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: Year-round measurements of nitrogen oxides and ozone in the tropical North Atlantic marine boundary layer
Authors: J.D. Lee, S.J. Moller, K.A. Read, A.C. Lewis, L. Mendes and L.J. Carpenter
Journal: J. Geophys. Res.
Year: 2009
Volume: 114
Pages: D21302
DOI: 10.1029/2009JD011878
Web URL: http://www.agu.org/pubs/crossref/2009/2009JD011878.shtml
Abstract: A highly sensitive chemiluminescence instrument has been deployed to measure nitric oxide (NO) and nitrogen dioxide (NO2) at the Cape Verde Atmospheric Observatory in the remote tropical North Atlantic marine boundary layer (MBL). Using two different methods, the instrument was assessed to have a detection limit of around 1.8 parts per trillion by volume (pptv) for NO and 5.5 pptv for NO2 for hour-long integration periods. The overall accuracy was estimated at ∼18% for NO and 30% for NO2. Measurements of NO, NO2, and ozone (O3) over a period of 12 months in 2007 show very low levels of NOx (typically <30 pptv) and net daytime ozone destruction on most days of the measurement period. Air originating over Africa exhibited the highest levels of NOx (∼35 pptv) and reduced daily O3 destruction, with O3 production observed on a few days. Air that had not originated over Africa showed lower NOx levels (∼25 pptv), with greater observed O3 destruction. A dependence of the observed O3 destruction on NO mixing ratios, averaged over all air masses, was observed and reproduced using a simple box model. The model results imply that the presence of between 17 and 34 pptv of NO (depending on the month) would be required to turn the tropical North Atlantic from an O3 destroying to an O3 producing regime.

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