2013-10-29: Retrieval of aerosols using the oxygen A band in the presence of vegetation fluorescence

Within ESA's Sentinel-5 Precursor / TROPOMI project researchers at KNMI are currently developing a new operational aerosol product that is specifically dedicated to retrieval of the height of tropospheric aerosols. Aerosol height information will contribute to ash-forecasting systems for aviation safety and it will improve our understanding of aerosol interactions in climate research. The aerosol layer height product is based on the oxygen A band in the near-infrared wavelength range. In the same wavelength region vegetation exhibits fluorescence emissions as a byproduct of photosynthesis. Studies shows that aerosol retrieval may be inaccurate if fluorescene emissions are not taken into account. In a recently published article we have investigated how vegetation fluorescence should be treated in an O2 A band aerosol retrieval algorithm.

We have investigated the precision of retrieved aerosol parameters for a generic aerosol retrieval algorithm over vegetated land using the O2 A band. Chlorophyll fluorescence is taken into account in the forward model. Fluorescence emissions are modeled as isotropic contributions to the upwelling radiance field at the surface and they are retrieved along with aerosol parameters. Precision is calculated by propagating measurement errors and a priori errors, including model parameter errors, using the forward model's derivatives. Measurement errors consist of noise and calibration errors. The model parameter errors considered are related to the single scattering albedo, surface pressure and temperature profile. We assume that measurement noise is dominated by shot noise; thus, results apply to grating spectrometers in particular. We describe precision for various atmospheric states, observation geometries and spectral resolutions of the instrument in a number of retrieval simulations. These precision levels can be compared with user requirements. A comparison of precision estimates with the literature and an analysis of the dependence on the a priori error in the fluorescence emission indicate that aerosol parameters can be retrieved in the presence of chlorophyll fluorescence: if fluorescence is present, fluorescence emissions should be included in the state vector to avoid biases in retrieved aerosol parameters.

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Precision of the main fit parameters as a function of spectral resolution for three values of the signal-to-noise ratio. Top left: mid pressure (Pmid); top right: aerosol optical thickness (τ); bottom left: surface albedo (As) at three wavelength nodes; bottom right: fluorescence emission (Fs) at three wavelength nodes. Results are for the following atmospheric scenario: aerosol layer at 700 hPa with optical thickness of 0.4, default aerosol model, surface albedo at 758 nm of 0.20, and a fluorescence emission at 758 nm of 0.46×10^12 photons s^-1 cm^-2 sr^-1 nm^-1 (or 1.2 mW m^-2 sr^-1 nm^-1). The solar zenith angle is 50° and the viewing direction is nadir.