DETERMINATION OF THE PARTICLE-SIZE DISTRIBUTION AND CHEMICAL-COMPOSITION OF SEMIVOLATILE ORGANIC-COMPOUNDS IN ATMOSPHERIC FINE PARTICLES WITH A DIFFUSION DENUDER SAMPLING SYSTEM

Correct assessment of the factors associated with visibility impairmen t is dependent on accurate determination of the chemical composition a s a function of size of particles present in the atmosphere. Collectio n of particles on a filter results in underestimation of particulate o rganic compounds due to losses from the semi-volatile organic fraction during sample collection, i.e. a ''negative sampling artifact''. Thes e semi-volatile organic compounds lost from particles collection a fil ter can be correctly measured using a diffusion denuder sampling syste m. This paper describes a multi-system, multi-channel, high-volume dif fusion denuder sampler for the accurate determination of the particle size distribution and chemical composition of semi-volatile organic co mpounds in atmospheric fine particles. Data obtained in Provo, UT and Los Angeles, CA on the chemical composition and particle size distribu tion of semi-volatile organic compounds in atmospheric fine particles indicate that the majority of the semi-volatile organic compounds lost from particles during sampling were present in particles in the 0.4-0 .8 mum size range in the atmospheres sampled. Organic compounds lost f rom the particles during sampling include paraffinic compounds, aromat ic compounds and organic acids and esters.