Exhaust Hydrocarbon Relationships with Photochemical Aerosol Formation

Abstract

The formation of photochemically derived aerosols from auto exhaust vapors has been studied in the Battelle-Columbus smog chamber for several years, and this paper summarizes our principal findings on the subject. In leading up to conclusions regarding exhaust composition effects on aerosol formation, salient features of secondary aerosol growth and the measurement of these aerosols by light scattering methods are discussed. Inclusion of primary exhaust aerosols during the irradiation of exhaust influences the ultimate size distribution of the secondary aerosols and may therefore have sizeable effects on light scattering. With unleaded fuel, relatively little primary aerosol is present, and the effect is minimal. In the absence of primary aerosols, substantial concentrations of secondary aerosols might be produced before aerosol growth extends into the light scattering size range.

When the smog chamber data are considered entirely, a significant correlation exists between the percent aromatic content of the exhaust and secondary aerosol formation measured by light scattering. The statistical significance of the relationship is only slightly improved by the use of a linear summation model to predict light scattering from hydrocarbon reactivities established for individual hydrocarbons. In the important region of exhaust aromatic content between 25 and 50%, neither the reactivity model nor the aromatic content of the exhaust accurately predicts the reactivity of the exhaust in forming aerosols. By analogy with results observed when mixtures of two hydrocarbons were irradiated, the lack of a hydrocarbon composition effect in the 25-50 % aromatic region is thought to involve reactions in which competition occurs between hydrocarbons having different reactivities for aerosol formation and NO oxidation. The chemistry of these processes is complex and little understood. In conclusion, it is apparent that a substantial reduction in aerosols derived photochemically from auto exhaust could be achieved with reasonable certainty only by taking steps which would reduce the aromatic content of the exhaust to some very low concentration.