Richard Högström

Study of the PM Gas-Phase Filter Artifact Using a Setup for Mixing Diesel-Like Soot and Hydrocarbons

The filter artifact is a significant source of error in gravimetric measurements of particulate matter (PM) exhaust. However, only a few studies on the subject exist. Results from these studies show a large discrepancy mainly because the experiments were performed using real diesel vehicle exhaust with varying exhaust composition. In this study, a setup for mixing diesel-like soot and hydrocarbon vapor was constructed for generating a stable exhaust aerosol with adjustable composition. The particle size distribution of the diesel-fueled soot generator (GMD [geometric mean diameter] adjustable between 27 and 164 nm) was found to represent “real” exhaust particulate emission. This setup was applied for studying the filter artifact on Teflon-coated glass fiber filters using pentadecane as the hydrocarbon vapor. Experiments were performed using particle and hydrocarbon concentrations of 130–700 μg/m3 and 10–12 ppm, respectively. It was found that the particle concentration of the aerosol affects the filter artifact. At lower particle concentrations, more hydrocarbon adsorption was detected. In the absence of particles, the adsorption was highest. Furthermore, filter soot load, corresponding to 0.13%–0.66% of the clean filter mass, was found to affect adsorption. Sooty filters adsorbed less vapor than clean filters. However, increasing the soot load resulted in more adsorption. Moreover, it was found that the backup filter serves as a reasonable estimate of the filter artifact only for low particle concentrations and filter soot loads. These results indicate that the filter soot load is an important parameter influencing the filter artifact, and therefore, it should be considered when performing gravimetric sampling. The setup was proven to be a unique tool for quantitative studies of the filter artifact. Copyright 2012 American Association for Aerosol Research

Validating the single charged aerosol reference (SCAR) as a traceable particle number concentration standard for 10 nm to 500 nm aerosol particles

Measurement of nanometre-sized aerosol particles is based on particle number concentration measurements. The commonly used method for providing traceability for these measurements involves charging and electrical counting of aerosol particles. This method requires that the particles are singly charged or that the average charge is exactly known, neither of which is easy to ensure. In the device called a single charged aerosol reference (SCAR), the fraction of multiply charged particles is minimal due to the novel operating principle of electrical charging and subsequent growth. In this study the SCAR was validated as a primary particle number concentration standard. The average charge of the output aerosol was evaluated for the whole operational particle size range. For this, the effect of the size distribution of the primary nanoaerosol and the output number concentration on the fraction of doubly charged and neutral particles was measured. It was found that the uncertainty caused by assuming singly charged particles is only 0.16%. A full uncertainty analysis was carried out for a condensation particle counter (CPC) calibration. According to the results, the relative expanded uncertainty of calibration was 3.0%. This represents a typical uncertainty level achieved in CPC calibrations performed with SCAR. As a result of this study, SCAR was validated as a particle number concentration standard suitable for traceable calibration of particle counting instruments in the particle size range from 10 nm to 500 nm.

Validation of a calibration set-up for radiosondes to fulfil GRUAN requirements

Interest in the precise measurement of water vapour in the upper-air is growing along with the consciousness of climate change. Because better knowledge of high altitude humidity levels improves the accuracy of climate models and weather forecasts, the Global Climate Observing System (GCOS) is putting more effort into the quality of these measurements. The GCOS established the Reference Upper-Air Network (GRUAN) and set the requirements for its measurements accuracy and traceability to the International System of Units (SI). To fulfil these requirements, improved radiosondes and methods to calibrate them are being developed. This paper presents a new calibration system for radiosondes based on mixing air flows from two independent dew-point generators. The system operates in the air temperature range down to −80 °C. Our results show that the new calibration set-up is able to provide calibrations for radionsondes at the uncertainty level of 2% in terms of the water vapour mixing ratio as required by GRUAN. A complete calibration covering the whole temperature and humidity range lasts less than three days. Additionally, the apparatus provides an option to characterize the behaviour of a radiosonde in changing temperatures and water vapour concentrations.

Atomic force microscopy studies of surface contamination on stainless steel weights

The SI unit of mass will probably be redefined within the next few years using an invariable natural constant. Nevertheless, dissemination of the kilogram will still be realized by weighing using physical weights prone to contamination. Published data on cleaning, humidity effects and long term stability of weights show large discrepancies, indicating that not all factors affecting adsorption characteristics of weights are known. In the work reported here, an atomic force microscope (AFM) was used to study surface effects of stainless steel weights at the nanometre scale. Effects of transfer between air and vacuum as well as effects of cleaning were studied by recording topography images of the surface before and after each procedure. An image processing method was developed for improving the sensitivity of detecting changes in images. Ultrasonic cleaning in ethanol removed contamination mainly from the grooves in the surface, while vacuum exposure caused contamination to build up in the grooves. The results show that the surface microstructure of stainless steel weights affects adsorption of contaminants in such a way that grooves seem to be preferential sites for adsorption. AFM has proven to be a valuable tool for studying surface effects of standard weights at ambient pressure with near nanometre resolution.

A Calibration System for Reference Radiosondes that Meets GRUAN Uncertainty Requirements

Abstract: A new International System (SI) traceable calibration set-up for reference radiosondes is presented here with a preliminary uncertainty analysis. By meeting the GRUAN requirements, this development fulfils the needs of the meteorological community for disseminating SI traceability to upper air humidity measurements with reduced uncertainty. The set-up was designed for calibrations from laboratory temperature down to 183 K and in terms of dew-point temperature from 193 K to 283 K. To enable rapid changes in humidity and to shorten the time needed for a single calibration, the set-up utilizes a hybrid humidity generator method in which two air flows with known water vapor concentrations are mixed. According to a preliminary uncertainty analysis, the relative expanded uncertainty (k = 2) of the set-up is less than 2 % expressed in terms of mixing ratio.

Erratum: Atomic force microscopy studies of surface contamination on stainless steel weights (2010 Metrologia 47 670–676)

The name of one author was omitted from the published article. Leena Stenlund should be added to the list of authors, with the same affiliation as the others.