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What Is The Function Of Condensation Nuclei In Cloud Formation

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Small Fraction Of Marine Cloud Condensation Nuclei Made Up Of Sea Spray Aerosol

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Submitted: 8 November 2019 / Revised: 29 November 2019 / Accepted: 4 December 2019 / Published: 6 December 2019

The activation of bacteria and the formation of cloud condensation nuclei (CCN) are studied using the classical theory of heterogeneous nucleation in the atmosphere. Simulations were performed using laboratory determined contact angles for the sulfuric acid/water binary system. Realistic model simulations were performed at different atmospheric heights for 140 different bacterial sets. Model simulations show that bacterial activation is a potentially favorable process in the atmosphere, which can be enhanced at low temperatures. The production of CCN from the nuclei of bacteria depends on the environmental conditions (temperature, relative humidity), the size of the bacteria and the concentration of sulfuric acid. In addition, a critical parameter for determining the activation of bacteria is the value of the intermolecular potential between the surface of the bacteria and the critical cluster formed on their surface. In classical nuclear theory, this is parameterized in terms of the contact angle between the substrate and the critical cluster. Therefore, a dataset of laboratory values ​​of water contact angle on different bacterial substrates should be enriched for realistic simulation of bacterial activation in the atmosphere.

Cloud Forming Ice Nucleating Particles

Primary biological aerosol particles (PBAPs) are an important source of naturally occurring particles in the atmosphere and include a wide variety of microorganisms, such as bacteria and viruses, as well as particles generated from abrasive processes [1, 2]. Airborne transmission of microorganisms can have a significant impact on public health, and thus bioaerosols have been characterized in various occupational settings (eg, [3, 4]).

In addition, airborne microorganisms have been studied in the scientific literature as potential candidates for cloud condensation nuclei (CCN) and ice nuclei (IN) in the atmosphere [2, 5, 6]. In particular, bacteria provide effective surfaces to act as CCN and IN [6, 7, 8, 9]. Frank and DeMott [9] found that specific strains of the pathogenic bacteria Erwinia carotovora isolated from plants were activated by supersaturation greater than 1%. The surface coating of bacteria and other microorganisms was also studied to determine changes in their nuclear properties [10, 11].

Typical bacterial sizes range from 0.5 to 5 microns in diameter, while bacterial agglomerates can reach diameters of 3 to 8 microns. Size distribution characteristics of airborne microbes are presented by Zhai et al. [12]. Hygroscopic growth of generally hydrophobic bacteria can be determined from the contact angle of water on the particle surface. The value of the contact angle determines the wettability of the surfaces, and values ​​as low as 20° have been reported in the literature, indicating their ability to act as CCN in the atmosphere [13]. Laboratory data have shown that ice cores without any coating have a contact angle below 20° for the bacterium Pseudomonas syringae, while the contact angle for mineral particles coated with sulfuric acid reaches 60° [10]. This is in agreement with previous studies showing that P. syringae has good IN nucleation potential [ 14 , 15 ]. Heterogeneous nucleation theory can be used to study CCN formation in bacteria [16, 17, 18], including measured contact angles. A numerical study of the immersion freezing contact angle parameterization was carried out by Ickes et al. [19] using classical nucleation theory. Similar parameterizations for various minerals have been adopted for use in general circulation models (GCMs).

Bauer et al. [6] also showed that a set of Gram-positive and -negative bacteria were activated at 0.07% to 0.11% supersaturation, which also highlights their ability to act as CCN in the atmosphere. Changes in CCN concentration can affect cloud structure and dynamics, which can further affect aerosol-cloud radiation and climate [ 20 ]. A significant influence on the formation of CCN can arise from the aging of airborne bacteria and their shape. Shape can affect the contact angle, and the aging process involves changes in the cell wall and extracellular materials that can affect cluster formation [ 10 , 11 ].

Collocated Observations Of Cloud Condensation Nuclei, Particle Size Distributions, And Chemical Composition

S) [7]. However, no significant effect was found on the influence of aerosol particles of biological origin on cloud formation, mainly due to their low atmospheric concentration [21]. However, quantification of microbial emission sources from different surfaces is far from complete, implying a possible underestimation of microbial impacts on climate [22]. These results were supported by recent findings on the influence of the fungus genus Fusarium, which may be important for ice nucleation and cloud formation [23]. In addition, several studies have identified the need to better characterize the environmental concentrations and biodiversity of biological particles in the atmosphere [1].

In the current study, binary nucleation theory of the sulfuric acid/water system was used to determine the conditions under which bacterial activation (CCN formation) occurs under tropospheric conditions. The surface diffusion mechanism was incorporated into the particle activation calculations using a previously developed kinetic approach of nucleation [ 18 , 24 ].

Activation of bacteria under atmospheric conditions can be described using the classical theory of heterogeneous nucleation, which occurs at lower saturation ratios than homogeneous nucleation. The surface of bacteria is considered to be wettable with water and at the same time insoluble in water. Heterogeneous nucleation theory on insoluble particles from vapor-liquid phase to vapor-liquid for vapor systems [17, 25] is adopted in the current study to evaluate the activation of bacteria in air. The scientific literature has recognized the importance of sulfuric acid on the activation of pre-existing insoluble particles [20] and therefore we studied the sulfuric acid/water system for particle activation and CCN formation. Specifically, sulfuric acid was used in CCN studies because of the interaction of molecules through hydrogen bonds, which is important in the formation of small atmospheric clusters.

Classical heterogeneous nucleation theory is a phenomenological theory that studies a drop of nuclei as a group of molecules that interact strongly with each other and weakly with the rest of the system. According to the classical theory, a nuclear cluster is considered by equilibrium thermodynamics as a macroscopic drop, the free energy of formation of which depends crucially on the surface tension. The rate of nucleation is exponentially dependent on the reversible performance of cluster formation, since nucleation is an activated process. The theory is based on the macroscopic description of the embryo in contact with the substrate using the concept of the contact angle θ [17, 25]. The interaction between the liquid embryo and the solid substrate is described in the classical model with the help of Young’s equation,

Effects Of Number Concentration Of Cloud Condensation Nuclei On Moist Convection Formation In: Journal Of The Atmospheric Sciences Volume 78 Issue 10 (2021)

, where θ is the contact angle between the substrate and the liquid embryo. The critical droplet composition is given by the composition at the saddle point [18].

In the classical theory, the free energy of the formation of a critical cluster on a curved surface can be expressed as follows [25]:

, which can vary from 0 to 180°, is the contact angle between the core and the solid substrate. For water nucleation, a solid is considered hydrophobic or hydrophilic depending on whether the contact angle is greater than or less than 90°.

The critical value of the Gibbs free energy in the uniform case is derived from the following expression [17]:

Pdf) External And Internal Cloud Condensation Nuclei (ccn) Mixtures: Controlled Laboratory Studies Of Varying Mixing States

Where Rgrowth is the cluster growth rate, Z is the Zeldovich disequilibrium factor, and N is the product of the total number of molecules adsorbed per unit surface area of ​​the seed particle (Nads) and the surface area available for adsorption (Aads) on the seed. particle [25].

A detailed description of the theory of heterogeneous nucleation can be found in [18, 25, 26, 27]. The surface diffusion mechanism is also involved in the nucleation process, which leads to higher nucleation rates [16, 18, 27, 28]. The kinetic model was validated against water nucleation on planar substrates [18]. In the atmosphere, it is important to know the activation probability Pd(t).

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