Seasonal Variations of Isoprene Emissions from Deciduous Trees Zhang Xiaoshan*, Mu Yujing, Song Wenzhi, Zhuang Yahui

Atmospheric Environment 34 (2000) 3027}3032

Seasonal variations of isoprene emissions from deciduous trees Zhang Xiaoshan*, Mu Yujing, Song Wenzhi, Zhuang Yahui

Research Center for Eco-Environmental Sciences, Academia Sinica, Beijing 100085, People's Republic of China Received 18 November 1998; accepted 23 June 1999

Abstract

Isoprene emission #uxes were investigated for 12 tree species in and around Beijing city. Bag-enclosure method was used to collect the air sample and GC-PID was used to directly analyze isoprene. Ginkgo and Magnolia denudata had negligible isoprene emissions, while signi"cant emissions were observed for Platanus orientalis, Pendula loud, Populus simonii, and Salix matsudana koidz, and other remaining trees showed no sign of isoprene emission. Variations in isoprene emission with changes in light, temperature and season were investigated for Platanus orientalis and Pendula loud. Isoprene emission rates strongly depended on light, temperature and leaf age. The maximum emission rates for the two trees were observed in summer with values of about 232 and 213 lgg\ dw h\, respectively. The measured emission #uxes were used to evaluate `Guenthera emission algorithm. The emission #uxes predicted by the algorithm were in relatively good agreement with "eld measurements. However, there were large di!erences for the calculated median emission factors during spring, summer and fall. The 25}75 percentiles span of the emission factor data sets ranged from !33 to #15% of the median values. ( 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Isoprene; Emission factors; Emission rates; Diurnal variation

1. Introduction gases such as methane by depleting OH radicals and producing CO; (2) contribute to aerosol back-scattering It is known that plants contain a number of volatile by the formation of aerosols and of cloud condensation organic compounds (VOC) including isoprene, mono- nuclei; (3) enhance acidic deposition in remote areas by and sesquiterpenes, alcohols, aldehydes, ketones and the formation of organic acids; (4) control tropospheric esters which may be widely distributed through plant ozone formation; (5) in#uence global carbon budgets. To organs. It appears that these compounds may be impor- assess the impact of reactive VOC on local, regional and tant in many functions, including defense against herbi- global ozone formation and its possible impact on the vory (Farentinos et al., 1981). The emissions of VOC annual temperature increase, accurate estimates of natu- from the vegetation contribute signi"cantly to the VOC ral VOC emission #uxes are necessary. budget in the troposphere (Steinbrecher, 1997; Guenther The two most abundant and best-studied VOC are et al., 1995). Guenther et al. (1995) estimated that vegeta- isoprene and monoterpenes because of their high reactiv- tion emitted over 90% of the global VOC emissions. ity and importance in determining the OH mixing ratios VOC play a key role in the short-term chemistry of the (Anderani-Aksoyoglu and Keller, 1995). In China, how- lower atmosphere and have an important impact on ever, there are few reports on the emission pattern and global change, including climate change (Hewitt and quantity of VOC from plant species. Street, 1992; Steinbrecher et al., 1997; Lerdau et al., 1997). Both isoprene and terpene emissions are sensitive to They may (1) increase the lifetime of radiative active temperature; only isoprene emission is strongly a!ected by the light intensity (Lerdau et al., 1997). Isoprene emission which takes place mainly during the daytime, is * Corresponding author. Tel.: #86-10-62925511-3190; fax: suggested to be associated with the photosynthetic activ- #86-10-629-23563. ities (Delwiche and Sharkey, 1993). Other factors which E-mail address: [email protected] (Z. Xiaoshan). have been reported to a!ect the emission of biogenic

