atmospheric fluxes and concentrations of monoterpenes in resin-tapped pine forests

Pergamon Atmospheric Environment Vol. 32, No. 4, pp. 683-691, 1998 © 1998 Elsevier Science Ltd

All rights reserved. Printed in Great Britain P I I : S1352-2310(97)00327-0 1352-2310/98 $19.00 + 0.00

ATMOSPHERIC FLUXES AND CONCENTRATIONS OF MONOTERPENES IN RESIN-TAPPED PINE FORESTS

CASIMIRO A. PIO* and ANTONIO A. VALENTE Departamento de Ambiente e Ordenamento, Universidade de Aveiro,

3810 Aveiro, Portugal

(Finest received 3 March 1997 and in final form 23 July 1997. Published February 1998)

Akstract--l~:easurement of VOC emission from exposed resin to the atmosphere was done along the 1-year cycle in resin-tapped pine forests. Emissions are composed mainly of ct- and fl-pinene. Emission rates depend in a complex way of the resination cycle, being influenced by ambient temperature and resin- tapping practices associated with the frequency of bark removal and the application of acidic active past in tree wounds. An algorithm with an exponential dependence of emission rate on temperature was calculated from the field experiments and used to estimate average annual emission fluxes from Portuguese resin- tapped pine forests. Emissions from resin are clearly dominant over evaporation from leaves in resin-tapped forests, being relevant to the atmospheric biogenic VOC budget in Portugal. © 1998 Elsevier Science Ltd. All rights reserved.

Key word index: Biogenic hydrocarbons, monoterpene, resin tapping.

INTRODUCTION

Volatile hydrocarbons emitted biogenically have an important role in atmospheric chemical processes, affecting the production of chemical oxidants, the budget of carbon monoxide and possibly the forma- tion of organic aerosols (Warneck, 1988; Chameides et al., 1992).

In regional or global inventories of biogenic hydrocarbons volatilization from leaves is the only source considered, because it is thought that they contribute more than 90% of the total isoprene/monoterpene fluxes to the atmosphere (Guenther et al., 1995).

Several field experiments have shown, however, the existence of other ,~ources of volatile hydrocarbons, principally of monoterpene-like compounds. These include the soil of forests, leaf litter deposited on forest floor and the rool:s of forest trees (Janson, 1992; Kotzias et al., 1995; Eichstraedter et al., 1993; Zimmerman, 1979).

In conifers, when branches and leaves are damaged, exposing resin channels, a dramatic increase in monoterpene emission fluxes has been observed (Cheniclet, 1987; Knoeppel et al., 1981; Juuti et al., 1990). The practice of resin tapping in live trees exposes large surfaces of extruded resin to the forest ambient air, increasing the evaporation of the volatile resin fraction. These non-leaf monoterpene emissions are presumably irrelevant at the global level. However, in

*Author to whom correspondence should be addressed.

areas where resin-tapping practices are common they may have an important effect on local photochemical atmospheric processes.

Resin tapping is common in Portuguese maritime pine (Pinus pinaster) forests, being the country's important producer of resin derivatives. The resin from maritime pine trees contains usually more than 70% of non-volatile matter, essentially formed by diter- penes, and a volatile fraction composed predomi- nantly by monoterpenes, but also containing small amounts of sesquiterpenes, alcohols, aldehydes and ketones (Cheniclet, 1987; Pauly et al., 1973).

An experimental study was performed along the various seasons of the year with maritime pine trees to evaluate hydrocarbon emissions from resin-tapping practices. This paper shows some of the results. A more detailed description is given in Valente (1995).

Resin-tapping practices

The emission of volatile organic compounds during resin tapping is strongly related with tapping activities, and therefore, a description of the process is convenient to fully understand the following discussion.

