Aerosol sample conditioning system

JAerosolSci. Vol. 31, Suppl. l, pp. $414-$415,2000

Pergamon www.elsevier.com/locate/jaerosci

Poster Session I. Sampling AEROSOL SAMPLE CONDITIONING SYSTEM I.A. MARSHALL and D.R. BOOKER Aerosol Science Centre, AEA Technology Enx~ironment, Culham, Abingdon, Oxfordshire, OX14 3ED, United Kingdom.

Keywords: Sampling, Instrument Evaluation, Emissions INTRODUCTION Work has been undertaken to develop a design standard for calibrating number-measuring aerosol instrumentation in the uitrafine region. Emphasis has been on achieving a robust and cost-effective capability that can be used both in specialist calibration laboratories as well as be able to be tal(en to the point of measurement. The potential of using accurate dilution, over a wide concentration range, as a means of providing a traceable and cost-effective means of calibrating the linearity of number-measuring aerosol instrumentation was identified as a key component of this study. An absolute dilution performance of better than 10 % was identified as a design target. An aerosol Sample Conditioning System (SCS) meeting this target has been designed and manufactured. The SCS has undergone comprehensive calibration and validation in terms of temperature and pressure sensor calibration, temperature compensation of measured pressures, flow calibration, pressure stability, leaktightness and dilution verification with a CO gas standard. An example validation of a TSI Condensation Nucleus Counter (CNC) was also undertaken utilising the SCS. DESCRIPTION The aerosol Sampling Conditioning System is based on the design of Brockmann et al. (1984). Brockmann et al. describe a diluter that was designed for sampling high concentrations (-10 ~° cm "3) of ultrafine aerosols. Particular care was taken to minimise the effects of coagulation and diffusional deposition within the diluter. The diluter was also designed so that the effects of these processes on the size distribution of the sampled aerosol could be calculated. A diagram of the diluter within the SCS is shown below. The sample is extracted through a needle and immediately mixed with the primary dilution flow. This diluted sample then flows through a second transport tube to the throat of the Venturi where it mixes with the secondary dilution air. The Venturi is used to create the suction needed to draw the sample. This allows a sample to be extracted and the diluted sample to be delivered at atmospheric pressure. Primary Mixing Chamber Secondary Mixing Needle Primary Transport Tube ,Secondary Transport Tube // //

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Abstracts of the 2000 European Aerosol Conference

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The SPS has been designed to ensure that traceable and accurate aerosol dilution can be carried out. To verify the performance of the instrument a tracer-gas dilution experiment was undertaken using a traceable high-concentration CO gas in Nitrogen (4000ppm). The diluted sample was measured using a calibrated CO detector. The reported dilutions are within 10% of the measured gas-dilution values. By using needle specific calibration data (rather than a needle type average data) the absolute error of dilution can be less than 5% over a dilution range of 500-5000. TSI CNC LINEARITY VALIDATION 2500

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The linearity o f a TSI CNC is validated in the single count mode. Ambient particluate was used as the challenge aerosol. The TSI CNC was subjected to a range of diluted and raw concentrations of the test aerosol. The resulting number concentration ratio between the diluted and un-diluted aerosols was compared with the absolute dilution ratio reported by the SCS. As can be seen the TSI instrument performance was validated over the range of conditions tested.

ACKNOWLEDGEMENTS This work has been funded by the National Measurement Systems Policy Unit (NMSPU) of the UK Government Department of Trade and Industry (DTI) under the Valid Analytical Measurement (VAM) scheme. REFERENCES Brockmann, J.E., Liu, B.Y.H. and McMurray, P.H. (1984) A Sample Extraction Diluter for Ultrafine Aerosol Sampling. Aerosol Sci. Technol. 3(4), 441-451.