JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. D24, PAGES 31,865-31,878, DECEMBER 27, 1998 
Measurements of irradiance attenuation and estimation of 
aerosol single scattering albedo for biomass burning 
al., 
al. 
aerosols in Amazonia 
Thomas F. Eck,' Brent N. Holben? Ilya Slutsker; and Albert0 SetzelA 
Abstract. Investigation of the effects of biomass burning aerosols on the surface irradiance were 
conducted as a part of the Smoke, Clouds, and Radiation - Brazil (SCAR-B) experiment during 
August - September 1995. Measurements of broadband and spectral irradiance, in conjuction with 
measurements of aerosol physical and optical properties (optical depth, phase function, and size 
distribution) were made under varying conditions of aerosol loading during the SCAR-B field 
campaign. Estimates of aerosol single scattering albedo (ao) were made from matching of the 
measured irradiance values to the model computed irradiances by varying w,,, for observations made 
under cloudless conditions. Values of a<,, at approximately 550 nm, estimated from this technique 
using broadband 400-700 nm irradiance measurements, ranged from approximately 0.82 to 0.94 for 
the dates and times of these SCAR-B measurements. Utilizing spectral irradiance data, the model 
retrieved values of o,, decreased with increasing wavelength, with the change of a, as a function of 
wavelength differing on different days. Reductions in photosynthetically active radiation (PAR; 
400-700 nm) incident at the surface were computed to range from about 20 to 45% compared to 
background aerosol conditions for the 2 month biomass burning season at several locations in the 
southern Amazon Basin. These large reductions in incident PAR at the surface due to the heavy 
aerosol loadings could have implications for primary production of sensitive ecosystems. In 
addition, reductions of total incident solar radiation from aerosol direct radiative effects may have 
significant impact on reducing surface heating and increasing aerosol layer heating from 
absorption. 
burning season, as a result of fires in both forest and cerrado 
regions, and these particles may be transported large distances 
a1.,1988; Hsu 19961. Holben et al. [1996] [Andreae et 
and Menzel Penner et al. Hobbs et al. 
a global basis. Both studies show negative forcing, although 
the magnitude of the computed direct forcing is much less for 
Hobbs et al. [I9971 due to differences in the assumed and/or 
measured values of the aerosol optical properties. Hobbs et al. 
(1997) also note that for biomass burning regions of Brazil, 
Sao Jose dos Campos, Sao Paulo, 
to the high optical depths over large regions. The solar 
radiation reaching Earth's surface is reduced and the energy 
balance in the surface-atmosphere system is modified due to 
scattering and absorption in the aerosol layer by carbonaceous 
aerosols. Robock [ 1991 ] analyzed the effects of large smoke 
plumes from forest fires in midlatitudes and showed that surface 
temperatures were reduced from 1.5"C to 7C during the daytime 
but with no nighttime effects. Christopher et [1996] 
utilized satellite measurements of broadband shortwave and 
longwave radiances at the top of the atmosphere to study the 
radiative forcing of smoke aerosols over Amazonia. They 
found that the net radiative forcing for smoke in cloudless 
conditions was negative, resulting in a cooling effect. 
1. Introduction 
Continuing agricultural expansion in Amazonia has led to et 
the clearing of 6% of the closed canopy forest in the Brazilian have measured the aerosol optical thickness (7,) from this 
Amazon as of 1988 [Skole and Tucker, 19931. Biomass biomass burning smoke to average approximately 1.0 to 1.5 
burning within tropical forest and cerrado (savanna) at 440 nm for monthly means in biomass burning areas of 
ecosystems in Brazil, associated with agricultural practices, Amazonia, versus background T , ~ values of about 0.2 at 440 nm. 
results in large carbonaceous aerosol loadings, especially The scattering and absorption of solar radiation by biomass 
within the primary biomass burning season of August and burning aerosols result in direct radiative forcing, which 
19961. Prins September [1994] [1992] and al., [1997] have computed on [Holben et 
utilized GOES satellite data to detect active fires in a large 
region of southern Amazonia and found that for the peak 
burning season, the amount of burning detected nearly doubled 
in the forested regions and tripled in the cerrado regions from 
1983 to 1991. Artaxo et al. [1994] have measured very large 
increases in fine particulates (diameter < 2.0 pm) during the the direct radiative forcing by these aerosols is substantial, due 
'Raytheon STX Corporation, NASA Goddard Space Flight Center, 
Greenbelt, Maryland 
'Biospheric Sciences Branch, NASA Goddard Space Flight Center, 
Greenbelt, Maryland 
'Science Systems and Applications Inc., NASA Goddard Space Flight 
Center, Greenbelt, Maryland 
41nstituto de Pesquisas Espaciais, 
Brazil 
Copyright 1998 by the American Geophysical Union, 
Paper number 98JD00399 
0148-0227/98/98JD-00399$09,00 
31,865 31,866 ECK ET AL.: BIOMASS BURNING AEROSOLS AND SURFACE IRRADIANCE 
Similarly, Anderson et a/. [ 19963 also computed a net cooling 
effect for biomass burning aerosols over cerrado regions of 
Brazil, based on aircraft measurements of aerosol properties 
and modeling. 
Heavy loadings of biomass burning smoke can lead to 
significant reductions in the solar radiation incident at the 
surface. For biomass burning regions of Africa, Konzelmann et 
al. [1996] suggest that monthly mean differences in satellite 
estimated versus surface measurements of total incident solar 
radiation of 25-40% may be due to the radiative effects of 
biomass burning aerosols. Lenoble [ 19911 evaluated the 
biomass burning aerosol parameters which influence solar 
radiation attenuation at the surface and found that the 
attenuation depends mainly on the aerosol optical thickness, 
2,) , and the single scattering albedo. The next parameters in 
order of importance were the asymmetry factor and vertical 
profile of the extinction coefficients. Of the two principal 
parameters, z, can be accurately measured from the surface by 
sunphotometer with uncertainties of -0.01 [Shaw, 19831, 
while the aerosol single scattering albedo is difficult to 
measure accurately. 
In order to investigate the effects of biomass burning 
aerosols on the incident solar radiation at the surface, we 
collocated narrowband and broadband solar irradiance 
measuring instrumentation with automatic tracking Sun-sky 
scanning spectral radiometers, during a portion of the Smoke, 
Clouds, and Radiation-Brazil (SCAR-B) field campaign from 
August 27 to September 11. 1995 (peak burning season 
period), at selected sites in biomass burning regions of Mato 
Grosso and Rondonia, Brazil. From analysis of data collected 
during cloudless time periods, these measurements are utilized 
to infer the column integrated value of the aerosol single 
scattering albedo and to study the attenuation of the 
photosynthetically active radiation (PAR; 400-700 nm) at the 
surface due to these aerosols. 
