Time-resolved size and element analysis of gas-borne nanoparticles

Physical and chemical characterization of gas-borne particles in the nanometer to sub-micrometer size range is important in many applications, such as nanoparticle related health studies, the monitoring of powder or dust distribution processes, assessing the release of engineered nanoparticles (ENP) from ENP reinforced materials due to abrasion or combustion, or exposure studies related to ENP containing spray products. Such investigations are also highly valuable in the development of technical processes, e.g. to determine distinct size fractions containing specific elements or isotopes in terms of dealing with nuclear waste, or to evaluate particulate matter in process gases and emissions from thermal waste treatment, gas turbines, or other combustion engines. Such particle speciation includes knowledge about particle size distribution and chemical composition, and if possible obtained with a high time resolution. However, most of the available techniques are offline methods and/or not able to provide such physical and chemical information simultaneously. The Scanning Mobility Particle Sizer (SMPS) is a well-established and widely used equipment for determining size distribution and number concentration of such particles, within scan durations of a few minutes. Inductively Coupled Plasma Mass Spectrometry (ICPMS) is a highly sensitive multi-element technique, which allows determining the elemental composition of normally liquid samples, with excellent detection limits and a wide dynamic concentration range. In this work, SMPS and ICPMS are coupled to one hyphenated system. A Rotating Disk Diluter (RDD) allows directing a well-defined flow of a diluted aerosol into subsequent measuring equipment, which besides is mainly argon based, due to the dilution effect. Therefore an RDD is implemented as sample introduction interface, and the SMPS flow concept is re-designed to fulfil the requirements of both different analytical methods. This coupling strategy allows achieving simultaneous information on particle size and elemental composition, with SMPS-typical time resolutions of a few minutes, and offers high flexibility in terms of dealing with different aerosols, loaded with particles in the nanometer to sub-micrometer size range. Proper SMPS operation under argon atmosphere instead of air is validated. The developed setup is tested with a model aerosol containing air-borne silver nanoparticles. The capabilities are then demonstrated on uniformly composited metal aerosols, as well as an aerosol mixture containing smaller gold and larger silver nanoparticles. Sensitivity and limit of detection, related to particle number and mass concentration, are determined for several elements and particle diameters. After the successful characterization, the instrumentation is applied in two research applications. The first is a study on aerosol particles, released by commercial consumer spray products. In the second application, particulate metal emissions from the thermal treatment of differently impregnated wood samples are investigated, with the focus on alkali metals.

