Characterization of polycyclic aromatic hydrocarbons (PAH) in particulate and gaseous emissions from agricultural burning.

摘要

Burning of agricultural residues is a common technique for land preparation and disposal of crop and wood wastes. The annual biomass burning in California amounts to 425,000 and 1,550,000 tons for orchards and crop residues, respectively. Rice and wheat straw, along with almond and walnut prunings comprise 95% of the agricultural biomass openly burned in California. Approximately 4800 tons of particulate matter is emitted from agricultural burning in California annually. Even though this figure represents only 1 percent of the total particulate matter emission in California, the temporal and spatial effect of this emission on air quality and public health is significant. Polycyclic aromatic hydrocarbons are mutagenic air pollutants formed as by-products of any incomplete combustion process. After formation and emission, PAH compounds partition between the gas phase and particulate matter phase. The environmental fate of these compounds, in part, depends on their distribution between gas and aerosol phase and among particle size fractions. Particle size affects the removal rate of the associated PAH from the atmosphere. The mechanism and location of deposition of particulate adsorbed PAH in the lungs are also affected by particle size. The large particles tend to deposit in the upper regions of the lung while smaller particles diffuse to the surface of the alveoli. The goal of this work is a better understanding of the distribution of PAH compounds in emissions from burning of rice straw and almond pruning, to determine the health risk of these operations.; A combustion chamber was built that simulates the burning conditions in the field including a spreading fire for the rice straw and a boundary layer in the air-flow over the fire. A Reduced Artifact Dilution System (RADS), prior to the sampling train, provided the thermal and chemical equilibrium conditions. Burn smoke emissions were sampled with a micro-orifice impactor in 10 aerodynamic size fractions and a Berner impactor with 8 aerodynamic size fractions for particulate matter size distribution profiles, and on a Hi-volume filter followed by a PUF plug for bulk particulate matter and gaseous emissions. The samples were analyzed for 16 priority pollutant PAHs.; PAH size distribution profiles showed a predominance of more volatile PAH compounds (3–4 rings) to the less volatile (5–6 rings) from both fuels. The less volatiles from both fuels showed unimodal distribution with peaks in the 0.08–0.17 μm range while more volatiles showed multimodal distribution. Higher emissions were observed from rice straw smoke which burned at lower temperature and at much faster rate.; Comparison of the data from the two impactors showed very good agreement as far as total PAH concentration collected on the impactor and size distribution profile. The impactor data were also compared with the bulk PAH concentrations from the Hi-volume measurements and the results were generally in good agreement. However, discrepancies were observed for size distribution and impactor-filter comparison for more volatile compounds (molecular weights 178 and 202).; The effect of particle size and organic carbon content were also investigated on gas-particle partitioning of PAH compounds. Results suggested that the partitioning behavior of PAH compounds will be different between particles of different size and the characteristics of the particles in each size class determines the relative importance of adsorptive and absorptive partitioning.