HAZE FORMATION: ITS NATURE AND ORIGIN
|Publication Date:||1 March 1975|
SCOPE OF PROGRAM
The initial objective of this program was to develop a method of distinguishing between light-scattering aerosols attributable to automotive sources as opposed to those from nonautomotive sources. Both chemical and optical approaches were considered initially, but upon reviewing the possibilities in more detail, emphasis was placed on chemical methods of analysis. Thus the first year of the program was devoted to seeking a chemical basis which would permit differentiation among the organic composition of aerosols according to precursors. For this purpose aerosols associated with the following sources were collected for analysis:
(1) Rural atmospheres. Blue Ridge Mountains, N.C.
(2) Controlled atmospheres, Battelle-Columbus smog chamber
(3) Diluted primary auto exhaust
(4) Urban atmospheres, Bronx, New York.
Chemical fractionation of particulate extracts was performed after which organic constituents were analyzed by gas chromatography combined with mass spectrometry. It became apparent that spectra of organic material present from each of the above sources is enormously complex. This is true even when a single hydrocarbon and NOX are irradiated to produce aerosols.
Irradiation of o-pinene (one of several terpenes emitted in abundance by trees and plants) and nitrogen oxides in a smog chamber produced aerosols having chemical properties similar to that of aerosols collected in a forested region. In this case, pinonic acid was identified as a product of both the natural-rural (Blue Ridge Mountains) and the simulated-rural (smog chamber) aerosols. This finding served to confirm the utility of the smog chamber in simulating atmospheric conditions conducive to secondary aerosol formation.
Analyses of primary auto-exhaust aerosols revealed two predominant compounds, namely benzoic and phenylacetic acid, among the complex acidic fraction of the aerosol matter. However, these aromatic acids were not detected in urban particulate similarly analyzed. This result is rationalized in terms of dilution of auto exhaust in the atmosphere, or on the basis of removal of these acids from the atmosphere by photochemical reactions. Indeed, decarboxylation of organic acids was found to occur during the progress of aerosol formation under smog-chamber conditions.
The principal mission during the first year was to determine if a chemical basis could be established for estimating the atmospheric burden of secondary aerosols related to automotive emissions. Combined gas chromatography - mass spectrometry analyses revealed that the organic constituency of atmospheric aerosols' was enormously complex - so complex that complete resolution was a formidable task. It was also obvious that automobile exhausts do not contain hydrocarbons uniquely related to automotive operations*. Thus at the conclusion of the first year, the approach of establishing unique precursor relationships between automotive emissions and the chemical composition of aerosols was abandoned, in spite of the evidence that aerosol precursor relationships could be established in smog-chamber simulations of simple systems. The problem as it related to auto exhaust was too complex, and too little was known about the chemical nature of aerosols in the atmosphere.
*There are a few hydrocarbons, such as acetylene and ethylene, which might be regarded as unique to automotive emissions on a relative basis, but these hydrocarbons do not participate in the formation at organic matter condensing into aerosols.