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

摘要

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.EC size distribution in urban Guangzhou was characterized by three significant accumulation modes with mass median aerodynamic diameter (MMAD) of &sim0.15 mum, &sim0.40 mum, and &sim0.90 mum, with 0.40 mum mode being most prominent. The coexistence of 0.15 mum and 0.40 mum 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 &sim0.40 mum 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 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 mum, 58-81% of total EC), while the condensation mode (0.22-0.33 mum, 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 mum, the depletion of the condensation-mode EC at 0.40 mum, and the presence of a droplet mode EC at the rural receptor locations. Coarse-mode EC (MMAD: 4-7 mum) 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.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 mum), condensation mode I (MMAD: 0.13-0.17 mum), condensation mode II (MMAD: 0.4-0.45 mum), droplet mode (MMAD: 0.9-1.2 mum), and coarse mode (MMAD: 4-6 mum). The distributions of PAHs in different size modes vary with the volatility of PAHs.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 mug/m 3).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 mum), droplet mode (0.7-1.0 mum), and coarse mode (3-5 mum). 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. (Abstract shortened by UMI.)