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MAX-DOAS observations of tropospheric aerosols and formaldehyde above China Tim Vlemmix Francois Hendrick Michel Van Roozendael Isabelle De Smedt Katrijn.

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Presentation on theme: "MAX-DOAS observations of tropospheric aerosols and formaldehyde above China Tim Vlemmix Francois Hendrick Michel Van Roozendael Isabelle De Smedt Katrijn."— Presentation transcript:

1 MAX-DOAS observations of tropospheric aerosols and formaldehyde above China Tim Vlemmix Francois Hendrick Michel Van Roozendael Isabelle De Smedt Katrijn Clémer

2 The motivation for this study is twofold: 1st - to provide a data set of tropospheric HCHO columns for the comparison with measurements from space (GOME-2) 2nd - to provide a data set of tropospheric HCHO profiles which can be used to (possibly) improve the comparison between satellite and ground based observations (using profile assumptions in the satellite retrieval that are consistent with ground based observations) Because of the large impact of aerosols on the MAX-DOAS HCHO measurements, this study cannot be performed without paying attention to the effect of uncertainties in the aerosol retrieval on the HCHO retrieval. This presentation shows only 'work in progress'

3 General approach The "bePRO" (optimal-estimation based) profiling algorithm developed at BIRA (Clémer et al., 2010) has been used to analyze MAX-DOAS measurements performed in Xianghe (~40 km East of Beijing). The retrieval is based on single wavelength radiative transfer simulations (LIDORT RTM) in the UV (at 360 nm). This wavelength which lies in the DOAS fitting window in which (among others) the absorbers HCHO and O 4 can be fitted simultaneously. Main subject of this presentation: which a-priori settings to use? When using the bePRO-algorithm, many choices can be made which characterize the retrieval. Since it is well known that the information content of MAX-DOAS measurements is generally quite low (typically 1.5 to 2.5 degrees of freedom for signal) it is essential to study the effect of different settings with respect to the a-priori aerosol and HCHO vertical profiles, as well as their respective uncertainty covariance matrices, on the retrieval outcome. This presentation shows the effect on the retrieval algorithm applied to one year of MAX-DOAS observations (Xianghe, 2011) for three different settings of the a-priori

4 For each of the three cases the a-priori is described by an exponentially decreasing profile, which is characterized by a certain ‘scale height’ (SH). In the following, each time the same scale height is used for the aerosol extinction and HCHO profile. First, we compare two extremes (case 1 and 2): SH=0.5km SH=2.5km In addition we study the effect of an intermediate choice: SH=BL, where BL is the monthly median value of the boundary layer height taken from ECMWF reanalysis data (provided by Jenny Stavrakou) The reason to consider this third alternative is that (as will be shown below) changing from case 1 to case 2 introduces systematic differences not only in the retrieved HCHO profile, but also in the retrieval tropospheric HCHO column. Not considering seasonal and diurnal variations might therefore artificially introduce a seasonality or diurnal cycle in the tropospheric columns. 3km 1km Boundary layer Heights from ECMWF reanalysis Grey: daily value Black: median Red: 5%-95% and 25%-75% ranges

5 Results For the AOD there seems to be a good mutual agreement, only some outliers among >2900 data points It appears that the impact of a different a-priori on the trop. HCHO column is larger, with values approx. 20-30% higher for SH=2.5km SH=0.5km SH=2.5km SH=0.5km SH=2.5km frequency

6 Comparison for aerosol extinction at surface (1), HCHO concentration at surface (2) 1 2 SH=0.5km SH=2.5km Frequency

7 So far we have considered integrated quantities (AOD,VCD HCHO ) and surface values. How can we (quantitatively) compare the profiles themselves?  we introduce the quantity “H75” meaning the altitude (in km) below which 75% of the integrated (aerosol/HCHO) profile resides >> If the retrieval is successful, one may expect to see diurnal and seasonal variations of H75 comparable to those in the ECMWF boundary layer heights (see earlier slide). >> If the retrieval is successful, one hopes to see that H75 is quite independent of the a-priori settings. H75 amount altitude 75% 25%

8 Retrieval outcome for selected days

9 All panels: SH=0.5 SH=2.5 SH=BL ECMWF Red: Geom. approx. 30  Black: Direct sun Red: ECMWF BLH for specific day Black: Monthly median BLH from ECMWF, prescribed for SH=BL ECMWF Black: AERONET extinction profiles HCHO profiles H75 aerosols AOD VCD HCHO HCHO surface 2011 day 015 (*)(*)

