Abstract
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.
Highlights
It has been well established that homogeneous oxidation of tropospheric gases is generally dominated by reactions with the hydroxyl (OH) radical during daylight hours and – in regions with significant nitrogen oxide (NOx) concentrations – with the nitrate (NO3) radical in the absence of sunlight (Stone et al, 2012)
The average atmospheric lifetime of H2SO4 with respect to condensational sink (CS) was estimated from scanning mobility particle sizer (SMPS) and aerodynamic particle sizer (APS) measurements using the approach of Fuchs and Sutugin (1971) and of Seinfeld and Pandis (1998) to be on the order of 7 min assuming an accommodation coefficient of 1.0 (Kolb et al, 2010; Hanson, 2005; Boy et al, 2005), a diffusion coefficient for H2SO4(2 H2O) of 0.075 atm cm2 s−1 at
Other candidates besides OH acting as SO2 oxidants m10i4g4ht FigurOe 7IO. + SO2. be halogen oxide radicals; to our knowledg1e04r5espective rate constants are available in the literature only for the reactions of IO and ClO with SO2 (Larin et al, 2000; around 30 times larger than [OH] at midday at Mace Head, DeMore et al, 1997), which are 3 and 6 orders of magni- the ratio of rate constants is 1 / 1050, so the OH reaction tude smaller compared to kSO2+OH, respectively
Summary
It has been well established that homogeneous oxidation of tropospheric gases is generally dominated by reactions with the hydroxyl (OH) radical during daylight hours and – in regions with significant nitrogen oxide (NOx) concentrations – with the nitrate (NO3) radical in the absence of sunlight (Stone et al, 2012). The time needed (in each case starting at the first injector) to reach and have significantly improved our understanding of tropo- the second injector is 73 ms, to the entrance of the ionization respheric chemistry (e.g., Stone et al, 2012; Huey, 2007; Heard gion 115 ms, and to the aperture in front of the mass spectrometer and Pilling, 2003). In some of these studies it has already 450 ms. Termediates because of their observed reactivity towards SO2 in the measurement system (e.g., Berresheim et al, 2002)
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