Weighting functions Box AMFs for Limb measurements of
Weighting functions (Box AMFs) for Limb measurements of stratospheric trace species using 3 D Monte Carlo RTM Christoph v. Friedeburg, A. Butz, F. Weidner, S. Sanghavi, K. Pfeilsticker, U. Platt and T. Wagner • Box AMF and profile retrieval • 3 D Monte Carlo RTM „AMFTRAC“ • AMF investigation example • Balloon-borne limb geometry • Outlook cvfriede@iup. uni-heidelberg. de IUP University of Heidelberg
Box AMF and profile retrieval • SCD and AMF do not tell us where along the light path the trace gas is located • But this is what we’d like to know. • Discretization of atmosphere into boxes i=1, . . , n • SCD box-wise • AMF box-wise: A(i) • Weighting Function • S(i) = c(i) * d(i) • V(i) = c(i) * v(i) • S(i) = V(i) * A(i) Altd [m] c(i), σ(c, i) d(i) v(i) • SCD = Σ S(i) • SCD = Σ c(i) * v(i)* A (i) • => into equation system C [cm-3]
Box AMF and profile retrieval Box AMF defined as: • sum over all intensity having traversed the layer/cell • divided by total intensity received by detector Altd [m] c(i), σ(c, i) d(i) • divided by layer‘s/cell‘s v(i) vertical extension C [cm-3]
Box AMF and profile retrieval • Multiple scattering increases retrieval difficulty since • geometrical approximations/estimations not valid and misleading • AMF and A(i) must be modelled with RTM • Behaviour of A(i) with relevant parameters must be investigated & understood θ ε
3 D Monte Carlo RTM „AMFTRAC“ (working title) features e. g. v. Friedeburg EGS 2002 • spherical 3 D geometry • supports arbitrary platform positions and viewing geometries ai • full MS by Rayleigh, aerosols, clouds, albedo • refraction, polarization and solar CLD (limb) principle vv, i di • backward Monte Carlo technique • N photon launched out of telescope • random numbers, scatt. centre ND & c/s govern light path => establish path sun->detector • molecular absorption calculated analytically • AMF computed from modelled av. intensity with/without absorber detector
3 D Monte Carlo RTM „AMFTRAC“ output • SCDs, SODs, AMFs, Box AMFs (A(i)) for a specified set of boxes/layers • abs. radiances • geometrical path length, traversed air column, O 4 • number of Rayleigh, Mie and albedo scattering events • altitudes of first and last scattering event, distance detector-last scattering event • entry angle of light into atmosphere, first scattering angle • Solar CLD effect parameter, polarization (under testing) • parameters as intensity weighted means • errors as intensity weighted std dev.
3 D Monte Carlo RTM „AMFTRAC“ radiance validation • In addition to validation against AMF by other RTMs: • validation against measurements with calibrated spectro-radiometer
AMF investigation example • λ = 352 nm ground based MAX DOAS scenario • atmosphere: 1 km vertical discret. , 0 -70 km • ε = 2°, 5°, 10°, 20°, 45°, 90°; • azimuth α to sun 90°, aperture 0. 1° • albedo values 0%, 30%, 50%, 70% • standard aerosol scenario • Br. O near ground: 3 profiles investigation of total AMFs in relation to scattering parameters
AMF investigation example Br. O AMF P 1, P 2: AMF highest for 2° elev. P 3: ?
Results: AMF->f(Albedo), O 4 AMF • P 3: AMF increases with albedo, but behaviour pertains: • AMF for smallest elevations not highest • same effect for O 4 - looks like P 1 & 2, but higher proportion (~3/4) located above 1 km.
AMF investigation example Number of scatterings • Single scattering approx. („ 1/sin(τ)“, „ 1/cos(θ)“) heavily limited • similar investigations for higher wavelengths useful
AMF investigation example Last Scattering Altitude LSA • LSA for small elevations between 300 and 400 m =>decreases light path within lowest boxes as comp. to 1/sin(ε)
AMF investigation example LSA -> AMF(i) LSA • LSA for small elevations < 1 km • A(i) for boxes above 1 km decrease for low elevations
Balloon-borne limb geometry • relevant to SCIAVAL balloon operations • SCIAMACHY limb mode SZA (at altd 0 below instrument position): 70° atmosph. discret. 1 km Variation of • altitude • elevation angle • azimuth angle • aperture angle • cloud cover
Balloon-borne limb geometry Error investigation Box AMF error’s absolute value depends on: ·Number of paths ·layer’s/grid cell’s shape & extension ·layer’s/grid cell’s distances from the instrument ·influence of multiple scattering on the way to & within the layer ·incl. albedo, clouds · 2000 PU was used for the calculations to follow.
