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.


Publication single view


Title: Measurement and modelling of tropospheric reactive halogen species over the tropical Atlantic Ocean
Authors: A.S. Mahajan, J.M.C. Plane, H. Oetjen, L. Mendes, R.W. Saunders, A. Saiz-Lopez, C.E. Jones, L.J. Carpenter and G.B. McFiggans
Journal: Atmos. Chem. Phys.
Year: 2010
Volume: 10
Pages: 4611
DOI: 10.5194/acp-10-4611-2010
Web URL:
Abstract: Although tropospheric reactive halogen chemistry is well studied in coastal and polar environments, the presence of halogens over the open ocean environment has not been widely reported. The impacts of halogens on the tropical open ocean marine boundary layer (MBL), in particular, are not well characterised. This paper describes observations of iodine monoxide (IO) and bromine oxide (BrO) over eight months in the tropical open ocean MBL, on the north-eastern side of São Vicente (Cape Verde Islands, 16.85° N, 24.87° W). The highest BrO mixing ratio observed was 5.6±1 pmol mol-1, while the maximum observed IO mixing ratio was 3.1±0.4 pmol mol-1. The average values seen between 09:00–17:00 GMT were ~2.8 pmol mol-1 for BrO and ~1.5 pmol mol-1 for IO; these averages showed little variability over the entire campaign from November 2006 to June 2007. A 1-dimensional chemistry and transport model is used to study the evolution of iodine species and quantify the combined impact of iodine and bromine chemistry on the oxidising capacity of the MBL. It appears that the measured fluxes of iodocarbons are insufficient to account for the observed levels of IO, and that an additional I atom source is required, possibly caused by the deposition of O3 onto the ocean surface in the presence of solar radiation. Modelling results also show that the O3 depletion observed at Cape Verde cannot be explained in the absence of halogen chemistry, which contributes ~45% of the observed O3 depletion at the height of measurements (10 m) during summer. The model also predicts that halogens decrease the hydroperoxy radical (HO2) concentration by ~14% and increase the hydroxyl radical (OH) concentration by ~13% near the ocean surface. The oxidation of dimethyl sulphide (DMS) by BrO takes place at a comparable rate to oxidation by OH in this environment. Finally, the potential of iodine chemistry to form new particles is explored and conditions under which particle formation could be important in the remote MBL are discussed.

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