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Acetone and monoterpene emissions from the boreal forest in northern Europe
Atmospheric Environment 35 (2001) 4629–4637
Acetone and monoterpene emissions from the boreal forest in northern Europe Robert Janson*, Claes de Serves Air Pollution Laboratory, Institute for Applied Environmental Research, Stockholm University, S-106 91 Stockholm, Sweden Received 21 September 2000; received in revised form 21 February 2001; accepted 7 March 2001
Abstract Acetone is a ubiquitous component of the atmosphere which, by its photolysis, can play an important role in photochemical reactions in the free troposphere. This paper investigates the biogenic source of acetone from Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) in the Scandinavian boreal zone. Branch emission measurements of acetone, monoterpenes, and isoprene were made with an all-Teflon flow-through branch chamber from five specimens of Scots pine at three sites in Sweden and Finland, and from one specimen of Norway spruce at one site in Sweden. Acetone samples were taken with SepPakTM DNPH cartridges, monoterpenes with Tenax TA, and isoprene with 3 l electropolished canisters. Acetone was found to dominate the carbonyl emission of both Scots pine and Norway spruce, as large as the monoterpene emissions and for Norway spruce, as the isoprene emission. The average standard emission rate (308C) and average b-coefficient for the temperature correlation for 5 specimens of Scots pine were 870 ng C gdw1 h1 (gdw=gram dry weight) and 0.12, respectively. For the monoterpenes the values were 900 ng C gdw1 h1 and 0.12, respectively. The standard emission rate (308C) for acetone from Norway spruce was 265 ng C gdw1 h1, but the sparsity of data, along with the unusual weather conditions at the time of the measurements, precludes the establishment of a summertime best estimate emission factor. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Carbonyls; BVOC; Biogenic emissions; Scots pine; Norway spruce
1. Introduction Atmospheric acetone has been found not only to be a ubiquitous component of the atmosphere, but also at relatively high concentrations compared to other nonmethane organic species (Singh et al., 1994, 1995). Model results suggest that acetone concentrations in the free troposphere are sufficient to render its photolysis an important source for peroxyacetylnitrate (PAN), which removes NOx from further photochemical activity, as well as an important source of free radicals in upper and dry regions (Singh et al., 1995). Thus, it is an important component of photochemical cycles in the upper troposphere, affecting the tropospheric ozone budget. *Corresponding author. E-mail address: [email protected] (R. Janson).
The sources of acetone are many, both primary and secondary, natural and anthropogenic (e.g. Singh et al., 1994). Estimates of the secondary production by the atmospheric oxidation of mainly anthropogenic, and of those mainly propane, but also natural hydrocarbons show it to be the largest source on a global basis. Primary anthropogenic sources include evaporative loss during its use as a solvent and as an intermediate in the chemical industry, automobile exhaust, and biomass burning, where biomass burning is thought to be the most important. Direct emissions from land vegetation, notably evergreens, contribute 9 Tg to the estimated global source of 40 Tg yr1. The uncertainties for most of the sources are large, in particular those for the biogenic sources, for which source strength estimates range from 4 to 18 Tg yr1 (Singh et al., 1994). Very few direct measurements of foliar emissions have been made
1352-2310/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 1 6 0 - 1
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(e.g. Simpson et al., 1999; Kesselmeier and Staudt, 1999; Fall, 1999). Isidorov et al. (1985) identified acetone in the emissions of a number of tree species and MacDonald and Fall (1993) reported small emissions from the buds of conifers. Martin et al. (1999) investigated emissions from 3 deciduous and 5 coniferous trees in New Mexico, USA, and reported the overall average emission of acetone (all species) to be 8% by mass. They do not report specific emission rates for acetone. In a screening experiment at three sites in the USA, in which one or two samples were taken from each of 63 species, Helmig et al. (1999) report acetone emission rates for 14 species. In this paper, we present results from the first field emission measurements of light carbonyl compounds made on Scots pine and Norway spruce and summarize those results as standard emission factors. Results from the complementary measurements of monoterpenes and isoprene are also presented and emission factors given.