Resin tapping is made on the trunks of fully grown trees. The activity is seasonal and extends from spring to autumn. In March, an area of approximately 10 cm width by 5 cm height is cut on the trunk surface, as near as possible to the ground. The bark is removed mechanically exposing the under-wood surface. Resin extrudes from resin channels and drips into an open bucket, or plastic bag, placed on the cutting base. To

683

684 C.A. PIO and A. A. VALENTE

increase resin flux and to extend the extruding period a chemically active paste containing sulphuric acid is applied to the exposed fresh surface. The extrusion of resin peaks immediately after the cutting, decreasing with time. As a result new bark removals are made in the trunk, in an area immediately above the previous cutting, at intervals that vary between 15 days and one month. At the end of the annual campaign, the wound resulting from bark removal has a height of up to 50 cm, as a consequence of 10-13 successive cutting operations. In October/qXlovember the process is fin- ished and the extruded resin collected and removed from the forest. During the campaign intermediate resin collection can happen when the buckets/bags are full. The collected resin is transported to processing industries for the production of essential oils, per- fumes and solvents.

EXPERIMENTAL

Field experiments were performed at three different sites, one in Ilhavo and two in Cantanhede, in the centre-north of Portugal. The resin emission measurements were made in forests of maritime pine, 35-40 years old, during the years 1992 and 1993, in trees that were tapped for the second year. Volatile organic compound (Cs-Ct2) concentrations were measured in resin, resin emissions and forest atmosphere.

Emission rates resulting from resin evaporation were measured using a bag enclosure technique. A Teflon transparent sheet was placed around the trunk, enclosing the resin extruding surface and the collecting recipient. The sheet was closed vertically with clamps and tape, and also above and below the extruding/collecting area, against the trunk, with rubber bands, forming a closed bag. Inlet and outlet tubes permitted the circulation of air. Thermopar sensors measured resin and gas temperatures. Air cleaned with charcoal was pumped into the bag at an approximate, but precisely measured, rate of 201 rain- 1. The outlet tube, all made up of Teflon, had a restriction which caused a small overpressure, maintaining the bag inflated, with an internal volume of 60-100 t.

Through a tee, a small fraction of the outlet airflow was deviated and sucked through a Chrompack glass tube containing 0.1 g of Tenax TA, 60-80 mesh, with a flow rate of 50 ml min- t, during 2 rain.

Exposed tubes were analysed by Gas Chromatography with FID using a 25 m length, 0.25 mm internal diameter, CP-SIL 19CB, capillary column. A Chrompack TCT injec- tor was employed to cryo-focus the sample at - 100°C, prior to injection. The method has a detection limit of 0.02 ng carbon.

The technique was calibrated with standards of ct-pinene, fl-pinene, myrcene, limonene, camphene, A3-carene, phelan- drene, p-cimene, terpinene, 1-8 cineol, etc., prepared from pure chemicals dissolved in methanol. The chromatographic technique permitted the identification and quantification of most of the compounds with exception of sabinene and fl-pinene that co-eluted partially.

Samples of resin were collected from the trunk surface and collecting buckets. The composition of resin in volatile VOCs was determined by dissolving a precise mass in methanol and transferring a known volume of the resultant solution to Tenax TA tubes, which were analysed by GC/FID after solvent evaporation.

The capacity of Tenax TA-filled tubes to collect monoterpenes was evaluated by sampling in the field and laboratory with two tubes in series. The adsorption capacity was dependent on the terpene concentration in the gaseous phase and increased appreciably for higher levels. Monoterpenes as high as 500/~g per gram of Tenax, could be collected with efficiencies higher than 99%, when sampling resin emissions for only 1-2 min, at 50 ml min-1. When sampling ambient air the Tenax capacity was orders of magnitude lower and break through was observed with values as low as 1 pg monoterpenes per gram of Tenax. Therefore, ambient air samples were always taken with two tubes in series, at flow rates of 150-200 ml min-1, during approximately 30 min.

