Chemical characterization of marine sub-micrometer aerosol particles during the MERIAN cruise MSM18/3 from the Cape Verde island Sao Vicente to Gabun (Atlantic Ocean)

Sub-micron marine aerosol particles (PM1) were collected during the MERIAN cruise MSM 18/3 between 22 June 2011 and 21 July 2011 from the Cape Verde island Sao Vicente to Gabun crossing the tropical Atlantic Ocean and passing equatorial upwelling areas. According to air mass origin and chemical composition of the aerosol particles, three main regimes could be established. Aerosol particles in the first part of the cruise were mainly of marine origin, in the second part was marine and slightly biomass burning influenced (increasing tendency) and in the in last part of the cruise, approaching the African mainland, biomass burning influences became dominant. Generally aerosols were dominated by sulfate (caverage = 1.99 µg/m**3) and ammonium ions (caverage = 0.72 µg/m**3) that are well correlated and slightly increasing along the cruise. High concentrations of water insoluble organic carbon (WISOC) averaging 0.51 µg/m**3 were found probably attributed to the high oceanic productivity in this region. Water soluble organic carbon (WSOC) was strongly increasing along the cruise from concentrations of 0.26 µg/m**3 in the mainly marine influenced part to concentrations up to 3.3 µg/m**3 that are probably caused by biomass burning influences. Major organic constituents were oxalic acid, methansulfonic acid (MSA) and aliphatic amines. MSA concentrations were quite constant along the cruise (caverage = 43 ng/m**3). While aliphatic amines were more abundant in the first mainly marine influenced part with concentrations of about 20 ng/m**3, oxalic acid showed the opposite pattern with average concentrations of 12 ng/m**3 in the marine and 158 ng/m**3 in the biomass burning influenced part. The alpha dicarbonyl compounds glyoxal and methylglyoxal were detected in the aerosol particles in the low ng/m**3 range and followed oxalic acid closely. MSA and aliphatic amines accounted for biogenic marine (secondary) aerosol constituents whereas oxalic acid and the alpha dicarbonyl compounds were believed to result mainly from biomass burning. N-alkane concentrations increased along the cruise from 0.81 to 4.66 ng/m**3, PAHs and hopanes were abundant in the last part of the cruise (caverage of PAHs = 0.13 ng/m**3, caverage of hopanes = 0.19 ng/m**3). Levoglucosan was identified in several samples of the last part of the cruise in concentrations around 2 ng/m**3, pointing to (aged) biomass burning influences. The investigated organic compounds could explain 9.5% of WSOC in the mainly marine influenced part (dominating compounds: aliphatic amines and MSA) and 2.7% of WSOC in the biomass burning influenced part (dominating compound: oxalic acid) of the cruise.