Lewis, S. R.

Mason, J. P.

Patel, M. R.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsEffects of aerosols on the observed irradiance from theultraviolet to near-infrared at the surface of MarsConference or Workshop ItemHow to cite:Mason, J. P.; Patel, M. R. and Lewis, S. R. (2011). Effects of aerosols on the observed irradiance from theultraviolet to near-infrared at the surface of Mars. In: Fourth International Workshop On the Mars Atmosphere:Modelling and Observations, 8-11 Feb 2011, Paris, France.For guidance on citations see FAQs.c© 2011 The AuthorsVersion: Accepted ManuscriptLink(s) to article on publisher’s website:http://www-mars.lmd.jussieu.fr/paris2011/Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukEFFECTS OF AEROSOLS ON THE OBSERVED IRRADIANCE FROM THE ULTRAVIOLET TO NEAR-INFRARED AT THE SURFACE OF MARS J. P. Mason , M.R Patel, PSSRI, The Open University, Milton Keynes, UK (j.p.mason@open.ac.uk), S.R Lew-is, Department of Physics & Astronomy, The Open University, Milton Keynes, UK   Introduction:    Aeolian dust is the major climate driver compo-nent of the Martian atmosphere with variations in dust loading having significant effect on the atmos-pheric dynamics [1, 2]. So far there have been sig-nificant studies on Martian aeolian dust using obser-vations from landers [3, 4], orbiting spacecraft [5, 6] and measurements from the Earth [7]. However, currently lacking is in situ information and charac-terization of the irradiance encountered at the surface of Mars in the UV, visible and NIR wavelengths and how different aerosols species influence the ob-served spectrum at the surface of Mars.    In this work, a radiative transfer model is used to simulate the irradiance observed at the surface of Mars in the 200 – 1100 nm region for different load-ings of various aerosols species. Of interest is the modification of the spectrum and determining what information can be obtained about the aerosol spe-cies’ single scattering properties. The aerosols con-sidered in this analysis are, dust, water ice, CO2 ice and water vapour covering the physical phenomenon of the background dust, dust clouds [8, 9], H2O ice clouds [9, 10] , CO2 ice clouds [11, 12] and near-surface morning fogs [13, 14].   Current models to determine the single scatter-ing properties of Martian atmospheric dust from spectrum observations assume a uniform dust com-ponent within the modeled atmosphere (i.e. constant optical properties as a function of pressure). In real-ity this is not the case, with smaller particles being more prevalent in the upper atmosphere and larger particles being present closer to the surface. Differ-ences in particle size will affect the single scattering properties of the dust, namely the extinction effi-ciency, single scattering albedo and asymmetry pa-rameter. In this work the accuracy of the uniform dust assumption is investigated.  Radiative transfer models:   Two radiative transfer models were developed for this study into Martian aerosols. Both models are modified versions of the radiative transfer model, [15, 16, 17], which uses the delta-Eddington ap-proximation to simulate the passage of solar radia-tion through the Martian atmosphere in the 200 -1100 nm region. Within the model the atmosphere is divided into 70 separate layers corresponding to an altitude range of 0 to 140 km. The altitude resolution is a definable parameter, allowing increased altitude resolution for the surface fog and CO2 ice cloud in-vestigations. For the aeolian dust analysis the Homogeneous Model (henceforth called HM) assumes a uniform dust component with the optical properties and parti-cle size constant with pressure. The Vertical Model (henceforth called VM) employs a vertically varying dust component which scales the effective radius of the dust size distribution with pressure.  The variation of effective radius as a function of altitude was taken from [18] and provides three dis-tributions for reff(z) for different values of the effec-tive variance which are held constant with altitude. The distributions only give values of reff(z) up to 24 km, for altitudes greater than 24 km the effective radius is scaled with pressure until a particle size of 0.2 µm is reached, after which the dust component is assumed uniform. Figure 1 shows the resultant effec-tive radius as a function of pressure and altitude.    Figure 1: Dependence of the effective radius on pressure (black lines) and altitude (blue lines) for the three distributions given by [18].  