Ground-based measurements on aerosol particles at Cape Verde (Sep-Oct 2017)

In the framework of the MarParCloud (Marine biological production, organic aerosol particles and marine clouds: a Process Chain) project, measurements were carried out on the islands of Cape Verde, to investigate the abundance, properties, and sources of aerosol particles in general and cloud condensation nuclei (CCN) in particular, both close to sea and cloud level heights. A thorough comparison of particle number concentration (PNC), particle number size distribution (PNSD) and CCN number concentration (NCCN) at the Cape Verde Atmospheric Observatory (CVAO, sea level station) and Monte Verde (MV, cloud level station) reveals that during times without clouds the aerosol at CVAO and MV are similar and the boundary layer is generally well mixed. Therefore, data obtained at CVAO can be used to describe the aerosol particles at cloud level. Cloud events were observed at MV during roughly 58% of the time and during these, a large fraction of particles were activated to cloud droplets. A trimodal parameterization method was deployed to characterize PNC at CVAO. Based on number concentrations in different aerosol modes, four well separable types of PNSDs were found, which were named the marine type, mixture type, dust type1 and dust type2. Aerosol particles differ depending on their origins. When the air masses came from the Atlantic Ocean, sea spray can be assumed to be one source for particles, besides for new particle formation. For these air masses, PNSDs featured the lowest number concentration in Aitken, accumulation and coarse mode. Particle number concentrations for the sea spray aerosol (SSA, i.e., the coarse mode for these air masses) accounted for about 3.7% of NCCN,0.30% (CCN number concentration at 0.30% supersaturation) and about 1.1% to 4.4% of Ntotal (total particle number concentration). When the air masses came from the Saharan desert, we observed enhanced Aitken, accumulation and coarse mode particle number concentrations and overall increased NCCN. NCCN,0.30% during the strongest observed dust periods is about 2.5 times higher than that during marine periods. However, the particle hygroscopicity parameter κ for these two most different periods shows no significant difference and is generally similar, independent of air mass. Overall, κ averaged 0.28, suggesting the presence of organic material in particles. This is consistent with previous model work and field measurement. There is a slight increase of κ with increasing particle size, indicating the addition of soluble, likely inorganic material during cloud processing.