Field Observations of In-Cloud Nucleation and the Modification of Atmospheric Aerosol Size Distributions after Cloud Evaporation

Alkezweeny, A.J.

doi:10.1175/1520-0450(1995)034<2649:fooicn>2.0.co;2

Cited by 12 publications

(9 citation statements)

References 11 publications

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“…Aerosol growth near clouds has been observed in both in situ measurements and model simulations. Alkezweeny [1995] measured a decreased aerosol number concentration for optically active aerosols with radii below 0.2 μ m, and an increased aerosol number concentration for aerosol radii between 0.2 and 1.5 μ m in the processed clear air. He argued that the in‐cloud chemical conversion of SO 2 to sulfate adds new material to droplet.…”

Section: Discussionmentioning

confidence: 98%

“…Since every droplet generates only one aerosol particle upon evaporation [ Mitra et al , 1992], the new size is therefore larger. Although Alkezweeny [1995] did not measure particle sizes greater than 1.5 μ m, we note that the increase in particle number concentrations altered the particle size distribution of the coarse mode ( r ≳ 0.5 μ m) as well as the accumulation mode.…”

Section: Discussionmentioning

confidence: 99%

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Aerosol and cloud interaction observed from high spectral resolution lidar data

Su¹,

Schuster²,

Loeb³

et al. 2008

J. Geophys. Res.

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[1] Recent studies utilizing satellite retrievals have shown a strong correlation between aerosol optical depth (AOD) and cloud cover. However, these retrievals from passive sensors are subject to many limitations, including cloud adjacency (or three-dimensional) effects, possible cloud contamination, uncertainty in the AOD retrieval. Some of these limitations do not exist in High Spectral Resolution Lidar (HSRL) observations; for instance, HSRL observations are not affected by cloud adjacency effects, are less prone to cloud contamination, and offer accurate aerosol property measurements (backscatter coefficient, extinction coefficient, lidar ratio, backscatter Angstrom exponent, and aerosol optical depth) at a fine spatial resolution (<100 m) in the vicinity of clouds. Hence the HSRL provides an important data set for studying aerosol and cloud interaction. In this study, we statistically analyze aircraft-based HSRL profiles according to their distance from the nearest cloud, assuring that all profile comparisons are subject to the same largescale meteorological conditions. Our results indicate that AODs from HSRL are about 8-17% higher in the proximity of clouds ($100 m) than far away from clouds (4.5 km), which is much smaller than the reported cloud three-dimensional effect on AOD retrievals. The backscatter and extinction coefficients also systematically increase in the vicinity of clouds, which can be explained by aerosol swelling in the high relative humidity (RH) environment and/or aerosol growth through in-cloud processing (albeit not conclusively). On the other hand, we do not observe a systematic trend in lidar ratio; we hypothesize that this is caused by the opposite effects of aerosol swelling and aerosol in-cloud processing on the lidar ratio. Finally, the observed backscatter Angstrom exponent (BAE) does not show a consistent trend because of the complicated relationship between BAE and RH. We demonstrate that BAE should not be used as a surrogate for Angstrom exponent, especially at high RH.

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Section: Discussionmentioning

confidence: 98%

Section: Discussionmentioning

confidence: 99%

Aerosol and cloud interaction observed from high spectral resolution lidar data

Su¹,

Schuster²,

Loeb³

et al. 2008

J. Geophys. Res.

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“…Two of the remaining major factors to possibly explain the correlation between the cloud cover and AOT, cloud‐processed aerosol particles (factor c) and newly generated particles (factor d) under a humid environment near clouds [ Hoppel et al , 1990; Hegg et al , 1990; Hoppel et al , 1994; Alkezweeny , 1995; Hegg et al , 2004] are discussed in this section. Here cloud‐processed particles are meant to be aerosols left after cloud droplets evaporate.…”

Section: Examination and Discussionmentioning

confidence: 99%

“…An order of 10% increase of aerosol scattering efficiency due to cloud processing of aerosols was reported [ Hegg et al , 2004]. On the other hand, new particle genesis is in reference to homogenous and heterogeneous nucleation such as sulfate production through a gas‐to‐particle conversion [e.g., Hoppel et al , 1990; Alkezweeny , 1995].…”

