Efficiency of immersion mode ice nucleation on surrogates of mineral dust

A differential scanning calorimeter (DSC) was used to explore heterogeneous ice nucleation of emulsified aqueous suspensions of two Arizona test dust (ATD) samples with particle diameters of nominally 0–3 and 0–7 μm, respectively. Aqueous suspensions with ATD concentrations of 0.01–20 wt% have been investigated. The DSC thermograms exhibit a homogeneous and a heterogeneous freezing peak whose intensity ratios vary with the ATD concentration in the aqueous suspensions. Homogeneous freezing temperatures are in good agreement with recent measurements by other techniques. Depending on ATD concentration, heterogeneous ice nucleation occurred at temperatures as high as 256 K or down to the onset of homogeneous ice nucleation (237 K). For ATD-induced ice formation Classical Nucleation Theory (CNT) offers a suitable framework to parameterize nucleation rates as a function of temperature, experimentally determined ATD size, and emulsion droplet volume distributions. The latter two quantities serve to estimate the total heterogeneous surface area present in a droplet, whereas the suitability of an individual heterogeneous site to trigger nucleation is described by the compatibility function (or contact angle) in CNT. The intensity ratio of homogeneous to heterogeneous freezing peaks is in good agreement with the assumption that the ATD particles are randomly distributed amongst the emulsion droplets. The observed dependence of the heterogeneous freezing temperatures on ATD concentrations cannot be described by assuming a constant contact angle for all ATD particles, but requires the ice nucleation efficiency of ATD particles to be (log)normally distributed amongst the particles. Best quantitative agreement is reached when explicitly assuming that high-compatibility sites are rare and that therefore larger particles have on average more and better active sites than smaller ones. This analysis suggests that a particle has to have a diameter of at least 0.1 μm to exhibit on average one active site

Ice nucleation properties of volcanic ash from Eyjafjallajökull

The ice nucleation ability of volcanic ash particles collected close to the Icelandic volcano Eyjafjallajökull during its eruptions in April and May 2010 is investigated experimentally, in the immersion and deposition modes, and applied to atmospheric conditions by comparison with airborne measurements and microphysical model calculations. The number of ash particles which are active as ice nuclei (IN) is strongly temperature dependent, with a very small minority being active in the immersion mode at temperatures of 250–263 K. Average ash particles show only a moderate effect on ice nucleation, by inducing freezing at temperatures between 236 K and 240 K (i.e. approximately 3–4 K higher than temperatures required for homogeneous ice nucleation, measured with the same instrument). By scaling the results to aircraft and lidar measurements of the conditions in the ash plume days down wind of the eruption, and by applying a simple microphysical model, it was found that the IN active in the immersion mode in the range 250–263 K generally occurred in atmospheric number densities at the lower end of those required to have an impact on ice cloud formation. However, 3–4 K above the homogeneous freezing point, immersion mode IN number densities a few days down wind of the eruption were sufficiently high to have a moderate influence on ice cloud formation. The efficiency of IN in the deposition mode was found to be poor except at very cold conditions (<238 K), when they reach an efficiency similar to that of mineral dust with the onset of freezing at 10 % supersaturation with respect to ice, and with the frozen fraction nearing its maximum value at a supersaturation 20 %. In summary, these investigations suggest volcanic ash particles to have only moderate effects on atmospheric ice formation

Progress in manufacturing the first 8.4 m off-axis segment for the Giant Magellan Telescope

ABSTRACT The first of the 8.4 m off-axis segments for the primary mirror of the Giant Magellan Telescope is being manufactured at the Steward Observatory Mirror Lab. In addition to the manufacture of the segment, this project includes the development of a complete facility to make and measure all seven segments. We have installed a new 28 m test tower and designed a set of measurements to guide the fabrication and qualify the finished segments. The first test, a lasertracker measurement of the ground surface, is operational. The principal optical test is a full-aperture interferometric test with a null corrector that includes a 3.75 m spherical mirror, a smaller sphere, and a computer-generated hologram. We have also designed a scanning pentaprism test to validate the measurement of low-order aberrations. The first segment has been cast and generated, and is in the process of loose-abrasive grinding

MKID Exoplanet Camera for Subaru SCExAO

We present the MKID Exoplanet Camera (MEC), a z through J band (800–1400 nm) integral field spectrograph located behind The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) at the Subaru Telescope on Maunakea that utilizes Microwave Kinetic Inductance Detectors (MKIDs) as the enabling technology for high contrast imaging. MEC is the first permanently deployed near-infrared MKID instrument and is designed to operate both as an IFU, and as a focal plane wavefront sensor in a multi-kHz feedback loop with SCExAO. The read noise free, fast time domain information attainable by MKIDs allows for the direct probing of fast speckle fluctuations that currently limit the performance of most high contrast imaging systems on the ground and will help MEC achieve its ultimate goal of reaching contrasts of 10⁻⁷ at 2 λ/D. Here we outline the instrument details of MEC including the hardware, firmware, and data reduction and analysis pipeline. We then discuss MEC's current on-sky performance and end with future upgrades and plans