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Evaluations of marine aerosol radiative forcing in the Southern coastal region of Portugal
.Z Aerosol Sci. Vol. 3 I, Suppl. I. pp $281 S282, 2000
Pergamon www.elsevier.com/locate/jaerosci
Poster Session I. Atmospheric aerosols: Optical properties EVALUATIONS OF MARINE AEROSOL RADIATIVE FORCING IN THE SOUTHERN COASTAL REGION OF PORTUGAL C. TOMASI, V, VITALE, A. CACCIARI, A. LUPI, S. MARANI Institute of Atmospheric and Oceanic Sciences - ISAO - C.N.R. Via Gobetti 101, 140129 Bologna (Italy)
Keywords: aerosol particle size-distributions, aerosol single scattering albedo, aerosol radiative forcing.
INTRODUCTION Direct solar irradiance measurements were carried out at 13 window-wavelengths from 0.4 to 3.7 ~tm using the 1R-RAD sun-radiometer at Sagres, southern Portugal, during the CLEARCOLUMN (ACE 2) experiment of June and July 1997 (Vitale et al., 2000). Examining more than 2100 spectral series of IRRAD output voltages taken on 21 clear-sky days, the corresponding spectral series of aerosol optical depth ~(2,) were determined. Three of the measurement days (June 19 and 23 and July 5, 1997) were characterised by a prevailing presence of marine aerosols in the atmospheric vertical column. The microphysical parameters and radiative properties of the columnar particulate matter were inferred from the solar radiation measurements carried out on those days, from which realistic evaluations of the marine aerosol radiative Iorcing were obtained.
RADIATIVE PROPERTIES OF MARINE AEROSOLS The 192 spectral series of N~,) measured on the three selected days were examined using the King (1982) inversion method to retrieve the size-distribution curves of the columnar particle number over the 0.1-10 ~tm radius range, using the mean daily values of real part n and imaginary part k of the particulate refractive index in the visible and near-infrared spectral range, as determined from the measurements of sky-brightness angular dependence in almucantar taken simultaneously by von Hoyningen-Huene (private communication). The retrieval procedure was applied assuming n = 1.43 and k = 0.003 at all the wavelengths on June 19, and n = 1.47 and k = 0.005 on both June 23 and July 5. For all the sizedistribution curves of the columnar total particle number N, we calculated the corresponding sizedistributions of the columnar particle volume V. The size-distribution curves of both columnar parameters N and V were found to present clearly bimodal characteristics in most cases, due to the presence of the accumulation mode in the radius range below 0.3 gm and of the coarse particle mode throughout the upper range. Moreover, we calculated the values of n and k as functions of wavelength within the 0.4-3.7 gm range, using the particulate extinction models defined by the 6S computer code (Vermote et aI., 1994) for the oceanic and water-soluble components and varying the mass concentrations of both components to obtain the mean daily values of n and k assumed above. We then determined the spectral curves of aerosol single scattering albedo co tbr all the 192 particle size-distributions, using in one case the values of n and k constant with wavelength and in the other the values of n and k varying with wavelength. The timepatterns of parameter co obtained for all the particle number size-distributions and for both spectral models of n and k are shown in Fig. l(a), presenting values ranging between 0.91 and 0.97 in the first case and between 0.88 and 0.96 in the second. In order to give the measure of the aerosol extinction "along the vertical atmsopheric path, the time-patterns of aerosol optical depth gg550 nm), as determined from the sun-radiometric measurements taken on the three selected days, are shown in Fig. 1(b).
