Quantification of non-volatile number and volume fractions of atmospheric aerosols in different environments

.1. Aerosol S¢i. Vol. 30, Suppl. 1, pp. S117-SI 18, 1999 © 1999 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0021-8502/99/$ - see front matter

Pergamon

QUANTIFICATION OF NON-VOLATILE NUMBER AND VOLUME FRACTIONS OF ATMOSPHERIC AEROSOLS IN DIFFERENT ENVIRONMENTS S. Philippin, F. Stratmann, A. Wiedensohler Institute for Tropospheric Research, Permoser Str. 15, D-04318 Leipzig, Germany KEYWORDS Non-volatile aerosol fraction, state of mixing, TDMA, volatility. INTRODUCTION Carbonaceous material within atmospheric aerosols significantly influences their optical properties. Absorption and scattering of light, particularly caused by elemental carbon, affects climate and visibility. Anthropogenically influenced aerosols can show high fractions of non-volatile carbon. The determination and quantification of non-volatile material, e.g. soot, in the fine particle size range (< 150 nm) and their mixing (external/internal) within the particles has been difficult in the past due to the size and accordingly small mass of the particles. A VTDMA (Volatility Tandem Differential Mobility Analyzer) was used to examine the volatility behavior of selected atmospheric particles with short time resolution. Measurements were performed at the source of combustion processes, in moderately as well as in low pollution areas. METHODS In a VTDMA selected aerosol particles between 15 and 150 nm are heated to specific temperatures (max. 280 °C). Volatile components within the particle evaporate and only nonvolatile components, e.g. soot, remain after the conditioning process. The resulting number size distribution is subsequently measured and inverted to recalculate the true size distribution after the conditioning process. Non-volatile number and volume fractions of the residual material can be derived from the ratio of the number and volume of the residual (at Tmax) to the initial number size distribution (at 25 °C). After the volatilization process the monomodal number size distribution of the initially selected particles is shifted to smaller particle diameters, or results in a bi- or multimodal structure. This additionally allows a classification of particles with respect to their external/intemal mixing state. RESULTS The volatility number size distributions were obtained from various VTDMA measurements at a source of combustion processes (vehicle-emitted Diesel particles), in a moderately to heavily polluted urban area (VOLE 99, IfT Leipzig, Germany), and in rural, little anthropogenically-influenced regions (ACE-2, Sagres, Portugal and LACE 98, Lindenberg, Germany). Results of individual volatility scans of selected 50 and 150 nm aerosol particles are shown in Figures la-c. Displayed in each figure is an initial (ambient) mode, centered at the selected 50 and 150 nm diameter, respectively, and a second mode resulting from thermal conditioning of these particles to 250 and 280 °C, respectively. The volatility scan of emitted 50 nm Diesel particles shows a monomodal shift of the initial distribution towards smaller particle diameters with a slight broadening of the peak. According to the combustion processes of diesel vehicles the residual material is assumed to be mostly comprised of nonvolatile carbon, i.e., soot, with large relative volume fractions between 60 and 100 %,

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Abstracts of the 1999 European Aerosol Conference

depending on vehicle type, speed, and selected particle size. Size distributions of anthropogenically-influenced atmospheric particles, however, reveal a multimodal structure with an external mode, purely consisting of pertinent non-volatile component and a residual, internally mixed size distribution•

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Figures la-c. Volatility size distributions of aerosol particles of different origin at 25 °C (dashed) and 250/280°C (solid), representing (a) vehicle-emitted 50 nm Diesel particles at 50 km/h, (b) urban 150 nm particles during rush hour, and (c) little-polluted 150 nm particles (averaged 10-hr sample)•

Depending on the extent of pollution the non-volatile volume fractions, most likely consisting of non-volatile carbon, account in average up to 20 to 40 % in urban areas and 0 to 12 % in more remote regions. Respective externally mixed fractions of the total refractory material within the aerosol comprise up to 30 % and below 10 %, respectively, a quantity large enough to effectively influence their radiative properties. Individual results of above scans are summarized in Table 1. Table 1. Derived non-volatile number and volume fractions of selected 50 and 150 nm particles of individual volatility scans Aerosol Type

nv Number Fraction (%)

nv Volume Fraction (%)

Externallymixed (%)

50 nm Diesel

98.9

77.8

150 nm urban

99.6

21.7

19

150 nm rural

92.3

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3.1

Furthermore, the volatility of particles at intermediate temperature steps, 80 °C and 180 °C, respectively, may give indications about their lifetime, i.e., aging processes, based on their proportions of sulfuric acid and its neutralized, volatile products• Results of extensive VTDMA measurements will be presented and discussed•