2. Instrumentation and Data 
The measurements which are presented in this study were 
made in several locations in Mato Grosso and Rondonia, 
Brazil. Three sites had instrumentation to measure narrowband 
and/or broadband irradiance, and all sites had automatic Sun irradiance and, similarly, an estimate at 615 nm was made with 
and sky scanning radiometers. Narrowband and broadband 610 and 620 nm measurements. Optronics measurements of 
irradiance measurements were made in Cuiaba (1 5"33'S, irradiance across the 10 nm range were relatively constant, 
56"4'W) in a cerrado region, and broadband only measurements differing by only about 0.5% at 610 to 620 nm and by about 
in a (9"16'S, 62"52'W), were made at Potosi Mine, Rondonia 3.0% between 410 and 420 nm. In addition, the bandpass of 
region of seasonal tropical forest, and at Fazenda Ipiranga in the Optronics is narrower at 2 nm, compared to 10 nm for the 
the northern Pantanal (a seasonal floodplain wetland), Mato MFRSR. For all but the 670 nm channel, the percent 
Grosso (16"23'S, 56"37'W). See Figure 1 for a map showing differences in the comparison between Optronics and the 
the location of instrumented sites reported in the paper. The MFRSR instrument falls within the expected accuracy of the 
automatic tracking Sun and sky scanning radiometers [Holben instruments, and also given the differences in spectral 
al., 19981 et made direct Sun measurements with a 0.8') full bandpasses of the instruments. The Optronics spectral 
field of view every 15 min at 339, 379, 437, 498, 669, 871, irradiances at 10 nm intervals were integrated over the 400 t o 
940, and 1021 nm, which were used to compute aerosol optical 
depth at each wavelength except 940 nm, which was used to 
retrieve total precipitable water in centimeters. The bandpass 
of the ultraviolet channels (339 and 379 nm) was 2.5 nm, two measurements of PAR ranged from -1.0% to +1.5% o n 
while the bandpass of all other channels was 10 nm. The sky August 28, 1995, in Cuiaba. 
radiance almucantar measurements at 437, 669, 871, and 1021 The two Eppley PSP pyranometers which we utilized were 
nm were used to retrieve aerosol phase function and size 
distribution, applying the methodology of Nakajiina et al. 
[1996]. Remer et a / . [in this issue] analyzed these almucantar 
inversions to develop lognormal volume size distribution 
smoke models which were applied in this study. The Sun-sky 
radiometers utilized in this study were either calibrated before 
and after SCAR-B at Mauna Loa Observatory (MLO) or 
intercalibrated with the M M calibrated instrument in Brazil 
during SCAR-B. The zero air mass voltages (instrument 
voltage for direct normal solar flux extrapolated to the top of 
the atmosphere [Shaw, 19831 of all channels except 379 nm 
changed by less than 1% between MLO calibrations in May 
and October 1995 for the reference instrument. The 379 nm 
data were not used in this analysis. 
In Cuiaba, spectral irradiance from 300 to 800 nm was 
measured at 10 nm intervals with 2 nm bandpass (Optronics OL 
754 Spectroradiometer) and total and diffuse irradiance 
measured in six spectral bands with 10 nm bandpass (Yankee 
Environmental Systems, Multi-Filter Rotating Shadowband 
Radiometer (MFRSR)), in addition to broadband total 
irradiance (0.28-2.8 pm; Eppley PSP pyranometer) and 
broadband PAR (0.4-0.7 pm; Skye Probetech SKE-510). The 
Optronics spectroradiometer took measurements every 6 min 
with a scan time of approximately 3 min to measure irradiance 
from 300 to 800 nm in 51 wavelength intervals. The Yankee 
MFRSR took measurements of spectral total and diffuse 
irradiance at 415, 500, 615, 670, 868, and 937 nm every 15 s 
and these were averaged into 1 min means. The broadband 
measurements of PAR and total irradiance were made at either 1 
min or 5 min intervals depending on location. In Rondonia 
and the Pantanal, only broadband total and PAR irradiance 
measurements were made. The Optronics OL754 
spectroradiometer was calibrated using a National Institute of 
Standards and Technology (NIST) traceable standard lamp, and 
the relative change in calibration from before SCAR-B t o 
during SCAR-B ranged from t0.5% to -1.5% from 390 nm t o 
790 nm. The spectral total irradiance as measured by the 
Optronics instrument was compared to the MFRSR instrument 
on August 30, 1995, in Cuiaba, and the percent 
difference was found to be 3.6% at 415 nm, -3.8% at 500 nm, 
1.7% at 615 nm, and -13.3% at 670 nm. However, the 
comparisons made at 415 and 615 nm are not direct 
comparisons, since the Optronics measured irradiance at 4 1 0 
and 420 nm, which were averaged to estimate 415 nin 
700 nm spectral range and compared to broadband 
measurements of 400-700 nm irradiance made by the Skye- 
Probetech instrument. The percent difference between these 
calibrated by Eppley Laboratories in 1994, and in order t o 
check their relative calibrations, we operated them side by side 31,867 ECK ET AL.: BIOMASS BURNING AEROSOLS AND SURFACE IRRADIANCE 
0' 
-5' 
-1 0' 
SPCTRAL2 
,. . . . . . . . . . . " .... . 
-1 5' 
Figure 1. Map of the study region showing the location of measurement sites and the capital of Brazil, 
Brasilia 
in Cuiaba for 5 hours on August 27, 1995. The irradiances 
measured by these two Eppley PSP pyranometers agreed with 
each other to within 1.3% or less during this time period. 
I , 
3. Cloudless Sky Irradiance Models 
give good agreement with a rigorous radiative transfer code. 
The aerosol optical parameters which are used as input to the 
SPCTRAL2 model are the aerosol optical depth at all measured 
wavelengths, the asymmetry factor, and the aerosol single 
scattering albedo. The values of total column water vapor and 
aerosol optical depths as measured by the Sun-sky scanning 
the irradiance measurements are made, the value of single 
scattering albedo is varied until agreement between measured 
and modeled irradiance is reached. The SPCTRALZ model 
We have utilized two spectral solar irradiance models in our 
In order to select cloudless conditions for analysis and analysis of the attenuation of solar irradiance under cloudless 
comparison to computed irradiance from cloudless sky models, conditions at the surface. The first model, called 
we utilized three cloud screening checks. First, we checked the [Bird and Riordun, 19861, is a simple parameterized model of 
temporal variability of the irradiance from pyranometers (1 the cloudless atmosphere which Bird and Riordan adjusted to 
min or 5 min intervals dependent on location), and time series 
of data with high-frequency temporal variation or decreasing 
(increasing) irradiance with decreasing (increasing) solar 
zenith angle were identified as influenced by the presence of 
cloud. Temporal variation of irradiance exceeding 2% per 
minute was identified as cloudy. In addition, the ratio of PAR 
to total irradiance is influenced by the presence of both radiometer are used as input. The asymmetry factor was 
aerosols and clouds. The PAR/total irradiance ratio is higher determined from phase function retrievals of the Sun-sky 
for cloudy conditions due to the absorption by water vapor in scanning radiometer. Therefore the only aerosol parameter 
the near infrared, and the ratio is lower for small particle used in the SPCTRAL2 model which is not inferred from 
aerosols due to higher aerosol optical depths at shorter measurements is the single scattering albedo. Using the 
wavelengths [Pinker and Laszlo, 19921. Therefore time series measured aerosol properties at the same time and location as 
of the PARltotal irradiance ratio are sensitive to the presence 
of cloud and aerosols. Temporal variation of PAR/total ratio 
, exceeding 0.003 per minute was attributed to cloud cover. The 
~~ ..-I-.-..-- I-..... ---I -I-. . . . . . .... .- - __ - - - - ..-, ...-- 
analysis of the wavelength variation of the aerosol optical for both narrowband ( 2 nm) and broadband spectral regions 
depth as measured by a colocated automatic sunphotometer. (0.4-0.7 pm). 
