Reactivity of stabilized criegee intermediates (sCI) from isoprene and monoterpene ozonolysis toward SO2 and organic acids

Abstract

Oxidation processes in Earth's atmosphere are tightly connected to many environmental and human health issues and are essential drivers for biogeochemistry. Until the recent discovery of the atmospheric relevance of stabilized Criegee intermediates (sCI), atmospheric oxidation processes were thought to be dominated by few main oxidants: ozone, hydroxyl radicals (OH), nitrate radicals and, e.g. over oceans, halogen atoms such as chlorine. Here, we report results from laboratory experiments at 293 K and atmospheric pressure focusing on sCI formation from the ozonolysis of isoprene and the most abundant monoterpenes (α-pinene and limonene), and subsequent reactions of the resulting sCIs with SO2 producing sulphuric acid (H2SO4). The measured sCI yields were (0.15 ± 0.07), (0.27 ± 0.12) and (0.58 ± 0.26) for the ozonolysis of α-pinene, limonene and isoprene, respectively. The ratio between the rate coefficient for the sCI loss (including thermal decomposition and the reaction with water vapour) and the rate coefficient for the reaction of sCI with SO2,k(loss) /k (sCI + SO2), was determined at relative humidities of 10% and 50%. Observed values represent the average reactivity of all sCIs produced from the individual alkene used in the ozonolysis. For the monoterpene derived sCIs, the relative rate coefficientsk(loss) /k(sCI + SO2) were in the range (2.0-2.4) × 1012 molecule cm-3 and nearly independent on the relative humidity. This fact points to a minor importance of the sCI + H2O reaction in the case of the sCI arising from α-pinene and limonene. For the isoprene sCIs, however, the ratiok(loss) /k(sCI + SO2) was strongly dependent on the relative humidity. To explore whether sCIs could have a more general role in atmospheric oxidation, we investigated as an example the reactivity of acetone oxide (sCI from the ozonolysis of 2,3-dimethyl-2-butene) toward small organic acids, i.e. formic and acetic acid. Acetone oxide was found to react faster with the organic acids than with SO2;k(sCI + acid) /k (sCI + SO2) Combining double low line (2.8 ± 0.3) for formic acid andk(sCI + acid) /k(sCI + SO2) Combining double low line (3.4 ± 0.2) for acetic acid. This finding suggests that sCIs can play a role in the formation and loss of several atmospheric constituents besides SO2. © Author(s) 2014. CC Attribution 3.0 License.