Retrieval of CHOCHO from MAXDOAS measurements in the

Retrieval of CHOCHO from MAX-DOAS measurements in the Beijing area F. Hendrick 1, C. Gielen 1, C. Lerot 1, T. Stavrakou 1, I. De Smedt 1, C. Fayt 1, C. Hermans 1, J. -F. Müller 1, G. Pinardi 1, R. Volkamer 2, P. Wang 3, and M. Van Roozendael 1 1 Belgian 2 Dept. Institute for Space Aeronomy (IASB-BIRA), Brussels, Belgium of Chemistry and Biochemistry, University of Boulder, Colorado 3 Institute of Atmospheric Physics/CAS, Beijing, China 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 1

Outline • Introduction – glyoxal in the atmosphere • CHOCHO vertical profile retrieval from MAXDOAS observations in the Beijing area • Description of correlative data sets: - CHOCHO VCD from OMI and from the 3 D-CTM IMAGES • Results: - CHOCHO seasonal and diurnal variations - Glyoxal : Formaldehyde ratio - Optimal Estimation versus geometrical approximation • Conclusions 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 2

Introduction: Glyoxal in the atmosphere Sinks Sources Photolysis Oxidation of NMVOCs Direct emissions from fuel combustion biomass burning CHOCHO (lifetime: 2 -4 h) OH oxidation Conversion to SOA Dry and wet deposition Fires (d), 12% Fires (i), 8% Biog. 51% Anthr. (d), 3% Measuring this species is of major importance in air quality issues Anthr. (i), 26% 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 3

MAX-DOAS observation sites Institute of Atmospheric Physics (06/2008 - 04/2009) (03/2010 till now ) -Elevation angles: 2°, 4°, 6°, 8°, 10°, 12°, 15°, 30°, 90° (zenith) -Measurements from ~85°SZA sunrise to 85°SZA sunset -15’ per scan 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 4

DOAS settings • Fitting window: 434 -460 nm • Fitted species: − CHOCHO (296 K) − NO 2 (220 and 298 K) − O 3 (223 K) − O 4 (293 K) − H 2 O (296 K) − Ring effect • CHOCHO XS: Volkamer et al. (2005) at 296 K 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 5

be. PRO profiling tool Clémer et al. , AMT, 2010 MAXDOAS measurements LIDORT Apriori DOAS CHOCHO DSCD Atm: P, T Surface albedo Trace gas: , profile Aerosol: optical prop. O 4 DSCD Weighting functions Optimal estimation Aerosol vertical extinction profiles Wavelength: • O 4: 477 nm Tropospheric HONO vertical profiles A priori profile: • CHOCHO: 455 nm with SH=0. 5 km and SH=1. 0 km 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 6

CHOCHO vertical profile retrieval Xianghe, 23 June, 2010, 16: 22 LT/54°SZA Mean daytime CHOCHO profile Xianghe (2010 -2013) Selection of ‘good’ scans based on: • RMS (SH=0. 5 km gives the lowest RMS) • DOFS (>1) • Multiple scattering (O 4 DSCDs) flag (Gielen et al. , 2014) 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 7

OMI glyoxal retrieval algorithm Ø Original algorithm developed for GOME-2 A in 2010 (Lerot et al. , ACP). Ø Recently updated and adapted to OMI. Ø Relies on the standard DOAS approach. 1. Slant column retrieval Fitting window: 435 -460 nm Updated Absorption cross-sections 2. CHOCHO + O 3, NO 2 (two temperatures as in Alvarado et al. , Miller et al. , 2014), H 2 O, O 4, Ring, liquid water (fixed) and VRS Conversion to vertical columns • Row-dependent background correction procedure (based on Pacific sector) • Air mass factors computed using a priori profile shapes provided by a. the CTM IMAGESv 2 over lands. b. Airborne MAX-DOAS TORERO profile over oceans. • No cloud correction applied. Only pixels with cloud fraction < 20% are kept. 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 8

3 D-CTM IMAGES § Resolution: 2°x 2. 5° § Meteorological fields: ECMWF § Emission inventories: − Biogenic: Anthropogenic: MEGAN REAS (Regional Emission inventory in Asia) − Pyrogenic: GFEDv 3. 1 § NMVOC species: 22 explicit IMAGES: Stavrakou et al. , ACP, 2013 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 9

Time-series of CHOCHO VCD Vertical column density • Good agreement between OMI (100 km overpass) and MAXDOAS Smoothed MAX-DOAS (x 1 E 15 molec/cm 2) Unsmoothed MAX-DOAS • Smoothing of MAXDOAS profiles with OMI AVKs improves the agreement between both data sets. • Observed seasonality can be mainly explained by larger biogenic and pyrogenic activities in summer (x 1 E 15 molec/cm 2) 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 10

