Cap Verde

The Cape Verde Atmospheric Observatory (CVAO), at Calhau on the island of São Vicente. The CVAO is a World Meteorological Organisation-Global Atmospheric Watch (WMO-GAW) global station and provides quality-assured atmospheric data.

An Aero-Laser AL5001 instrument for the measurement of carbon monoxide mixing ratios in background air has been installed at the National Centre for Atmospheric Science (NCAS) Cape Verde Atmospheric Observatory (CVAO) since October 2006 and there are no plans for the measurements to stop in the foreseeable future. At the University of York we also have an AL5002 instrument which is used for shorter deployments (e.g. for NAMBLEX and OP3 and has in the past been used as a back-up for both the Cape Verde system and the FAAM aircraft system. 

The aim of the project is 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.  The CVAO site is a “Global” Global Atmospheric Watch site which means that it meets the requirements to provide data required to address environmental issues of global scale and importance.

Some of these requirements include the following:

  1. The station location is regionally representative and is normally free of the influence of significant local pollution sources.
  2. There are adequate power, air conditioning, communication and building facilities to sustain long term observations with greater than 90% data capture (i.e. <10% missing data). 
  3. The GAW CO observation made is of known quality and linked to the GAW CO Primary Standard. 
Inside the CVAO station.

CO data is presently submitted in near-real-time to the MACC (Monitoring Atmospheric Composition and Climate) project which is part of the European GMES (Global Monitoring for Environment and Security) programme. 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 Canary Islands.

Instrument rack containing an AL5001 CO-monitor.

Publications

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Article

Title: Intra-annual cycles of NMVOC in the tropical marine boundary layer and their use for interpreting seasonal variability in CO
Authors: K.A. Read, J.D. Lee, A.C. Lewis, S.J. Moller, L. Mendes and L.J. Carpenter
Journal: J. Geophys. Res.
Year: 2009
Volume: 114
Pages: D21303
DOI: 10.1029/2009JD011879
Web URL: http://www.agu.org/pubs/crossref/2009/2009JD011879.shtml
Abstract: Carbon monoxide and nonmethane volatile organic compounds (NMVOC) have been measured for the first time on a continuous basis in the tropical marine boundary layer of the Atlantic Ocean. CO and ethane, which have similar lifetimes with respect to OH degradation, vary intra-annually with the sinusoidal variability expected due to a primary hydroxyl radical (OH) sink, but with CO showing a smaller cycle amplitude. The ethane measurements were used to derive the seasonal variation in the nominal OH concentration (n[OH]) experienced along the air mass trajectories arriving at Cape Verde. The n[OH] represents the variability in both the true OH concentration and any intra-annual changes in ethane emissions, and was subsequently used to simulate the seasonal cycle of CO, allowing identification of potentially differing sources and sinks from those of ethane. Deviation of the observed CO from the n[OH] fit indicated that summer sources of CO are approximately 60% (30–68%) higher than winter, assuming values of ±8% and ±49% seasonal variability, respectively, in fossil fuel and biomass burning emissions of CO and ethane, and including −29% uncertainty due to the losses of ethane through reaction with chlorine atoms. The evidence suggests that secondary production of CO from the oxidation of methane (CH4) and NMVOC and in particular from methanol, acetone and acetaldehyde may be dominant summer CO sources in this region.

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