Experimental artifacts of size distributions of H 2O-H 2SO 4 aerosol particles formed in a photochemical vertical laminar flow reactor due to its design

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J. Aerosol Sci. Vol. 28, Suppl. I, pp. $341-S342, 1997 ©1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0021-8502197 $17.00+0.00

PII:S0021-8502(97)00218-8

EXPERIMENTAL AEROSOL

ARTIFACTS OF SIZE DISTRIBUTIONS

O F H=O-H2SO4

P A R T I C L E S F O R M E D IN A P H O T O C H E M I C A L

VERTICAL

L A M I N A R F L O W R E A C T O R D U E T O ITS D E S I G N Beate Roth *, Rita V a n Dingenen and Frank Raes European Commission, JRC Ispra, Environment Institute 1-21020 Ispra (Va), Italy * c/o Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305, Japan KEYWORDS atmospheric aerosols, H2SO4 particle, flow reactor, nucleation, condensation, coagulation ABSTRACT As aerosol particles have a direct and indirect effect on the radiation budget of the earth more detailed knowledge of the their formation and modification processes ist needed in order to be able to improve their recognition in numerical transport models. A photochemical reactor which provides controlled experimental conditions is a useful tool for gas-to particle conversion studies. Therefore the vertical laminar flow reactor AERREA2 [Roth al., 1994] was set up to study nucleation, condensation and coagulation processes of H20-H2SO4 aerosol particles, which are formed in situ by oxidation of SO2. The reactor has a special design to ensure laminar flow conditions in its reaction section. The design and the used chemical system was described in detail by Roth (1996). The 9 m long reaction section of the flow tube is divided in 5 sections of actinic lamps, which can be switched on independently. The samples are always taken at the end of the whole reaction section from the core of the reactor. In Figure 1 the size distributions of particles, which were measured after irradiating different sections of the reactor, are shown. The effect of coagulation can be seen clearly. The particles which were formed at the first part of the reactor and transported before sizing, showed a lower total number concentration and a larger diameter than particles measured directly after irradiation in an identically long section at the lower part of the reactor.

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Fig. 1: Aerosol size distributions measured with a differential mobility analyser (TSI), different sections of the reactor were irradiated with maximum intensity. Residence time in the reactor core=222 s. Initial conditions: RH=50%, T=25°C, [S02]=0. 6ppm, [HONO]=O. 8ppm AS 28:SI-M

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Abstractsofthe1997EuropeanAerosolConference

In order to achieve more information of the nucleation process of the sulphuric acid particles the intensity of the UV-lamps was decreased to 25% of the previously used value. The particles formed under these conditions were sized by a ultrafine differential mobility analyser which enables measuring particles down to a diameter of 3nm. Figure 2 shows the size distributions achieved for irradiating different sections with reduced intensity. The fact that the total number concentration of particles formed in the first section was found to be higher than of particles formed in the last identically long section must be caused by an experimental artifact. 1.00E+08 I:: u

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Fig. 2: Aerosol size distributions measured with an ultrafine differential mobility analyser, different sections of the reactor were irradiated with 25% of the maximum intensity. Residence time in the reactor core=72s Initial conditions:, RH=50%, T=25°C, [S O2]=O.6ppm, [HONO]=O. 8ppm Figure 3 shows calculated gaseous H2SO,-concentrations, for the irradiation conditions used in the experiment with reduced UV-intensity, as a function of distance from the reactor core. The H2SO4-profiles suggests the formation of sulphuric acid near the wall of the reactor (last section irradiated) and transport by radial diffusion to the core of the reactor, when the residence time is sufficient (first section irradiated). The cross section of the reactor from which samples were taken is indicated by rsamp. This effect might be an explanation for the observed artifact of measuring too low total number concentrations of new formed very small particles in the core of the reactor, because those particles are formed near the wall of the reactor. o

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Fig. 3: Calculated gaseous H2SO4-concentration produced for different irradiation sections with 25% of maximum intensity as a function of radial distance from the reactor core. A multicomponent aerosol model was used to get a better understanding of the processes which led to the experimental results discussed here [Wiick et al., 1997] REFERENCES Roth, B., Van Dingenen, R. and Raes, F. (1994) J. Aerosol Sci., 25 suppl.1, Sl1-S12. Roth, B. (1996) Ph.D. Thesis, University of Hamburg. Wilck et al. (1997) this conference