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Title:
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Humidity dependence of the aerosol light-scattering by mixtures of ammonium nitrate, ammonium sulfate and soot
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Author(s):
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Published by:
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Publication date:
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ECN
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1997
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ECN report number:
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Document type:
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ECN-RX--97-008
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Article (scientific)
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Number of pages:
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33
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Published in: To be submitted for publication in the Journal of aerosol science (), , , Vol., p.-.
Abstract:
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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