The second model which we have used in this study is called For biomass burning aerosols in Brazil, the typical range of 
Angstrom waveiength exponent is 1.6 to 2.0, with lower the Second Simulation of the Satellite Signal in the Solar 
SpectIum (6s) [Vemzote et al., 1996, 19971. This model is a values resulting from cloud contamination, due to the large 
size of the cloud particles (water droplets or ice crystals). radiative transfer model which relies on the successive order of 
When all of these cloud screening tests were passed for at least 
a 1 hour interval, the data were identified as being cloudless. 
scattering method. The spectral resolution of the model is 2.5 
nm, and the aerosol layer is divided into 13 layers with a scale ECK ET AL.: BIOMASS BURNING AEROSOLS AND SURFACE IRRADIANCE 31,868 
height of 2 km. The aerosol inputs for this model are the reflectance values we used in the SPCTRAL2 model were 4.3% 
aerosol optical depth at 550 nm, the aerosol size distribution, for vegetation and 16.9% for soil, while for the 6s model the 
and the aerosol refractive index. We have utilized the smoke values at 630 nm were 7.5% for vegetation and 16.3% for soil. 
aerosol models of Remer et al. [this issue] for the aerosol 
lognormal volume size distributions. These smoke size 
distributions were based on analysis of almucantar 
measurements made in cerrado and forest regions in Brazil. The 
aerosol optical depth at 550 nm is obtained by wavelength 
interpolation of the measured values at 500 and 670 nm. The Figure 2 shows the aerosol optical thickness measured from 
real part of the refractive index was kept constant at 1.53 or direct Sun observations by the Sun-sky scanning radiometer 
1.40 (values representative of the probable range of the real from August 27 (day 238) through September 1, 1995 (day 
refractive index of biomass burning aerosols [Anderson et al., 243), in Cuiaba, which shows the large range in aerosol 
1996; Lenoble, 1991]), and the imaginary part of the loading which occurred during this time period. These large day 
refractive index is varied until the modeled irradiance is equal to day variations in T ' ~ at this site are due primarily to changes 
to the measured irradiance. Model 6 s computes the phase in air mass trajectories, with advection from burning regions 
function and extinction and scattering coefficients (and single on some days and advection from regions with few fires on 
scattering albedo) from Mie theory as an output based on the others [Remer et al., this issue]. The large wavelength 
input aerosol parameters. The Mie calculations in 6s assume dependence of the T,! for these biomass burning aerosols i s 
that the scattering of solar radiation is from a mixture of evident, especially for days with the highest aerosol loadings, 
homogeneous isotropic spheres, whose sizes are comparable August 29 and 30 (days 240 and 241). The Angstrom 
to or larger than the wavelength. These assumptions in the wavelength exponent (a) computed from the 440 and 870 nm 
Mie code of 6s are in accordance with those used in the Sun- channels for these days of highest aerosol loading ranged from 
sky scanning radiometer size distribution retrieval algorithm approximately 1.8 to 2.0, indicative of particles with small 
used with almucantar observations. Measurements of smoke radius. The Angstrom wavelength exponent was computed as: 
particle shape in Brazil during SCAR-B by Martins et al. 
[ 19961 suggest that smoke in regional hazes over Amazonia 
consists largely of spherical particles, and thus the use of the 
Mie calculations is valid. 
In both models, we have input a range of different surface 
spectral albedo values, which were not measured at the site, in 
order to analyze the model sensitivity to surface albedo 
magnitude. For the SPCTRAL2 model, we utilized the spectral 
reflectance data of Deering [1989] for a bare soil and a 
vegetated crop canopy and for the 6 s model there is a set of 
spectral reflectance data within the model which are 
representative of a vegetated surface and a sandy surface 
[Vermote et al., 19961. For example, at 630 nm, the 
3.0 
2.5 
2.0 
1.5 
1.0 
n.n _._ 
4. Irradiance Measurements and Retrieval of 
Aerosol Single Scattering Albedo 
( T ~ ~ , ~ ~ , = 
where h is the wavelength of the measured T~ values. 
In order to investigate directly the spectral attenuation of 
irradiance from these aerosols, we computed the ratio of 
spectral irradiance (measured by the Optronics instrument at 10 
nm intervals) from the day of highest T,,, August 30 
2.28), to the day of lowest T,!, August 28 (T,,,,, = 0.37) (see 
Figure 2), for a cloudless time period on each day. Figure 3a 
shows the values of these irradiance ratios from 300 to 800 nm 
for the solar zenith angle range of 37.6" to 44.5". It is seen 
244 243 242 241 
0.5 
238 240 239 
DAY OF YEAR 
Figure 2. The aerosol optical thickness (AOT) measured at Cuiaba, Brazil, from August 27 to September 1 , 
1995, by the direct Sun measurements from the Sun-sky scanning spectral radiometer. ECK ET AL.: BIOMASS BURNING AEROSOLS AND SURFACE IRRADIANCE 31,869 
0.9 
................................ 0.8 ............................................................................................... 
0.7 
0.6 
0.5 
+SZA 
0.4 
.............................................................................................. 
I 5 I ! 
= 41.0 
- . o - - S Z A = 37.6 
! , , , , I , , , , I , , , , 
500 800 700 600 400 
0.31 
300 
0.90 , , , , i 
.--o.-- SZA = 42.2 
b. 
- SZA = 44.6 
= 41.0 i -SZA 
- + . - S Z A = 39.8 .SZA = 38.7 ........ 
WAVELENGTH (nm) 
I , I I I ! , I l " " I " " 
.. 
.,; ................................................... 
1 
__a. .. SZA = 43.4 ........ -- 
I , , 
WAVELENGTH (nm) 
Figure 3. (a) Ratio of spectral irradiance (measured by the Optronics 754 spectroradiometer instrument at 10 
nm intervals from 300 to 800 nm) from the day of highest T,, August 30, 1995 (z,,,,,,, = 2.28) in Cuiaba (see 
Figure 2), to the day of lowest T,,, August 28, 1995 (T,,,,,,, = 0.37), for a cloudless time period on each for the 
solar zenith angle (SZA) range of 37.6" to 44.5" (b). The irradiance ratio of September 1 (T,,,,, = 1.29) t o 
August 28 computed, with higher ratios than for the August 30/August 28 irradiance ratios since the aerosol 
loading is less on September 1. 
I irradiance ratio of September (z,,,,, = 1.29) to August 28 was 
also computed and is shown in Figure 3b. As expected, the 
ratios are higher than for the August 30/August 28 irradiance 
ratios, since the aerosol loading is less on September 1 than 
on August 30, but the percentage reduction in irradiance per 
0.1 increment of 7,) is greater on September 1 than on August 
z,$ 30 (3.3% per 0.17,) on September 1 versus 2.2% per 0.17, at 
that the 550 nm irradiance is reduced by about 40% on the hazy 
day relative the clear day, and that for the PAR region (400- 
700 nm) the reduction ranges from about 47% at 400 nm to 
about 32% at 700 nm. For a portion of the UVB region, 300- 
320 nm, the irradiance reduction, comparing these 2 days, is 
about 63% at these solar zenith angles. These reductions would 
be significantly larger if compared to background values of 
0.15 [Holben al., 1996]), since the on August 500 nm on August 30). This can be accounted for by the smoke 
aerosol on September 1 being more strongly absorbing than 
f 0.50 
........................ 
T,l,,,, et (z,,,~~) = 
28, the clear day, was 0.37 during this time period. The ECK ET AL.: BIOMASS BURNING AEROSOLS AND SURFACE IRRADIANCE 31,870 
1200 
1000 
800 
u 
I , I 
0.5 
600 
400 1 
aerosol volume size distribution. 
AEROSOL OPTICAL DEPTH (550 nm) 
Figure 4. Irradiance at Earth's surface at 550 nm computed by the 6s model at 40" solar zenith angle. 
