual DS for a langley event must be atleast 100 (i.e. 20 summaries).We removed ozone values lower (greater) than 100 DU (600 DU)from the data set.We limit the analysis to half-days (airmass range 1.15-3.75) withstable Ozone and little AOD variability (std(AOD) < 0:02).We use robust linear regression by means of an iterative linear-t(Harrison and Michalsky [1994])Aerosol Optical Depth Algorithm 8 / 28Langley MethodAerosol Optical Depth Algorithm 9 / 28We applied a physically-based method to screen cloudy half-days(since March 2009) from the data set based on a modied Long andAckerman [2000] clear sky detection algorithm (Garcia, R.D., [2014]).We dene (reject) sample outliers as those ETC values more than1.5 standard deviations from the sample mean.Langley MethodThe Brewer spectrophotometer shows a good stability in time of the AODcalibration, within 2% (0.02 in AOD).Aerosol Optical Depth Algorithm 10 / 28WORCC { RBCC-ENext we compare the AOD as obtained from both the RBCC-E and theWORCC algorithm. The rst step was to select a good day forindependently ETC determination through the zero-air mass extrapolationmethod.We look for clear days matching the following conditions:total ozone half-day variation less than 2.5 DUlow aerosol optical depth (AOD at 340nm < 0.1) and a diurnalSTD(AOD) < 0:05The linear regression is performed on the [1.1 - 3.0] air mass rangeAerosol Optical Depth Algorithm 11 / 28WORCC { RBCC-EDierent degrees of agreement between the RBCC-E and the WORCCalgorithms are observed, depending on which instrument we analyze.07:12 09:36 12:00 14:24 16:48 19:120.20.250.30.350.40.450.50.55Natural Day 130, 2014, IZO, Brewer IZO#157Aerosol Optical Depth  AOD RBCC-E AOD WORCC04/21 04/26 05/01 05/06 05/11-0.01-0.008-0.006-0.004-0.00200.0020.0040.0060.0080.01AOD absolute differencesIZO#157: AODWORCC-AODRBCC-E (Slit#6)  A possible cause for the observed discrepancies would be dierentcalibration constants used for data-processing, including the calibrationfactors independently calculated through the Langley method.Aerosol Optical Depth Algorithm 12 / 28WORCC { RBCC-EDierent degrees of agreement between the RBCC-E and the WORCCalgorithms are observed, depending on which instrument we analyze.07:12 09:36 12:00 14:24 16:48 19:120.020.030.040.050.060.070.080.090.10.11Natural Day 130, 2014, IZO, Brewer IZO#183Aerosol Optical Depth  AOD RBCC-E AOD WORCC04/21 04/26 05/01 05/06 05/1100.0050.010.0150.020.0250.030.035AOD absolute differencesIZO#183: AODWORCC-AODRBCC-E (Slit#6)  A possible cause for the observed discrepancies would be dierentcalibration constants used for data-processing, including the calibrationfactors independently calculated through the Langley method.Aerosol Optical Depth Algorithm 12 / 28WORCC { RBCC-EDierent degrees of agreement between the RBCC-E and the WORCCalgorithms are observed, depending on which instrument we analyze.07:12 09:36 12:00 14:24 16:48 19:1200.010.020.030.040.050.060.070.080.09Natural Day 130, 2014, IZO, Brewer IZO#185Aerosol Optical Depth  AOD RBCC-E AOD WORCC04/21 04/26 05/01 05/06 05/1100.0050.010.0150.020.025AOD absolute differencesIZO#185: AODWORCC-AODRBCC-E (Slit#6)  A possible cause for the observed discrepancies would be dierentcalibration constants used for data-processing, including the calibrationfactors independently calculated through the Langley method.Aerosol Optical Depth Algorithm 12 / 28WORCC { RBCC-EDierent degrees of agreement between the RBCC-E and the WORCCalgorithms are observed, depending on which instrument we analyze.07:12 09:36 12:00 14:24 16:48 19:1200.010.020.030.040.050.060.070.080.09Natural Day 130, 2014, IZO, Brewer IZO#185Aerosol Optical Depth  AOD RBCC-E AOD WORCC04/21 04/26 05/01 05/06 05/1100.0050.010.0150.020.025AOD absolute differencesIZO#185: AODWORCC-AODRBCC-E (Slit#6)  A possible cause for the observed discrepancies would be dierentcalibration constants used for data-processing, including the calibrationfactors independently calculated through the Langley method.Aerosol Optical Depth Algorithm 12 / 28WORCC { RBCC-EWe found large AOD discrepancies for the Brewer #157, independently ofthe AOD algorithm used.00.050.10.150.2Brewer AOD(320 nm)  IZO#157 IZO#183 IZO#18507:12 08:24 09:36 10:48 12:00 13:12 14:24 15:36 16:48 18:00 19:12260265270275280285290RBCC-E Triad, Izana, 24th Sep. (267), 2014Ozone [DU]We will not be able to ensure an AOD calibration unless we cantake all the instruments into a reasonably agreement (within lessthan 0.02 in AOD).Aerosol Optical Depth Algorithm 13 / 28WORCC { RBCC-EWe found large AOD discrepancies for the Brewer #157, independently ofthe AOD algorithm used.00.050.10.150.2Brewer AOD(320 nm)  IZO#157 