Final Report: Formation and Physical Properties of Secondary Organic Aerosol

EPA Grant Number: R823514
Title: Formation and Physical Properties of Secondary Organic Aerosol
Investigators: Pandis, Spyros N.
Institution: Carnegie Mellon University
EPA Project Officer: Hahn, Intaek
Project Period: September 28, 1995 through September 27, 1998
Project Amount: $382,668
RFA: Exploratory Research - Chemistry and Physics of Air (1995) RFA Text |  Recipients Lists
Research Category: Air Quality and Air Toxics , Air , Safer Chemicals


The objective of this research project was the study of the formation of secondary organic aerosol (SOA) compounds, their transport from the gas to the aerosol phase, and their interaction with water, and the inorganic aerosol components. Specific goals included: (1) quantification of the ability of organic particles to serve as cloud condensation nuclei leading to droplet formation and faster removal from the atmosphere; (2) study of the mass transfer rates of organic and inorganic aerosol components between the gas and particulate phase; and (3) investigation of the dynamics of organic aerosol formation in a controlled environment (smog chamber). The measured variables were incorporated in a state-of-the-art secondary organic aerosol formation model (SOAM2). The ability to model the SOA formation process in the ambient atmosphere was evaluated by comparing the model output with results of a comprehensive field study in central California.

Summary/Accomplishments (Outputs/Outcomes):

Submicrometer atmospheric particles can serve as nuclei on which water condenses to form cloud droplets at supersaturations lower than those required for homogeneous water nucleation. Understanding the cloud condensation nuclei (CCN) activity of atmospheric aerosols is important in order to quantify their lifetime in the atmosphere, their transport distances, and their effect on cloud properties. In this study (Cruz and Pandis, 1997), the ability of model submicron aerosols consisting of pure organic species to become CCN was investigated. The CCN activity of glutaric acid, adipic acid, and dyoctylphathalate (DOP) aerosols was determined by producing a nearly monodisperse distribution of submicron particles and comparing total CCN concentrations to total number concentrations. The measurements were performed using a Tandem Differential Mobility Analyzer (TDMA) in combination with a cloud condensation nuclei counter at supersaturations of 0.3 percent and 1.0 percent. The uncertainty in the measurements was determined by using NaCl and (NH4)2SO4 aerosols; the results indicated that activation diameters could be measured within 15 percent. Adipic acid and glutaric acid aerosols served as CCN as both supersaturations and their behavior was in fair agreement with Kohler theory. On the other hand, DOP aerosol as large as 0.2 micrometers in diameter did not become activated, even at supersaturations as high as 1.2 percent. These results indicate that the CCN activity of hygroscopic organic aerosols may be comparable to that of some inorganic particles.

The ability of mixed atmospheric particles consisting of inorganic salts coated with organic films was investigated experimentally (Cruz and Pandis, 1998). We used two types of coatings around solid ammonium sulfate cores. The first was glutaric acid, a CCN active organic found in the atmosphere, and the second was dyoctylphathalate, a nonhygroscopic organic. Glutaric acid can be viewed as representative of secondary organic material, while DOP is often used as a model for primary organic aerosol. The CCN activation of ammonium sulfate-glutaric acid particles was measured at a supersaturation of 0.3 percent, for different inorganic core sizes and organic film thickness. We found that a coating of glutaric acid increases the CCN activation of an ammonium sulfate particle and that this behavior can be predicted by Kohler theory. The deviation from theory for the mixed aerosol was determined by comparing theoretical and experimental CCN activation diameters for the particles and was found to be within experimental error. A thick coating of DOP (at least 70 percent per mass) did not hinder the activation of ammonium sulfate particles at supersaturations of 0.5 and 1.0 percent. The values for the measured activation diameters for the DOP-coated ammonium sulfate particles were within the experimental error determined by the pure inorganic experiments, indicating that DOP was most likely acting as inert mass during activation. These results provide strong evidence that the current estimates of significantly longer lifetimes of organic particles compared to their inorganic counterparts are erroneous. Particles containing secondary organic aerosol matter will activate readily, while the hydrophobic primary organic aerosol matter appears not to inhibit the activation of the inorganic salts.

The condensation of organic vapors on a nearly monodisperse externally mixed aerosol population was measured in order to test the hypothesis that organic species may preferentially condense on specific substrates (Cruz and Pandis, 1999). The organic species tested were glutaric acid, a typical secondary organic species, and DOP, a model organic for primary species. A series of organic and inorganic substrates were investigated: sodium chloride, ammonium sulfate, glutaric acid, and adipic acid. The growth of these populations was measured using a TDMA and the results were analyzed using transition regime mass transfer theory. The measurements show that no detectable preferential condensation occurs for either species on any of the substrates studied. Analysis of the results suggest that the accommodation coefficients of the organics studied on the different substrates are of order unity (greater than 0.2) and do not depend strongly on the substrate composition. Formation of the droplet mode in the ambient SOA distribution cannot be explained by preferential condensation of the organics. These results can be used in the development of atmospheric aerosol models that do not assume an internally mixed aerosol population.

