Environmental Context. Atmospheric particulate material can affect climate by absorbing and scattering solar radiation and by altering the properties of clouds. They are also implicated as a health risk. Secondary organic aerosol (SOA) material makes an important contribution to this particulate burden. SOA material results from the transfer of gas-phase species into a particle state after the formation of products from the reaction of atmospheric volatile organic compounds (VOCs) with oxygen. SOA from the oxidation of aromatic hydrocarbons, such as toluene, a gasoline fuel component, is important in the polluted urban environment and yet formation mechanisms are not well understood.

Abstract. The formation and composition of secondary organic aerosol (SOA) from the gas-phase photooxidation of toluene has been simulated using the Master Chemical Mechanism version 3.1 (MCM v3.1) coupled to a representation of the transfer of organic material from the gas phase to a particle phase. The mechanism was tested against data from a series of toluene photooxidation experiments performed at the European Photoreactor (EUPHORE) outdoor smog chamber in Valencia, Spain. Simulated aerosol mass concentrations compared reasonably well with the measured SOA data after absorptive partitioning coefficients were increased by a factor of between 20 and 80, although the simulated onset of SOA growth was delayed with respect to the experiments. A simplified representation of peroxyhemiacetal adduct formation, from the reaction of organic hydroperoxides with aldehydes in the condensed organic phase, was included in the mechanism and this reduced the required scaling of partitioning coefficients and reduced the time-lag in simulated SOA growth. These observations, and the dependence of SOA formation efficiency upon the initial NO concentration, strongly imply the significant occurrence of association reactions in the condensed organic phase and the important role of organic hydroperoxides in SOA formation. Aerosol data from photooxidation experiments of intermediate degradation products (butenedial, 4-oxo-pentenal, and ortho-cresol) were also simulated using the developed mechanism.