Insights into the Chemistry of Iodine New Particle Formation: The Role of Iodine Oxides and the Source of Iodic Acid.

Iodine chemistry is an important driver of new particle formation in the marine and polar boundary layers. There are, however, conflicting views about how iodine gas-to-particle conversion proceeds. Laboratory studies indicate that the photooxidation of iodine produces iodine oxides (IxOy), which are well-known particle precursors. By contrast, nitrate anion chemical ionization mass spectrometry (CIMS) observations in field and environmental chamber studies have been interpreted as evidence of a dominant role of iodic acid (HIO3) in iodine-driven particle formation. Here, we report flow tube laboratory experiments that solve these discrepancies by showing that both IxOy and HIO3 are involved in atmospheric new particle formation. I2Oy molecules (y = 2, 3, and 4) react with nitrate core ions to generate mass spectra similar to those obtained by CIMS, including the iodate anion. Iodine pentoxide (I2O5) produced by photolysis of higher-order IxOy is hydrolyzed, likely by the water dimer, to yield HIO3, which also contributes to the iodate anion signal. We estimate that ∼50% of the iodate anion signals observed by nitrate CIMS under atmospheric water vapor concentrations originate from I2Oy. Under such conditions, iodine-containing clusters and particles are formed by aggregation of I2Oy and HIO3, while under dry laboratory conditions, particle formation is driven exclusively by I2Oy. An updated mechanism for iodine gas-to-particle conversion is provided. Furthermore, we propose that a key iodine reservoir species such as iodine nitrate, which we observe as a product of the reaction between iodine oxides and the nitrate anion, can also be detected by CIMS in the atmosphere.

The reaction between HgBr and O3: kinetic study and atmospheric implications.

The rate constants of many reactions currently considered to be important in the atmospheric chemistry of mercury remain to be measured in the laboratory. Here we report the first experimental determination of the rate constant of the gas-phase reaction between the HgBr radical and ozone, for which a value at room temperature of k(HgBr + O3) = (7.5 ± 0.6) × 10-11 cm3 molecule s-1 (1σ) has been obtained. The rate constants of two reduction side reactions were concurrently determined: k(HgBr + O) = (5.3 ± 0.4) × 10-11 cm3 molecule s-1 and k(HgBrO + O) = (9.1 ± 0.6) × 10-11 cm3 molecule s-1. The value of k(HgBr + O3) is slightly lower than the collision number, confirming the absence of a significant energy barrier. Considering the abundance of ozone in the troposphere, our experimental rate constant supports recent modelling results suggesting that the main atmospheric fate of HgBr is reaction with ozone to form BrHgO.

Limb lengthening in the treatment of posteromedial bowing of the tibia.

PURPOSE: Posteromedial bowing of the tibia is an uncommon but recognized congenital lower extremity deformity in children that can lead to limb length discrepancy (LLD) and residual angulatory deformity. The purpose of this study is to report a series of children at a single institution with posteromedial bowing treated by lengthening. METHODS: A retrospective review was carried out at our institution identifying 16 patients who were treated with limb lengthening for posteromedial bowing of the tibia and followed to skeletal maturity. Projected LLD was a mean of 7.7 cm (range 5.0 cm to 14.2 cm). Three patients were treated in a staged fashion with lengthening and deformity correction at age three to four years and subsequent definitive tibial lengthening. The remaining 13 patients were treated with limb lengthening approaching adolescence using circular external fixation. RESULTS: All patients were pain free and ambulated without a limp at final follow-up. The mean final LLD was 0.3 cm short. In spite of correction of distal tibial shaft valgus in 11 of the 16 patients, eight of the 16 (50%) required later correction of persistent, symptomatic ankle valgus by either hemiepiphyseodesis (seven patients) or osteotomy (one patient). CONCLUSIONS: Children with posteromedial bowing of the tibial with projected LLD over 5cm can be effectively treated with lengthening. Patients with severe valgus of more than 30° of shaft valgus and difficulty ambulating at age three years can be successfully treated with a two-stage lengthening procedure. Attention should be paid in patients with posteromedial bowing to ankle valgus. LEVEL OF EVIDENCE: IV.

A gas-to-particle conversion mechanism helps to explain atmospheric particle formation through clustering of iodine oxides.

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.

CH2OO Criegee intermediate UV absorption cross-sections and kinetics of CH2OO + CH2OO and CH2OO + I as a function of pressure.

