In situ aerosol characterization at Cape Verde: Part 1: Particle number size distributions, hygroscopic growth and state of mixing of the marine and Saharan dust aerosol

Particle number size distributions and hygroscopic properties of marine and Saharan dust aerosol were investigated during the SAMUM-2 field study at Cape Verde in winter 2008. Aitken and accumulation mode particles were mainly assigned to the marine aerosol, whereas coarse mode particles were composed of sea-salt and a variable fraction of Saharan mineral dust. A new methodical approach was used to derive hygroscopic growth and state of mixing for a particle size range (volume equivalent) from dp_ve = 26 nm to 10 μm. For hygroscopic particles with dp_ve < 100 nm, the median hygroscopicity parameter κ is 0.35. From 100 nm < dp_ve < 350 nm, κ increases to 0.65. For larger particles, κ at dp_ve = 350 nm was used. For nearly hydrophobic particles, κ is between 0 and 0.1 for dp_ve < 250 nm and decreases to 0 for dp_ve > 250 nm. The mixing state of Saharan dust in terms of the number fraction of nearly hydrophobic particles showed the highest variation and ranges from 0.3 to almost 1. This study was used to perform a successful mass closure at ambient conditions and demonstrates the important role of hygroscopic growth of large sea-salt particles.