The software package OPAC

    The data base on Optical Properties of Aerosols and Clouds (OPAC) and associated FORTRAN code has been developed by a German group from  Meteorolgisches Institut der Universitat Munchen and from Max-Plank-Institut fur Meteorologie (Hess et al., 1998).
    The properties of clouds and aerosol particles are highly variable, both in time and space, that is why it is impossible to model aerosols and clouds in detail. It is necessary to reduce the variability of naturally occuring aerosols and clouds to typical cases. This goal is achieved in OPAC by the use of a dataset of typical clouds and aerosol components listed in Tables 1 and 2. Table 1 lists the names and types of clouds (first column), and corresponding references (second column). The basic aerosol components and their composition are listed in one-column Table 2.

Table 1. Clouds used in the OPAC software package.

Clouds
References
Stratus (continental) (water clouds) Diem, 1948; Hofmann and Roth, 1989; Tampieri and Tomasi 1976.
Stratus (maritime) (water clouds) Tampieri and Tomasi 1976; Stephens et al., 1978.
Cumulus (continental, clean) (water clouds) Tampieri and Tomasi 1976; Squires 1958; Leatich et al., 1992.
Cumulus (continental, polluted) (water clouds) Tampieri and Tomasi 1976; Diem, 1948; Fitzgerald and Spyers-Duran, 1973. 
Cumulus (maritime) (water clouds) Tampieri and Tomasi 1976.
Fog Tampieri and Tomasi 1976.
Cirrus 1 (T=-25° C) (ice clouds) Heymsfield and Platt, 1984; Strauss et al., 1997; Hess and Wiegner, 1994.
Cirrus 2 (T=-50° C) (ice clouds) Heymsfield and Platt, 1984; Strauss et al., 1997; Hess and Wiegner, 1994.
Cirrus 3 (T=-50° C)+ small particles (ice clouds) Heymsfield and Platt, 1984; Strauss et al., 1997; Hess and Wiegner, 1994.

Table 2. Aerosol components used in the OPAC software as they were described in Deepak and Gerber, 1983; Shettle and Fenn, 1979; d'Almedia et al., 1991; Koepke et al., 1997.


Aerosol components (composition)
  • Insoluble (soil particles with a certain amount of organic material)
  • Soot (absorbing black carbon)
  • Water-soluble (sulfates, nitrates&other water-soluble substances)
  • Sea salt -acc. mode(*) (various kinds of salt contained in seawater)
  • Sea salt -coa. mode(*) (various kinds of salt contained in seawater)
  • Mineral -nuc. mode(**) (a mixture of quartz and clay minerals) 
  • Mineral -coa. mode(**) (a mixture of quartz and clay minerals)
  • Mineral -acc. mode(**) (a mixture of quartz and clay minerals)
  • Mineral-transported (desert dust transported over long distances with a reduced amount of large particles)
  • Sulfate droplets (75% solution of H2SO4)

 (*) Two sea-salt modes are given to allow for a different wind-speed-dependent increase of particle number for particles of different size (Koepke et a., 1997).
(**) Three mineral modes are given to allow one to consider the increase of relative amount of large particles for increase of turbidity (Hess et al., 1998).

    OPAC consists of two parts:
 

Dataset of the microphysical and optical properties of aerosols and clouds

    The radiative properties in OPAC are modeled on the basis of components of aerosols and clouds. Each component is described by an individual particle size distribution and spectral reflectance index. Its radiative properties are modeled with Mie theory (see e.g. Van de Hulst, 1981) in the case of water clouds and aerosols, and with geometric optics in the case of ice crystals, which are assumed to be hexagonal comumns for calculations in the solar spectrum range (Hess and Weigner, 1994). In the terrestrial spectral range, ice crystals are considered to be spheres because the geometric optics assumption is only valid for particles, which are considerably larger than the wavelength of the incident radiation (Hess et al., 1998). Radiative properties of a real atmospheric aerosol is modeled as weighted sums of the radiative properties of its components listed in Table 2 (see paragraphe "Modeled optical properties of aerosols and clouds"). Microphysical properties of aerosols and clouds are described below.

Microphysical properties of aerosol components

    Aerosol particles result from different sources and processes. To describe the wide range of possible composition the aerosol particles are modeled as components (Deepak and Gerber, 1983), each of them meant to be an internal mixture of all chemical substances that have a similar origin. These components, listed in Tables 2and 3, that were referred as basic aerosol constituents  on the LITMS aersol data page, may be externally mixed (it means that there is no physical or chemical interaction between particles of different components) to form aerosol types.