The dust particle radii follow a log-normal distri-bution, though this is somewhat arbitrary since the effective radius and variance are used to describe the first and second moments of the distribution [19]. The single scattering dust properties are deter-mined within the models by Mie scattering theory for spherical particles [19] or T-matrix code to simu-late random orientated non-spherical particles. The ice cloud particle single scattering properties are calculated using a Mie scattering algorithm for coated spheres [19]. Martian Dust Investigations: Irradiance sensitivity to dust properties.  The to-tal irradiance for different particle size distributions was produced using the HM to determine the sensi-tivity of the total irradiance to changes in (1) the effective radius and (2) the effective variance. For the first case the effective variance was held constant and the effective radius was given values of 1.0, 1.5, 2.0, 3.0 and 5.0 µm. For the second case the reff was held constant and veff was given the values 0.1, 0.2,   0.4, 0.8, and 1.0. Figure 2 shows the changes in the total irradiance for each case. 00.20.40.60.81200 400 600 800 1000Irradiance (Wm-2 nm-1 )Wavelength (nm)reff = 1.0 reff =1.5reff = 2.05 reff =3.0reff = 5.0  00.20.40.60.81200 400 600 800 1000Irradiance (Wm-2 nm-1 )Wavelength (nm)veff = 0.1 veff = 0.2veff = 0.4 veff = 0.8veff = 1.0 Figure 2: Changes in the total received irradiances for different values of (a) effective radius, (b) effec-tive variance.   As the effective radius increases there is a de-crease in the observed total irradiance from 450 nm to 1100 nm. This decrease is related to the scattering nature of the particles, as the particles become larger compared to the wavelength of interest they scatter more favorably in the forward direction thus reduc-ing the amount of diffuse flux received at the sur-face. Interestingly, the total irradiance at wave-lengths <450 nm is less sensitive to changes in the effective radius. This is explained by the reduced important of scattering at these wavelengths, were martian dust exhibits stronger absorption which is shown by the single scattering albedo and imaginary refractive index [20]. This result indicates that trying to determine the effective radius of the dust particle size distribution by attempting to fit a model spec-trum to an observed spectrum within the wavelength region 180 – 450 nm would be extremely difficult. Fortunately above 450 nm the differences in total irradiance for different particle size distributions can be clearly seen. While size distributions with an ef-fective radius value of 3.0 and 5.0 µm are extreme for suspended Martian dust, the difference in spectra between the more realistic values of reff (1.5 µm and 2.05 µm) is large enough in the wavelength region 450 to 800 nm to allow determination of different dust size distributions. For smaller particle sizes the difference in total irradiance becomes less obvious even at longer wavelengths, which reflects the re-duced importance of the particle size relative to wavelength in scattering processes at small size pa-rameters, (2piR⁄λ).  The sensitivity of the total observed irradiance to the dust particle’s effective radius implies that the uniform dust assumption could be introducing errors into the simulated irradiances by having a larger value of reff at higher altitudes than would be realisti-cally present as a result of sedimentation. As stated, the particle size affects its scattering properties, thus allowing reff to scale vertically with pressure would see smaller particle present at higher altitudes which will in turn have an impact on the received diffuse flux at the shorter wavelengths.     As illustrated in Figure 2 changes in the effec-tive variance has little effect on the total received irradiance, there is negligible difference in the total flux for particle distributions with variances of 0.2 and 0.1. As the effective variance increases the dif-ference with respect to the 0.1 case are greater, how-ever, even at extreme variances (0.8 and 1.0) these differences only amount to difference of 7% within the 200- 600 nm wavelength region. H2O ice clouds:  Previous investigations into Martian H2O ice clouds have revealed significant variation in the derived size distribution for the cloud particles with the effective radius ranging from 0.5 µm [14], 1- 2 µm [6] and 3- 4 µm [21]. It has been hypothesized that these variations are a result of po-tentially different cloud types. Pure ice particles. The first set of simulations makes the assumption a pure water ice particles ex-isting with the cloud structure. A range of size distri-butions with reff = 0.5, 1.0, 3.0 and 4.0 µm were used to cover the range of potential different cloud types and the optical depth of the cloud was varied from 0.05 to 0.67. The simulated spectra are normalised to 800 nm to help emphasize the differences. Figure 3a shows for a cloud of low optical depth the attenua-tion of the observed irradiance spectrum is small, for higher optical depths, Figure 3b, the differences are more noticeable especially 400 – 700 nm wavelength range.  