Aerosols

What is the aerodynamic diameter?

Defined as the diameter of a spherical particle of unit density having the same settling velocity as the particle in question. The aerodynamic diameter is useful for particles in the inertial size range (those larger than about 0.5 microns).

What are the basic assumptions of kinetic theory?

Gases contain a large number of molecules. The molecules are small compared with the distances between them. The molecules are rigid spheres traveling in straight lines between elastic collisions. Often described as a 'billiard ball' model of gas molecules to describe properties such as temperature, pressure, viscosity, diffusion, thermal conductivity, etc.

What is particle density?

Particle density refers to the mass per unit volume of the particle itself, not of the aerosol. Liquid particles and crushed or ground solid particles have a density equal to that of their parent material. Smoke and fume particles may have apparent densities significantly less than that predicted from their chemical composition.

What is the most important parameter for characterizing the behavior of aerosols?

Particle size. All properties of aerosols depend on particle size, some very strongly. Not only do aerosol properties depend on particle size, but the nature of the laws governing these properties may change with particle size.

Why are particle sizes of greater than 10 micrometers of little importance in the environment?

Particles greater than 10 micrometers have limited stability in the atmosphere. But, they can still be an importance source of occupational exposure because of a worker's proximity to a source.

What is the definition of a nanoparticle?

Particles less than 50nm are called nanometer particles or nanoparticles.

What are ultrafine particles?

Particles that are smaller than 0.1 micrometers (i.e. 100 nanometers - typically the smallest types of aerosols).

What is the thermophoretic force with relation to aerosols?

Temperature gradients in a gas create a force on an aerosol particle called the thermophoretic force. Aerosol particles share energy with the gas molecules it is suspended within (and the particle motion resistance changes with particle size and becomes especially important when the particle sizes approach the spacing between gas molecules). Aerosol particles exhibit Brownian motion.

What is a cascade impactor?

A cascade impactor is used when a particulate substance is moving through an opening with the use of aerosol. The impactor is designed to measure the range of the substance's reach. Cascade impactors are strictly measurement-related devices. The impactor can be used to measure the range of substances moved through an opening by aerosol, and to determine the particle size of the distributed substance.

What is a spray?

A droplet aerosol formed by mechanical breakup of a liquid. Particles are larger than a few micrometers.

What is smog?

A general term for visible atmospheric pollution in certain areas. The word was originally derived from the words smoke and fog. Particles are generally less than 1 or 2 micrometers.

What is meant by the term agglomerate?

A group of particles held together by van der Waals forces or surface tension.

What is meant by the term flocculate?

A group of particles very loosely held together, usually by electrostatic forces. Flocculates can easily be broken apart by shear forces within the air.

What is meant by the term aggregate?

A heterogeneous particle in which the various components are not easily broken apart. The term heterogeneous indicates that the individual components may differ from each other in size, shape, and chemical composition.

What is the definition of a cloud?

A high-density suspension of particles in air, often with a well-defined boundary.

What are the different types of particle suspension based on the states of the suspending medium and particles involved?

A liquid medium with gas particles is called a foam. A solid medium with suspended gas particles is called a sponge. A Liquid medium with suspended liquid particles is an emulsion. A solid medium with suspended liquid particles is a gel. A gas medium with liquid particles suspended in it is a fog, mist, or spray. A gas medium with solid particles is a fume, or dust. A liquid medium with solid particles is a colloid, suspension, or slurry. A solid medium with solid particles is an alloy.

What is eddy covariance measurement?

A method whereby the fluctuating velocities (u') are determined by measurement and this is used to determine the vertical flux of a component, in order to gain estimates of momentum, heat transfer etc.

What is a nanoparticle?

A particle in the size range of 1 to 100nm.

What is the difference between primary and secondary aerosols?

A primary aerosol has particles that are introduced directly into the atmosphere, whereas a secondary aerosol has particles that are formed in the atmosphere by chemical reactions of gaseous components (gas-to-particle conversion).

What is a seiche?

