OZONE AND STRATOSPHERIC NITROGEN DIOXIDE MONITORING
IN THE SOUTH OF PORTUGAL FROM GROUND BASED AND
SATELLITE MEASUREMENTS
D. Bortoli1, 2, G. Giovanelli1, A.M. Silva2
1
Geophysics Centre, University of Évora
R. Romão Ramalho, 59, 7000 Évora, Portugal
2
Institute of Atmospheric Sciences and Climate (ISAC-CNR)
Via Gobetti, 101, 40129 Bologna, Italy
Email: [email protected]
ABSTRACT
Thanks to the new satellite technology, we have global scenes of the dynamical and chemical processes of
the atmospheric compounds. Ground-based equipments have to be used for the validation of the satellite
retrievals, particularly when there are clouds in the observed pixels. The SPATRAM (SPectrometer for
Atmospheric TRacers and Aerosol Measurement) is a UV-Vis spectrometer allowing for the measurements
of spectral sky radiance along the zenith direction, in its standard configuration. This equipment is installed
at the CGE observatory at Évora, in south of Portugal. The application of Differential Optical Absorption
Spectroscopy methodology to the measured raw data, allows for the retrieval of the total column content of
several traces gases.
The mid-latitudes atmosphere is a complex region of our planet, where air masses coming from lower and
higher latitudes meet, essentially, to balance the temperature gradient. The study of the relationship
between the Polar Regions and the mid-latitudes is well developed nowadays, due to the polar ozone loss.
On the other hand the interaction between tropics\subtropics and mid-latitude regions still remains
insufficiently understood. Stratospheric ozone is continuously subordinated to chemical processes of
creation and destruction by chemically active compounds as chlorofluorocarbons, and also from other
gases like nitrogen, bromine, chlorine and iodine oxides. The NOx compounds manifest a strong diurnal
variability for the photochemical nature of the dissociation reactions. Recent iodine monoxide observations
(IO) seem to confirm the hypothesis that it plays a not negligible role in the ozone depletion processes.
Therefore, studies of total column and vertical profile of atmospheric gases, obtained with DOAS
methodology, in zenith sky or off-axis configuration, are of extreme importance as from the satellite
observations as from the ground based ones.
Ground-based stations have for many years been providing a large amount of useful measurements to
study the atmospheric chemistry and dynamics and to monitor the environmental pollution. From such
stations long term data-set was built-up in many regions of the planet to study the time variations of
stratospheric trace gases, aerosol loading and other important physical parameters as a function of
particular natural events such as volcanic eruptions. The recent launch of the ENVISAT satellite, have
enlarged the number of existing sensors (SCIAMACHY, GOMOS) for the retrieval of atmospheric gases
total column variations. Here, the first results obtained with the SPATRAM spectrometer, for the South of
Portugal, and comparisons with GOME and TOMS data will be presented.
1. INTRODUCTION
In the last thirty years, the stratospheric ozone in the Northern Hemisphere (NH) mid-latitudes has suffered a
meaningful decrease. The analysis of results obtained from TOMS (Total Ozone Mapping Spectrometer),
installed on the Nimbus 7 satellite, underlines the significant reduction of the ozone total column also before
the Pinatubo eruption. During the eighties, studies on ozone profile temporal series in the NH show that the
annual average ozone concentration increases in troposphere and decreases in the low stratosphere
(Angell, 1983). Other authors confirm these results (Bojkov and Fioletov, 1995; Wang, 1993), underlining
that in Southern Europe, the values of the stratospheric ozone trend per decade are higher than those found
in the United States, also considering areas at the same latitudes and with equal values of superficial
nitrogen oxides. Comparisons of the results obtained for both hemispheres lead to the conclusion that ozone
loss is smaller at northern high latitudes than the Antarctic depletion phenomena. At mid-latitudes this
difference is balanced with equivalent values between 5% and 8% per decade (Randel, 1999), but it would
seem greater in the NH. Nevertheless, the physical processes that can justify these losses in the ozone
budget of the north mid-latitude, is not completely explained. The hypothesis of the air masses transport from
the polar vortex towards the South Hemisphere (SH) cannot be completely applied for the NH. In fact, Arctic
regions show very different meteorological conditions as higher average temperatures that do not favour the
Polar Stratospheric Clouds (PSCs) formation. On the other hand, the lower polar vortex strength allows for
losing its shape causing air masses intrusions until the mid-latitudes.
For some years the nitrogen compounds were thought to be the greatest responsible for the ozone catalytic
destruction. The understanding of the dynamical processes, chemical and photochemical reactions involving
nitrogen compounds in the stratosphere has improved. The existence of a winter drop in NO2 total column at
high latitudes has been explained by the coupling of dynamical and chemical phenomena (Solomon and
Garcia, 1983). As clearly shown by Noxon (1979), the horizontal transport plays a very important role at midlatitudes: air can move very rapidly from the high to the low latitudes, depending on the weather conditions.
Sometimes this process of dislocation is slow so the chemistry of NO, NO2, N2O5 and HNO3 can play the
primary role in the nitrogen compounds partitioning, that follow the change in temperature and in insolation
time caused by the slow air movement. Besides, now we know more about the interference between ClOx
and NOx family cycle. Chlorine nitrate (ClONO2) is formed by the NO2 that reacts with ClO, giving a long-lived
reservoir, thus inhibiting the large catalytic ozone depletion potential of the chlorine monoxide as
hypothesized by Molina and Rowland (1974).
This study presents trace gases vertical column results obtained from DOAS (Differential Optical Absorption
Spectroscopy) methodology applied to the first data of diffused zenith sky radiation obtained with the
SPATRAM (SPectrometer for Atmospheric Tracers Measurements) instrument, installed at the observatory
of the Geophysics Centre of Evora – South of Portugal. In addition, the comparison with equivalent results
obtained from satellite borne equipment (GOME and TOMS) are presented and discussed.
2. GROUND BASED INSTRUMENTAL SET UP
The SPATRAM is a UV-Vis spectrometer for the measurements of the solar scattered radiation along
different directions. The instrument was developed by the Atmospheric physics group of the University of
Evora (UE) thanks to a collaboration with the ISAC-CNR Institute in Italy. It is installed in the Observatory of
the Geophysics Centre of Evora (Fig. 1) since the end of March 2004.
The spectrometer is installed inside a thermostatic box able to keep the internal temperature within the
working range of the instrument (typically 15°C for the Optical Unit).
The equipment allows for multiple input of the radiation: from the primary input that is composed of a series
flat and spherical mirror that focalize the light beam on the entrance slit (0.1 x 8 mm); from the optic fibre
inputs where the signal is carried to the entrance slit with an optic fibre linked to a very simple optic system.
A rotating mirror, driven by a stepper motor, allows for choosing between the primary input. the optic fibre
ones and the additional input for spectral or radiometric calibration.
The spectrometer is composed of a holographic spherical diffraction grating of 1200 grooves/mm and a Back
Illuminated CCD sensor array by Marconi. The spectral dispersion is approximately 2.4 nm/mm (depending
on the investigated spectral region), and the overall typical spectral resolution is about 0.5 nm. The grating is
controlled by a stepper motor which, by varying the angle of incidence of light, allows the instrument to
obtain measurements from 250 to 950 nm. in spectral windows of 60 nm each. The CCD camera has a big
quantum efficiency allowing to perform the so-called ‘vertical binning’ of the vertical pixels in order to obtain
an higher signal to noise ratio. In order to minimize the effects of the dark and read current one double
stadium Peltier circuit cools the CCD chip till –40°C. Band-pass filters are used to reduce the stray light
inside the spectrometer. The integration time is automatically selected for each reading and ranges from
tenths of milliseconds at noon to a tenth of seconds at twilight. An internal mercury lamp is used for periodic
checks of the diffraction grating positions, ensuring a spectral accuracy of better than 0.2 nm.
