||HUmic-LIke Substances (HULIS) is a class of poorly defined organic compounds isolated from aerosols and cloud/fog droplets. Although the bulk-level physiochemical properties such as water affinity, surface activity, light absorption and reactivity of HULIS have drawn our attention for several years, as an unresolved complex mixture, the chemical characterization of HULIS remains rudimentary in comparison with other aerosol constituents, which hamper our meaningful and quantitative understandings on their sources and structure-property relationships. Work in this thesis aims to improve the scientific understanding on the abundance, composition and environmental impacts of HULIS, with special emphasis on the aerosols of PRD region. Multiple analytical approaches were employed for HULIS characterization and the main findings are summarized below: (1) An analytical method for isolation and determination of HULIS in aerosol samples has been characterized and exhibits its advantages. By this method, the concentration of HULIS was determined for various types of aerosol samples. Atmospheric HULIS was abundant in the aerosol of PRD region, accounting for ~60% of the water-soluble organic carbon and ~10% of fine particle mass. The size distribution of HULIS was characterized by a dominant droplet mode, a minor condensation mode and a tiny coarse mode, which indicated that soil-derived HULIS was a very minor source. In the particles in the range of 0.32-1.8 μm, the abundance of HULIS was 40-90% of the combined abundance of sulfate and ammonium, suggesting that HULIS should be considered when quantifying the role of sulfate aerosols serving as CCN. (2) The amount of HULIS was also determined in various fresh biomass burning (BB) smoking particles and a relative constant emission ratio of HULIS/OC was measured under different burning conditions. The influence of BB on ambient HULIS levels was investigated by examining the spatial and temporal variation of HULIS and other aerosol constituents and interspecies relations in ambient PM2.5. Significant correlation between HULIS and water-soluble K+ concentrations supported that BB was an important source of HULIS. HULIS also correlated well with sulfate, oxalate, and oxidant (the sum of O3 and NO2), implying that secondary formation was also an important source of HULIS in the atmosphere. The urban-suburban spatial gradient of HULIS was opposite to that of elemental carbon, negating vehicular emissions as a significant source of HULIS. (3) The HULIS fraction of aerosol samples collected in harvest season at a rural location of PRD were analyzed by electrospray ionization (ESI) coupled with a high resolution mass spectrometer (HRMS). With the remarkable resolving power and mass accuracy of HRMS, thousands of individual peaks were detected and assigned with unambiguous elemental formulas. Tandem MS measurements were performed someone select intensive ions to get structural information. Besides the typical CHO molecular series indicative of carboxyl, carbonyl and/or hydroxyl groups, nitrogen and sulfur containing organic compounds likely contribute a large percentage of organic matters in the HULIS fraction. Alkaloids, amino acids and their derivatives were presumed to be the major reduced nitrogen organic matters, which mainly came from biomass burning emissions. Secondary organic aerosol components such as ogranosulfate and nitrooxy-organosulfate were identified to be among constituents of HULIS. These molecule tracers reconfirm that biomass burning and SOA formation are two important sources of HULIS in this region. (4) A cell-free dithiothreitol (DTT) assay was used to measure ROS production mediated by HULIS. Under conditions of DTT consumption not exceeding 90%, the HULIS-catalyzed oxidation of DTT follows the zero-order kinetics with respect to DTT concentration and the rate of DTT oxidation is proportional to the dose of HULIS. The ROS activity of the aerosol HULIS, on a per unit mass basis is 2% of the ROS activity by a reference quinone compound, 1,4-naphthoquinone and exceeds that of two tested aquatic fulvic acids. The HULIS fraction in the ambient samples tested exhibits comparable ROS activities to the organic solvent extractable fraction, which would contain compounds such as quinones, a known organic compound class capable of catalyzing generation of ROS in cells. HULIS was found to be the major redox active constituent of the water-extractable organic fraction in PM. It is plausible that HULIS contains reversible redox sites, thereby serving as electron carriers to catalyze the formation of ROS. This work suggests that HULIS could be an active PM component in generating ROS and further work is warranted to characterize its redox properties. This thesis work is the first study reporting abundance and major sources of atmospheric HULIS in the Pearl River Delta Region. This thesis research also for the first time reveals that HULIS were among aerosol constituents that are linked to generation of reactive oxygen species, providing important information for health effect studies of particulate matter. In addition, data on the molecular level elemental composition of HULIS obtained through ultra-high resolution mass spectrometric analysis provides valuable scientific basis for further elucidating chemical and physical properties of HULIS and organic aerosol in general.