On the effect of the chemical composition of atmospheric aerosol particles on nucleation scavenging and the formation of a cloud interstitial aerosol

Nucleation scavenging and the formation of a cloud interstitial aerosol (CIA) were theoretically studied in terms of the chemical composition of atmospheric aerosol particles. For this study, we used our air-parcel cloud model, which includes the entrainment of air and detailed microphysics, for determining the growth and interaction of aerosol particles and drops. Maritime and remote continental aerosol particle spectrums were used whose size distributions were superpositions of three log-normal distributions, each of a prescribed chemical composition. Our results show (1) that the CIA exhibits a size distribution with a distinctive cut-off at a specific radius of the dry as well as of the wet particle size distribution. All particles above this limiting size become activated to cloud drops and, thus, are not present in the CIA spectrum. This limiting size was found to be independent of the chemical composition of the particles and only dependent on the prevailing supersaturation. Below this specific size, the CIA spectrum becomes depleted of dry aerosol particles in a manner which does depend on their chemical composition and on the supersaturation in the air. (2) The number of aerosol particles nucleated to cloud drops depends critically on the chemical composition of the particles and on the prevailing supersaturation.     

Hygroscopic growth of aerosol particles and its influence on nucleation scavenging in cloud: Experimental results from Kleiner Feldberg

The hygroscopic growth of individual aerosol particles has been measured with a Tandem Differential Mobility Analyser. The hygroscopic growth spectra were analysed in terms of diameter change with increasing RH from 20% to 85%. The measurements were carried out during the GCE cloud experiment at Kleiner Feldberg, Taunus, Germany in October and November 1990.Two groups of particles with different hygroscopic growth were observed. The less-hygroscopic group had average growth factors of 1.11, 1.04 and 1.02 for particle diameters of 50, 150 and 300 nm, respectively. The more-hygroscopic group had average growth factors of 1.34, 1.34, and 1.37 for the same particle diameters. The average fraction of less-hygroscopic particles was about 50%. Estimates of the soluble fractions of the particles belonging to the two groups are reported.Hygroscopic growth spectra for total aerosol, interstitial aerosol and cloud drop residuals were measured. A comparison of these hygroscopic growths of individual aerosol particles provides clear evidence for the importance of hygroscopic growth in nucleation scavenging. The measured scavenged fraction of particles as a function of diameter can be explained by the hygroscopic growth spectra.     

Below-cloud rain scavenging of atmospheric aerosols for aerosol deposition models

Below-cloud aerosol scavenging is generally estimated from field measurements using advanced instruments that measure changes in aerosol distributions with respect to rainfall. In this study, we discuss various scavenging mechanisms and scavenging coefficients from past laboratory and field measurements. Scavenging coefficients derived from field measurements (representing natural aerosols scavenging) are two orders higher than that of theoretical ones for smaller particles (D_p < 2 μm). Measured size-resolved scavenging coefficients can be served as a better option to the default scavenging coefficient (e.g. a constant of 10^?4 s^?1 for all size of aerosols, as used in the CALPUFF model) for representing below-cloud aerosol scavenging. We propose scavenging correction parameter (C_R) as an exponential function of size-resolved scavenging coefficients, winds and width in the downwind of the source–receptor system. For a wind speed of 3 m s^?1, C_R decrease with the width in the downwind for particles of diameters D_p < 0.1 μm but C_R does not vary much for particles in the accumulation mode (0.1 < D_p < 2 μm). For a typical urban aerosol distribution, assuming 3 m s^?1 air-flow in the source–receptor system, 10 km downwind width, 2.84 mm h^?1 of rainfall and using aerosol size dependent scavenging coefficients in the C_R, scavenging of aerosols is found to be 16% in number and 24% in volume of total aerosols. Using the default scavenging coefficient (10^?4 s^?1) in the CALPUFF model, it is found to be 64% in both number and volume of total aerosols.     

Thin coating on ultrafine aerosol particles

This article presents the results of an experimental investigation on gas-phase coating of nanometer-sized NaCl aerosol particles with a condensing vapor of ZnCl₂. The coating process has been carried out in a commercially available Venturi aspirator, where the NaCl at ambient temperature is mixed with the supersaturated ZnCl₂ vapor. The operating conditions (ZnC1₂ vapor temperature, and ZnCl₂/NaCl flow rate ratio) at which the ZnCl₂ vapor preferentially nucleates onto the surface of the seed NaCl particles forming a coating layer on them, have been determined. Particle size growth as a function of the original seed diameter has also been determined. Smaller particles undergo a larger relative size growth ratio. It has also been shown that the presence of a coating layer does not affect the charge distribution of the original seed particles.     

