CLIMATE AND AIR POLLUTION MONITORING IN TWO FOREST ECOSYSTEMS IN
BULGARIA
Nikolina Tzvetkova, Christina Hadjiivanova - University of Forestry - Sofia, Bulgaria
e-mail: [email protected]
Abstract
The relationship between month and daily mean concentrations of ozone and NOx were
established. Month values of ozone dose (AOT40 values) for the vegetation period as well as the
short-term ozone episodes and some climatic factors (air temperature and humidity, global radiation)
and NOx concentrations is established with direct reference to ICP-forest data. The monitoring of air
pollution is carried out in two forest ecosystems: Vitinia and Jundola. The values of the correlation
coefficients in Pearson correlation matrixes are used as the basis of an estimation of the variability in
these relations. There was not clear trend between month mean ozone concentrations and global
radiation. The coefficient values for the air temperature increased along with the month means of
these parameters. Daily means of the temperature and NOx concentrations are in a good correlation
with the short-term ozone means during the summer months. The tendencies established are
explained in relation to climate peculiarities in two forest ecosystems during the different parts of the
vegetation season.
Introduction
The ground level ozone is responsible for a variety of adverse effects on both human being and
plant life (Lu et al. 2002). Many studies suggest that ozone have a major role in some types of forest
decline (Karlson et al. 2005). Deciduous species appeared to be more sensitive to ozone exposure
while conifers are more sensitive to the concentrations of SO2 and NO2 (Ozolincius et al., 2005). To
protect the forest trees from such adverse effects, early information and precautions about the high
ozone level need to be ascertained (Lu et al., 2002). There remains a great need for information
about NOx and stratospheric ozone
Ground level ozone is formed in the atmosphere by the reactions of volatile organic compounds
and nitrogen oxides in the presence of heat and sunlight. Increased anthropogenic emissions are
likely to raise the background level of ozone. The photochemical formation of tropospheric ozone from
increased concentrations of methane and nitric oxides may also lead to a higher ozone level on a
global scale. A significant increase in the background concentration of ozone has been observed at
several sites in Europe although the underlying causes are not settled (Solberg, 2005).
Measurements and modelling results indicate that there is a strong link between climate and seasonal
factors and surface ozone (Balls et al.1995; Benton et al. 1996; Grünhage and Jäger1994, Grendfelt,
1996). Ozone is formed during all the year but most readily during hot summer weather (Lelieveld et
al., 2002).
Ozone turned to be the most important air pollutant in both relatively clear and urban areas in
Bulgaria (Tzvetkova and Doncheva 2001). However data for annual pattern of distribution of the
elevated ozone values especially in forest stands in Bulgaria are scarce.
This paper reports the average month values for ozone and nitrogen oxide concentrations along
with some climatic factors during the vegetation period of 2001. The data used for the estimation are
received on the basis of continuous pollution and climate monitoring in two typical for Bulgaria forest
ecosystems. The study aims to investigate the importance of meteorological factors and their impact
on relevant pollutant concentration levels. Rough estimation was made of the effect of some potential
ozone sources at the experimental stations.
Acknowledgment: This investigation was carried out with the technical equipment and financial
support of the Swiss Government as a part of Bulgarian-Swiss cooperative project “Monitoring of the
forest ecosystems”.
Key words: ozone, nitrogen oxides (NOx), climatic factors
Materials and Methods
The study was made in typical for Bulgaria coniferous and deciduous ecosystems - Jundola and
Vitinia.
Station Vitinia. The experimental station Vitinia is situated at an altitude of 950 m. The main
sample plot there is with 130 years old beech (Fagus sylvatica L.) of a natural origin. As a part of the
West Balkan region of the Moderate Continental forest vegetation zone Vitinia belongs to the
Mountain climatic region, where the relief and its underlying surface have an important role for the
climate formation. These factors are in relation with the orographic situation, range, altitude, depth of
uneven, slop bias, slop exposition, density of shading and duration of snow covering. This region is
distinguished not only by the specific state of the separate climatic elements but with their peculiar
daily, seasonal and annual course. The territory is under the influence of air masses coming from big
industrial source - Kremikovci as well as from Sofia city and the nearby high road Hemus. To a certain
extend the effect on the orientation of this transfer is in relation to the movement of the air masses,
generated by the specificity of the microclimate, which is typical for the urban territories, enclosed by
high mountains.
