Correction of atmospheric scattering effects in space-based observations of carbon dioxide: Model study of desert dust aerosol

https://doi.org/10.1016/j.jqsrt.2008年02月01日2 Get rights and content

Abstract

We describe an original methodology for CO2 retrievals using space-based measurements of reflected sunlight spectra. The effects of optical-path modification by aerosols were considered in terms of photon path-length statistics. First, the general approach was verified using a representative set of photon trajectories produced by the Monte Carlo technique. This method enabled accurate consideration of optical-path modification by aerosols and was effective in CO2 retrievals if aerosol optical properties were assumed. The next approach involved a limited number of parameters that describe the photon path-length distribution function (PPDF) and which were retrieved simultaneously with the CO2 amount. This approach was efficient under conditions of strong path modification by desert dust aerosol. The retrieval procedure included the following: estimation of PPDF parameters from radiance spectra in the O2 A-band; the necessary correction to use these estimated parameters in the 1.6-μm band; and, finally, CO2 retrievals from the 1.6-μm band. The procedure was verified by numerical simulations using an independent radiative transfer approach to produce radiance spectra expected for the Greenhouse Gases Observing Satellite (GOSAT) sensor.

Section snippets

1. Introduction

Space-based measurements of atmospheric carbon dioxide (CO2) are crucial for better understanding and predicting the global carbon cycle. In particular, column-averaged amounts of CO2 retrieved from reflected sunlight in the near-infrared region should help reduce current uncertainties in the spatial distribution and temporal variability of CO2 sources and sinks [1], [2]. For spaced-based measurements to have an advantage over existing ground-based measurements, precision of 2.5 ppmv (better

Desert dust aerosol optical model

The aerosol optical model was based on data from the Aerosol Robotic Network (AERONET) for desert sites [15]. In particular, the bimodal particle volume size distribution was used to produce single light scattering characteristics in the radiative transfer modelling, as follows: dV(r)dlnr=∑i=12CV,i2πσiexp[-(lnr-lnrV,i)22σi2].The numerical simulations presented here were performed for the following values of median radius rV,i, standard deviation σi, and relative fraction for fine (f) and coarse

Direct Monte Carlo PPDF simulation

Correction of the aerosol effects within the PPDF approach requires determination of the function P(L1,L2, ..., LM). Formally, this function is equivalent to the representative set of the trajectories for N photons that contribute to the measured radiance: {Lmn}, n=1,2, ..., N; m=1,2, ..., M, Teff=1EN∑n=1Nenexp(-∑m=1MLmnτmhm),where τm and hm are the gaseous optical and geometrical thicknesses of layer m, respectively; Lmn is path length of the nth photon within the mth atmospheric layer; EN=∑n=1Nen, where e

Estimation of the PPDF parameters from the O2 A-band

The PPDF parameters can be determined from spectral channels outside of the target gas absorption bands [13]. In the case of atmosphere with cirrus cloud, important information on the PPDF parameters comes from the H2O-saturated area of the 2.0-μm band. However, this spectral region is not suitable for assessment of path-length modification due to near-surface aerosol effects because the measured radiance in this case depends on the water vapour content, which is generally unknown. Another

Summary and conclusions

In this paper, we have presented an original methodology for atmospheric scattering correction of CO2 retrievals from reflected sunlight spectra. The approach considers aerosol-scattering effects in terms of photon path-length statistics. Practical implementation of the methodology has been demonstrated under conditions of strong optical-path modification due to desert dust aerosol.
The forward model for the retrieval algorithm was developed based on the equivalence theorem. Scattering processes

Acknowledgements

This study was conducted in cooperation with the Ministry of Environment of Japan through the GOSAT project at the National Institute for Environmental Studies (NIES). We express our sincere thanks to all NIES GOSAT project members for their helpful comments and discussions.

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