Computations are made for five values of single scattering albedo (SSA) from 1 .O to 0.80, with a smoke model 
the aerosol on August 30. This will be investigated further i n 
the retrieval of a,, for these two cases. 
The 6 s model was used to compute the changes in irradiance 
at 550 nm, for a solar zenith angle of 40", which result from 
both variations in aerosol optical depth and single scattering 
albedo (Figure 4). We utilized the cerrado smoke model 
lognormal size distribution of Remer et al. [this issue], with 
modal particle radius of 0.132 ym, as input to 6s and fixed the 
real part of the imaginary refractive index at 1.53, which 
resulted in an asymmetry factor of 0.63. From this analysis, 
we can see that the irradiance change, at Earth's surface at 550 
nm, resulting from a change in aerosol single scattering 
albedo from 1.0 to 0.80, is relatively small at z,of 0.2 (about 
5%), while at z, of 2.0 we see a decrease in irradiahce of 
approximately 46% from w,, of 1.0 to 0.80. Therefore for cases 
of moderate to high aerosol loading, measurements of solar 
irradiance are very sensitive to the magnitude of aerosol 
absorption and thus to the value of w(,. For example, at z, of 
0.80 at 550 nm, an error in irradiance of 1% will result in 
approximately a 0.01 error in the retrieval of a,,, retrieved 
using the technique described in the previous section. 
Therefore errors in w,, retrieval from this technique can be 
expected to range from 0.02 to 0.03 for aerosol optical 
thickness of 0.80 at 40" solar zenith, based on our 
approximate irradiance measurement accuracy of 2-3%. Errors 
in o, retrieval, due to irradiance measurement errors, at larger 
values of 2, will be smaller (e.g., w,, error of -0.005 per 1% 
irradiance measurement error at T~, = 1.6) and errors at lower 
values of z;, will be larger than this (e.g., a,, error of -0.02 per 
1 % error in irradiance at z;, = 0.4) for this solar zenith angle. 
We made a comparison of the spectal irradiance measured by 
the Optronics instrument to values computed by the two 
atmospheric models for a case with relatively low aerosol a,, based on matching the 
loading (with respect to burning season values), August 28, modeled irradiance to measured irradiance. The results of w,, 
1995, in Cuiaba. The observation time which we selected was 
1400 Gh4T on August 28 with solar zenith angle of 36.3", 
precipitable water of 1.87 em, and aerosol optical thickness of 
0.37 at 500 nm and 0.14 at 670 nm. The measured and modeled 
irradiances at this observation time are shown in Figure 5 . For 
the modeled irradiances, from both the 6 s and SPCTRALZ 
models, we assumed a constant value of single scattering 
albedo with wavelength of 0.94, which yielded good 
agreement with the measurements at most wavelengths. 
However, at the relatively low aerosol loadings on this day the 
irradiance sensitivity to the magnitude of a,, is small (Figure 
4). As can be seen from the irradiance plot, both models 
reproduce the major features of the measured irradiance from 
390 to 780 nm with differences at most wavelenghths of 4% or 
less. However, both models do show larger differences from 
the measurements at 440 and 520 nm. Both of these 
wavelengths are at regions of large gradients in the solar 
spectral irradiance. The discrepancy may be due to a calibration 
uncertainty in the Optronics instrument, a temperature 
dependent instrumental wavelength shift, or possibly the 
extraterrestrial solar flux data, since both models use the 
values of Neckel and Labs [1981]. We do not know which of 
these is the most likely cause of the discrepancy, however. It 
is also noted that there are large departures between the 
computed irradiances and measurements at 390 and 400 nm. 
Owing to these anomalies, we will present only results of high 
spectral resolution analysis for 6 s from 410 nm upward and 
not present the results of either model at 440 and 520 nm i n 
the following spectral analyses. 
For two days in Cuiaba (September 1 and August 30, 1995) 
which had heavy smoke loading, we made spectral irradiance 
measurements with the Optonics instrument, which were then 
used to estimate spectral values of 
l , , , , l 
1.0 2.0 1.5 
6s ECK ET AL.: BIOMASS BURNING AEROSOLS AND SURFACE 1RRADlANCE 
1400 
1200 
1000 
800 
600 ie... 
+MEASURED IRRADIANCE 
SPCTRALZ SSA = 0.94 
6s SSA = 0.94 
' ' ' 1 ' ' ' ' " ' " ' I " ' " " 4 0 0 1 8 " " ' 
400 450 500 550 600 650 700 750 800 
%.. -lo 
r 
WAVELENGTH (nm) 
6 s SSA = 0.94 
AOT 440 = 0.46 
AOT 500 = 0.37 
AOT 670 = 0.19 1 7 1 - 
' I ' " . Z O t . ' l " ' 1 " " 1 " 500 400 
Figure 5 . Spectral irradiance comparison for August 28, 
1995, at 1400 GMT (solar zenith 36.3") in Cuiaba of measured 
Optronlcs spectral irradiance from the OL 754 
spectroradiometer to modeled irradiance from the SPCTRAL2 
and 6s models. Measured spectral T, was used as input to the 
models. Measured T , ~ was relatively low for this case with T , ~ ~ ~ ~ ~ = 
0.31. 
0.95 
" " ' " ' " ' 600 700 
1 
0.90 30 
WAVELENGTH (nm) 
retrievals from the Optronics measurements made o n 
September 1, 1995, at 1330 GMT (solar zenith angle 40.6) in 
Cuiaba are given in Figure 6. There was a heavy smoke aerosol 
loading on this day with T$% at 550 nm of 1.06. The value of the 
asymmetry factor which was used in the SPCTRAL2 model was 
0.58, based on almucantar retrievals for smoke aerosol at high 
Both models retrieved a,, values which showed significant z,. 
decreases from 400 to 740 nm, with SPCTRAL2 showing a 
drop of about 0.13 over this range and 6s showing a 
decrease of approximately 0.10. The exact aerosol size 
distribution and real part of the refractive index (which are 
input parameters to the 6 s model) are not known for these 
observations, due in part (for the size distribution) to a lack of 
almucantar inversions, which are not successful above T,, of 
approximately I .O due to multiple scattering effects 
[Nakujirna, et al., 19831. Therefore we used two values of 
lognormal aerosol volume size distributions for smoke aerosol 
from Rerner et al. [this issue] which spanned a wide range of surface and for a sand surface. 
o,, Figure 6 . (a) Single scattering albedo estimates inferred 
from the 6s and SPCTRAL2 models for September 1 , 1995, at 
1330 GMTin Cuiaba, when T,,,,, was 1.06. The solar zenith 
angle for this case was 40.6". Measured T,, were input to the 
models, and measured irradiances from the Optronics OL 7 5 4 
spectroradiometer were also used in the retrievals. The particle 
radius (lognormal volume size distribution) was varied from 
0.132 to 0.170 pm, and the real part of the refractive index was 
varied from 1.53 to 1.40. (b) Both the 6.9 and SPCTRAL2 
models run with spectral albedo specified for a vegetation 
WAVELENGTH (nm) 
- 
a. . 