A seiche is a standing wave in an enclosed or partially enclosed body of water. The key requirement for formation of a seiche is that the body of water be at least partially bounded, allowing the formation of a standing wave. Seiches can be considered long period or infragravity waves (surface gravity waves with frequencies lower than wind waves), which are due to subharmonic nonlinear wave interaction with the wind waves, having periods longer than the accompanying wind-generated waves.

What is dust?

A solid-particle aerosol formed by mechanical disintegration of a parent material, such as by crashing or grinding. Particles range in size from submicrometer to more than 100 micrometers and are usually irregular.

What are fumes?

A solid-particle aerosol produced by the condensation of vapors or gaseous combustion products. These submicrometer particles are often clusters or chains of primary particles. The latter are usually less than 0.05 micrometers.

What is a hydrosol?

A stable liquid suspension of solid particles.

What is a thermocline or metalimnion?

A thermocline (aka. thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid in which temperature changes more rapidly with depth than it does in the layers above or below. In the ocean, the thermocline divides the upper mixed layer from the calm deep water below. Depending largely on season, latitude and turbulent mixing by wind, thermoclines may be a semi-permanent feature of the body of water in which they occur or they may form temporarily in response to phenomena such as the radiative heating/cooling of surface water during the day/night. Factors that affect the depth and thickness of the thermocline include seasonal weather variations, latitude and local environment conditions, such as tides and currents. The water layer is called the epilimnion and the cold layer is called the hypolimnion.

What is haze?

A visibility-reducing aerosol.

What is smoke?

A visible aerosol resulting from incomplete combustion. Particles may be solid or liquid, are usually less than 1 micrometer in diameter, and may be agglomerated like fume particles.

What is a cloud?

A visible aerosol with defined boundaries.

What is acoustic coagulation?

Acoustic coagulation is where intense sound waves are used to create relative motion between particles. Depending on their size, particles respond to high intensity sound waves differently; large particles may be unaffected, whereas small particles oscillate with the sound wave. The relative motion that results leads to collisions, and the process is called acoustic coagulation. Generally, sound pressure levels exceeding 120dB are required to produce significant coagulation.

What is adsorption?

Adsorption is the process whereby vapor molecules attach to solid surfaces. It is most important for porous solids, such as activated charcoal, that have large surfacea rea.

What are some other names for aerosols?

Aerosols are also referred to as suspended particulate matter, aerocolloidal systems, and disperse systems.

What is a homogeneous aerosol?

An aerosol in which all particles are chemically identical.

What is the definition of an aerosol?

An aerosol is defined in its simplest form as a collection of solid or liquid particles suspended in a gas. Aerosols are two-phase systems, consisting of the particle and the gas in which they are suspended. They include a wide range of phenomena such as dust, fume, smoke, mist, fog, haze, clouds, and smog.

What is a bioaerosol?

An aerosol of biological origin. Bioaerosols include viruses, viable organisms, such as bacteria and fungi, and products of organisms, such as fungal spores and pollen.

What is an air elutriator?

An air elutriator is a simple device which can separate particles into two or more groups. Material can be separated by means of an elutriator, which consists of a vertical tube up which fluid is passed at a controlled velocity. When the particles are introduced, often through a side tube, the smaller particles are carried over in the fluid stream while the larger particles settle against the upward current. If one starts with low flow rates, small less dense particles attain their terminal velocities, and flow with the stream. The particle from the stream is collected in overflow and hence will be separated from the feed. Flow rates can be increased to separate higher size ranges. Further size fractions may be collected if the overflow from the first tube is passed verticall upwards through a second tube of greater cross-section, and any number of such tubes can be arranged in series. It is used in mineral processing for size classification. The elutriation dust value is a usual measure for the quantification of dust, generated by testing wherein mechanical forces such as vibration are applied to granules. Elutriation is a common method used by biologists to sample meiofauna.

What is a haze?

An atmospheric aerosol that affects visibility.

What is the optical diameter?

An optical diameter is defined as the diameter of a calibration particle having the same response in an instrument detecting particles by their interaction with light.

Why is the term ppm not used for aerosols?

Because two phases are involved and aerosol concentrations are numerically very low when expressed in this way.

