Abstract. 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 × 10⁴ 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 μm (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(O¹D), 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 significantly to the observed CIMS background signal. A detailed analysis of this CIMS background signal in context with recently published kinetic data currently suggests that Criegee intermediates (CIs) produced from ozonolysis of alkenes play no significant role for SO₂ oxidation in the marine atmosphere at Mace Head. On the other hand, SO₂ oxidation by small CIs such as CH₂OO produced photolytically or possibly in the photochemical degradation of methane is consistent with our observations. In addition, H₂SO₄ formation from dimethyl sulfide oxidation via SO₃ as an intermediate instead of SO₂ also appears to be a viable explanation. Both pathways need to be further explored.