Intercomparison of aerosol extinction profiles retrieved from MAX-DOAS measurements

Atmos. Meas. Tech., 9, 3205–3222, 2016 www.atmos-meas-tech.net/9/3205/2016/ doi:10.5194/amt-9-3205-2016 © Author(s) 2016. CC Attribution 3.0 License. Intercomparison of aerosol extinction profiles retrieved from MAX-DOAS measurements U. Frieß1 , H. Klein Baltink2 , S. Beirle3 , K. Clémer4,a , F. Hendrick4 , B. Henzing5 , H. Irie6 , G. de Leeuw5,7,8 , A. Li9 , M. M. Moerman5 , M. van Roozendael4 , R. Shaiganfar3 , T. Wagner3 , Y. Wang9,3 , P. Xie9 , S. Yilmaz1 , and P. Zieger10,b 1 Institute of Environmental Physics, University of Heidelberg, Heidelberg, Germany Netherlands Meteorological Institute (KNMI), De Bilt, the Netherlands 3 Max Planck Institute for Chemistry, Mainz, Germany 4 BIRA-IASB, Brussels, Belgium 5 Netherlands Organization for Applied Scientific Research (TNO), Utrecht, the Netherlands 6 Center for Environmental Remote Sensing, Chiba University, Chiba, Japan 7 Finnish Meteorological Institute (FMI), Helsinki, Finland 8 Department of Physics, University of Helsinki, Helsinki, Finland 9 Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, China 10 Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen, Switzerland a now at: Institute of Astronomy, Leuven University, Leuven, Belgium b now at: Department of Environmental Science and Analytical Chemistry, Stockholm University, Stockholm, Sweden 2 Royal Correspondence to: U. Frieß () Received: 19 November 2015 – Published in Atmos. Meas. Tech. Discuss.: 15 January 2016 Revised: 20 June 2016 – Accepted: 6 July 2016 – Published: 22 July 2016 Abstract. A first direct intercomparison of aerosol vertical profiles from Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations, performed during the Cabauw Intercomparison Campaign of Nitrogen Dioxide measuring Instruments (CINDI) in summer 2009, is presented. Five out of 14 participants of the CINDI campaign reported aerosol extinction profiles and aerosol optical thickness (AOT) as deduced from observations of differential slant column densities of the oxygen collision complex (O4 ) at different elevation angles. Aerosol extinction vertical profiles and AOT are compared to backscatter profiles from a ceilometer instrument and to sun photometer measurements, respectively. Furthermore, the near-surface aerosol extinction coefficient is compared to in situ measurements of a humidity-controlled nephelometer and dry aerosol absorption measurements. The participants of this intercomparison exercise use different approaches for the retrieval of aerosol information, including the retrieval of the full vertical profile using optimal estimation and a parametrised approach with a prescribed profile shape. Despite these large conceptual differences, and also differences in the wavelength of the observed O4 absorption band, good agreement in terms of the vertical structure of aerosols within the boundary layer is achieved between the aerosol extinction profiles retrieved by the different groups and the backscatter profiles observed by the ceilometer instrument. AOTs from MAX-DOAS and sun photometer show a good correlation (R>0.8), but all participants systematically underestimate the AOT. Substantial differences between the near-surface aerosol extinction from MAX-DOAS and from the humidified nephelometer remain largely unresolved. 1 Introduction Aerosols play an important role in the atmospheric system. Aerosol particles scatter and absorb radiation but also affect the formation, optical properties, and lifetime of clouds and therefore have an impact on the radiation balance of the Earth’s atmosphere. However, the impact of aerosols on the climate system is still only poorly understood (Stocker et al., 2013). Direct emission of soot particles, as well the forma- Published by Copernicus Publications on behalf of the European Geosciences Union. 3206 tion of secondary organic aerosols and the condensation of atmospheric gases on aerosol particles (e.g., sulfuric acid or organic vapours), affect air quality and human health. Various chemical processes in the atmosphere can be strongly affected by aerosols, since these provide surfaces for heterogeneous reactions. Examples are the heterogeneous formation of nitrous acid on soot particles (Ammann et al., 1998), the autocatalytic release of reactive bromine on sea salt aerosols in polar regions (Simpson et al., 2007), and the stratospheric ozone depletion as a consequence of halogen activation on polar stratospheric clouds (Crutzen and Arnold, 1986). A quantification of the optical properties, spatial distribution, and chemical composition of aerosols is crucial for an understanding of these processes. Therefore, measurement techniques for the determination of the amount, vertical distribution, and optical properties of aerosols using relatively simple and cost-effective instrumentation are highly desirable. Furthermore, knowledge on the spatial distribution of aerosols and their impact on the radiative transfer is also important for the interpretation of passive atmospheric remote sensing observations from ground and satellite. MultiAxis Differential Optical Absorption Spectroscopy (MAXDOAS) measurements allow for the retrieval of aerosol extinction profiles, and are sensitive to aerosol microphysical and optical properties, in the planetary boundary layer. The usage of MAX-DOAS measurements for the retrieval of atmospheric aerosol properties (Hönninger et al., 2004; Wagner et al., 2004; Frieß et al., 2006) has found a growing number of applications during recent years (e.g., Irie et al., 2008, 2009; Lee et al., 2009; Takashima et al., 2009; Clémer et al., 2010; Li et al., 2010; Vlemmix et al., 2010; Zieger et al., 2011; Frieß et al., 2011; Wagner et al., 2011; Sinreich et al., 2013; Wang et al., 2014; Hendrick et al., 2014; Vlemmix et al., 2015). As part of these studies, MAX-DOAS aerosol profiles, aerosol optical thickness (AOT), and/or surface extinction were compared to established instrumentation, such as lidar, sun photometer, and in situ aerosol instruments. These intercomparison studies are of great value for the validation of MAX-DOAS aerosol retrievals but suffer from several difficulties. A comparison of the AOT from MAX-DOAS and sun photometer does not allow for a validation of the retrieved profile shape. Compared to lidar profiles, MAX-DOAS has a much coarser vertical resolution and a different altitude sensitivity. Backscatter lidar instruments only provide information on the backscatter signal, and a determination of the actual aerosol extinction from these measurements is subject to large uncertainties. Therefore comparisons of backscatter lidar with MAX-DOAS extinction profiles can generally only be performed on a qualitative basis. Raman lidar systems can directly measure aerosol extinction profiles but suffer from a low signal-to-noise ratio during daylight, while MAX-DOAS measurements cannot be performed at night. A further shortcoming (...truncated)