 |
| Titel |
MAX-DOAS observations of aerosols, formaldehyde and nitrogen dioxide in the Beijing area: comparison of two profile retrieval approaches |
| VerfasserIn |
T. Vlemmix, F. Hendrick, G. Pinardi, I. Smedt, C. Fayt, C. Hermans, A. Piters, P. Wang, P. Levelt, M. Roozendael |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1867-1381
|
| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 8, no. 2 ; Nr. 8, no. 2 (2015-02-25), S.941-963 |
| Datensatznummer |
250116145
|
| Publikation (Nr.) |
 copernicus.org/amt-8-941-2015.pdf |
|
|
|
|
|
| Zusammenfassung |
A 4-year data set of MAX-DOAS observations in the Beijing area
(2008–2012) is analysed with a focus on NO2,
HCHO and aerosols. Two very different retrieval
methods are applied. Method A describes the tropospheric profile with
13 layers and makes use of the optimal estimation method. Method B
uses 2–4 parameters to describe the tropospheric profile and an
inversion based on a least-squares fit. For each constituent
(NO2, HCHO and aerosols) the retrieval
outcomes are compared in terms of tropospheric column densities, surface
concentrations and "characteristic profile heights" (i.e. the
height below which 75% of the vertically integrated
tropospheric column density resides).
We find best agreement between the two methods for tropospheric
NO2 column densities, with a standard deviation of relative
differences below 10%, a correlation of 0.99 and a linear
regression with a slope of 1.03. For tropospheric
HCHO column densities we find a similar slope, but also
a systematic bias of almost 10% which is likely related to
differences in profile height. Aerosol optical depths (AODs) retrieved
with method B are 20% high compared to method A. They are
more in agreement with AERONET measurements, which are on average only
5% lower, however with considerable relative differences
(standard deviation ~ 25%). With respect to near-surface
volume mixing ratios and aerosol extinction we find considerably
larger relative differences: 10 ± 30,
−23 ± 28 and −8 ± 33% for aerosols,
HCHO and NO2 respectively. The frequency
distributions of these near-surface concentrations show however
a quite good agreement, and this indicates that near-surface
concentrations derived from MAX-DOAS are certainly useful in
a climatological sense. A major difference between the two methods is
the dynamic range of retrieved characteristic profile heights which is
larger for method B than for method A. This effect is most pronounced
for HCHO, where retrieved profile shapes with method
A are very close to the a priori, and moderate for NO2
and aerosol extinction which on average show quite good agreement for
characteristic profile heights below 1.5 km.
One of the main advantages of method A is the stability, even under
suboptimal conditions (e.g. in the presence of clouds). Method B is
generally more unstable and this explains probably a substantial part
of the quite large relative differences between the two methods.
However, despite a relatively low precision for individual profile
retrievals it appears as if seasonally averaged profile heights
retrieved with method B are less biased towards a priori assumptions
than those retrieved with method A. This gives confidence in the
result obtained with method B, namely that aerosol extinction profiles tend on
average to be higher than NO2 profiles in spring and
summer, whereas they seem on average to be of the same height in
winter, a result which is especially relevant in relation to the
validation of satellite retrievals. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|