2. Experimental 2.1. Branch emissions Scots pine (Pinus sylvestris) emission measurements were made in the spring and summer of 1997, at Asa Research Park (578N, 128E) in southern Sweden, and at Mekrij.arvi Research Station (638N, 318E), in southeastern Finland, respectively, as part of the BIPHOREP (Biogenic VOC emissions and photochemistry in the boreal regions of Europe) project (Laurila and Lindfors, 1999). A few measurements were also made on Norway spruce (Picea abies) at Asa. Asa is a mixed pine and spruce forest in the southern boreal zone and Mekrij.arvi a pine forest at the border of the southern and middle boreal zones. Measurements from Scots pine were also made during the summer of 1998 and spring of 1999, at the Hyyt.ıa. l.a Research Station in southern Finland (628N, 248E) as part of the BIOFOR (BIOgenic aerosol formation in the boreal FORest) project (Kulmala et al., 2001). The forest at Hyyt.ıa. l.a is predominantly pine with some Norway spruce and birch. The monoterpene data from Hyyt.ıa. l.a are reported in Janson et al. (2001) but are included here for comparison and calculation of the emission factor. The measurements were made with a 25 cm diameter, 18 liter all-Teflon chamber made of 0.05 mm transparent FEP-Teflon film enclosing a 20–30 cm branch segment. A fan, with the motor externally mounted, ensured mixing of the air inside the chamber. Care was taken to avoid contact between the branch and the walls of the chamber in order to avoid mechanical abrasion which can temporarily affect emission rates (Juuti et al., 1990). The chamber was continuously flushed with 9 NL min1 (NL=normal liters) ambient air (Bronkhorst HI-TEC massflow controller), purified of ozone with a KI
scrubber, and of particles with a 47 mm Teflon filter (2 mm). The water content of the inlet airflow was reduced by cooling with a Peltier element, regulated manually to keep the chamber humidity at about the same as ambient humidity. Ambient, inflow, and chamber temperatures and relative humidity were measured continuously during all experiments with Rotronic MP-100 sensors. Photosynthetic active radiation (PAR) was measured with a LiCOR LI-190SA quantum sensor mounted above the chamber. During the Hyyt.ıa. l.a experiments, the CO2 flux was recorded with a Model 41H Ambient Gasfilter Correlation CO2 Instrument (Thermal Environmental Instrument Inc.). Simultaneous samples were taken from a constant flow tapped from the chamber inlet and outlet lines via 8-way Teflon ports and 20 cm 0.0300 i.d. Teflon tubing. In this way, the flow through the chamber was not disturbed every time sampling commenced (0.8 l min1 for DNPH cartridges). Sampling times and volumes were controlled by a system of timers, solenoid valves, and Honeywell mass flow sensors. Sample flow, meteorological and CO2 data were continuously collected on a PC via ADAM1 Data Acquisition Modules (ADAM 4017 and 4520, Advantech Co., Ltd.). The branch emission was determined from the concentration difference between the outlet and inlet, the airflow through the chamber, and the branch needle dry weight. Rates are given as mass carbon, ng C gdw1 h1 (gdw=gram dry weight). The all-Teflon chamber was found to have small and variable artifacts for formaldehyde and 2-butanone, which may have been due to adsorption and desorption effects. However, they were at all times below the detection limits in ng C gdw1 h1 for these gases. No such effects were seen for acetaldehyde or acetone. The uncertainty in the chamber emission data for acetone, as determined by the blank variability, see below, was 4 ng C gdw1 h1. 2.2. Ambient air concentrations Daily concentration measurements were made inside the forest at Hyyt.ıa. l.a during the BIOFOR campaigns in the summer of 1998 and spring of 1999. Samples were taken automatically with a system of timers and solenoid valves, and the flows monitored with Honeywell air mass sensors. 2.3. Sampling and analysis The light carbonyl compounds were sampled on SepPakTM DNPH cartridges at a flow rate of 0.8 l min1 for 2 h (4 h for concentration measurements). All cartridges were stored at 8–108C. In the laboratory, the compounds were eluted with acetonitrile, and analyzed by HPLC. Liquid standards included formaldehyde, acetaldehyde, acetone, propanal, and 2-butanone.
