Ammonium nitrate aerosol is volatile under typical ambient conditions and thus difficult to measure. In the field the scattering coefficient of the dry aerosol is measured with a nephelometer by heating the ambient aerosol to a low reference relative humidity (typically around 40%). In this paper we examine the decrease in the light scattering coefficient of ammonium nitrate aerosol due to evaporation in a heated nephelometer. Changes in the scattering coefficient of a laboratory generated monodisperse ammonium nitrate aerosol are measured as a function of mean residence time and temperature within the nephelometer sample volume. At the same time the change in the aerosol size distribution due to ammonium nitrate evaporation is directly measured with a laser particle counter. The change in the aerosol size distribution and scattering coefficient is modeled as a function of mean residence time and temperature. Model results for the change in the aerosol scattering coefficient due to evaporation agree with measurements to within 10%. Application of the theory to conditions typical of NOAA field sites suggests that the decrease in the aerosol scattering coefficient due to the evaporation of ammonium nitrate is typically less than 20%.
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