Postfrontal nanoparticles at Cape Grim: observations

Environmental context. Clouds and the factors controlling cloud properties are essential components in understanding and accurately predicting global climate change. This work examines nanometre-sized atmospheric particles, particularly bursts of enhanced particle concentrations following cold fronts over the Southern Ocean. The properties of these events have been established to enable modelling of their significance as a source of cloud-droplet-forming nuclei. Abstract. Nanoparticles (diameter <10 nm) were studied in clean maritime air at Cape Grim over a 2-year period. Concentrations were determined using a condensation nucleus counter (CNC) and an ultra-CNC (UCNC), requiring careful treatment of drifts in counter efficiency. This is the first extended examination of nanoparticles following cold fronts and shows that nanoparticle enhancements were present following 94% of 121 cold fronts studied. Typical enhancements were ~100 cm–3 with maxima ~300–500 cm–3, occur 9–11 h after the front and contain multiple peaks with peak-to-peak separation of 8–11 h. Most enhancements were associated with drier conditions, indicative of increased entrainment of free-tropospheric air after the front. The quasi-periodicity of the enhancements may be related to mesoscale structures in cloud fields following fronts but this requires testing. This quantification of event properties allows evaluation of the significance of these events for the cloud nucleating particle (CCN) population.

X-Ray Techniques in the 1 to 400 Å Range

X-ray work in the 10 to 400 Å range usually requires, for maximum sensitivity, new methods of X-ray generation, analysis, and detection. A comparative survey includes X-ray generation by fluorescence, electron bombardment, and proton bombardment. Dispersive analysis with crystals and gratings is described. Finally, window less photoelectric multipliers are evaluated as X-ray detectors.Results show that typical efficiences range from 10−4 to 10−2 photons/electron or photons/proton, with the latter value as a practical upper limit. An efficiency of 8.5% has been measured for a KAP crystal covering the wavelength range up to 25 Å, while an original gold-plated grating lias a maximum measured efficiency of 20%. Counter efficiency may range from 1 to 50%, depending on wavelength. Total efficiency for a grating-type spectrometer in the 20 to 400 Å range has been measured to 2 × 105 photons/cm2-count.