Size distribution characteristics of ambient aerosol in urban Hong Kong : major chemical components and light extinction estimation

Abstract

Aerosol size distribution as a key property of ambient aerosols, plays a very important role in understanding properties, formation process and visibility degradation. Humic-like substances (HULIS) as the hydrophobic part (carbon in humic-like substances, HULIS-C) of water soluble organic carbon (WSOC) has been increasingly concerned recently due to its abundance and associated importance in atmospheric activities. From August 2013 to August 2014, one-year size-segregated aerosol samples in the range of 0.056-18 μm were collected using an impactor sampler at an urban site in Tsuen Wan, Hong Kong. HULIS, WSOC and other major aerosol components - organic carbon (OC), elemental carbon (EC) and ion species were measured in the study. A tri-modal size distribution with a dominant droplet mode was observed for OC, EC, WSOC, HULIS and several ion species (NH₄⁺, SO₄^2-, K⁺ and oxalate) with mass median aerodynamic diameter (MMAD) of 0.6-0.9 μm. For HULIS, the coarse mode (MMAD 4.2-5.2 μm) made up a larger fraction (26%) than the condensation mode (13%, MMAD 0.16-0.23 μm) on year average. The droplet-mode HULIS was more predominant in winter (77%, 3.6 μg m^-3) when total HULIS mass concentration was highest. The hydrophilic part of WSOC (WSOC_h) was obtained by subtracting HULIS-C from WSOC and was more evenly distributed in spring and summer compared with HULIS. The correlation coefficient results between HULIS, WSOC_h and other species in three modes suggested secondary formation involving in-cloud processing and particle growth from condensation mode to droplet mode through heterogeneous reactions were the major formation pathways for both droplet-mode HULIS and WSOC_h. Ship emission and fresh sulfate formation were the possible major sources for the condensation-mode HULIS. Coarse-mode HULIS was probably formed from coagulation, resuspension with soil particles. While the formation processes and possible sources of WSOC_h were more complicated and multiple than HULIS and need further investigation. A second objective of this thesis work is to take advantage of the size distribution data to estimate light the extinction coefficient (b_ext) using a modified Mie calculation method. The results showed that estimated b_ext agreed well with the observed b_ext and particles ranging from 0.32 to 1.8 μm contributed most to the total b_ext.

Date of Award	2015
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology