Reaction Kinetics of Meteoric Sodium Reservoirs in the Upper Atmosphere

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 HO_x 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 (I_xO_y), 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.

“How iodine-bearing molecules contribute to atmospheric aerosol formation is not well understood. Here, the authors provide a new gas-to-particle conversion mechanism and show that clustering of iodine oxides is an essential component of this process while previously proposed iodic acid does not play a large role.”

Reaction Kinetics of CaOH with H and O2 and O2CaOH with O: Implications for the Atmospheric Chemistry of Meteoric Calcium

The ablation of cosmic dust particles entering the Earth's upper atmosphere produces a layer of Ca atoms around 90 km. Here, we present a set of kinetic experiments designed to understand the nature of the Ca molecular reservoirs on the underside of the layer. CaOH was produced by laser ablation of a Ca target in the fast flow tube and detected by non-resonant laser-induced fluorescence, probing the D(²Σ⁺) ← X(²Σ¹) transition at 346.9 nm. The following rate constants were measured (at 298 K): k(CaOH + H → Ca + H₂O) = (1.04 ± 0.24) × 10^-10 cm³ molecule^-1 s^-1, k(CaOH + O → CaO + OH) < 1 × 10^-11 cm³ molecule^-1 s^-1, and k(CaOH + O₂ → O₂CaOH, 1 Torr) = (5.9 ± 1.8) × 10^-11 cm³ molecule^-1 s^-1 (uncertainty at the 2σ level of confidence). The recycling of CaOH from reaction between O₂CaOH and O proceeds with an effective rate constant of k_eff(O₂CaOH + O → CaOH + products, 298 K) = 2.8_-1.2^+2.0 × 10^-10 cm³ molecule^-1 s^-1. Master equation modeling of the CaOH + O₂ kinetics is used to extrapolate to mesospheric temperatures and pressures. The results suggest that the formation of O₂CaOH slows the conversion of CaOH to atomic Ca via reaction with atomic H, and O₂CaOH is likely to be a long-lived reservoir species on the underside of the Ca layer and a building block of meteoric smoke particles.