Upload
lily-holland
View
217
Download
0
Embed Size (px)
Citation preview
Validation of OMI NO2 data using ground-based spectrometric NO2 measurements
at Zvenigorod, Russia
A.N. Gruzdev and A.S. Elokhov
A.M. Obukhov Institute of Atmospheric Physics Moscow
Zvenigorod (55.7°N, 36.8°E)
• The station is located in a rural area 50 km west of Moscow.
• The station is exposed to pollution episodes most frequent and intensive in winter
Zvenigorod
Kislovodsk
Lovozero
Minsk
Issyk-Kul
Tomsk
Petrodvorets
ZhiganskSalekhard
The method of NO2 measurements• Measurements are done by a zenith viewing grating scanning monochromator MDR-23 in 435-450 nm wavelength spectral range (versus 415-465 nm for OMI)• Spectral resolution is 0.7 nm (versus ~0.5 nm for OMI)•Measurements are done in morning and evening twilight at solar zenith angles 84-96°•The instrument and method of measurements of a slant column NO2 were certified by the NDACC following the results of intercomparison carried out at Zvenigorod in 1997• Using measured slant column NO2 abundances, vertical distribution of NO2 is retrieved by solving inverse problem with Chahine method• A NO2 vertical profile is represented by NO2 abundances in ten 5-km thickness layers and in the thin surface layer• The method allows separating unpolluted and potentially polluted parts of the NO2 column content
Examples of NO2 vertical profiles at Zvenigorod retrieved for conditions of clean and polluted boundary layer
During pollution episodes, the NO2 abundance within the atmospheric boundary layer can be significantly larger than in the above troposphere-stratosphere layer
The profiles correspond to solar zenith angle 84°
0 0.5 1 1.5 2 30 40N O 2 layer content (1015 cm -2)
0
10
20
30
40
50
Alti
tude
(km
)
18 June, 2007, evening23 June, 2007, m orn ing
The axis is broken
Comparison with data of SAGE II and CRISTA 2 satellite measurements
CRISTA 2 август 1997 г.
SAGE II
Alti
tude
(km
)
Alti
tud
e
(km
)A
ltitu
de
(km
)
NO2 concentration (108 mol/cm3)
NO2 concentration (108 mol/cm3)
7 April 1993
23 March 1992
Zvenigorod
Zvenigorod
SAGE II (52N, 39.2E)
SAGE II (57.7N, 41.5E)
August 1997
ZvenigorodCRISTA 2
Methodology of comparison with data of OMI measurements
• In deriving slant column NO2 abundances, the NO2 absorption cross sections by Vandaele et al. (1998) are used corresponding to temperature 220 K• The distance of OMI ground pixel centers from the Zvenigorod station is chosen to be less than 45 km• NO2 contents retrieved from ground-based measurements correspond to solar zenith angle 84° • Data of ground-based measurements are then interpolated to time of OMI overpass measurements with the help of photochemical modeling
0 3 6 9 12 15 18 21 24Loca l tim e (hour)
0.4
0.8
1.2
1.6
Str
atos
phe
ric c
olum
n N
O2
(
rela
tive
units
)
D ecem ber 22M arch 22June 22Septem ber 22
Stratospheric co lum n N O 2 d iurna l varia tion
Quantities used for comparison
OMI Ground-based
1 “Unpolluted” vertical column NO2 abundance
(NO2Unpol product)
Vertical column NO2 abundance above the surface layer (includes NO2 contents in ten 5-km thickness layers)
2 Tropospheric vertical column NO2 abundance
(NO2Trop product)
Tropospheric (0-10 km) vertical column NO2 abundance (includes NO2 contents in two tropospheric 5-km thickness layers and in the surface layer)
Comparison of OMI and ground-based measurements
“Unpolluted” column NO2
2005 2006 2007 2008Year
0
1
2
3
4
5
6
NO
2 c
onte
nt (
101
5 c
m-2
)
O M Iground-based, m orn ingground-based, evening
OMI data are generally between morning and evening ground-based data
“Unpolluted” column NO2
2005 2006 2007 2008Year
0
1
2
3
4
5
6N
O2 c
onte
nt (
101
5 c
m-2
)O M IG round-based, in terpolated
Difference (OMI - ground-based)
Mean difference: –(0.300.03)∙1015 cm-2
or –(11.2 1.2)%
Mean square root diff.:
0.6∙1015 cm-2 or 22%
Mean difference:–(0.170.04)∙1015 cm-2
or –(6.9 1.6)%
Mean square root diff.: 0.47∙1015 cm-2 or 19%
-3
-2
-1
0
1
2
Diff
eren
ce,
OM
I-G
B (
101
5 с
м2)
For a ll data
2005 2006 2007 2008Year
-3
-2
-1
0
1
2
For O M I tropospheric co lum n N O 2 < 2 .10 15 cm -2
Correlation of “unpolluted” column NO2 values
Correlation coefficient ~0.9
Linear regression equation
OMI = 0.82GB + 0.2∙1015 cm-2
0 1 2 3 4 5G round-based unpolluted colum n NO 2 (10 1 5 cm -2)
0
1
2
3
4
5
OM
I un
pollu
ted
colu
mn
NO
2 (
101
5 c
m-2
)
Possible reasons of discrepancies between OMI and ground-based stratospheric NO2 data
• Different spectral regions used for measurements• Different spectral resolutions of the two instruments• Different methods of measurements resulting, in
particular, in different spatial resolution (smoothing) of results of measurements
• Different sensitivity to tropospheric pollution
• Temperature dependence of NO2 cross sections that may differently affect derived NO2 contents due to different spectral regions and spectral resolutions of the two instruments
Comparison of OMI and ground-based measurementsTropospheric column NO2
• There is a significant discrepancy between OMI and ground-based estimates of tropospheric NO2 contents• It is related to strong spatial inhomogeneity ant temporal variability of pollution part of NO2 and is due to different spatial averaging of satellite and ground-based observations
2005 2006 2007 2008
0
20
40
60
80
100
120
140
NO
2 c
onte
nt (
101
5 c
m-2
)
O M Iground-based, m orn ingground-based, evening
2005 2006 2007 2008Year
0
20
40
60
80
100
120
140
NO
2 c
onte
nt (
101
5 c
m-2
)
g round-based, in terpola ted
Mean OMI-GB difference: –(1.40.5)∙1015 cm-2
Mean square root difference7.9∙1015 cm-2 (~200%)
Correlation of tropospheric column NO2 values
Correlation coefficient for daily data
~0.4
Linear regression equation
OMI = 0.22GB + 2.2∙1015 cm-2
Correlation coefficient for monthly mean data
~0.45
Linear regression equation
OMI = 0.16GB + 2.3∙1015 cm-2 0 10 20 30 40 50 60 70
G round-based tropospheric co lum n NO 2 (10 1 5 cm -2)
0
10
20
30
40
50
OM
I tr
opos
pher
ic c
olum
n N
O2 (
101
5 c
m-2
) da ilym onth ly m ean
Conclusions
• OMI “unpolluted” NO2 columns underestimate ground-based measurements at Zvenigorod by (0.300.03)∙1015 molecules/cm-2 (~11%), if all data are used, and by (0.170.04)∙1015 molecules/cm-2 (~7%), if a part of data are used for comparison for which OMI tropospheric NO2 columns is not large (< 2∙1015 cm-2)
• OMI tropospheric NO2 columns are on average by (1.40.5)∙1015 molecules/cm-2 (~35%) less than those derived from ground-based measurements
• On the whole, more detail investigation of lower tropospheric pollution effects is needed for more accurate validation of OMI data