Role of Criegee intermediates in the formation of sulfuric acid at a Mediterranean (Cape Corsica) site under influence of biogenic emissions.

Reaction of stabilized Criegee intermediates (SCIs) with SO 2 was proposed as an additional pathway of gaseous sulfuric acid (H 2 SO 4) formation in the atmosphere, supplementary to the conventional mechanism of H 2 SO 4 production by oxidation of SO 2 in reaction with OH radicals. However, because of a large uncertainty in mechanism and rate coefficients for the atmospheric formation and loss reactions of different SCIs, the importance of this additional source is not well established. In this work, we present an estimation of the role of SCIs in H 2 SO 4 formation at a western Mediterranean (Cape Corsica) remote site, where comprehensive field observations including gas-phase H 2 SO 4 , OH radicals, SO 2 , volatile organic compounds (VOCs) and aerosol size distribution measurements were performed in July–August 2013 as a part of the project ChArMEx (Chemistry-Aerosols Mediterranean Experiment). The measurement site was under strong influence of local emissions of biogenic volatile organic compounds, including monoterpenes and isoprene generating SCIs in reactions with ozone, and, hence, presenting an additional source of H 2 SO 4 via SO 2 oxidation by the SCIs. Assuming the validity of a steady state between H 2 SO 4 production and its loss by condensation on existing aerosol particles with a unity accommodation coefficient, about 90 % of the H 2 SO 4 formation during the day could be explained by the reaction of SO 2 with OH. During the night the oxidation of SO 2 by OH radicals was found to contribute only about 10 % to the H 2 SO 4 formation. The accuracy of the derived values for the contribution of OH + SO 2 reaction to the H 2 SO 4 formation is limited mostly by a large, at present factor of 2, uncertainty in the OH + SO 2 reaction rate coefficient. The contribution of the SO 2 oxidation by SCIs to the H 2 SO 4 formation was evaluated using available measurements of unsaturated VOCs and steady-state SCI concentrations estimated by adopting rate coefficients for SCI reactions based on structure–activity relationships (SARs). The estimated concentration of the sum of SCIs was in the range of (1–3) × 10 3 molec. cm -3. During the day the reaction of SCIs with SO 2 was found to account for about 10 % and during the night for about 40 % of the H 2 SO 4 production, closing the H 2 SO 4 budget during the day but leaving unexplained about 50 % of the H 2 SO 4 formation during the night. Despite large uncertainties in used kinetic parameters, these results indicate that the SO 2 oxidation by SCIs may represent an important H 2 SO 4 source in VOC-rich environments, especially during nighttime. [ABSTRACT FROM AUTHOR]