Thin coating on ultrafine aerosol particles

This article presents the results of an experimental investigation on gas-phase coating of nanometer-sized NaCl aerosol particles with a condensing vapor of ZnCl₂. The coating process has been carried out in a commercially available Venturi aspirator, where the NaCl at ambient temperature is mixed with the supersaturated ZnCl₂ vapor. The operating conditions (ZnC1₂ vapor temperature, and ZnCl₂/NaCl flow rate ratio) at which the ZnCl₂ vapor preferentially nucleates onto the surface of the seed NaCl particles forming a coating layer on them, have been determined. Particle size growth as a function of the original seed diameter has also been determined. Smaller particles undergo a larger relative size growth ratio. It has also been shown that the presence of a coating layer does not affect the charge distribution of the original seed particles.     

An experimental study of aerosol scavenging by hexagonal plate ice crystals

The scavenging efficiency, E, of small hexagonal plate ice crystals for aerosol particles has been measured in a series of laboratory cloud experiments. The ice crystal diameters, D μm ranged from 35 to 150 μm with aerosol particles in the range 4 to 6 μm. An ice crystal replication technique made possible the individual examination of more than 43m500 individual crystals from which the relation: log₁₀ E = 1.554 − 1.047 log₁₀D was established with values of E in the range 0.2–0.9 corresponding to the range of crystal sizes investigated. For some crystal collectors, values of E extended above unity and this was attributed to wake capture, oscillations and the extended sweep out path of the ice crystals associated with their spiral fall pattern.     

Experimental investigation of the scavenging of gaseous mercury by sea salt aerosol

Sea salt aerosol may be an important sink for reactive gaseous mercury (RGM) in the marine boundary layer, reducing ambient RGM concentrations and transferring the mercury (Hg) to the oceans and coastal ecosystems. The goal of this study was to determine the affinity of gaseous mercury for sea salt aerosol (SSA) by conducting adsorption experiments with sea salt-coated sampling denuders. In the first set of experiments, ambient outdoor air was passed through denuders coated with either KCl, as in the widely accepted method to sample RGM, or with NaCl, a primary component of sea salt aerosols. On the one sampling day in which RGM was above the MDL, the NaCl coated denuders removed Hg from the ambient air, equivalent to 87% of the RGM in the air (as determined by KCl denuders). For the second set of experiments HgCl₂ generated in the laboratory was passed through denuders coated with KCl and either NaCl or sea salt. The NaCl denuders collected an average of 99 ± 16% of the mercury that the KCl denuders collected. Newly coated sea salt denuders collected 88 ± 17% of the amount of mercury that the KCl denuders collected, but interestingly the sea salt denuders capacity decreased with repeated use. These experiments demonstrate that HgCl₂, a major component of RGM has a strong affinity for NaCl and sea salt and is therefore likely to be scavenged by SSA. This study adds to the growing evidence that RGM is scavenged by sea salt aerosols and therefore more quickly deposited to the ocean and coastal environment.     

Large aerosol particles in cirrus type clouds

Research flights in November 1990 over the central parts of the United States, Wyoming and Colorado, were aimed to the investigation of the properties and microstructure of cirrus clouds (mainly cirrocumulus lenticularis). Among the other parameters measured on board the NCAR Saberliner were the concentration and size distribution of submicron particles and, in some cases, the particle deliquescence. For coarse insoluble particles found inside and outside of cloud elements, size distributions and morphology information were obtained by evaluating inertial impactor samples with an optical microscope and scanning electron microscope. In addition, the coarse particle composition was determined by x-ray energy spectrum analysis. The following conclusions from these measurements are:The large and coarse particle size distribution can be roughly simulated by a log-normal function with the modus around r=0.5 μm. Particle concentrations are very variable between several tenths and several particles per cm³. Particle volume distribution features a distinct maximum around 0.75 μm without a broad plateau which was observed in the case of sampling at lower altitude. Aerosol composition heterogeneity at cirrus cloud level is well documented by the evaluation of the fine particle sampling taken with the UMR sampling system. This heterogeneity can be partly explained by the interaction between aerosol and cloud elements, which is documented by the measured particle size distribution curves inside and outside of cloud elements. Assuming that particle deliquescence is caused by H₂SO₄ and/or by (NH₄)₂SO₄, particle soluble mass fractions were found to be around 30% in the first case and about 40% in the second. The most frequently occurring elements in large and coarse particles at cirrus cloud level were Si, Cl, Ba, S, Ca and C.     

