Mass conversion theory

The conversion of a component aerosol mass m into a component aerosol optical depth rby a model is expressed by the mass extinction efficiency b, defined as where re is the effective radius, p is the aerosol density and Q is the extinction efficiency.

Given a component (dry) mass m (no aerosol water), these three aerosol properties, and Q (in addition to ambient relative humidity and to permitted aerosol humidification) are critical for the derivation of component optical depths.

Figure 8a-d. Regional comparisons of modeled monthly averaged optical depth contributions from dust, sulfate, carbon and sea-salt for five models (ECHAM4, MIRAGE, GOCART, CCSR and GISS). Component comparisons are presented for regions over oceans: (1) for the north-western Atlantic (30-55N deg latitude and 285-330E deg longitude), (2) for the eastern

Atlantic (15-30N, 300-350E), (3) for the south-eastern Atlantic (5-15N, 320-10E) and for the western Pacific (1 OS-ION, 100-150E). AVHRR-giss retrievals of (total) aerosol optical depths based on NOAA9 data are given for reference in the first (black) column

Figure 8a-d. Regional comparisons of modeled monthly averaged optical depth contributions from dust, sulfate, carbon and sea-salt for five models (ECHAM4, MIRAGE, GOCART, CCSR and GISS). Component comparisons are presented for regions over oceans: (1) for the north-western Atlantic (30-55N deg latitude and 285-330E deg longitude), (2) for the eastern

Atlantic (15-30N, 300-350E), (3) for the south-eastern Atlantic (5-15N, 320-10E) and for the western Pacific (1 OS-ION, 100-150E). AVHRR-giss retrievals of (total) aerosol optical depths based on NOAA9 data are given for reference in the first (black) column re: The effective radius re is the representing size for a distribution of sizes (re is the ratio between third moment (r*r*r) and second moment (r*r) integrals of a size-distribution). Suggestions for re based on inversion methods of AERONET sun/sky-photometer data are of limited use, because they relate to aerosol mixtures, rather than pure components. (Nonetheless, AERONET inversions can provide good estimates at sites and seasons, when a component clearly dominates). The component size assumptions of the tested models are summarized in Table 9. For assumed mono-modal size-distributions the associated effective radius is presented, for model inter-comparison. Comparisons of size-assumptions, however, are not possible for size-classes (many without knowing the associated weights.

Table 9. Comparison of size-assumptions for aerosol components in models. Either assumed aerosol size classes or effective radius re of assumed size-dis tri but ions are listed for dry aerosol [reat 100% relative humidity")_

Aero sol-type

ECHAM 4

MIRAGE

GOCART

CCSR

G1SS

Dust

2 size-classes

2 size-classes

4 size-classes

/ 0 size-classes

S size-classes

,21pm

,13pm

.50pm

,13pm, ,20pm.

,15pm, ,25pm,

1.3pm

1.2 jim

1.4pm

,33pm, ,52pm,

,40pm, ,80pm,

2.4 pm

.82pm, 1.3pm,

1.5pm, 2.5pm,

4.5pm

2.0pm, 3.2pm,

4.0pm, 8.0pm

5.1pm, 8.0pm,

Org. Carbon

.llpm[.27pm]

.13pm[.30pm]

llpm[.27pm]

.29pm [.55pm]

6 size-classes

Black Carbon

.04 pm

,023pm

,04pm

(in org.carb)

None

Sea Salt

2 size-classes

2 size-classes

4 size-classes

! 0 size-classes

None

.73pm[3.5pm]

,13pm[.34pm]

.26pm,1.2pm,

0.1 - 10pm

5.9pm[l7.pm]

3.3pm[IO.pm]

2.4pm,7,6pm

[ca 3times larger]

Sulfate

,24pm[.81pm]

.13pm[.34pm]

.24pm[.8lpm]

,49pm[1.61pm]

.iOpm

ECHAM4 and GOCART model aerosol sizes are based on the GADS data-set (Koepke et al., 1997) MIRAGE predicts size for each mode within size-bounds - this table lists nominal sizes of the modes

ECHAM4 and GOCART model aerosol sizes are based on the GADS data-set (Koepke et al., 1997) MIRAGE predicts size for each mode within size-bounds - this table lists nominal sizes of the modes

Aerosol sizes for carbon and sulfate aerosol in CCSR are much larger than for MIRAGE or ECHAM4. Sulfate aerosol sizes of MIRAGE are smaller than those of ECHAM4.

- For hydrophilic aerosol components (sulfate, sea-salt and organic carbon) increases in ambient relative humidity cause a non-linear increase in aerosol size. Size increases are larger at higher relative humidity. In Table 9, expected effective radii re at 100% relative are given in square brackets next to the assumed effective radii of the dry aerosol.

