Sulfur dioxide oxidation induced mechanistic branching and particle formation during the ozonolysis of ß-pinene and 2-butene.

Recent studies have suggested that the reaction of stabilised Criegee Intermediates (CIs) with sulfur dioxide (SO(2)), leading to the formation of a carbonyl compound and sulfur trioxide, is a relevant atmospheric source of sulfuric acid. Here, the significance of this pathway has been examined by studying the formation of gas phase products and aerosol during the ozonolysis of ß-pinene and 2-butene in the presence of SO(2) in the pressure range of 10 to 1000 mbar. For ß-pinene at atmospheric pressure, the addition of SO(2) suppresses the formation of the secondary ozonide and leads to highly increased nopinone yields. A complete consumption of SO(2) is observed at initial SO(2) concentrations below the yield of stabilised CIs. In experiments using 2-butene a significant consumption of SO(2) and additional formation of acetaldehyde are observed at 1 bar. A consistent kinetic simulation of the experimental findings is possible when a fast CI + SO(2) reaction rate in the range of recent direct measurements [Welz et al., Science, 2012, 335, 204] is used. For 2-butene the addition of SO(2) drastically increases the observed aerosol yields at higher pressures. Below 60 mbar the SO(2) oxidation induced particle formation becomes inefficient pointing to the critical role of collisional stabilisation for sulfuric acid controlled nucleation at low pressures.

Pressure dependent aerosol formation from the cyclohexene gas-phase ozonolysis in the presence and absence of sulfur dioxide: a new perspective on the stabilisation of the initial clusters.

The ozonolysis of cyclohexene is studied with respect to the pressure dependent formation of stable gas-phase products and secondary organic aerosol (SOA) as well as the influence of the presence of SO(2). In addition the rate coefficient for the initial reaction cyclohexene + O(3) was determined at 295 K. The observed increase in CO and ethene yields at low pressures and the absence of ketene in the product spectrum confirm previously proposed reaction pathways forming these decomposition products. An enhanced ethene formation at pressures below 300 mbar coincides with drastically decreased aerosol yields pointing to a high influence on SOA formation of chemical activation driven dynamics in the vinylhydroperoxide channel. The static reactor experiments at 450 mbar in the presence of SO(2) in the present study showed a similar sensitivity of additional particle formation to H(2)SO(4) number densities as found in near-atmospheric flow reactor experiments [Sipiläet al., Science, 2010, 327, 1243], a surprising result with regard to the very different experimental approaches. At low pressures (around 40 mbar) no significant new particle formation is observed even at high H(2)SO(4) concentrations. These findings indicate that the collisional stabilisation of initial clusters is an important aspect for SOA formation processes involving sulfuric acid and organic compounds. The results may have implications for geo-engineering strategies based on stratospheric sulfur injection, but caution is mandatory when room temperature laboratory results are extrapolated to stratospheric conditions.