MAXDOAS observations in Beijing

G. Pinardi, K. Clémer, C. Hermans, C. Fayt, M. Van Roozendael

BIRA-IASBPucai Wang & Jianhui Bai

IAP/CAS

24 June 2009, AMFIC meeting, Barcelona

Overview

MAXDOAS instrument MAXDOAS retrieval strategies Comparisons with satellite and CHIMERE

model (NO2, HCHO,…) Conclusions and perspectives

• 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: O3, NO2, BrO, HCHO, glyoxal, SO2, 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

UV channel

VIS channel

O3

SO2

HCHO, HONO

BrO

O4, NO2

NO2, glyoxal

O3

O4, NO2

O4O4 O2

H2O

MAXDOAS analysisDOAS analysis:

O3 (Huggins)SO2 HCHO HONO

BrO NO2 Glyoxal O3 (Chappuis)

O4 (360 nm) O4 (470 nm) O4 (570 nm) O4 (630 nm)

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 1sin ( ) 1

off axis zenithtrop

SC SCVC

LOS

Assuming that the NO2 layer is below the scattering altitude, a geometrical approximation can be used to obtain tropospheric vertical columns:

Line of Sight

MAXDOAS retrieval strategies

1rst 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 NO2, HCHO, CHOCHO and SO2

Trop SC = SCoff - SCzenith

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 1sin ( ) 1

off axis zenithtrop

SC SCVC

LOS

Assuming that the NO2 layer is below the scattering altitude, a geometrical approximation can be used to obtain tropospheric vertical columns:

Line of Sight

MAXDOAS retrieval strategies

1rst 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 NO2, HCHO, CHOCHO and SO2

Using radiative transfer modeling and optimal estimation method (involving aerosols profile retrieval from O4 DSCD) to invert tropospheric NO2 profiles

MAXDOAS retrieval strategies2nd approach: MAXDOAS retrieval algorithm

AOD retrieved from 4 bands of O4 (treated independently)Correction by a factor 1/0.8 to correct the O4 xs, so the

measured O4 DSCD measured O4 DSCD*0.8MAXDOAS AOD compared with available CIMEL

measurements (only two wavelengths)

MAXDOAS retrieval strategies2nd approach: MAXDOAS retrieval algorithmFirst step: derivation of aerosol AODs from O4 and comparison with CIMEL

Second step: derivation of NO2 profiles and comparison of the tropospheric columns with the geometrical approximation

MAXDOAS retrieval strategies2nd approach: MAXDOAS retrieval algorithm

Comparisons with satellitesNO2: OMI and GOME-2 (TEMIS algorithm)

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 NO2, HCHO, Glyoxal, SO2 profiles (8 levels), for each cell, 1 output per hour

Comparison with CHIMERE

Work under-progress: comparison with MAXDOAS at Beijing «simulation» of the NO2 field seen by OMI

Use CHIMERE as transfer standard to link satellite and ground-based column measurements

Bas Mijling presentation

Comparison with CHIMERE CHIMERE cell: 0.25°x0.25° (which at the Beijing latitude is ~28x21km²)

CHIMERE cells

50 and 100 Km

Beijing

GOME-2 pixel: 40x80km² OMI pixel: 13x24km²

Spatial and temporal variations (1)

GOME-2 pixel: 40x80km² OMI pixel: 13x24km²

Spatial and temporal variations (2)

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!!

HCHO: (Glyoxal, SO2)

MAXDOAS vs CHIMERE ☺

SCIAMACHY and GOME-2 will be presented by I. De Smedt

Other trace gases…

day 20080802day 20081130

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 NO2 With CIMEL for the aerosols (AOD)

Tropospheric NO2 and HCHO time series are compared to satellites (CHOCHO, SO2 also possible)

Comparisons with the CHIMERE model is under development

Idea: study the temporal and spatial variability and the effects of horizontal smoothing