MAX-DOAS observations of tropospheric aerosols and formaldehyde above China Tim Vlemmix

MAX-DOAS observations of tropospheric aerosols and formaldehyde above China

Tim VlemmixFrancois HendrickMichel Van RoozendaelIsabelle De Smedt Katrijn Clmer

The motivation for this study is twofold:

1st - to provide a data set of tropospheric HCHO columns for the comparison with measurements from space (GOME-2)2nd - to provide a data set of tropospheric HCHO profiles which can be used to (possibly) improve the comparison between satellite and ground based observations (using profile assumptions in the satellite retrieval that are consistent with ground based observations)

Because of the large impact of aerosols on the MAX-DOAS HCHO measurements, this study cannot be performed without paying attention to the effect of uncertainties in the aerosol retrieval on the HCHO retrieval.

This presentation shows only 'work in progress'

General approach

The "bePRO" (optimal-estimation based) profiling algorithm developed at BIRA (Clmer et al., 2010) has been used to analyze MAX-DOAS measurements performed in Xianghe (~40 km East of Beijing).

The retrieval is based on single wavelength radiative transfer simulations (LIDORT RTM) in the UV (at 360 nm). This wavelength which lies in the DOAS fitting window in which (among others) the absorbers HCHO and O4 can be fitted simultaneously.

Main subject of this presentation: which a-priori settings to use?

When using the bePRO-algorithm, many choices can be made which characterize the retrieval.

Since it is well known that the information content of MAX-DOAS measurements is generally quite low (typically 1.5 to 2.5 degrees of freedom for signal) it is essential to study the effect of different settings with respect to the a-priori aerosol and HCHO vertical profiles, as well as their respective uncertainty covariance matrices, on the retrieval outcome.

This presentation shows the effect on the retrieval algorithm applied to one year of MAX-DOAS observations (Xianghe, 2011) for three different settings of the a-priori

For each of the three cases the a-priori is described by an exponentially decreasing profile, which is characterized by a certain scale height (SH). In the following, each time the same scale height is used for the aerosol extinction and HCHO profile.

First, we compare two extremes (case 1 and 2):SH=0.5kmSH=2.5kmIn addition we study the effect of an intermediate choice:SH=BL, where BL is the monthly median value of the boundary layer height taken from ECMWF reanalysis data (provided by Jenny Stavrakou)

The reason to consider this third alternative is that (as will be shown below) changing from case 1 to case 2 introduces systematic differences not only in the retrieved HCHO profile, but also in the retrieval tropospheric HCHO column. Not considering seasonal and diurnal variations might therefore artificially introduce a seasonality or diurnal cycle in the tropospheric columns.

3km1kmBoundary layer Heights fromECMWF reanalysis

Grey: daily valueBlack: medianRed: 5%-95%and25%-75%ranges

Results

For the AOD there seems to be a good mutual agreement, only some outliers among >2900 data points

It appears that the impact of a different a-priori on the trop. HCHO column is larger, with values approx. 20-30% higher for SH=2.5km SH=0.5kmSH=2.5kmSH=0.5kmSH=2.5kmfrequencyfrequency

Comparison for aerosol extinction at surface (1), HCHO concentration at surface (2) 1 2SH=0.5kmSH=2.5kmFrequency

So far we have considered integrated quantities (AOD,VCDHCHO) and surface values.

How can we (quantitatively) compare the profiles themselves? we introduce the quantity H75 meaning the altitude (in km) below which 75% of the integrated (aerosol/HCHO) profile resides

>> If the retrieval is successful, one may expect to see diurnal and seasonal variations of H75 comparable to those in the ECMWF boundary layer heights (see earlier slide).