RESULTS AND DISCUSSION

Resin composition

Measurements of resin composition, resin emissions and monoterpene concentrations in ambient air are summarized in Table 1, together with previously

Table 1. Average percent composition of terpenic compounds in the volatile fraction (Cs-Ct2) of Pinus pinaster resin, emissions and forest atmosphere (between brackets the standard deviation)

Resin (%)

Bernard-Dagan et al. (1971)* Kubeczka and

This Carvalho Schultze Emissions Atmosphere Compound study Portugal Landes (1986) (1987) (%) (%)

~t-pinene 76.5 (6.6) 45 45 79 44.1 64.5(15.0) 48.4 (20.9) Campbene 0.9 (0.3) 0.5 0.7 2.3 (2.7) 0.7 (0.9) fl-pinene 17.2 (8.0) 38 31 15 29.5 23.1 (8.9) 13.8 (6.0) A3-carene n.d. 2 11 n.d. 3.3 n.d. n.d. Myrcene 0.9 (0.3) 10 7 0.7 4.7 2.5 (2.0) 0.3 (0.3) Terpinene n.d. n.d. traces 1.0 (0.6) 3.4 (3.2) Limonene 1.4 (0.5) 1.4 3.2 2.3 (3.2) 0.8 (1.1) P-cimene n.d. n.d. 0.1 0.1 (0.2) n.d. Cineol 0.4 (0.1) 3.2 1.4 1.6 (3.2) 12.5 (10.8) Other VOCs 2.7 (1.9) 9.6 2.6 (3.0) 20.1 (13.3)

*Average value for resin composition inside the trunk. n.d. non-detected.

Atmospheric fluxes and concentrations of monoterpenes 685

published values. Some differences in resin composition were found by different groups, probably as a result of genetic, phenological and geographic vari- ations (Bernard-Dagan et al., 1971; Birks and Kanowski, 1988). The monoterpenes measured in our experiments constitute in average 19% of the resin mass, value very similar to the 18-20% of volatile fraction for Portuguese pine resin published by the Portuguese Fores1: Direction (DGF, 1991). Our measurements show that approximately 94% of the resin volatile fraction is formed by two monoterpenes, ~-pinene and fl-pinene, ~-pinene being clearly predominant.

Emission fluxes

Emission of monoterpenes from exposed resin was measured for two incomplete year cycles, in 1992 and 1993. During the measurements ambient temperature varied from 7°C in winter to 32°C in summer. Tem- perature inside the Teflon bag was always higher than outside and resin temperature could reach values as high as 57°C, in summer. From measurements the emission rates were calculated on the basis of both exposed resin mass and resin/air interface area. Emis- sion fluxes were compared with air and resin temperatures to obtain a best-fit equation. Fluxes estimated on the basis of res~in exposed mass showed a lower dispersion than flux es calculated on the basis of resin surface, probably because of the irregular character of resin dripping along the trunk surface which makes the measurement of resin/air interface area inaccurate. As a consequence, results are presented only on the basis of extruded resin mass.

Measured fluxes varied between a minimum of 15 to a maximum of 63'00 pg VOC, per gram of resin per hour. The compounds identified in resin emissions were ~-pinene, fl-pinene, myrcene, limonene, camphene, terpinene, cineol and p-cimene, (which constitutes more than 97% of the total VOC emissions), ~- and fl-pinene being highly predominant. By comparison with the resin composition inside the tree it is found that there is a higher loss (volatilization) of myrcene and fl-pinene, than ¢-pinene, during resin exposition to the atmosphere.

Figure 1 shows the emission rates for ~-, fl-pinene, and total emitted VOCs, as a function of temperature. Emission increases exponentially with temperature with a rate increase that is higher than the variation of vapour pressure. This is possibly a consequence of the effect of temperature on parameters, such as the diffusion coefficient of monoterpenes in resin phase, affecting the evaporation rate. The effect of other parameters, besides vapour pressure, is evident from the observed enrichment of/~-pinene, relative to c(- pinene in the gaseous phase (see best fitting curves in Fig. 1).

The ratio of fl-pinene/~-pinene emission rates de- creases with increasing temperature, from an average value of 0.43, at 15°C, to 0.27, at 35°C. ~-Pinene has a lower boiling point (B.P. 154-156°C) than fl-pinene

10000 ~ o - 19~2 23X~93 . . r 10000

1 0 0 0 1 L°g E = 0"041 + 0"087 T O ~ ~ o • 1000 ,~-~

" -- Log P = 0.0943 + 0.0227 T 1 1 I t I I I r 1

.~ 1 0 0

1

lOOOO - ~ o - ' 1 ~

~" Log E = 0.494 + " ~ 1 0 0 0

j,ol !

1

1 0

o - 1 ~ b) o - 19E;3

Log E = - 0.178 + 0.0T/T ~ 9 ~ ~ "

ooo; Log P = - 0.0328 + 0.(~29 T

J I J I t I i I i I

E=0.494+0,078T ~ - O [

D "%

i [ i i i i

15 20 25 30 35 4 0 4 5

10000

¸1000

• 100

Bag air tnmnat~ 0c)

Fig. 1. Terpene emission rates resulting from the resin- tapping practices in Pinus pinaster forests, presented as a function of temperature. The lower lines in figs (a) and (b)

represent the vapour pressure of pure ~- and #-pinene.

(B.P. = 164-165°C) and other measured terpenic compounds, and a reverse emission behaviour should be expected if vapour pressure alone was the interven- ing factor controlling volatilization.

The evaporation rate may be controlled by limitations to transference, in the gaseous phase or in the resin material. Resin is a highly viscous liquid, which at lower temperatures has even a solid character. After a certain period of exposure to ambient air and hu- midity the resin crystallizes and acquires a white solid-looking aspect. Loss of volatile compounds, presence of liquid water and lower ambient temperatures favour crystallization.

The diffusion coefficient of molecules in resin depends in a complex way on temperature and is a function of resin viscosity (Liley et al., 1984). Resin viscosity is influenced by the chemical composition, increasing with the loss of more volatile hydrocarbons, and decreasing strongly with temperature, principally in the range 5-35°C (Carvalho, 1979).

The effect of temperature on resin emissions may manifest also indirectly, through an increase in the resin extrusion flow from wounds in the trunk surface. Resin extrusion flux is controlled by internal factors such as resin viscosity and resin pressure in resin channels (Dyer, 1955). Carvalho, (1979), found

686 C.A. PIe and A. A. VALENTE

800

600

.=

v 400

E 200

---e- Total t e r p e n e s

i/'~'\ ---O-- A-pinene / '" k --o- B-pinene

/I ~. B a g a i r t e r n p e r a t u r e

/ / \ ~:'. - - - R e s i n t e m p e r a t u r e

.... / / / ) a \\ "( ~ --

....... ;, / ,

." // \I '< . . . . ,

~i ,~ "--..... / ~.. ...... ~ . _ /

i i i I i i i i i 0

6 9 12 15 18 21 24 3 6 9 12 H o u r ( U T )

50 4000

40 3000

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2000

10 ~ 1000

23106/93 (3)

0 [] t l ) i i i

A M J D

(3) O (4) t 1 (3)

(4)

I[I (3) (3) •

i i i i i

J A S O N

Fig. 2. Variation in emission rates from resin during the days 21 and 22/07/93. The figure also shows the resin and air

temperature inside the bag.

Fig. 3. Seasonal variability in emission rates from samples taken at 12-15H and standardized to 20°C. Between brackets are the number of samples. The vertical bars represent the range of emission rates observed. Measurements on

23/06/93 were made 2 days after the bark removal.

a strong correlation between resin extrusion rates and ambient maximum diurnal temperatures. Higher extrusion fluxes of resin, enriched in volatile compounds, result in higher emission fluxes of monoterpenes.