Are the bulk properties of aerosols similar to that of air or not?

Bulk properties of aerosols, such as viscosity and density, differ imperceptibly from air. This is because the particulate phase of an aerosol represents only a very small fraction of its total mass and volume, less than 0.0001%. Consequently, to study the properties of aerosols, one must adopt a microscopic point of view (i.e. one particle at a time).

What is coagulation?

Coagulation is an aerosol growth process that results from the collision of aerosol particles with each other. If the collisions are the result of Brownian motion, the process is called thermal coagulation; if they are the result of motion caused by external forces, the process is termed kinematic coagulation. Thermal coagulation is in some ways analogous to growth by condensation except that it is other particles diffusing to a particle's surface rather than molceules that causes the growth. It differs from condensation in that a supersaturation is not required, and it is a one-way process of growth with no equivalent process corresponding to evaporation. The result of many collisions between particles is an increase in particle size and a decrease in aerosol number concentration. In the absence of any loss or removal mechanisms there is no change in mass concentration as a result of coagulation. To understand the process we look first at a simplified description of coagulation called simple monodisperse coagulation or Smoluchowski coagulation. The latter is named after the person who developed the theory in 1917. This approach illustrates the process well, is useful for analyzing many situations, and is the basis for further refinements.

What mechanisms can cause a transfer between aerosol particles and the surrounding gas?

Condensation, evaporation, nucleation, adsorption, and chemical reaction, or it may result from interparticle mass transfer, such as by coagulation.

How do we overcome the errors introduced by aerosols not being exactly spherical?

Correction factors and the use of equivalent diameters enable these theories to be applied to nonspherical particles. An equivalent diameter is the diameter of the sphere that has the same value of a particular physical property as that of an irregular particle. Particles with extreme shapes, such as long, thin fibers, are treated as simplified nonspherical shapes in different orientations. The complex shape of some fume and smoke particles can be characterized by their fractal dimension.

What is elutriation?

Elutriation is a process for separating particles based on their size, shape and density, using a stream of gas or liquid flowing in a direction usually opposite to the direction of sedimentation. This method is mainly used for particles smaller than 1 micron. The smaller or lighter particles rise to the top (overflow) because their terminal sedimentation velocities are lower than the velocity of the rising fluid. The terminal velocity of any particle in any medium can be calculated using Stokes' law if the particle's Reynolds number is below 0.2. Counterflow centrifugation elutriation is a related technique to separate cells.

What is shear or gradient coagulation?

Gradient or shear coagulation occurs for particles moving in a flow velocity gradient. Particles on slightly different streamlines in a velocity gradient travel at different velocities and faster particles eventually overtake the slower ones. If the particles are big enough, particle contact occurs by interception.

Why do industrial hygienists make aerosol measurements?

In order to ensure that the public and industrial work force are not exposed to hazardous aerosols at undesirable concentration levels.

What is the dynamic shape factor within Stoke's law?

In the derivation of Stoke's law, we assume the particles are spherical. The particle drag for shapes other than spheres is usually difficult to predict theoretically. Therefore, for particles of other shapes, a dynamic shape factor 'chi' is introduced that relates the motion of the particle under consideration to that of a spherical particle.

What is turbulent coagulation?

In turbulent flow, particles follow a complex path having strong velocity gradients. Relative motion between particles arises from these gradients and from inertial projection of the particles. The resulting coagulation is called turbulent coagulation. This mechanism is most effective when the turbulent eddy size is the same order of magnitude as the particle stopping distance. This mechanism is only important for particles larger than about 1 micron. Generally, the more intense the turbulence, the more coagulation that results from this mechanism.

What is the Cunningham slip correction factor?

Introduced into the calculation of the settling velocity of a particle because the suspending gas is not a continuous fluid but consists of discrete molecules.

What is drainage-basin management?

It covers water-storage, in the form of reservoirs, and flood protection.

What is the ideal gas law a combination?

It is a combination of Boyle's law, Charle's law, and Avogadro's principle. Boyle's law states that Pv = constant for constant T and n_m. It can also be derived from kinetic theory.