Station Jundola. The experimental station Jundola is situated in the Rodopi mountain at an
altitude of 1600 m. The stand is natural with 90 – 170 years old 40% spruce (Picea abies L.) and 40%
fir (Abies alba L.) and single trees of Scots pine (Pinus sylvestris L.) and beech (Fagus sylvatica L.).
This region is accepted as non-polluted by local sources.
The concentrations of the NO2, NOX and O3 were measured automatically. The ML®9841A
nitrogen oxides analyzer works on the basis of gas-phase chemiluminescence detection to perform
continuous analysis of nitric oxide (NO), total oxides of nitrogen (NOx), and nitrogen dioxide (NO2).
Non-dispersive ultraviolet photometer serves as the basis for the ML®9812 Ozone Analyzer. The
hourly means of O3 levels for daylight hours (with a radiation above 50 W/m2) are used as a basis for
estimation of ozone doze AOT40.
The study deals with the characteristics of daily and month mean ozone and nitric oxide levels
under different climatic conditions such as temperature, solar radiation, relative air humidity. The
values of 24-hour and month averages for O3 and NOx as well as the cumulative ozone exposure
(AOT40) for the growing seasons are established. The AOT40 (accumulated exposure over a
threshold of 40 ppb) is calculated as the sum of the differences between the hourly concentration (in
ppb) and 40 ppb for each hour when O3 concentration exceeds 40 ppb. The ozone exposure was
determined for the period of six months - from April to September - the period of maximal sensitivity of
the tree plants because of their high physiological activity and intensive shoot growth.
Throughout the monitoring period the main climatic conditions (air temperature - Tair, relative
humidity - RHair, and solar radiation - Rad) were recorded continuously too. Air humidity was
measured by hygrometer MP 100A (Rotronic) and radiation - by piranometer Li-200SZ.
The data are proceeded statistically (Systat 7.0). Specific command files are created in order to
put the logged values as a separate variable for statistical analysis. A half-hour means of the
concentrations for every pollutant and the level of the climatic factors has been logged in data files as
a separate cases. The relationship between month and daily mean concentrations of ozone and NOx,
air temperature and global radiation were established.
The values of the correlation coefficients in Pearson correlation matrix are used as the basis of an
estimation of the variability in these relations.
Due to the relatively short period of the meteorological observations at the region of the station
the information represented is mainly ascertained, without defining of a tendency in a deviation of
some climatic elements.
Results and Discussion
In station Vitinia month means of ozone concentrations varied from 40 to 48 ppb and the range for
nitrogen oxides is between 1.9 and 4.4 ppb.
The highest month average value for ozone concentration was observed in August, the month with
hot and very dry weather (Table 1). Month value of cumulative ozone exposure AOT40 increases
more than 2.5 times from April to August along with an increase of mean air temperature and
radiation.
Over the growing season the daily means of ozone concentrations were only during a few days
below the critical level of 25 ppb. Almost all of the 24-hour means for ozone concentration during the
entire period were above the critical level of 30 ppb.
Total value of cumulative ozone exposure for the growth period in station Vitinia is about 1.8 times
above the threshold (10 000 ppb.h) – Laurila and.Tuovinen (1996).
