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Cape Verde Observatory - University of York
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CO
continuous |
NOx
continuous |
O3
continuous |
VOCs *
continuous |
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| Category |
Air sampling observation |
| Sampling Type |
continuous |
| Sampling Height/Depth |
5 |
| Sampling and Analysis Frequency |
Measurement of both NO and NO + NO2 occurs on the same instrument channel. The chemiluminesence analyser runs on a 10 minute measurement cycle, 5 minutes of the signal received from NO and 5 minutes of the signal from NO + NO2. Each 5 minute period consists of 150 s of pre-chamber zero readings, followed by 150 s of NO signal. Here we just report NOx (NO + NO2). |
| Sampling Environment |
Except from local source emissions from passing ships, the level of NOx in the Cape Verde region (tropical marine boundary layer) is mainly influenced by emissions in the northern hemisphere. There, emissions of NOx are dominated by anthropogenic sources from urban or industrial regions, with little or no emission from oceanic or uninhabited areas. Oxidation of emitted NOx to nitric acid (HNO3), peroxyacetyl nitrate (PAN) and other alkyl nitrate compounds occurs in less than a day in the continental boundary layer, therefore NOx levels in remote regions are usually very low (<100 pptv).
In the presence of NO, VOC oxidation leads to catalytic production of O3, where peroxy radicals are recycled to OH, leading to further VOC oxidation. At higher levels of NOx, such as those found in urban or industrials areas of Europe or North America, OH radicals can be lost via reaction with NO2 to produce HNO3, which can be a reservoir species for NOx. At low levels of NOx, peroxy radicals self-react leading to net destruction of O3 via its photodissociation in the presence of water vapour. It is the competition between the photochemical loss and production of O3 that leads to a non-linear dependency of O3 production on NOx with the O3 production efficiency being greater for lower NOx levels.
A particularly crucial NOx range is 5 – 100 pptv, which depending on the time of year, location in the troposphere and levels of O3 and H2O, contains the so called O3 compensation point, a level of NOx at which there is a net production of O3. |
| Measurement Method |
Chemiluminescence |
| Current status and history of Instruments |
Measurements obtained since October 2006.
Single channel Chemiluminescence instrument manufactured by Air Quality Design (AQD) Inc, Colorado, USA. |
| Description of Instruments |
This technique is highly selective and sensitive, and has a linear response over a wide dynamic range (1 ppt – 500 ppb).
The NO2 converter used in the set-up at CVAO utilises a solid-state light source with a narrow band of emission wavelengths to achieve interference-free NO2 to NO conversion.Typically the NO2 to NO conversion efficiency of this set-up is between 35 - 40 %, with a gas resonance time in the irradiated area of around 1 second. The instrument runs on a 10 minute measurement cycle of which 5 minutes is with the LEDs off followed by 5 minutes with the LEDs on. Each 5 minute period consists of 150 s of pre-chamber zero readings, followed by 150 s of NO signal.
The instrument sensitivity is determined every 37 hrs when the instrument is calibrated. The sensitivity to NO varies from 2.2 – 3.1 counts s-1 pptv-1.
The Limit of Detection (LOD) can either be determined from the zero count rate variability, directly related to the photon counting of the detector; or by considering the signal scatter of the processed 10 minute nighttime data (little or no local pollution so assume all NO is converted to NO2 during transport). These methods give values of 1.5 pptv and 1.7 pptv respectively and so we quote the LOD of NO as below 2 pptV.
The accuracy of the measurement considers the Sample and calibration gas mass flow controller accuracies, the concentration of the NO standard, and the error in determining the NO artifact which gives a total uncertainty of 23 %. The NO2 artifact is assessed and combined with the extra uncertainty of the conversion of NO2 to NO, means that the overall uncertainty for NO2 measurements is around 30%.
We report measurements of NOx (NO+NO2) |
| Time Zone |
UTC |
| Data Period |
2006-10-01 - 2008-12-31 |
| Data Type |
daily, monthly |
| Parameter detail |
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| Other Descriptions for Sampling and Analyses |
10 m 1/4 inch teflon tubing. Filter fitted on inlet. |
| Scientific Aim |
To obtain one of the first sets of longterm measurements of NOx in the tropical marine boundary layer, a region previously data sparse.
NOx levels and therefore tropospheric O3 destruction, are dependent on air mass origin, with higher NOx observed in air from the African continent compared to Atlantic oceanic air masses. In model simulations calculated for average annual conditions, the O3 compensation point would occur at about 25 pptv NO, which is higher than early measurements suggest (7-8 pptv). Data taken at the observatory in subsequent years should allow a more complete investigation into the O3 compensation point and hence how the O3 destroying capability of the region may change with possible future changing NOx. |
| Supporting Contributor(s) |
University of York, UK |
| Last update |
2009-03-25 |
| Situation |
ongoing |
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This site is maintained by the Japan Meteorological Agency
in cooperation with the World Meteorological Organization
(Created : 2001/07/02 Modified : 2010/07/31)
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WMO World Data Centre for Greenhouse Gases
c/o Japan Meteorological Agency
1-3-4, Otemachi, Chiyoda-ku
Tokyo 100-8122, Japan
| Tel: |
+81-3-3287-3439 |
| Fax: |
+81-3-3211-4640 |
| E-mail: |
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WDCGG Data Submission and Dissemination Guide (PDF 1.2Mbyte)
WDCGG leaflet (PDF 2.7MByte, July 2008)