R. Janson, C. de Serves / Atmospheric Environment 35 (2001) 4629–4637
SepPakTM DNPH cartridges are known to have high and variable blank values. Blank values must therefore be carefully determined from every batch. In order to test the variability, 2 boxes were purchased and analyzed for their blank values. The average ( 1 S.D.) blank values were 13 11 ng cartridge1 for formaldehyde, 73 49 ng cartridge1 for acetaldehyde, 158 74 ng cartridge1 for acetone, and 50 29 ng cartridge1 for 2-butanone (n=35 cartridges). Cartridges stored at room temperature, at 8–108C, and at 208C for 3, 8, and in some cases 12 weeks, showed no significant differences in their blank values. In any given experiment, the detection limits for the carbonyls are a function of the blank value of the particular batch DNPH cartridges, as well as the experimental setup as regards sample volume and for emission measurements chamber flow and needle dry weight. In the experiments reported here, detection limits for concentration and emission measurements varied from 50 to 80 pptv and 3 to 24 ng C gdw1 h1, respectively, for formaldehyde, 40 to 200 pptv and 9 to 140 ng C gdw1 h1 for acetaldehyde, 40 to 60 pptv and 11 to 370 ng C gdw1 h1 for acetone, and 100 to 200 pptv and 24 to 230 ng C gdw1 h1 for 2-butanone. Ozone is known to create artifacts when sampling with silica gel DNPH cartridges (e.g. Kleindienst et al., 1988, 1998; Gilpin et al., 1997; Apel et al., 1998). We conducted parallel sampling with and without KI ozone scrubbers in front of the cartridges. Ozone levels were generally below 50 ppbv and carbonyl mixing ratios quite low. The samples without an ozone scrubber all showed lower values for formaldehyde, acetaldehyde, and acetone, but the differences were generally below the level of significance set by the variability of the DNPH cartridge blanks. Nevertheless, all concentration sampling was done through ozone scrubbers (KI coated annular denuders) placed at the air inlet and immediately in front of the cartridges. KI scrubbers take up water at high relative humidities and therefore no sampling was done under rainy conditions. For the chamber emission measurements, the O3-scrubber was placed at the chamber air inlet. Thus, the chamber air was free of ozone, which serves the twofold purpose of avoiding sampling artifacts and avoiding ozone reactions in the chamber. From 10 experiments in which two cartridges were placed in series, we estimate sampling efficiency for acetaldehyde and acetone to be better than 95% and 90%, respectively, while that for formaldehyde was variable and two trials gave values as low as 62%. These results are in agreement with those of the intercomparison reported by Gilpin et al. (1997), which indicated that silica gel DNPH cartridges trap formaldehyde at only about 75% efficiency, even without interfering gases such as ozone. Kleindienst et al. (1998), on the other hand, reported 95% recovery.
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Monoterpenes were sampled with stainless steel tubes containing 200 mg Tenax TA at an air flow rate of 100 ml min1 for 12–40 min. After sampling, the tubes were stored dry until analysed by GC-MS analysis at the ITM Air Pollution Laboratory. Compound identification and quantification was accomplished with terpene standards in methanol. The standards included tricyclene, a-pinene, camphene, sabinene, b-pinene, myrcene, D3-carene, limonene, and ocimene. Isoprene was sampled from air samples collected in 3 l electropolished stainless steel canisters. The canisters were evacuated to 106 bar at 1508C for 6 h. The sampling flow rate was regulated to 0.8 l min1 and the canister filled to a final overpressure of 2 bars with a TFE-Teflon membrane pump (BRC, model FC-1121). Sampling took 10– 15 min. The samples were stored at room temperature and analyzed by GC-FID. Identification was done with authentic standards and quantification with a propane standard.
