Variations in the size distribution of non‐sea‐salt sulfate aerosol in the marine boundary layer at Barbados: Impact of African dust

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Abstract
Four African mineral dust episodes occurred during a program of daily aerosol measurements at Barbados (13°15′N, 59°30′W) in April 1994. Non‐sea‐salt sulfate (nss SO4=) and dust were highly correlated (r2 = 0.93) and ranged from 0.5 to 4.2 μg m−3 and 0.9 to 257 μg m−3, respectively. Day‐to‐day variations in the size distributions of mineral dust and sea salt were relatively small. However, the coarse‐particle (aerodynamic diameter > 1 μm) fraction (CPF) of nss SO4= varied substantially, from 21% to 73%. The highest CPF SO4= values were associated with dust events; the lowest CPF was associated with the air mass from the central North Atlantic when the dust concentration was lowest, 0.9 μg m−3. We suggest that large CPF SO4= values are a consequence of SO2 in European pollutants that heterogeneously react with the suspended dust over North Africa. Nonetheless, the association of pollutants and dust does not always result in a large CPF; low CPF values suggest that SO2 may have been oxidized to SO4= prior to mixing with dust‐laden air. On those days when dust and pollution concentrations were low, the dominant source of nss SO4= was ascribed to oceanic dimethylsulfide (DMS) and the CPF remained close to 20%. Because such large variations can occur in the particle size distribution of nss SO4= in association with dust events, the role of mineral dust on nss SO4= size must be taken into account when estimating the impact of nss SO4= on radiation transfer in the atmosphere.