There are other, less easily detected, factors affecting resin emission rates such as resin-tapping practices. An important increase in emission rate was noticed in periods immediately after the periodic bark removal, when more abundant and fresh resin was extruding from the new wounds in the trunk surface. The sampling of 23/6/93, made 2 days after a new incision in the tree trunk, shows this rate increasing.

Factors such as rainfall also affects the emission rates. The samples of 25/11/92 were made after a period of rainfall and a water layer covered the resin surface in the sampling buckets. The water seems, unexplainably, to favour the emission of monoterpenes, compounds that are not water soluble, to the atmosphere.

As a result of ambient temperature variation there is a daily pattern of resin emissions to the forest atmosphere, with maximum values in the middle of the day and minimum emissions at night. Figure 2 is an example of resin emissions along a summer day, showing that emission rates are 10 times higher in the beginning of the afternoon, when temperatures reach a maximum, than during night and early morning when temperatures are lower.

There is also a seasonal pattern on resin emissions as shown in Figs 1 and 3. Figure 3 was drawn by considering only emissions measured in the middle of the day, between 12 and 15 h, to eliminate the influence of the daily cycle on resin extrusion characteristics. The direct effect of temperature on monoterpene vapour pressure was also removed from the data by normalizing emissions at 20°C, following the meth-

odology of Tingey et al. (1991), and Janson (1992), based on the Clausius-Clapeyron equation. Figure 3 clearly shows an increase in standard emissions (at 20°C) during summer months, when resin extrusion rates are higher.

The variability in emission composition during the sampling campaigns can be further investigated by studying the fl-pinene/~-pinene ratio in exposed resin and in emissions, along the various seasons of the year. Figure 4 presents this ratio for samples collected in the middle of the day (between 12 and 15 h) during 1992 and 1993. The figure shows that while in the

1.0

0.8

0.6 E

~. 0.4

0.2

0.0

O Emissions 1992

El Emissions 1993 • Resin 1992

El Resin 1993

~(3) (~3) ~)(5)

(4~

D

o ~(3) d ') 0(1) [~13) r(3) •

i i I I i I i I

A M J J A S O N D

Fig. 4. Seasonal variation of the relative composition of fl-pinene and ct-pinene in resin and resin emissions, for measurements performed between 12 and 15 H (UT). Be- tween brackets are the number of samples in resin emissions.

The vertical bars represent the range of values observed.


resin there is a small decrease in the/~-pinene/ct-pinene ratio, from summer to winter, a complete opposite behaviour is observed in resin emissions. During 1993, emissions had approximately the same /3-pinene/ct- pinene composition, with even a relative decrease in fl-pinene concentration, than resin mass. This behaviour is in accordance with the expectations from observation of the monoterpene vapour pressures alone. In 1992, the first two samples also gave a similar ratio pattern. However, in the end of August, suddenly, the/~/ct-pinene ratio in emissions increased to much higher values, that were maintained until the end of the year.

Coincident. with this change there was a modifica- tion in the composition of the paste applied to the tree wounds to ease resin extrusion, with an increase in the concentration of sulphuric acid. It is then reasonable to conclude that the change in emission composition is a consequence of the chemical interaction between the resin and paste, substances. It is known that in acidic conditions there is isomerisation in resin between c~-pinene and/~-pinene, although it is expected that the reaction will be directed to the production of ~-pinene, which is, in these conditions, thermodyn- amically more stable, (Ferreira and Farelo, 1987). ~-Pinene may also suffer isomerization/hydration processes, leading to the production of other organic compounds, maintaining the original bi-cycle struc- ture. This reaction may consume almost all the ~- pinene present and is reversible, but usually do not produce ~-pinene.

The experimenta] set-up used in field experiments, as a result of practical limitations, could not repro- duce the ambient air turbulence conditions which exist in the forest atmosphere under the canopy. Dur- ing the experiments the flow rate through the bag was equivalent to an air velocity of only 0.05 m s- 1, while wind speed inside the forests can range between 0.5 and 2.0 m s- 1, in conditions of neutral stability (Oke, 1987; Li et al., 1990).