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. Measurements were made using pulsed laser flash photolysis of CH2I2/O2/N2 gas mixtures coupled with time-resolved broadband UV absorption spectroscopy at pressures between 6 and 300 Torr. Results give a peak absorption cross-section of (1.37 ± 0.29) × 10-17 cm2 at ∼340 nm and a rate coefficient for the CH2OO self-reaction of (8.0 ± 1.1) × 10-11 cm3 s-1, with no significant pressure dependence of the absorption cross-sections or the self-reaction kinetics over the range investigated. The rate coefficient for the reaction between CH2OO and I demonstrates pressure dependence over the range investigated, with a Lindemann fit giving k0 = (4.4 ± 1.0) × 10-29 cm6 s-1 and k∞ = (6.7 ± 0.6) × 10-11 cm3 s-1. The origins of IO in the system have been investigated, the implications of which are discussed.

Effect of Side Struts on the Strength of Long Arm Plaster Splints: A Biomechanical Study.

BACKGROUND: There are multiple methods of achieving upper extremity immobilization after pediatric elbow injuries; however, no biomechanical study has established an optimal construct. The goal of this study was to compare the strength of commonly used long arm splints and to evaluate the effect of reinforcing plaster splints with side struts. METHODS: Five categories of long arm posterior slab splints were tested: 4-inch plaster without side struts, 4-inch plaster with a medial side strut, 4-inch plaster with medial and lateral side struts, 5-inch plaster without side struts, and 4-inch fiberglass splint material without side struts. There were 4 splints in each group. As a control, 4 half fiberglass long arm casts were also tested. Each splint or cast was mounted on a single-column tensile tester and a 3-point bending load was applied to simulate an extension moment at the elbow. The maximum load before failure was measured and an ANOVA model was used to analyze the differences between groups. Additionally, a retrospective chart review was performed of pediatric patients who were immobilized postoperatively in a long arm plaster splint with side struts. We collected data on patient age, type of fracture, time from splint application in the operating room to removal in clinic, length of follow-up, and any complications. RESULTS: The 4-inch plaster splints reinforced with 2 struts had the highest average maximum load to failure (731±143 N), which was significantly higher than the 4-inch plaster splints with one strut (505±48 N) (P=0.01) and the 4-inch plaster splints without struts (100±10 N) (P<0.001). The half fiberglass casts failed at an average maximum load of 655±96 N, however there was no statistically significant difference compared with 4-inch plaster splints with 2 struts (P=0.10). The 5-inch plaster splints without side struts failed at a greater average maximum load (341±110 N) compared with the splints constructed with fiberglass material without side struts (233±61 N) (P=0.03). A total of 140 patients were identified in the retrospective review. Splint-related complications occurred in 2 patients. CONCLUSIONS: The addition of both 1 and 2 side struts to a 4-inch long arm plaster splint significantly increased the load to failure. The strength of 4-inch plaster splints with 2 side struts was comparable to that of half fiberglass casts. LEVEL OF EVIDENCE: NA (biomechanical study).

Femoral lengthening over a pediatric femoral nail: results and complications.

BACKGROUND: Limb lengthening by callotasis as described by Ilizarov has become the standard method of lower extremity lengthening. Lengthening over an intramedullary nail to allow early removal of the external fixator has also become common in adults but few studies have addressed the efficacy in children. METHODS: A retrospective review of 37 consecutive children who had undergone femoral lengthening with external fixator over an intramedullary nail was performed. Charts were reviewed for demographics, surgical details, and complications. Radiographs were examined to determine magnitude of lengthening and to calculate lengthening index. RESULTS: The average age of the 37 patients was 11.6 years (range, 8.1 to 17.0). The amount of lengthening averaged 7.0 cm (range, 3.0 to 11.4 cm), which represented a mean 20.4% increase in length. The mean time in the fixator was 81 days. The lengthening index was 1.21 days/mm. Thirteen patients developed major complications (37.8%) including 4 limbs that failed to lengthen initially, 3 fractures (1 before fixator removal and 3 after fixator removal), 2 nail failures, 4 deep infections, and 2 joint subluxations requiring operative care. The 3 fractures after fixator removal were treated with exchange nailing as were the 2 intramedullary nail failures. Four patients (10.8%) developed deep infections requiring irrigation, debridement, and IV antibiotics. One patient developed a late hip subluxation, which was treated with a shelf osteotomy but resulted in pain and limitation of motion. One patient developed knee subluxation during lengthening requiring operative intervention. The technique was successful in obtaining a good result with a functional lengthened femur without unresolved problems in 94% of the patients despite a significant rate of major complications, particularly in those with a congenital etiology. Only 2 of the 37 patients ultimately had results that were ultimately compromised by complications. CONCLUSIONS: Femoral lengthening over an intramedullary nail with the aid of an external fixator has shown to be an effective method for correcting limb length discrepancy. The technique has a high complication rate similar to other methods of lengthening. LEVEL OF EVIDENCE: Level IV-Case Series.