The effect of dioctyl phthalate films on the ammonium nitrate aerosol evaporation rate

The evaporation of submicrometer ammonium nitrate (NH4NO3) aerosol coated with an organic film was measured in order to determine the effect of the film on mass transfer rate and equilibration time of the semi-volatile inorganic. Ammonium nitrate (NH4NO3) particles 100–200 nm in diameter were coated with dioctyl phthalate (DOP) and allowed to evaporate in a constant temperature laminar flow reactor. Evaporation rates for the organic-coated particles were compared to pure ammonium nitrate evaporation at 22 and 27°C. A decrease, up to 50%, in NH4NO3 evaporation rate due to the presence of the DOP film was observed. The decrease in evaporation due to the DOP can be described mathematically at 22°C by a decrease in the accommodation coefficient for NH4NO3, αNH4NO3, from 0.4 (the pure NH4NO3 value) to 0.25 for the DOP-coated NH4NO3. Similarly, at 27°C, a decrease in αNH4NO3 from 0.3 for the pure NH4NO3 to 0.25 for the DOP-coated particles was estimated. The decrease in evaporation rates can also be explained by a decrease in NH4NO3 effective diffusivity. The implications to NH4NO3 formation and evaporation in the atmosphere are discussed.