3. Results and discussion 3.1. Emissions from Scots pine (Pinus sylvestris) Acetone was found to dominate the light carbonyl emission from Scots pine, accounting for between 64% and 94% (by carbon mass) of the light carbonyl emission. As most of the emission data for formaldehyde and acetaldehyde were below the detection limit for these compounds, they will not be discussed in any detail here. The values from the spring-99 data from Hyyt.ıa. l.a, being the most reliable (i.e. with the largest number of values above detection limits), were 64 13% acetone, 23 13% acetaldehyde, and 12 7% formaldehyde (n=42). The corresponding mole fractions are 26 13%, 26 14%, and 48 14%, respectively. 2butanone was also identified, but always below the detection limit. The few nighttime samples from Hyyt.ıa. l.a with values above the detection limits posed by the cartridges, indicate a significant shift towards a slightly higher percentage of acetone in the emission during the night, 70 18% (n=8), as compared to the daytime, 60 9% (n=34). These results are in contrast to those of Martin et al. (1999), who measured carbonyl emissions from 3 deciduous and 5 coniferous trees in New Mexico, USA. While they do not report the distribution for individual species, they found the emission of formaldehyde and acetaldehyde always to be greatest. On average, acetone accounted for only 8% of the carbonyl emissions in that study. Acetone emission rates ranged from below 20 to more than 1000 ng gdw1 h1, (Table 1). The diurnal variation and absolute values were similar to those of a-pinene, with maximum values around noontime, (Fig. 1). The mechanism of acetone production and emission, as well
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Table 1 Emission rates of acetone (ngC gdw1 h1) from branches of Scots pine at Asa, southern Sweden, June, 1997; Mekrij.arvi, southeastern Finland, August, 1997; and Hyyti.al.a, southern central Finland, 1998 and 1999. Minimum, maximum, and median temperatures for each data set are given in column 2. In the 4th–6th columns are given the b coefficient (Eq. (1)) and the correlation coefficient for the temperature dependences, as well as standardized emission rates (E(208C)) for each period Site
(n)
Temp. (8C) min–max, median
Acetone (ngC gdw1 h1)
b
R2
E (208C)
Asa, June 1997 Mekri, Aug 1997 Hyyti.al.a May 1998 August 1998 Mar–Apr 11 1999
(18) (22)
9–29, 20 11–28, 22
520–640 520–760
0.1566 0.097
0.6329 0.6413
113 214
(09) (31) (40)
2–18, 10 5–25, 17 0–23, 9
520–240 520–1400 520–780
0.0799 0.1058 0.1625
0.7399 0.2398 0.7569
270a 340 330 250 90
Grand average a
Few data above detection limit during the spring-98 campaign.
Fig. 1. (a) Branch emissions of acetone and a-pinene from Scots pine, and (b) chamber air temperature (8C) and photosynthetic active radiation (PAR: mmol m2s1). Hyyt.ıa. l.a, March 29–April 5, 1999.
as the factors which regulate that mechanism, are still largely unknown (e.g. Fall, 1999). With the exception of the summer data from Hyyt.ıa. l.a, the acetone emissions from Scots pine were found to correlate fairly well with chamber temperature according to ln E ¼ T þ c;
ð1Þ 1
1
where E is the emission rate (ng C gdw h ) at temperature T, b is the slope of the correlation curve, and c is a constant (Fig. 2). b-coefficients were between 0.10 and 0.16 and R2 values between 0.63 and 0.76
(Table 1). Considerable scatter was seen in the few data when temperatures were below 08C. Reasons for poor correlation in the summer data could not be found in the temperature, relative humidity, PAR, or H2O flux data, but it can be noted that the monoterpene data also showed an abnormally poor correlation. Emission rates did not correlate to the H2O flux, as determined from the temperature and relative humidity data from the inlet and chamber air. Neither did they correlate to CO2 assimilation, (Fig. 3), which is contrary to some laboratory studies in which acetone emissions
R. Janson, C. de Serves / Atmospheric Environment 35 (2001) 4629–4637
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(Fig. 2). The standard rate by the emission algorithm for isoprene is defined by Es* ¼
Fig. 2. The natural logarithm of the acetone emission (ngC gdw1 h1) from Scots pine plotted against air temperature inside the branch chamber. T>08C. Hyyt.ıa. l.a, March– April, 1999.