Physical characterization of aerosol particles during nucleation events

Particle concentrations and size distributions have been measured from different heights inside and above a boreal forest during three BIOFOR campaigns (14 April–22 May 1998, 27 July–21 August 1998 and 20 March–24 April 1999) in Hyytiälä, Finland. Typically, the shape of the background distribution inside the forest exhibited 2 dominant modes: a fine or Aitken mode with a geometric number mean diameter of 44 nm and a mean concentration of 1160 cm⁻³ and an accumulation mode with mean diameter of 154 nm and a mean concentration of 830 cm⁻³. A coarse mode was also present, extending up to sizes of 20 μm having a number concentration of 1.2 cm⁻³, volume mean diameter of 2.0 μm and a geometric standard deviation of 1.9. Aerosol humidity was lower than 50% during the measurements. Particle production was observed on many days, typically occurring in the late morning. Under these periods of new particle production, a nucleation mode was observed to form at diameter of the order of 3 nm and, on most occasions, this mode was observed to grow into Aitken mode sizes over the course of a day. Total concentrations ranged from 410–45 000 cm⁻³, the highest concentrations occurring on particle production days. A clear gradient was observed between particle concentrations encountered below the forest canopy and those above, with significantly lower concentrations occurring within the canopy. Above the canopy, a slight gradient was observed between 18 m and 67 m, with at maximum 5% higher concentration observed at 67 m during the strongest concentration increases.     

A role of aerosol particles in atmospheric ice nucleation

Considered is the process of ice nucleation in the atmosphere as a result of heterogeneous condensation of water vapor in surface inhomogeneities of aerosol particles and of subsequent heterogeneous crystallization of supercooled water accumulation. It is revealed that the size, structure, and composition of aerosol particles determine the thermal regime of crystallization.     

雾天气向霾天气转化时气溶胶颗粒物的动力学特性

针对自然界中雾天气向霾天气转化的现象,提出建立和发展雾环境气溶胶颗粒物多效应的动力学模型。基于离散系统的颗粒群平衡方程和多重Monte Carlo算法,分别对雾形成阶段（凝并、冷凝和成核3个动力学事件占主导）、雾发展阶段（凝并、破碎、沉积和成核4个动力学事件占主导）和雾消散阶段（破碎、蒸发和沉积3个动力学事件占主导）气溶胶颗粒物平均体积、颗粒数目随时间的演变过程进行研究。结果表明：气溶胶颗粒物的初始体积尺度为1,经过雾的生命周期进入消散阶段时,尺度为0.0156的细小气溶胶颗粒数目迅速增加,1000 s时间已发展至初始值的8.12倍。研究结果解释了自然界中雾天气向霾天气转化的过程和物理机制。     

Phase partitioning of aerosol particles in clouds at Kleiner Feldberg

The partitioning of aerosol particles between cloud droplets and interstitial air by number and volume was determined both in terms of an integral value and as a function of size for clouds on Mt. Kleiner Feldberg (825 m asl), in the Taunus Mountains north-west of Frankfurt am Main, Germany. Differences in the integral values and the size dependent partitioning between two periods during the campaign were observed. Higher number and volume concentrations of aerosol particles in the accumulation mode were observed during Period II compared to Period I. In Period I on average 87±11% (±one standard deviation) and 73±7% of the accumulation mode volume and number were incorporated into cloud droplets. For Period II the corresponding fractions were 42±6% and 12±2% in one cloud event and 64±4% and 18±2% in another cloud event. The size dependent partitioning as a function of time was studied in Period II and found to have little variation. The major processes influencing the partitioning were found to be nucleation scavenging and entrainment.     

