A Nine Year Study Of Background Surface Ozone Concentrations
On The Island Of Gozo In The Central Mediterranean
a
a
b
c
Martin Saliba , Raymond Ellul , Hans Güsten , Liberato Camilleri
a
Department of Physics, University of Malta, Msida MSD 06, Malta
b
Institut für Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe,
Universität Karlsruhe, D-76021 Karlsruhe, Germany
c
Department of Statistics and Operations Research, University of Malta
Msida MSD 06, Malta
Atmospheric Research, Physics Department UNIVERSITY OF MALTA Msida MSD 06, Malta
ray.ellul@um.edu.mt . http://projects.um.edu.mt/apr
The second approach involved the plotting of the daily maximum and minimum
means for the sequence of all 9 years and examining the difference in the slopes of
the plots; slopes were computed both with and without the year 2003 which was a
particular year with mean ozone readings higher than expected:
Ozone
mean
maximum
minimum
Slope excluding 2003
0.413
0.382
0.470
0.241
0.150
0.334
Both methods show that the minimum ozone is increasing at a higher rate than the
maximum ozone with a small upward trend of just under 0.5 ppbv per annum over
the nine years.
Since, in the early days, we did not measure all
meteorological parameters we have plugged these gaps
in our own data using local meteorological data from the
meteorological office at Malta International Airport and
global radiation measurements from the institute of
Energy Technology of the University of Malta.
These results are in direct contrast to the results of Gerasopoulos et. al (2005) who
find a decrease in the ozone trend for the Eastern Mediterranean for a seven year
study between 1997 and 2004.
All data collected are averaged every 15 minutes and for
the purpose of this work we have again averaged these
data to hourly values.
The background CO (1997-2005) and SO2 (2004-2005) measurements have also
been analyzed in relation to ozone concentrations and wind sector.
The ozone daily mean observations over the past nine years (1997-2005) are shown
in Fig.1 while Fig.2 shows the ozone monthly means as a simple line graph; the latter
clearly brings out the spring ozone maximum in April- May with a strong shoulder/s
over the summer months lasting into September /October. A clear diurnal cycle is
observed. All 9 years conform to this behavior with the maximum and minimum of
the diurnal cycle varying with the time of the day between winter and summer such
that the trends fit well to what is expected of photochemical ozone formation
(Güsten, 1986; Sonnemann, 1992; Nolle et al., 2002)
The annual averages of the ozone mixing ratios for all nine years are shown in Fig.3
It is clearly evident that we have a slight upward increase of ozone mixing ratios
from 1997 to 2003 with this change leveling off and possibly decreasing very slightly
after this period. Outstanding is the year 2003 with a mean of 54.6ppbv.
To analyze this long term trend we have adopted two approaches:
The first approach computed the percentiles for each individual year. The ozone
percentiles for each individual year were than combined into one data set and a
linear regression analysis was performed for each percentile with the year taken as
CORRELATED CO AND SO2 MEASUREMENTS
A clear background variation with wind sector is visible. It is also clear that the
peaks of both CO and SO2 emissions coincide (Fig.5). In the case of both CO and SO2
the peaks are anti correlated with ozone when prevailing winds blow over the
Maltese islands from the South East. Apart from the wind direction of the main
island of Malta, the observations are due to regional emissions from the Sicilian
mainland as well as ships' traffic in the straits of Sicily- Gozo.
The link between ozone and the meteorological parameters that influence ozone
mixing ratios has been studied using regression analysis with the help of an SPSS
(Statistical package for Social Sciences) software package.
This work has focused on the development of two regression models which predict
the daily maximum and minimum ozone concentrations given information about
the relevant predictors. These predictors include temperature, relative humidity,
wind speed, atmospheric pressure and global radiation and were all assumed to be
covariates. Since it is known that ozone exhibits both diurnal and seasonal cycles,
other predictors such as time of day, time of year and wind direction were included
as factors in the model fit. In order to explain more adequately this ozone variability
these factors were respectively categorized into 24 hours, 12 months and 6 wind
sectors (Fig.6). The coefficient of determination for the maximum and minimum
Ozone mixing ratios (in ppbv)
70
60
50
We have recorded and examined a nine year series of ozone mixing ratios between
1997 and 2005 and find a small upward trend under 0.5ppbv per annum. This is in
contrast to the Eastern Mediterranean where the trend shows a decrease in ozone
over the same period. We attribute this increase of ozone over the Maltese Islands to
(a) long range transport of air masses from the North West as well as (b) higher
regional pollution by ships' traffic in the Gozo-Sicily straits and emissions from the
Sicilian mainland.
REFERENCES
Gerasopoulos, E., Kouvarakis, G., Vrekoussis, M., Kanakidou, M., Mihalopoulos, N.,
2005:
Ozone variability in the marine boundary layer of the Eastern Mediterranean based
on 7-year observations.
