1. Cloud screening and aerosol retrievals from MAX-DOAS measurements at Ukkel Clio Gielen Michel Van Roozendael, Francois Hendrick, Caroline Fayt, Christian Hermans, Gaia Pinardi, Didier Gilllotay, and Tim Vlemmix + Hugo De Backer, Veerle De Bock, and Quentin Laffineur from RMI BIRA contribution to WP3:

2. Goals Qualify sky and cloud conditions from MAX-DOAS data Apply to Ukkel station to improve aerosol retrievals Tasks 3.1 (algorithm improvements) and 3.2 (operating a MAX-DOAS instrument at Ukkel)

3. Sky conditions: the colour index Trace gas and aerosol retrievals affected by clouds → introduce: additional photon absorption multiple scattering strong temporal variation Degenerate problem: thin/thick clouds, broken clouds, fog, haze, white clouds, grey clouds, high/low aerosol load,.... Use observed colour of the sky for characterisation → Colour Index = ratio of intensities

4. Colour Index characteristics Investigation of the colour index shows that: - the CI = high for clear skies - the CI < in the presence of clouds, aerosols, fog,... - the CI = a constant value over time in the presence of optically thick clouds - the CI = highly variable in the presence of broken/scattered clouds

5. Cloud-Screening flags Using the colour index we define two flags: - a flag to describe the general sky condition (clear/polluted/full-cloud cover) → Colour-Index flag - a flag to note the presence of scattered clouds → Broken-Cloud flag

6. Broken-Cloud flag Temporal variation to determine presence of scattered clouds → model observations with double-sine function and detect outliers → no information on presence of full-cloud cover!

7. Colour-Index measurements: Ukkel

8. Colour-Index flag: sky condition Good Mediocre Bad By combining measurements and simulations we define 3 sky-condition regions Good= clear sky Med= thin clouds/ low aerosols Bad= thick clouds/ high aerosols

9. Impact on aerosol retrievals + no cloud screening + CI-flag=good/med + CI-flag=good/med and no broken clouds ◊ AOD measurements (Aeronet + Brewer) + AOD retrievals (MAX-DOAS + bePRO profiling code)

10. Impact on aerosol retrievals + no cloud screening + CI-flag= good/med + CI-flag= good/med and no broken clouds

11. Cloud-cover from thermic infrared data

12. Interpretation of aerosol data at Ukkel Exploiting the ensemble of data at BIRA-RMI (task 3.3): -BIRA:  Aerosol AOD and extinction profiles (MAX-DOAS)  Aerosol AOD (CIMEL)  Cloud screening (MAX-DOAS)  Cloud-cover fraction (thermic infrared) -RMI:  Aerosol AOD (Brewer)  Backscatter profiles/mixing-layer height (lidar)  Cloud screening (lidar/Brewer) + Aethalometer in the future

13. Vertical Profiles: BIRA-RMI Aerosol AOD (BIRA/RMI) Backscatter/ cloud/MLH (RMI) Aerosol vert.prof+ cloudscreening (BIRA) Aerosol vert.prof (BIRA)

14. Status - Operational cloud-screening method for MAX-DOAS measurements - Aerosol retrievals for full Ukkel dataset (2011-2013): AOD and extinction profiles - Applying cloud screening strongly improves aerosol retrievals - Good agreement between BIRA cloud screening and RMI data - Good agreement between MAX-DOAS cloud screening and thermal infrared cloud-cover data

15. Future tasks for the last part of the project - Further improvement of cloud-screening method:  Include O 4 DSCDs → distinguish clouds/aerosols/fog... + cloud bottom height - Detailed study of aerosol retrievals at Ukkel (BIRA + RMI):  Trends (seasonal/daily/weekly)  Correlation with meteorology - Estimation of the radiative forcing effect at Ukkel (task 3.3)

17. Colour-Index simulations Aerosol load Cloud optical depth Simulations performed by Tim Vlemmix, with DAK plan-parallel model