Month means of NOx varies between 1.9 and 4,4 ppb and average value of NO2 for the summer
period is far below the critical value of 30 ppb (Caporn, 1992)
Table 1
Month values of AOT40 and means of air pollutants and climatic factors in station Vitinia during
growth period of 2001
Month
April
May
June
July
August
September
Growth period
AOT 40
О3
(ppb.h)
(ppb)
2124
46.4
1757
40.4
2570
45.1
3614
46.9
6065
48.1
1614
44.1
17744
NOx
(ppb)
4.4
1.9
4.1
3.2
2.4
3.3
TAIR
(ºC)
9.7
17.3
18.3
22.1
20.4
21.7
RHAIR
(%)
52.2
56.3
48.8
47.4
32.4
47.2
Rad
(W/m 2)
377.8
427.2
475.1
465.4
546.8
412.9
There was not clear trend between month means of ozone concentrations and global radiation
(Table 2). The coefficient values for the correlation between ozone concentration and air temperature
increased along with the month means of these parameters. Some deviations from the tendency were
observed in July and September and may be due to the different values of relative air humidity. There
was no significant correlation between month average values of ozone and NOx for the entire period.
Table2.
Correlation coefficients for the relation between mean values of air pollutants and
some climatic factors in separate months during the growth period in station Vitinia
Month
O3/Tair
O3/Rad
April
May
June
July
August
September
0.390
0.604
0.580
0.751
0.792
0.800
0.435
0.445
0.389
0.424
0.546
0.362
NOx/O3
-0.210
-0.332
-0.462
0.224
0.117
-0.057
Daily means of air temperature and NOx concentrations are in a good correlation with the shortterm ozone average values during the summer months (Table 3).
Table 3.
Days with the highest ozone and nitrogen oxides mean concentrations and correlation
coefficients for the relation of O3 to NOx, air temperature and solar radiation (along with 24-means of
climatic factors) in station Vitinia
Date
O3/Tair
O3/Rad
NOx/O3
28.05
25.06
29.06
10.07
0.933
0.936
0.578
0.945
0.387
0.693
0.545
0.618
-0.943
-0.934
-0.929
-0.945
О3
NOx
TAIR
(ppb)
(ppb)
(ºC)
54.4
2.8
18.9
44.9
9.9
18.5
49.7
6.4
23.3
51.4
5.6
23.6
Rad
(W/m 2)
482.8
471.3
509.6
584.4
In station Jundola month means of ozone concentrations varied from 44 to 54 ppb and the range
for nitrogen oxides is between 1.0 and 4.3 ppb.
The highest month value of mean ozone and NOx concentrations as well as of AOT40 was
observed also in August. The total sum of ozone doze during the entire growth period in this station is
1.6 times above the threshold about forest ecosystems (Laurila andTuovinen (1996).
Regardless of almost equal mean values of ozone concentration and air temperature in July and
August there was a big difference between values of AOT40 for these months. May be the differences
is due to the variability in relative air humidity and solar radiation at that time from the growth period.
Table 4.
Month means of AOT40 and average values of air pollutants and climatic factors in station Jundola
during the growth period of 2001.
Month
April
May
June
July
August
September
Growth period
AOT 40
(ppb.h)
1884
1800
2499
3587
5487
1176
16433
О3
(ppb)
48.0
44.3
45.6
52.4
54.0
44.1
NOx
(ppb)
1.0
1.3
1.7
2.3
4.3
3.6
TAIR
(ºC)
13.2
17.2
21.9
26.0
25.6
21.7
RHAIR
Rad
(%)
(W/m 2)
65.0
374.3
63.5
481.4
50.2
542.4
47.8
492.8
41.2
514.6
47.2
412.9
The correlation between month average ozone concentrations and air temperature is high during
the summer month - especially in July and August, but there was not any significant dependence
between ozone means, solar radiation and NOx average value (Table 5).
Table 5.
Correlation coefficients for the relation between air pollutants and some climatic factors in
separate months during the growth period in station Jundola
Month
O3/Tair
O3/Rad
NOx/O3
April
May
June
July
August
September
0.388
0.551
0.723
0.788
0.805
0.737
0.269
0.334
0.365
0.283
0.319
0.327
0.160
-0.220
0.107
- 0.304
- 0.443
0.234
Similarly, in station Jundola the 24-hour ozone means are in a good correlation with air
temperature and NOx. The negative correlation between short time ozone exposure and daily NOx
concentrations is due to ozone formation during NOx depletion This effect is more pronounced in the
periods with high levels of air temperature (Table 6).
