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Cape Verde Observatory
-O3-
| Observation |
| Category |
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Air sampling observation (Marine boundary layer) |
| Situation |
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ongoing |
| Time zone |
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UTC |
| Sampling |
| Sampling height |
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7.5 |
| Description |
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continuous |
| Sampling and analysis frequency |
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1 minute |
| Sampling environment |
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Tropospheric ozone is produced via the catalytic oxidation of carbon compounds in the presence of nitrogen oxides (NOx = NO + NO2), and has an additional smaller source from stratospheric influx of ozone into the free troposphere. Ozone is lost to the surface due to deposition and can be destroyed throughout the atmosphere by photochemical processes, predominantly via photolysis and the subsequent reaction of electronically excited oxygen atoms with water vapour. Ozone is formed predominantly in continental regions where there are sources of NOx, and is typically lost in marine regions where sources are small. Due to its high water vapour content, enhanced radiation field and large geographical extent, the tropical marine boundary layer is the most important global region for loss of ozone. |
| Description for sampling analysis |
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Ozone is sampled through 5m through 2m of heated 1/4" teflon tubing at a flow rate of 1.5 L/min. The air passes through a teflon filter before it enters the instrument.
From August 2010, the sample is taken from the main heated glass sampling inlet (40mm diameter). Then the air passes through 2m of 1/4" teflon tubing at a flow rate of 1.5 L/min. The air passes through a teflon filter before it enters the instrument.
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| Instrument and Analysis |
| Measurement method |
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Light absorption analysis (UV) |
| Current status and history of instrument |
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The instrument (Thermo Scientific UV Ozone Analyser model 49C, S/N:0536414593) ) was new in January 2006 ran continuously at the Observatory between October 2006 and August 2010 as the main instrument. (Instrument 1)
A new instrument (Thermo Scientific UV Ozone Analyser model 49i, S/N:0708621224) was installed 14/02/09 - 31/10/09 whilst the original 49i was being repaired. (Instrument 2)
A new instrument (Thermo Scientific TEI 49i S/N: 0730525149) was installed 01/03/2010 - present (Instrument 4).
Instrument 1 finally came back to the UK for repair in August 2010 and was replaced with Instrument 2 (Thermo Scientific TEI 49i). Instrument 2 is now run as a duplicate instrument for instrument 4.
The majority of data submitted is now from Instrument 4. If Instrument 4 ever has problems data is submitted from Instrument 2. The instruments are calibrated every 3 months and their atmospheric data is checked to make sure that it agrees within <5% (but usually<2%). |
| Description of instrument |
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http://www.thermo.com/com/cda/product/detail/1,1055,10122762,00.html
Range: 0-500 ppb Detection Limit: 0.5 ppb
Since it is an absolute measurement technique, the accuracy of the instrument is purely determined by the random error (or precision) and there is no assumed systematic error. The measurements show a 1 sigma standard deviation of 0.9 ppb for 10 second averaged data which, allowing for the dual cell photometer set-up, equates to a precision of 0.07 ppb for hourly averaged data. |
| Calibration |
| Current scale employed in the measurement |
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TEI49PSI Calibrated against the NIST calibrated standard in 2007, May 2010, May 2011, April 2013.
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| Measurement calibration |
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Zero
The instrument zero is checked every 2 weeks using an external scrubber (activated charcoal) on the inlet (20 minutes with the last 10 minutes of data being used) and as part of the calibration procedure. Results from this are recorded but the instrument is not rezeroed unless it is clear that there is a problem with the instrument, i.e. if the standard deviation of the zero data is large.
The standard difference between two SRPs measuring zero air should not exceed (root 2 x 0.25 ppbV) = 0.49 ppbV (1 sigma), 1 minute averaging time.
Since it is an absolute measurement technique, the accuracy of the instrument is purely determined by the random error (or precision) and there is no assumed systematic error. The routine zero data (1 minute) is used to calculate the error on the data (√2σ) and if this is > 3 ppbV for minute data then the data is flagged.
To give the precision for hourly averaged data (√2σ)/ (√60).
Calibrations
External calibrations are done using a photometer standard (2b model 606) every 4 months.
The transfer lines are flushed through with at least 2ppm Ozone for 1 hour before calibrating.
The instrument is fed with a range of in sequential levels of ozone from the calibrator (0, 25, 50, 75, 100, 125, 150 ppbv) for a 20 minute period. The last 10 minute averages are used for the calibration plot. This data is logged using the usual DAQ factory logging software. The coefficient is measured but only recorded and not applied to the instrument. The zero is not adjusted at this time.
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| Scale and calibration(treasability) |
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This instrument is almost always run alongside a second duplicate instrument. Two independent ozone instruments agreeing within 3% can replace the need for frequent calibrations.
A transfer standard (CVTS), 2B model 606, is used to calibrate the ozone instrument and this is done every 3-4 months during routine maintenance trips to the Cape Verde Atmospheric Observatory. The CVTS is checked against the NIST calibrated transfer standard (TS) and brought in-line with it before and after each Cape Verde trip.