- 
- 
- (WlmZlum) 
" " 
' 
1 
AOT 870 = 0.14 
i 
800 
aerosol size, with the modal radius of the aerosol at 0.132 and 
0.17 bm. We also used two values of the real part of the 
refractive index, 1.53 and 1.40, since the true value i s 
unknown. As is shown in Figure 6a there is a range in a,, of 
about 0.03-0.04 due to combinations of these two aerosol 
parameters. The retrieved a,, decreases with increase in particle 
size and also decreases with decreases in the real part of the 
refractive index. Therefore differences between retrievals of o,, 
made with the SPCTRAL2 and 6.7 models range from about 
0.05 to 0.01 for this case, dependent on the input size 
distribution and real refractive index utilized in the 6s model. 
Retrieved a,, values from 6s as a function of surface albedo 
(Figure 6b) for sand albedo and vegetation albedo differed the 
most for the near-infrared wavelengths of 710 and 740 nm, 
,. ,..... . : .... 0.90 - t 
WAVELENGTH (nm) 
, , , , , , , , , , , , I , , , , , , , , , , , , , , , 
L 
31,871 
i ........... ....... . ...... . ....... . ..._ 
I , , I , , , , , , , , , , , , , , , , , , , , , , , 
31.872 
90 , , , 
100, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
400 450 500 
Figure 7 . The computed values of the percent diffuse 
irradiance from the 6 s and SPCTRAL2 models compared with 
the measured values at four wavelengths from the MFRSR 
shadowband radiometer for (a) September 1, 1995 in Cuiaba 
and (b) August 30 in Cuiaba (see Figure 6 and 8, same cases). 
since the vegetation albedo was approximately 50% versus 
about 25% for the sand albedo at these wavelengths. At other 
wavelengths, the differences in the albedo for the two surfaces 
were considerably smaller. For the SPCTRAL2 model the 
differences between the retrieved values of a,, ranged from 0.0 
to 0.02 for the two different surface spectral reflectances, soil 
and vegetation. Therefore both models showed only a small to 
moderate change of retrieved a, over the expected range of 
surface reflectance in the PAR wavelengths (400-700 nm), 
with larger differences in W, (0.03) due to larger surface 
relectance differences of 25% absolute. The computed values of 
the percent diffuse irradiance from the 6s and SPCTRAL2 
models were in very good agreement with the measured values 
from the MFRSR shadowband radiometer (Figure 7a), with 
differences of 1-3% at the four measured wavelengths. 
WAVELENGTH (nm) 
6s and SPCTRAL2 
On August 30, 1995 in Cuiaba the smoke aerosol loading 
was extremely high, with the 2, = 1.93 at 550 nm at 1332 GMT GMT in Cuiaba when z ~ ~ , , 
(solar zenith angle 40.8O). On this day the wavelength 
variation in a,,as retrieved by both models, zenith angle for this case was 40.8". 
WAVELENGTH (nm) 
Figure 8. Same as Figure 6 but for August 30, 1995, at 1332 
was 1.93, significantly higher than 
the September 1, 1995, case shown in Figure 6. The solar 
ECK ET AL.: BIOMASS BURNING AEROSOLS AND SURFACE RRADIANCE 
, , , , , , , 
750 550 700 650 600 
(Figure 8a) was less than for September 1, and the difference i n 
inferred w,, between the two models was greater, ranging from 
about 0.03 to 0.08, dependent on wavelength and the values of 
real refractive index and aerosol size distribution utilized in the 
6s model. It is noted that the aerosol volume size distribution 
models of Remer et al. [this issue] only include almucantar data 
analysis of cases with smoke 7, up to about 0.7 at 670 nm (due 
to the previously mentioned limits on the retrievals), while for 
this case the Z, at 670 nm was 1.35, approximately twice as 
high. Therefore there may be considerable uncertainty in the 
aerosol volume size distribution for cases such as this with 
extremely high aerosol loading. Also for the SPCTRAL2 
model there may be a greater uncertainty in the asymmetry 
parameter, since this parameter is also retieved from 
almucantar measurements with maximum 2, of approximately 
0.7 at 670 nm. Both the 6 s and SPcTRAL2 irradiance models 
also may have larger uncertainties for cases with very high T~ 
and resultant high multiple scattering due to approximations 
in the radiative transfer code. The sensitivity of the retrievals 
to change in surface spectral reflectance for soil/sand and 
vegetated surfaces is similar for this case (Figure 8b) as it was 
0.95 
1.00 . , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , 
- w . 
...... :.-.?.. :: ...= . '.'. . ..% a: '.. 
........ ................. ....... 
.... ," - *. .. - - _ 
........... ...... - a .a.... A.- 
%. 
~ & - - - . + - - - ........ Q - - 1 ...................................................................... - - 4- - - - 
....... 
... . 
I ' I 
Y 
6s r=0.132 Rk1.53 SAND REFL. 
6s ~ 0 . 1 7 0 Rk1.40 SAND REFL. 
I , ~, 
.... .................................... : ................................... 0.95 
0.90 
0.85 
t 
t 
i- 
1 
L O O , , , , 
- - h - - L S I 
L 
t 
WAVELENGTH (nm) 
I , , I , , , , , , , , , , , , 
.^ ................................ 
Rk1.53 VEG. REFL. r d . 1 3 2 
, I , , , I , , I 
o.80 
' P - - - , ... t. . . . . . 
I 1 
. I 
, , 
........... .j 
I ECK ET AL.: BIOMASS BURNING AEROSOLS AND SURFACE IRRADlAhTE 
Table 1. Single Scattering Albedo (0,) Values Derived From Broadband PAR Irradiance Measurements Utilizing 
SPCTRAL2 and 6 s Models, Showing the Influence of Surface Reflectance for Both Models and Variation in the Real 
Refractive Index n, for 6 s 
Solar Zenith Site and 
deg Date in 1995 
41.95 Cuiaba, 
Aug. 27 
40.64 Cuiaba, 
Sept. 1 
42.14 Pantanal, 
Aug. 30 
27.45 Rondonia, 
Sept. 6 
On August 27, 1995, in Cuiaba, we did not have spectral 
inadiance measurements, but we did take broadband 
measurements of the PAR irradiance from 400 to 700 nm. For 
observations taken at 1800 GMT (solar zenith angle 41.9') 
both models were utilized to retrieve the 61~~. Measured values of 
The asymmetry parameter g and imaginary part of the refractive index n, is shown for the 6s retrievals. V, vegetation: S, soil. 
for the September 1 case (Figure 6b) Also, the spectral size distribution at Cuiaba in 1995 by Ji et al. [1996] showed 
percentage of diffuse irradiance computed from the two models that the particle size increased as aerosol loading increased. 
is in excellent agreement with the measurements made with the Reid et al. [this issue] also report significant variability i n 
MFRSR shadowband radiometer, within about 1-2% at the four aerosol size distributions for regional hazes in Amazonia, with 
measurement wavelengths (Figure 7b). 
at 339, 437, 498, 669, and 871 nm were input to the 
SPCTRAL2 model, while the zn at 550 nm was computed from 
interpolation to be used as input for the 
retrieved value of single scattering albedo in the PAR difference in a,, is 0.02-0.05. As discussed earlier, the 
waveband using 6 s was 0.83 for both sand and vegetation differences in the retrievals between the two models is due i n 
albedo (Table 1). This compares quite closely with the values part to uncertainty in input parameter values to the 6s model 
retrieved with the SPCTRAL2 model of 0.84 assuming (real refractive index and volume size distribution) and also 
vegetation albedo and 0.83 assuming soil albedo. probably due to larger uncertainty in both models at very high 
volume mean radius over Rondonia (during the first week of 
September 1995) and the Pantanal (on August 30, 1995) about 
0.03 pm larger than over Cuiaba (last week of August 1995). 
