THEORETICAL EXPERIMENTAL STUDIES AIHA Journal 64 630 639 2003 AUTHORS Jeff Wagnera Janet M Macherb a University of North Carolina Department of Environmental Sciences and Engineering CB 7400 Rosenau Hall Chapel Hill NC 27599 b California Department of Health Services Environmental Health Laboratory 850 Marina Bay Parkway G365 Richmond CA 94804 Present address California Department of Health Services Environmental Health Laboratory 850 Marina Bay Parkway G365 Richmond CA 94804 Ms 367 Comparison of a Passive Aerosol Sampler to Size Selective Pump Samplers in Indoor Environments The objective of this work was to investigate the ability of the Wagner Leith passive aerosol sampler to measure indoor exposures over periods of 24 hours to 2 weeks An automated analysis technique was developed so that lower aerosol concentrations could be sampled over shorter time periods A test of the new analytical method against a manual method showed good agreement The passive sampler was tested alongside three pump operated sizeselective samplers in indoor environments Generally good correlation with the active samplers was observed Correlation with a personal impactor with uncoated substrates was not statistically significant but the cyclone MS T impactor and overall correlations had R2 values of 0 73 0 88 Combining these data with a previous study produced an R2 of 0 96 between passive and active results Large discrepancies up to 147 between passive and personal impactor results were observed and were attributed to particle bounce in the impactor passive sampler imprecision due to few collected fine particles and problems with detection of organic particles in the passive sampler The Wagner Leith sampler has now been tested over five orders of magnitude in mass concentration in which it has proved useful for obtaining aerosol size distributions mass fractions qualitative elemental analysis and morphology of individual particles The sampler has several limitations including increased sensitivity to contamination when fewer particles are collected uncertainties in sampling semi volatile particles and the need for some expertise and expense to analyze the passive samples Keywords aerosol sampling indoor air passive sampling sampler comparison SEM he passive aerosol sampler described by Wagner and Leith 1 is a potentially useful tool for exposure assessments The sampler Figure 1 does not require a pump and is capable of sampling for periods of minutes to weeks depending on the ambient aerosol concentration The passive sampler is silent and unobtrusive weighs 1 7 g and is 1 5 cm wide Thus it interferes minimally with human activities The sampler consists of a standard scanning electron microscope SEM stub a collection substrate and a protective mesh cap During sampling particles are transported by gravity diffusion and inertia through the 157 mm diameter holes of the mesh cap and deposit on a substrate mounted on the stub The stub is oriented such that the substrate is horizontal After sampling the mesh cap is removed the stub is placed in T This work was supported by a NIOSH UNC Occupational Safety and Health Training Pilot Project research training grant T42CCT41042306 630 AIHA Journal 64 September October 2003 an SEM and the particles are counted and sized to determine the particle flux Other microscope types may be used but SEM is most convenient A semiempirical particle size dependent deposition velocity model is used in conjunction with the measured flux to obtain the ambient mass size distribution C5 F F 5 vdep vambgm 1 where C is the average mass concentration over the sampling period F is the mass flux particle mass time substrate area vdep is the deposition velocity vamb is the ambient deposition velocity and gm is the mesh factor All are functions of particle aerodynamic diameter da The ambient deposition velocity represents the theoretical deposition rate to a flat plate 1 The mesh factor represents the effect of the mesh on the particle Copyright 2003 American Industrial Hygiene Association THEORETICAL EXPERIMENTAL STUDIES FIGURE 1 Passive aerosol sampler deposition rate and was determined with wind tunnel testing for 0 1 da 10 mm 2 These tests also verified that the deposition rate was independent of wind speed at low turbulence levels such as would be found commonly indoors Comparison of the passive sampler to conventional samplers in various environments is needed to demonstrate its strengths and limitations Previously the passive sampler had been field tested only in an industrial environment with high concentration coarsemode dusts 3 Results correlated well with those of an eight stage cascade impactor Discrepancies were attributed to the small number of fine particle counts in the passive sampler and particle bounce in the impactor Aerosols in indoor environments such as homes and offices offer a significantly different challenge to the passive sampler Typically these aerosols are present at much lower levels than problem areas in industrial workplaces Indoor aerosols often contain a variety of organic and inorganic species and a significant fraction of fine and submicron particles 4 The objective of this work was to investigate the passive aerosol sampler s ability to measure indoor particle exposures over periods of 24 hours to 2 weeks The work consisted of two tasks First an automated analysis technique was developed so that large portions of the collection surface could be examined more efficiently This capability allowed lower aerosol concentrations to be sampled over shorter time periods Second the passive sampler was tested alongside three conventional pump operated size selective samplers in indoor environments By integrating passive sampler size distributions over the appropriate size ranges comparisons could be made to the size fractions measured by each active sampler This article describes the findings and their implications for the utility of the passive sampler METHODS Automated SEM Analysis The number of particles collected by the passive sampler depends linearly on the particle concentration and sampling time When few particles are collected observation of more microscope fields is necessary to measure the flux In previous tests of the passive sampler 2 3 aerosol concentrations were high leading to large numbers of collected particles The small number of SEM images needed to count and size the particles were obtained and processed manually In this work lower aerosol