$281
$282
Abstracts of the 2000 European Aerosol (ZOlll~:rttllC¢
CALCULATION OF AEROSOL RADIATIVE FORCING AND CONCLUSIONS For these estimates of 09 and ~(~), the change AFI" in the outgoing flux of solar radiation (produced by the above columnar contents of aerosol particles over the solid angle 2n) was calculated tot the corresponding solar zenith angles 0 and for the spectral albedo characteristics of a clear water (sea) surface, using the 6S computer code (Vermote et al,, 1994). The time-patterns of AF~ are presented in Fig. l(c), showing that the radiative Iorcing term zlF'l" does not change significantly passing I?om one spectral model of n and k to the other. The results also suggest that a close dependence of AFT on both fi(2) and co exists. Correspondingly, the radiative forcing per unit aerosol optical depth 5(550 nm) was evaluated to vary mainly between 50 and 100 W/m e, slowly decreasing as 0 increases from 15 ° to 80 °. In addition, using the present estimates of AFI" together with the corresponding vaiues of V obtained as integrals of the above volume particle size-distributions and values of marine particulate matter density ranging between 1.97 and 2.13 g/cm 3 (H~tnel, 1976), we lbund values of the instantaneous direct aerosol radiative forcing per unit columnar particulate mass varying between 40 and 80 W g-~, without important variations in the range of 0 observed on those days.
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8 6 2
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8
10
12
14
16
18
20
Time (hours)
Figure 1.- Time-patterns of (a) aerosol single scattering albedo co, (b) aerosol optical depth 6(550 nm), and (c) change AFI" in the outgoing flux of solar radiation, the quantities co and AF']"being calculated using the 6S computer code (Vermote et aL, 1994) for the spectral measurements of 6(~) found on June 19 (circles), June 23 (squares) and July 5 (triangles), 1997, and for both cases with n and k constant with wavelength (open symbols) and with n and k varying with wavelength (solid symbols). REFERENCES H'~net, G. (1976). The properties of atmospheric aerosol particles as functions of the relative humidity at thermodynamic equilibrium with the surrounding moist air, Adv. Geophys. 19, 73. King, M.D. (1982). Sensitivity of constrained linear inversions to the selection of the Lagrange multiplier, J. Atmos. Sci. 39, 1356. Vermote, E., Tanr6, D., Deuz~, J.L., Herman, M. and J.J. Morcrette (1994). 6S User Guide Version, (April 18, 1994. Universit6 de Lille, France). Vitale, V., Tomasi, C., Bonaf~, U., Marani, S., Lupi, A.and A. Cacciari (2000), Spectral measurements of aerosol particle extinction in the 0.4-3.7 ~tm wavelength range, performed at Sagres with the IRRAD sun-radiometer, Tellus B (in press).
Pergamon www.elsevier.com/locate/jaerosci
Poster Session I. Atmospheric aerosols: Optical properties EVALUATIONS OF MARINE AEROSOL RADIATIVE FORCING IN THE SOUTHERN COASTAL REGION OF PORTUGAL C. TOMASI, V, VITALE, A. CACCIARI, A. LUPI, S. MARANI Institute of Atmospheric and Oceanic Sciences - ISAO - C.N.R. Via Gobetti 101, 140129 Bologna (Italy)
Keywords: aerosol particle size-distributions, aerosol single scattering albedo, aerosol radiative forcing.
INTRODUCTION Direct solar irradiance measurements were carried out at 13 window-wavelengths from 0.4 to 3.7 ~tm using the 1R-RAD sun-radiometer at Sagres, southern Portugal, during the CLEARCOLUMN (ACE 2) experiment of June and July 1997 (Vitale et al., 2000). Examining more than 2100 spectral series of IRRAD output voltages taken on 21 clear-sky days, the corresponding spectral series of aerosol optical depth ~(2,) were determined. Three of the measurement days (June 19 and 23 and July 5, 1997) were characterised by a prevailing presence of marine aerosols in the atmospheric vertical column. The microphysical parameters and radiative properties of the columnar particulate matter were inferred from the solar radiation measurements carried out on those days, from which realistic evaluations of the marine aerosol radiative Iorcing were obtained.