The Electronic Control Unit (ECU) is equipped with 1GHz CPU. The SPATRAM is handled by a software tool
(DAS – Data Acquisition System) that was developed in order to manage all the spectrometer devices and
for automatic scheduled measurements and unattended mode as per predefined measurements cycle; the
ECU provides also to the storage of the recorded spectra and to their first analysis, with DOAS methodology,
in order to obtain the vertical content of the species presenting absorptions features in the selected spectral
range.
Stara Zagora
Stara Zagora
Mt. Cimone
Evora
Lampedusa
Fig.1 – Geo-location of Evora where the SPATRAM is installed since the end of March 2004.
3. SATELLITE DATA SOURCE – GOME and TOMS
ERS-2 was launched in April 1995 into a near-polar sun-synchronous orbit at a mean altitude of 795 km. The
descending node crosses the equator every 2800 km at 10:30 local time. GOME is a nadir-scanning double
monochromator covering the 237 nm to 794 nm wavelength range with a spectral resolution of 0.17-0.33 nm.
The spectrum is split into four spectral channels, each recorded quasi-simultaneously by a 1024-pixel
photodiode array. The global spatial coverage is obtained within 3 days at the equator by a 960 km acrosstrack swath (4.5 s forward scan, 1.5 s back scan). The ground pixel size of the measurements is 320 X 40
km2. The solar irradiance is measured daily. Details of the overall scientific objectives of GOME, instrument
concept, and some first scientific results are reported elsewhere (Burrows et al., 1999).
The Nimbus 7 spacecraft was in a south-to-north, sun-synchronous polar orbit so that it was always close to
local noon/midnight below the spacecraft. Thus, ozone measurements were taken for the entire world every
24 hours. TOMS directly measures the ultraviolet sunlight scattered by the Earth's atmosphere. Total column
ozone is inferred from the differential absorption of scattered sunlight in the ultraviolet range. Ozone is
calculated by taking the ratio of two wavelengths (312 nm and 331 nm, for example), where one wavelength
is strongly absorbed by ozone while the other is absorbed only weakly. The instrument has a 50 kilometres
square field of view at the sub-satellite point. TOMS collects 35 measurements every 8 seconds as it scans
right to left producing approximately 200,000 ozone measurements daily.
4. RESULTS AND DISCUSSION
Ground-based measurements collocated in time and space with the satellite ones are of great importance in
estimating the real accuracy of satellite and ground based data. Comparison of GOME NO2 total content
measurements with the simultaneous ground-based observations at the CGE observatory was accomplished
The SPATRAM data of diffused solar radiation are analysed for the period 1st of April – 19th of May 2004. In
order to obtain the slant column density (SCD) of the investigated species, the DOAS algorithms are applied
in the 425-455 nm spectral window where the strongest absorbers are NO2 and Ozone. The absorbers
vertical column density (VCD) are obtained applying to the SCD the air-mass factor (AMF) calculated with
the AMEFCO RT model (Petritoli et al., 1999). In order to avoid bias due to the seasonal dependency of the
vertical profile and hence in the AMF, different boundary condition are applied to the model with the aim to
distinguish summertime and wintertime air masses. The value at 90 degrees of Solar Zenith Angle (SZA) is
calculated with a cubic interpolation of the data, because the SPATRAM is not able to take measurements at
a fixed angle. The errors associated to the presented results are in the range of 5-8% for the vc at 90°.
Values with larger errors, due to very low signal intensity, are rejected.
The plot of figure 2 is an example of NO2 SCD time series for 10 days of the whole period of measurements.
Analysing the plot, where the abscissa’s values are date and time, it seems that the AM and PM values are
asymmetric in respect to the local noon. Simply, this fact is due to the daylight saving time. In fact, looking at
figure 3, where the SCD are plotted versus the Solar Zenith Angle, the maximum values for AM and PM SCD
are retrieved for about 93° of SZA. The higher PM values than the AM are caused by the photochemical
activity of the Nitrogen Dioxide as explained below.
NO2 can be oxidized by O3 to form NO3, a strong atmospheric oxidant and a precursor to the formation of
dinitrogen pentoxide, N2O5. Because NO3 is rapidly photolyzed at visible wavelengths, both its daytime
concentration and chemistry are of relatively minor importance in the lower and middle stratosphere. In
contrast, N2O5 can have morning concentrations in the lower stratosphere comparable to NOx (NO+NO2)
even though its production requires the formation of NO3.
