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ECN publication
Humidity dependence of the aerosol light-scattering by mixtures of ammonium nitrate, ammonium sulfate and soot
Published by: Publication date:
ECN 1997
ECN report number: Document type:
ECN-RX--97-008 Article (scientific)
Number of pages:

Published in: To be submitted for publication in the Journal of aerosol science (), , , Vol., p.-.

The light-scattering by aerosols of ammonium nitrate, ammonium sulfateand internal mixtures of the two salts was investigated. The aerosols, produced by nebulizing of solutions, were of a size and in concentrations similar to those at ambient aerosols in The Netherlands. Ammonium sulfate showed the expected hysteresis behaviour, with a decrease in light-scattering when the air was dried to below the crystallisation point (42% RH) and a large increase in light-scattering when the humidity of the dried air was subsequently raised at the deliquescence point (80% RH). In contrast, the light-scattering by ammonium nitrate aerosols showed a continuous change in light-scattering signal as a function of humidity. This was evidence that crystallisation did not occur when the aerosol was dried to the typical relative humidities used in ambient measurements of aerosols. Crystallisation was even absent when the air was dried to 8% RH, an unrealistic low value for ambient measurements. Next, aerosols consisting of (internal) mixtures of ammonium nitrate and ammonium sulfate were investigated. Those with a mixing ratio of 2.5 and higher did not dry. Ambient aerosol with similar mixing ratios behaved similarly. Laboratory aerosols with a mass ratio of ammonium nitrate to ammonium sulfate of 1.2 could be dried and they thus also had a deliquescence point, between 55% and 60% RH, close to that of ammonium nitrate. Ambient aerosols with the same mixing ratios showed similar humidity behaviour. Laboratory aerosol with intermediate mixing ratios (between 1.5 and 2.5) did not dry, but ambient aerosols with similar composition did. It was hypothesized that the insoluble 'soot' in ambient particles acted as crystallisation nuclei in the aerosols. However, tests with realistic amounts of 'soot' added to (laboratory) mixtures of ammonium nitrate/ammonium sulfate did not produce crystallisation. 9 figs., 20 refs.

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