The potential of airborne imaging spectrometry for assessing and monitoring natural resources is studied. Therefore, an AVIRIS scene of the NASA's MacEurope 1991 campaign - acquired in Central Switzerland - is used. The test site consists of an urban area, the Lake Zug with its surrounding fields, the Rigi mountain in the center of the test site, and the Lake of Four Cantons. The region is covered by the AVIRIS flight #910705, run 6 and 7 of the NASA ER-2 aircraft resulting in an average nominal pixel size of about 18 m. Simultaneous to the ER-2 overflight spectroradiometric measurements have been taken in various locations. Preselected reference targets were measured in the field with a GER Mark V spectroradiometer, and radiance measurements were taken to the lake using a Li-Cor LI 1800UW specroradiometer below and above the water surface. A comprehensive meteorological data set was obtained by joining the POLLUMET experiment which carried out measurements to investigate the summer smog in Switzerland on the same day. The quality assessment for the actual data set can be found in detail in Meyer et al. A parametric approach calculating the location of the airplane was used to simulate the observation geometry. This parametric preprocessing procedure, which takes care of effects of flight line and attitude variations as well as the pixel-by-pixel topographic corrections is described in Meyer

Itten, K. I.

Keller, J.

Kurer, U.

Schaepman, M.

Schlaepfer, D.

Veraguth, S.

