The retrieval algorithm for aerosol layer height makes a spectral fit of reflectance spectrum across the entire O2-A band (see Figure 2). The retrieval technique is the Optimal Estimation Method (OEM), which ensures proper error propagation. The main fitted parameters are the aerosol pressure (which corresponds to the mid pressure of the layer), the aerosol optical thickness (valid for all wavelength of O2-A band) and the surface albedo. In this algorithm, aerosols are assumed to be uniformly distributed in a single layer with a fixed pressure difference between the top and bottom of the layer, and an aerosol fraction of 1 (see Figure 1). The pressure of the interval is retrieved keeping the pressure thickness constant. The reported pressure is the layer's mid pressure (mean of the top and bottom pressure). The Level-2 aerosol layer height product contains error estimated for its retrieval parameters as well as other relevant diagnostics so that the user can evaluate the retrieval result.
The aerosol layer height product is retrieved for cloud-free pixels only. Pixels covered by snow or (sea-) ice are initially excluded from analysis. In addition, pixels known to contain significant amounts of aerosol are processed first. For example, pixels can be selected based on the aerosol index. High values for this index indicate the presence of elevated absorbing aerosol.
Figure 1: Aerosol Profile Parameterization for the TROPOMI Aerosol Layer Height Level-2 Product
The baseline profile parameterization assumes an elevated layer with fixed pressure difference between the top and bottom of the layer. The layer's mid pressure is retrieved.
Fitting Window
O2-A band: 758-770 nm (spectral band 6)
Figure 2: Simulated Reflectance Spectra of the O2-A Band for a Scene Containing a Representative Aerosol Layer
Figure 2 shows simulated reflectance spectra in the O2-A band at a Spectral Resolution (FWHM) of 0.5 nm for a scene containing an aerosol layer between 950 and 850 hPa (pink line) or between 540 and 475 hPa (blue line).