Fig. 3. The natural logarithm of the acetone emission (ngC gdw1 h1) from Scots pine plotted against the CO2 flux (mmol m2s1). Hyyt.ıa. l.a, March–April, 1999.
were determined in relation to CO2 uptake (Singh et al., 1994). We have also tested the ability of current emission rate algorithms to reproduce the results of the spring-99 data from Hyyt.ıa. l.a. We calculated two emission factors, one by averaging standard rates determined by the temperature algorithm and one by the ‘‘isoprene’’ algorithm introduced by Guenther et al. (1993), which includes both temperature and PAR. The standard emission rate, Es , by the temperature algorithm is defined by Es ¼
E ; expðbðT Ts ÞÞ
ð2Þ
where Es is the standard emission rate at a reference temperature Ts (308C), E is the emission at temperature T, and b is the slope of the temperature correlation
E ; ðCL CT Þ
ð3Þ
where Es is the standard rate at a reference temperature and PAR flux (308C and 1000 mmol m2 s1), CL is the light dependence factor and CT is the temperature dependence factor (Guenther et al., 1993). We found that including PAR in the emission rate algorithm did not improve the agreement between modelled and measured values. The average error introduced into the spring-99 data by the ‘‘isoprene’’ algorithm was 15( 60)% (n=28) for PAR>100 mmol m2 s1, while the error for the temperature algorithm was 7( 70)% (n=42). However, it should be remembered that PAR fluxes are difficult to define under field conditions. Although the thin Teflon film is transparent to PAR, sun conditions are never the same on all needles of even a small branch segment as that enclosed in the branch chamber. Temporary shadow effects from adjacent needles, the experimental scaffold, and adjacent trees are unavoidable during the course of a day. In the light of the relatively good correlation to temperature and in the absence of more information, we will use the temperature algorithm to determine an emission factor for acetone. We would prefer to call it a best estimate emission factor, considering all the uncertainties. The 208C standard emission rates for acetone varied between 110 and 350 ng C gdw1 h1 with an average of 250 90 ng C gdw1 h1 for the five specimens of Scots pine (Table 1). Monoterpene emissions from Scots pine are generally dominated by a-pinene and D3-carene, with smaller rates of camphene, limonene, myrcene, b-pinene, and tricyclene, (Table 2) (Janson, 1993). Interestingly, very little D3-carene was observed in the emission of the Scots pine tree at Mekrij.arvi and that of the tree at Hyyt.ıa. l.a measured in the spring of 1999. In both cases, the emissions from these trees were abnormal in the sense that concentration measurements at both sites showed D3-carene to be a predominant compound of the forest emission (Hakola et al., 2000; Rinne et al., 2000; Janson et al., 2001). The higher percentage of D3-carene in the spring-98 emission at Hyyt.ıa. l.a as compared to the august-98 emission, taken from different branches of the same tree, is probably due to the seasonal variation of the relative composition (Janson, 1993; Hakola et al., 2000). As was the case for the carbonyl compounds, the monoterpene emissions showed good correlation to temperatures above 08C and much poorer correlation to temperatures below. Table 3 lists the data for the temperature correlation (T>08C) for Asa, Mekrij.arvi, and Hyyt.ıa. l.a. With the exception of the summer-98 data, correlation coefficients ranged from 0.63 to 0.76
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Table 2 Relative composition (%) of the monoterpene emission from Scots pine at Asa, Mekrij.arvi, and Hyyti.al.a. Number of samples are indicated in parentheses in the first columna
Asa: June, 1997 Mekri: August 1997 Hyyti.al.ab April–May, 1998 August 1998 March–April 1999
tri.
a-pin.
cam.
b-pin.
myr.
