Dept. of Environmental and Biomolecular Systems
Oregon Health & Science University
The overall composition and concentration of atmospheric particulate matter (PM) influences the effects of PM on visibility, cloud formation, and human health. Aerosol PM in the ambient atmosphere may be comprised of a mixture of non-polar and relatively polar organic compounds, inorganic salts, and water. When significant amounts of water, non-polar compounds related to primary anthropogenic emissions and/or plant wax debris, and organic species are present in an aerosol system, the aerosol PM may be more stable as two liquid phases than as one homogeneous liquid phase. When phase separation to two liquid phases can occur, with one phase relatively more polar and the other relatively less polar, it can be expected that the increased stability of the system that is accomplished will generally lead to higher total PM (TPM, Î¼g m-3) concentrations than would otherwise be the case. On the other hand, the additional presence of PM-phase dissolved inorganic salts has been observed to decrease the organic portion of TPM. A modeling method was developed for use in predicting the stability of multiple liquid phases in atmospheric PM. The method utilizes a pseudo-diffusion process that simulates the multicomponent inter-phasic movement of constituents between adjacent PM phases. It can be used as a stand-alone application, and can also be incorporated in overall gas/particle (G/P) partitioning models of aerosol PM formation. The well-known UNIFAC (UNIQUAC Functional-group Activity Coefficients) method was used to calculate the necessary activity coefficient (Î¶) values of all diffusing constituents. UNIFAC applies only to mixtures of non-electrolytes. In order to predict the effects of dissolved inorganic salts on the formation of aerosol PM, a UNIFAC-based method (Ionic-UNIFAC.1) was developed for calculating activity coefficients of neutral compounds in general PM comprised of a mixture of organic compounds, dissolved inorganic salts, and water, with total salt concentrations as much as 2 mol kg-1. The Î¶ values are considered to be determined by a combination of short- and long-range interactions. The expression utilized for Î¶ involves both a Debye-HÃ¼ckel term and conventional UNIFAC terms. This method was then implemented in a G/P partitioning model to predict the formation of general PM and changes in PM concentrations and compositions relative to cases in which no salt was present. It was predicted that the combined presence of low-polarity compounds together with higher polarity compounds can lead to phase separation to two phases, even in the absence of water in the PM. The assumption of a single PM phase when in fact the PM is more stable as two phases will lead to errors in the predicted TPM value; the examples considered here gave errors in the range -3.9% to -21.8%. The relative error in Î¶ values predicted with Ionic-UNIFAC.1 was found to be independent of the ionic strength (I). On average, the relative error tended to decrease with increasing activity (a) over the range -2 < log a < 0.5, and did not exceed 20% for the data fitted. Average errors in predicted aerosol yields and hygroscopic growth factors in general aerosol systems with dissolved salt concentrations â 2 mol kg-1 were ~20%, which is within the expected range of error for Ionic-UNIFAC.1. Excellent agreement was obtained between predicted and measured decreases in aerosol yield in the Î±-pinene/O3 system at RH = 50% with aqueous (NH4)2SO4 and CaCl2 seed: 45% vs. 44%, and 21% vs. 24%, respectively. The methods developed here can be used as tools to predict concentrations and compositions of aerosol PM, and can be applied within three-dimensional air-shed models to assist in air quality management.
OGI School of Science and Engineering
Erdakos, Garnet Bailey, "Effects of multiple phases and ionic constituents on gas/particle partitioning in atmospheric and other aerosols" (2004). Scholar Archive. 44.