Long-term chemical characterization of tropical and marine aerosols at the Cape Verde Atmospheric Observatory (CVAO) from 2007 to 2011

Abstract

Abstract. The first long-term aerosol sampling and chemical characterization results from measurements at the Cape Verde Atmospheric Observatory (CVAO) on the island of São Vicente are presented and are discussed with respect to air mass origin and seasonal trends. In total 671 samples were collected using a high-volume PM10 sampler on quartz fiber filters from January 2007 to December 2011. The samples were analyzed for their aerosol chemical composition, including their ionic and organic constituents. Back trajectory analyses showed that the aerosol at CVAO was strongly influenced by emissions from Europe and Africa, with the latter often responsible for high mineral dust loading. Sea salt and mineral dust dominated the aerosol mass and made up in total about 80% of the aerosol mass. The 5-year PM10 mean was 47.1 ± 55.5 μg m−2, while the mineral dust and sea salt means were 27.9 ± 48.7 and 11.1 ± 5.5 μg m−2, respectively. Non-sea-salt (nss) sulfate made up 62% of the total sulfate and originated from both long-range transport from Africa or Europe and marine sources. Strong seasonal variation was observed for the aerosol components. While nitrate showed no clear seasonal variation with an annual mean of 1.1 ± 0.6 μg m−3, the aerosol mass, OC (organic carbon) and EC (elemental carbon), showed strong winter maxima due to strong influence of African air mass inflow. Additionally during summer, elevated concentrations of OM were observed originating from marine emissions. A summer maximum was observed for non-sea-salt sulfate and was connected to periods when air mass inflow was predominantly of marine origin, indicating that marine biogenic emissions were a significant source. Ammonium showed a distinct maximum in spring and coincided with ocean surface water chlorophyll a concentrations. Good correlations were also observed between nss-sulfate and oxalate during the summer and winter seasons, indicating a likely photochemical in-cloud processing of the marine and anthropogenic precursors of these species. High temporal variability was observed in both chloride and bromide depletion, differing significantly within the seasons, air mass history and Saharan dust concentration. Chloride (bromide) depletion varied from 8.8 ± 8.5% (62 ± 42%) in Saharan-dust-dominated air mass to 30 \\textpm 12% (87 ± 11%) in polluted Europe air masses. During summer, bromide depletion often reached 100% in marine as well as in polluted continental samples. In addition to the influence of the aerosol acidic components, photochemistry was one of the main drivers of halogenide depletion during the summer; while during dust events, displacement reaction with nitric acid was found to be the dominant mechanism. Positive matrix factorization (PMF) analysis identified three major aerosol sources: sea salt, aged sea salt and long-range transport. The ionic budget was dominated by the first two of these factors, while the long-range transport factor could only account for about 14% of the total observed ionic mass.

Impacts of bromine and iodine chemistry on tropospheric OH and HO<sub>2</sub>: comparing observations with box and global model perspectives

“…In this work we use a value of γ HO 2 = 0.1 in order to maintain consistency with previous modelling studies at the site and to account for potential impacts of ions of copper and iron in aerosol particles influenced by mineral dust Müller et al, 2010;Fomba et al, 2014;Matthews et al, 2014;Lakey et al, 2015).…”

How well can we quantify dust deposition to the ocean?

“…Duce et al [18] Mahowald [31] Aerosol 2010 Aerosol 2011 7 Be-norm. Patey [40] Fomba [33] dust flux (g m -2 yr -1 ) Figure 2. Dust deposition in the Eastern Tropical North Atlantic Ocean estimated from surface aerosol concentrations.…”

“…Grey line is the dust deposition calculated from the average (July 2007-July 2008) aerosol Al concentration measured at the Cape Verde Atmospheric Observatory (CVAO, 16.87°N, 24.87°W [40]) assuming 8% Al in dust and an aerosol settling velocity of 1000 m day −1 . Tan line is the dust deposition calculated from the 5-year average (2007-2011) aerosol dust concentration measured at the CVAO [33] assuming an aerosol settling velocity of 1000 m day −1 . Blue triangles represent annual average desert dust deposition from the reference model of Mahowald et al [31].…”

“…Dust deposition is calculated to be 9.5 g m −2 yr −1 using the 5-year (2007-2011) average aerosol dust concentration of Fomba et al [33] and 13.2 g m −2 yr −1 using the average aerosol Al concentration reported by Patey et al [40] for the period July 2007-July 2008, assuming 8% Al in the dust (figure 2). The larger dust deposition estimated for 2007-2008 reflects the greater aerosol concentrations during that year compared to the full 5-year record [33]. These estimates are consistent with dust deposition in this region in the reference model (figure 2).…”

Bathypelagic particle flux signatures from a suboxic eddy in the oligotrophic tropical North Atlantic: production, sedimentation and preservation

“…Typically, mineral dust flux correlates with the satellitebased annual aerosol optical index . High dust fluxes have been found at the oligotrophic EU-MELI site far north of the CVOO (Bory et al, 2001 Fomba et al (2014). The mooring is equipped with a set of core sensors for hydrography (temperature, salinity sensors at different depth), currents (profiling in the upper 100 m and single rotor current meter (RCM-8) instruments at approximately 600, 1300, and 3400 m depth), and oxygen (typically two single sensors at 50 and 180 m depth).…”