Experiments with resin emissions were made in the laboratory by covering with a uniform layer of resin a ring section of the internal surface of a glass tube maintained at ~20"C. A flow of clean air was drawn through the tube at different flow rates and emission fluxes measured. The results are presented in Fig. 5 for both fluid and crystallized resin samples. The figure shows that crystallized resin emits at much lower rates than non-crystallized resin and that, in the conditions of the laboratory, there is an increase in emi,,;sions when air speed increases, reaching a maximum for speed values of approximately 4 m s- 1.

It is difficult to translate laboratory results into field conditions, in the forest, because of the existence of non-controlled parameters that affect differently the gaseous- and condensed-phase conductances controlling VOC emissions. It is possible to conclude, however, that the emission algorithm obtained in these field conditions only estimates a minimum value for

20

15

4

3

2

1

I []

Non-crystallised r e s i n

Crystallised r e s i n o o1"1

Q 130 dj o OI3

o °R8 o

8 8 zx zx z~

I I I I I I

Non-crystallised resin o [3 °

C P / s t a l l i s e d r e s i n m

0

a)

m 13

O O

[] O

O [] []

Oj~OD A ~' O~ A

Z~ /2, Z~

i I i r i I I

0 1 2 3 4 5 6

Ai r ve loc i t y (m/s )

b)

Fig. 5. Emission rates of ct-pinene (fig. a) and fl-pinene (fig. b) in experiments performed in the laboratory at approximately 20°C with crystallized and non-crystallized resin

samples.

the VOC emission rate from resin tapping which will be also affected by air turbulence and wind intensity.

Atmospheric concentrations in resin-tapped forests

From known needle emission rates, (Valente, 1995; Nunes, 1996), it is estimated that each resinated pine tree emits 10 to 100 times more monoterpenes from exposed resin than from the leaf canopy (depending on the temperature). It is expected then that the resin emissions will control monoterpene levels in the ambient air of resin-tapped pine forests.

Monoterpene concentrations were measured at 2 m height in the under canopy ambient air of resin- tapped pine forests to evaluate the effect of resin tapping practices on local ambient VOC loading. Average values are presented in Table 2. Average relative monoterpene composition is given in Table 1. The amount of monoterpenes identified in ambient air is only 80% of total VOCs measured (C8-C12), by comparison with 97% in emissions. Similarly to emissions also in ambient air ~t-pinene and fl-pinene are the main compounds measured, constituting on average 60% of the total VOCs detected.

The concentrations of ct-pinene, fl-pinene and the total VOCs detected, in the resin-tapped forest ambient air, are presented in Fig. 6 for a day cycle in the summer of 1993. The daily pattern of concentration is characteristic and has been observed in other forest atmospheres ( H o v e t al., 1983; Riba et al., 1987; Lopez et al., 1988), with maximum monoterpene concentrations observed during nighttime periods

688 C. A. PIO and A. A. VALENTE

Table 2. Average ambient concentrations of total terpenes measured at 2 m height in resin-tapped forests (between brackets the number of samples)

Ambient Atmospheric Sampling Sampling temperature concentration site date (°C) (ppbTerp.)

Cantanhede I

Cantanhede II

26/08/92 26.0-28.0 3.3-5.4 (2) 16/09/92 21.5-31.5 1.7-3.6 (6) 06/11/92 19.0-22.5 0.8-5.2 (6) 25/11/92 17.5-18.5 1.7-3.5 (6) 17/02/93 7.0-16.5 0.7-0.8 (4)

19/03/93 17.5-20.5 0.6-1.9 (5) 20/05/93 13.0-18.0 0.4-1.1 (5) 23/06/93 22.5-24.5 2.2-4.8 (4) 21/07/93 21.0-30.0 1.0-4.0 (5) 22/07/93 14.5-29.0 1.9-13.6 (5)

10

2

- ~ ' - Total t ~ o e n ~

- - o - A-pirmr~ 8

--o- B-pinene

-,O.. OtherVOCs

6

4

2

0

\ \

\ \

÷

.-" ,..,¢, M/ . . " ,.. \\ / ~ . , . • . ~ ,

I I I I I i i I

6 9 12 15 18 21 24 3 6 9 12

Ho~" (UT)

Fig. 6. Atmospheric concentrations of C8-C~2 VOCs in a resin-tapped pine forest, measured at 2 m height, during

21-22/07/93.

