Seasonal characteristics of tropical marine boundary layer air measured at the Cape Verde Atmospheric Observatory

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 year-round 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 CO 2 and CH 4 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 O 3 and anthropogenic non-methane hydrocarbons (NMHC) in air originating from North America. Ozone/CO ratios were highest (up to 0.42) in European air masses that contain relatively less well-aged air. In air heavily influenced by Saharan dust the O 3 /CO ratio was as low as 0.13, possibly indicating O 3 uptake to dust. Nitrogen oxides (NO x and NO y ) 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 HO 2 of 9 × 10 6 molecule cm -3 and 6 × 10 8 molecule cm -3 , respectively. After the primary photolysis source, the chemistry of IO and BrO, the abundance of HCHO, and aerosol uptake are important for the HO x budget in this region.3

Isotopic composition of atmospheric nitrate in a tropical marine boundary layer

“…Although the models appeared to calculate a more pronounced seasonality than the observations, it is probably insignificant considering the large uncertainty. On the other hand, the identical concentrations predicted by both models strongly support the premise that the prevailing marine conditions at the CVAO are also valid for the air masses transported to this site, in agreement with recent studies (23,27), making our observed seasonal trends weakly sensitive to transport and air mass origin. The best concentration match between observations and model predictions was actually found for a nitrate lifetime of ca.…”

“…The CVAO was established as an atmospheric observatory in 2006. Intensive and long-term studies conducted at the CVAO have already been reported (e.g., 23), and this well-characterized site allows for a thorough evaluation of the isotopic composition of nitrate in terms of the chemical state and physical state of the tropical MBL. This study combines seasonal variations of Δ 17 O and δ 15 N in the tropical MBL.…”

Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer

“…There are several inconsistencies in the observations of reactive iodine, especially under conditions representative of the open ocean: for example, Carpenter et al (2010) highlighted the discrepancy between observations of IO made by different groups at Cabo Verde, while Mahajan et al (2010) found that IO concentrations in the eastern Pacific did not agree with the observations of chlorophyll a and dissolved organic matter, and were much lower than the SCIAMACHY satellite observations. Additionally, the measured emission rates of iodinated VOCs are unable to explain the observed levels of IO and I 2 Jones et al, 2010b;Großmann et al, 2013;Lawler et al, 2014).…”

Aerosol time-series measurements over the tropical Northeast Atlantic Ocean: Dust sources, elemental composition and mineralogy

“…Aerosol sampling was undertaken at the Cape Verde Atmospheric Observatory (CVAO) (Carpenter et al, 2010) in the period between 2 July 2007 and 11 July 2008. The CVAO is situated on the northwest coast of the island of São Vicente (16° 51' N, 24° 52' W), around 800 km off the west-African coast.…”