A comprehensive model was developed for the description of the dynamic gas- particle partitioning of semivolatile organics generated from the oxidation of atmospheric hydrocarbons (Bowman, et al., 1997). Vapor transport to aerosol particles, nucleation, and particle deposition were accounted by the model, allowing the aerosol size distribution to be predicted both in the atmosphere and in a laboratory smog chamber. The results were compared with m-xylene/NOx outdoor smog chamber experiments conducted in the chamber of the California Institute of Technology. The model was successful in reproducing the evolution of the aerosol number and mass concentration as well as its size distribution during the 5-hour long experiments. When inorganic seed particles were present, gas-particle partitioning exhibits a threshold for aerosol growth. Until sufficient hydrocarbon reacts to create a concentration of condensable products that will exist in equilibrium with an organic aerosol phase, no aerosol is formed. An examination of characteristic times suggested conditions where an assumption of instantaneous gas-particle equilibrium is justified. Based on the time delay for the onset of aerosol growth and the observed yield curves, the condensing vapor species from m-xylene were predicted to be second- generation (products of the products of the parent hydrocarbon) rather than first-generation. This was supported by the chemical structure of the few identified organic aerosol components. The mass accommodation coefficient of the m-xylene produced secondary organic aerosol components was estimated to have a value between 0.1 and 1.0.

The mass transfer rate of pure ammonium nitrate between the aerosol and gas phases was quantified experimentally by the use of the TDMA/SMPS (Tandem Differential Mobility Analyzer/Scanning Mobility Particle Sizer) technique. Ammonium nitrate particles 80 to 220 nm in diameter evaporated in purified air in a laminar flow reactor under temperatures of 20 to 27oC and relative humidities in the vicinity of 10 percent. The evaporation rates were calculated by comparing the initial and final size distributions. A theoretical expression of the evaporation rate incorporating the Kelvin effect and the effect of relative humidity on the equilibrium constant is developed. The measurements were consistent with the theoretical predictions, but there was evidence of a small kinetic resistance to the mass transfer rate. The discrepancy can be explained by a mass accommodation coefficient ranging from 0. 8 to 0.5 as temperature increases from 20 to 27oC. The corresponding timescale of evaporation for submicron NH4NO3 particles in the atmosphere is of the order of a few seconds to 20 minutes. The potential inhibition of this mass transfer of ammonium nitrate by organics is the subject of current studies.

The above model, which evaluated laboratory experiments, was extended for the simulation of secondary organic aerosol formation in the ambient atmosphere (Strader, et al., 1999). The new model (Secondary Organic Aerosol Model 2, SOAM2) is a Lagrangian trajectory model that simulates the formation, transport, and deposition of secondary organic aerosol. The model was applied to the Integrated Monitoring Study (IMS 95) in the San Joaquin Valley, California. Under suitable conditions (clear skies, low winds, low mixing heights) as much as 15 g C m-3 of SOA can be produced mainly due to the oxidation of aromatics. The low mixing heights observed during the winter in that area allow accumulation of SOA precursors and the acceleration of SOA formation. Clouds and fog slow down the production of secondary compounds, reducing their concentrations by a factor of two or three from the above maximum levels. In addition, it appears that there is strong diurnal variation of SOA concentration. A strong dependence of SOA concentrations is observed, along with the existence of an optimal temperature for SOA formation. These SOAM2 results were in general agreement with estimates of the SOA concentration by using elemental carbon as a tracer of primary organic carbon.

Journal Articles on this Report : 5 Displayed | Download in RIS Format

Other project views: All 5 publications 5 publications in selected types All 5 journal articles
Type Citation Project Document Sources
Journal Article Bowman FM, Odum JR, Seinfeld JH, Pandis SN. Mathematical model for gas-particle partitioning of secondary organic aerosols. Atmospheric Environment 1997;31(23):3921-3931. R823514 (Final)
R824970 (Final)
  • Full-text: ScienceDirect-Full Text PDF
  • Abstract: ScienceDirect-Abstract
  • Journal Article Cruz CN, Pandis SN. A study of the ability of pure secondary organic aerosol to act as cloud condensation nuclei. Atmospheric Environment 1997;31(15):2205-2214. R823514 (Final)
    not available
    Journal Article Cruz CN, Pandis SN. The effect of organic coatings on the cloud condensation nuclei activation of inorganic atmospheric aerosol. Journal of Geophysical Research.D.Atmospheres 1998;103(D11):13111-13124. R823514 (Final)
    not available
    Journal Article Dassios KG, Pandis SN. The mass accommodation coefficient of ammonium nitrate aerosol. Atmospheric Environment 1999;33(18):2993-3003. R823514 (Final)
    not available
    Journal Article Strader R, Lurmann F, Pandis SN. Evaluation of secondary organic aerosol formation in winter. Atmospheric Environment 1999;33(29):4849-4863. R823514 (Final)
    not available

    Supplemental Keywords:

    air, chemical transport, particulates, organics, modeling, RFA, Scientific Discipline, Air, Geographic Area, particulate matter, air toxics, Physics, Chemistry, State, Engineering, Chemistry, & Physics, EPA Region, ambient aerosol, ambient air quality, fate and transport, particulates, PM10, PM 2.5, Pennsylvania, atmospheric particles, aerosol particles, emission control technologies, exposure, Region 3, spectroscopic studies, VOCs, chemical mixtures, ambient emissions, organic compounds, air pollution modeling system, Tandem Differential Mobility Analyzer, chemical kinetics, secondary organic aerosol, urban air , aerosol production, aerosols

    Progress and Final Reports:

    Original Abstract
  • 1996
  • 1997