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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 06 Aug 2019

Submitted as: research article | 06 Aug 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

A MAX-DOAS aerosol profile retrieval algorithm for high altitude measurements: application to measurements at Schneefernerhaus (UFS), Germany

Zhuoru Wang1,2, Ka Lok Chan1, Klaus-Peter Heue1, Adrian Doicu1, Thomas Wagner3, Robert Holla4, and Matthias Wiegner5 Zhuoru Wang et al.
  • 1Remote Sensing Technology Institute, German Aerospace Center (DLR), Oberpfaffenhofen, Germany
  • 2Faculty of Civil, Geo and Environmental Engineering, Technical University of Munich (TUM), Munich, Germany
  • 3Max Planck Institute for Chemistry, Mainz, Germany
  • 4German Meteorological Office (DWD), Hohenpeißenberg, Germany
  • 5Meteorological Institute, Ludwig Maximilian University of Munich (LMU), Munich, Germany

Abstract. We present a new aerosol profile retrieval algorithm for Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements at high altitude sites. The study is based on the long-term measurement (February 2012 to February 2016) at the Environmental Research Station Schneefernerhaus (UFS), Germany, which is located near the summit of Zugspitze, at an altitude of 2,650 m. Due to the low signal to noise ratio, commonly used MAX-DOAS retrieval algorithms based on the optimal estimation method are not suitable for the retrieval of high altitude measurements. We developed a new retrieval algorithm using an O4 differential slant column density (DSCD) look-up table. The look-up table consists of simulated O4 DSCDs corresponding to numerous possible aerosol profiles. The sensitivities of O4 absorption to several parameters were investigated for the design and parameterization of the look-up table. In the retrieval, the simulated O4 DSCDs for each possible profile are derived by interpolating the look-up table to the observation geometries. The cost functions are calculated for each aerosol profile in the look-up table based on the simulated O4 DSCDs, the O4 DSCD observations as well as the measurement uncertainties. Valid profiles are selected from all the possible profiles according to the cost function, and the optimal solution is defined as the weighted mean of all valid profiles. A comprehensive error analysis is performed to better estimate the total uncertainty. Based on the assumption that the look-up table covers all the possible profiles under clear sky conditions, we determined a set of O4 DSCD scaling factors for different elevation angles and wavelengths. The dependence of the scaling factors on elevation angle might be partly related to the specific properties of the high altitude station, e.g. the highly structured topography, horizontal gradients of the aerosol extinction and the systematic dependence of the surface albedo on altitude. The retrieved aerosol optical depths (AODs) are compared to coincident and co-located sun photometer observations. High correlation coefficients of 0.733 and 0.798 are found for measurements at 360 and 477 nm, respectively. However, especially in summer the sun photometer AODs are systematically higher than the MAX-DOAS retrievals by a factor of 2. The discrepancy might be related to the limited measurement range of the MAX-DOAS, and is probably also related to the decreased sensitivity of the MAX-DOAS measurements at higher altitudes. Our results also show maximum AOD and maximum Ångström exponent in summer which is consistent with observations from an AERONET station located ~ 43 km of the MAX-DOAS.

Zhuoru Wang et al.
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Status: final response (author comments only)
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Zhuoru Wang et al.
Zhuoru Wang et al.
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Short summary
We present a new aerosol profile retrieval algorithm for MAX-DOAS measurements at high altitude sites and applied to the MAX-DOAS measurements at UFS. The retrieval algorithm is based on O4 DSCD look-up table which is dedicated for high altitude MAX-DOAS measurements. The comparison of retrieved aerosol optical depths (AODs) to sun photometer observations shows good agreement with correlation coefficient (R) of 0.733 and 0.798 at 360 and 477 nm, respectively.
We present a new aerosol profile retrieval algorithm for MAX-DOAS measurements at high altitude...