VOC are plant age, water de"cit and air pollution within the enclosure (g), and < is the volume of the (Anderani-Aksoyoglu and Keller, 1995; Kesselmeier and enclosure system (m). Studt, 1999). The investigations presented in this paper, focus on the in#uence of environmental parameters on 2.2. Analytical method the isoprene emission rate from two typical tree species in and around Beijing city. The emission algorithm for- VOC were usually measured by GC-FID methods in mulated by Guenther et al. (1993) has been tested for most laboratories. Since the concentration of VOC in the isoprene emission from Platanus orientalis and Pendula enclosure system is lower than the detection limit of FID; loud. preconcentration was needed. These processes are relatively complex and will cause contamination and loss for VOC (O can react with VOC because of their reactivity, 2. Experimental methods Larsen et al., 1997). Photoionization detector and reduction detector were also used for analyzing isoprene 2.1. Sampling method (Lerdau and Keller, 1997; Monson et al., 1994). These analytical techniques do not require any preconcentra- The 5.24 l cuboid enclosure (14.2;14.2;26 cm) used tion for isoprene mixing ratios above 1 ppbv. for the measurement of isoprene emission, was construc- In this study, photoionization detector (PID-100C, ted of a Velton "lm over a wire-supported frame. The made by the Institute of RCEES) was employed for losses of isoprene in the enclosure system were tested by directly analyzing isoprene. using standard isoprene gas (350 ppbv). The average A packed Te#on column of 5% b,b-Oxydipropionit- losses of isoprene in the system for 20 min are about vile coating on Chromosorb P NAN (60}80 mesh) was 1.6% (the injection interval is about 20 min, and 12 used to separate the air sample. The column is 3 m long, injections). Because the enclosure time in our experiment and with an inner diameter of 3 mm. Compressed air is less than 10 min, the losses of isoprene in the system is which passed through dry Molecular Sieve and Active less than 0.8% during our experiment. This value is Carbon was selected as the carrier gas. The #ow rate was within the precision of the analytical system. about 10 ml per min. The column and injection temper- The twigs investigated were generally exposed on the atures were room temperature (&253C). The quantity of south side. The enclosure system was vibrated for mixing the injection was 1 ml. The column can e$ciently separ- the air volume in it before sampling. During a given time ate isoprene from other biogenic VOC compounds (e.g. interval (1}2 min for daytime, and 20 min for night), Butane, n-butene, butadiene and monoterpenes). 10}20 ml of the air sample in the bag was collected using The standard gas of isoprene (74.7 ppmv, provided by a glass syringe. After collection, the syringes were stored the Institute of Standard Material, Beijing) was diluted to in the dark, to prevent photochemical reactions. The di!erent concentrations using N and was used to test contents of the syringe were quickly analyzed by GC- the linear relationship of PID. The results are shown in PID within half an h of collection. The loss of isoprene in Fig. 1. There is a good linear correlation for isoprene the syringe was tested by using samples of known iso- from 3.73 to 4500 ppbv, the correlation coe$cient (R)is prene content, there was almost no loss for at least 1 h. higher than 0.98. The accuracy of the GC-PID system for To isolate the main controlling factors for isoprene isoprene was about 2.4% (standard deviation of the emission, a set of environmental parameters (photosyn- mean; n"15) and the time resolution of the analytical thetic active radiation [PAR] and temperature) were cycle was 10 min. Hence GC-PID is an easy, quick and determined simultaneously from the emission measure- reliable method for analyzing isoprene. ments. The temperatures inside and outside the bag were measured using mercury thermometers. PAR (Li 190SB 2.3. Emission parameterization Quantum Sensor Lincoln, NB) was measured outside the bag. Light and temperature are the main controlling factors The dry weight was determined after drying the col- for isoprene emission. To predict isoprene emission from lected leaves at 1003C for 24 h. The isoprene emission vegetation, Guenther et al. (1993) developed an algorithm rates, E (lgg\ h\), for individual branches were cal- based on leaf-temperature and light. Isoprene emission culated as rate E* can be calculated by the following equation: (C !C ) < " ; ; E" G ; , E* S C* C2, (1) ! (tG t) = where S is the isoprene emission factor at a standard ! ¹ where (CG C) is the di!erence in the concentrations of temperature of 303 K, and a standard PAR of l \ ! l isoprene for a given time interval ( gm ), (tG t) is the 1000 E, C* and C2 are the correction terms for light and given time interval (h), = is the dry weight of the leaves temperature relative to standard conditions, respectively. Z. Xiaoshan et al. / Atmospheric Environment 34 (2000) 3027}3032 3029

Table 1 Isoprene emission rates of Beijing tree species in May, and comparison of the results of this study with others for the same genera. n is the number of emission rate measurements by this study, N represent negligible or no isoprene emission, Eiso (lgCg\ h\) are representative of leaf temperature of 303C and PAR #ux of 1000 lmol m\ s\

Eiso (lgCg\ h\)

