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Abstract:
Observations of the tropical atmosphere are fundamental to the understanding of
global changes in air quality, atmospheric oxidation capacity and climate, yet the tropics are
under-populated with long-term measurements. The first three years (October 2006–September
2009) of meteorological, trace gas and particulate data from the global WMO/Global
Atmospheric Watch (GAW) Cape Verde Atmospheric Observatory Humberto Duarte Fonseca (CVAO; 16° 51′ N, 24° 52′ W) are presented, along with a characterisation of the origin and
pathways of air masses arriving at the station using the NAME dispersion model and
simulations of dust deposition using the COSMO-MUSCAT dust model. The observations
show a strong influence from Saharan dust in winter with a maximum in super-micron aerosol
and particulate iron and aluminium. The dust model results match the magnitude and daily
variations of dust events, but in the region of the CVAO underestimate the measured aerosol
optical thickness (AOT) because of contributions from other aerosol. The NAME model also
captured the dust events, giving confidence in its ability to correctly identify air mass origins
and pathways in this region. Dissolution experiments on collected dust samples showed a
strong correlation between soluble Fe and Al and measured solubilities were lower at high
atmospheric dust concentrations. Fine mode aerosol at the CVAO contains a significant fraction
of non-sea salt components including dicarboxylic acids, methanesulfonic acid and aliphatic
amines, all believed to be of oceanic origin. A marine influence is also apparent in the yearround presence of iodine and bromine monoxide (IO and BrO), with IO suggested to be confined mainly to the surface few hundred metres but BrO well mixed in the boundary layer.
Enhanced CO2 and CH4 and depleted oxygen concentrations are markers for air-sea exchange
over the nearby northwest African coastal upwelling area. Long-range transport results in
generally higher levels of O3 and anthropogenic non-methane hydrocarbons (NMHC) in air
originating from North America. Ozone/CO ratios were highest (up to 0.42) in relatively fresh
European air masses. In air heavily influenced by Saharan dust the O3/CO ratio was as low as
0.13, possibly indicating O3 uptake to dust. Nitrogen oxides (NOx and NOy) show generally
higher concentrations in winter when air mass origins are predominantly from Africa. High
photochemical activity at the site is shown by maximum spring/summer concentrations of OH
and HO2 of 9×106 molecule cm−3 and 6×108 molecule cm−3, respectively. After the primary
photolysis source, the most important controls on the HOx budget in this region are IO and BrO chemistry, the abundance of HCHO, and uptake of HOx to aerosol.
