Missing so 2 oxidant in the coastal atmosphere? – observations from high-resolution measurements of oh and atmospheric sulfur compounds

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Berresheim, H., Adam, M., Monahan, C., O'Dowd, C., Plane, J. M. C., Bohn, B., Rohrer, F.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Copernicus GmbH 2014
• Subjects: marine boundary-layer; stabilized criegee intermediate; volatile organic-compounds; master chemical mechanism; mcm v3 part; methanesulfonic-acid; tropospheric degradation; photochemical data; north-atlantic; rate constants; Mace; North Atlantic
• Online Access: http://hdl.handle.net/10379/10419; https://doi.org/10.5194/acp-14-12209-2014

Diurnal and seasonal variations of gaseous sulfuric acid (H2SO4) and methane sulfonic acid (MSA) were measured in NE Atlantic air at the Mace Head atmospheric research station during the years 2010 and 2011. The measurements utilized selected-ion chemical ionization mass spectrometry (SI/CIMS) with a detection limit for both compounds of 4.3 x 10(4) cm(-3) at 5 min signal integration. The H2SO4 and MSA gas-phase concentrations were analyzed in conjunction with the condensational sink for both compounds derived from 3 nm to 10 mu m (aerodynamic diameter) aerosol size distributions. Accommodation coefficients of 1.0 for H2SO4 and 0.12 for MSA were assumed, leading to estimated atmospheric lifetimes on the order of 7 and 25 min, respectively. With the SI/CIMS instrument in OH measurement mode alternating between OH signal and background (non-OH) signal, evidence was obtained for the presence of one or more unknown oxidants of SO2 in addition to OH. Depending on the nature of the oxidant(s), its ambient concentration may be enhanced in the CIMS inlet system by additional production. The apparent unknown SO2 oxidant was additionally confirmed by direct measurements of SO2 in conjunction with calculated H2SO4 concentrations. The calculated H2SO4 concentrations were consistently lower than the measured concentrations by a factor of 4.7 +/- 2.4 when considering the oxidation of SO2 by OH as the only source of H2SO4. Both the OH and the background signal were also observed to increase significantly during daytime aerosol nucleation events, independent of the ozone photolysis frequency, J((OD)-D-1), and were followed by peaks in both H2SO4 and MSA concentrations. This suggests a strong relation between the unknown oxidant(s), OH chemistry, and the atmospheric photolysis and photooxidation of biogenic iodine compounds. As to the identity of the atmospheric SO2 oxidant(s), we have been able to exclude ClO, BrO, IO, and OIO as possible candidates based on ab initio calculations. Nevertheless, IO could contribute ...