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Determination of climate-relevant aerosol properties of Pinatubo event by sun and sky radiometer
J. Aerosol Sci., Vol. 26. Suppl 1, pp. $373-S374, 1995
ls P e r " a m o n
Elsevier Science Ltd Printed in Great Britain 0021-8502/95 $9.50 + 0.00
Determination of climate-relevant aerosol properties of Pinatubo event by sun and sky radiometer Peter Posse, Wolfgang von Hoyningen-Huene Institut ~ r Meteorologie, Universit~it Leipzig, StephanstraBe 3, D-04103 Leipzig, Germany
The eruption of Mt. Pinatubo in June 1991 was the most important volcanic event in this century. Great masses of sulphur dioxid were injected into the stratosphere, which converted to more than 20 Mt of sulphat aerosol particles (McCormick & Veiga, 1992). This aerosol has distributed in the stratosphere around the globe within a few month and it resides there a few years. A time series of spectral optical thickness of aerosol/5 A(;~) in the wavelength range from 350 to 1200 nm was measured by sun photometer on top of the mountain Zugspitze in the Alps to determine the contribution of Pinatubo volcanic aerosol. An inversion program is used to calculate a discrete aerosol size distribution dN/dlogr from the measured data. Then lognormal distributions are fitted to the discrete size distribution and mode parameters are derived. Besides values for the column particle surface-area, particle volume and mass are calculated. Additionally the sky radiances Psky(0) in the almueantar were measured simultaneously as a function of scattering angle 0. Such combined measurements give a possibility to determine almost all climate-relevant aerosol properties except absorption from experimental remote sensing data. A Coupled Inversion Radiation Transfer program CIRATRA (Wendisch &von Hoyningen-Huene, 1992, 1994, yon Hoyningen-Huene & Posse, 1995) is used for the closure between spectral optical thickness and sky radiances in the almucantar. By means of this technique other unknown aerosol parameters are determined. Besides information about the particle shape are derived. Some examples for the spectral optical thickness of Pinatubo aerosol are presented in Figure 1. From the early state the spectral slope in aerosol optical thickness decreased fast and during maximum state in 1992 it was found a nearly neutral spectral behaviour with a slight maximum around 600 nm wavelength and an optical thickness value greater than 0,1. During maximum state of Pinatubo event the small particles of stratospheric background aerosol are not present. The mode radius for the main volcanic mode was around 0.3 lam during 1992 and increased up to 0.8 ~tm now in the late state at end 1994. In 1994 the mode of small particles of stratospheric background aerosol has formed again. The spectral behaviour in November 1994 is similar to the state before the Pinatubo eruption (September 1990), but the level of optical thickness is still significantly higher. From the time series was determined a time constant of 15 month for an exponential decrease of optical thickness. An annual trend with a maximum value of optical thickness in spring and a minimum value in late autumn can be seen. From measurements of sky radiances at 870 nm and 550 nm wavelength, performed on 4. 11. 1993, combined with the spectral optical thickness the real part of refractive index was found to be 1.425 q- 0.025. No significant absorption occur. The particle shape is spherical. These results matches the assumption of spherical droplets of sulphuric acid. From the aerosol phase functions the asymmetry parameters of Pinatubo aerosol were determined to be 0.75 at 870 nm and 0.73 at 550 nm wavelength. $373
S374
P. POSSE and W. yon H o ~ n ~ a ~ - H t m ~
The complete knowledgeof aerosol properties allows to estimate the caused shortwave radiative forcing, which gives the main climatic impact. As input quantities for our radiative forcing calculations we used the aerosol size distribution and the spectral behaviour of optical thickness as derived from our measurements in March 1992. On top of the mountain Zugspitze in the Alps we have measured an optical thickness value of 0,12 at 550 nm wavelength for the contribution of Pinatubo aerosol. To get a realistic global value of radiative forcing, the optical thickness at 550 nm wavelength was set to 0,13 according to the mean global value of aerosol optical thickness given by Stowe et al., 1992. We derived a value of-5.7 Wm"2 for the shortwave radiative forcing, which is somewhat larger than the -4.3 Wm2 given by Stowe et al. A comparison with the observed decrease in global radiation of 4% at the high mountain station Jungfraujoch, Switzerland (Blumthaler& Ambach, 1994) during 1992 gives a good coincidence. 2 -
0.10
m m
Zugspitze
5 -
-+-
m
3
o
s~,9o M,~92
-
Jtm92 2
--
N o v 93 May94
Nov94 0.01 3
I
f
I
I
l'
I
4
5
6
7
8
9
J 1.00
wavelength (l~m)
Figure 1: Spectral optical thickness of Pinatubo aerosol measured on top of mountain Zugspitze. References Blumthaler, M. & W. Ambach, 1994: Changes in solar radiation fluxes after the Pinatubo eruption. Tellus, 46B, 76-78. McCormick, M.P. & R.E. Veiga, 1992: SAGE II measurements of early Pinatubo aerosols. Geophys. Res. Lett., 19, 155-158. Stowe, L.L., R.M. Carey, P.P. Pellegrino, 1992: Monitoring the Mt. Pinatubo aerosol layer with NOAA/11 AVHRR data. Geophys. Res. Lett., 19, 159-162. Wendisch, M. & W. von Hoyningen-Huene, 1992: Optically equivalent refractive index of atmospheric aerosol particles. Beitr. Phys. Atm., 65, 293-309. Wendisch, M. & W. yon Hoyningen-Huene, 1994: Possibility of refractive index determination of atmospheric aerosol particles by ground based solar extinction and scattering measurements. Atmospheric Environment, 28, 785-795.
