Abundance and sources of hydrophilic and hydrophobic water-soluble organic carbon at an urban site in Korea in summer

Abstract

In this study, the characteristics of total water-soluble organic carbon (WSOC) and isolated WSOC fractions were examined to gain a better understanding of the pathway of organic aerosol production. 24 h PM_2.5 samples were collected during the summer (July 28–August 28, 2009) at an urban site in Korea. A glass column filled with XAD7HP resin was used to separate the filtered extracts into hydrophilic (WSOC_HPI) and hydrophobic (WSOC_HPO) fractions. The origins of air mass pathways arriving at the sampling site were mostly classified into three types, those originating over the East Sea of Korea that passed over the eastern inland urban and industrial regions (type I); those from the marine (western/southwestern/southern marine) and passed over the national industrial complex regions (type II); and those from northeastern China that passed through North Korea and metropolitan areas of South Korea (type III). Measurements showed an increase in the average WSOC fraction of total OC from the type II to III air mass (53 to 64%) periods. Also, higher SO₄²⁻/SO_x (=SO₂ + SO₄²⁻) was observed in the type III air mass (0.70) than those in the types I (0.49) and II (0.43). According to the average values of WSOC/OC and SO₄²⁻/SO_x, measurements suggest that the aerosols collected during the type III air mass period were more aged or photo-chemically processed than those during the types I and II air mass periods. The relationship between the SO₄²⁻/SO_x and WSOC/OC (R² = 0.64) suggests that a significant fraction of the observed WSOC at the site could be formed by an oxidation process similar to SO₄²⁻ aerosols, probably the oxidation process using OH radicals, or in-cloud processing. The photochemical production of WSOC_HPO was also observed to significantly contribute to the total OC.