Measuring and testing – Particle size
Reexamination Certificate
1999-07-13
2001-07-24
Noland, Thomas P. (Department: 2856)
Measuring and testing
Particle size
C356S037000, C356S335000
Reexamination Certificate
active
06263744
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to aerosol measurements and technology. More particularly, the present disclosure describes a precision aerosol detection system for fast characterization of fine particle size distributions in a pressure-changing environment.
BACKGROUND AND SUMMARY OF THE INVENTION
Aerosol measurements characterize the size, concentration and composition of particles suspended in the atmosphere. Measuring the particle size distribution provides the concentration of particles as a function of size. Atmospheric particles influence climate change, radiative transfer, visibility, and air quality. Measurements of the concentration and sizes of particles present in the atmosphere allow quantification of pollutant effects and monitoring particulate growth.
Aerosol instruments can be used for carrying out high-resolution, in-situ aerosol measurements from aircraft and ships. These measurements can probe the spatial and temporal variability of the tropospheric aerosol. A striking example of the effect of spatial variability of aerosol characteristics on cloud properties is provided by ship tracks, first observed from satellites, and later from aircraft measurements. Ship tracks provide a dramatic example of the ability of aerosols to alter the resulting cloud characteristics; measuring this microphysical evolution by means of in-situ measurements was an important goal of the Monterey Area Ship Track (MAST) Experiment. These spatially well-defined perturbations of the aerosol concentration and composition and of cloud properties provide an opportunity to study the broader question of the impact of anthropogenic emissions on cloud properties.
Theoretical studies have predicted that both marine and anthropogenically-influenced tropospheric aerosols should vary diurnally as a result of photochemical reactions resulting in secondary new particle formation and aerosol growth. Such work suggests that the aerosol size distribution will evolve during the day through a series of characteristic size distributions indicative of periods of nucleation and condensation. Providing in-situ evidence for such direct dependence of aerosol properties on other atmospheric variables suggests studies of marine boundary layer and free tropospheric aerosol with aircraft instrumented to measure size distributions quickly and automatically.
Airborne measurements of submicron aerosol size distributions at a frequency capable of resolving the differences, for example, between the cloud line features and surrounding clouds are desirable in order to characterize small-scale or ephemeral features in the atmospheric aerosol. Many of the commercially-available submicron aerosol classification and counter designs are not suited for this application partially because of the long sampling times required to characterize the submicron size distribution extending over two or more decades in particle diameter.
One type of the prior-art airborne aerosol instruments use optical particle counters aboard the aircraft. These instruments provide valuable insight into the variation of aerosol with altitude, and the character of aerosol in and above the clouds. One such system was described by Radke et al. for obtaining size distribution information with an optical particle counter (OPC) for particles greater than 0.1 mm diameter in “Direct and remote sensing observations of the effects of ships on clouds”, Science, Vol.246, pp.1146-1149, 1989. Clarke et al. introduced the Thermo-Optical Aerosol Detector (TOAD) to characterize both the dry aerosol distribution and its volatility in 1991 (“A thermo-optic technique for in-situ analysis of size-resolved aerosol physicochemistry”, Atmos. Env., Vol.25A, pp. 635-644). Hegg et al. and Clarke extended the effective size range of aerosol measurement using mobility-classification to below 20 nm diameter. Detailed descriptions of their work can be found in “Aerosol size distributions in the cloudy atmospheric boundary layer of the North Atlantic Ocean”, J. Geophys. Res., Vol.98, pp.8841-8846,1993 and “Airborne measurements of aerosol properties in clean and polluted air masses during ASTEX”, EOS Proceedings of the 1993 AGU Spring Meeting, Apr. 20, 1993.
Several constraints are inherent to aircraft-based submicron aerosol measurement, including limitations on size, weight, and power as well as the necessity for making fast measurements while adjusting rapidly for changing pressure, temperature, and humidity conditions. The need for rapid measurements derives from the aircraft's speed relative to the spatial scale of changes in aerosol properties. The spatial resolution possible with an airborne instrument is determined both by the speed of the instrument and the speed of the aircraft. Conventional differential mobility analysis requires a sampling period of about 10 min. If continuous sampling methods were employed, the resulting size distribution would represent, for example, at a speed of 100 m/s, a composite distribution of sized aerosol concentrations for a 60-km flight leg. Since air mass characteristics can change drastically over 60 km, several prior-art systems employed a grab sampling approach in which air is drawn into a holding chamber and stored while a single measurement is processed. Radke et al. employed a 90 l steel cylindrical chamber with a floating piston filled by ram pressure to store the aerosol for size classification, and were thus able to store a sample collected in 5 seconds (see, the above referenced publication in 1989). Hegg et al. also employed a large (about 2.5 m
3
) polyethylene bag for analysis over a ten minute period of size classification (see, the above referenced publication in 1993).
The approach of grab sampling has successfully provided in-flight snapshots of aerosol in air masses, which have been coupled with continuous condensation nuclei (CN) measurements to determine the aerosol's spatial variability. Measurement speed still limits both the frequency with which complete distributions can be acquired and the instrument's lower detection limit. Diffusional deposition of aerosol particles on the walls of a sampling vessel can reduce the number concentrations dramatically for long counting times. Consequently, the chamber's volume must be chosen such that particle losses during sampling and analysis are minimized. Particle losses in a chamber are also exacerbated by electrostatic enhancement of charged particles on the chamber walls. Hegg et al. measured ultra fine particles during a 10-min. sample measurement protocol by employing a 2.5-m
3
chamber.
To size particles smaller than 0.1 &mgr;m in diameter, a differential mobility analysis is usually employed. This technique is described in detail by Knutson and Whitby, in “aerosol classification by electrical mobility” in J. Aerosol Sci., vol.6, p.453, 1975. A differential mobility analyzer separates charged particles according to their migration velocities in an applied electric field. Differential mobility analysis is accomplished by introducing a small aerosol flow near one electrode of a two-electrode apparatus, with a larger particle-free sheath flow separating that aerosol from the second electrode. An electrical potential drives particles of appropriate polarity across the sheath flow toward the opposite electrode. At a location downstream from the aerosol inlet, small classified aerosol sample flow is extracted, which the remaining flow is discharged to an exhaust. Only particles that migrate within a narrow range of velocities are included in the classified aerosol sample flow. Particles with higher migration velocities deposit on the counter electrode while those with lower migration velocities are discharged with the exhaust flow. In measurements of differential mobility size distribution, the classified aerosol particles are transported to a detector for counting. Because the particles of interest are too small to be efficiently detected optically, they are commonly grown by vapor condensation in a detector known as a condensation nucleu
Flagan Richard C.
Russell Lynn M.
Zhang Shouhua
California Institute of Technology
Fish & Richardson P.C.
Noland Thomas P.
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