>> If the retrieval is successful, one hopes to see that H75 is quite independent of the a-priori settings.H75amountaltitude75%25%

Retrieval outcome for selected days

All panels:SH=0.5SH=2.5SH=BLECMWFRed: Geom. approx. 30 Black: Direct sunRed: ECMWF BLH for specific dayBlack: Monthly median BLH from ECMWF, prescribed for SH=BLECMWFBlack: AERONETextinctionprofilesHCHOprofilesH75 aerosolsAODVCD HCHOHCHO surface2011 day 015(*)

All panels:SH=0.5SH=2.5SH=BLECMWFRed: Geom. approx. 30 Black: Direct sunRed: ECMWF BLH for specific dayBlack: Monthly median BLH from ECMWF, prescribed for SH=BLECMWFBlack: AERONETextinctionprofilesHCHOprofilesH75 aerosolsAODVCD HCHOHCHO surface2011 day 023

Horizontal: ECMWF boundary layer height (=value for specific date&time, not the monthly median) SH=BLECMWF SH=0.5km SH=2.5km AOD

Preliminary CONCLUSIONS

One year of data (aerosols/HCHO) has been analyzed using three different a-priori settingsA good agreement is found with respect to the AOD and aerosol extinction at the surfaceThe HCHO tropospheric column for SH=2.5 is about 20-30% higher than for SH=0.5Because of this it is important to improve on the a-prioriThe agreement between the HCHO tropospheric column from the MAX-DOAS retrieval(s) and the direct sun is reasonable. No quantitative comparison has been performed yet.Agreement for HCHO at surface is not as good as aerosol extinctionOnly for conditions with low AOD(e.g. < 0.2) the algorithm reproduces the fluctuations of the boundary layer height , and only if this is prescribed in the a-priori using monthly median values of ECMWF boundary layer heights For higher AODs, the algorithm does not reproduce the heights that are prescribedFuture work:

Similar study with in the visible region using Xianghe NO2/aerosols data Similar study in Uccle (Brussels) using combined miniDOAS and ceilometer instuments.

Thank you very much for your attention!

Special thanks to:Francois HendrickMichel Van RoozendaelJenny StavrakouKatrijn Clmer

I will present the preliminary results of a study made by Tim Vlemmix aboutThe scaling height is the altitude (distance) where the vertical column decreases by a factor of e.

Note that the (median) BL peaks at 3 km in May and 1 km in December. Also mention diurnal variations. In addition: large variations from day to day*Almost no impact on aerosol extinctionConsiderable impact on HCHO surface concentrationLittle difference in the residuals (note: not corrected for errors -> not to be confused with 2 )*The following 9 days are selected to showI:4 days with relatively low AODII:3 days with high AOD or clouds (-> no AERONET data)III:2 days which start with low and end with high AOD or vice versaNote: for each three groups only discuss the first day in some detail (with focus on H75), and only shortly mention the similarities the other days of this group *Slides 9-12: low AOD

Blue curve follow the black and the red curves more than the orange and grey ones.

Note that H75 and BLH (ECMWF) does not have to be the same: only it would be nice if it follows the same diurnal variation(*) Note that the retrieved profile using SH=2.5 as a-priori does not go to zero at 3.5 km. This is probably cause of the systematic difference of 0.05 in the AOT with respect to SH=0.5. This is a technical issue and can be solved by changing the definition of the a-priori (such that it always goes to zero at 4 km, independent of scale height).

Agreement with aeronet for the shape of the diurnal variation*The next three selected days correspond to days with high AOD and the presence of clouds (aeronet and direct sun data are missing).Agreement between the different retrievals regarding extinction profiles and AOD.The three retrievals do not reproduce the diurnal variation of the boundary layer height.We see a clear differenc ebetween mornig and afternoon regarding HCHO VCD derived using the geometrical approximation due to the presence of clouds.Regarding HCHO, we see a reasonably good agreement between SH=0.5 and SH=BL ECMWF.For the next two selected days we have a strong gradient in AOD between morning and afternoon maybe due to a wind direction change.

The variation in profile shape is very large

We see that the blue curve follows first the black curve then it follows the orange curve. We have here a similar behaviour as for high AODs.Good agreement of all retrievals with aeronet.H75 is given for the aerosol extinction profile (not for HCHO)For AOD 0.2 H75 is almost independent of a-priori, but does not look convincing (almost no relation to BLH)*

MAX-DOAS observations of tropospheric aerosols and formaldehyde above China Tim Vlemmix

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Transcript of MAX-DOAS observations of tropospheric aerosols and formaldehyde above China Tim Vlemmix