We study hydrogen-bond dynamics in liquid water at low temperatures using molecular dynamics simulations, and find results supporting the hypothesized continuity of dynamic functions between the liquid and glassy states of water. We find that average bond lifetime ͑ϳ1 ps͒ has Arrhenius temperature dependence. We also calculate the bond correlation function decay time ͑ϳ1 ns͒ and find powerlaw behavior consistent with the predictions of the mode-coupling theory, suggesting that the slow dynamics of hydrogen bonds can be explained in the same framework as standard transport quantities.[S0031-9007 (99)08707-4] PACS numbers: 61.43.Fs, 61.20.Ne Both experiments on [1-3] and simulations of [4][5][6] water have focused on understanding various aspects of hydrogen bond (HB) dynamics, such as the structural relaxation time t R and average HB lifetime t HB . Experiments show that characteristic relaxation times commonly obey power laws on supercooling, but that t HB follows a simple Arrhenius law [1][2][3]. Simulations are particularly useful for investigating supercooled water since nucleation does not occur on the time scale of the simulations. Furthermore, quantitative HB information is available in simulations. We find Arrhenius behavior ( Fig. 1) of t HB , as expected from experimental results [3,7]. We also find complex behavior of t R (Fig. 2), consistent with the predictions of the mode-coupling theory (MCT) [1,8] and with the hypothesized continuity of the liquid and glassy states of water [9][10][11].We perform lengthy molecular-dynamics simulations (up to 70 ns) at seven temperatures between 200 and 350 K [12] using the extended simple point charge (SPC/E) potential for water [13]. We calculate the HB dynamics by considering two definitions of an intact HB: (i) an energetic definition, which considers two molecules to be bonded if their oxygen-oxygen separation is less than 3.5 Å and their interaction energy is less than a threshold energy E HB , and (ii) a geometric definition [6], which uses the same distance criterion but no energetic condition, instead requiring that the O H · · · O angle between two molecules must be less than a threshold angle u HB . We select these criteria to roughly reproduce the experimental value for the activation energy needed to break bonds through librational motion.We calculate t HB using both bond definitions with threshold values E HB 210 kJ͞mol and u HB 30 ± over the entire temperature range simulated, and find Arrhenius behavior of t HB (Fig. 1). Measurements of t HB using depolarized light scattering techniques [3,7] find Arrhenius behavior [14]. The activation energy E A associated with t HB has been interpreted as the energy required to break a HB via librational motion, a "fast" motion [3,7]. Comparison of experimental and simulated values of E A provides a primitive test of the bonding criteria in our simulations; we obtain reasonable agreement between experimental and simulated values of E A using thresholds of E HB 210 kJ͞mol for the energetic definition and u HB 30 ± ...

[1] The evolution of the aerosols in the tropical stratosphere since the beginning of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission in June 2006 is investigated using Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) lidar data. It is shown that the current operational calibration requires adjustment in the tropics. Indeed, on the basis of the assumption of pure Rayleigh scattering between 30 and 34 km the current calibration leads to an average underestimation of the scattering ratio by 6% because of the significant amount of aerosols up to 35 km altitude in the tropics, in contrast to midlatitudes. A better result is obtained by adjusting the calibration to higher altitudes, 36-39 km, where past Stratospheric Aerosol and Gas Experiment (SAGE) II extinction measurements showed an almost complete absence of aerosols. After recalibration the tropical stratospheric aerosol picture provided by CALIOP during the first 2 years of the mission reveals significant changes in the aerosol concentration associated with different transport processes. In the stratosphere the slow ascent of several volcanic layers and their meridional transport toward the subtropics are very consistent with the Brewer-Dobson circulation. The near-zero vertical velocity observed around 20 km during the Northern Hemisphere (NH) summer is in good agreement with radiative heating calculation. In the Tropical Tropopause Layer (TTL), weak depolarizing particles are observed during land convective periods, particularly intense over South Asia during the monsoon season. Finally, seasonal fast occurrence of apparent clean air in the TTL during the NH winter requires more investigations to understand its origin.

We study hydrogen-bond dynamics in liquid water at low temperatures using molecular dynamics simulations. We analyze the dynamics using energetic and geometric definitions of a hydrogen bond, and employ two analysis methods: ͑i͒ a history-dependent correlation function, related to the distribution of bond lifetimes, and ͑ii͒ a history-independent correlation function. For method ͑i͒ we find an approximately Arrhenius temperature dependence of the bond lifetime, and find that the distribution of bond lifetimes is extremely sensitive to the choice of bond definition. For method ͑ii͒ we find-independent of bond definition-that the dynamics are consistent with the predictions of the mode-coupling theory, suggesting that the slow dynamics of hydrogen bonds can be explained in the same framework as standard transport quantities. Our results allow us to clarify the significance of the choice of both bond definition and analysis technique.