Ultrafast photochemistry of methyl hydroperoxide on ice particles.

Simulations show that photodissociation of methyl hydroperoxide, CH(3)OOH, on water clusters produces a surprisingly wide range of products on a subpicosecond time scale, pointing to the possibility of complex photodegradation pathways for organic peroxides on aerosols and water droplets. Dynamics are computed at several excitation energies at 50 K using a semiempirical PM3 potential surface. CH(3)OOH is found to prefer the exterior of the cluster, with the CH(3)O group sticking out and the OH group immersed within the cluster. At atmospherically relevant photodissociation wavelengths the OH and CH(3)O photofragments remain at the surface of the cluster or embedded within it. However, none of the 25 completed trajectories carried out at the atmospherically relevant photodissociation energies led to recombination of OH and CH(3)O to form CH(3)OOH. Within the limited statistics of the available trajectories the predicted yield for the recombination is zero. Instead, various reactions involving the initial fragments and water promptly form a wide range of stable molecular products such as CH(2)O, H(2)O, H(2), CO, CH(3)OH, and H(2)O(2).

Breath sulfides and pulmonary function in cystic fibrosis.

We have determined the concentrations of carbonyl sulfide (OCS), dimethylsulfide, and carbon disulfide (CS(2)) in the breath of a group of cystic fibrosis (CF) patients and one of healthy controls. At the detection sensitivity in these experiments, room air always contained measurable quantities of these three gases. For each subject the inhaled room concentrations were subtracted from the time-coincident concentrations in exhaled breath air. The most significant differences between the CF and control cohorts in these breath-minus-room values were found for OCS. The control group demonstrated a net uptake of 250 +/- 20 parts-per-trillion-by-volume (pptv), whereas the CF cohort had a net uptake of 110 +/- 60 pptv (P = 0.00003). Three CF patients exhaled more OCS than they inhaled from the room. The OCS concentrations in the CF cohort were strongly correlated with pulmonary function. The dimethylsulfide concentrations in breath were greatly enhanced over ambient, but no significant difference was observed between the CF and healthy control groups. The net (breath minus room) CS(2) concentrations for individuals ranged between +180 and -100 pptv. They were slightly greater in the CF cohort (+26 +/- 38 pptv) vs. the control group (-17 +/- 15 pptv; P = 0.04). Lung disease in CF is accompanied by the subsistence of chronic bacterial infections. Sulfides are known to be produced by bacteria in various systems and were therefore the special target for this investigation. Our results suggest that breath sulfide content deserves attention as a noninvasive marker of respiratory colonization.