Diurnal variation of CHOCHO Vertical column density Local time (h) Near-surface concentration (0 -200 m layer) Local time (h) 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 11

Comparison with 3 D-CTM IMAGES Vertical column density normalized WRT local noon Liu et al. , GRL, 2012: underestimation of aromatics (BTX) by a factor of 5 -6 in the current anthropogenic emission inventories over China 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 12

Glyoxal : Formaldehyde ratio CHOCHO: HCHO ratio RGF often used as an indicator of changes in VOC mixture: Previous studies on this ratio: 1. Vrekoussis et al. (2010) – based on GOME-2: • RGF < 0. 04 in urban areas • RGF > 0. 06 in rural environments 2. Di Gangi et al. (2012) – based on in situ measurements during two campaigns: • RGF < 0. 02 for biogenic regimes • RGF ~= 0. 025 -0. 035 for polluted air masses 3. Kaizer et al. (2015) – based on airborne in situ measurements: • RGF < 0. 025 for areas with high isoprenes emissions • RGF >0. 03 for areas with high monoterpenes emissions • RGF can increase in case of local emissions (oil and gas production) 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 13

MAX-DOAS and OMI HCHO observations over Beijing/Xianghe De Smedt et al. , AMTD, 2015 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 14

Glyoxal : Formaldehyde ratio • CHOCHO/HCHO VCD ratio is ranging from 0. 025 to 0. 055, except in DJF (0. 05 -0. 09) • A good consistency is found with OMI • Higher RGF values are found during the winter where anthropogenic emissions are the largest Results more consistent with Di Gangi and Kaizer than with Vrekoussis 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 15

Geometrical approximation vs OEM: CHOCHO OEM SH=0. 5 km OEM SH=1. 0 km 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 16

Geometrical approximation vs OEM: HCHO OEM SH=0. 5 km OEM SH=1. 0 km 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 17

Geometrical approximation vs OEM: CHOCHO OEM SH=0. 5 km OEM SH=1. 0 km 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 18

Geometrical approximation vs OEM: CHOCHO 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 19

Concluding remarks • Tropospheric CHOCHO vertical profiles and corresponding column densities have been retrieved in the Beijing area by applying an OEM-based profiling tool to MAX-DOAS observations from June 2008 till December 2013 • A good consistency is found between MAX-DOAS and OMI in terms of CHOCHO VCD seasonality (maximum in summer and a minimum in winter) • MAX-DOAS CHOCHO VCD has a maximum around local noon in the Beijing City centre, in reasonably good agreement with simulations from the 3 D-CTM IMAGES; no marked diurnal variation is found in Xianghe • CHOCHO/HCHO VCD ratio is ranging from 0. 025 to 0. 055, except in DJF (0. 05 -0. 09). A good consistency is found with OMI. • Further investigations: – Consistency between OEM and geometrical approximation – Comparison with the 3 D-CTM IMAGES (surface concentration + different emission scenarios) 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 20

Acknowledgements • Belgian Federal Science Policy Office, Brussels (PRODEX and AGACC-II projects) • EU FP 7 projects NORS and Marco. Polo Thank you for your attention ! 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 21

CHOCHO and HCHO diurnal variations Vertical column density normalized WRT local noon 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 22

Geometrical approximation vs OEM: CHOCHO and HCHO at Bujumbura (Burundi) CHOCHO OEM SH=1. 0 km HCHO OEM SH=1. 0 km 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 23

Multiple scattering (O 4) flag O 4 flag Gielen et al. , AMT, 2014 214. 2 214. 3 214. 4 214. 5 214. 6 214. 7 Fractional day Strong temporal variability of O 4 DSCDs due to enhanced multiple scattering commonly occurs in optically thick clouds, and is seen less for high aerosol optical depth 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 24

OMI retrieval of CHOCHO VCDs - Original algorithm for GOME-2 A developed in 2009 (Lerot et al. , ACP, 2010) - Two steps: slant column retrieval (DOAS) + conversion of SCDs to VCDs - Algorithm updates: 1. Strong focus on GOME-2 A/OMI consistency: Application of common settings to both sensors. 2. - DOAS fit Update of cross-section data base. On-line convolution of cross-sections with a fitted slit function (for GOME-2 A). Daily mean background radiance spectrum as the reference instead of solar spectra. 3. Normalization procedure - Row- (OMI) or VZA-dependent (GOME-2) procedure 4. - AMF computation Over lands: A priori profiles provided by an updated version of the CTM IMAGES. Over oceans: use of a fixed a priori profile (TORERO). Extension of the weighting function LUT. No cloud correction any more, only pixels with CF<0. 2 are kept. Reduction of the influence of the a priori information. 7 th International DOAS Workshop, 6 -8 July 2015, Brussels, Belgium 25