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1 ) Satellite group, Max Planck institute for Chemistry, Mainz, Germany

Explore the application of MAX-DOAS observations for validating satellite and model data in Wuxi, China, focusing on aerosols and trace gases. Compare results to OMI data, analyze annual variations, and discuss the challenges.

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1 ) Satellite group, Max Planck institute for Chemistry, Mainz, Germany

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  1. MAX-DOAS observations and their application to validations of satellite and model data in Wuxi, China Yang Wang, Thomas Wagner, Pinhua Xie, Ang Li, Steffen Beirle, Nicolas Theys, Isabelle De Smedt, Trissevgeni Stavrakou 07.07.15 1) Satellite group, Max Planck institute for Chemistry, Mainz, Germany 2) Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, China 3) Belgian Institute for Space Aeronomy – BIRA-IASB, Brussels, Belgium

  2. Overview: • Where is Wuxi city? • Motivation, satellite data and MAX-DOAS measurements in Wuxi • MAX-DOAS results, profiles of aerosols and trace gases • Effects of Aerosol and shape factor of trace gases on box AMF and AMF of satellite retrieval • Comparison of daily averaged OMI VCD with MAX-DOAS VCD • Annual variation of aerosol and trace gases from MAX-DOAS, OMI and CTM • Conclusion 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  3. Where is Wuxi? 2011-2014 mean OMI NO2 DOMINO SO2 BIRA HCHO BIRA Wuxi city (circle) is about 130 km north-west of Shanghai (triangle) and by the Yangtze river. The population in this city is about six millions. It is located at the boundary of the area with high pollution adjoined to Shanghai. 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  4. Motivation • NO2, SO2, HCHO are important for environment and climate science • Satellite is valuable way to obverse the global distribution. • Some challenges for the retrieval of the Trop. VCD for satellite: • SO2, HCHO SCD retrieval is influence by ozone and low absorption signal. • Tropospheric AMF calculations : • Shape factor of trace gases from chemistry transfer model • Not including aerosols • cloud products sensitive to aerosols MAX-DOAS => aerosol and trace gases profiles and Trop. VCD => validate satellite products 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  5. Satellite data • Ozone monitor instrument: • Resolution: 13 x 24 km, daily global coverage, overpass time 13:30 • Data sets (2011-2014): • 1) NO2, DOMINO2.0 product, trace gas shape factor from TM4 model (KNMI) • 2) SO2, • BIRA-IASB (N. Theys et al. JGRD., 2015), trace gas shape factor from IMAGESv2, horizontal resolution of 2° × 2.5° (Stavrakou, Atmos. Chem. Phys. 2013) • NASA, NickolayKrotkov, http://disc.sci.gsfc.nasa.gov/ • 3) HCHO, BIRA-IASB (I. De Smedt et al. ACPD., 2015), trace gas shape factor from IMAGESv2 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  6. Our measurements in Wuxi station MINI MAX-DOAS from 2011 to 2014: • Spectral range: 290 – 425 nm (NO2, SO2, HCHO and O4). • Elevation angle: 5°, 10°, 20°, 30° and 90° • Azimuth angle: Exact north SCD retrieval: • O4 and NO2, 350 nm – 391 nm; SO2, 307.8 – 330; HCHO, 324.6 nm – 359 nm • Filters: shift < 1 pixel, offset correction < 2%, RMS < 0.05, SZA < 80 VCD and profile: • Nonlinear optimal estimation method to retrieve profiles of aerosol extinction and trace gas VMR, then integrate profiles to acquire VCD. • Filters: difference between measured and retrieved dSCD < a threshold; measured dSCD > 0 (avoid effect of clouds); keep trace gas profiles with convincing aerosol profiles 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  7. Normalized profiles of aerosol extinction and trace gas VMR from MAX-DOAS 13:00 to 14:00 • Different from Apriori profile • The profile shapes: • Aerosol: Gaussian • NO2: exponential • SO2: exponential but higher layer (box in Winter) • HCHO: a box shape • Different seasons: • Shapes similar shape factor 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  8. Discrepancy of shape factors from MAX-DOAS and CTM, and its effect on AMF clear sky: SZA: 40 SAA:-140 VZA:30 VAA:40 Totally mean 13:00 to 14:00 decrease AMF => increase VCD by including MAX-DOAS shape factor Consistent for NO2 and SO2 but quite different for HCHO 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  9. Effects of Aerosol on box AMF of satellite retrieval- compared with clear sky AOD: 0.83 SSA: 0.9 g: 0.72 Geometry: SZA: 40 SAA:-140 VZA:30 VAA:40 HCHO at 337nm SO2 at 319nm NO2 at 435nm mean aerosol profile from MAX-DOAS altitude / km box AMF • depended on wavelength, stronger at short wavelength (SO2) • Shading effect occurred below 1km, its magnitude up to 50% • Enhancing effect above 1km, up to 10% 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  10. Effects of Aerosol on box AMF of satellite retrieval- effect of fake clouds due to aerosols on box AMF Lambertian clouds CF=10% -> CRF=25% fake low clouds => CTP=1000 hPa (0.1km) fake high clouds => CTP=900 hPa (1km) CF up to 15%, CTP 900 – 1000 hPa in high aerosol load (AOD>0.4) Poster #20, Cloud and aerosol classification for 2 ½ years of MAX-DOAS observations in Wuxi (China) and comparison to independent data sets. Atmos. Meas. Tech. Discuss. 8, 4653–4709, 2015 NO2 at 435nm HCHO at 337nm SO2 at 319nm box AMF • Treating aerosol as clouds, especially low clouds can overestimate the boxAMF strongly, up to 300% near the ground. • This overestimation is stronger at short wavelength (SO2) 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  11. Effects of Aerosol on AMF of satellite retrieval Mean shape factors from MAX-DOAS 4 AMFs for NO2, SO2,HCHO 4 types of boxAMFs • aerosol profile => decrease AMF by few percent • Treating aerosol as low clouds increase AMF by up to 100%. As high clouds increase AMF by up to 30% Distinguish aerosol and clouds much more important than including aerosol information Suggest: calculate AMF in clear sky when CTP>900 hPa 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  12. Comparison of daily averaged OMI VCD with MAX-DOAS VCD- NO2 Cloud fraction Cloud top height High cloud shading effect: Using CTP, Improvement of linear regression by excluding the data with CTP<900 Aerosol effect: Using AOD, Improvement of linear regression by excluding the data for AOD>0.5 CF<10%, CTP>900 R2=0.84 Slope=0.78 Coincident criteria: MAX-DOAS: +- 30 minutes around overpass time OMI: distance from pixel center to station < 50 km • The high cloud shading effect underestimate NO2 VCD strongly, slopes improved by 15% by excluding high clouds • Aerosol effect underestimate NO2 VCD strongly, slopes improved by 17% by excluding high clouds, treating aerosol as clouds. • For clear sky, OMI VCD lower than MAX-DOAS VCD by 5% 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  13. Comparison of daily averaged OMI VCD with MAX-DOAS VCD- SO2 BIRA • High clouds shading effect: excluding high clouds, slope is improved by 10%. • Aerosol effect: excluding large aerosols, slope is improved by 11%. • In clear sky underestimation by 40%? 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  14. Comparison of daily averaged OMI VCD with MAX-DOAS VCD-HCHO Random error mean OMI VCD =11.2 mean OMI VCD random error=9.3 mean MAX-DOAS VCD=13.8 Cloud properties The random error from the DOAS fitting of SCD causes the large scattering points. In clear sky OMI underestimate HCHO by 8%. Aerosols belong to second organic aerosol. 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  15. Annual variation of aerosol and trace gases- Bimonthly mean AOD from MAX-DOAS and AERONET 13:00 to 14:00 90% 75% mean Median 25% 10% 2011 2012 2013 MAX-DOAS AERONET • The maximum of AOD in summer • Well agreement, but MAX-DOAS > AERONET Month 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  16. Annual variation of aerosol and trace gases- Bimonthly mean VCD of trace gases 2011 2012 2013 2014 NO2: Poster #18: MAX-DOAS observations of NO2, SO2 and HCHO in the Yantzi River Delta (YRD) and their use for the validation of satellite and model data Trop. VCD [10^15 molecs/cm^2] HCHO: Comparison: Variation trend agree well MAXDOAS>OMI> CTM Maximum: NO2 in winter; HCHO in summer; SO2 in winter SO2: 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  17. Difference between the VCDs from geometry approximations and profile retrievals NO2 SO2 HCHO relative azimuth angle between sun and line of sight • Their differences depended on RAA • Around noon (overpass time of OMI), geometrical VCDs agree well with profile VCDs: • NO2 and HCHO => EA of 20° • SO2 => EA of 20 ° 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  18. Conclusion: • Profile shapes from MAX-DOAS are consistent with CTM for NO2 and SO2 but quite different for HCHO => decrease AMF by 13% • Treating aerosol as clouds, especially low clouds cause the boxAMF overestimated up to 300% and AMF up to 100%. So distinguish aerosol and clouds is much more important than including aerosol information. • We suggest to calculate AMF in clear sky when CTP>900 hPato avoid the large error from treating aerosol clouds. • Cloud shading effect and aerosol effect (treating aerosol as clouds) make OMI underestimate NO2, SO2 strongly. We can use CTP>900 and AOD<0.5 to exclude high clouds and strong AOD to improve validation. 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  19. Great thanks for your attention! - 19 - 30.04.2014 - EGU General Assembly 2014, Yang Wang