IZO#183 IZO#18507:12 08:24 09:36 10:48 12:00 13:12 14:24 15:36 16:48 18:00 19:12260265270275280285290RBCC-E Triad, Izana, 24th Sep. (267), 2014Ozone [DU]We will not be able to ensure an AOD calibration unless we cantake all the instruments into a reasonably agreement (within lessthan 0.02 in AOD).Aerosol Optical Depth Algorithm 13 / 28WORCC { RBCC-EAfter analyzing the Langley residuals data, we thought of the Brewerpolarization sensitivity as responsible for AOD discrepancies.1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3-0.1-0.08-0.06-0.04-0.0200.020.040.060.080.1Ozone air mass factorRes. to Langley, log(pps): AM + PM11th Nov. (315), 2013, IZO  IZO#157IZO#183IZO#1851 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3-0.1-0.08-0.06-0.04-0.0200.020.040.060.080.1Ozone air mass factorRes. to Langley, log(pps): AM + PM24th Sep. (267), 2014, IZO  IZO#157IZO#183IZO#185The next step was to analyze the polarization curve for each triad member.Aerosol Optical Depth Algorithm 14 / 28IntroductionMotivationOutlineAerosol Optical Depth AlgorithmAlgorithm DescriptionRBCC-E Langley MethodologyWORCC { RBCC-E AlgorithmsPolarization SensitivityMethodologyPolarization ResultsCimel ComparisonAOD Calibration TransferConclusionsPolarization Sensitivity 15 / 28Brewer PolarizationTwo polarization sensitive elements were found in the Brewer:1. The at quartz window (QW) as the rst optical element, mountedat an angle of 35 with respect to the horizontal plane2. The internal diraction gratingPolarization Sensitivity 16 / 28Brewer PolarizationTwo polarization sensitive elements were found in the Brewer:1. The at quartz window (QW) as the rst optical element, mountedat an angle of 35 with respect to the horizontal plane2. The internal diraction gratingPolarization Sensitivity 16 / 28Brewer PolarizationTwo polarization sensitive elements were found in the Brewer:1. The at quartz window (QW) as the rst optical element, mountedat an angle of 35 with respect to the horizontal plane2. The internal diraction gratingPolarization Sensitivity 16 / 28MethodologySolar direct irradiance measurements at several UV wavelengths werecollected for solar zenith angles (SZA) 80 to 30 with and withoutthe quartz window (QW)We used a normal DS measurement for measurements through theQW, while the same routine renamed to DK was used to retrieve solardirect irradiance measurements without the QW.We followed the same procedure described in [Cede et al., 2006] toobtain the polarization curve (eld measurements, method 4)We tested an experimental setup designed to characterize the Brewerpolarization sensitivity during routine Brewer intercomparisons.This allowed us to get some insight into the dierences betweendierent QWsPolarization Sensitivity 17 / 28Polarization ResultsWe observed a similar change in Brewer spectrophotometer sensitivity withSZA as in previous studies (Bais et al, 2005, Cede et al, 2006).30 35 40 45 50 55 60 65 70 75 80-10-5051015Sensitivity Change [%]Solar Zenith AngleIzana, 12th to 13th March, 2015. Experimental QW  slit#0slit#2slit#3slit#4slit#5slit#6Brewer IZO#157slit#0slit#2slit#3slit#4slit#5slit#6Brewer IZO#185On the contrary, we observed a clear wavelength dependence in the SZAdependence for SZA > 65.Polarization Sensitivity 18 / 28Polarization ResultsLarge dierences are found between dierent QW's, which, in principle,invalidate the experimental setup designed to characterize the instrument'spolarization sensitivity during routine Brewer intercomparisons.30 35 40 45 50 55 60 65 70 75 80-20-15-10-5051015Sensitivity Change [%]Solar Zenith AngleIzana, 17th March, 2015  slit#0slit#2slit#3slit#4slit#5slit#6IZO#157: Exp.QWslit#0slit#2slit#3slit#4slit#5slit#6IZO#157: Orig. QWPolarization Sensitivity 19 / 28Polarization ResultsWe correct solar direct irradiance measurements for SZA dependence andcalculate new calibration factors from the corrected data set.Fit the polarization data to a 6-degree polynomialCalculate the zero air mass factorUsing the new calibration factors we achieved a very good agreementbetween the RBCC-E Brewer TriadPolarization Sensitivity 20 / 28slit#0slit#2slit#3slit#4slit#5slit#620 30 40 50 60 70 800.850.90.9511.051.11.151.21.251.31.35QW sza correction FactorSolar zenith angle  -0.02-0.015-0.01-0.00500.0050.010.0150.020.025Brewer  #157: 17th March, 2015, Izaæa. Original QWResiduals to polarization curvePolarization ResultsWe correct solar direct irradiance measurements for SZA dependence andcalculate new calibration factors from the corrected data set.Fit the polarization data to a 6-degree polynomialCalculate the zero air mass factorUsing the new calibration factors we achieved a very good agreementbetween