Comparison of diffusion charging and mobility-based methods for measurement of aerosol agglomerate surface area

Abstract

We compare different approaches to measure surface area of aerosol agglomerates. The objective was to compare field methods, such as mobility and diffusion charging based approaches, with laboratory approach, such as Brunauer, Emmett, Teller (BET) method used for bulk powder samples. To allow intercomparison of various surface area measurements, we defined 'geometric surface area' of agglomerates (assuming agglomerates are made up of ideal spheres), and compared various surface area measurements to the geometric surface area. Four different approaches for measuring surface area of agglomerate particles in the size range of 60-350 nm were compared using (i) diffusion charging-based sensors from three different manufacturers, (ii) mobility diameter of an agglomerate, (iii) mobility diameter of an agglomerate assuming a linear chain morphology with uniform primary particle size, and (iv) surface area estimation based on tandem mobility-mass measurement and microscopy. Our results indicate that the tandem mobility-mass measurement, which can be applied directly to airborne particles unlike the BET method, agrees well with the BET method. It was also shown that the three diffusion charging-based surface area measurements of silver agglomerates were similar within a factor of 2 and were lower than those obtained from the tandem mobility-mass and microscopy method by a factor of 3-10 in the size range studied. Surface area estimated using the mobility diameter depended on the structure or morphology of the agglomerate with significant underestimation at high fractal dimensions approaching 3.

Toward Developing A New Occupational Exposure Metric Approach for Characterization of Diesel Aerosols

“…The assumption is that all the primary particles are equally exposed to the surrounding gas: the aggregates need to be linear-chain assumption with a low fractal dimension. Recently, Ku and Kulkarni (2012) showed that the geometric surface area of aggregates is underestimated by the IA method applied to mobility analysis when the chains have a fractal dimension of 2.37-2.63. Because no statistical morphology analysis was carried out in this study, the SMPS surface area can be considered only an estimation of geometric surface area.…”

“…The size distribution measurement, and thus the estimated surface area, is affected by the morphology and aggregation of the particles (Lall and Friedlander 2006) but these are typically not known with precision in occupational environments without in-depth analysis. Studies that compared the results of these methods for measuring the surface area of ultrafine and nanoaerosols have been performed (Ku 2010) and recently concluded (Ku and Kulkarni 2012).…”

Characteristics of airborne gold aggregates generated by spark discharge and high temperature evaporation furnace: Mass–mobility relationship and surface area

“…Although methods that measure surface area on-line are available (Asbach, Fissan, Stahlmecke, Kuhlbusch, & Pui, 2009;Fierz, Houle, Steigmeier, & Burtscher, 2011;Ntziachristos, Giechaskiel, Ristimäki, & Keskinen, 2004;Wang et al, 2010), these were typically not developed for non-spherical and porous particles, and thus do not work in an optimal way for all types of aggregates (LeBouf et al, 2011). Also there is data suggesting that the techniques based on unipolar diffusion charging does not relate directly to the surface area of the aerosol particles as suggested (Gopalakrishnan, Thajudeen, Ouyang, & Hogan, 2013;Ku & Kulkarni, 2012;Ku & Maynard, 2005). Transmission electron microscopy (TEM) imagery is often used to characterize primary particle size (d pp ) of aggregates.…”

“…Various techniques have been used for specific surface area characterization, whereof nitrogen adsorption, or Brunauer Emmett Teller (BET) is the most common (Brunauer, Emmett, & Teller, 1938). BET has been successfully used for nanoparticle surface area characterization (Eggersdorfer, Groehn, Sorensen, McMurry, & Pratsinis, 2012;Ku & Kulkarni, 2012). However, the technique can prove challenging for many applications and nanoparticle sources since it is offline and a relatively large amount of material is needed (min.…”

Determining the effective density of airborne nanoparticles using multiple charging correction in a tandem DMA/ELPI setup

“…Ehara et al, 1996) tandem DMA/APM (e.g. Mc Murry et al, 2002;Ku et al, 2006;Pagels et al, 2009;Kim et al, 2009;Ku and Kulkarni, 2012;Shapiro et al, 2012;Rissler et al, 2013;Charvet et al, 2014). Measured particle diameter and mass are basic parameters which should simplify data collection and interpretation.…”