Balloon-borne limb geometry Altitude variation Line Of Sight Parameters: -4° elevation, 90° azimuth, 0. 5° aperture, 30% albedo ·above instrument: Box AMF governed by SZA ·below instrument: Box AMF increases due to LOS geometry, ·at altd<25 km LOS hits ground ·below altitude of highest Box AMF: fall-off depends on aperture (see aperture var. ) ·near ground AMFs dependent on multiple scattering
Balloon-borne limb geometry Altitude variation: scattering parameters ·NRS (MS importance) decreases with increasing altitude ·Last Scattering Distance (LSD) affected by MS ·LSA complies well with altitude of highest Box AMF
Balloon-borne limb geometry LOS Parameters: 90° azimuth, 0. 5° aperture, altitudes 10 and 30 km Elevation variation ·strong variation in sensitivity for tangent altitude ·tangent altitude moves upwards ·important for limb scanning geometry - total Box AMF as weighted average ·below tangent altitude Box AMF largely unaffected
Balloon-borne limb geometry LOS Parameters: 90° azimuth, 0. 5° aperture, altitudes 10 and 30 km Elevation variation ·strong variation in sensitivity for tangent altitude ·tangent altitude moves upwards ·important for limb scanning geometry - total Box AMF as weighted average ·below tangent altitude Box AMF largely unaffected
Balloon-borne limb geometry Azimuth variation LOS Parameters: -4° elevation, 90° azimuth, 0. 5° aperture, altitudes 10 and 30 km ·impact small as compared to e. g. elevation influence ·for az. 90° Box-AMF largest in tangent alt, above for az 180° “sun beam” has to travel longer distance to reach LOS intersection point
Balloon-borne limb geometry Aperture variation LOS Parameters: -4° elevation, 90° azimuth, 0. 5° aperture, altitudes 10 km ·fall-off below tangent altd influenced by aperture for geometrical reasons ·for ap. angles <1° effect small but: ·depends on chosen discretization ·changing elevation (scanning) equals a higher effective ap. angle
Balloon-borne limb geometry Cloud cover variation LOS Parameters: -4° elevation, 90° azimuth, 0. 5° aperture, altitudes 10 and 30 km ·cloud cover: · 1. grid cell filled with Mie particles - high CPU time · 2. layer with altitude, coverage, albedo, transmission ·multiple layers, vertical cloud surfaces easy to implement ·accuracy depends on cloud effects impact on measurement ·cloud layer altitude 5 km albedo 80%, transmission zero
Balloon-borne limb geometry Cloud cover variation: O 4, radiance LOS Parameters: -4° elevation, 90° azimuth, 0. 5° aperture, altitude 10 km ·radiance: smooth increase with cloud cover ·O 4: decrease due to lower troposphere shielding
Conclusion/Outlook • AMFTRAC capable of handling limb geometry in relevant LOS parameters • output parameters help understanding AMF‘s and A(i)‘s behaviour quantitatively • Investigation of use of polarization and CLD effects • Implementation of realistic clouds and aerosols • Inclusion of basic LES-based retrieval module
MAX-DOAS, AMF and profile retrieval • DOAS: Differential Optical Absorption Spectroscopy • Measured quantity: optical density τ of trace gases investigated • integrated over light path • σ(λ) absorption cross section c(s) concentration • along “slant” light path : Slant Column Density SCD = τ(λ) / σ(λ) • along vertical path from location: Vertical Column Density VCD • related by Air Mass Factor AMF • VCD=SCD / AMF • Measure of sensitivity for the trace gas profile
MAX-DOAS, AMF and profile retrieval • Solar light enters atmosphere on straight path • gets scattered by e. g. Rayleigh, Mie, albedo • enters telescope => AMFs depend on: • Solar Zenith Angle (SZA θ) and Solar Azimuth Angle • scattering centre number density, cross section, phase fct. • elevation ε, aperture angle, . . . θ ε
Investigation: Influence of aerosol how to retrieve aerosol data - radiance? • radiance series R(ε) : Spectrographs not usually absolutely calibrated • d. R/dε not significantly different to allow for concl. on aerosol
Investigation: Influence of aerosol LSA • without aerosol impact effect present, but much weaker • aerosol scenario largely governs AMF -> f(ε) • use of std. scen. Risky => need hard data on local aerosol
Investigation: Influence of aerosol albedo in many cases known; aerosol load not. investigation on AMF->f(aerosol) for albedo 30%
Investigation: Influence of aerosol how to retrieve aerosol data - O 4? • But O 4 -AMF->f(ε) signif. tly different for large aerosol ld. differences • parametrize the O 4 -AMF(ε) behaviour as f(aerosol ext. coeff. ) => scale aerosol ext. coeff. • uncertainties: effect of phase function ?
MAX model scenario • aerosols: • continental scenario (F. Hendrick, IASB, pers. comm. ) • ext. coeff. Value for 0 km varied for investigation
Monte Carlo Approach for MS • calculation of distance d to next voxel boundary • extinctors (Rayleigh, Mie particles) yield probability p(x) for free passage up to x 1. p(d)=p 0 prob. of unscattered passage along d 2. map random number p‘ to x by the inverse of function p(x): 3. determines location of scattering event [0, d] 4. use a second random number to decide between scatterers according to the relative probabilities d p‘ 1 p 0 X d
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