Table 6.
Correlation coefficients for the relation of ozone to NOx and air temperature in the days with the
highest mean ozone concentrations and 24-hour means of climatic factors in station Jundola
Date
O3/Tair
O3/Rad
NOx/O3
02.05
05.06
08.08
24.08
0.768
0.645
0.857
0.837
0.300
0.381
0.290
0.304
0.977
-0.607
-0.593
-0.725
О3
NOx TAIR Rad
(ppb) (ppb)
(ºC)
(W/m 2)
51.4
7.6
18.8
599.4
63.2
3.8
19.6
586.4
61.1
5.6
26.4
455.1
65.5
4.1
28.1
476.9
Higher correlation between ozone and nitrogen oxides was found in days with more intensive solar
radiation.
The minimal and maximal month means of air temperature in Vitinia (respectively, 9.7 and 22 ºC)
were lower than in Jundola (13.2 and 26 ºC). In the same time relative air humidity was higher in
station Jundola. There are some differences in the summer pattern of NOx concentrations between
two stations. In station Vitinia the higher mean values of these pollutants were observed during the
first part of the vegetation period, but in Jundola there is an opposite situation. There was no such
difference between two stations in relation to the dynamics of ozone mean values – they are higher in
the second part of the growth period.
Highway emissions of NOx near station Vitinia along with the big industrial source Kremikovci are
the main reasons for the higher summer concentrations of nitrogen dioxides there. The dryer climate
along with the higher NOx level in Vitinia are more favourable for ozone formation in the station
(Suppan and Schadler, 2004). But the month means of ozone concentration within the growth period
were lower than in station Jundola. It may be due to the stronger influence of remote air pollutant
source and the open relief of the forest ecosystem in Jundola. The higher mountains, which enclose
station Vitinia, probably serve as an effective protector against the air masses coming from nearby
highway “Hemus” and Kremikovci. The forest ecosystem in station Jundola is consisted of coniferous
tree species, which produce huge amount of hydrocarbons. These compounds are an effective
precursors for ozone and may be a reason for higher ozone levels in Jundola.
According to Suppan and Schadler (2004) in regions with low NOx concentration ozone formation
is dependent entirely on NOx (NOx sensitive regions) while in urbanic regions where NOx
concentrations are usually high ozone formation depends on the concentration of volatile organic
compounds.
Some scientists have suggested that there are natural buffers and various unknown sources of
NOx that make it impossible to ascertain. The intensive fumigation with nitrogen oxides significantly
increase stratospheric ozone levels (Lelieveld et al., 2002).
The correlation between ozone and NOx concentration is better for short-term exposures while the
correlation with temperature is well seen also for the longer periods.
In two stations the highest mean concentrations of ozone were observed in August – the month
with very dry and hot weather.
The patterns of month average concentrations of O3 during the growth period in two stations were
similar, but the seasonal dynamics of mean NOx values has some specificity. It may be due also to
the peculiarities in growth and physiological activity of conifers and deciduous forest ecosystems.
Consequently, the type of vegetation may plays an important role in the dynamics of air pollutants.
Conclusions
From the results and discussion some suggestions could be made for the pattern of ozone
episodes during the vegetation period at the two forest locations.
The distribution of the ozone concentrations during the growing season shows that the highest
ozone levels in 2001 coincided with a period when the trees were exposed to high temperature and
drought stress.
A good correlation was observed between month mean ozone values and air temperature in both
forest ecosystems.
Hourly ozone concentrations above the level of 40 ppb are measured more often at the station
Jundola. It can be related with the transfer of precursors for photochemical ozone production, may be
hydrocarbons compounds.
There was not clear trend between month mean ozone concentrations and global radiation. The
coefficient values for the air temperature increased along with the month means of these parameters.
Daily means of the temperature and NOx concentrations are in a good correlation with the short-term
ozone doses during the summer months. The tendencies established are in relation to tree species,
climatic factors and NOx levels during the different parts of the vegetation season.
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