Why do large aerosol particles have different dynamic behavior compared to small aerosols?

Large aerosol particles are constantly being bombarded from all directions by a great number of gas molecules. When a particle is small, less than 1 micron in size, its location in space may be affected by bombardment of individual gas molecules. Its motion is then no longer determined by continuum flow considerations, but by gas kinetics. This transition in dynamic behavior is determined by the Knusden number, which relates the gas molecular mean free path to the physical dimension of the particle, usually the particle radius. Kn = lambda/r, where lambda ~ 0.0664 microns at 293K and atmospheric pressure. The value of lambda depends on the pressure, temperature, and a value called the Sutherland constant, which is different for each gas. For Kn >> 1 indicates free molecular flow, whereas Kn << 1 indicates continuum flow, i.e. the particle collides much more often than the particle size. 0.4 < Kn < 20 is the slip flow regime, where the flow is highly dependent on its initial conditions.

What is a major difference between liquid and solid aerosol particles?

Liquid aerosol particles are nearly always spherical (since this is the most stable shape for bubbles). Solid aerosol particles usually have complex shapes. It is usually necessary to assume that the particles are spherical in the theory of aerosol properties.

What is mist and fog?

Liquid-particle aerosols formed by condensation or atomization. Particles are spherical with sizes ranging from submicrometer to about 200 micrometers.

What is a monodisperse aerosol?

Monodisperse aerosols have particles that are all the same size and can be produced in the laboratory for use as test aerosols.

What is a polydisperse aerosol?

Most aerosols are polydisperse, with a wide range of particle sizes, and statistical measures should be used to characterize their particle size.

What units are bioaerosols and fibers typically expressed in terms of?

Number concentration.

Why can one instrument not measure the entire 5 decade size range of aerosols?

On a macroscopic scale, this would be equivalent to measuring a 1mm distance (which is six orders of magnitude larger than 1mm). When sensing with optical techniques utilizing white light, the wavelength of visible light from about 0.4 to 0.7 microns limits the observation of particles to about this size range and larger. Inertial techniques become inefficient below 0.5 microns at normal temperature and pressure. In an electron microscope, the observational tool is EM radiation with a much smaller wavelength that can see much smaller particles.

How does one estimate the toxicity of an aerosol entering the lung?

One needs to know the size-dependent diffusion, gravitational settling, impaction, and interception properties of the particles to determine the deposition rate within the lung. In addition, the chemistry, surface area, and fibrosity of the particles may indicate their interactions with the lung tissues once they are deposited.

What is the diffusive diameter?

Particles smaller than about 0.5 microns undergo Brownian diffusion and are characterized by the diffusive diameter, the diameter of a particle of unit density have the same rate of diffusion as the particle in question.

What is cross pollination?

Pollination of a flower or plant using pollen from another flower or plant.

What techniques can be used to characterize particles by shape parameters, light-scattering properties, chemical properties, surface, or dynamic behavior?

Shape parameters - microscope Light-scattering properties - optical particle counter or a nephelometer Elemental or chemical properties - X-ray fluorescence or IR spectroscopy Surface properties - pycnometer or by an adsorption measurements Dynamic behavior - measurement of settling velocity or diffusion Note that that characteristic may not correlate well with another characteristic.

Why is silica dangerous in the workplace and what is the current exposure standard in the US?

Silica is one of the most toxic dusts encountered in the workplace. The current exposure standard in the US is 50 micrograms/m^3. In comparison, the workplace standard of the least toxic materials are 10 mg/m^3. The environmental air quality standard in the US is 80 micrograms/m^3.

What is silicosis?

Silicosis is a lung disease caused by breathing in tiny bits of silica, a mineral and mineral ores such as quartz. Over time, exposure to silica particulates causes scarring in the lungs which can harm your ability to breathe.

What is thermohaline circulation?