Microphysical properties of aerosol components mean the following characteristics (as it was referred in Hess et al., 1998):

    In the OPAC software, lognormal distributions (see, e.g., Deepak and Gerber, 1983) are applied for each aerosol component:
                                        (1)
where rmodN is the mode radius,  is the width of the distribution, and N is the total particle number density of the component in particles per cubic centimeter. The mode radius of volume distribution  is calculated from rmodN using the following expression:
                                                                                (2)
The detailed description of the aerosol components can be found in a paper by Hess et al., 1998. The default values of the above mentioned parameters used in Eq. 1 are listed in Table 3.

Microphysical properties of clouds

   Five water-cloud models, one fog model, and three ice-cloud models have been considered in OPAC.
The modified Gamma function is used as the size distribution function of water clouds and fog (Deirmendjian, 1969):

                                                                                (3)
with
where N is the total number density in particles per cubic centimeter and rmod is the mode radius in micrometers. The constants   and  describe the slope of the size distribution, while a is a normalization constant ensuring that the integral of the size distribution over all radii yields N. The parameters of the normalized size distributions (Eq. 3), selected from a paper by Tampieri and Tomasi, 1976, together with the effective radius reff  (see Eq. 4), the liquid water content L, and particle number density N are listed in Table 4.
                                                                                    (4)
The size distribution functions used for cirrus clouds (ice clouds) are analytical functions after Heymsfield and Platt, 1984, calculated with help of the parametrization after Liou (1992), which relates the parameters a, b and I of the two-mode size distribution function (Eq. 5) to the temperature in cloud:
for x < x0
(5)
for x > x0
      Here x is the maximum dimension of the ice crystals (the length of the columnar crystals used for calculating the optical properties), N is the number density, I is the ice water content, a, b, f are the parameters of the distribution. The additional factor f  provides the normalization of the size distribution (Eq. 5). The size distributions for cirrus clouds are are mixtures of eight columnar ice crystals of different size and correspondingly different aspect ratio that are randomly orietated in space (Height and Wiegner, 1994).
    Properties of three cirrus clouds (for three different temperatures) are provided in Table 5.

Modeled optical properties of aerosols and clouds

    Optical properties of aerosols, water clouds, and fog are calculated with Mie theory (Quenzel and Muller, 1978). Optical properties of ice clouds are calculated with geometric optics (Hess and Weigner, 1994). In the OPAC software package, the optical properties values are stored in separate files (listed in second column of Tables 3, 4, 5) for each component and relative humidity. The following optical properties have been archived (see, e.g. Van de Hulst, 1981 for explicit formulas and definitions):

    In the OPAC software, the extinction coefficient, the scattering coefficient, the absorption coefficient, and the volume phase function are normalized to a number density of 1 particle cm-3. To get the absolute values of these parameters, the stored date have to be multiplied by the total particle number density, e.g. the absolute value of the extinction coefficient of an aerosol sample with total particle number concentration N particles per 1 cm-3 can be expressed in terms of the archived data  like this: =N. The other optical parameters are calculated according to the same principle.
    The aerosol optical depth in case of the non-homogenious atmosphere is calculated from the extinction coefficient of the chosen aerosol type (see Table 6) in combination with the height profile N(h) of the particle number density in particles per cubic centimeter at the height h (km), provided in OPAC (see section 5 in a paper by Hess et al., 1998), or given by the user for four discrete layers. The height profile N(h) is approximated by the formulae:
                                                                                            (6)
where N(0) is the particle number density in particles per cubic centimeter at sea level, h is the altitude above ground in kilometers, and Z is the scale height in kilometers.
    The aerosol optical depth of an aerosol can be computed in terms of the data archived in the OPAC software package using the following expression:
                                                                                (7)
where m is the number of aerosol layers (four maximum in the OPAC software package), is the extinction coefficient of the aerosol in layer j, normalized to 1 particle cm-3, Hj, min, Hj, max are the lower and upper limits of j-th aerosol layer, Zj is scale height of the jth layer, Nj(0) is sea level concentration of jth component.
    The optical depth of clouds is calculated assuming one homogeneous layer (m=1) with cloud droplet density independent of height (Z=infinity). Thus, the (Eq. 7) for clouds is reduced to:
                                                                                                (8)
where  is the geometrical thickness of the cloud.