The densities of assumed dry aerosol components are compared in Table

Table 10. Comparison of assumed density p in models for components of dry aerosol (in

g/cm3)

Aerosol-type

EC HAM 4

MIRAGE

GOCART

CCSR

GISS

Dust

2.6

2.6

2,6

2.5

2.5

Org. Carbon

1.9

1.7

1.9

1.55

None

Black Carbon

1.0

1.0

1.0

1.25

None

Sea Salt

2.2

1.9

2.2

2,25

None

Sulfate

1.7

1.77

1,7

1.77

None

Densities of explicitly size-resolving aerosol components usually agree among models. Only different CCSR carbon densities and a smaller MIRAGE sea-salt density stand out.

- For hydrophilic aerosol components (sulfate, sea-salt and organic carbon) water uptake in aerosol decreases the aerosol density. Ultimately at high ambient relative humidities aerosol densities will be close to that of water (of 1g/cm3). Assuming aerosol sizes of the GADS data-set (Koepke et al., 1997), they are used in ECHAM4 and MIRAGE, densities at 80% relative humidity reduce to 1.15g/cm3 for sulfate, 1.50g/cm3 for organic carbon and 1.18g/cm3 for sea-salt.

Q: The extinction efficiency Q is the ratio between extinction cross-section and the geometric cross-section. Q depends on aerosol size and composition (e.g. Lacis and Mishchenko, 1994, Tegen and Lacis, 1996). Q is largest, if particle radius and interacting wavelength have similar values. Maximum values for Q of near 3 are common for size-distributions with effective radii of about 0.5|im at mid-visible wavelengths. Q converges towards 2 for increasingly larger radius-to-wavelength ratios. For increasingly smaller radius-to-wavelength ratios, Q decreases sharply (inverse proportional to 4th power) for scattering aerosol but only moderately (inverse proportional) for absorbing aerosol.

- For hydrophilic aerosol components (sulfate, sea-salt and organic carbon) water uptake impacts Q in two ways: Water uptake increases the aerosol size, thereby increasing the radius-to-wavelength ratio. And water uptake decreases the aerosol absorption, which is less important if the effective aerosol radius is larger than the wavelength.

- b : Mass extinction efficiency b (as illustrated by the formula above) is proportional to Q and inverse proportional to and Moreover, all three properties Q, p and re are function of relative humidity. Thus, for the evaluation of b assumptions on aerosol component humidification and data for ambient relative humidities are important as well.

Observed and calculated values for b are presented in Table 11. Calculations are based on assumptions for size, humidification and density by the GADS data-set and by the CCSR model (see Tables 9 and 10).

Table II. Comparison of observed and calculated mass extinction efficiencies b (m2/g) at visible wavelengths_

Aerosol-type relative humidity

Observations 'ambient' conditions

Catc., GADS sizes ** 0%/ 80%/ 100%

Calc., CCSR sizes ** 0%/80%/ 100%

Dust< lum

1.-1.7 (Malm 94)

0.6-2.4 *

Dust> lum

0.25 (Carlson 77)

0,1-0.4 *

Org. Carbon

4 (Liousse 96)

2/13/82

5/11 /25

Black Carbon

9 (Liousse 96)

10*

5.7*

Sea Salt < lum

0.4 (Andreae 95)

1.3/8.6/18

Sea Salt > lum

.12 / .69/2.2

Sulfate

5 (Liousse 96)

3.6/21/90

2.4/11/40

* hydrophobic aerosol type,

** sizes are given in Table 9 [Global Aerosol Data Set (Koepke et al., 1997)]

* hydrophobic aerosol type,

** sizes are given in Table 9 [Global Aerosol Data Set (Koepke et al., 1997)]

Calculated values for b are not necessarily correct. However, the range and differences for b demonstrate the importance of assumptions for dry aerosol size selection and for ambient relative humidity in the determination of the component aerosol optical depth (for sulfate, see also Kiehl et al., 1999). Thus, a model with accurate data for aerosol mass is going to compromise its ability to predict aerosol optical depth (and climatic impacts) with a poor selection for b. The assumption of a constant for b, as for a few components in some models ('None' in Tables 9 or 10), will certainly introduce errors. For hydrophilic aerosol, in addition, the strong sensitivity to size at higher ambient relative humidities creates a big problem: Already small deviations in ambient relative humidity create large differences in component aerosol size and optical depth. Thus, rather than predicting ambient relative humidity (e.g. ECHAM4), less variable data from assimilations (e.g. GOCART) are often adopted or simply prescribed (e.g. MIRAGE).

The effective b can be deduced from the ratio between component optical depth and component mass. Thus, modeled component mass are introduced next.

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