2.0

m.,

I .== ,:L

1.5

1.0

0.5

0.0

i ! I

e--- ~ ~ / I !

! I

I

/ \ / \

Q f~/ /ll--~e__~ll, i r l@\ \ i I X ) ~ k x._ I I \0

i I i i I I i I i

9 12 15 18 21 24 3 6 9 12

Hour (UT)

Fig. 7. Variation in the fl/~-pinene ratio, in resin emissions and atmospheric concentrations in a resin-tapped pine forest

during the days 21-22/07/93.

when dispersions caused by atmospheric turbulence and photochemical decomposition are minimum.

Figure 7 shows the fl-pinene/~-pinene ratio for ambient concentrations and resin emissions in the same summer daily cycle period. While in resin emissions the/~/~-pinene ratio varies between 0.3, during day- time, and 0.5, at night, in ambient air the ratio increases to the values as high as 1.9 in the middle of night.

The enrichment of nocturnal ambient air in /~- pinene can be an indication that pine leaf emissions, which have an ~-pinene/fl-pinene composition ratio equal, or slightly superior, to 1, (Nunes, 1996), play an important role in the nocturnal increase of monoterpene ambient concentrations. More probably, however, the observed daily pattern ratio in ambient air is a consequence of chemical decomposition processes in the atmosphere. The oxidation of ~-pinene and

fl-pinene in the atmosphere results from hydroxyl radical attack during the day and nitrate radical decomposition at night, the role of ozone at rural concentrations usually considered minor (Altshuller, 1983; Atkinson et al., 1985; Wayne et al., 1990). The rate of fl-pinene removal by OH is 1.5 times higher than the equivalent process for ~-pinene, during the day. On the contrary, c~-pinene is consumed 2.5 times more rapidly than fl-pinene, by nitrate radicals, during the night.

Inventory of VOC emissions from resin-tapped forests Resin-tapping industry has changed largely during

the last 40 years in Portugal as a result of market forces controlling the price of resin products. Produc- tion of resin increased from 30ktony -1, in 1945, to 130ktony-1, in 1980, when Portugal was the third world producer of resin derivative products. During


4000

3500

3000

O

2500

2000

.~ 15oo

E LLI 1000

500

Jan. Feb. Mar. April May June July

Month

Aug. Sept. Oct. Nov. Dec.

Fig. 8. Aver~.ge monthly VOC emissions from resin-tapping practices in Portuguese maritime pine forests during the decade 1980-1990. Values calculated from the emission algorithm, using a daily temperature

pattern taken from maximum and minimum average daily temperatures.

the decade 1980-1990 there was a stabilization in resin production. In recent years, although there are no precise statistics, it is known that a strong decrease in resin-tapping activities has happened as a consequence of the low competitive costs from China and other third-world countries.

As a result of this variability and lack of enough accurate informatio:a in the recent years we will evaluate only the average emissions during the decade from 1980 to 1990 from resin extraction operations. During this decade the ave:rage production of resin gum by Portuguese forests was 102.3 k tony -~ (DGF, 1991), corresponding to an average number of 35 × 106 resin-tapped trees.

To estimate the emissions from resin tapping, we adapted the emission algorithm presented in Fig. 1, in order to obtain the emission of total VOCs as a function of temperature (in °C) by each resin-tapped tree using the equation

loglo E(T) = 0.631 + 0.06T

where E is the emission rate in mg VOC per hour per tree.

