Comparison of measured and calculated collision efficiencies at low temperatures

Nagare, Baban; Marcolli, Claudia; Stetzer, O.; Lohmann, Ulrike

doi:10.5194/acp-15-13759-2015

Cited by 16 publications

(51 citation statements)

References 54 publications

(103 reference statements)

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“…The aerosol particles were generated by atomizing this suspension and dried (for details, refer to Nagare et al, 2015). The suspension was usually prepared the day before a measurement series was started and used for a measurement series performed during, typically, 2 days.…”

Section: Aerosol Generation and Samplingmentioning

confidence: 99%

“…The collision rate between particles and droplets is a function of the collision efficiency, which is not well determined in the accumulation mode size range as pointed out by Nagare et al (2015). Collision efficiency (CE) is defined as the fraction of particles in the cylindrical volume swept out by a falling droplet that collides with it.…”

Section: Introductionmentioning

confidence: 99%

“…At each temperature, a blank experiment without aerosol parti-8902 B. Nagare et al: Comparing contact and immersion freezing from continuous flow diffusion chambers cles was performed before the aerosol stream was turned on. A more detailed description of the instrument and experiment is given in Nagare et al (2015).…”

Section: Introductionmentioning

confidence: 99%

“…For coarse-mode particles, impaction and interception of particles are the dominant contributors to CE. The minimum of CE (Greenfield gap) is in the accumulation mode where thermophoresis and diffusiophoresis may also contribute to CE (Nagare et al, 2015).…”

Section: Introductionmentioning

confidence: 99%

“…Droplets with diameters of 80 ± 3 µm are generated with a droplet generator (Ulmke et al, 2001) at the top center of the chamber with a frequency of 100 Hz. The droplets are generated with pure water (Milli-Q, 18.2 M ) at a temperature of 281 K. The relaxation time for a droplet to reach its terminal velocity (0.186 ms −1 ) is 0.2 s, and the time needed to reach the target temperature is about 0.1 and 0.6 s when the chamber is kept at 261 and 235 K, respectively (Nagare et al, 2015). While performing the experiment, the walls of the chamber are coated with a thin layer of ice creating an ice-saturated environment inside the chamber.…”

Section: Introductionmentioning

confidence: 99%

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Comparing contact and immersion freezing from continuous flow diffusion chambers

et al. 2016

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Abstract. Ice nucleating particles (INPs) in the atmosphere are responsible for glaciating cloud droplets between 237 and 273 K. Different mechanisms of heterogeneous ice nucleation can compete under mixed-phase cloud conditions. Contact freezing is considered relevant because higher ice nucleation temperatures than for immersion freezing for the same INPs were observed. It has limitations because its efficiency depends on the number of collisions between cloud droplets and INPs. To date, direct comparisons of contact and immersion freezing with the same INP, for similar residence times and concentrations, are lacking. This study compares immersion and contact freezing efficiencies of three different INPs. The contact freezing data were obtained with the ETH CoLlision Ice Nucleation CHamber (CLINCH) using 80 µm diameter droplets, which can interact with INPs for residence times of 2 and 4 s in the chamber. The contact freezing efficiency was calculated by estimating the number of collisions between droplets and particles. Theoretical formulations of collision efficiencies gave too high freezing efficiencies for all investigated INPs, namely AgI particles with 200 nm electrical mobility diameter, 400 and 800 nm diameter Arizona Test Dust (ATD) and kaolinite particles. Comparison of freezing efficiencies by contact and immersion freezing is therefore limited by the accuracy of collision efficiencies. The concentration of particles was 1000 cm −3 for ATD and kaolinite and 500, 1000, 2000 and 5000 cm −3 for AgI. For concentrations < 5000 cm −3 , the droplets collect only one particle on average during their time in the chamber. For ATD and kaolinite particles, contact freezing efficiencies at 2 s residence time were smaller than at 4 s, which is in disagreement with a collisional contact freezing process but in accordance with immersion freezing or adhesion freezing. With "adhesion freezing", we refer to a contact nucleation process that is enhanced compared to immersion freezing due to the position of the INP on the droplet, and we discriminate it from collisional contact freezing, which assumes an enhancement due to the collision of the particle with the droplet. For best comparison with contact freezing results, immersion freezing experiments of the same INPs were performed with the continuous flow diffusion chamber Immersion Mode Cooling chAmber-Zurich Ice Nucleation Chamber (IMCA-ZINC) for a 3 s residence time. In IMCA-ZINC, each INP is activated into a droplet in IMCA and provides its surface for ice nucleation in the ZINC chamber. The comparison of contact and immersion freezing results did not confirm a general enhancement of freezing efficiency for contact compared with immersion freezing experiments. For AgI particles the onset of heterogeneous freezing in CLINCH was even shifted to lower temperatures compared with IMCA-ZINC. For ATD, freezing efficiencies for contact and immersion freezing experiments were similar. For kaolinite particles, contact freezing became detectable at higher temperatures than imm...

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Section: Aerosol Generation and Samplingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Comparing contact and immersion freezing from continuous flow diffusion chambers

et al. 2016

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Active prevention of snow accumulation on cameras of autonomous vehicles

et al. 2021

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Accumulation of atmospheric icing, particularly wet snow, on the visual sensors/navigators of autonomous vehicles (AVs) increases the possibility of accidents by obstructing the lenses of the sensors. Here, two navigator designs were suggested that use airflow across the lens surfaces of the AVs to prevent snow accumulation on them. The impact of airflow intensity across the lens, wind velocity (relative velocity of wind with respect to vehicle), and liquid water content of snow on prevention of snow accumulation on the lenses of the AVs was explored experimentally. Here, artificial snow grains were formed using a novel snow gun and their average sizes at low liquid water content (LWC of ≈ 8%) and high liquid water content (LWC of ≈ 28%) were measured to study the impact of grain sizes on snow accumulation on camera lenses. The effects of wind velocity, snow density, and diameter of the snow grains on their trajectory in the testing section were also studied numerically. The results indicated that the snow grains with higher velocity, density, or diameter possessed higher inertia forces and were more prone to collide with the navigator, increasing collision efficiency of snow grains. We realized that the airflow across the lens effectively prevented snow accumulation on the lens at vehicle/wind velocities of up to 20 mph. The proposed designs actively reduced the snow accumulation on the camera lens, promising to be applied in future AVs. Graphic abstract

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Role of Organic Hydrocarbons in Atmospheric Ice Formation via Contact Freezing

Collier

2016

J. Phys. Chem. A

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Comparison of measured and calculated collision efficiencies at low temperatures

Cited by 16 publications

References 54 publications

Comparing contact and immersion freezing from continuous flow diffusion chambers

Comparing contact and immersion freezing from continuous flow diffusion chambers

Active prevention of snow accumulation on cameras of autonomous vehicles

Role of Organic Hydrocarbons in Atmospheric Ice Formation via Contact Freezing

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