English

NASA Technical Reports Server

N95- 33770
EXTRACTION OF OZONE AND CHLOROPHYLL-A DISTRIBUTION FROM AVIRIS
DATA
M. Schaepman 1, K.I. Itten l, D. Schl_ipfer l, U. Kurer l, S. Veraguth I and J. Keller 2
lRemote Sensing Laboratories, University of Zurich-Irchel, 8057 Zurich, Switzerland
2paul Scherrer Institute, 5232 Villigen PSI, Switzerland
Data Basis and Preprocessing
The potential of airborne imaging spectrometry for assessing and monitoring natural re-
sources is studied. Therefore, an AVIRIS scene of the NASA's MacEurope'91 campaign - acquired
in Central Switzerland - is used. The test site consists of an urban area, the Lake Zug with its sur-
rounding fields, the Rigi mountain in the center of the test site, and the Lake of Four Cantons. The
region is covered by the AVIRIS flight #910705, run 6 and 7 of the NASA ER-2 aircraft resulting
in an average nominal pixel size of about 18 m.
Simultaneous to the ER-2 overflight spectroradiometric measurements have been taken in
various Iocations.Preselected reference targets were measured in the field with a GER Mark V
spectroradiometer, and radiance measurements were taken on the lake using a Li42or LI 1800UW
specroradiometer below and above the water surface [Itten et al., 1992].
A comprehensive meteorological data set was obtained by joining the POLLUMET expe-
riment which carried out measurements to investigate the summer smog in Switzerland on the same
day [Neininger, t991].
The quality assessment for the actual data set can be found in detail in Meyer et al. [1993].
A parametric approach calculating the location of the airplane was used to simulate the observation
geometry. This parametric preprocessing procedure, which takes care of effects of flight line and
attitude variations as well as the pixel-by-pixel topographic corrections is described in Meyer,
[1994].
Atmospheric Correction
The atmospheric correction is based on the radiative transfer code MODTRAN2. The cor-
rection itself is performed using a model proposed by Woodham [Woodham, 1985] and a pixel-
by-pixel approach. The atmospheric profiles used, have been taken from radiosonde data. Where
no data were available the standard midlatitude summer profile was used.
Another important factor was to estimate the visibility. The values obtained from the
Swiss Meteorological network, and the estimations made in the field vary from 2-14 km for July
5th. Since small changes of the visibility have a siginficant impact on the output of the calculation
in the radiative transfer code, a method to extract the visibility from AVIRIS data is tested. Two
AVIRIS channels (53 / 71; 872 nm/ 1045 nm) are selected. In these bands, the total attenuation of
the radiance depends only on aerosol scattering. The meteorological visibility from this approach is
estimated to be 5 km (eq. horizontal visibility of 3.85 km). The 5S code restricts however the hori-
zontal visibility to 5 km, therefore this approach will have to be evaluated further also considering
adjacency effects.
The pixel-by-pixel approach bases not on the reconstruction of the path through the atmo-
sphere but relies on modelling the possible maximum and mininimum radiance at the sensor. As-
suming that the objetct's reflectance is linear with the received radiance at the sensor, we did run
the radiative transfer code twice for each pixel [Veraguth, 1994].
A comparison of two different corrections (visibilitiy decreasing with altitude and constant
within the inversion layer) in a corrected AVIRIS scene using MODTRAN2 is shown in figure 1.
149
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Extraction of Ozone
Ozone absorbs particulary in the ultraviolet and visible range of the spectrum. Hence,
spectrometry is expected to be a promising tool to extract the ozone contents in a given air column
by using the correlation between cumulative trace gas amount and absorption strength in the sensor
channels located within the absorption bands. The following investigations were initiated to detect
tropospheric ozone in the Chappuis-Band (500 - 700 nm), based on the POLLUMET data set, the
AVIRIS specifications, and MODTRAN2 simulations.
Normally, the abundance of water vapor from imaging spectrometry data is usually deter-
mined by ratio methods [Frouin, 1990; Bruegge, 1990]. Two or more channels are combined to
yield the total column water vapor abundance. The same method can be used to detect ozone.
First, the best channels within the Chappuis-Band have to be determined. It must be dis-
criminated between 'Reference channels' which should not be affected by any absorbing gases
including ozone and, on the other hand, 'Measurement channels' which should be sensitive to the
change of ozone amount and not be disturbed by other absorbing gases. An empirical comparison
of MODTRAN2 transmission calculations in the AVIRIS channels yielded a number of suitable
AVIRIS ozone channels:
Measurement channel numbers: 22 (608nm), 23 (618 nm), 17 (558 nm), 16 (549 nm)
Reference channel numbers: 1-12(< 515 ran), 28 (667 nm), 29 (677 nm), 40 (746 nm)
These selected channels could be combined both by three known splitting window tech-
niques and by new approaches. To date, the following methods are used: Ratio of two channels
[King, 1992], continuum interpolated band ratio (CIBR) using 2 reference channels [Bruegge,
1990] and the Narrow/Wide technique [Frouin, 1990] with two channels of different bandwith
centered at the same wavelength. Two new approaches are introduced: The first is a quadratic
interpolated band ratio (QIBR) which is analogous to the CIBR but which uses three reference
channels. The second is a statistical approach (TOTAL) by ratioing several measurement channels
by the same number of reference channels. Each of these five methods can be tested with different
channel combinations, using the reference and measurement channels.
From radiosonde data and other POLLUMET in-situ measurements a reference atmo-
sphere for July 5th ( 199 I) was modelled, ending up in a complete profile of pressure, temperature,
water vapor and ozone. The total ozone column in the different atmospheric layers was integrated
with height and compared to standard atmospheric profiles. None of them represented the tropo-
spheric ozone amounts in a satisfying manner; the ozone column was nearly twice as high as gues-
sed by the tropical or the midlatidude summer model. This shows, that tropospheric ozone had a
bigger influence on the total ozone signal than assumed.
In the evaluation of the methods, the influence of disturbing factors and the sensitivity to
varying amounts of ozone were investigated. For this purpose the amount of the respective ele-
ments (aerosols, water vapor and ozone) were varied systematically in the atmospheric reference
profiles. MODTRAN2 allows to simulate the corresponding radiance at the sensor channels and all
the methods could be applied to the simulated radiance. A good sensitivity to ozone variations is
obtained by the CIBR, the channel ratio and the TOTAL methods. The Narrow/Wide technique is
only half as sensitive to ozone as the others. Changing visibility is proportional to the aerosol con-
tents and affects especially the TOTAL and some of the CIBR-Methods. The Narrow/Wide and
QIBR methods are not influenced that much. If tropospheric humidity is doubled, the influence on
all investigated methods is only about one fifth of the change that is due to varying aerosol optical
thickness. This evaluation step led to the elimination of most of the methods. The remaining ones
were quantified using the ozone variation procedure described above. An absolute quantification
was only possible for one CIBR (channels 10,28 and 22) and one Narrow/Wide ratio (channels 10,
28, 17 and 22). All the others yielded irrealistic high or low ozone contents. Finally the CIBR was
selected because of the better sensitivity to ozone variations. However, the application to the
AVIRIS scene (see figure 2) was only possible over water, which is due to the high sensitivity to
changing ground response and the very low absorption in the Chappuis Band. The error which must
be taken into account considering the disturbing factors is still about +/- 30%.
150
Extraction of Chlorophyll-a
During the overflight of the ER-2 aircraft, a wealth of limnologic measurements has been
collected on the Lake Zug. For the results presented here, the measurements of chlorophyll con-
centrations using a two-filter method in various depths [Schanz, 1982] and the use of the Li-Cor
1800UW underwater spectroradiometer are of partic ul ar interest. Down welli ng and upwell ing
irradiance measurements have been taken underwater as well as incident global radiation. The
photosynthetically active radiation measurements of the solar irradiance are used to calculate the
absorption coefficients.
The ER-2 overflight on July 5th 1991 was not during an algae bloom, hence very low
chlorophyll concentrations were expected. In a first processing step, the AVIRIS radiance data are
converted in upwelling irradiance data just below the water surface. Using MODTRAN2, the path
scattered radiance was subtracted from the AVIRIS data and divided by the atmospheric trans-
mission. Then the AVIRIS signal was transformed in upwelling radiance just unter the water sur-
face. In the last step, the radiance had to be converted into irradiance data using a conversion pro-
posed by Kirk [1983].
Different combinations of channels of the AVIRIS have been selected to extract chloro-
phyll-a. A CIBR method using the AVIRIS channels 26 / 29 / 31 (647 nm / 677 nm / 697 nm)
shows promising results [Kurer, 1994] (see figure 3).
Conclusion
It is expected, that the presented applications to extract ozone and chlorophyll-a can be
extended to many more environmentally sensitive parameters. Some efforts still have to be made in
respect to adjacency effects and improvements of the atmospheric correction.
Acknowledgements
We would like to acknowledge the contribution of all other scientists and institutions in-
volved in the AVIRISwiss'91 campaign. The support of the NASA - JPL, Ames and HQ - is great-
fully acknowledged.
References
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Figures
Figure I' Atmospheric correction results by modelling the visibility decreasing
with altitude (left), and constant within the inversion layer (right)
< 8 g/m2
8 - I0 g/m2
10-12 g/m2
> 12 g/m2
Figure 2: Total column tropospheric ozone Figure 3: Chlorophyll-a distribution
distribution over Lake Zug in Lake Zug
152


Extraction of ozone and chlorophyll-A distribution from AVIRIS data

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