NO2 SlantColumnDensity [molec/cm2]
1.2E+17
1.0E+17
8.0E+16
6.0E+16
4.0E+16
2.0E+16
0.0E+00
11/04/04 00.00
10/04/04 12.00
10/04/04 00.00
09/04/04 12.00
09/04/04 00.00
08/04/04 12.00
08/04/04 00.00
07/04/04 12.00
07/04/04 00.00
06/04/04 12.00
06/04/04 00.00
05/04/04 12.00
05/04/04 00.00
04/04/04 12.00
04/04/04 00.00
03/04/04 12.00
03/04/04 00.00
02/04/04 12.00
02/04/04 00.00
01/04/04 12.00
01/04/04 00.00
Date Time [dd/mm/yy hh.mm]
Fig.2 – Time series of NO2 slant column density obtained with SPATRAM for the period 21/04-01/05/2004 at
the CGE Observatory.
1.E+17
NO2 SlantColumnDensity [molec/cm2]
NO2 AM SCD
NO2 PM SCD
1.E+17
8.E+16
6.E+16
4.E+16
2.E+16
0.E+00
60
65
70
75
80
85
90
95
100
Solar Zenith Angle [deg]
Fig.3 – NO2 slant column density versus Solar Zenith Angles for the 13th of May 2004.
8.0E+15
NO2 VerticalColumnDensity [Molec/cm2]
7.5E+15
SPATRAM AM
SPATRAM PM
GOME
7.0E+15
6.5E+15
6.0E+15
5.5E+15
5.0E+15
4.5E+15
4.0E+15
24/03/2004
03/04/2004
13/04/2004
23/04/2004
03/05/2004
13/05/2004
23/05/2004
Date [dd/mm/yyyy]
Fig.4 – NO2 VCD at 90° of SZA (AM - sunrise and PM - sunset) obtained with SPATRAM and GOME results
for the period of activity of the ground based equipment.
NO2 + O3 → NO3 + O2
NO3 + hν → NO + O2
→ NO2 + O
NO3 + NO2 + M → N2O5 + M
(1.1)
(1.2)
(1.3)
(1.4)
The explanation results primarily from the difference in photolysis rates of N2O5 and NO3. During the night
N2O5 builds at the expense of NO, or more precisely at the expense of NO2
After sunrise, N2O5 can be photolyzed to essentially two NOx molecules
N2O5 + hν → NO3 + NO2
(1.5)
but the photolysis time constant is dependent on both the usual factors (zenith angle,altitude, albedo, etc.)
and the ambient temperature.
600
SPATRAM
500
TOMS
Ozone Vertical Column [DU]
GOME
400
300
200
100
0
29/03/2004
08/04/2004
18/04/2004
28/04/2004
08/05/2004
Date [dd/mm/yyyy]
Fig.5 –SPATRAM, GOME and TOMS results for O3 VCD during the period of activity of the ground based
spectrometer.
The satellites and the ground-based spectrometers capture major stratospheric features similarly. Although it
is difficult to evaluate precisely the accuracy of the NO2 total column due to various problems such as the
diurnal variation of NO2 and the profile shape effect on the Air Mass Factor (AMF), the overall accuracy in
areas of low tropospheric NO2 is estimated to fall within the 5% to 20%.
Figure 4 shows the comparison of the results obtained with the SPATRAM spectrometer and the GOME
instrument for this firs period of activity of the ground-based equipment. The GOME data are closer to the
SPATRAM AM results due to the fact that the overpass of the ERS-2 satellite over Evora is around 11:30 in
the morning, when the photochemical process of NO2 creation is just started. It is also possible to appreciate
the positive seasonal trend of NO2 VCD, that will reach the maximum in the summer and the minimum in the
winter.
