Universal Spectrum for Atmospheric Suspended Particulates: Comparison with Observations: Data Set I - Rain Formation in Warm Clouds: General Systems Theory - page 54

and absorb solar and terrestrial radiation, and via the indirect effect whereby they
modify the microphysical properties of clouds thereby affecting the radiative prop-
erties and lifetime of clouds (Haywood et al. 2003 ). At present empirical models for
the size distribution of atmospheric suspended particulates are used for quantitative
estimation of earth-atmosphere radiation budget related to climate warming/cooling
trends. The empirical models for different locations at different atmospheric con-
ditions, however, exhibit similarity in shape implying a common universal physi-
cal mechanism governing the organization of the shape of the size spectrum. The
pioneering studies during the last three decades by Lovejoy and his group (Love-
joy and Schertzer 2008 , 2010 ) show that the particulates are held in suspension
in turbulent atmospheric flows which exhibit self-similar fractal fluctuations on
all scales ranging from turbulence (mm-sec) to climate (kms-years). Lovejoy and
Schertzer ( 2008 ) have shown that the rain drop size distribution (DSD) should show
a universal scale invariant shape. In this study, a general systems theory for frac-
tal space--time fluctuations developed by the author (Selvam 1990 , 2005 , 2007 ,
2009 ) (see Sect. 1.3) is applied to derive a universal (scale independent) spectrum
for suspended atmospheric particulate size distribution expressed as a function of
the golden mean τ (≈ 1.618), the total number concentration and the mean volume
radius (or diameter) of the particulate size spectrum. A knowledge of the mean
volume radius and total number concentration is sufficient to compute the total
particulate size spectrum at any location. Model predicted spectrum is in agree-
ment with the following four experimentally determined data sets: (i) CIRPAS mis-
sion TARFOX_WALLOPS_SMPS aerosol size distributions, (ii) CIRPAS mission
ARM-IOP (Ponca City, OK) aerosol size distributions, (iii) SAFARI 2000 CV-580
(CARG Aerosol and Cloud Data) cloud DSDs, and (iv) TWP-ICE (Darwin, Austra-
lia) rain DSDs (Selvam 2012 ).

3.2 Atmospheric Suspended Particulates: Current

State of Knowledge

3.2.1 Aerosol Size Distribution

As aerosol size is one of the most important parameters in describing aerosol prop-
erties and their interaction with the atmosphere, its determination and use is of fun-
damental importance. Aerosol size covers several decades in diameter and hence a
variety of instruments are required for its determination. This necessitates several
definitions of the diameter, the most common being the geometric diameter d . The
size fraction with d > 1-2 μm is usually referred to as the coarse mode, and the
fraction d < 1-2 μm is the fine mode. The latter mode can be further divided into
the accumulation d ~ 0.1-1 μm, Aitken d ~ 0.01-0.1 μm, and nucleation d < 0.01 μm
modes. Due to the d 3 dependence of aerosol volume (and mass), the coarse mode
is typified by a maximum volume concentration and, similarly, the accumulation