In-cloud and below-cloud scavenging of aerosol ionic species over a tropical rural atmosphere in India

The temporal variation in concentrations of major water soluble ionic species has been studied from several rain events occurred over Gadanki (13.5 °N, 79.2 °E), located in tropical semi arid region in southern India. The contribution from rain-out (in cloud) and wash-out (below cloud) processes to the total removal of ionic species by rain events is also estimated using the pattern of variations of ionic species within an individual event. A number of rain samples were collected from each rain event during June–November in 2006, 2007 and 2008. On average, nearly 20% of the total NH ₄ ⁺ and non-sea SO ₄ ^2? is removed by in-cloud scavenging, suggesting that their removal by “below cloud” washout is relatively dominant. In contrast Na⁺, Ca²⁺, Mg²⁺, NO ₃ ^? and sea-SO ₄ ^2? are mainly removed by below-cloud scavenging or wash-out process. A significant variation in the acidity was observed within rain events with successive precipitation showing higher acidity at the final stage of the precipitation due to partial neutralization of non-sea SO ₄ ^2? . Overall, greater influence of both terrestrial and anthropogenic sources is recorded in the rain events compared to that from marine sources.     

Changes in scavenging of particles by droplets due to weak electrification in clouds

Electrical charges on aerosol particles and droplets modify the droplet–particle collision efficiencies involved in scavenging, and the droplet–droplet and particle–particle collision efficiencies involved in coalescence of droplets and particles, even in only weakly electrified clouds and aerosol layers. This work places electrically enhanced scavenging, and the electrical inhibition of scavenging in the context of the microphysics of weakly electrified clouds.Collision efficiencies are calculated by numerical integration to obtain particle trajectories, that are determined by the complex interplay of electrical, gravitational and phoretic forces together with inertia. These modify the trajectory of a particle as it is carried by flow around the falling droplet. Conversely, the flow around the particle also modifies the trajectory of the droplet. The flows are specified analytically, using a hybrid of the Proudman–Pearson stream function for that region close to the droplet or particle, where it is accurate, merging into the exact Oseen stream function for larger distances, where that becomes accurate. The effect of the flow around the particle on the motion of the droplet was simulated using Langmuir's superposition technique on the hybrid stream functions. The treatment of inertia in the present calculations allows an extension of the scope of our previous work by a factor of 10 larger in particle size (10³ in mass). The coverage is extended to a wide range of atmospheric conditions and particle densities.The pressures and temperatures used in the models ranged from a representation of the lower troposphere at  1 km altitude (900 hPa, 10 °C) to that of the middle stratosphere at  30 km altitude (12 hPa, − 47 °C). The particles considered range from 0.1 μm to 10 μm radius; the droplet radii range from 4 μm to 50 μm; particle densities range from 300 kg m^− 3 to 2500 kg m^− 3; particle charges range from 2e to 100e with droplet charges of like sign of 100e; and relative humidities range from 10% to 100%.For the larger particles (radii greater than about 3 μm) interacting with the larger droplets (radii greater than about 15 μm) the effects of inertia increase with particle density and dominate at the larger densities. For particles with radii in the range 1–3 μm the ‘Greenfield Gap’ of very low collision efficiencies was found, and was determined to be due to the effects of the gravitational force causing a reduction of collisions of particles with the front of the droplet, and the effect of inertia overcoming the tendency for the weight to produce a collision in the slow velocity region in the rear. When the electrical or phoretic forces are sufficiently large the Greenfield Gap is closed.When the particles have radii < 3 μm inertial effects no longer dominate the collisions, although inertia modifies the weight effects for particles with radii down to about 0.5 μm. For charged aerosol particles with radii smaller than about 0.1 μm interacting with droplets or background aerosol particles smaller than a radius of about 15 μm, the long range electrical repulsive force is effective in opposing the phoretic forces and keeping the particle out of range of the short range attractive image force. Thus ‘electroscavenging’ gives way to ‘electroprotection’ against the scavenging or coagulation processes otherwise caused by Browninan diffusion or phoretic forces.In an atmosphere of temperature 10 °C and pressure 900 hPa the net phoretic force reduces to zero and becomes repulsive for particles with radii above about 2 μm (depending on particle conductivity). This enhances the development of the Greenfield Gap. However, the value of this radius (at which the net phoretic force is zero) increases strongly with decreasing temperature and pressure (increasing altitude) as expected from theory, and is about 5 μm in the middle troposphere and more than 10 μm in the stratosphere. Thus a net attractive phoretic force on particles extends into the 1–3 μm radius range in the upper troposphere; however, the weight and inertial effects can ensure the presence of the Greenfield Gap in that range for 2000 kg m^− 3 particles up to the middle stratosphere.     

The Effect of Weak Shear－induced Motion on Brownian Coagulation of Aerosol Particles

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