A three-dimensional (3D) ion velocity imaging method was developed to measure the product velocity distributions in crossed molecular beam experiments. While maintaining conventional two-dimension velocity mapping, the third velocity component was mapped linearly to the ion time of flight. A weak extraction field was used to spread the ion turnaround time to several hundred nanoseconds, which permits good resolution for selection of the longitudinal velocity. A fast gated (⩾5 ns) intensified charge coupled device camera was used to record time-sliced ion images. Calibration of the apparatus was done by measuring O+ images from the multiphoton dissociation/ionization of O2. The resolution in velocity achieved was about 1% (Δv/v) in slicing through the center of a Newton sphere. The overall performance was examined by observing product ion images from the F+CD4→DF+CD3 reaction. To detect CD3+ with kinetic energy release of about 1 eV, 50 ns time slicing provides sufficient velocity resolution, such that resolution of the image is mainly limited by the spread in velocity of the two molecular beams. These ion optics can focus on a large volume of ion cloud, which is crucial in crossed molecular beam experiments. Direct 3D imaging also simplifies data analysis. This direct 3D ion imaging method provides a powerful tool with which to study systems with no cylindrical symmetry.

Carbonyl oxides, or Criegee intermediates, are important transient species formed in the reactions of unsaturated hydrocarbons with ozone. Although direct detection of Criegee intermediates has recently been realized, the main atmospheric sink of Criegee intermediates remains unclear. We report ultraviolet absorption spectroscopic measurements of the lifetime of the simplest Criegee intermediate, CH2OO, at various relative humidity levels up to 85% at 298 kelvin. An extremely fast decay rate of CH2OO was observed at high humidity. The observed quadratic dependence of the decay rate on water concentration implied a predominant reaction with water dimer. On the basis of the water dimer equilibrium constant, the effective rate coefficient of the CH2OO + (H2O)2 reaction was determined to be 6.5 (±0.8) × 10(-12) cubic centimeters per second. This work would help modelers to better constrain the atmospheric concentrations of CH2OO.

Criegee intermediates are thought to play a role in atmospheric chemistry, in particular, the oxidation of SO 2 , which produces SO 3 and subsequently H 2 SO 4 , an important constituent of aerosols and acid rain. However, the impact of such oxidation reactions is affected by the reactions of Criegee intermediates with water vapor, because of high water concentrations in the troposphere. In this work, the kinetics of the reactions of dimethyl substituted Criegee intermediate (CH 3 ) 2 COO with water vapor and with SO 2 were directly measured via UV absorption of (CH 3 ) 2 COO under near-atmospheric conditions. The results indicate that (i) the water reaction with (CH 3 ) 2 COO is not fast enough (k H2O < 1.5 × 10 −16 cm 3 s −1 ) to consume atmospheric (CH 3 ) 2 COO significantly and (ii) (CH 3 ) 2 COO reacts with SO 2 at a near-gas-kinetic-limit rate (k SO2 = 1.3 × 10 −10 cm 3 s −1 ). These observations imply a significant fraction of atmospheric (CH 3 ) 2 COO may survive under humid conditions and react with SO 2 , very different from the case of the simplest Criegee intermediate CH 2 OO, in which the reaction with water dimer predominates in the CH 2 OO decay under typical tropospheric conditions. In addition, a significant pressure dependence was observed for the reaction of (CH 3 ) 2 COO with SO 2 , suggesting the use of low pressure rate may underestimate the impact of this reaction. This work demonstrates that the reactivity of a Criegee intermediate toward water vapor strongly depends on its structure, which will influence the main decay pathways and steady-state concentrations for various Criegee intermediates in the atmosphere. Ozonolysis of unsaturated hydrocarbons produces highly reactive Criegee intermediates (CIs) (1), which may (i) decompose to radical species like OH radicals or (ii) react with a number of atmospheric species, for example, with SO 2 to form SO 3 and with NO 2 to form NO 3 (2, 3). The SO 2 oxidation by CIs has gained special attentions because the SO 3 product would be converted into H 2 SO 4 , an important constituent of aerosols and acid rain (4-8). For example, Mauldin et al. (4) (2) demonstrated an efficient method to prepare a CI in a laboratory by the reaction of iodoalkyl radical with O 2 (for example, CH 2 I + O 2 → CH 2 OO + I). This method can produce a CI of high enough concentration that allows direct detection. With photoionization mass spectrometry (PIMS) detection, Welz et al. (2) measured the rate coefficients of the simplest CI (CH 2 OO) reactions with SO 2 and NO 2 . Notably, these new rate coefficients, confirmed by a few later investigations (9-11), are orders of magnitude larger than those previously used (12, 13) in atmospheric models (e.g., MCM v3.3, available at mcm.leeds. ac.uk/MCM/browse.htt?species=CH2OO), suggesting a greater role of CIs in atmospheric chemistry. This result also indicates previous ozonolysis analyses may be affected by complicated and partly unknown side reactions and may contain errors in some of the reported rate coefficients.Typical...