Introduction

El Chichón volcano located in northwestern Mexico (17.3N, 95.2W) erupted from March 28 to April 4, 1982, and spewed a large amount of ash and gaseous products into the atmosphere. The large ash particles fell off within a few days, but the gaseous products, particularly SO2, were converted into sulfuric acid and resided in the stratosphere for a few years (Kruger, 1983). Airborne lidar data showed that by July 1982 the aerosol cloud had spread up to 30N and by October 1982 the El Chichón cloud boundaries were from 10S to 35N (Bandeen and Fraser, 1982; Robock and Matson, 1983). 

Soon after the eruption, the aerosol optical thickness measured at the Mauna Loa Observatory was the largest in the 25-year history of observations (Deluisi et al, 1983, King et al., 1984). The effect of volcanic eruption was observed in satellite-measurements of geophysical parameters including sea-surface temperature by the Advanced Very High Resolution Radiometer (AVHRR) and ozone measurements by the Solar Backscatter Ultraviolet (SBUV) instrument and the Total Ozone Mapping Spectrometer (TOMS). The sea surface temperature was biased by as much as 2.5° C for many months (Bandeen and Fraser, 1982), and column ozone amount derived with the operational algorithm was much greater than previously observed (Bandeen and Fraser, 1982, Kruger, 1983).

Based on a simulation study, Howard and Castano (1988) had concluded that the El Chichón aerosols would not have any appreciable effect on the determination of water-leaving radiances from the Coastal Zone Color Scanner Experiment (CZCS) instrument. However, recent reprocessing of the CZCS data clearly show significant increase in the retrieved water-leaving radiances in 520 and 550-nm bands during the El Chichón period. This prompted researchers at NASA's Ocean Biology Distributed Active Archive Center (OB.DAAC) to re-examine the effect of El Chichón aerosols on water-leaving radiances by the CZCS instrument.

Using CZCS to Study the Change in Water-Leaving Reflectance

As a part of the re-examination, we simulated the top of the atmosphere (TOA) radiances for the CZCS bands with an aerosol layer at 26 km in the stratosphere representing El Chichón aerosols embodied in a molecular (Rayleigh) atmosphere with a flat ocean as the lower boundary. This height is consistent with the larger eruption on April 4, 1982 (Bandeen and Fraser, 1982). We used Deirmendjian's (1969) modified gamma distribution to characterize the aerosol size distribution. The parameters of this distribution were determined by DeLuisi et al [1983] by inverting the spectral solar transmission measurements obtained at the Mauna Loa Observatory using King's [1982] constrained linear inversion method. The retrieved size distribution for the month of July 1982 is shown in Figure 2a. We also computed the TOA radiances with the Gordon's M90 aerosol distribution (Gordon and Wang, 1994) that is operationally used to process the CZCS data. M90 is a maritime aerosol model for a relative humidity of 90 percent. It is bi-modal lognormal distribution that is constructed from the Shettle and Fenn's aerosol distributions (1979) for tropospheric and oceanic aerosols. The phase functions in the backward direction for the two distributions are shown in Figure 2b. Fopr both figures, red indicates M90 aerosol and green indicates El Chichón sulphate aerosol.

We simulated the reflectances for all the four bands of the CZCS instrument. Figure 3 shows the spectral dependence of Rayleigh corrected reflectance for the two aerosol models. The results are for Θ₀=30⁰, Θ=36⁰, and Φ=96⁰. This sun-view angle geometry corresponds to a scattering angle (Χ) of 137⁰. From the graph we find that M90 aerosol reflectance increases almost linearly with a decrease in wavelength from 670 to 443 nm. On the other hand, the El Chichón reflectance increases almost linearly up to 520 nm and then levels off for wavelengths shorter than 520 nm. Also, in the range of 670 to 520 nm, the rate of increase in the reflectance for El Chichón aerosols is much larger (-1.161x10^-5/nm) than for M90 aerosols (-3.806x10^-6/nm).

Figure 4 shows the change in the reflectance value when M90 aerosols are replaced by the El Chichón aerosols in the atmosphere. We find that at large solar zenith angle (~60<°) and small view angles (<30<°), the percent change in the reflectance value 443(550) nm is about 0.8(3-3.5) percent. Whereas, for solar zenith angle of ~30 degrees and nadir viewing geometry, the percent change in the 443(550) nm band is ~2.5 (2.8), and the change is about 1.5(3.6) at large view angles (~50<°). We attribute these results to the significant difference in the phase functions of the two aerosol models. Credit: OB.DAAC

Major Findings

Based on our previous radiative transfer studies, we find that in the oligotrophic waters, 1 percent change in the TOA radiance is equivalent to about 10 percent change in the water-leaving radiance. Based on the Sun-view angle geometry and aerosol optical thickness, the change in the water-leaving reflectance can vary from 8 to 35 percent. These simulations, contrary to the previously reported results, do show the effect of El Chichón aerosols on the retrieval of water-leaving radiances observed in the CZCS data.

References

Bandeen, W. R., R S Fraser, Radiative effects of the El Chichon volcanic eruption, Preliminary results concerning remote sensing, National Aeronautics and Space Administration. Goddard Space Flight Center, Greenbelt, MD, 1982.

DeLuisi, J. J., E. G. Dutton, K. l. Coulson, T. E. DeFoor, and B. G. Mendonca,On some radiative features of the El Chichon volcanic stratospheric dust cloud and a cloud of unknown origin observed at Mauna Loa, J. Geophys. Res., 88, 6769-6772, 1983.

Deirmendjian, D., Electromagnetic Scattering on Spherical Polydispersions, Elsevier, New York, 1969.

Gordon, H. R., D. J. Castano, Coastal zone color scanner atmospheric correction: Influence of El-Chichon, Appl. Opt., 27(16), 3319-3322, 1988.

Gordon, H. R., and M. Wang, Retrieval of water-leaving radiance and aerosol optical thickness over the oceans with SeaWiFS: a preliminary algorithm, Appl. Opt., 443-452, 1094.

King, M. D., Harshvardhan, A. Arking,A model of the radiative properties of the El Chichon stratospheric aerosol layer, Journal of Climate and Applied Meteorology, 23, 1121-1137, 1984.

Krueger, A.J., Sighting of El Chichon sulfur dioxide clouds with the Nimbus 7 total ozone mapping spectrometer, Science, 220: 1377-1379, 1983.

Robock, A., M. Matson, Circumglobal transport of the El Chichon volcanic dust cloud, Science, 221, 195-197, 1983.

Shettle, E. P. and R. W. Fenn, Models for the aerosols of the lower atmosphere and the effects of humidity variations on their optical properties, AFGL-TR-79-0214 (U.S. Air Force Geophysics Laboratory, Hanscomb Air Force Base, Mass., 1979).