Fig. 1. The linear range of PID for isoprene. Genus species n ab c

Platanus orientalis 13 25.2 20 10.9}27.5 Guenther et al. formulate these terms as Pendula loud 36.3** Populus simonii 222.64037}100 aC ¸ Salix matsudana Koidz 2 19.2 20 12.5}115 " * C* , (2) Ginkgo biloloa 1 (0.1 (0.1 N, or (1 (1#a¸ Paulownia tomentosa 1N** Magnolia denudata 1 (0.1 (0.1 N a where (0.0027) and C* (1.066) are empirical constants Cedrus deodara 1N(0.1 N ¸ l and is the PAR( E). C2 is described by Firmiana simplex 1N** Fraxinus chinensis 1N(0.1 N exp[c (¹!¹ )/R¹ ¹] Duplex 1N** C " 2 , (3) 2 # ¹!¹ ¹ ¹ ( 1 exp[c2( +)/R ] Juglans regia 1N 0.1 N where R is the gas constant (8.314 J K\ mol\), a, This study; b, Guenther et al. (1994); c, Kesselemer and \ \ ¹ Staudt (1999). C2 (95000 J mol ), C2 (230000 J mol ) and + (314 K) are empirical coe$cients based on measurements of three plant species: eucalyptus, aspen and velvet bean. During 1998, the leaf emergence in Populus occurred These parameters and algorithms were used to calculate about the "rst week of April in Beijing. When isoprene the emission factors (measured isoprene emission rates at emission measurements were made, the leaf age did not ' l l PAR values 100 E normalized to 303C and 1000 E exceed 40 d. The lower emission rate of Populus should PAR) for isoprene emission from the tree species. The leaf be ascribed to the low ability of isoprene synthesis at this temperature was considered as the average temperature time. in the bag during the measurement period. To examine the role of the developmental stages of leaves on the isoprene emission rate, the instantaneous isoprene emission rates of Platanus orientalis and Pendu- 3. Results and discussion lar loud were measured in three di!erent seasons. The results indicated that, "rst, isoprene emission rates de- Isoprene emission rates were investigated from 12 typi- pended strongly on light and temperature. On sunny cal tree species in and around Beijing city in May 1998. days, the PAR values reached 1800 lE and temperatures The average isoprene emission rates are shown in Table 1, up to 393C were observed in the bag. Isoprene emission normalized to 303C and 1000 lmol m\ s\. Four of rates were highest for Platanus orientalis and Pendula the 12 genera listed in Table 1 have signi"cant isoprene loud and reached values of up to 232 and 213 lg emission rates, while the remaining tree species have g\ dw h\, respectively. On a rainy and cloudy day, the negligible or no isoprene emissions. isoprene emission from these trees was low and did not The isoprene emission rate factors of seven related plant exceed 1.5 lgg\ dw h\. At night, the isoprene emis- species reported in literature are also listed in Table 1. sion rates decreased to zero. Second, the developmental Considering the errors in using `Guenthera emission stages of the leaves have a strong in#uence on the isop- algorithms (35%, Guenther et al., 1993), the results of rene emission rates. Under almost the same environ- this study agree well with theirs, except for Populus. mental conditions (PAR and temperature), isoprene The isoprene emission factor of Populus in this study is emission rates of the two trees were much higher in about 44% lower than that of Guenther et al. (1994). It summer than in spring and fall. should be noted that isoprene emissions were measured The `Guenther algorithmsa were used to calculate in spring (10 May, 1998) for this study. Numerous pre- emission factors (measured isoprene rates at PAR values vious studies have shown that the isoprene emission '100 lE normalized to 303C and 1000 lE PAR) potential was in#uenced by leaf age (e.g. Monson et al., and the parameters of the algorithm were used for the 1994; Steinbrecher et al., 1997). parameterizations of isoprene emission from Platanus 3030 Z. Xiaoshan et al. / Atmospheric Environment 34 (2000) 3027}3032

Table 2 Isoprene emission rates and emission factors of Platanus orientalis and Pendula loud

Tree species Platanus orientalis Pendula loud

Month (1998) May Aug. Oct. May Aug. Oct. PAR (mE) 0.1}1.02 0.1}1.7 0.1}0.75 0.2}0.8 0.1}1.8 0.1}0.7 ¹ (K) 292}308 303}312 294}302 298}304 298}312 294}298 n 13 16 19 3 18 10 \ \ E* (g g dw h )1.1}72 20.5}232 6.34}83 1.7}9.1 6.1}213 4.3}19.8 Emission factor! (g g\ dw h\) 28.6 80.6 (48&112) 63.4 (46&73) 7.1 63.4 (37&96) 28