RADIATIVE PROPERTIES OF MARINE AEROSOLS The 192 spectral series of N~,) measured on the three selected days were examined using the King (1982) inversion method to retrieve the size-distribution curves of the columnar particle number over the 0.1-10 ~tm radius range, using the mean daily values of real part n and imaginary part k of the particulate refractive index in the visible and near-infrared spectral range, as determined from the measurements of sky-brightness angular dependence in almucantar taken simultaneously by von Hoyningen-Huene (private communication). The retrieval procedure was applied assuming n = 1.43 and k = 0.003 at all the wavelengths on June 19, and n = 1.47 and k = 0.005 on both June 23 and July 5. For all the sizedistribution curves of the columnar total particle number N, we calculated the corresponding sizedistributions of the columnar particle volume V. The size-distribution curves of both columnar parameters N and V were found to present clearly bimodal characteristics in most cases, due to the presence of the accumulation mode in the radius range below 0.3 gm and of the coarse particle mode throughout the upper range. Moreover, we calculated the values of n and k as functions of wavelength within the 0.4-3.7 gm range, using the particulate extinction models defined by the 6S computer code (Vermote et aI., 1994) for the oceanic and water-soluble components and varying the mass concentrations of both components to obtain the mean daily values of n and k assumed above. We then determined the spectral curves of aerosol single scattering albedo co tbr all the 192 particle size-distributions, using in one case the values of n and k constant with wavelength and in the other the values of n and k varying with wavelength. The timepatterns of parameter co obtained for all the particle number size-distributions and for both spectral models of n and k are shown in Fig. l(a), presenting values ranging between 0.91 and 0.97 in the first case and between 0.88 and 0.96 in the second. In order to give the measure of the aerosol extinction "along the vertical atmsopheric path, the time-patterns of aerosol optical depth gg550 nm), as determined from the sun-radiometric measurements taken on the three selected days, are shown in Fig. 1(b).
$281
$282
Abstracts of the 2000 European Aerosol (ZOlll~:rttllC¢
CALCULATION OF AEROSOL RADIATIVE FORCING AND CONCLUSIONS For these estimates of 09 and ~(~), the change AFI" in the outgoing flux of solar radiation (produced by the above columnar contents of aerosol particles over the solid angle 2n) was calculated tot the corresponding solar zenith angles 0 and for the spectral albedo characteristics of a clear water (sea) surface, using the 6S computer code (Vermote et al,, 1994). The time-patterns of AF~ are presented in Fig. l(c), showing that the radiative Iorcing term zlF'l" does not change significantly passing I?om one spectral model of n and k to the other. The results also suggest that a close dependence of AFT on both fi(2) and co exists. Correspondingly, the radiative forcing per unit aerosol optical depth 5(550 nm) was evaluated to vary mainly between 50 and 100 W/m e, slowly decreasing as 0 increases from 15 ° to 80 °. In addition, using the present estimates of AFI" together with the corresponding vaiues of V obtained as integrals of the above volume particle size-distributions and values of marine particulate matter density ranging between 1.97 and 2.13 g/cm 3 (H~tnel, 1976), we lbund values of the instantaneous direct aerosol radiative forcing per unit columnar particulate mass varying between 40 and 80 W g-~, without important variations in the range of 0 observed on those days.
1.00 O "12
0.95
.,O
,,( 0.90
"~
0.2 0.1
I3_
C~
~
b)
0.0
8 6 2
0 6
8
10
12
14
16
18
20
Time (hours)
Figure 1.- Time-patterns of (a) aerosol single scattering albedo co, (b) aerosol optical depth 6(550 nm), and (c) change AFI" in the outgoing flux of solar radiation, the quantities co and AF']"being calculated using the 6S computer code (Vermote et aL, 1994) for the spectral measurements of 6(~) found on June 19 (circles), June 23 (squares) and July 5 (triangles), 1997, and for both cases with n and k constant with wavelength (open symbols) and with n and k varying with wavelength (solid symbols). REFERENCES H'~net, G. (1976). The properties of atmospheric aerosol particles as functions of the relative humidity at thermodynamic equilibrium with the surrounding moist air, Adv. Geophys. 19, 73. King, M.D. (1982). Sensitivity of constrained linear inversions to the selection of the Lagrange multiplier, J. Atmos. Sci. 39, 1356. Vermote, E., Tanr6, D., Deuz~, J.L., Herman, M. and J.J. Morcrette (1994). 6S User Guide Version, (April 18, 1994. Universit6 de Lille, France). Vitale, V., Tomasi, C., Bonaf~, U., Marani, S., Lupi, A.and A. Cacciari (2000), Spectral measurements of aerosol particle extinction in the 0.4-3.7 ~tm wavelength range, performed at Sagres with the IRRAD sun-radiometer, Tellus B (in press).