10 All panels: SH=0.5 SH=2.5 SH=BL ECMWF Red: Geom. approx. 30  Black: Direct sun Red: ECMWF BLH for specific day Black: Monthly median BLH from ECMWF, prescribed for SH=BL ECMWF Black: AERONET extinction profiles HCHO profiles H75 aerosols AOD VCD HCHO HCHO surface 2011 day 023

11 All panels: SH=0.5 SH=2.5 SH=BL ECMWF Red: Geom. approx. 30  Black: Direct sun Red: ECMWF BLH for specific day Black: Monthly median BLH from ECMWF, prescribed for SH=BL ECMWF Black: AERONET extinction profiles HCHO profiles H75 aerosols AOD VCD HCHO HCHO surface 2011 day 025

12 All panels: SH=0.5 SH=2.5 SH=BL ECMWF Red: Geom. approx. 30  Black: Direct sun Red: ECMWF BLH for specific day Black: Monthly median BLH from ECMWF, prescribed for SH=BL ECMWF Black: AERONET extinction profiles HCHO profiles H75 aerosols AOD VCD HCHO HCHO surface 2011 day 030

13 All panels: SH=0.5 SH=2.5 SH=BL ECMWF Red: Geom. approx. 30  Black: Direct sun Red: ECMWF BLH for specific day Black: Monthly median BLH from ECMWF, prescribed for SH=BL ECMWF Black: AERONET extinction profiles HCHO profiles H75 aerosols AOD VCD HCHO HCHO surface 2011 day 036

14 All panels: SH=0.5 SH=2.5 SH=BL ECMWF Red: Geom. approx. 30  Black: Direct sun Red: ECMWF BLH for specific day Black: Monthly median BLH from ECMWF, prescribed for SH=BL ECMWF Black: AERONET extinction profiles HCHO profiles H75 aerosols AOD VCD HCHO HCHO surface 2011 day 051

15 All panels: SH=0.5 SH=2.5 SH=BL ECMWF Red: Geom. approx. 30  Black: Direct sun Red: ECMWF BLH for specific day Black: Monthly median BLH from ECMWF, prescribed for SH=BL ECMWF Black: AERONET extinction profiles HCHO profiles H75 aerosols AOD VCD HCHO HCHO surface 2011 day 053

16 All panels: SH=0.5 SH=2.5 SH=BL ECMWF Red: Geom. approx. 30  Black: Direct sun Red: ECMWF BLH for specific day Black: Monthly median BLH from ECMWF, prescribed for SH=BL ECMWF Black: AERONET extinction profiles HCHO profiles H75 aerosols AOD VCD HCHO HCHO surface 2011 day 061

17 All panels: SH=0.5 SH=2.5 SH=BL ECMWF Red: Geom. approx. 30  Black: Direct sun Red: ECMWF BLH for specific day Black: Monthly median BLH from ECMWF, prescribed for SH=BL ECMWF Black: AERONET extinction profiles HCHO profiles H75 aerosols AOD VCD HCHO HCHO surface 2011 day 067

18 Horizontal: ECMWF boundary layer height (=value for specific date&time, not the monthly median) SH=BL ECMWF SH=0.5km SH=2.5km AOD<0.2 0.2<AOD<0.6 AOD>0.6 H75 H75 H75

19 Preliminary CONCLUSIONS One year of data (aerosols/HCHO) has been analyzed using three different a-priori settings A good agreement is found with respect to the AOD and aerosol extinction at the surface The HCHO tropospheric column for SH=2.5 is about 20-30% higher than for SH=0.5 Because of this it is important to improve on the a-priori The agreement between the HCHO tropospheric column from the MAX-DOAS retrieval(s) and the direct sun is reasonable. No quantitative comparison has been performed yet. Agreement for HCHO at surface is not as good as aerosol extinction Only for conditions with low AOD(e.g. < 0.2) the algorithm reproduces the fluctuations of the boundary layer height, and only if this is prescribed in the a-priori using monthly median values of ECMWF boundary layer heights For higher AODs, the algorithm does not reproduce the heights that are prescribed Future work: Similar study with in the visible region using Xianghe NO2/aerosols data Similar study in Uccle (Brussels) using combined miniDOAS and ceilometer instuments.

20 Thank you very much for your attention! Special thanks to: Francois Hendrick Michel Van Roozendael Jenny Stavrakou Katrijn Clémer

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