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Knudsen cell construction, validation and studies of the uptake of oxygenated fuel additives on soot
Jacob Mønster1, Thomas Rosenørn, Ole John Nielsen
1Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen O, Denmark.
Intention, Goal, Scope, Background:
The properties of atmospheric particles are important to public health, radiative forcing of the atmosphere and to elucidating the chemical reactivity of atmospheric particles. We have constructed a Knudsen cell to study the uptake of organic compounds on soot. This article describes the construction and validation of the instrument, and our results on commercial soot concerning the uptake coefficient of ethanol, acetone, 1-butanol and diethoxymethane.
Objectives:
First, a technical description of the instrument is presented. Next, its performance is validated by measuring the uptake of NO2 on hexane soot. Finally, the uptake coefficients of four oxygenated hydrocarbons on commercial soot are presented. The objective is to contribute to the understanding of the formation of particles in motor vehicle exhaust.
Methods:
A Knudsen cell is used to measure the uptake of specific gas-surface systems. A quadrupole mass spectrometer is used to determine the decay rate of a pulse of reagent gas in the reaction chamber.
Results And Discussion:
The BET surface area of the commercial soot was 12.6 m2/g. The uptake coefficient (gamma) has been determined for ethanol (gamma0,BET = 7.7 +/- 4.8 x 10(-8)), 1-butanol 1.4 +/- 0.54 x 10(-7)), acetone (gamma0BET = 1.5 +/- 0.15 x 10(-7)) and diethoxymethane (gamma0,BET = 2.6 +/- 0.61 x 10(-7). These results are characteristic of the specific soot sample used. The ordering of the uptake coefficients, ethanol < 1-butanol approximately acetone < diethoxymethane, can be ascribed to a combination of physical (size and mass) and chemical effects. In addition, the initial uptake coefficient for NO2 on fresh hexane soot was determined to be gamma0,BET = 1.7 +/- 1.1 x 10(-4).
Conclusions:
In conclusion, we demonstrate that this instrument is able to measure uptake coefficients that are in agreement with accepted literature values. New data is presented concerning four light oxygenated hydrocarbons.
Recommendations And Outlook:
A large amount of detailed information concerning individual heterogeneous reactions is necessary in order to model the composition of motor vehicle emissions. We look forward to increasing the size of this database. Results for a series of alcohols and alkanes will be presented in a forthcoming publication.