Three other cases of a,, retrieval from broadband PAR 
irradiance measurements are shown in Table 1. It is seen that 
the values of w,, retrieved from the two models, 6s and 
SPCTRAL2, diverge as z, increases, with excellent to good 
agreement for the cases with z,l,,,, of 0.89 and 1.28, 
6s model. The respectively, while for two cases with T,~,,,, = 2.08 the 
n, 
1.53 
0.83(S) 1.53 
0.82(V) 1.40 
0.84(V) 1.53 
0.83(S) 1.53 
0.82(V) 1.40 
0.92(V) 1.53 
0.92(S) 1.53 
0.90(V) 1.40 
0.94(V) 1.53 
0.94(S) 1.53 
0.93(V) 1.40 
a,, inferred ranged from about 0.85 to 0.91, with 
a,, of only about 0.005 to 0.01 resulting from a 
z,, 
Measurements of w,, made from aircraft from in situ air samples aerosol loadings. In a related application, the computation of 
for this date and location by Reid et al. [this issue] were also in atmospheric backscattered radiance to satellite, Royer et al. 
very close agreement to these values, w,, = 0.83 from the [ 19881 found that the predominantly used atmospheric 
integrating plate technique and w,, = 0.82 from the particle radiative transfer codes tend to disagree significantly only at 
soot/absorption photometer. The aerosol optical depth for large aerosol optical thicknesses. 
this case was 0.89 at 500 nm and 0.47 at 670 nm. These w,, with the SPCTRAL2 model, utilizing Retrievals of 
aerosol loadings are lower than for the previous two cases broadband PAR irradiance measurements, for the tropical forest 
discussed, and the smoke volume size distribution model (at 2, region of Jamari in Rondonia are shown in Figure 9a. The data 
= 0.6 at 670 nm) of Remer et al. [this issue] should be shown are for the cloudless periods of four consecutive days 
representative for this case, while its applicability for the with very high aerosol loadings, September 4-7, 1995 (days 
other cases studied is less certain. The lognormal volume size 247-250), with T,,,,, ranging from 1.6 to 4.5 (Figure 9b). The 
distribution model of Remer et al. [this issue] used for this case Angstrom wavelength exponent shows very little variation 
had a volume modal radius of 0.132 pm, while for the other over this 4 day period, ranging from about 1.7 to 1.9. The 
another smoke three cases in Table 1 (which have higher 7,) values of 
model of Remer et al. [this issue] with modal radius of 0.17 differences in 
was used. Analysis of ground-based measurements of aerosol change in assumed surface reflectance of soil or vegetation. 
7 m n 
0.89 
1.28 
2.09 
2.08 
ail 
SPCTRAL2 
0.84(V) 
0.83(S) 
0.82(V) 
0.8 1 (S) 
0.88(V) 
0.87(S) 
0.90(V) 
0.89(S) 
pm 
31,873 
a, 
6s 6s 
0.83(V) 
R 
6s 
0.63 0.029 
0.63 0.030 
0.67 0.022 
0.65 0.029 
0.65 0.030 
0.70 0.024 
0.65 0.0118 
0.65 0.0122 
0.70 0.0106 
0.65 0.0075 
0.65 0.0080 
0.70 0.0075 ECKET AL.: BIOMASS BURNING AEROSOLS AND SURFACE IRRADIANCE 
I 
31,874 
0.850 f 1 A SOIL REFLECTANCE 
E VEGETATION REFLECTANCE 
I , , , I , 
247.5 248 248.5 249 249.5 250 250.5 251 
DAY OF YEAR 
I " ' I I " ' I " " I " " I ' 5.0 , 5.0 , , 
4s 4s 
r! 
i- i- 1 
' , I , 1 , , I , , 
ALPHA (440/870) 
- , 
3.0 
2.5 
1.0 
Figure 9 . (a) Retrievals of a,, for Potosi Mine, Rondonia, a,, of the 
near Jamari from September 4-7, 1995, utilizing the fresh smoke. There may be many other processes at work, 
SPCTRAL2 model and measured broadband PAR (400-700 nm) however, in the evolution of the a,, with time, including 
irradiances (b) Extremely high aerosol optical thickness for absorption of water by the aerosols, gas to particle 
this 4 day period, ranging from 1.6 to 4.5 at 440 nm, with conversion, coagulation of aerosols into larger particles, and 
Angstrom wavelength exponent (alpha) remaining very stable cloud processing of aerosols possibly modifying the aerosol 
from 1.7 to i.9. size distribution and their absorption properties with aging 
The values of o,, which we have retrieved with the above 
techniques fall within the range of values previously reported 
for biomass burning smoke. For example, Radke et al. [ 19911 
measured an average w,, at visible wavelengths of 0.83 for 
plumes of several North American temperate zone prescribed 
fires and wildfires, with most values in the range 0.80 to 0.90. 
The value of the asymmetry parameter (g) assumed here was 
0.58, and the sensitivity to this value was relatively small, 
with w,, retrievals decreasing by about 0.01 for an assumed g = 
a,,, 0.85, occurred for the case of 0.65. The lowest values of In order to assess the impact of the biomass burning smoke 
T , ~ ~ ~ , ~ on the afternoon of September 4 (day 247), the highest aerosols on the attenuation of PAR irradiance for the peak 
when a very dense aerosol cloud was advected into the area. burning season (August-September), we computed the 
This very dense aerosol cloud was possibly from recent and irradiance based on aerosol optical depths measured by Sun- 
active burning taking place upwind of the site. The values of sky scanning radiometers which were located at four sites 
w,, inferred from the 6 s model for September 6 and 7 at this within Amazonia (Table 2). The SPCTRAL2 model was used to 
site were about 0.03 to 0.05 higher than the SPCTRAL2 model 
values shown here (see Table 1, the Rondonia case for 
September 6, 1995). 
5. Biomass Burning Season Estimates of PAR 
Irradiance Attenuation 
calculate the ratio of irradiance computed with measured T,~ 
versus the background (preburning season) values of T,~. 
Measured aerosol optical depths at wavelengths of 339, 437, 
498,669, and 871 nm were input to the SPCTRAL2 model. The 
value of the asymmetry parameter which was assumed for the 
burning season at all sites was 0.58, which is based on 
almucantar retrievals made at T , ~ ~ ~ ~ ~ of approximately 1.0 in 
Amazonia. Values of w,, assumed for the burning season were 
based on retrievals made with the SPCTRAL2 model and 
b'l "1 
For biomass burning fires in the Amazon basin, Kaufian et af. 
computed the w,, to be 0.90 [1992] +- 0.01 (wavelength not 
reported), assuming graphitic carbon in an external mode 
mixture. Horvath [1993] also computed a value of 0.90 for w,, 
(wavelength not reported) of biomass burning aerosols i n 
Amazonia based on the measurements of aerosol composition 
of Andreae et al. [1988]. Reid et al. [this issue] present the 
results of many aircraft-based in situ measurements of regional 
smoke haze in Amazonia utilizing four different techniques, 
which show w,, ranging from 0.80 to 0.90 at 550 nm. A 
comparison between these in situ based methods and the 
ground based remote sensing technique presented here was 
done for three separate cases during SCAR-B [Reid et al., this 
issue]. This comparison showed the a,, values retrieved with 
the SPCTRAL2 model to be in excellent agreement with the i n 
situ techniques (within about 0.01) and the retrievals of a,, 
from 6s to be in excellent agreement with in situ 
determinations for a case of moderate aerosol loading, and 
values inferred for very high aerosol loadings (T,,,,,,, = 2.08) 
from 6 s to be higher by approximately 0.02 to 0.05. 
