Page 73 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: FWD Revised Pages
Encyclopedia of Physical Science and Technology En001c-14 May 7, 2001 18:25

296 Aerosols

TABLE III Remote Sensing Methods for Atmospheric Aerosol Characterization
Method Measurable light Light source
Ground-based, passive
Photometry and radiometry Optical thickness; sky brightness Sun
Polarimetry Polarization of skylight Sun; diffuse sky
Polar nephelometry Extinction coefficient Sun; skylight
Teleradiometry Horizon brightness; relative contrast Reflected and scattered light
Airborne, passive
Spectrophotometry Albedo; optical thickness Reflected sunlight
Limb occultation (satellite) radiometry Optical thickness; polarization Sunlight
Active sensing
Transmission/backscatter Optical thickness Searchlight; LIDAR a
a
LIDAR denotes light detection and ranging.
spectroscopists, meteorologists, and space engineers for a matrix of many different, often unknown components.
many years. The publications listed in the bibliography New methods have been introduced that involve spectro-
contain the details of the methodology and the intercom- scopic examination by a variety of techniques including
parisons between remote sensors of different kinds and X-ray fluorescence, plasma emission spectroscopy, neu-
direct aerosol observations. tron activation analysis, photoelectron spectroscopy, and
In general, the methods are difficult to interpret quan- mass spectroscopy.
titatively in terms of aerosol properties because of am- The wet chemical methods are summarized in Table IV.
biguities in the size distribution–concentration–distance Basically this approach centers on the water-soluble ex-
profiles and variations in chemical properties contributing tract obtained from filter substrates, impactor plates, or
to the index of refraction. Nevertheless, remote sensing other collection surfaces. The extraction process has to
continues to be important for the surveillance of aerosol be done with some care to ensure that all of the water-
behavior in planetary atmospheres. soluble material is removed. Standard extraction methods
now involve the use of ultrasonic devices to maximize
extraction efficiency. Once the extract is obtained, it can
B. Methods for Chemical Characterization be subjected to a number of the methods listed in Ta-

After the collection of particles, it is useful to determine ble IV, such that a detailed elemental breakdown by inor-
the chemical characteristics of the material. This can be ganic and (water-soluble) organic carbon is accomplished.
accomplishedintermsofanalysisofawholesamplecorre-
sponding to the total mass concentration, or it can be done
on a size-fractionated basis. In some cases, individual par- TABLE IV Wet Chemical Methods for Particle Analysis
ticles can also be examined. Chemical characterization is Particle Method
very important when one is considering a heterogeneous
collection of aerosol particles such as those found in the Cations
ambient air or in the workplace. These include whole sam- Ammonium Indol phenol blue colorimetry,
ion chromatography
ple microscopic analysis by collected batch, as well as
Anions
continuous measurement.
Sulfate Methylthymol blue colorimetry,
ion chromatography
1. Macroscopic Techniques Nitrate Cadmium reduction, ion
chromatography
Macroscopic methods for chemical analysis essentially Chloride Ferric thiocyanate, ion
take either all of the particulate matter sampled or a sig- chromatography
nificant protion of it for bulk analysis. Traditionally, this Elements
has been approached by the application of standard mi- Pb, Fe, Na, K, Ca, Cr, Atomic absorption spectroscopy,
crochemical techniques of wet chemistry. The unique an- Ni, As, Mn, Si plasma emission spectroscopy
alytical requirement for aerosol particle samples is the Carbonaceous material
microgram quantities collected. The analytical methods Solvent-soluble organics Extraction and carbon detection;
differential thermal analysis
adopted must be capable of detecting these quantities in