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Cape Verde Observatory
-CO-
| 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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10 |
| Description |
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continuous |
| Sampling and analysis frequency |
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1 second sampling with 1 minute averages calculated and stored on external data logger. |
| Sampling environment |
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The concentration of CO in the Marine Boundary Layer is mainly controlled by the Hydroxyl radical (OH) concentration. Deviations occur as a result of long-range transport from more polluted areas and the occasional Biomass burning input from the Camary islands. |
| Description for sampling analysis |
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CO is sampled through 10m of 1" of teflon tubing followed by 3m of 1" stainless steel tubing and then finally 2m of 1/8" stainless steel tubing. The air passes through a nafion dryer (PERMAPURE) and a teflon filter before it enters the instrument.
From August 2010, the stainless steel inlet was replaced with a 40mm diameter heated glass inlet. The air then passes through 2m of 1/8" stainless steel tubing. The air passes through a nafion dryer (PERMAPURE) and a teflon filter before it enters the instrument.
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| Instrument and Analysis |
| Measurement method |
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VUV Fluorescence |
| Current status and history of instrument |
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The instrument (Aerolaser 5001) was manufactured in January 2006 and has been running continuously at the Observatory since October 2006. |
| Description of instrument |
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Measuring range: up to 100,000.00 ppbV
Detection limit: < 0.80 ppbV at Integration time = 10 s
Sensitivity: about 30 to 100 counts/ppbV
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| Calibration |
| Current scale employed in the measurement |
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NOAA-GMD/WMO 2004 |
| Measurement calibration |
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Calibration of CO is performed with reference to a single gas standard containing a known amount of CO. It is well understood that the instrument behaves linearly.
Calibrations are performed every 9 hours and the data is applied to the subsequent 9 hours of measurements. The data is quality assured and if step changes are observed in the concentration data (e.g. due to humidity effects) then data is recalibrated or rejected accordingly. Instrument parameters (temperatures, flows, and gas pressures) are logged alongside those of the concentrations and used to diagnose problems. Instrument sensitivity is monitored and when it drops below 15Hz/ppbV a thorough cleaning of the instrument is performed.
The carbon monoxide measurements are fully traceable to the international Global Atmospheric Watch (GAW) scale (currently NOAA/ESRL WMO-2000). This means that the gas calibration standards used to calculate the final concentrations are referenced to a common standard held at the NOAA/ESRL laboratory. To ensure this is upheld, the designated world calibration centre for Carbon Monoxide: WCC-Empa (http://www.empa.ch/plugin/template/empa/*/7571), carries out regular audits of the measurements using travelling standards.
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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| Scale and calibration(treasability) |
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13/04/2008 NOAA Filled 29/06/2007 CA07823 (219 ppbV)
An adjustment to the NOAA standard NOAA CA07823 200ppbV (manufactured
29/06/2007) has been applied to all the data between 13/04/08 and 17/11/09.
17/11/2009 BOC Filled 20/04/2010 116004 (1ppm).
26/11/2009 NOAA Filled 9/11/2008 CA08495 (221.3 ppbV)
CA08495 was returned and recalibrated at NOAA. An adjustment has not yet been made to the data between 26/11/09 and 23/11/12.
23/11/12 NOAA Filled Jun 12 CB09507 (232.2 ppbV)
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| Data Processing |
| Measurement unit |
: |
ppb |
| Data processing |
: |
Data Treatment
Zero data are removed and analysed off-line (average and standard deviation.
The calibration data is also removed (inf data, filter using sensitivity column). Sometimes not all the data is flagged as inf so needs to be checked. The database is checked for any other potential problems to the data.
During a power cut the logger records noise and so a very small signal is observed. This is not a background so does not need to be subtracted from the main data. Comparisons can always be made with the ad logged data if in doubt.
If the instrument has been off for any time it is allowed a two hour warm-up period and this data is removed.
If the instrument experiences a large change (> 5%) in sensitivity during a calibration (perhaps as a result of a water droplet finding its way into the instrument), this is seen in a sudden drop in concentration. Once the instrument has been calibrated again the data recovers. The data for this period is usually removed or else the appropriate calibration data is applied.
The Aerolaser instrument is not well insulated therefore is affected by sudden changes in temperature. Data when the laboratory temperature has changed rapidly during calibrations needs to be either removed or adjusted using calibration data which has been collected when the lab temperature is stable.
Considering the treatment of outliers, data <40ppbV and >200ppbV are removed before submission to database. A concentration of 250 ppbv may indicate lab air leak.
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| Processing for averaging |
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Processing for Hourly Data:
Zero, calibration and invalidated data is removed prior to averaging. Hourly data is obtained by averaging all the data within a 1 hour period for example between 00:00 - 01:00. Standard deviation and number of points used to obtain the average are provided.
Processing for Daily Data:
Zero, calibration and invalidated data is removed prior to averaging. Daily data is obtained by averaging all the data within a 24 hour period for example between 01/01/14 00:00 - 02/01/2014 01:00. Standard deviation and number of points used to obtain the average are provided.
Processing for Monthly Data:
Zero, calibration and invalidated data is removed prior to averaging. Daily data is obtained by averaging all the data within a 24 hour period for example between 01/01/14 00:00 - 01/02/2014 01:00. Standard deviation and number of points used to obtain the average are provided. |
| 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 |
| Data remarks |
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| Other Information |
| Scientific aim |
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To monitor the background concentration of CO (along with other trace gases) in the Tropical Marine Boundary Layer to gain increased understanding of the oxidation capacity in this region. |
| Reference |
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The designated world calibration centre for Carbon Monoxide: WCC-Empa (http://www.empa.ch/plugin/template/empa/*/7571), carries out regular audits of the measurements using travelling standards.
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
Read, K.A. et al., Extensive halogen-mediated ozone destruction over the tropical Atlantic Ocean, Nature, 453, 1232-1236, 2008.
Read, K. A., J. D. Lee, A. C. Lewis, S. J. Moller, L. Mendes, and L. J. Carpenter, Intra-annual cycles of NMVOC in the tropical marine boundary layer and their use for interpreting seasonal variability in CO, J. Geophys. Res., 114, D21303, doi:10.1029/2009JD011879, 2009.
Cape Verde Atmospheric Observatory Webpage: https://www.ncas.ac.uk/index.php/en/cvao-home |
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| submitted by University of York |
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