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Size distributions of elemental carbon, organic carbon, and polycyclic aromatic hydrocarbons in ambient aerosols of the Pearl River Delta region

Authors Yu, Huan
Issue Date 2009
Summary Knowledge of size distributions of Elemental carbon (EC) and polycyclic aromatic hydrocarbons (PAHs) is essential when studying aerosol health effects, aerosol light extinction, the sources and deposition of atmospheric aerosols, and regional/global climate. This thesis work focuses on the determination of size distribution characteristics of EC, organic carbon (OC), and thirteen PAHs in the Pearl River Delta (PRD) region and identifying the major sources and atmospheric processes that affect the size distributions. Size-segregated aerosol samples were collected in urban Guangzhou and two rural receptor sites in PRD (Backgarden (BG) in the north and HKUST in the south) during the period of 2006-2008. EC, OC, PAHs, and major ionic species were analyzed to help the understanding of EC and PAH size distribution characteristics. EC size distribution in urban Guangzhou was characterized by three significant accumulation modes with mass median aerodynamic diameter (MMAD) of ~0.15 μm, ~0.40 μm, and ~0.90 μm, with 0.40 μm mode being most prominent. The coexistence of 0.15 μm and 0.40 μm condensation modes could be explained to be a result of emissions from vehicles operating at different loadings. The dominance of the EC accumulation mode at ~0.40 μm has not often been reported in studies conducted in developed countries, but our observation is consistent with EC size distributions measured in a roadway tunnel in this region. The 0.90 μm mode, which had the highest OC/EC ratio, was postulated to be a result of in-cloud processing of soot particles. The EC size distribution characteristics at the two rural receptor locations were different from those at the urban locations. The most significant mode was the droplet mode (MMAD: 0.7-1.1 μm, 58-81% of total EC), while the condensation mode (0.22-0.33 μm, 15-33% of total EC) became the second largest mode. The combined result of condensation growth and in-cloud processing explains the observation of a condensation-mode EC at an MMAD of 0.22-0.33 μm, the depletion of the condensation-mode EC at 0.40 μm, and the presence of a droplet mode EC at the rural receptor locations. Coarse-mode EC (MMAD: 4-7 μm) are postulated to derived from resuspension of EC-containing soil/dust particles or tire abrasion. The contributions of the coarse mode decreased from 20% at the urban sites to 3.5%-12.7% at the rural receptor sites. By highlightling the partly internal-mixing state of EC and sulfate in the PRD region, calculations using Mie theory and the measured size distributions of EC, OC, and major inorganic ions indicated that the contribution of EC-containing particles was important (76%) in the overall light extinction of urban Guangzhou atmosphere due to their more abundant presence from vehicular emissions. In comparison, contribution of light extinction due to EC-containing particles in the rural locations was reduced but still significant (38-41%). Further measurement and modeling work are required to reconstruct light extinction of the PRD region, especially at the rural sites. Despite the limited sample numbers, analysis of correlations between PAHs, EC, and potassium indicated that the PAHs at the summer downwind site (BG) were mainly from vehicle emissions while the PAHs at the winter downwind site (HKUST) were dominated by biomass burning source. In urban Guangzhou, PAH size distributions were fit with five modes and the respective MMADs are: nuclei mode (MMAD: 0.05 μm), condensation mode I (MMAD: 0.13-0.17 μm), condensation mode II (MMAD: 0.4-0.45 μm), droplet mode (MMAD: 0.9-1.2 μm), and coarse mode (MMAD: 4-6 μm). The distributions of PAHs in different size modes vary with the volatility of PAHs. The least volatile seven-ring PAH was mainly in condensation mode II and a minor portion in the droplet mode. Five-and six-ring PAHs had significant presence in all the three accumulation modes. The more volatile three- and four-ring PAHs were also found in coarse mode in addition to significant presence in the three accumulation modes. Size-segregated gas-particle partition coefficients of PAHs (Kp) were estimated using measured EC and OM data, on the basis of the concept of Pankow’s dual adsorption-absorption model. The Kp values for a given PAH could differ by a factor of up to 18 on particles in different size modes, with the highest value associated with the condensation-mode I particles and the lowest value associated with the coarse-mode particles. Adsorption onto soot carbon was found the dominant mechanism driving the gas-particle partitioning of PAHs in Guangzhou urban atmosphere. Adsorption on soot particles accounted for 69-90% of three- to seven-ring PAHs in particle phase, due to the high EC concentration (5.6 μg/m3). We also derived from Kp the equilibrium timescales of repartitioning of PAHs to particles in different modes after they were emitted from vehicles. The calculations show that five- to seven-ring PAHs could not achieve equilibrium partitioning within their typical residence time in urban atmospheres. Consequently, they resided mostly in accumulation modes (condensation I, II and droplet mode) and little of them would volatilize and partition to coarse mode. Three- and four-ring PAHs could readily reach new equilibrium state in particles of all sizes corresponding to the changing atmospheric conditions. As a result, their presence in all size modes is predicted, consistent with their measured size distribution. As the equilibrium assumption does not hold for five- to seven-ring PAHs, a partitioning flux J is more appropriate in describing the gas/particle partitioning of PAHs in air quality models for PAHs. Size distributions of PAHs did not show much difference in the two receptor sites, but the difference between the receptor sites and the Guangzhou urban site is striking. At the receptor sites, the size distributions of three- and four-ring PAHs consisted of three modes: condensation mode (0.2-0.3 μm), droplet mode (0.7-1.0 μm), and coarse mode (3-5 μm). The less volatile five- to seven-ring PAHs were mainly distributed in the condensation mode and droplet mode, with little presence in the coarse mode. The dominant droplet mode is attributed to a result of in-cloud processing of vehicular soot particles and biomass burning particles during the transport of source aerosols to the receptor sites. It is very possible that presence of sulfate aqueous coating on the droplet-mode particles could make PAHs trapped inside inaccessible to volatilization from particle phase and to oxidation by gaseous oxidants. This in turn implies: (1) the lifetime of droplet-mode PAHs in the atmosphere could be longer than PAHs in the gas phase or PAHs on particles in the condensation mode or coarse mode; and (2) more PAHs would be found in the particle phase than predicted by gas-particle equilibrium theory. More evidence on the chemical composition and morphology of single particles is needed to prove the speculations arising from the size distribution observations in this study.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2009
Language English
Format Thesis
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