MAX-DOAS observations of aerosols, formaldehyde and nitrogen dioxide in the Beijing area: comparison of two profile retrieval approaches

Atmos. Meas. Tech., 8, 941–963, 2015 www.atmos-meas-tech.net/8/941/2015/ doi:10.5194/amt-8-941-2015 © Author(s) 2015. CC Attribution 3.0 License. MAX-DOAS observations of aerosols, formaldehyde and nitrogen dioxide in the Beijing area: comparison of two profile retrieval approaches T. Vlemmix1,2 , F. Hendrick2 , G. Pinardi2 , I. De Smedt2 , C. Fayt2 , C. Hermans2 , A. Piters3 , P. Wang4 , P. Levelt3,1 , and M. Van Roozendael2 1 Delft University of Technology (TU-Delft), Delft, the Netherlands 2 Belgian Institute for Space Aeronomy (IASB-BIRA), Brussels, Belgium 3 Royal Netherlands Meteorological Institute (KNMI), De Bilt, the Netherlands 4 Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China Correspondence to: T. Vlemmix () Received: 27 July 2014 – Published in Atmos. Meas. Tech. Discuss.: 19 September 2014 Revised: 22 December 2014 – Accepted: 30 December 2014 – Published: 25 February 2015 Abstract. A 4-year data set of MAX-DOAS observations in the Beijing area (2008–2012) is analysed with a focus on NO2 , HCHO and aerosols. Two very different retrieval methods are applied. Method A describes the tropospheric profile with 13 layers and makes use of the optimal estimation method. Method B uses 2–4 parameters to describe the tropospheric profile and an inversion based on a leastsquares fit. For each constituent (NO2 , HCHO and aerosols) the retrieval outcomes are compared in terms of tropospheric column densities, surface concentrations and “characteristic profile heights” (i.e. the height below which 75 % of the vertically integrated tropospheric column density resides). We find best agreement between the two methods for tropospheric NO2 column densities, with a standard deviation of relative differences below 10 %, a correlation of 0.99 and a linear regression with a slope of 1.03. For tropospheric HCHO column densities we find a similar slope, but also a systematic bias of almost 10 % which is likely related to differences in profile height. Aerosol optical depths (AODs) retrieved with method B are 20 % high compared to method A. They are more in agreement with AERONET measurements, which are on average only 5 % lower, however with considerable relative differences (standard deviation ∼ 25 %). With respect to near-surface volume mixing ratios and aerosol extinction we find considerably larger relative differences: 10 ± 30, −23 ± 28 and −8 ± 33 % for aerosols, HCHO and NO2 respectively. The frequency distri- butions of these near-surface concentrations show however a quite good agreement, and this indicates that near-surface concentrations derived from MAX-DOAS are certainly useful in a climatological sense. A major difference between the two methods is the dynamic range of retrieved characteristic profile heights which is larger for method B than for method A. This effect is most pronounced for HCHO, where retrieved profile shapes with method A are very close to the a priori, and moderate for NO2 and aerosol extinction which on average show quite good agreement for characteristic profile heights below 1.5 km. One of the main advantages of method A is the stability, even under suboptimal conditions (e.g. in the presence of clouds). Method B is generally more unstable and this explains probably a substantial part of the quite large relative differences between the two methods. However, despite a relatively low precision for individual profile retrievals it appears as if seasonally averaged profile heights retrieved with method B are less biased towards a priori assumptions than those retrieved with method A. This gives confidence in the result obtained with method B, namely that aerosol extinction profiles tend on average to be higher than NO2 profiles in spring and summer, whereas they seem on average to be of the same height in winter, a result which is especially relevant in relation to the validation of satellite retrievals. Published by Copernicus Publications on behalf of the European Geosciences Union. 942 1 T. Vlemmix et al.: Comparison of profile retrieval algorithms for MAX-DOAS Introduction Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a ground-based passive remote sensing technique that is used to detect tropospheric trace gases such as nitrogen dioxide (NO2 ), formaldehyde (HCHO), sulfur dioxide (SO2 ), nitrous acid (HONO), iodine oxide (IO), glyoxal (CHOCHO), bromine oxide (BrO) and aerosols (aerosol extinction) (e.g. Wittrock et al., 2004; Wagner et al., 2004, 2009; Irie et al., 2011; Coburn et al., 2011; Pinardi et al., 2013; Hendrick et al., 2014; Wang et al., 2014). MAX-DOAS instruments take spectral measurements of scattered sunlight in the ultraviolet (UV) and visible (Vis) part of the electromagnetic spectrum. Profile information is obtained from a scan which comprises spectral measurements at different elevation angles but in the same azimuthal direction. The main retrieval products are tropospheric column densities, concentrations near the surface and estimates of the vertical profile shape. Because of this versatility MAX-DOAS is complementary to ground-based in situ observations (in a spatial sense) as well as to satellite observations (in a temporal and spatial sense, i.e. the vertical) and it can play an important role in bridging the gap between those techniques (Richter et al., 2013). Knowledge of the relationship between surface concentrations and integrated tropospheric column densities (in urban, suburban and rural regions) is important for the use of satellite observations in studies of air quality (e.g. Boersma et al., 2009; Mendolia et al., 2013). MAX-DOAS has great potential to be used in regional or global networks similar to the AERONET (sun photometer) and EARLINET (lidar) networks because of its versatility, the relatively low cost per instrument, the fact that a radiometric calibration is not required, and the fact that instruments can operate autonomously. Long-term data sets can be used for e.g. air quality monitoring, validation of chemical transport models, validation of satellite tropospheric column density retrievals and potentially as input in data assimilation systems for air quality forecasts. With respect to satellite validation it is interesting to note that MAX-DOAS can provide not only tropospheric trace gas column densities for direct comparison, but also profile shape estimates for trace gases and aerosol extinction. These can replace the a priori profile shapes assumed for the satellite retrieval, such that one can assess the impact of the a priori profile shape assumption (both for aerosols and for the trace gas of interest) on the satellite retrieval accuracy (Rodgers and Connor, 2003). Proper knowledge of the accuracy of the profile shape assumptions that are used in the satellite retrieval is crucial for a realistic estimate of the potential biases in the retrieved tropospheric column density. Mostly in the last decade, much progress has been made wi (...truncated)