EVAPORATION OF AMMONIUM-NITRATE AEROSOL IN A HEATED NEPHELOMETER - IMPLICATIONS FOR FIELD-MEASUREMENTS

Ammonium nitrate is a semivolatile aerosol component under typical amb ient conditions and thus difficult to measure. In the field, the aerso sol scattering coefficient is usually measured with a nephelometer by heating the ambient aerosol to a low reference relative humidity (simi lar to 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 scatter ing coefficient of ammonium nitrate aerosol due to evaporation in a he ated nephelometer. Changes in the scattering coefficient of a laborato ry-generated ammonium nitrate aerosol are measured as a function of me an residence time and temperature within the nephelometer sample volum e. At the same time, the change in the aerosol size distribution due t o ammonium nitrate evaporation is directly measured with a laser parti cle counter. The change in the aerosol size distribution and scatterin g coefficient is modeled as a function of mean residence time and temp erature. Model results for the change in the aerosol scattering coeffi cient due to evaporation agree with measurements to within 10%. Applic ation of the theory to conditions typical of NOAA field sites suggests that the decrease in the aerosol scattering coefficient due to the ev aporation of ammonium nitrate is generally less than 20%.