Enhanced Evaporation of Microscale Droplets With an Infrared Laser

Enhancement of water droplet evaporation by added infrared radiation was modeled and studied experimentally in a vertical laminar flow channel. Experiments were conducted on droplets with nominal initial diameters of 50 lm in air with relative humidities ranging from 0% to 90% RH. A 2800 nm laser was used with radiant flux densities as high as 4 Â 10 5 W/m 2 . Droplet size as a function of time was measured by a shadowgraph technique. The model assumed quasi-steady behavior, a low Biot number liquid phase, and constant gas-vapor phase material properties, while the experimental results were required for model validation and calibration. For radiant flux densities less than 10 4 W/m 2 , droplet evaporation rates remained essentially constant over their full evaporation, but at rates up to 10% higher than for the no radiation case. At higher radiant flux density, the surface-area change with time became progressively more nonlinear, indicating that the radiation had diminished effects on evaporation as the size of the droplets decreased. The drying time for a 50 lm water droplet was an order of magnitude faster when comparing the 10 6 W/m 2 case to the no radiation case. The model was used to estimate the droplet temperature. Between 10 4 and 5 Â 10 5 W/m 2 , the droplet temperature changed from being below to above the environment temperature. Thus, the direction of conduction between the droplet and the environment also changed. The proposed model was able to predict the changing evaporation rates for droplets exposed to radiation for ambient conditions varying from dry air to 90% relative humidity

Comprehensive characterization of mainstream marijuana and tobacco smoke

Abstract: Recent increases in marijuana use and legalization without adequate knowledge of the risks necessitate the characterization of the billions of nanoparticles contained in each puff of smoke. Tobacco smoke offers a benchmark given that it has been extensively studied. Tobacco and marijuana smoke particles are quantitatively similar in volatility, shape, density and number concentration, albeit with differences in size, total mass and chemical composition. Particles from marijuana smoke are on average 29% larger in mobility diameter than particles from tobacco smoke and contain 3.4× more total mass. New measurements of semi-volatile fractions determine over 97% of the mass and volume of the particles from either smoke source are comprised of semi-volatile compounds. For tobacco and marijuana smoke, respectively, 4350 and 2575 different compounds are detected, of which, 670 and 536 (231 in common) are tentatively identified, and of these, 173 and 110 different compounds (69 in common) are known to cause negative health effects through carcinogenic, mutagenic, teratogenic, or other toxic mechanisms. This study demonstrates striking similarities between marijuana and tobacco smoke in terms of their physical and chemical properties

Generation of a Monodisperse Size-Classified Aerosol Independent of Particle Charge

<div><p>Copyright 2014 American Association for Aerosol Research</p> </div

Relative Humidity Dependence of Soot Aggregate Restructuring Induced by Secondary Organic Aerosol: Effects of Water on Coating Viscosity and Surface Tension

Soot aggregates have a significant warming effect on climate, and their structural and optical properties may evolve in the presence of coatings. Here, the relative humidity (RH) dependence of soot aggregate restructuring induced by secondary organic aerosol (SOA) coatings was investigated in a series of photo-oxidation experiments. Burner-generated soot aggregates were classified by mobility diameter and injected into a smog chamber, where they were exposed to oxidation products of <i>p</i>-xylene; coated aggregates were subsequently conditioned at one of the following RHs: <12%, 20%, 40%, 60%, or 85%. Changes in diameter and mass were monitored using differential mobility and centrifugal particle mass analyzers, respectively. At RH < 12%, the SOA coating was too viscous to induce restructuring, so the particle diameter increased uniformly with coating mass. At RH ≥ 20%, the SOA coating induced restructuring, and the degree of restructuring increased with RH, indicating that the decreased viscosity and increased surface tension of SOA have significant implications on SOA-induced restructuring of soot aggregates. At RH ≥ 60%, appreciable water uptake occurred, and the hygroscopicity parameter of the SOA coating was derived. Our results provide crucial insights into the complex interactions between soot, SOA, and water in the atmosphere

Measuring aerosol size distributions with the aerodynamic aerosol classifier

<p>The Aerodynamic Aerosol Classifier (AAC) is a novel instrument that selects aerosol particles based on their relaxation time or aerodynamic diameter. Additional theory and characterization is required to allow the AAC to accurately measure an aerosol’s aerodynamic size distribution by stepping while connected to a particle counter (such as a Condensation Particle Counter, CPC). To achieve this goal, this study characterized the AAC transfer function (from 32 nm to 3 μm) using tandem AACs and comparing the experimental results to the theoretical tandem deconvolution. These results show that the AAC transmission efficiency is 2.6–5.1 times higher than a combined Krypton-85 radioactive neutralizer and Differential Mobility Analyzer (DMA), as the AAC classifies particles independent of their charge state. However, the AAC transfer function is 1.3–1.9 times broader than predicted by theory. Using this characterized transfer function, the theory to measure an aerosol’s aerodynamic size distribution using an AAC and particle counter was developed. The transfer function characterization and stepping deconvolution were validated by comparing the size distribution measured with an AAC-CPC system against parallel measurements taken with a Scanning Mobility Particle Sizer (SMPS), CPC, and Electrical Low Pressure Impactor (ELPI). The effects of changing AAC classifier conditions on the particle selected were also investigated and found to be small (<1.5%) within its operating range. </p> <p>Copyright © 2018 American Association for Aerosol Research</p