Retrieval of Aerosol Height from the Oxygen A Band with TROPOMI

The Tropospheric Monitoring Instrument (TROPOMI), to be launched in 2015, will feature a new aerosol product that is specifically dedicated to retrieval of the height of aerosol layers. The Aerosol Layer Height product will be based on absorption by oxygen in the A-band (759-770 nm). Algorithm development for the aerosol height product is currently underway at KNMI. In this presentation we will introduce the product, highlight the algorithm and some of its development issues and discuss possible applications and example aerosol cases.

TROPOMI is a hyperspectral imager with channels in the ultraviolet, visible, near-infrared and shortwave infrared wavelength ranges, dedicated to remote sensing of the troposphere. It will have daily global coverage with small 7 x 7 km ground pixels. It is the successor to OMI and the precursor to ESA's Sentinel-5 mission. Aerosol Layer Height and the Absorbing Aerosol Index will be TROPOMI's main aerosol products.

Aerosol height observations from the near-infrared wavelength range will help to interpret the Absorbing Aerosol Index. Furthermore, they will improve retrieval of other aerosol properties, particularly retrieval of absorption optical thickness in the ultraviolet wavelength range. An increase in absorption can be due to a higher imaginary part of the refractive index or to the aerosol layer being at a higher altitude. Independent height observations will therefore further constrain retrieval of the single scattering albedo.

In addition, aerosol height information is an important parameter when estimating radiative forcings and climate impacts of aerosol, it is a significant source of uncertainty in trace gas retrieval and it helps in understanding atmospheric transport mechanisms. Finally, timely available, global observations of aerosol height will be of interest to aviation safety agencies.

The retrieval algorithm will be based on a spectral fit of reflectance (resolution 0.5 nm) across the O2 A absorption band. Aerosols are assumed to be confined to a single layer. The retrieval method will be optimal estimation to ensure a proper error analysis. The retrieval algorithm will be particularly sensitive to elevated, optically quite thick aerosol layers.

Sensitivity studies have indicated that accuracy and precision of retrieved height for cloud-free scenes will be well below the TROPOMI science requirements (1 km). They have also shown that retrieval is robust against inaccurate knowledge of the single scattering albedo and that precise knowledge of the phase function or the surface albedo is not needed. Thus, specific knowledge of the aerosol type is not needed for a reliable height retrieval.