Aerosol formation and growth in atmospheric organic/NO_x systems—I. Outdoor smog chamber studies of C_7- and C_8-hydrocarbons

Abstract

Outdoor smog chamber experiments have been performed to determine the aerosol-forming potential of selected C_7- and C_8-hydrocarbons in sunlight-irradiated hydrocarbon-NO_x mixtures. Measured aerosol size distributions were used to determine the rates of gas-to-particle conversion and to study the effects of the addition of SO_2 and/or NH_3 on aerosol formation and growth. The average aerosol yields by mass for the hydrocarbons studied were (the range of measured values for methylcyclohexane and 1-octene are in parentheses): • methylcyclohexane 9.2% (0.12–18.8); • 1-octene 4.2% (0.17–6.9); • toluene 18.6%; • n-octane <0.001%. The average yields are accompanied by large standard deviations (see Table 3) and depended strongly on the conditions, particularly the hydrocarbon to NO_x initial concentration ratio. Addition of SO_2 to the organic/NO_x systems led to an early nucleation burst and subsequent rapid growth of the newly formed aerosol. In the presence of NH_3, the gas-to-particle conversion rate of the organic/NO_x system was enhanced perhaps due to the formation of NH_4NO_3 or the reaction of NH_3 with carboxylic acids. Sustained particle formation was observed when both SO_2 and NH_3 were present, presumably as a result of (NH_4)_2SO_4 formation. We have estimated the complexity of the 1-octene aerosol and identified 5-propyl furanone as a component of the aerosol.

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