!Given are the median values from n emission rate data sets and in brackets the 25 and 75 percentiles. Measured emission rates (PAR values'100 lE) were normalized to 303C and 1000 lE PAR (see Eqs. (1)}(3)).

orientalis and Pendula loud. The instantaneous emission members of a plant genus tend to have similar foliar rates and basal emission rates (emission factors) are listed VOC emission rates. However, since the biogenic VOC in Table 2. It is obvious that the basal emission rates emission rates are a!ected by many factors besides PAR depend on the developmental stages of the leaves. These and temperature (e.g. plant development, growth regime, results are similar to those observed for aspen plants leaf nitrogen concentration, air pollution and so on, (Monson et al., 1994). For this study, in spring, where the Monson et al., 1994; Kesselmeier and Staudt, 1999), the age of the leaves was about 40 d, the ability of isoprene temporal and spatial variation of the basal emission rate synthesis is low; in summer, when the leaves' age was for the same genus are obvious. about 140 d, and the leaves reached full expansion, it was Earlier studies indicated that the release of some VOC the opportune time for isoprene production by the trees; from diverse plant species was found to be enhanced in fall, when the leaves' age was about 200 d, the lower under SO exposure, anaerobic conditions, or general air isoprene emission is due to leaf senescence. pollution conditions (recent reviews by Kesselmeier and In addition to PAR, temperature and the developmen- Staudt). In China, the air pollution of SO and O are tal stage of the leaves, the short-term weather also had an very serious in recent years, and it may be responsible for in#uence on isoprene emission rates. For example, the the larger isoprene emissions rates from the tree species. median emission factor on August 14 (clear day) was about 41 and 74% larger than that on August 15 (clear 3.1. Diurnal cycle and emission parameterization with clouds) for Platanus, and Pendula, respectively. Surprisingly, the median emission factor on October The diurnal cycles of isoprene emission rate from 15 was 30% higher than that on October 14 for Platanus, Platanus orientalis and Pendula loud were studied in while the PAR and the temperature were almost at the spring, summer and fall. During sunny days, isoprene same level. It should be noted that the day of October 14 emission exhibited pronounced diurnal cycles with emis- was the "rst sunny day after long cloudy days (about sion maximum from 9 : 00 to 16 : 00. The maximum 15 d, the maximum temperature on those days was below values were much higher in summer than in spring and 203C). fall due to the higher peak of the temperature and PAR. Regarding the impact of short-term weather on iso- The enhancement of the isoprene emission during morn- prene emission, it may be speculated that short-term ing hours was relatively slow when compared to the changes in weather could a!ect carbon partitioning and decrease during the afternoon hours. The di!erent emis- the equivalents of energy and reduction within the plant sion pattern in the morning and afternoon was similar to metabolism. the variation of PAR. This indicated that PAR played an The average basal emission rates of Platanus for three important role for isoprene emission when the light seasons was about 50.7 lgCg\ h\. Guenther et al. intensity was below the light saturation of isoprene emis- (1994) have recommended using a basal isoprene emission ((1000 lmol m\ s\). sion rate of 20 lgC g\ h\ for members of the genus Fig. 2 shows the diurnal variations of Platanus orien- Platanus (Table 1). The value of the current study is talis in summer and fall, as well as Pendula loud in about 154% larger than the value estimated by Guenther summer. In general, the diurnal variations of isoprene et al. (1994). This di!erence re#ects the fact that although emission were in correlation with the variations of PAR an emission rate of 20 lgCg\ h\ approximately pre- and temperature. However, there are a number of devi- dicts the spring emission rate from Plantanus (Table 1), ations (dips and peaks) for measured data throughout the it does not accommodate seasonal or developmental day (especially in the case of fall). Past studies have variation. Guenther et al. (1994) pointed out that the shown that a large di!erence in the isoprene emission Z. Xiaoshan et al. / Atmospheric Environment 34 (2000) 3027}3032 3031

Fig. 2. Calculated isoprene emission and measured isoprene emission of Platanus orientalis and Pendula loud, as well as the corresponding PAR and temperature. The open circle represents the calculated emission rate by using the median emission factor. The dotted lines represent the 25 and 75 percentiles of the emission factor data set.