The comparison of the O3 results obtained with the SPATRAM instrument with other satellite borne
equipment are showed in figure 5. In this plot the SPATRAM results for Ozone VCD are the daily averages
of all the data obtained during the period of activity of the instrument during the day ( for SZA in the range
65°-95°, in order to maximize the signal to noise ratio of the measurements)
Both TOMS and GOME results are in good agreement with the SPATRAM data. The episodically events of
under or over estimation of the ozone VCD of the ground based equipment are mainly due to the different
meteorological conditions of the zenith sky measurements and to the point of view of the instruments (from
the space and from the ground.
5. CONCLUSIONS
The goal to install an automatic DOAS instrument at the Observatory of Evora for the measurements of
atmospheric tracers in unattended mode was reached. Vertical columns of NO2 and Ozone for the first 40
days of measurements are presented and discussed. The seasonal trend and the diurnal variations for the
column values of NO2 show a good agreement with both the photochemical theory of the NOx family and
values observed by other authors in mid-latitude sites. Observation of nitrogen dioxide can improve our
understanding of the complex physical and chemical processes in the stratosphere.
No evident problems occur to the optical equipment during the period of activity.
The comparison of the SPATRAM results with satellite borne equipment shows a good agreement,
encouraging the possibility to continue with the work of validation of the satellite results. This activity could
be enlarged as the SPATRAM is similar to other spectrometers installed at the climatic Station of Lampedusa
(south of Italy), at the Ottavio Vittori Station over the Mt Cimone (Italy), at the Bulgarian Atmospheric Station
in Stara Zagora and at the Italian Antarctic Station.
6. ACKNOWLEDGMENTS
The authors thank the scientists who proposed GOME and TOMS, the ESA team that ensured the success
of ERS-2 satellite and the NASA for the Earth Probe satellite. Daniele Bortoli was financially supported by
the Subprograma Ciência e Tecnologia do 3° Quadro Comunitário de Apoio.
7. BIBLIOGRAPHIC REFERENCES
ANGELL, J.K.(1983): Global variation in total ozone and layer mean ozone: an update through 1981, J. Clim.
And Appl. Meteorol., vol. 22, pp. 1611-1626.
BOJKOV R., V.E. Fioletov, (1995):Estimating the global ozone characteristics during the last 30 years, J.
Geophys. Res., vol. 100, pp. 16537-16551.
BURROWS, J. P., M. Weber, M. Buchwitz, V. Rozanov, A. Ladstätter-Weißenmayer, A. Richter, R. de Beek,
R. Hoogen, K. Bramstedt, K.-U. Eichmann, M. Eisinger, and D. Perner, (1999) The Global Ozone
Monitoring Experiment (GOME): Mission concept and first scientific results, J. Atmos. Sci., vol. 56, pp.
151-175.
MOLINA M.J. and Rowland F.S. (1974): Stratospheric sink for chlorofluoromethans: chlorine atoms
catalysed destruction of ozone. Nature, vol. 249, p. 810.
NOXON J.F. (1979): Stratospheric NO2, Observational Method and behaviour at mid latitude, Geophys. Res.
Lett., vol. 84, pp. 5047-5065.
PETRITOLI A., G. Giovanelli, P. Bonasoni, T. Colombo, F. Evangelisti, U.Bonafe, D. Bortoli, Iv. Kostadinov
and F. Ravegnani, (1999) “Ground Based NO2 and O3 Analysis at Mt. Cimone Station during 19951996: a case study for spring 1995 NO2 concentration profile”, in Spectroscopic Atmospheric Monitoring
Techniques, K.Schafer, ed., Proc. EUROPTO 3867, pp. 280-289.
RANDEL, W. J., R.S. Storalski, D.M. Cunnold, J.A.Logan, M.J.Newchurch, J.M. Zawodny (1999): Trends in
the vertical distribution of Ozone, Science, vol. 285, pp. 1689-1692.
SOLOMON, S. and R. Garcia, (1983): On the distribution of Nitrogen Dioxide in the High latitude
stratosphere, J. Geophys. Res., vol. 88, pp. 5229-5239.
WANG, W.C.(1993): Climate implication of observed changes in ozone vertical distribution at middle and
high latitudes in the northern hemisphere, Geophys. Res. Lett., vol. 20, pp. 1567-1570.