J. Geophys. Res. Vol 110, No D15, D15309.
Güsten, H. ; 1986:
Formation, transport and control of photochemical smog. In: Hutzinger, O. (Ed.)
The Handbook of Environmental Chemistry, Vol 4/ Part a Air Pollution. Springer,
Berlin, Germany. pp. 53 106.
Nolle, M.; Ellul, R.; Güsten, H.; Heinrich, G.; (2001):
Long Term Background Ozone and Carbon Monoxide Measurements on the
Maltese Islands.
Proceedings of 8th European Symposium on the Physico Chemical behaviour of
Atmospheric Pollutants. September 2001, Turin, Italy.
Nolle, M., Ellul, R., Heinrich, G., Güsten, H., 2002:
A long term study of background ozone concentrations in the Central
Mediterranean- diurnal and seasonal variations on the island of Gozo.
Atmos. Environ. 36, 1391-1402.
REGRESSION MODELLING
80
CONCLUSION
Sonneman, G.: 1992:
Ozon Natürliche Schwankungen und anthropogene Einflüsse.
Akademie Verlag GmbH, Berlin, Germany.
ACKNOWLEDGMENTS
Meteorological Office, Malta International Airport.
Institute of Energy Technology, University of Malta.
120
160
100
150
80
140
6.5
6
5.5
CO mixing ratios (ppbv)
TRENDS IN SURFACE OZONE LEVELS
Ozone mixing ratios (in ppbv)
period 1997-2005
period 1997-2005
period 1997-2005
Slope
ozone models was improved to r2 = 62.3% and 52.9% respectively (Figs.7a,7b). The
sufficiency of the regression assumptions was also analyzed using Studentized plots.
These show that approximately 95% of residuals lie between +2 and -2 indicating
that the assumptions applied to both regression models are correct.
60
40
5
130
4.5
120
SO2 mixing ratios (ppbv)
A nine year study of ozone concentrations was carried out at a background regional
GAW station (36° 4⁄ 24⁄ north; 14° 13⁄ 9⁄⁄ east) known as Giordan lighthouse on the
island of Gozo in the Central Mediterranean. Details of the station and experimental
setup may be found in Nolle et al. 2002. Measurements are made at a height of 167
meters above sea level with the anemometer being set
above the highest point of the lighthouse at 181 meters
above sea level. The meteorological parameters ( wind
speed and direction, relative humidity and temperature)
as well as the ozone mixing ratio are also measured at
our main laboratory in Xewkija where, in addition, the
pressure and global radiation are measured using a
calibrated Vaisala PTB 101 B barometric pressure sensor
and a Kipp and Zoned SP-Lite pyranometer. For all
intents and purposes we assume that the pressure and
global radiation measurements at Xewkija are valid for
the lighthouse which is only 10 km away.
4
20
40
110
0
1996
30
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
3.5
100
0
Year
20
40
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
3
360
Wind direction in degrees
100
98
50
2
0
Linear (100)
Linear (98)
Linear (50)
Linear (2)
Linear (0)
Carbon monoxide
20
1997
1998
1999
2000
2001
2002
2003
2004
2005
Sulphur dioxide
2006
Year
Figure 1: Plot of daily ozone mixing ratios from January 1997
till December 2005.
Figure 4: Ozone percentiles from January 1997 till December
2005
Figure 5: Variation of CO and SO2 mixing ratios with wind
direction
60
Ozone mixing ratios (ppbv)
55
50
45
40
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Month
Figure 7a: Maximum ozone model scatter plot
Figure 2: Plot of ozone mixing ratios from January 1997 till
December 2005
60
56.6
52.2
50
49.7
49.0
52.4
52.3
50.8
50.4
50.7
40
Mean Ozone (ppbv)
G AW
Station - Giordan Lighthouse
INTRODUCTION
parameter. The variations of the ozone percentiles together with the linear
regression (represented by a trend line) are shown in Fig.4. The 100th percentile is
decreasing at an annual rate of 1.1ppbv while the median and 0th percentile are both
increasing at a rate of 0.5ppbv per annum.
30
20
10
0
1997
1998
1999
2000
2001
2002
2003
2004
2005
Year
Figure 3: Annual averages of ozone mixing ratios from
January 1997 till December 2005
Figure 6: Classification of wind sectors for air masses arriving at
Giordan lighthouse station. The Maltese islands are shown inset
within the map delineating the Mediterranean
and
surrounding countries. The wind sectors are numbered 1 to 6.
sector 3 indicates polluted wind from the main island while
sectors 5, 6, 1 indicate wind sectors relatively unaffected by the
local topography.
Figure 7b: Minimum ozone model scatter plot