THC is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water. Wind-driven surface currents (such as the Gulf Stream) travel polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes (forming North Atlantic Deep Water). This dense water then flows into the ocean basins. .While the bulk of it upwells in the Southern Ocean, the oldest waters upwell in the North Pacific. Extensive mixing takes place between the ocean basins, reducing difference between them and making the Earth's oceans a global system. On their journey, the water masses transport both energy and matter around the globe. As such, the state of circulation has a large impact on the climate of the Earth. The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyor, or the global conveyor belt. On occasion, it is used to refer to the meridional overturning circulation (MOC). The term MOC is more accurate and well defined, as it is difficult to separate the part of the circulation which is driven by temperature and salinity alone as opposed to other factors such as the wind and tidal forces. Moreover, temperature and salinity gradients can also lead to circulation effects that are not included in the MOC itself.

What is the Bowen ratio?

The Bowen ratio is used to describe the type of heat transfer in a water body. Heat transfer can either occur as sensible heat (differences in temperature without evapotranspiration) or latent heat (the energy required during a change of state, without a change in temperature). The Bowen ratio is the mathematical method generally used to calculate heat loss (or gained) in a substance; it is the ratio of energy fluxes from one state to another by sensible heat and latent heat respectively. B = Q_h / Q_e Q_h = sensible heat, Q_e = latent heat. Typically, over tropical oceans this values is less than 0.1, and can be above 10 for deserts (since there is little water available and hence Q_e is small).

What is the Gulf Stream?

The Gulf Stream, together with its northern extension the North Atlantic Drift, is a warm and swift Atlantic ocean current that originates in the Gulf of Mexico and stretches to the tip of Florida, and follows the eastern coastline of the US and Newfoundland before crossing the Atlantic ocean. The process of western intensification causes the Gulf Stream to be a northward accelerating current off the east coast of North America. At about 40 degrees latitude 30 degrees longitude, it splits into two, with the northern stream, the North Atlantic Drift, crossing to Northen Europe and the southern stream, the Canary Current, recirculating off West Africa. The Gulf Stream may be slowing down as a result of climate change. The Gulf Stream is also a significant potential source of renewable power generation.

What is the Peclet number?

The Peclet number is the amount of convective transport of particles toward an object related to the diffusive transport. Pe = Ud_c / D where d_c is the significant dimension of the particle collecting surface and U is the upstream gas velocity toward the surface. The larger the value of Pe, the less important is the diffusional process. Pe is often used in the description of diffusional deposition on filters.

What is the definition of the Schmidt number?

The Schmidt number is the ratio of the Peclet number to the Reynolds number. It expresses the ratio of kinematic viscosity (inertial forces) to the diffusion coefficient (diffusion forces). As the Schmidt number increases, convective mass transfer increases relative to Brownian diffusion of particles. It has been used for describing diffusive transport in flow fluids (convective diffusion), especially in the development of filtration theory. Sc is relatively indepedent of T and P near STP.

What is the Stoke's diameter?

The Stokes diameter is the diameter of a spherical particle having the same density and settling velocity as the particle in question.

What is hydroinformatics?

The adaptation of information technology to hydrology and water resource applications

How does one choose the appropriate particle size definition for an aerosol?

The appropriate definition depends primarily on the type of measurement made. For example, the aerodynamic diameter would be used to analyze the data from a cyclone, impact, or an Aerodynamic Particle Sizer (APS; TSI). The diffusive diameter would be used for a diffusion battery measurement, the Stokes diameter for a DMA, and the optical diameter with an optical particle counter. Stokes diameter - DMA Diffusive diameter - diffusion battery optical diameter - OPC aerodynamic diameter - APS, cyclone, or impactor.

What are some important applications of aerosol technology?

The field expanded rapidly in the 1980s to include the use of aerosols in high-tech production processes and a concern for aerosol contamination in the semiconductor industry (clean technology).

How does homogeneous nucleation work?

The initial formation of the droplet from vapor is a more complicated process. Droplets can be formed in the absence of condensation nuclei, but this process, called homogeneous nucleation, or self-nucleation, requires large saturation ratios, usually in the range of 2 to 10, which normally occur only in special laboratory or chemical process situations. Pure water vapor at 293K and at a saturation of 3.5 or greater spontaneously forms droplets by homogeneous nucleation. This corresponds to a Kelvin diameter of 0.0017 microns and suggests that molecular clusters of about 90 molecules are necessary for this process.

