One year of, almost continuous, measurements of aerosol optical properties and chemical composition were performed at the outskirts of Praia, Santiago Island, Cape Verde, within the framework of CV-DUST (Atmospheric aerosol in Cape Verde region: seasonal evaluation of composition, sources and transport) research project, during 2011. This article reports the aerosol number and mass concentration measurements using a GRIMM Optical Aerosol Spectrometer that provides number size discrimination into 31 size ranges from 0.25 to 32 μm. Time series of 5 min average PM10 concentrations revealed peak values higher than 1000 μg.m-3 during winter dust storm events originating over Northern Africa. The 24 hours average concentrations exceeded the World Health Organization (WHO) guidelines for PM2.5 and PM10 in 20% and 30% of the 2001 days, respectively. Annual average mass concentrations (±standard deviation) for PM1, PM2.5 and PM10 were 5±5, 19±21 and 48±64 μg.m-3, respectively. The annual PM2.5 and PM10 values were also above the limits prescribed by the WHO (10 and 20 μg.m-3, respectively). The aerosol mass size distribution revealed two main modes for particles smaller than 10 μm: a fine mode (0.7-0.8 μm), which possibly results of gas to particle conversion processes; and a coarse mode with maxima at 3-4 μm, which is associated with desert dust and sea salt sources. Within the coarse mode two sub-modes with maxima at 5-6 μm and 10-12 μm were frequently present.