Determination of the Size Distribution and Chemical Composition of Fine Particulate Semivolatile Organic Material in Urban Environments Using Diffusion Denuder Technology

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Abstract
Collection of particles on a filter results in underestimation of particulate organic compounds due to losses from the semivolatile organic fraction during sample collection, that is, a “negative sampling artifact.” This sampling-induced change in the phase distribution of semivolatile organic material resulted in the loss of an average of 35% of the particulate organic material in samples collected at the South Coast Air Quality Management District (SCAQMD) sampling site at Azusa, CA, in the Los Angeles Basin. These semivolatile organic compounds lost from particles were measured using two diffusion denuder sampling systems. A multichannel diffusion denuder sampling system (BOSS) was used to determine the concentrations of fine particulate carbonaceous material, and a multisystem, multichannel, high-volume diffusion denuder sampler (BIG BOSS) was used for the determination of the particle size distribution and chemical composition of semivolatile organic compounds in fine particles. Results obtained with the two sampling systems agreed. A smaller artifact associated with the absorption of gas-phase organic compounds by the filter during sampling was corrected for using two tandem quartz filters in the BOSS sampling system. The quartz filter artifact was not present after removal of gas-phase compounds by the diffusion denuder. Comparison of the denuder results with those obtained by the SCAQMD using filter pack sampling systems showed that the observed concentration of the quartz filter artifact was dependent on the sample flow rate. However, the denuder sampler results on the concentration of particulate carbonaceous material retained on a quartz filter during sampling, corrected for the quartz filter artifact, were also in agreement with results reported by the SCAQMD. The loss of semivolatile organic material from particles during sampling resulted in an underestimation of particulate organic material by collection with tandem quartz filters by an average of 55% of the measured concentration. Semivolatile particulate organic compounds lost from the particles during sample collection included paraffinic compounds, aromatic compounds, and organic acids and esters. Underestimation of the concentration of semivolatile organic compounds in particles is a function of molecular weight, chemical compound class, and particle size. The maJority of the organic compounds in fine particles 0.8–2.5 μm in size are semivolatile organic compounds lost from the particles during sampling onto a filter. The maJority of carbonaceous material in particles smaller than 0.4 μm is not lost from the particles during sample collection. The results obtained using the diffusion denuder sampling systems indicate that the fine particulate organic constituents to which an urban population is exposed have not been well characterized or quantified in previous studies.