What applications do aerosols have in industry?

The manufacture of spray-dried products, fiber optics, and carbon black; the production of pigments; and the application of pesticides.

What is the definition of mass concentration?

The mass of particulate matter in a unit volume of aerosol. Common units are g/m^3, mg/m^3. The mass concentration is equivalent to the density of the ensemble of aerosol particles in air.

How does heterogeneous nucleation work?

The more common formation mechanism is nucleated condensation or heterogeneous nucleation. This process relies on existing submicrometer particles, called condensation nuclei, to serve as sites for condensation. Our natural atmosphere contains thousands of these nuclei in each cubic centimeter of air. To a first approximation, insoluble nuclei serve as passive sites on which condensation occurs for supersaturated conditons. Under supersaturated conditions, a solid nucleus with a wettable surface will have on its surface an adsorbed layer of vapor molecules. If the nucleus has a diameter greater than the Kelvin diameter for a particular condition of supersaturation, the nucleus 'looks like' a droplet to surrounding vapor molecules and vapor will condense on its surface. Once condensation starts, droplet growth continues as described by equations 5.3 and 5.4. The situation with soluble nuclei is more complex and important. Our normal atmosphere contains large numbers of soluble nuclei, formed as the solid residue left behind after the water has evaporated from a droplet containing dissolved material. Many are sodium chloride nuclei formed from droplets of sea water created by the action of waves and bubbles in the oceans. Because these soluble nuclei have a strong affinity for water, they facilitate the initial formation of droplets and enable their growth to occur at lower saturation ratios than would be the case for insoluble nuclei. Because of the complex effect the presence of dissolved salt has on the rate of growth of a droplet, equations 5.3 and 5.4 cannot be used to determine growth rates for such droplets. The stabilization time for droplets containing salt is described by Ferron and Soderholm (1990). In general dissolved salts increase the rate of growth and decrease the rate of evaporation compared with that for pure liquids. As a dropet grows by the addition of water vapor, the concentration of salt becomes more and more dilute. Consequently, it is convenient to characterize the amount of salt in a droplet not by its concentration but by the mass of salt in the droplet, a quantity that remains constant during condensation and evaporation processes. The mass of salt is also equal to the mass of the original nucleus upon which the droplet formed. When a dissolved salt is present in a droplet there are two competing effects at work as the droplet evaporates or grows. As a droplet evaporates the concentration of salt increases, because only the water leaves. This enhances the affinity of the dissolved salt to hold water in the droplet. The other effect is the Kelvin effect that results in an increase in the equilibrium vapor pressure required for a droplet as its size decreases. The relationship between saturation ratio and particles size for droplets containing dissolved salts is illustrated by the three lines called Kohler curves.

What is the definition of number concentration?

The number of particles per unit volume of aerosol, commonly expressed as number/cm^3 or number/m^3.

What is the mechanism for evaporation of a droplet?

The process of evaporation of a pure liquid droplet is similar to the process of growth except that it proceeds in the opposite direction. Evaporation will occur when the ambient partial pressure of vapor is less than the saturated vapor pressure (p < p_s). The rate of particle shrinkage due to evaporation can be predicted by equation 5.4. During evaporation, the term in parentheses will be negative, giving a negative growth rate, which represents shrinkage due to evaporation. For volatile particles such as water or alcohol, the quantity p_d must be evaluated at the cooler conditions prevailing at the droplet surface, T_d, which is given by equation 5.5. For particles larger than about 50 microns an additional correction must be included to account for the disruption in the diffusion of vapor away from the droplet surface caused by the settling of the droplet. This effect increases the rate of evaporation of 50 and 100 microns droplets by 10% and 31%.

What is the difference between absorption and adsorption?

