Real-Time Monitoring of the Surface and Total Composition of Aerosol Particles

Laser desorption ionization is used to characterize the products of an aerosol reaction in real time. Sodium chloride aerosols are mixed with vapor phase ammonia and nitric acid to produce an ammonium nitrate surface layer. The product aerosols are sampled directly into a mass spectrometer where the surface and total compositions are distinguished by laser desorption ionization. With a low laser irradiance, only material near the particle surface is desorbed and ionized. In this case, the mass spectra are independent of the amount of ammonium nitrate deposited, and simply indicate a nitrate·enriched surface layer. With a higher laser irradiance, both the surface layer and core are ablated. The relative intensities of ions originating from the surface layer and core reflect the total composition. That is, the relative intensities of ions produced from the ammonium nitrate surface layer increase with increasing surface layer thickness. The laser irradiance dependences of the mass spectra allow the surface and total composition to be qualitatively distinguished, but particle-to-particle variations of the relative signal intensities preclude a quantitative measurement of the amount of ammonium nitrate deposited.