the RBCC-E Brewer TriadPolarization Sensitivity 20 / 281 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3-0.1-0.08-0.06-0.04-0.0200.020.040.060.080.1Air mass factorRes. to Langley, log(pps): AM + PM Brewer #157: 24th Sep. (267), 2014, IZO  Polarization ResultsWe correct solar direct irradiance measurements for SZA dependence andcalculate new calibration factors from the corrected data set.Fit the polarization data to a 6-degree polynomialCalculate the zero air mass factorUsing the new calibration factors we achieved a very good agreementbetween the RBCC-E Brewer TriadPolarization Sensitivity 20 / 2807:12 08:24 09:36 10:48 12:00 13:12 14:24 15:36 16:48 18:00 19:1200.010.020.030.040.050.060.070.080.090.1Aerosol Optical DepthRBCC-E Brewer Triad: 24th Sep. (267), 2014, IZO  IZO#157 IZO#183 IZO#185Cimel ComparisonThe AOD measurements at 320 nm derived with the Brewerspectrophotometer were compared with the AOD at 340 nm from a Cimelsun-photometer.0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.400.050.10.150.20.250.30.350.4AOD Brewer (320 nm)AOD CIMEL (340 nm)Brewer #185 1st August - 20th October, 2014  y=1.054 x+ -0.034 r=0.992  The two data sets are highly correlated (r=0.992)An oset of 0.03 is observed.Polarization Sensitivity 21 / 28Cimel ComparisonThe AOD dierences between the Brewer and the Cimel appear to becorrelated to the AOD measured by the Brewer.The better agreement is found for high AOD levels (about AOD >0.02).0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4-0.0100.010.020.030.040.050.06AOD Brewer320 - AOD CIMEL340AOD CIMEL (340 nm)Brewer #185 1st August - 20th October, 201400.050.10.150.20.250.30.350.40.45Brewer #185: August, 2015, IzanaAOD  08/24 08/25 08/26 08/27 08/28 08/29 08/30 08/31 09/01CIMEL (340 nm)Brewer (320 nm)Polarization Sensitivity 22 / 28IntroductionMotivationOutlineAerosol Optical Depth AlgorithmAlgorithm DescriptionRBCC-E Langley MethodologyWORCC { RBCC-E AlgorithmsPolarization SensitivityMethodologyPolarization ResultsCimel ComparisonAOD Calibration TransferConclusionsAOD Calibration Transfer 23 / 28Calibration TransferThe Aerosol calibration transfer method is done similar to the ozone one:the (espectral) extraterrestrial constants are obtained by comparison withthe reference brewer using near-simultaneous AOD measurements. Bydoing  refaod = instaod , and after solving for log(I0()), we getlog(I0()) = log(I()) + refaod () instaod+O3()ext1000 log(10)+R()scatt  log(10)AOD Calibration Transfer 24 / 28Using the simultaneous AOD data from the reference instrument, thespectral ETCs can be derived for each near-simultaneous [ refaod ; instaod ] pairand then averaged.We have used a time window of5 minutes for near-simultaneousAOD measurementsaod range to be used is aninput to the algorithmMeasurements such thatjszaref   szainstj > sza0 will beremoved from the analysisAOD Calibration Transfer 25 / 281 1.5 2 2.519.719.7219.7419.7619.7819.8Air Mass factorETC=τrefaodµaod+log(Iλ)+O3λ+RayλeETC = 371723757EC#145 ETC Transfer from RBCC-E reference IZO#185: Slit#2instaod < 3szasync = 0:003AOD Calibration Transfer 26 / 28Data from Brewer #145 collected at the Iza~na Observatory during theperiod from 20th to 30th April, 2014 were chosen to test the calibrationtransfer algorithm, using the Brewer #185 as a reference.04/19 04/21 04/23 04/25 04/27 04/29 05/01-0.04-0.03-0.02-0.0100.010.020.030.04Brewer #145 - Brewer #185AOD absolute diferencesUsing the transferred AOD calibration factors we achieved a goodagreement between both spectrophotometers, within 0:02.IntroductionMotivationOutlineAerosol Optical Depth AlgorithmAlgorithm DescriptionRBCC-E Langley MethodologyWORCC { RBCC-E AlgorithmsPolarization SensitivityMethodologyPolarization ResultsCimel ComparisonAOD Calibration TransferConclusionsConclusions 27 / 28ConclusionsWe found some inconsistencies when checking the RBCC-E AODalgorithm against the WORCC algorithm. AOD deviations were of theorder of 0.025 in the worst caseA possible cause for the observed AOD discrepancies should bedierent calibration constants used for data-processingThe Brewer quartz window does not necessarily show the same SZAdependence for dierent instruments. Correcting for the polarizationeect can be a key factor to obtain reliable AOD measurementsA preliminary methodology to transfer the absolute calibration ofdirect spectral irradiance measurements from a reference standard toother instruments has been developed.Conclusions 28 / 28

Brewer AOD Calibration and Retrieval in the UV-B

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Brewer AOD Calibration and Retrieval in the UV-B

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