14-16 April 2003International Limb Workshop, Bremen On two potential sources of systematic error in retrievals using limb-scattered sunlight Chris A. McLinden (1) and Christopher E. Sioris (2) (1) Meteorological Service of Canada, Toronto, Ontario, Canada (2) Atomic and Molecular Physics Division, Harvard- Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA

14-16 April 2003International Limb Workshop, Bremen Outline 1.Allowing for a varying atmosphere along the line-of-sight (LOS) when performing retrievals 2.Systematic errors in plane-parallel models, including so-called “pseudo-spherical” limb scattering models.

14-16 April 2003International Limb Workshop, Bremen Varying atmosphere along the LOS (1)  Perform multiple plane-parallel calculations, each one for a particular atmosphere & SZA along the LOS  From these, calculate LOS sources function – integrate to obtain limb radiance  Using vector RT model of McLinden et al. (Can. J. Phys., 80, , 2002)

14-16 April 2003International Limb Workshop, Bremen Varying atmosphere along the LOS (2) - in practice, slices narrower near tangent point OS or SCIA N 1 2

14-16 April 2003International Limb Workshop, Bremen Varying atmosphere along the LOS (3)  Want to estimate magnitude of errors when atmosphere is not homogeneous along LOS, but assumed to be in retrieval (‘2D’ forward, ‘1D’ inversion)  If species diurnal, can a ‘2D’ retrieval improve result for scans near terminator and SZA variations along LOS ?

14-16 April 2003International Limb Workshop, Bremen Varying atmosphere along the LOS (4)

14-16 April 2003International Limb Workshop, Bremen Varying atmosphere along the LOS (4) - Flittner algorithm used -‘2D’ forward calculation, ‘1D’ inversion

14-16 April 2003International Limb Workshop, Bremen Varying atmosphere along the LOS (5)

14-16 April 2003International Limb Workshop, Bremen Varying atmosphere along the LOS (6) -Sioris NO 2 inversion algorithm used -Uses an iterative onion peel technique SZA=27º, dAZ=92º; Aug., 25ºN, albedo=0

14-16 April 2003International Limb Workshop, Bremen Varying atmosphere along the LOS (7) SZA=27º, dAZ=92º; Aug., 25ºN, albedo=0

14-16 April 2003International Limb Workshop, Bremen Varying atmosphere along the LOS (8) SZA=27º, dAZ=92º; Aug., 25ºN, albedo=0

14-16 April 2003International Limb Workshop, Bremen Varying atmosphere along the LOS (9) Comparison of ‘1D’ and ‘2D’ OSIRIS inversions with HALOE coincidence LOS scaling factors from photochemical box model 4 June 2002 lat=4ºN, lon=55ºW

14-16 April 2003International Limb Workshop, Bremen Systematic Errors in “Pseudo- Spherical” Limb Radiances (1) -Comparison of several “pseudo-spherical” (PS) radiative transfer model calculations with 3-D, spherical Monte-Carlo (MC) model calculations show systematic errors -These errors increase with (a) tangent height and (b) surface albedo

14-16 April 2003International Limb Workshop, Bremen Systematic Errors in “Pseudo- Spherical” Limb Radiances (2) SZA=39º; albedo=0.95

14-16 April 2003International Limb Workshop, Bremen Systematic Errors in “Pseudo- Spherical” Limb Radiances (3) -PS calculations based on plane-parallel source function & integrated through spherical-shell atmosphere -These errors are driven by an overestimation of the surface reflected component – either the surface proper or the effective Rayleigh “surface”

14-16 April 2003International Limb Workshop, Bremen Systematic Errors in “Pseudo- Spherical” Limb Radiances (4) -root of problem is how much of the surface a point in the atmosphere “sees”: -PP atmosphere: point sees 2  [str] -spherical atmosphere: point sees <2  [str]; this decreases with altitude

14-16 April 2003International Limb Workshop, Bremen Systematic Errors in “Pseudo- Spherical” Limb Radiances (5) Point under consideration Direct Surface reflected

14-16 April 2003International Limb Workshop, Bremen Systematic Errors in “Pseudo- Spherical” Limb Radiances (6) - try simple, near-transparent atmosphere correction factor using ratio of PS to spherical solid angles Where is cos(SZA) and is surface albedo

14-16 April 2003International Limb Workshop, Bremen Systematic Errors in “Pseudo- Spherical” Limb Radiances (7) 800 nm SZA=39º

14-16 April 2003International Limb Workshop, Bremen Systematic Errors in “Pseudo- Spherical” Limb Radiances (8) -Trace gas retrievals should be minimally impacted since there will be cancellation/removal of this effect - e.g., the Flittner radiance-ratio method there will be a large degree of cancellation due to form of retrieval vector; - in DOAS it will not cancel but it would be included or removed in closure polynomial or high pass filter

14-16 April 2003International Limb Workshop, Bremen Systematic Errors in “Pseudo- Spherical” Limb Radiances (9) -Aerosol retrievals may suffer systematic errors if the retrieval involves a model- measurement matching of radiances -If aerosol signal small, the PP errors can lead to amplified aerosol errors; unclear how “optimal”-type solutions would handle this

14-16 April 2003International Limb Workshop, Bremen Systematic Errors in “Pseudo- Spherical” Limb Radiances (10) -If aerosol signal is: S = I(measure) – I(model w/o aerosol) -But P-P aerosol signal is: S PP = I(measure) – I(model w/o aerosol) / c Where c=

14-16 April 2003International Limb Workshop, Bremen Systematic Errors in “Pseudo- Spherical” Limb Radiances

14-16 April 2003International Limb Workshop, Bremen Systematic Errors in “Pseudo- Spherical” Limb Radiances (12)

14-16 April 2003International Limb Workshop, Bremen Systematic Errors in “Pseudo- Spherical” Limb Radiances (13)

14-16 April 2003International Limb Workshop, Bremen Conclusions & Future Work -systematic PS errors may be important in OS aerosol retrievals + other RT applications including photochemistry (& other planetary atmospheres) -attempt to obtain “universal” correction valid at all wavelengths, tangent heights, albedo

14-16 April 2003International Limb Workshop, Bremen Systematic Errors in “Pseudo- Spherical” Limb Radiances (14)