00.20.40.60.811.21.41.61.8200 500 800 1100Irradiance (normalised to 800 nm)Wavelength (nm) (a) (b) (a) 00.20.40.60.811.21.41.61.8200 500 800 1100Irradiance (normalised to 800 nm)Wavelength (nm)reff = 0.5 reff = 1.0reff = 2.0 reff = 3.0reff = 4.0 Equivalent Dust ODBackground Figure 3: Attenuation of the total irradiance spectra for H2O cloud optical depths of (a) 0.1 and (b) 0.45. The Legend in (b) is applicable to (a).  If the cloud feature is an isolated incident the ra-tio of the irradiance spectrum prior to the cloud en-counter and during the encounter can be used to de-termine the transmission through the cloud, figure 4. 0.70.80.911.1200 500 800 1100Transmission (Cloud)Wavelength (nm)Reff = 0.5Reff = 1.0Reff = 3.0Reff = 4.0Equivalent Increase in Dust Opacity  Figure 4: The transmission through the cloud for optical depths of 0.1 (black lines) and 0.45 (grey lines).  For clouds of low optical depth (0.1), little change is seen in the observed irradiances. For in-creasing optical depth the attenuation caused by the H2O ice cloud is more noticeable especially for lar-ger particle sizes (3-4 µm). Comparing to an equiva-lent increase in the dust optical depth (solid black and gray lines in figure 4) the characteristic absorp-tion at bluer wavelength as a result of dust absorp-tion is significantly reduced; this is a result of in-creased scattering by the ice particles at these wave-lengths resulting in a higher diffuse flux. The in-crease in irradiance at redder wavelengths as a result of dust scattering is also reduced with little net in-crease observed in the presence of H2O ice.  This change in the attenuation of the incoming light could be used to indirectly infer the presence of a water ice cloud, however, as Figure 4 illustrates it would be difficult to deduce the particle size distribution and the optical properties of the cloud particles for low opacity clouds.  CO2 ice clouds:  The presence of CO2 ice clouds at altitude >50 km have been observed by both orbit-ing spacecraft [11, 22] and anders [12], however, the conclusions and estimates of the cloud particles properties are far from definitive. The formation of CO2 ice clouds is a result of atmospheric tempera-tures at high altitudes being low enough for to allow condensation of the CO2 atmosphere [12].    The same technique for modeling H2O ice cloud effects was used to model CO2 ice clouds. CO2 ice clouds, however, exist much higher in the Martian atmosphere (> 50 km) [11]. The cloud particles were chosen to have a size distribution with an effective radius of 0.2 [12], 1.0 and 1.5 µm [22] and an effec-tive variance of 0.1 and consist of pure CO2 [23]. The optical depth of the CO2 ice cloud was chosen to be 0.1. The vertical extent of the cloud was 10 km at an altitude of 70 km. The ratio of the background irradiance compared to the irradiance during a CO2 cloud event is shown in Figure 5. Also shown for comparison is the passage of a H2O cloud with the same optical depth and various size distributions and the case for an equivalent increase in the dust back-ground haze.  0.90.920.940.960.9811.021.041.06200 500 800 1100Transmission (Cloud)Wavelength (nm)Reff = 1.5Reff = 0.2Reff =1.0  Figure 5: Transmission through CO2 ice clouds (black lines), H2O ice clouds (gray lines, see figure 4 legend for details) and dust cloud (solid Gray line) for an optical depth of 0.1.  The attenuation caused by the CO2 clouds is small, but the H2O and CO2 spectral signatures are quite different. The CO2 clouds exhibit increased flux at all wavelengths > 350 nm. Below 350 nm there is a sharp decline in transmittance due to in-creased absorption by the CO2 ice particles. It should be noted that using Mie theory to calcu-late the single scattering properties of the CO2 ice particles is far from accurate and the ice particles are likely to be cubic in nature [24]. However, since the opacities of the clouds are small and have little im-pact on the observed irradiance at the surface of Mars, the errors introduced by using Mie theory should be small. Another method can be used to distinguish be-tween dust, water ice clouds and CO2 ice clouds, (b)   provided direct and diffuse fluxes can be determined. The ratio of the diffuse and direct flux for different optical depths and a particle size distribution of 1.0 µm is shown in Figure 6. 