Abstract. Reaction of stabilized Criegee intermediates (SCIs) with SO2 was proposed as an additional pathway of gaseous sulfuric acid
(H2SO4) formation in the atmosphere, supplementary to the
conventional mechanism of H2SO4 production by oxidation of
SO2 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 H2SO4 formation at a western Mediterranean
(Cape Corsica) remote site, where comprehensive field observations including gas-phase H2SO4, OH radicals, SO2, 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 H2SO4 via SO2 oxidation by the SCIs. Assuming the validity of
a steady state between H2SO4 production and its loss by
condensation on existing aerosol particles with a unity accommodation
coefficient, about 90 % of the H2SO4 formation during the day
could be explained by the reaction of SO2 with OH. During the night the
oxidation of SO2 by OH radicals was found to contribute only about
10 % to the H2SO4 formation. The accuracy of the derived values
for the contribution of OH + SO2 reaction to the H2SO4
formation is limited mostly by a large, at present factor of 2, uncertainty in the OH + SO2 reaction rate coefficient. The contribution of the SO2 oxidation by SCIs to the H2SO4 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) × 103 molec. cm−3. During the day the reaction of SCIs with
SO2 was found to account for about 10 % and during the night for
about 40 % of the H2SO4 production, closing the H2SO4
budget during the day but leaving unexplained about 50 % of the
H2SO4 formation during the night. Despite large uncertainties in
used kinetic parameters, these results indicate that the SO2 oxidation
by SCIs may represent an important H2SO4 source in VOC-rich environments, especially during nighttime.
Efficient Formation Pathway of Methyl Formate: The Role of OH Radicals on Ice Dust
