Enrichment of submicron sea-salt-containing particles in small cloud droplets based on single-particle mass spectrometry

The effects of the chemical composition and size of sea-salt-containing particles on their cloud condensation nuclei (CCN) activity are incompletely understood. We used a ground-based counterflow virtual impactor (GCVI) coupled with a single-particle aerosol mass spectrometer (SPAMS) to characterize chemical composition of submicron (dry diameter of 0.2-1.0 mu m) and supermicron (1.0-2.0 mu m) sea-salt-containing cloud residues (dried cloud droplets) at Mount Nanling, southern China. Seven cut sizes (7.5-14 mu m) of cloud droplets were set in the GCVI system. The highest number fraction of sea-salt-containing particles was observed at the cut size of 7.5 mu m (26%, by number), followed by 14 mu m (17%) and the other cut sizes (3%-5%). The submicron sea-salt-containing cloud residues contributed approximately 20% (by number) at the cut size of 7.5 mu m, which was significantly higher than the percentages at the cut sizes of 8-14 mu m (below 2%). This difference was likely involved in the change in the chemical composition. At the cut size of 7.5 mu m, nitrate was internally mixed with over 90% of the submicron sea-salt-containing cloud residues, which was higher than sulfate (20%), ammonium (below 1%), amines (6%), hydrocarbon organic species (2%), and organic acids (4%). However, at the cut sizes of 8-14 mu m, nitrate, sulfate, ammonium, amines, hydrocarbon organic species, and organic acids were internally mixed with > 90%, > 80%, 39%-84%, 71%-86%, 52%-90%, and 32%-77% of the submicron sea-salt-containing cloud residues. The proportion of sea-salt-containing particles in the supermicron cloud residues generally increased as a function of cut size, and their CCN activity was less influenced by chemical composition. This study provided a significant contribution towards a comprehensive understanding of sea-salt CCN activity.