Abstract
Abstract. We present a new aerosol extinction profile retrieval algorithm for multi-axis differential optical absorption spectrometer (MAX-DOAS) measurements at high-altitude sites. The algorithm is based on the lookup table method. It is applied to retrieve aerosol extinction profiles from the long-term MAX-DOAS measurements (February 2012 to February 2016) at the Environmental Research Station Schneefernerhaus (UFS), Germany (47.417∘ N, 10.980∘ E), which is located near the summit of Zugspitze at an altitude of 2650 m. The lookup table consists of simulated O4 differential slant column densities (DSCDs) corresponding to numerous possible aerosol extinction profiles. The sensitivities of O4 absorption to several parameters were investigated for the design and parameterization of the lookup table. In the retrieval, simulated O4 DSCDs for each possible profile are derived by interpolating the lookup table to the observation geometries. The cost functions are calculated for each aerosol profile in the lookup table based on the simulated O4 DSCDs, the O4 DSCD observations, and 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 the valid profiles. A comprehensive error analysis is performed to better estimate the total uncertainty. Based on the assumption that the lookup table covers all possible profiles under clear-sky conditions, we determined a set of O4 DSCD scaling factors for different elevation angles and wavelengths. The profiles retrieved from synthetic measurement data can reproduce the synthetic profile. The results also show that the retrieval is insensitive to measurement noise, indicating the retrieval is robust and stable. The aerosol optical depths (AODs) retrieved from the long-term measurements were compared to coinciding and co-located sun photometer observations. High correlation coefficients (R) 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. The MAX-DOAS measurements indicate the aerosol extinction decreases with increasing altitude during all seasons, which agrees with the co-located ceilometer measurements. Our results also show maximum AOD and maximum Ångström exponent in summer, which is consistent with observations at an AERONET station located ∼43 km from the UFS.
Highlights
Atmospheric aerosols play an important role in atmospheric physics and chemistry
From the O4 differential slant column densities (DSCDs) lookup table, we found that O4 DSCD at 5◦ is almost negatively correlated with aerosol optical depths (AODs), while it is insensitive to the shape of the profile
The so-called median O4 DSCDs were simulated with profiles with the aerosol extinction coefficient between 2 and 4 km equal to 0.5σ3 (50 % of the aerosol extinction coefficient between 1 and 2 km), while the extreme values were simulated with the aerosol extinction coefficient between 2 and 4 km set equal to σ3. 2–4 km was derived from the relative difference between the extreme and median results
Summary
Atmospheric aerosols play an important role in atmospheric physics and chemistry. They affect the atmospheric radiation budget by absorbing and scattering radiation, as well as providing nuclei for the formation of clouds (Haywood and Boucher, 2000; Bellouin et al, 2005; Li and Kou, 2011; Heald et al, 2014). Vertical profile information can be retrieved from MAXDOAS observations using parameterized approaches (e.g., Lee et al, 2009; Li et al, 2010; Vlemmix et al, 2011; Wagner et al, 2011; Sinreich et al, 2013) These methods simplify aerosol profiles as limited parameters, e.g., aerosol optical depth (AOD), layer height, shape parameter (Wagner et al, 2011; Hartl and Wenig, 2013). We present a new MAX-DOAS aerosol profile retrieval algorithm suitable for high-altitude measurements. It is based on an O4 DSCD lookup table.
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