D3-c.
lim.
(15) (19)
21 10
35 4 63 4
13 6 10 3
23 41
43 31
30 8 12
96 13 4
(44) (36) (77)
11 11 21
23 9 33 6 64 11
32 62 10 4
73 52 75
21 51 44
60 12 44 7 13
35 0 12 13
a
tri.=tricyclene, a-pin.=a-pinene, cam.=camphene, b-pin.=b-pinene, myr.=myrcene, D3-c.=D3-carene, lim.=limonene, bph.=b-phellandrene. b Janson et al. (2001). Table 3 Monoterpene emission rates (ngC gdw1 h1) observed in the branch emission measurements made on Scots pine at Asa, southern Sweden, June, 1997; Mekrij.arvi, southeastern Finland, August, 1997; and Hyyti.al.a in 1998 and 1999. In the 4th–6th columns are given the b coefficient (Eq. 1) and the correlation coefficient for the temperature dependences, as well as standardized emission rates (E(208C)) for each period Site
(n)
Temp. (8C) min–max, median
Terpenes (ng C gdw1 h1)
b
R2
E(208C)
Asa, June 1997 Mekri, Aug 1997 Hyyti.al.aa May 1998 August 1998 March–April 1999
(19) (23)
9–29, 20 11–28, 22
10–450 05–600
0.0943 0.1488
0.6336 0.8542
128 135
(28) (36) (93)
2–25, 11 6–22, 16 –3–23, 6
10–1193 60–450 0–1554
0.1365 0.0676 0.1448
0.7933 0.5458 0.6723
384 204 392
Grand average a
250 130
Janson et al. (2001).
and b-coefficients from 0.08 to 0.16. The b-coefficients fall into the range of most monoterpene emission studies (Guenther et al., 1993). Interestingly, a stronger temperature dependence for D3-carene was seen in May, which is consistent with its higher fraction of the total monoterpene emission (Table 2) and with the seasonal variation of the composition of the emission previously reported by Janson (1993). The standard emission rates (Eq. (2)) varied between 130 and 390 ng C gdw1 h1, and their average, the emission factor at 208C, was 250 130 ng C gdw1 h1, which is consistent with earlier results from central Sweden (608N) (Janson, 1993). The canister samples taken at Asa, Mekrij.arvi, and Hyyt.ıa. l.a have confirmed that Scots pine is not an isoprene emitter. Monoterpenes are known to be stored in special storage cells in the needles of conifers. Their emission is primarily a function of the effect of needle temperature on their vapor pressures and diffusion pathways, although other factors are also important (e.g. Fall, 1999). For acetone and other carbonyls, plant physiological factors are expected to be more important, since these compounds, like isoprene, are not stored in the
needles. Still, acetone emission rates did not correlate to CO2 assimilation or H2O flux, nor did the isoprene emission algorithm describe the emissions better than a simple temperature algorithm. The b-coefficient is also seen to lie in the same range and have the same average as that for the monoterpenes for the five trees measured, 0.12. However, while the June b-coefficient at Asa was low for the monoterpenes, it was high for acetone, an anti-correlation which holds for several of the other experiments (Tables 1 and 3). Midday acetone mixing ratios at Hyyt.ıa. l.a were always found to be higher than monoterpene mixing ratios, as seen in Fig. 4 for the spring-99 campaign. Also, while monoterpene mixing ratios decrease rapidly above the forest, no significant difference was observed for acetone between daytime mixing ratios inside the forest and 10 m above. The relatively high acetone mixing ratios reflect the longer lifetime of acetone as well as the larger diversity of sources. Acetone has biogenic, anthropogenic, and secondary sources and a lifetime of the order of weeks, while the terpenes have only biogenic sources and lifetimes of the order of hours or less. The few data available for comparison of concentrations and fluxes
R. Janson, C. de Serves / Atmospheric Environment 35 (2001) 4629–4637
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Fig. 4. Mid-day (10:00–15:00) mixing ratios of acetone and monoterpenes (pptv) and ambient air temperatures at a coniferous forest in southern Finland. Hyyt.ıa. l.a, March–April, 1999.