The TS (Transfer Standard – TS Model 49I-PS S/N: 0703820627) is held at the University of York, and is itself calibrated annually using the primary standard reference photometer #2 (SRP#2) from the National Physical Laboratory (NPL) (traceable through the National Institute of Standards and Technology (NIST). 3% uncertainty is acceptable for reference to standards e.g. to the SRP.
Instrument parameters (e.g. cell intensities, temperatures, flows) are logged alongside those of the concentrations and used to diagnose problems.
The ozone measurements are fully traceable to the international Global Atmospheric Watch (GAW) scale (NIST Standard reference photometer SRP#2). This means that the ozone photometer used to generate ozone for the calibrations is referenced to a photometer held at NIST. To ensure this is upheld, the designated world calibration centre for Ozone: WCC-Empa (http://www.empa.ch/plugin/template/empa/*/7571), carries out regular audits of the measurements using travelling photometers.
For more information please see the report from the most recent of these audits:
http://www.wmo.int/pages/prog/arep/gaw/documents/CVO_2012.pdf
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| Data Processing |
| Measurement unit |
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ppb |
| Data processing |
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Data from the two instruments is compared and periods of disagreement (>5%) are evaluated using the database resource (i.e. inlet filter changes, instrument maintenance etc). Zero data is removed and analysed off-line (standard deviation, precision of instrument calculated, see earlier). Calibration data is removed. Calibration data is also analysed off-line and coefficient and intercept data is recorded. The database is checked for any other potential problems to the data.
After the instrument has been off for a significant power cut (>12 hours) we remove data for the first hour of operation. Considering the treatment of outliers, data <10 ppbV and >80 ppbV is removed before submission.
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| Processing for averaging |
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Processing for Hourly Data:
Zero, calibration data and any data which is invalid is removed before averaging. Hourly data is obtained by averaging all the data within a 1 hour period for example between 00:00 - 01:00.
Processing for Daily Data:
Zero, calibration data and any data which is invalid is removed before averaging. Daily data is obtained by averaging all the data within a 1 day period for example between 01/01/2014 00:00 - 02/01/2014 00:00.
Processing for Monthly Data:
Zero, calibration data and any data which is invalid is removed before averaging. Monthly data is obtained by averaging all the data within a 1 month period for example between 01/01/2014 00:00 - 01/02/2014 00:00. |
| Data flag |
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Error Flag = 0 Good data
Error Flag = 1 Reduced quality data
Error Flag = 2 Below detection limit
Error Flag = 3 Invalid or missing data
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| Data remarks |
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| Other Information |
| Scientific aim |
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To monitor the (increasing) background concentration of Ozone in the tropical Marine Boundary Layer, a region characterised by high water vapour (high OH), and high radiation. This data is particularly important for the validation of climate models. At present the region is an efficient daytime destroyer of Ozone but this is highly dependent on NOx concentrations which may increase over time leading to the region becoming less efficient at this process. |
| Reference |
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The designated world calibration centre for Ozone: WCC-Empa (http://www.empa.ch/plugin/template/empa/*/7571), carries out regular audits of the measurements using travelling photometers.
For more information please see the report from the most recent of these audits:
http://www.wmo.int/pages/prog/arep/gaw/documents/CVO_2012.pdf
Ozone TEI49PSI calibrator information can be found on http://www.ncas.ac.uk/fgam/index.php?option=com_content&task=view&id=185&Itemid=1
http://www.thermo.com/com/cda/product/detail/1,1055,10122762,00.html
Some publications using the ozone data:
Seasonal observations of OH and HO2 in the remote tropical marine boundary layer, Vaughan et al., Atmos. Chem. Phys.,12, 2012, 2149-2172
Seasonal characteristics of tropical marine boundary layer air measured at the Cape Verde Atmospheric Observatory L.J. Carpenter et al. , J. Atmos. Chem. DOI 10.1007/s10874-011-9206-1, 2010.
Measurement and modelling of reactive halogen species over the tropical Atlantic Ocean. Mahajan, A. S. et all. Atmos. Chem. Phys. 10, 2010, 4611-4624
The Chemistry of OH and HO2 Radicals in the Boundary Layer over the Tropical Atlantic Ocean Whalley, L. K. et al. Atmos. Chem. Phys., 10, 1555-1576, 2010
Pollution-enhanced Cl chemistry in the eastern tropical Atlantic boundary layer. Lawler, M.J. et al. Geophys. Res.Lett. 36, L08810, 2009
Read, K.A. et al., Extensive halogen-mediated ozone destruction over the tropical Atlantic Ocean, Nature, 453, 1232-1236, 2008.
Lee, J. D., S. J. Moller, K. A. Read, A. C. Lewis, L. Mendes, and L. J. Carpenter, Year-round measurements of nitrogen oxides and ozone in the tropical North Atlantic marine boundary layer, J. Geophys. Res., 114, D21302, doi:10.1029/2009JD011878, 2009.
Cape Verde Webpage: https://www.ncas.ac.uk/index.php/en/cvao-home
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| submitted by University of York |
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