Size distributions of hydrophilic and hydrophobic fractions of water-soluble organic carbon in an urban atmosphere in Hong Kong

Water-soluble organic carbon (WSOC) is a significant part of ambient aerosol and plays an active role in contributing to aerosol's effect on visibility degradation and radiation budget through its interactions with atmospheric water. Size-segregated aerosol samples in the range of 0.056-18 gm were collected using a ten-stage impactor sampler at an urban site in Hong Kong over one-year period. The WSOC samples were separated into hydrophilic (termed WSOC_h) and hydrophobic fractions (i.e., the humic-like substances (HULIS) fraction) through solid-phase extraction procedure. Carbon in HULIS accounted for 40 +/- 14% of WSOC. The size distribution of HMS was consistently characterized in all seasons with a dominant droplet mode (46-71%) and minor condensation (9.0-18%) and coarse modes (20-35%). The droplet mode had a mass median aerodynamic diameter in the range of 0.7-0.8 mu m. This size mode showed the largest seasonal variation in abundance, lowest in the summer (0.41 mu g/m(3)) and highest in the winter (3.3 mu g/m(3)). WSOC_h also had a dominant droplet mode, but was more evenly distributed among different size modes. Inter-species correlations within the same size mode suggest that the condensation-mode HULLS was partly associated with combustion sources and the droplet mode was strongly associated with secondary sulfate formation and biomass burning particle aging processes. There is evidence to suggest that the coarse-mode HULIS largely originated from coagulation of condensation-mode HULIS with coarse soil/sea salt particles. The formation process and possible sources of WSOC_h was more complicated and multiple than HULIS and need further investigation. Our measurements indicate that WSOC components contributed a dominant fraction of water-soluble aerosol mass in particles smaller than 032 mu m while roughly 20-30% in the larger particles. (C) 2017 Elsevier Ltd. All rights reserved.