Dry and ambient aerosol properties at the Jungfraujoch

Remo Nessler

Atmospheric aerosol is defined as a suspension of solid or liquid particles in air. The major concerns about aerosol particles are their adverse health effects and their role in the Earth's climate system. Atmospheric particles influence the Earth's radiation budget in two ways: either directly by scattering and absorbing incoming solar radiation (and to a far less extent longwave radiation emitted from the Earth) or indirectly by their ability to act as cloud condensation nuclei (CCN). With increasing particle concentrations, caused by, e.g., anthropogenic emissions, more CCN are available to form cloud droplets. For a fixed water content in the atmosphere this leads to more but smaller droplets and therefore to enhanced cloud albedos as well as to rain suppression. Both effects have been recognized in recent years to be of great importance for understanding the observed climate change. Nevertheless, they remain poorly understood and quantified. The Jungfraujoch (JFJ, 3580 m asl; 46.548°N, 7.984°E) High Alpine Research Station is equipped with instruments that have for decades performed important Global Climate Observations. A major part of the aerosol instruments perform in-situ measurements. However, due to the harsh weather conditions, they usually cannot be run outdoors. The ambient air has to be inducted into a housing, such that the aerosol is sampled at a temperature (T) and relative humidity (RH) different from the ambient values. Therefore, the measured aerosol properties may considerably differ from the ambient — the climate-relevant — ones. During the first Cloud and Aerosol Characterization Experiment (CLACE) at the JFJ in-situ aerosol size distributions were measured simultaneously indoor at dry conditions (T ≈ 25 °C and RH < 10%) and, for the first time in such an exposed place, outdoor at ambient conditions (T < –5 °C) by means of two scanning mobility particle sizers (SMPS). The data set was completed by measurements of hygroscopic growth factors using a hygroscopicity tandem differential mobility analyzer (H-TDMA). A comparison of dry and ambient size distributions shows two main features: First, the dry total number concentration is often considerably smaller (on average 28%) than the ambient total number concentration. This is most likely due to the evaporation of volatile material at the higher indoor temperature. These particle losses mainly concern small particles (dry diameter Ddry ≤ 100 nm), and therefore have only a minimal affect on the surface and volume concentrations. Second, the dry number size distribution is shifted towards smaller particles, reflecting the hygroscopic behavior of the aerosols. This shift was modeled using a modified Köhler equation adapted to the measured hygroscopic growth factors. The corrected dry surface and volume concentrations are in good agreement with the ambient measurements for the whole RH range, but the correction works best for RH < 80%. The results indicate that size distribution data measured at indoor conditions (i.e. dry and warm) may be successfully corrected to reflect ambient conditions. By combining these results with earlier JFJ aerosol measurements (e.g. chemical composition), it was possible to model dry and ambient optical aerosol properties for a summer and a winter case. The model calculations were based on a total of 496 size distributions representative for the JFJ aerosol, hygroscopic growth parameterizations and the following simplified aerosol structures: The fine mode (Ddry ≤ 1 µm) aerosol was assumed to consist of a water-insoluble spherical core concentrically encased by a coating comprising all water-soluble compounds, and the coarse mode (Ddry > 1 µm) particles were assumed to be water-insoluble homogeneous spheres. Ambient scattering coefficients σsca(RH) were found to be considerably different from the dry scattering coefficients σsca(RH = 0). The scattering enhancement factors ξ(RH) = σsca(RH)/σsca(RH = 0) strongly depend on the particle size. At RH = 85% they vary for example between ≈ 1.2 and ≈ 3.8. It was possible to establish a parameterization of ξ(RH) with the dry Ångström exponent å (based on scattering only). Since å follows directly from the dry scattering measurements, the parameterization allows to derive ambient scattering coefficients from the dry scattering coefficients without the need of any additional measurement other than ambient RH. RH also affects the absorption coefficient σabs. Depending on particle size and wavelength (investigated between 370 nm and 950 nm) absorption enhancement factors χ(RH) = σabs(RH)/σabs(RH = 0) were found to range from 0.84 to 1.78. However, it was possible to demonstrate that, even though the humidity effect on absorption is substantial, its contribution to the humidity effect on extinction and the single scattering albedo is negligible at the JFJ. The EPFL Raman lidar installed at the JFJ provides profiles of extinction and backscattering coefficients at the wavelengths 355 nm, 532 nm and 1064 nm for altitudes between typically 4 km and 15 km asl. These measurements are ambient but afflicted with more uncertainties than the dry in-situ measurements. A method was developed to derive microphysical aerosol properties from optical lidar data. This is a so-called ill-posed problem, i.e., small errors in the lidar data lead to huge errors in the inverted microphysical properties, unless special mathematical tools are applied. Therefore the method makes use of regularization techniques. It is shown that microphysical parameters can be inverted with acceptable accuracy from an optical dataset as it is provided by a lidar of e.g. the EPFL type.

EPFL2004

A hyphenated SMPS-ICPMS coupling setup: Size-resolved element specific analysis of airborne nanoparticles

Adrian Hess, Christian Ludwig, Mohamed Tarik

Physical and chemical characterization of gas borne particles in the nanometer to sub micrometer size range includes knowledge about particle size distribution and chemical composition, possibly with a high time resolution. However, most of the available techniques are offline methods and/or not able to provide such physical and chemical information simultaneously. Scanning Mobility Particle Sizer (SMPS) is a well established and widely used technique for determining size distribution and number concentration of such particles, within scan durations of a few minutes. Inductively Coupled Plasma Mass Spectrometry (ICPMS) is a highly sensitive multi element technique, allowing the determination of elemental composition with excellent defection limits and a wide dynamic concentration range. In this work, SNIPS and ICPMS were coupled to one hyphenated system which allows achieving on-line and simultaneously size and chemical information with SMPS-Lypical time resolutions. A Rotating Disk Diluter was used as sample introduction interface, and the SMPS flow concept was re designed to fulfill the requirements of both different analytical methods. The developed setup was successfully tested with a model aerosol containing airborne silver nanoparticles. Proper SMPS operation under argon atmosphere instead of air was validated. The presented setup was applied to scan the particle size range from 7 to 156 um within 120s, Similar sensitivities and efficiencies compared to that of state-of-the-art SMPS and ICPMS instruments were achieved. (C) 2015 Elsevier Ltd. All rights reserved.

Elsevier Sci Ltd2015

Dry and ambient aerosol properties at the Jungfraujoch

Remo Nessler

EPFL2004