The process whereby gas molecules dissolve in a liquid droplet is called absorption. In this process the transfer at the interface is usually not controlling, but diffusion in either the gas phase or liquid phase may be. The process can continue until the limit of solubility of the gas in the liquid is reached. This limit may change with temperature or the presence of other dissolved components. Adsorption is the transfer of gas molecules from the surrounding gas to a solid surface. There are two types of adsorption that can occur on the surface of a solid particle: physical adsorption, or physisorption; and chemical adsorption, or chemisorption. Physisorption is a physical process where gas molecules are held to a particle's surface by van der Waals forces. It occurs for all gases when the ambient temperature is below the critical temperature. It is a rapid and readily reversable process.

How can information be obtained about aerosols by scattering light from them?

The scattering pattern produced by spheres is quite predictable, while that from glass fibers is predictable only if their orientation relative to the light beam is known. The light pattern from asbestos fibers might be similar to that from glass fibers in some cases, but may be much more complex when the fibers are splayed bundles or matted clumps. Much less light might be scattered by carbon black due to its light-absorbing properties.

What is hydrology?

The scientific study of the movement, distribution, and quality of water on Earth and other planets. It subdivides into surface water hydrology, groundwater hydrology (hydrogeology), and marine hydrology. Domains of hydrology include hydrometeorology, surface hydrology, hydrogeology, drainage-basin management and water quality. Oceanography and meteorology are not included because water is only one of many important aspects within those fields

What is the nuclei mode?

The smallest mode in the atmospheric aerosol, in terms of both particle size and mass concentration, is the nuclei mode, which, however, contains the highest number of particles. The mode diameter increases with time by less than a factor of three because the nuclei mode particles coagulate more rapidly with particles in the condensation mode than with the other particles in the nuclei mode. The nuclei mode is formed by photochemical reactions on gases in the atmosphere and by combustion. A striking demonstration of the photochemistry is afforded by the rapid appearance and growth of the nuclei mode at dawn. Because of its transient nature, the nuclei mode is significant only in the immediate vicinity of sources, such as on a freeway.

What is surface hydrology?

The study of hydrologic processes that operate at or near Earth's surface

What is ecohydrology?

The study of interactions between organisms and the hydrologic cycle

What is chemical hydrology?

The study of the chemical characteristics of water

What is isotope hydrology?

The study of the isotopic signatures of water

What is hydrogeology?

The study of the presence and movement of groundwater

What is hydrometeorology?

The study of the transfer of water and energy between land and water body surfaces and the lower atmosphere.

What is the accumulation mode?

This size range contains most of the fine particle mass. The combustion of fossil fuels produces gases containing sulfur, nitrogen, and organic compounds. Complex reactions in the atmosphere result in the oxidation of the sulfur and nitrogen to produce particles in the accumulation mode containing inorganic compounds such as ammonium sulfate and ammonium nitrate. Organic and elemental carbon particles are also produced in the accumulation mode size range. Some of these chemicals are externally mixed (i.e. they are in separate particles) and some are internally mixed, being in the same particles. Because an internally mixed compound may be different from that of the mode of the particles in question. Whether a compound is internally or externally mixed can sometimes be inferred indirectly from the size distribution, but is best determined directly by single particle analysis techniques such as microscopy or recently developed on-line spectrometers. Whitby described a single accumulation mode with a mass median diameter of about 0.3 microns. In a study of sulfur aerosols in LA, Hering and Frieldander (1982) found the size distributions in the accumulation mode size range on different days to fall into two different types, depending on atmospheric conditions, namely, whether the air was relatively clean and dry or polluted and humid. One mode, designated the condensation mode by John et al. had an average aerodynamic diameter of 0.2 microns. The other mode, named the droplet mode, had an average aerodynamic diameter of 0.7 microns. Both modes contained sulfate, nitrate, and ammonium ion. Size distributions measured with DMAs and OPCs showed similar modal structure. This was possible even though the instruments used do not speciate chemical because the inorganic ions dominated the total particle concentration, as is typical for urban air.

What is the Kelvin effect?

