Satellite remote sensing of NO 2

Transcription

1 Satellite remote sensing of NO 2 views from outside Steffen Beirle Satellite Group MPI Mainz

2 UV-vis satellite instruments Current nadir UV/vis satellite instruments: GOME 1/2, SCIAMACHY, OMI Nadir: probing the troposphere! LEO (low earth orbiter, ~800 km) Polar orbits Global cover in 1-6 days Sun-synchronous: local time ~10 a.m. / 2 p.m.

3 Retrieval I: DOAS Measured UV/vis Earth spectra Ring Vegetation O 3 UV HCHO OClO O 4 H 2 O O 2 Intensity [rel. units] Wavelength [nm] SO 2 NO 2 BrO O 3 vis Keep in mind: no NO x/y, just NO 2! DOAS: retrieval of total slant column densities (SCDs) (previous talk by Udo Frieß)

4 Retrieval II: Stratospheric Correction DOAS: total SCD Tropospheric column: total stratospheric column Different stratospheric estimation schemes: Models Reference sector (RS) (1D) / Reference regions (2D) Stratospheric measurements: e.g. from SCIAMACHY limb

5 Retrieval II: Stratospheric Correction SCIAMACHY NO 2 total column on Jan 28, Longitudinal variations of integrated profiles from SCIAMACHY limb measurements on Jan 28, 2006.

6 Retrieval II: Stratospheric Correction Mean NO 2 tropospheric column January : Simple RS correction Additional correction for longitudinal limb variations

7 Retrieval III: Air-Mass factors Measured: Slant column densities (Integrated concentrations along effective light path) Wanted: Vertical column densities (vertically integrated concentrations) Transformation via Air mass factor AMF ( sensitivity ) Requires radiative transfer calculations Depends on ground albedo, aerosols, clouds, profiles! cloud free clouded shielding albedo multiple scattering Clouds also lead to Discontinuities in the NO/NOx profile! Profiles (of NO 2!) are essential!!!

8 UV/vis sat. instr.: History and perspective GOME on ERS-2 SCIAMACHY on ENVISAT OMI on AURA GOME-2 on METOP year GOME-I 320x40 km 2 60x30 km 2 SCIAMACHY 26x13 km 2 OMI 80x40 km 2 GOME-II GOME-II GOME-II 7x7 km 2 5x5 km 2 Sent. 5 prec. Sent. 4 (GEO) Sent. 5

9 Summary satellite measurements Final product: tropospheric vertical column density molec/cm 2 ~ 0.37 ppb for 1km BLH ~ 0.74 ppb for 0.5 km BLH Typical errors: (individual pixels) <5x10 14 molec/cm 2 absolute, <40% relative Highest uncertainty in retrieval: Step III (calculation of tropospheric AMFs) Strong impact of clouds (even for cloud free pixels...) Typical spatial dimensions: 80x40 km 2 (GOME-2) down to 5x5 km 2 (Sentinels) Continous measurements over decades (but different spatial resolution, different local time etc.)

10 Potential I: The global view *10 15 molec/cm 2 Mean tropospheric NO 2 column density (Sep 2007-Aug 2008) derived from GOME-2 spectra rather boring for many GAW sites

11 Potential II: Temporal patterns *1015 molec/cm2

12 Potential III: Trends Average annual changes in tropospheric NO2 as observed by GOME from (Richter et al., 2005)

13 Potential IV: Spatial patterns NO x from international shipping: molec/cm 2 Spatially highpass-filtered NO 2 column density, clearly illustrating the NO x emissions from ships. Continents are masked out since they show much higher signals.

14 Potential IV: Different NO x sources Increased NO 2 column over Gulf of Mexico due to lightning

15 Potential V: Lifetime information Signal "travels" 200km in 24h! Tokyo Exponential fit e-folding distance lifetime!

16 Towards quantitative statements: Validation "Relative" signals (e.g. weekly cycles etc.): "relatively" easy Absolute "truth": "absolutely" hard... needs validation Comparisons with aircraft measurements ground-based column measurements (direct sun meas., zenith sky DOAS, MAX-DOAS) ground-based in-situ measurements

17 Satellite vs. ground based meas. Comparison of satellite columns with ground conc.: high variability (in both datasets!) moderate correlations Blond et al., 2007: EMEP Chen et al., 2009: Shanghai Polluted sites: also an issue for GAW? - or not our business?

18 Satellite vs. ground based meas. Satellite TVCDs are spatial averages! systematic effects for horizontal gradients quality of TVCDs depends on quality of AMF albedo/aerosols/clouds/profile translation of TVCD into surface concentration depends also on profile! no trivial link between TVCDs and surface concentrations!

19 Bridging the gaps The integrative perspective: The missing link: NO 2 profiles! (MAX-DOAS, NO 2 -sondes? CINDI-campaign) Models The Satellite perspective: ground-based measurements provide ground truth absolute concentrations diurnal cycle Validation of my measurements! The ground-based perspective: satellites provide Measurements from one instrument: consistent global measurements reference for all stations fill the gaps Validation of my measurements! The pragmatic perspective: Compare! consistent temporal patterns? (trends/seasonal cycles) discrepancies might indicate biases

20 Wishing list More stations e.g. in Russia (would help to judge on stratospheric estimation schemes) Chain (or even network?) of stations downwind from strong sources ~5 km distance: spatial resolution of sentinels! Validation of satellite pixel: grid of ground based & profile measurements (e.g. 10x10 km with 1 km resolution) To link satellite column measurements to surface concentrations, we need (relative) profile information!