For two prescribed fires, Radke et al. [I9911 measured the 
w,, to be under 0.7 for the flaming phase of the fires compared 
to a value of approximately 0.90 for the smoldering phase. 
Under laboratory conditions, Patterson and McMahon [ 19841 
,',i 
observed much higher rates of elemental carbon produced in 
the flaming phase relative to the smoldering phase. Therefore 
the lower a,, values which we measured in the Cuiaba region 
may be due to the greater production of smoke from flaming 
phase combustion in cerrado fires, versus the higher values of 
w,, in Potosi Mine, Rondonia, which is in a region of tropical 
forest conversion to agriculture. The felled .trees which are 
subsequently burned in these areas tend to smolder for a long 
period of time after the flaming phase ends. The ratio of smoke 
production from flaming verus smoldering phases in part 
determines the amount of black carbon in the aerosol and thus 
directly influences the absorption properties and 
[Radke eta[., 19911. ECK ET AL.: BIOMASS BURNING AEROSOLS AND SURFACE IRRADIANCE 
Attenuation by Aerosols 
2.14 
~ ' ~ S O ( 1 Assumed w,, Average Dates Location % PAR 
Reduction 
19-Sept. 30, 45 
38 
0.85 
0.90 
Aug. 
1995 
Ji-Parana 
Rondonia, Brazil 
10.85 "S, 61.78"W 43 days 
1.61 36 
30 
0.85 
0.90 
Aug. 5-Sept. 30, 
1995 
Alta Floresta 
Mato Grosso, Brazil 
9.92"S, 56.00"W 57 days 
1.33 33 
27 
0.85 
0.90 
Aug. I-Sept. 28, 
I994 
59 days 
0.94 Aug. I-Sept. 30, 
1995 
Cuiaba 
Mato Grosso, Brazil 
27 
23 
20 
0.80 
0.85 
0.90 61 days 15.50"S, 56 00"W 
13-Sept. 29, 1.08 32 
28 
Aug. 
1994 
23 
0.80 
0.85 
0.90 48 days 
1.08 Aug. 2-Sept. 21, 
1993 
31 
27 
23 
0.80 
0.85 
0.90 56 days 
15 
1.35 El Refugio 33 
28 
0.85 
0.90 
Aug. 15-29 
1995 
days 
Bolivia 
14.75"S, 62.03"W 
31,875 
Table 2. Computed Reduction in PAR (400-700 nm) Irradiance During the Biomass Burning Season Resulting From 
of assumed w,, . SPCTRAL2 model calculations of percent PAR reductions are integrations over the entire time periods with values 
assumed values of w,, = 0.945 and g = 0.65. As was shown in for w,, values of 0.90 and 0.85, respectively. Alta Floresta i n 
w,, value does not have a large influence on 1994 shows lower average aerosol optical depth (T,,,,, = 1.33) 
described in the previous section. The preburning season 45% for assumed a,, values of 0.90 and 0.85, respectively 
aerosol optical properties were assumed the same for all sites (Table 2). For Alta Floresta in 1995, the seasonal average T~~~~~~ 
with T , ~ , ~ ) = 0.15 and Angstrom wavelength exponent of 1.10 is not as high, at 1.61 for a 57 day period, and the resultant 
based on the measurements of Holben et al. [1996], and computed reductions in PAR irradiance range from 30% to 36% 
than in 1995 and therefore lesser reductions in PAR irradiance, 
1995, for Cuiaba, which is located in a cerrado biomass 
Figure 4, the 
irradiance at these low preburning season aerosol loadings. 
The daily integrated totals of irradiance based on the measured ranging from 27% to 33% for w,, of 0.90 and 0.85, 
z, within the burning season were computed for all days within respectively. Calculations were made for three years, 1993- 
the time period, and the sum ratioed to the sum of daily 
integrals computed for the background, preburning season burning region and which also is affected by biomass burning 
aerosol conditions. The length of the time period analyzed at smoke which is advected from forest burning areas to the north 
each site and year varied from 15 days to 61 days and is not the such as the Alta Foresta region. The burning season average T,, 
length of the biomass burning season at each site, but the did not change very much from year to year during this period, 
length of the data record available during August-September ranging from 0.94 in 1995 to 1.08 in both 1994 and 1993. 
when the Sun-sky scanning radiometer was operational. 
The sites of Ji-Parana, Rondonia and Alta Floresta, Mato 
Calculations of reductions in PAR irradiance were made with 
the a,, value of 0.80 in addition to 0.85 and 0.90, since 
Grosso, are located in tropical forest regions of Amazonia retrievals of w, in the previous section showed lower values for 
(Figure 1) and are also in major regions of agricultural activity this site, which may be the result of a greater proportion of 
and thus biomass burning. Therefore these sites would be flaming phase combustion and higher black carbon content in 
representative of levels of aerosol loading which would the aerosol from cerrado burning. The resultant estimated 
approach the maximum for burning regions within the Amazon reductions in PAR irradiance at Cuiaba ranged from 20% to 
Ji- Basin. The largest reductions in PAR were computed for 32% dependent mainly on the assumed value of w,, with the 
Parana in 1995, which was a dry year [Mattos et al., 19961 and most likely reductions ranging from 23% to 28%, associated 
thus a year with more intense and prolonged burning than a with a W, value of 0.85. Even locations which have very little 
z,~,,,, at this site for the 43 days of typical year. The average local biomass burning may experience significant reductions 
data availability was 2.14, which is very high, and the in PAR irradiance due to advection of smoke from adjacent 
resultant computed reduction in PAR irradiance (compared to regions of biomass burning. For example, El Refugio, Bolivia 
preburning season aerosol conditions) ranged from 38% to is located within Noel Kempf Mercado National Park, which is ECK ET AL.: BIOMASS BURNING AEROSOLS AND SURFACE IRRADIANCE 31,876 
8 8 , 0.48 
0.46 0.46 
J 1 0.43 
z 
g 
2 
5 
5 
k 
k 
g 0.45 0.45 
0.42 
0.47 1 
t 
-A- 
0.41 
1 
1 I 
i 
3 
3 
j 
, , I , 
I AOT 500 =0.86 8/27/95 CUIABA 
- - -E+ - - 8/28/95 CUIABA ...*-.. 8/30/95 PANTANAL AOT(500)=2.13 
AOT[500{=0.50 
" " 8 ' 8 8 I , 
17.5 
1 
18 17 
0.401'18 
16.5 
TIME (UTC) 
10. Measured PAR fraction, or the ratio of PAR (400-700 nm) irradiance, to "total" (280-2800 nm) 
irradiance for three cases of varying aerosol optical thickness (AOT) in Cuiaba and northern Pantanal (world's 
Laszlo [1992] computed the PAR faction reduction from 0.45 
to 0.42, for a range of aerosol optical depth from 0.0 to I . 2 , 
for a continental aerosol model. The magnitude of the PAR 
fraction reduction with increases iii T:, depends also on the 
value of w,, and asymmetry parameter, both of which exhibit 
wavelength dependence. The reduction of UV irradiance (300- 
400 nm) is greater than that for PAR irradiance (Figure 3), due 
to higher T,, at UV wavelengths, while the reduction in near 
infrared irradiance is less than that for the PAR reduction due to 
lower T:, as wavelength increases (Figures 2 and 3). However, 
from the measured PAR fractions shown in Figure 10, we can 
infer that the total irradiance reduction is also high, although 
lower than PAR reduction, for cases of high aerosol loading. 