Vapor pressure has been defined as the partial pressure required for mass equilibrium (no net evaporation or condensation) for a flat liquid surface. Because liquid aerosol particles have a sharply curved surface, a greater partial pressure is required to maintain mass equilibrium for a droplet than for a flat liquid surface at a given temperature. The effect is called the Kelvin effect. The saturation ratio required for mass equilibrium (no net condensation or evaporation) for a droplet of diameter d_p is given by the Kelvin equation: S_R = exp(4*/gamma*M/ (rho_p * R * T * d_p)) where /gamma is the surface tension, M is molecular weight, and density rho_p is density of the liquid. Thus, 0.1 and 0.01 micron diameter water droplets require an environment with a saturation ratio of atleast 1.022 and 1.24, respectively, to prevent evaporation. Evaporation will occur if the saturation ratio is less than that given by equation 5.2, even if the saturation ratio is greater than one. Likewise, if the ratio is greater than that required by the Kelvin equation, then condensation and growth will occur. For a given super-saturation, the minimum droplet size required to prevent evaporation is given by equation 5.2 and is referred to as the Kelvin diameter for that condition.

What is the ratio of the volume of a 10-micrometer spherical particle to that of a 0.1-micrometer particle?

Volume = PI/6 * d^3 Ratio = [(PI/6)*d_10^3]/[(PI/6)*d_0.1^3] = (d_10/d_0.1)^3 = (10/0.1)^3 = 10^6

What is western intensification?

Western intensification is the intensification of the western arm of an oceanic current, particularly a large gyre in an ocean basin. The trade winds blow westward in the topics, and the westerlies blow eastward at mid-latitudes. This wind pattern applies a stress to the subtropical ocean surface with negative curl in the northern hemisphere and positive curl in the southern hemisphere. The resulting Sverdrup transport (theoretical relationship between wind stress exerted on the surface of the open ocean and the vertically integrated meridional transport of ocean water) is equatorward in both cases.

Describe the concept of growth rate for aerosol particles.

When a droplet of pure liquid is in a supersaturated environment that exceeds the requirement given by the Kelvin equation, the droplet grows by condensation of vapor on its surface. The rate of growth depends on the saturation ratio and the particle size. It is controlled by the rate of arrival of vapor molecules at the droplet surface. Initially the droplet will usually be less than the mean free path of the surrounding gas (0.066 microns at standard conditions) and the rate of arrival of vapor molecules is governed by the kinetic theory of gases. The growth rate, the rate of increase in droplet diameter, is given by the Hinds (1999) equation (for d_p < mean free path). /alpha_c = condensation coefficient, which is the fraction of arriving molecules that stick, approximately 0.04. Once a droplet's size is greater than the mean free path, the rate of arrival of vapor molecules is governed by the rate of molecular diffusion to the droplet surface. Under these conditions the rate of growth is given by a different equation. This equation has a correction factor called the Fuchs correction, which corrects for complications in the calculation of mass transfer by diffusion within one mean free path of the particle surface. The Fuchs corrector factor /phi is given by Davies (1978). We can neglect this factor for particles above 2 microns. The equations only apply to pure materials, i.e. single-component liquids without any dissolved salts or impurities. The growth rate for droplets less than the mean free path is independent of droplet size, but it is inversely proportional to droplet size for droplets larger than the mean free path. The time required for a droplet to grow from d_1 to d_2 can be obtained by integration the previous equations over the size limits and rearranging.

What is the Whitby model?

Whitby (1978) described a trimodal distribution consisting of a nuclei mode in the size range of 0.005 to 0.1 microns, an accumulation mode from 0.1 to 2 microns, and a coarse mode of more than 2 microns. Each mode was fitted by a lognormal function. Typical ambient particle size distributions have a minimum concentration between the accumulation and coarse modes (i.e. near 2 microns). Whitby divided the particles into two main fractions: fine particles, with diameters less than 2 microns; and coarse particles, with diameters of more than 2 microns. These two fractions have major differences in both origins and in physical and chemical characteristics. The fine fraction derives mainly from combustion, whereas the coarse fraction is generated by mechanical processes. This includes the nuclei mode, which are transient particles formed by condensation and coagulation. The nuclei rapidly grow into the accumulation mode. According to Whitby, the accumulation mode also contains droplets formed by the chemical conversion of gases to vapors that condense. The coarse particle fraction contains wind blown dust, sea spray, and plant particles.