Integrated and real-time diffusion denuder sample for PM 2.5

Abstract

Particulate matter (PM) is a complex mixture of stable condensed phases, adsorbed or dissolved gases, and semi-volatile materials, i.e. compounds that transfer between the gas and condensed phases. Fine particles in both rural and urban environments contain substantial quantities of semi-volatile material that can be lost from the particles during sample collection on a filter and during subsequent handling, storage and conditioning (or equilibration). These include ammonium nitrate, semi-volatile organic compounds and particle bound water. In addition, gas phase organic compounds can also be absorbed by a quartz filter. As a result, the concentrations of these species are often significantly in error for results obtained with a conventional single filter sampler. The accurate measurement of the mass and chemical components of fine particles will become more important as a result of the recent promulgation of fine particle standards by the US Environmental Protection Agency and the additional monitoring that will be required for implementation of those standards as well as for additional research on the sources and effects of PM. Past organic compound diffusion denuder samplers developed by us (BOSS, BYU Organic Sampling System) are not amenable to routine field use because of the need to independently determine the gas phase semi-volatile organic material breakthrough of the denuder for each sample. This problem has been eliminated in the Particle Concentrator–Brigham Young University Organic Sampling System (PC–BOSS) using a combined virtual impactor, particle-concentrator inlet to provide a concentrated stream of 0.1–2.5 μm particles. This is followed by a BOSS diffusion denuder and filter packs containing particle collecting and sorbent filters to collect particles, including any semi-volatile organic material or ammonium nitrate lost from the particles during sampling. The PC–BOSS contains a sequential sampler to allow for the routine collection of several samples. A second sampler, the Real-time Total Ambient Mass Sampler (RAMS), has been developed by combining the PC–BOSS with TEOM technology. In this sampler, a diffusion dryer to remove gas phase water follows the diffusion denuder. The dried aerosol stream is then sampled with a “sandwich” (TX40 and sorbent) filter on the TEOM tapered element to collect particles, including any semi-volatile species. Laboratory and field validation data indicate that the precision of determination of fine particulate material, including ammonium nitrate and semivolatile organic material is better than ±10%.