Atmosphere aerosol/single particle

Vented Underground Burst. An underground detonation which produces no visible fireball, but which results in the release of volatile radionuclides through fissures or other vents, produces a single particle class— atmospheric aerosol particles with condensable radionuclides deposited on their surface. Radionuclide abundance is independent of particle size. 

More ambitious attempts at measuring the heterogeneity of the atmospheric aerosol have been undertaken as well. Single-particle analysis by mass spectrometry was demonstrated by Sinha and co-workers (31, 32). In this technique, an aerosol sample is introduced into a vacuum chamber in the form of a particle beam. The particles are injected into a Knudsen cell oven, where they undergo many collisions with the cell wall and are ultimately vaporized and ionized. The ions are then mass-analyzed with a quad-rupole or sector mass spectrometer. So that individual particles can be analyzed, the flux of particles into the Knudsen cell is limited so that coincidence errors are minimized. Ion pulses from individual particles allow the determination of the amount of the species being analyzed in the particular particle. The sensitivity of the technique is limited. For sodium, the detection... 

Single-particle optical analyzers are especially useful for continuous measurement of particles of uniform physical properties. However, as discussed earlier, uncertainties develop in the measurement of particle clouds that are heterogeneous in composition because the refractive index may vary from particle to particle. Thus, in making atmospheric aerosol measurements, workers have assumed an average refractive index characteristic of the mixture to estimate a calibration curve or have reported data in terms of the equivalent particle diameter for a standard aerosol, such as suspended polystyrene latex spheres. 

An important advance in continuous analyzers uses both particle size data and single-particle chemical composition. These instruments employ a method of rapid depressurization of the aerosol that produces a particle beam and irradiation of particles to generate ions that are analyzed by mass spectroscopy. The single particle analyzers have been employed in atmospheric research recently but have not reached the stage where they are used routinely in air monitoring. 

The theoretical analysis of atmospheric aerosols just described focuses on a population of typical yet hypothetical particles that have the average chemical characteristics of the ambient mix. Yet, the nature of actual, individual particles remains largely a mystery due to the difficulties in collecting and probing single submicron particles. 

The total mass of particulate matter per unit volume of air is perhaps the simplest integral property, and it is on this quantity that U.S. federal standards for particulate pollution have been based. Until recently there was a single primary (health related) standard of 50 g/m (annual geometric mean) and 150 /rg/m- (maximum 24-hr concentration not to be exceeded more than once per year), with an upper cutoff in panicle size of 10 /ttm (PMio). However, epidemiological studies indicate an association between adverse health effects, including enhanced mortality, and submicron aerosol concentrations in many U.S. cities (Pope cl al.. 1995). This has led to the establishment of an additional mass ba.sed standard for particles smaller than 2.5 /im (PMj.j) (U.S. EPA, 1996). There is also a separate health-based standard for lead, one component of the atmospheric aerosol. 

An aerosol distribution can be described by the number concentrations of particles of various sizes as a function of time. Let us define Nk(t) as the number concentration (cm-3) of particles containing k monomers, where a monomer can be considered as a single molecule of the species representing the particle. Physically, the discrete distribution is appealing since it is based on the fundamental nature of the particles. However, a particle of size 1 pm contains on the order of 1010 monomers, and description of the submicrometer aerosol distribution requires a vector (N2, N-j,..., N10io) containing 1010 numbers. This makes the use of the discrete distribution impractical for most atmospheric aerosol applications. We will use it in the subsequent sections for instructional purposes and as an intermediate step toward development of the continuous general dynamic equation. 

The radiative effects of a population of particles depend on the composition of the particles, through their refractive indices. As we know, the atmospheric aerosol seldom consists exclusively of a single component it is generally a mixture of species from a number of sources. How all the components are distributed among the particles constitutes the mixing state of the aerosol. Two extremes of mixing state are depicted in Figure 24.9. One extreme is termed an external mixture, where, in the aerosol population, each particle... 

The composition of single aerosol particles in the micrometer range is important for assessing their environmental health hazard and for studying the atmospheric chemistry of particle formation and transformation during atmospheric transport. Both... 

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