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.
Highlights
The interest in research on atmospheric aerosols is limited to heavily polluted megacities and other strongly anthropogenically polluted areas; concerns naturally mobilized dust and sea salt aerosols which are in the focus of marine chemistry, biology and atmospheric chemistry
(B) The air mass spent less than 48 h over the ocean in the last 96 h before arriving at Cape Verde Atmospheric Observatory (CVAO) and originated from the African continent, crossing over the Saharan, urban sites (Nouakchott, Dakar, etc.) as well as biomass burning regions through the Mauritanian upwelling region to the CVAO (22.2 % of all samples)
The higher amounts of Saharan dust together with anthropogenic gaseous and particulate compounds from the African continent are responsible for the winter elevated PM10 mass concentrations, while marine and non-African air mass inflow were responsible for low mass loadings
Summary
The interest in research on atmospheric aerosols is limited to heavily polluted megacities and other strongly anthropogenically polluted areas; concerns naturally mobilized dust and sea salt aerosols which are in the focus of marine chemistry, biology and atmospheric chemistry The investigation of the role of mineral dust in the ocean has been the focus of a number of research and ship cruises along the tropical Atlantic Ocean (Bates et al, 2001; Chen and Siefert, 2003; Allan et al, 2009) These measurements have mostly focused on short-term measurements during intensive field campaigns that last for 3-6 weeks, making predictions about seasonal variability and long-term understanding of atmospheric processes quite difficult. Such long-term data sets have been often requested (Mahowald et al, 2005), but only a few data exist for the region of the tropical North Atlantic (e.g., Kandler et al, 2007; Chiapello et al, 1995). Related works from the CVAO include first investigations from short-term experiments of PM characterization (Fomba et al, 2013; Müller et al, 2010) and of specific organic single compounds (Müller et al, 2009)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