For example, assuming a PAR fraction of 0.46 for background 
conditions and 0.42 for high aerosol loading, and an assumed 
40% reduction (compared to background conditions) in PAR 
irradiance at T;,,,,= 
Figure 
largest wetland). 
an extensive protected region with little burning, yet smoke i s 
advected here from Rondonia, Brazil and biomass burning 
regions within Bolivia. For 15 days in August 1995 when a 
Sun-sky scanning radiometer was operational at the site, there 
was a measured average T,lsix, = 1.35, which resulted in PAR 
irradiance reductions estimated to be from 28% to 33% for 
values of 0.90 and 0.85, respectively. 
For all of the above calculations of PAR irradiance 
reduction, due to attenuation by biomass burning aerosols, the 
model calculations are valid for cloudless sky conditions. 
Since the sky is not always cloudless in the dry season in these 
regions of southern Amazonia, these calculations for cloudless 
conditions will differ from the actual reductions in irradiance. 
Computing the effects on irradiance of combined cloud and 
aerosol effects is beyond the scope of this paper. However, the 
authors have often observed relatively low cloud fractions 
while taking measurements in these regions of heavy aerosol 2.13, we can estimate that the reduction in 
loading in the Amazonian, dry season (August-September). total irradiance is about 34%. Of course, this is a general 
Moreover, reductions in surface incident solar radiation from- estimate which would be modified for differing values of the 
absorbing aerosols in conjunction with absorption within the. wavelength dependence of w,, and g. This case would be similar 
aerosol layer may result in a reduction of local cloud cover, to the aerosol loading measured at Ji-Parana in 1995 for 43 
which Hansen et al. [1997] call the "semidirect" aerosol 
radiative forcing effect. 
o,, 
days in August-September with the resulting computed PAR 
reduction of 38-45% (Table 2). 
7. Summary and Conclusions 6. Aerosol Effects on the Fraction of PAR to 
Total Irradiance 
Measurements of both broadband and spectral irradiance in 
The fraction of PAR (400-700 nm) to total (280-2800 nm) Amazonia were used in conjuction with measured aerosol 
irradiance as measured by the combination of Skye-Probetech optical properties (T,), g, and aerosol size distributions) to 
PAR sensors and Eppley PSP pyranometers is shown in Figure retrieve values of single scattering albedo (a,!) for biomass 
10 for a solar zenith angle range of about 28" to 42". burning aerosols. Irradiance under cloudless sky conditions is 
Measurements were made within a 4 day period in Cuiaba and very sensitive to the magnitude of aerosol absorption and 
northern Pantanal (about 100 km SSW of Cuiaba) with therefore w,, for cases of middle to high aerosol loading, which 
differing levels of aerosol loading, with T;,,,,, ranging from frequently occur in biomass burning regions of Amazonia 
0.50 to 2.13. The PAR fraction ranged from about 0.46 at T,,,,,,, during August-September. Retrievals of o,, using broadband 
= 0.50 to about 0.42 at t;,5iXI= 2.13. In comparison, Pinker and PAR (400-700 nm) irradiance measurements in conjuction with ECKET AL.: BIOMASS BURNING AEROSOLS AND SURFACE IRRADIANCE 
measured aerosol optical properties and cloudless s k y 
irradiance models yielded a,, values which ranged from 0.81 t o 
0.94, using two different irradiance models. Agreement 
between the a,, retrievals using t h e SPCTRAL2 and 6s models 
was good for aerosol loading corresponding to z:,,,,, of 0.9 t o 
1.3, while for very high aerosol loadings (T;,,,,, = 2.1) the 6s 
model retrievals of a,, were 0.02 to 0.05 higher than 
SPCTRAL2 retrievals. The GS model retievals of a,, depend i n 
part on the assumed real part of the refractive index and t h e 
aerosol size distribution and associated modal particle radius. 
For t w o different days on which spectral irradiance data were 
available, both models retrieved a similar magnitude of 
decrease with increasing wavelength, with the magnitude of 
the decrease in wavelength differing on these 2 days. 
a,, 
Reductions in PAR irradiance at the surface were computed 
for cloudless conditions f o r the burning season months of 
August-September at sites located in biomass burning regions 
i n Amazonia. Measurements of aerosol optical depth made 
with automatic Sun and sky scanning radiometers were input t o 
the SPCTHALZ model along with a range of w,, values 
representative o f the region, based on previous retrievals. 
These computations show large reductions in PAR irradiance 
for cloudless skies over the 2 month burning season, ranging 
from 20 to 45%, dependent on the aerosol optical depth at t h e 
site and t h e value of w,,. Due to long-range transport of 
biomass burning aerosols, even areas which do not experience 
local burning may be impacted by the radiative effects of h i g h 
smoke aerosol loading. Ecologically significant and sensitive 
regions of Amazonia and t h e Pantanal are subjected to events 
with large aerosol optical depth. 
[I9971 have suggested that rainforest 
clearing, i n regions with large cleared areas such as i n 
Rondonia, Brazil, may increase local cloud cover through 
differential surface heating and resultant mesoscale 
circulations with a deeper mixed layer over the cleared area. 
They suggest that this may result in increased cloud cover over 
the cleared area. The results of our analysis presented here 
show significant reductions in surface irradiance and therefore 
likely reductions in surface heating which result from smoke 
aerosols. It also follows that as a result of the absorbing nature 
of the aerosols there would likely b e an increase in heating 
within the mixed layer, leading to increased atmospheric 
stability. Therefore during the burning season months of 
August-September the aerosol radiative effects may act t o 
suppress convection and therefore may decrease local cloud 
cover. 
Gash aid Nobre 
Acknowledgments. This project was supported by Diane Wickland, 
NASA Headquarters, and Mike King, EOS Project Science Office. We 
gratefully acknowledge the assistance. of Joe Vaughan (Washington 
State University) in field experiment participation during SCAR-B, and 
in providing us the data from his MFRSR shadowband radiometer 
located in Cuiaba during the experiment. We also thank Alfreido Periera 
and Jose Chagas (INPE) for setting up Sun and sky scanning radiometers 
at field sites in Brazil and in assisting in experiment logistics. We are 
indebted to the efforts of several site managers, who maintained 
instruments in several locations in Brazil and Bolivia. These include 
Santurnino in Alta Floresta, Brazil; Levi in Ji-Parana, Brazil; and Tim 
Killeen (Missouri Botanical Gardens) in El Refugio, Bolivia. We also 
thank the reviewers for their thoughtful comments, which helped in 
improving the manuscript. 
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20771. MD Greenbelt, Flight 
T.F. Eck, Raytheon STX Corporation, Code 923, NASA/Goddard 
Space Center, 
mail:tom@spamer.gsfc.nasa.gov) 
(e- MD 
(e- 
B.N. Holben, Biospheric Sciences Branch, Code 923, NASNGoddard 
20771. Space Flight Center, Greenbelt, 
maikbrent @spamer.gsfc.nasa.gov) 
A. Setzer, Instituto de Pesquisas Espacias, Sao Jose dos Campos, Sao 
Paulo, Brazil 12201-970. (e-mail:asetzer@ltid.inpe.br) 
I. Slutsker, Science Systems and Applications Inc., Code 923, 
NASAIGoddard Space Flight Center, Greenbelt, MD 20771. (e-mail: 
ilya@spamer.gsfc.nasa.gov) 
(Recieved August 15, 1997; revised January 16, 1998; 
accepted January 29, 1998.)