Earlier experimental studies by one of us (Kelso, 1981a, 1984) have shown that abrupt phase transitions occur in human hand movements under the influence of scalar changes in cycling frequency. Beyond a critical frequency the originally prepared out-of-phase, antisymmetric mode is replaced by a symmetrical, in-phase mode involving simultaneous activation of homologous muscle groups. Qualitatively, these phase transitions are analogous to gait shifts in animal locomotion as well as phenomena common to other physical and biological systems in which new "modes" or spatiotemporal patterns arise when the system is parametrically scaled beyond its equilibrium state (Haken, 1983). In this paper a theoretical model, using concepts central to the interdisciplinary field of synergetics and nonlinear oscillator theory, is developed, which reproduces (among other features) the dramatic change in coordinative pattern observed between the hands.

Abstract. The EU-project MINATROC (MINeral dust And TROpospheric Chemistry) aims at enabling an estimation of the influence of mineral dust, a major, but to date largely ignored component of tropospheric aerosol, on tropospheric oxidant cycles. Within the scope of this project continuous atmospheric measurements of gas-phase HNO 3 and SO 2 were conducted in June and July 2000 at the CNR WMO station, situated on Monte Cimone (MTC) (44 • 11 N -10 • 42 E, 2165 m asl), Italy. African air transporting dust is occasionally advected over the Mediterranean Sea to the site, thus mineral aerosol emitted from Africa will encounter polluted air masses and provide ideal conditions to study their interactions. HNO 3 and SO 2 were measured with an improved CIMS (chemical ionization mass spectrometry) system for ground-based measurements that was developed and built at MPI-K Heidelberg. Since HNO 3 is a very sticky compound special care was paid for the air-sampling and backgroundmeasurement system. Complete data sets could be obtained before, during and after major dust intrusions. For the first time these measurements might provide a strong observational indication of efficient uptake of gas-phase HNO 3 by atmospheric mineral-dust aerosol particles.

Supersaturations, microphysics and nitric acid partitioning in a very cold subvisible tropical cirrus cloud observed on 2 February 2006, during the field campaign CR-AVE, are studied by comparing a simulated set of possible cloud development scenarios with the in situ observations. The scenario that best matches the observations is a cirrus cloud forming by heterogeneous freezing of a small number of ice nuclei with subsequent unimpeded mass accommodation of water on ice. Variation of the freezing process, the accommodation coefficient or the amount of available water leads to simulated clouds that differ microphysically from the observed cloud in important respects. In particular, the simulations suggest that heterogeneous ice nucleation or another freezing mechanism producing only a low number of ice crystals could be an important process for cold cirrus cloud formation, possibly explaining the frequent observations of high supersaturations inside the cirrus cloud in this temperature regime.

Abstract. The low temperature aerosol chamber AIDA was used to study the nucleation of nitric acid dihydrate (NAD) in super-cooled nitric acid aerosols under simulated stratospheric conditions in the temperature range 192 K-197 K. The nucleating solution droplets had median diameters between 225 and 290 nm and molar fractions of nitric acid between 0.26 and 0.28. Nucleation of solid particles was unambiguously observed in two out of three experiments during time periods of up to five hours. The newly formed crystals could be clearly distinguished from the remaining liquid droplets by their increasing size with an optical particle spectrometer. The solid particles could be unequivocally identified as strongly aspherical nitric acid dihydrate crystals (α-NAD) by in-situ FTIR-spectroscopy. From our experimental data set there is no indication of direct nucleation of NAT or a conversion of NAD into NAT while having saturation ratios with respect to NAT of about 20-26. The temporal evolutions of the NAD particle concentrations were used to derive individual nucleation rates for NAD. The measured volume nucleation rates ranged from 3.9×10 5 cm −3 s −1 at 195.8 K and X NA =0.27 to 1.9×10 7 cm −3 s −1 at 192.1 K and X NA =0.28. The corresponding hypothetical surface nucleation rates of 2×10 0 to 1×10 2 cm −2 s −1 are smaller than the parameterization of Tabazadeh et al. (2002) by factors between 25 and >10 3 .

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