ls P e r " a m o n
Elsevier Science Ltd Printed in Great Britain 0021-8502/95 $9.50 + 0.00
Determination of climate-relevant aerosol properties of Pinatubo event by sun and sky radiometer Peter Posse, Wolfgang von Hoyningen-Huene Institut ~ r Meteorologie, Universit~it Leipzig, StephanstraBe 3, D-04103 Leipzig, Germany
The eruption of Mt. Pinatubo in June 1991 was the most important volcanic event in this century. Great masses of sulphur dioxid were injected into the stratosphere, which converted to more than 20 Mt of sulphat aerosol particles (McCormick & Veiga, 1992). This aerosol has distributed in the stratosphere around the globe within a few month and it resides there a few years. A time series of spectral optical thickness of aerosol/5 A(;~) in the wavelength range from 350 to 1200 nm was measured by sun photometer on top of the mountain Zugspitze in the Alps to determine the contribution of Pinatubo volcanic aerosol. An inversion program is used to calculate a discrete aerosol size distribution dN/dlogr from the measured data. Then lognormal distributions are fitted to the discrete size distribution and mode parameters are derived. Besides values for the column particle surface-area, particle volume and mass are calculated. Additionally the sky radiances Psky(0) in the almueantar were measured simultaneously as a function of scattering angle 0. Such combined measurements give a possibility to determine almost all climate-relevant aerosol properties except absorption from experimental remote sensing data. A Coupled Inversion Radiation Transfer program CIRATRA (Wendisch &von Hoyningen-Huene, 1992, 1994, yon Hoyningen-Huene & Posse, 1995) is used for the closure between spectral optical thickness and sky radiances in the almucantar. By means of this technique other unknown aerosol parameters are determined. Besides information about the particle shape are derived. Some examples for the spectral optical thickness of Pinatubo aerosol are presented in Figure 1. From the early state the spectral slope in aerosol optical thickness decreased fast and during maximum state in 1992 it was found a nearly neutral spectral behaviour with a slight maximum around 600 nm wavelength and an optical thickness value greater than 0,1. During maximum state of Pinatubo event the small particles of stratospheric background aerosol are not present. The mode radius for the main volcanic mode was around 0.3 lam during 1992 and increased up to 0.8 ~tm now in the late state at end 1994. In 1994 the mode of small particles of stratospheric background aerosol has formed again. The spectral behaviour in November 1994 is similar to the state before the Pinatubo eruption (September 1990), but the level of optical thickness is still significantly higher. From the time series was determined a time constant of 15 month for an exponential decrease of optical thickness. An annual trend with a maximum value of optical thickness in spring and a minimum value in late autumn can be seen. From measurements of sky radiances at 870 nm and 550 nm wavelength, performed on 4. 11. 1993, combined with the spectral optical thickness the real part of refractive index was found to be 1.425 q- 0.025. No significant absorption occur. The particle shape is spherical. These results matches the assumption of spherical droplets of sulphuric acid. From the aerosol phase functions the asymmetry parameters of Pinatubo aerosol were determined to be 0.75 at 870 nm and 0.73 at 550 nm wavelength. $373
S374
P. POSSE and W. yon H o ~ n ~ a ~ - H t m ~
The complete knowledgeof aerosol properties allows to estimate the caused shortwave radiative forcing, which gives the main climatic impact. As input quantities for our radiative forcing calculations we used the aerosol size distribution and the spectral behaviour of optical thickness as derived from our measurements in March 1992. On top of the mountain Zugspitze in the Alps we have measured an optical thickness value of 0,12 at 550 nm wavelength for the contribution of Pinatubo aerosol. To get a realistic global value of radiative forcing, the optical thickness at 550 nm wavelength was set to 0,13 according to the mean global value of aerosol optical thickness given by Stowe et al., 1992. We derived a value of-5.7 Wm"2 for the shortwave radiative forcing, which is somewhat larger than the -4.3 Wm2 given by Stowe et al. A comparison with the observed decrease in global radiation of 4% at the high mountain station Jungfraujoch, Switzerland (Blumthaler& Ambach, 1994) during 1992 gives a good coincidence. 2 -
0.10
m m
Zugspitze
5 -
-+-
m
3
o
s~,9o M,~92
-
Jtm92 2
--
N o v 93 May94
Nov94 0.01 3
I
f
I
I
l'
I
4
5
6
7
8
9
J 1.00
wavelength (l~m)
Figure 1: Spectral optical thickness of Pinatubo aerosol measured on top of mountain Zugspitze. References Blumthaler, M. & W. Ambach, 1994: Changes in solar radiation fluxes after the Pinatubo eruption. Tellus, 46B, 76-78. McCormick, M.P. & R.E. Veiga, 1992: SAGE II measurements of early Pinatubo aerosols. Geophys. Res. Lett., 19, 155-158. Stowe, L.L., R.M. Carey, P.P. Pellegrino, 1992: Monitoring the Mt. Pinatubo aerosol layer with NOAA/11 AVHRR data. Geophys. Res. Lett., 19, 159-162. Wendisch, M. & W. von Hoyningen-Huene, 1992: Optically equivalent refractive index of atmospheric aerosol particles. Beitr. Phys. Atm., 65, 293-309. Wendisch, M. & W. yon Hoyningen-Huene, 1994: Possibility of refractive index determination of atmospheric aerosol particles by ground based solar extinction and scattering measurements. Atmospheric Environment, 28, 785-795.