Ammonium nitrate is a semivolatile aerosol component under typical ambient conditions and thus difficult to measure. In the field, the aerosol scattering coefficient is usually measured width a nephelometer by heating the ambient aerosol to a low reference relative humidity (~40%) in order to measure a light scattering coefficient that is intrinsic to an aerosol rather than dependent on atmospheric relative humidity. 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 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 generally less than 20%.