  20. MAX-DOAS results compared to other independent ground-base instruments AOD vs AERONET Aerosol extinction • AOD: • vs AERONET Taihu, 18km south, level 1.5 cloud screened data • Aerosol extinction near surface: • vs the results from visibility meter in the same station (550nm ->360 nm with Angstrom of 1) • MAX-DOAS mean value from surface to 300m • NO2 and SO2 VMR near surface: • vs long-path DOAS, around noon LP-DOAS has larger error of NO2, because of low signal. NO2 VMR near surface SO2 VMR near surface 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  21. seasonally averaged diurnal circles of AOD and VCD of trace gases • AOD: obvious diurnal variation, variation magnitude in summer > spring > autumn > winter, generally maximum value around noon, probably due to second aerosol formation • NO2: obvious diurnal variation, variation magnitude in summer > spring > autumn > winter, generally maximum value in the morning, but during 15-16 in winter, due to its photolysis and traffic emission. • SO2: weak diurnal variation, variation magnitude in spring and autumn > winter > summer, maximum value in the morning. • HCHO: obvious diurnal variation, variation magnitude in autumn > summer > spring > winter, maximum value in the afternoon due to photo chemistry 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

  22. Comparison of daily averaged OMI VCD with MAX-DOAS VCD-HCHO Random error mean OMI VCD =11.2 mean OMI VCD random error=9.3 mean MAX-DOAS VCD=13.8 Cloud properties The random error from the DOAS fitting of SCD causes the large scattering points. Aerosol effect underestimate VCD by 13%. In clear sky underestimation by 8%. 07.07.2015 - 7th DOAS Workshop 2015, Yang Wang

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