ONLINE ACCESS TO THE OPAC SOFTWARE PACKAGE DATA

Table 3. Microphysical and optical(*) properties of aerosol components in dry state (from Hess et. al., 1998)


Component
Online access  data files(**)
rmodN
micrometers
rmodV 
micrometers
rmin
micrometers
rmax
micrometers

g cm-3
M*
(milligram m-3)/(part. cm-3)
Insoluble
INSO
2.51
0.471
6.00
0.005
20.0
2.0
23.7
Water-soluble
WASO(+)
2.24
0.0212
0.15
0.005
20.0
1.8
1.34 10-3
Soot
SOOT
2.00
0.0118
0.05
0.005
20.0
1.0
5.99 10-5
Sea salt (acc.mode)
SSAM(+)
2.03
0.209
0.94
0.005
20.0
2.2
0.802
Sea salt (coa. mode)
SSCM(+)
2.03
1.75
7.90
0.005
60.0
2.2
224
Mineral (nuc. mode)
MINM
1.95
0.07
0.27
0.005
20.0
2.6
0.0278
Mineral (acc. mode)
MIAM
2.00
0.39
1.60
0.005
20.0
2.6
5.53
Mineral (coa. mode)
MICM
2.15
1.90
11.00
0.005
60.0
2.6
324
Mineral-transported
MITR
2.20
0.50
3.00
0.02
5.0
2.5
15.9
Sulfate droplets
SUSO(+) 
2.03
0.0695
0.31
0.005
20.0
1.7
0.0228

(*) Optical properties of aerosol components as well as their complex refractive indices are stored in the data files listed in column   2 of the table.
(**) Click with the mouse left button on a file of interest to view its content. To download a file, click it with the mouse right button and select the "Save as" item of the pop-un menu.
(+) For aerosol components which are able to take up water, the data for eight values of relative humidity are available.

Table 4. Microphysical and optical(*) properties of the water-cloud and fog models (from Hess et al., 1998)


Component
Online access  data files(**)
rmod
(micrometers)
a
B
reff
(micrometers)
N
(cm-3)
L
(g m-3)
Stratus (continental)
STCO
4.7
5
1.05
9.792 10-3
0.938
7.33
250
0.28
Stratus (maritime)
STMA
6.75
3
1.30
3.818 10-3
0.193
11.30
80
0.30
Cumulus (cont., clean)
CUCC
4.8
5
2.16
1.105 10-3
0.0782
5.77
400
0.26
Cumulus (cont., polluted)
CUCP
3.53
8
2.15
8.118 10-4
0.247
4.00
1300
0.30
Cumulus (maritime)
CUMA
10.4
4
2.34
5.674 10-5
0.00713
12.68
65
0.44
Fog
FOGR
8.06
4
1.77
3.041 10-4
0.0562
10.70
15
0.058

(*) Optical properties of water clouds components and of the fog model are stored in the data files listed in column 2 of the table.
(**) Click with the mouse left button on a file of interest to view its content. To download a file, click it with the mouse right button and select the "Save as" item of the pop-un menu.

Table 5. Microphysical and optical(*) properties of ice cloud model (from Hess et al., 1998)

Component
Online access  data files(**)
a1
b1
a2
b2
x0
f
reff
(micrometers)
N
(cm-3)
I
(g m-3)
Cirrus 1: -25° C
CIR1
4.486 108
-2.417
1.545 x 1014
-4.376
670
0.909
91.7
0.107
0.0260
Cirrus 2: -50° C
CIR2
5.352 x 1010
-3.545
---
---
---
3.48
57.4
0.0225
0.00193
Cirrus 3:
-50° C
+ small particles(***)
CIR3
5.352 x 1010
-3.545
---
---
---
3.48
34.3
0.578
0.00208

(*) Optical properties of ice clouds components are stored in the data files listed in column 2 of the table.
(**) Click with the mouse left button on a file of interest to view its content. To download a file, click it with the mouse right button and select the "Save as" item of the pop-un menu.
(***) Cirrus 3 is the same distribution as cirrus 2 between 20 and 2000 micrometers. Additionally, there are 0.169 particles m-3 between 2 and 6 micrometers and 0.387 particles m-3 between 6 and 20 micrometers.

References

GEISA AEROSOLS PAGE


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