Emitted from the oceans, iodine-bearing molecules are ubiquitous in the atmosphere and a source of new atmospheric aerosol particles of potentially global significance. However, its inclusion in atmospheric models is hindered by a lack of understanding of the first steps of the photochemical gas-to-particle conversion mechanism. Our laboratory results show that under a high humidity and low HOx regime, the recently proposed nucleating molecule (iodic acid, HOIO2) does not form rapidly enough, and gas-to-particle conversion proceeds by clustering of iodine oxides (IxOy), albeit at slower rates than under dryer conditions. Moreover, we show experimentally that gas-phase HOIO2 is not necessary for the formation of HOIO2-containing particles. These insights help to explain new particle formation in the relatively dry polar regions and, more generally, provide for the first time a thermochemically feasible molecular mechanism from ocean iodine emissions to atmospheric particles that is currently missing in model calculations of aerosol radiative forcing.

The low temperature kinetics of the reactions of OH with ethanol and propan-2-ol have been studied using a pulsed Laval nozzle apparatus coupled with pulsed laser photolysis -laser induced fluorescence (PLP-LIF) spectroscopy. The rate coefficients for both reactions have been found to increase significantly as the temperature is lowered, by~a factor of 18 between 293 and 54 K for ethanol, and by~10 between 298 and 88 K for OH + propan-2-ol. The pressure dependence of the rate coefficients provide evidence for two reaction channels; a zero pressure bimolecular abstraction channel leading to products and collisional stabilization of a weakly bound OH-alcohol complex. The presence of the abstraction channel at low temperatures is rationalized by a quantum mechanical tunneling mechanism, most likely through the barrier to hydrogen abstraction from the OH moiety on the alcohol.

The UV absorption cross-sections of the Criegee intermediate CH2OO, and kinetics of the CH2OO self-reaction and the reaction of CH2OO with I are reported as a function of pressure at 298 K.

Recent, direct studies have shown that several reactions of stabilized Criegee intermediates (SCI) are significantly faster than indicated by earlier indirect measurements. The reaction of SCI with SO2 may contribute to atmospheric sulfate production, but there are uncertainties in the mechanism of the reaction of the C1 Criegee intermediate, CH2OO, with SO2. The reactions of C1, CH2OO, and C2, CH3CHOO, Criegee intermediates with SO2 have been studied by generating stabilized Criegee intermediates by laser flash photolysis (LFP) of RI2/O2 (R = CH2 or CH3CH) mixtures with the reactions being followed by photoionization mass spectrometry (PIMS). PIMS has been used to determine the rate coefficient for the reaction of CH3CHI with O2, k = (8.6 ± 2.2) × 10-12 cm3 molecule-1 s-1 at 295 K and 2 Torr (He). The yield of the C2 Criegee intermediate under these conditions is 0.86 ± 0.11. All errors in the abstract are a combination of statistical at the 1σ level and an estimated systematic contribution. For the CH2OO + SO2 reaction, additional LFP experiments were performed monitoring CH2OO by time-resolved broadband UV absorption spectroscopy (TRUVAS). The following rate coefficients have been determined at room temperature ((295 ± 2) K):CH2OO + SO2: k = (3.74 ± 0.43) × 10-11 cm3 molecule-1 s-1 (LFP/PIMS),k = (3.87 ± 0.45) × 10-11 cm3 molecule-1 s-1 (LFP/TRUVAS)CH3CHOO + SO2: k = (1.7 ± 0.3) × 10-11 cm3 molecule-1 s-1 (LFP/PIMS)LFP/PIMS also allows for the direction observation of CH3CHO production from the reaction of CH3CHOO with SO2, suggesting that SO3 is the co-product. For the reaction of CH2OO with SO2 there is no evidence of any variation in reaction mechanism with [SO2] as had been suggested in an earlier publication (Chhantyal-Pun et al., Phys. Chem. Chem. Phys., 2015, 17, 3617). A mean value of k = (3.76 ± 0.14) × 10-11 cm3 molecule-1 s-1 for the CH2OO + SO2 reaction is recommended from this and previous studies. The atmospheric implications of the results are briefly discussed.