In Situ Particle Formation/Reaction Mechanisms

Abstract

A significant fraction of the atmospheric particulate matter is not emitted directly as particles, but it is formed in-situ by gas-to-particle conversion. This section provides an overview of the fundamental and applied aspects of the processes leading either to the formation of new particles or to the transfer of material from the gas phase onto preexisting aerosols. The thermodynamic principles that govern the partitioning of species between the gas and aerosol phases are discussed first. An overview of the gas-phase reaction pathways leading to the formation of the main gaseous aerosol precursors (H2SO4, HNO3, NH3 and condensible organic gases) is presented next. The basic theory for the formation of new particles either from a single vapor or from two gaseous species is presented including brief discussion of the observed discrepancies with laboratory results. The next paragraph is a basic treatment of the mass transport of material from the gas to the particulate phase, with emphasis on the interaction of the gas molecules with the particle surface. The framework established in the above paragraphs is then used for the discussion of inorganic and organic aerosol formation mechanisms and the discussion of the gas-aerosol phase equilibrium assumption.