Effect of environmental variables on collection of atmospheric sulfate

Abstract

The performances of various filter media are determined relative to their use in collection of ambient sulfates. The effects of operational parameters and atmospheric variables on the collection of sulfates are investigated. Results show that sorption of ambient SO2 by basic filter components can contribute to appreciable errors in apparent ambient sulfate levels. This interaction is affected by SO2 concentration, relative humidity, and temperature. A chemical model relating the effects of these variables is presented.The need for accurate determination of ambient sulfate levels has been intensified in recent years because of the identification of sulfates as significant health hazards. While progress has been made in reducing atmospheric levels of sulfur dioxide, corresponding reduction in ambient sulfate levels has not, in general, been well correlated with the reductions in sulfur dioxide (1). This fact, along with both direct and indirect criticisms of the standard Hi-Vol sampling techniques for ambient sulfate collection, has made obvious the need for a thorough investigation of sulfate sampling and analysis procedures.Investigations such as those conducted by Lee and Wagman (2) and more recently by Radian personnel (3) and workers at Ford ( 4 j have shown that filters commonly used for sulfate sampling can absorb SO2 a t ambient levels, and that this can lead to generation of extraneous sulfate during the sampling process. It can be inferred from other investigations ( 5 ) that the presence of potential SO2 oxidation catalysts in ambient aerosols also may affect ambient sulfate measurements. Still other questions have been raised concerning the mechanical and dynamical factors involved in Hi-Vol sampling.The present work includes consideration of the effects of filter type and composition, ambient SO2 concentration, temperature, relative humidity, ambient gas composition, sampling time and velocity, sample storage time and conditions, and particulate composition (SO2 oxidation catalysts) on the formation of extraneous sulfate, to provide a quantitative base for the assessment of sulfate sampling methodology. ExperimentalApparatus. Ambient air was used as a base for all of the laboratory experiments. This was drawn into the laboratory at about 520 cfm through a stainless steel duct from a point some 20 ft above the building (ca. 50 ft above ground level). In the laboratory the air stream was divided through two parallel stainless ducts. At the top of each inside duct, injection ports and mixing orifices were added to enable spiking of the ambient air with various gases as needed for specific experiments. Sampling nozzles were fitted near the bottom of the parallel ducts, with provision for four identical sampling points in each duct. This arrangement permitted simultaneous isokinetic collection of as many as eight filter samples in any given run. Standard 142-mm Millipore filter holders were fitted with modified conical inlets to achieve uniform distribution of particles over the filter surface. Outlets from...

Aerosol Inorganic Composition at a Tropical Site: Discrepancies Between Filter-Based Sampling and a Semi-Continuous Method

“…Sulfur values from Apiezon-coated SFU filters and impactor filters agreed within 10%. However, the presence of alkaline species may provoke adsorption of SO 2 on filter substrates and subsequent oxidation with ozone (O 3 ) and/or hydrogen peroxide (H 2 O 2 ) (Appel et al 1984;Coutant 1977;Eatough et al 1995). High RHs could promote trapping of small droplets on the filter substrates leading to retention of gaseous species.…”

Inferring the Sources of Fine and Ultrafine Particulate Matter at Downwind Receptor Sites in the Los Angeles Basin Using Multiple Continuous Measurements Special Issue ofAerosol Science and Technologyon Findings from the Fine Particulate Matter Supersites Program

“…These methods are more prone to sample contamination due to the manual handling and transportation of filters and substrates. Furthermore, long sampling periods can increase sampling artifacts due to volatilization and adsorption of labile PM species such as ammonium nitrate and semivolatile organic compounds (Appel et al 1984;Coutant 1977;Hering and Cass 1999).…”

Sulphate aerosol size distributions at Mumbai, India, during the INDOEX-FFP (1998)

“…Glass-"bre "lters were avoided as conversion of sulphur dioxide on high-alkaline glass-"bre "lters during air sampling has been shown to give erroneously high particulate sulphate concentrations (Coutant, 1977;Stevens et al, 1978;Pierson et al, 1980). Cellulose-"bre "lters have not shown such artefact sulphate formation and have been used extensively in remote environments (Savoie et al, 1987).…”