The development of improved sampling and analysis techniques for
aerosol physical and chemical characteristics is an active and diverse
field that cannot possibly be done justice in this review; a few
U.S. developments in aerosol physical characterization methods are
noted here. Stolzenburg and McMurry [1991] report the
development of a particle counter for the measurement of ultrafine
aerosol (that is, particles with diameter less than 0.01 
m).
Advances in methods for obtaining size distribution information for
ultrafine particles are reported by Wiedensohler et al.
[1994]. Any ultrafine particles which are produced in the atmosphere
from bursts of nucleation have a very short lifetime; they are rapidly
lost to surfaces, or coagulate and grow to larger sizes. Their
detection is important in the evaluation of whether and where in the
atmosphere homogeneous nucleation of new particles takes place. A
technique for measurement of the sphericity of particles is described
by Sachweh et al. [1993]; many sampling techniques and signal
inversion methods are based upon the assumption of particle
sphericity, and the degree of departure from this idealized shape may
be important in interpreting measurements. Faster measurement of
submicron aerosol size distributions from differential mobility
analysis techniques was realized by the development of a scanning
instrument ( Quant et al. [1993]), making it possible to observe
rapid evolution of the aerosol size spectrum in a chemically-reacting
system ( Wang et al. [1992]).
Satellite observation of tropospheric and stratospheric aerosol properties will be an important component of future assessments of aerosol climate impacts ( NRC (National Research Council) [1993] and Penner et al. [1993a]). The Stratospheric Aerosol and Gas Experiment (SAGE I and SAGE II) and Stratospheric Aerosol Measurement (SAM II) instruments were designed to obtain vertical profiles of aerosol extinction (e.g., Brogniez and Lenoble [1991]). It has also been demonstrated that retrieval of some aerosol information from other existing instruments is possible; for example, analyses of data from the Advanced Very High Resolution Radiometer (AVHRR) ( Durkee et al. [1991]) showed the transport of dust plumes, as well as plumes of aerosol extending downwind from urban sources. Additional capabilities, new retrieval methods, and observations for validation efforts are urgently needed for the future. A summary of planned and proposed spacecraft instruments for aerosol monitoring is given by Penner et al. [1993a].