potentials exists between shade and sun leaves (Guenther tile of the emission factor data sets compared to the et al., 1996; Harley et al., 1996). These deviations may be parameterization with the median values. The range of ascribed to self-shading within the branch-enclosure sys- uncertainty for the average calculated median values, tem. The total uncertainty was estimated to be less than given as the 25 and 75 percentiles, is in the range of 50% because of self-shading. !33}#15% for Platanus orientalis and !30}#13% Fig. 2 also shows the variations in the isoprene emis- for Pendula loud. It should to be noted that the para- sion rate simulated by the temperature- and PAR-depen- meterizations based on the median emission factors dent algorithms developed by Guenther et al. (1993). underestimate the isoprene emission maximum by 25% Comparing the output of the algorithm to the measured for Platanus and 30% for Pendula in summer. emission rates of both the trees in summer, it is obvious In fall, the median isoprene emission factor for that the diurnal cycle of isoprene emission from both the Platanus orientalis was based on a larger data set, result- trees is better described by a run based on the 75 percen- ing in a smaller uncertainty range for the predicted time 3032 Z. Xiaoshan et al. / Atmospheric Environment 34 (2000) 3027}3032 maximum (53}77 lgg\ dw h\). The variations pre- and emission rate algorithm development. Journal of Geo- dicted by the median factor and the observed variations physical Research 96, 10799}10808. were closely correlated (r"0.78), and about half of the Guenther, A., Zimmerman, P., Harley, P.R., Monson, R.K., Fall, predicted variation in the emission rates was within 15% R., 1993. Isoprene and monoterpene emission rate variabil- of the observed and three-fourths were within 35% of the ity: model evaluation and sensitivity analysis. Journal of Geophysical Research 98D, 12609}12617. observed. Guenther, A., Zimmerman, P., Wildermuth, M., 1994. Natural volatile organic compound emission rate estimates for U.S. woodland landscapes. Atmospheric Environment 28, 4. Conclusions 1197}1210. Guenther, A., Hewitt, C., Erickson, D., Fall, R., Geron, C., GC-PID is an easy, reliable and quick method for Graedel, T., Harley, P., Klinger, L., Lerdau, M., McKay, analyzing isoprene emission from vegetation. Measure- W.A., Pierce, T., Scholes, B., Steinbrecher, R., Tallamraju, R., ments of isoprene emission on 12 tree species revealed Taylor, J., Zimmerman, P., 1995. A global model of natural volatile organic compound emissions. Journal of Geophysi- that Platanus orientalis, Pendula loud, Populus simonii cal Research 100, 8873}8892. and Salix matsudana koidz are strong emitters of isoprene. Guenther, A., Greenberg, J., Harley, P., Helmig, D., Klinger, L., In contrast, the remaining trees only emitted little or no Vierling, L., Zimmerman, P., Geron, C., 1996. Leaf, branch, isoprene. stand and landscape scale measurements of volatile organic Isoprene emission rate strongly depends on light as compound #uxex from US woodlands. Tree Physiology 16, well as temperature. The `Guenthera emission algorithm 17}24. is able to predict the diurnal cycles of isoprene emission. Harley, P., Guenther, A., Zimmerman, P., 1996. E!ects of light, However, there were large di!erences for the median temperature and canopy position on net photosynthesis and emission factors calculated during spring, summer and isoprene emission from leaves of sweetgum (liquidambar fall, even between consecutive days. In all these cases, the styraci#ua L.). Tree Physiology 16, 25}32. Hewitt, N.C., Street, R., 1992. A qualitative assesment of the inadequacies of using one basal emission rate to repres- emission of non-methane hydrocarbon compounds from the ent an entire season are apparent. Therefore, in order to biosphere to the atmosphere in the U.K.: present knowledge accurately estimate the isoprene emission budget from and uncertainties. Atmospheric Environment 17, 3069}3077. vegetation, long-term studies are needed. Kesselmeier, J., Staudt, M., 1999. Biogenic volatile organic compounds (VOC): an overview on emission, physiology and ecology. Journal of Atmospheric Chemistry 33 (1), 23}88. Acknowledgements Lerdau, M., Keller, M., 1997. Control on isoprene from trees in a subtropical dry forest. Plant, Cell, and Environment 20, This work was made possible by funds from Chinese 560}578. Lerdau, M., Guenther, A., Monson, R.K., 1997. 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