Section: Examination and Discussionmentioning

confidence: 99%

“…However, some other studies indicated that clouds can have a real impact on AOT. For example, increased new particle genesis and cloud processed particles near clouds have been reported [ Hegg et al , 1990; Hoppel et al , 1994; Alkezweeny , 1995; Hegg et al , 2004]. In addition, it has also been argued that an increased cloud fraction accompanied by enhanced relative humidity (RH) can affect aerosols through the aerosol humidification effect (AHE; also known as aerosol swelling effect), resulting in correlation between cloud faction and AOT [e.g., Ignatov and Nalli , 2002; Jeong and Li , 2005; Kaufman et al , 2005].…”

Section: Introductionmentioning

confidence: 99%

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Separating real and apparent effects of cloud, humidity, and dynamics on aerosol optical thickness near cloud edges

Jeong

2010

J. Geophys. Res.

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[1] Aerosol optical thickness (AOT) is one of aerosol parameters that can be measured on a routine basis with reasonable accuracy from Sun-photometric observations at the surface. However, AOT-derived near clouds is fraught with various real effects and artifacts, posing a big challenge for studying aerosol and cloud interactions. Recently, several studies have reported correlations between AOT and cloud cover, pointing to potential cloud contamination and the aerosol humidification effect; however, not many quantitative assessments have been made. In this study, various potential causes of apparent correlations are investigated in order to separate the real effects from the artifacts, using well-maintained observations from the Aerosol Robotic Network, Total Sky Imager, airborne nephelometer, etc., over the Southern Great Plains site operated by the U.S. Department of Energy's Atmospheric Radiation Measurement Program. It was found that aerosol humidification effects can explain about one fourth of the correlation between the cloud cover and AOT. New particle genesis, cloud-processed particles, atmospheric dynamics, and aerosol indirect effects are likely to be contributing to as much as the remaining three fourth of the relationship between cloud cover and AOT.

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Supersaturation and diffusional droplet growth in liquid clouds: Polydisperse spectra

et al. 2014

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Evolution of droplet size distribution (DSD) due to the water vapor diffusion in a vertically moving adiabatic parcels is investigated. Analytical expressions for height dependences of the main DSD parameters and DSD moments are obtained. The asymptotic behavior of the DSD parameters at large heights above cloud base is determined. It is shown that during diffusion growth, the width and the relative dispersion of the DSD decrease with height as z− 1/3 and z− 2/3, respectively. The paper presents examples of DSD evolution in cases DSD forms on aerosols with a three‐mode lognormal distribution. The aerosol distribution parameters used in the study correspond to four aerosol types: “Marine,” “Clean continental,” “Background,” and extremely polluted “Urban.” The vertical profiles of DSD parameters are compared with the asymptotic profiles. It is shown that in case of polydisperse DSD evolution, the vertical profile of supersaturation within several hundred meters above the cloud base can be approximated by a supersaturation profile corresponding to the “equivalent” monodisperse DSD. The initial radius of this equivalent DSD is equal to the mean radius of polydisperse DSD (haze size distribution) at cloud base, which is estimated using the Kohler theory. This result of the relation between the polydisperse and monodisperse solutions is universal. A new equation for estimation of supersaturation maximum for polydisperse case is obtained. The obtained analytical expressions and numerical results are useful for understanding the mechanisms of DSD formation in clouds and for parameterization of warm microphysical processes in cloud models.

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Field Observations of In-Cloud Nucleation and the Modification of Atmospheric Aerosol Size Distributions after Cloud Evaporation

Cited by 12 publications

References 11 publications

Aerosol and cloud interaction observed from high spectral resolution lidar data

Aerosol and cloud interaction observed from high spectral resolution lidar data

Separating real and apparent effects of cloud, humidity, and dynamics on aerosol optical thickness near cloud edges

Supersaturation and diffusional droplet growth in liquid clouds: Polydisperse spectra

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