Radiative impact of mineral dust on
surface energy balance and PAR,
implication for land-vegetationatmosphere interactions
Xin Xi
Advisor: Irina N. Sokolik
School of Earth and Atmospheric Sciences
College of Science
Georgia Institute of Technology
6th Graduate Student Symposium
Nov.14, 2008
1
Motivation
1. Climatic link of vegetation: global carbon cycle (photosynthesis and respiration),
global energy balance (surface reflection), hydrological cycle (evapotranspiration).
2. Aerosol affects vegetation growth through direct (light scattering and
absorption) and indirect (cloud and precipitation) effects. (aerosol deposition
also disturbs plant functioning)
3. Aerosol diffuse effect: Aerosol reduces total photosynthetically active radiation
(PAR, 0.4µm ~ 0.7µm), but increase the diffuse component, which uniformly
distributes among the leaves, thus increasing the total photosynthetic rate. (Cohan
etal 2002; Gu etal 2003; Yamasoe etal 2006)
e.g. Mount Pinatubo eruption in 1991  increase of noontime photosynthesis of
Harvard forest by 23% in 1992
a). Past studies didn’t consider the aerosol-induced change in both
surface net radiation and PAR.
b). No study in the dust aerosol.
This study is a starting point to investigate the dust aerosol effect in both
surface net radiation and PAR, and how this effect potentially relates to
vegetation functioning.
2
Approach
1. Optical modeling
Mie-theory: complex refractive indices of each species and particle size
distribution (lognormal)
dust composition: calcite, quartz and two clay-iron oxide aggregates
(illite-geothite and illite-hematite) (Lafon, et al 2006, JGR)
2. Dust surface forcing
1-D radiative transfer model: SBDART (Ricchiazzi et al 1998, BAMS)
Net radiative flux:
Fnet  F down  F up
Surface net radiation (0.2µm~100µm):
Dust surface radiative forcing:
aerosol
net
F

Rn  Ssfc
1     L T    Tsfc4
clear  sky
net
F
3
Factors to be considered:
Size distribution
Dust loading
Vertical profile
Surface cover
Reid et al 2008
high
mixed
dryland
Lafon et al 2006
moderate
multilayer
rangeland
Clarke et al 2004
low
lifted
grassland
-
5
Lafon
etal 2006
-
Clarke
etal 20043
0.35µm::1.46
55.6%
4
2
AOD
0.5µ
Reid
0.45µm::1.93
90.9%
0.84µm::1.78
9.1%
0.4µm::2.0
91.1%
1.05µm::2.15
8.9%
0.89µm::1.85
44%
4.3µm:1.5
0.4%
Lafon
m
0.5
0.03
0.4
0.3
0.84
2.06
2.0
1.92
Moderate
1.34
1.3
1.25
Mixed
0.41
Multilayer
0.4
0.28
0
Lifted
0.83
0.82
0.8
0.81
0.1
1High
Low
0.88
0.6
0.028
0.2
Clarke
dryland
rangeland
0.84
grassland
0.9
Reid
Lafon
Clarke
0.7
asymmetry parameter
Reid etal
2008
0.8
0.032
single scattering albedo
(km)
Coarse mode
normalized extinction coefficient
reflectance
Fine mode
0.5
0.026
0.3 .5
1
wavelength (micron)
1
1.5
0.76wavelength (um)
0.3 0.5
2
2.5
1
0.8
.3
3
.5
1
4
Dust surface forcing in SW+LW, SW, LW and PAR, and downward
diffuse PAR: comparison of dust loading
solar zenith: 20 degree
surface: grassland
dust size: Lafon etal 2006
vertical profile: mixed
diffuse PAR (38.1)
PAR (486.06)
LW (-76.74)
high
SW (716.29)
moderate
low
SW+LW (637.95)
-300
-200
-100
0
100
200
300
1. Negative forcing in shortwave (SW) and positive forcing in longwave (LW).
2. Net PAR is reduced, but the diffuse component dramatically increases e.g.,
by 139 Wm-2 at low dust loading case (AOD0.5µm=0.4).
5
Dust surface forcing in SW+LW, SW, LW and PAR, and downward
diffuse PAR: comparison of dust size distribution
surface: grassland
dust loading: moderate
(AOD0.5µm=1.34)
vertical profile: mixed
diffuse PAR (38.1)
PAR (486.06)
1 Wm-2 difference
LW (-76.74)
Reid etal 2008
SW (716.29)
Lafon etal 2006
Clarke etal 2004
SW+LW (637.95)
-150
-100
-50
0
50
100
150
200
250
300
1. “Reid” contains largest fraction of coarse particles, which are more
efficient in absorption and extinction (SW and PAR) than fine particles.
2. Coarse particles also cause larger LW forcing than fine particles (e.g.,
“Reid” is about 1 Wm-2 larger than “Clarke”), due to stronger absorption
and scattering.
6
Dust surface forcing in SW+LW, SW, LW and PAR, and downward
diffuse PAR: comparison of dust vertical profile
surface: grassland
size: Lafon et al 2006
dust loading: high
(AOD0.5µm=2.0)
diffuse PAR (38.1)
PAR (486.06)
LW (-76.74)
SW (716.29)
lifted
multilayer
mixed
SW+LW (637.95)
-220
-120
-20
80
180
280
1. Compared with “mixed” case, “lifted” dust layer causes less LW forcing (by
6 Wm-2), and as a result, a larger forcing in SW+LW.
- dust forcing varies during transport, not only due to composition change.
2. “lifted” case induces more diffuse PAR (by about 2 Wm-2 at high loading
case).
7
Dust surface forcing in SW+LW, SW, LW and PAR, and downward
diffuse PAR: comparison of surface albedo
size: Lafon et al 2006
dust loading: moderate
vertical profile: mixed
diffuse PAR (38.1)
PAR (486.06)
different surface emissivities
LW (-76.74)
grassland
rangeland
SW (716.29)
dryland
SW+LW (637.95)
-160
-110
-60
-10
40
90
140
190
1. The spectral dependence of
surface reflectance causes different
forcing in, e.g., SW vs. PAR.
800
2. Surface structure (e.g., canopy
shape) significantly alters the
radiation direction field, which is not
resolved in 1D model.
400
240
grassland
rangeland
700
600
dryland
500
300
200
100
0
SW+LW
SW
PAR
diffuse PAR
8
Implication for land-vegetation-atmosphere interactions
aerosol effects on vegetation
vegetation feedback
9
Conclusion and discussion
1.
Composition, size, vertical profile and surface properties all
affect dust surface forcing, which need to be constrained by
measurements in real case studies.
2.
Dust forcing differs in SW (-) from LW (+). This is important for
estimating diurnal dust radiative forcing.
3.
Even at low loading, dust substantially increases diffuse PAR.
Coarse particles cause more scattering and diffuse light. This
may significantly modify vegetation behaviors.
4.
Need to consider particle shape for more realistic scattering phase
function (e.g., T-matrix, DDA).
Need to consider surface 3D structure (Bi-directional reflectance
distribution function or BRDF) in the radiative transfer scheme for
plant canopies, and couple it to the ecological models. (Kobayashi &
Iwabushi, 2008, Matsui etal, 2008)
5.
10