| dc.description.abstract |
A ``humidograph{''} with an air flow rate of 0.4 m(3) s(-1) was built to investigate
the uptake of water and its effect on sizing, collection and light scattering of ambient
aerosol. In the humidograph the relative humidity (RH) can be scanned over a large
RH trajectory. Its performance was assessed with laboratory particles of ammonium
nitrate, ammonium sulfate and sodium chloride that are the major hygroscopic components
of ambient aerosol. The increase in size at the deliquescence points, which ideally
is a stepwise function of RH, occurs over a range of 3\% RH units. This is shown to
be an optimum value in a system of such large dimensions. Because the vapor pressure
of ammonium nitrate increases significantly with temperature, its evaporative loss
was investigated as a function of heating/drying temperature. The loss of pure test
aerosol, with a mass distribution similar to that in the ambient atmosphere, was found
to be acceptable for drying temperatures of up to 40 degrees C. The sizing of deliquesced
aerosol by LAS-X monitors was tested and found to be a complex function of RH. In
Berner low-pressure impactors growth of hygroscopic aerosol was not observed, not
even at an RH approaching saturation. (C) 2000 Elsevier Science Ltd. All rights reserved.
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