From the distribution of pine forests and production of resin in each year, the number of resin-tapped trees in each region (Distrito) could be calculated. The emission from resin can then be estimated with the algorithm if ambient temperature variability is known along the year for all the country. Presently, only isoplets for monthly daily-average temperatures and monthly daily maxiraum and minimum temperatures are available (SMN, 1974). The utilization of average

temperatures affects the results as a consequence of the exponential effect of temperature on emission rate. We estimated that the application of daily, instead of hourly, average temperatures can in a summer day result in an under estimation of emissions of 20-50%. Therefore, from maximum and minimum daily temperatures we developed a sinusoidal mathematical function to represent the hourly temperature variation. This function used the published monthly- average maximum and minimum daily temperatures to produce a daily temperature profile for each region and month of the year. The profile was used in con- junction with statistics on the number of resin-tapped trees and the emission algorithm to calculate the monthly-average emissions for each region.

Figure 8 shows the variability of VOC emissions along the year for all the country. As expected, the higher fluxes happen during summer months when the temperature is maximum. A total of 16.6 kton of VOCs was estimated to be emitted from the resin- tapping practices in Portugal, per year, in average, during the 1980s. Estimations for leaf emissions of monoterpenes and isoprene, from pine and eucalyptus forests, are of the order of 80 kton y - 1 (Nunes, 1996). Therefore, even in national terms the VOC emissions from resin tapping can reach relevant values.

The average amount of volatile compounds in pine resin inside the trunk of pine trees is considered to vary between 29% and 35% of resin mass (Carvalho, 1978). Resin composition measured in resin processing industries (personal communication from the industry) shows on average 18-20% of volatile com- ponents. This implies that during collection and

690 C.A. PIO and A. A. VALENTE

transport 50% of the volatile resin component is lost. For the decade 1980-1990, taking into account the amount of resin produced, an annual average loss of 19.4 kton of volatile compounds can be estimated using a simple material balance. This value is 17% higher than the annual emission of VOCs calculated from the developed emission algorithm.

This under prediction of algorithm application to VOC emissions is in accordance with expectations, taking into account that the effect of wind on emission fluxes is not considered. The small difference is an indication that the major factor controlling VOC va- porization from resin is the resistance to transference inside resin and that the resistance in the gaseous phase only plays a minor role. Therefore, the algorithm per- mits a reasonable estimation of VOC emissions from resin-tapping practices in Portuguese pine forests.

CONCLUSIONS

Measurements of VOC emission fluxes from exposed resin during resin-tapping practices in maritime pine forests showed that ct- and ~-pinene are the main organic compounds vaporized, constituting more than 90% of VOC emissions.

Emissions are mainly dependent on temperature with an exponential dependence that is higher than the increase in vapour pressure.

Emissions also show a complex dependence from the season of the year, frequency of bark removal and composition of activating paste. Emissions peak immediately after the periodic bark removal and decrease steadily with time until the next cutting. The characteristics of the activating past have a strong effect on resin emission composition. A relative enrichment of fl-pinene on emissions was observed when a paste with a higher concentration of sulphuric acid was applied on the trees.

Estimation of VOC emissions from resin, for the decade 1980-90, in Portugal, using the algorithm obtained from our experimental studies, gave a total emission flux that was only 17% lower than the amount of volatile resin component loss obtained through a material balance.

During the decade 1980-90 resin emissions corre- sponded to ~20% of total VOC emitted from eucalyptus and pine forests in Portugal. As a result of the recent reduction in tapping activities the importance of resin volatilization at regional level is smaller but still relevant. Emissions from resination activities are presumably also relevant in countries such as China and Russia where resin tapping is common.

Concentrations of monoterpenes in the ambient air of resin-tapped pine forests have seasonal and daily patterns that are influenced by emission rates, atmospheric dispersion and chemical consumption. Con- centrations are higher during summer when emission rates peak. The highest concentration values are observed at night when atmospheric turbulence is lower.

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atmospheric fluxes and concentrations of monoterpenes in resin-tapped pine forests

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