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The humidified nephelometer system allows us to look at the value of light scattering as a function of relative humidity (RH). To be able to compare how this indicator of hygroscopic growth changes for different types of aerosol (e.g., dust, smoke, sea salt, or pollution) we normalize the wet scattering using the scattering at low humidity to derive the dimensionless parameter 'f(RH)'. We define f(RH) as the ratio of light scattering at 85% humidity to the scattering at relative humidities < 40%. (Other research groups use slightly different values relative humidity in their definitions of f(RH). Some examples of how f(RH) changes as a function of humidity are available for Bondville and Southern Great Plains (scroll down to hygroscopic growth section).
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This is an example of a humidity scan. The thin black line shows the set points for humidity. The yellow dots show the actually humidity achieved by the humidification system. The red dots at the bottom show the cut points over the humidity scan as the airstream switches between going through both the 1 and 10 micrometer impactor and going through just the 10 micrometer impactor. On aircraft systems, the humidification in the wet nephelometer is controlled at a single relative humidity (~82%) rather than scanning through a range of humidities. While this results in the shape of the hygroscopic growth curve being unknown, it allows for comparison of hygroscopic growth as the aircraft can quickly move from one type of aerosol to the next.
