1. MAXDOAS observations in Beijing G. Pinardi, K. Clémer, C. Hermans, C. Fayt, M. Van Roozendael BIRA-IASB Pucai Wang & Jianhui Bai IAP/CAS 24 June 2009, AMFIC meeting, Barcelona

2. Overview  MAXDOAS instrument  MAXDOAS retrieval strategies  Comparisons with satellite and CHIMERE model (NO 2, HCHO,…)  Conclusions and perspectives

3. New 2-channels MAXDOAS system with direct-sun pointing capability developed at BIRA Instrument funded by Belspo as part of bilateral research agreement for DRAGON-AMFIC Aim: provide complementary measurements of AQ-related gases and contribute to satellite validation (Target data products: O 3, NO 2, BrO, HCHO, glyoxal, SO 2, aerosol, etc) Operation: in Beijing at IAP/CAS from June 2008 to April 2009. Longer term: permanent installation in background site North of Beijing (Xinglong) MAXDOAS instrument

4. UV channel VIS channel O3O3 SO 2 HCHO, HONO BrO O 4, NO 2 NO 2, glyoxal O3O3 O 4, NO 2 O4O4 O4O4 O2O2 H2OH2O MAXDOAS analysis DOAS analysis: O 3 (Huggins) SO 2 HCHOHONO BrONO 2 GlyoxalO 3 (Chappuis) O 4 (360 nm) O 4 (470 nm) O 4 (570 nm) O 4 (630 nm)

5. Multi axis pointing constant light path through the stratosphere long light path through the lower troposphere, especially for the low elevation angles Multi-Axis geometry: by collecting light at different elevations, from the horizon to the zenith, stratospheric and tropospheric contributions can be separated : Stratosphere Low Troposphere Assuming that the NO 2 layer is below the scattering altitude, a geometrical approximation can be used to obtain tropospheric vertical columns: Line of Sight MAXDOAS retrieval strategies 1 rst approach:  QA/QC: columns derived from 2 different elevations angles (15° and 30°) are eliminated if they differ by more than 30 %.  Tropospheric columns have been derived for NO 2, HCHO, CHOCHO and SO 2 Trop SC = SC off - SC zenith

6. Multi axis pointing constant light path through the stratosphere long light path through the lower troposphere, especially for the low elevation angles Multi-Axis geometry: by collecting light at different elevations, from the horizon to the zenith, stratospheric and tropospheric contributions can be separated : Stratosphere Low Troposphere Assuming that the NO 2 layer is below the scattering altitude, a geometrical approximation can be used to obtain tropospheric vertical columns: Line of Sight MAXDOAS retrieval strategies 1 rst approach:  QA/QC: columns derived from 2 different elevations angles (15° and 30°) are eliminated if they differ by more than 30 %.  Tropospheric columns have been derived for NO 2, HCHO, CHOCHO and SO 2

7. Using radiative transfer modeling and optimal estimation method (involving aerosols profile retrieval from O 4 DSCD) to invert tropospheric NO 2 profiles MAXDOAS retrieval strategies 2 nd approach: MAXDOAS retrieval algorithm

8.  AOD retrieved from 4 bands of O 4 (treated independently)  Correction by a factor 1/0.8 to correct the O 4 xs, so the measured O 4 DSCD  measured O 4 DSCD*0.8  MAXDOAS AOD compared with available CIMEL measurements (only two wavelengths) MAXDOAS retrieval strategies 2 nd approach: MAXDOAS retrieval algorithm First step: derivation of aerosol AODs from O 4 and comparison with CIMEL

9. Second step: derivation of NO 2 profiles and comparison of the tropospheric columns with the geometrical approximation MAXDOAS retrieval strategies 2 nd approach: MAXDOAS retrieval algorithm

10. Comparisons with satellites NO 2 : OMI and GOME-2 (TEMIS algorithm)

11. CHIMERE model:  Resolution 0.25°x0.25°  Emissions: adapted for China  Levels (8, until 500 hPa (5.5 km))  ECMWF data set (0.5°) Type of output:  One file per day  NO 2, HCHO, Glyoxal, SO 2 profiles (8 levels), for each cell, 1 output per hour Comparison with CHIMERE Work under-progress:  comparison with MAXDOAS at Beijing  «simulation» of the NO 2 field seen by OMI Use CHIMERE as transfer standard to link satellite and ground-based column measurements Bas Mijling presentation

12. Comparison with CHIMERE  CHIMERE cell: 0.25°x0.25° (which at the Beijing latitude is ~28x21km²) CHIMERE cells 50 and 100 Km Beijing

13. GOME-2 pixel: 40x80km² OMI pixel: 13x24km² Spatial and temporal variations (1)

14. GOME-2 pixel: 40x80km² OMI pixel: 13x24km² Spatial and temporal variations (2)

15. Depending on the relative position of the satellite closest pixel, a weighted average of several CHIMERE grid cells is performed in order to reproduce the spatial averaging performed by the satellite. Under-development!!

16. HCHO: (Glyoxal, SO 2 )  MAXDOAS vs CHIMERE ☺  SCIAMACHY and GOME-2  will be presented by I. De Smedt Other trace gases… day 20080802 day 20081130

17. Summary and Perspectives  MAXDOAS has measured in Beijing from June 2008 to April 2009  future re-installation outside Beijing  2 retrieval strategies show good agreement  One wrt the other for tropospheric NO 2  With CIMEL for the aerosols (AOD)  Tropospheric NO 2 and HCHO time series are compared to satellites (CHOCHO, SO 2 also possible)  Comparisons with the CHIMERE model is under development  Idea: study the temporal and spatial variability and the effects of horizontal smoothing