00.20.40.60.811.21.4200 400 600 800 1000Ratio (Diffuse/Direct) FluxWavelength (nm)CO2 Cloud CO2 CloudH2O cloud H2O cloudDust DustNo Cloud Figure 6: The diffuse / direct flux ratio for H2O, CO2 and dust clouds with an optical depth of 0.1 and 0.3. The effective radius was set to 1.0 µm for both cases.   The different cloud optical depths can be clearly seen as distinct offsets from the no cloud case. For low optical depths the observation of CO2 ice clouds is difficult to distinguish from H2O ice clouds with both showing very similar profiles, however,  sig-nificant differences are seen when compared to a dust cloud event which has a much weaker diffuse flux at wavelengths <500 nm resulting from greater absorption at these wavelengths. At higher optical depths the CO2 cloud departs from the H2O cloud profile showing a higher ratio as a result of a higher diffuse flux. The result of the analysis into CO2 ice clouds concludes that it would be extremely difficult to accurately determine the presence of CO2 ice clouds from attenuation of surface irradiance spectra using the diffuse / direct flux ratio due to the similar-ity of its profile to H2O ice clouds  Conclusions: The sensitivity of the total ob-served irradiance to the dust particle effective radius implies that the uniform dust assumption may intro-duce errors into the simulated irradiance. The resul-tant underestimation in the diffuse flux at shorter wavelengths as a result of large particles at higher altitudes could be of significant importance consider-ing the low attenuation caused by the H2O and CO2 clouds and methods used for their detection.  It has been shown that the ratio of the diffuse and direct fluxes can be used to infer the presence of H2O and CO2 clouds from the background dust com-ponent; however, as shown in Figure 6 it would be difficult differentiate between the two cloud types as they both exhibit very similar profiles.   Future Work: The VM will be used to investi-gate the accuracy of the uniform dust assumption by scaling the surface effective radius with altitude and comparing the simulated irradiance components to those produced by the HM.  A fitting routine has been developed to extract the dust optical properties from irradiance spectra and will be used to determine the effective radius as seen by the HM model from the VM output irradian-ces.  The studies into H2O and CO2 clouds will be re-peated using the VM and the observed irradiances and corresponding attenuation will be compared with the outputs produced by the HM using the retrieved size distribution from the fitting routine.  Acknowledgments:   This work is support by the UK Science and Technology Facilities Council (STFC). References:  [1] Haberle, R.M., et al., 1982, Icarus 50, 322-367. [2] Haberle, R.M., et al., 1993, J. Geophys. Res 98, 3093-3123. [3] Smith, P.H. and M. Lemmon, 1999, J. Geophys. Res 104, 8975-8985. [4] Lemmon, M.T., et al. 2004, American Association for the Advance-ment of Science. p. 1753-1756. [5] Smith, M.D., 2008, [6] Clancy, R.T., et al., 2003, J. Geophys. Res 108,  [7] Wolff, M.J., et al., 1999, J. Geophys. Res 104,  [8] Cantor, B.A., et al., 2001, J. Geophys. Res 106, 23653-23687. [9] Cantor, B., et al., 2002, J. Geophys. Res 107, 5014. [10] Clancy, R.T. and S.W. Lee, 1991, Icarus 93, 135-158. [11] Montmessin, F., et al., 2006, Icarus 183, 403-410. [12] Clancy, R.T. and B.J. Sandor, 1998, Geophysical Research Letters 25, 489-492. [13] Smith, P.H. and M. Lemmon, 1999, J. Geophys. Res 104, 8975–8985. [14] Pet-rova, E., et al., 1996, Plan. Sp. Sci. 44, 1163-1176. [15] Otter, S., 2010, Thesis, Open University [16] Patel, M.R., et al., 2002, Plan. Sp. Sci. 50, 915-927. [17] Patel, M.R., et al., 2004, Icarus 168, 93-115. [18] Chassefière, E., et al., 1995, J. Geophys. Res 100, 5525-5539. [19] Bohren, C.F. and D.R. Huff-man, Absorption and Scattering of Light by Small Particles. 2004. [20] Ockert-Bell, M.E., et al., 1997, J. Geophys. Res 102, 9039-9050. [21] Wolff, M.J. and R.T. Clancy, 2003, J. Geophys. Res 108, 5097. [22] Montmessin, F., et al., 2007, J. Geophys. Res 112, E11S90. [23] Warren, S.G., 1986, Applied op-tics 25, 2650-2674. [24] Wergin, W.P., et al., 2000, Microscopy and Microanalysis-New York 3, 1235-1236.   

Effects of aerosols on the observed irradiance from the ultraviolet to near-infrared at the surface of Mars

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Effects of aerosols on the observed irradiance from the ultraviolet to near-infrared at the surface of Mars

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