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Abstract
Over the last years, ground-based multi-axis differential absorption spectroscopy (MAXDOAS) has been proved to be a very promising tool for the automated retrieval of tropospheric pollutants. The MAXDOAS instruments are designed to allow for the quasi simultaneous observation of the scattered sun light in a range of different line-of-sight (LOS) directions from the horizon to the zenith, resulting in an increased sensitivity towards atmospheric absorbers present close to the surface.
One of the main obstacles for the tropospheric trace gas vertical profile retrievals is the sensitivity of the length of the light path –and thus the observed slant column densities (SCD) of an atmospheric absorber– to the presence of aerosol in the atmosphere. To overcome this difficulty we developed an algorithm to retrieve, in a first step, the aerosol extinction vertical profiles from measurements of the O4 absorptions for different LOS. In a second step, the obtained aerosol profiles are used as input for the retrieval of tropospheric trace gas (e.g., NO2) vertical profiles.
The profiles are retrieved from the measured spectra using the optimal estimation method for the inversion step. The forward model used is the linearized discrete ordinate radiative transfer model (LIDORT). One major advantage of this code is that it includes an analytical calculation of the weighting functions needed for the inversion step. Consequently the algorithm is relatively fast, which is a major advantage when one aims for real-time automated retrievals.
We will present the retrieval algorithm and discuss its capabilities and limitations by means of the aerosol extinction and NO2 vertical profiles we retrieved from MAXDOAS measurements made in Beijing during the period July 2008 – April 2009.