indicate that the mid-day molar ratio of concentrations, acetone/monoterpenes, was considerably enhanced in relation to that of the emissions when winds were in the southern sector, the sector more likely to contain pollutants, but not when winds were from the clean air sector in the NW. In a study at a coniferous forest site (not Scots pine) in the Sierra Nevada mountains (USA), Goldstein and Schade (2000) attributed 45% of the acetone mixing ratios to biogenic sources, of which 35% were attributed to direct forest emissions, and the remaining 65% to secondary production from biogenic precursors. During the spring campaign, the daytime acetone mixing ratios at Hyyt.ıa. l.a averaged 490 330 pptv (n=23 days), while in August they were 1400 480 pptv (n=20 days). Corresponding monoterpene mixing ratios were 200 180 pptv in the spring and 450 160 pptv in August (Janson et al., 2001). The acetone concentrations in August were similar to those found at a remote forest site in southeastern Finland (Janson, unpublished data) and to those reported by Singh et al. (1994) for August values in the lower troposphere (0–6 km) over eastern Canada (1140 413 pptv), while they were generally lower than summertime concentrations in rural areas of the USA (Goldstein and Schade, 2000; Reimer et al., 1998; Goldan et al., 1995). In Tables 1 and 2, we have given standard 208C emission rates as calculated from Eq. (2) using the temperature correlation observed in each experiment, and the average standard rates for the five experiments, 250 90 ng C gdw1 h1 for acetone and 250 130 ng C gdw1 h1 for the monoterpenes. Together with the average b-coefficient, 0.120 ( 0.04) and 0.118 ( 0.04), respectively, these values should be the best estimate emission factors for acetone and monoterpenes from boreal Scots pine. We have chosen to use 208C as
the standard temperature emission factor as it is closer to summertime temperatures in the boreal zone than the 308C which is usually used as the reference temperature. For the sake of comparison, we can redo the calculation of emission rates using 308C as the reference temperature. Averaging over the five experiments gives us 308C emission factors of 870 480 and 900 640 ng gdw1 h1 for acetone and monoterpenes, respectively. The monoterpene factor is lower than the 1300 ng C gdw1 h1 used by Simpson et al. (1999) in a recent emission inventory. It is 25% lower than the results obtained by Rinne et al. (2000), who used the micrometeorological gradient method at Hyyt.ıa. l.a and at a site near Mekrij.arvi in eastern Finland. Considering the difficulties and uncertainties involved in both chamber and micrometeorological techniques, we consider the agreement to be good. 3.2. Emissions from Norway spruce (Picea abies) A few branch emission measurements were also made on Norway spruce (Picea abies) at Asa in southern Sweden, in the beginning of June. As with Scots pine, the carbonyl emission was dominated by acetone, which generally accounted for more than 80% by carbon mass. We also found the acetone emission to be as large as or larger than the terpene and isoprene emissions (Fig. 5 and Table 4). The monoterpene emissions were dominated by a-pinene (64( 23)% by mass, n=25) and limonene (14( 11)%), respectively, which is in good agreement with earlier findings (Janson, 1993). The standard 208C emission rate listed in Table 4 is lower than previous results, but probably not representative of summertime rates as the entire spring period up to the days of the experiments was unusually cold for that time of the year, the weather for the most part being lousy,
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Fig. 5. Branch emissions of acetone, a-pinene, and isoprene (ng C gdw1 h1) from Norway spruce in southern Sweden, May–June, 1997.
Table 4 Acetone, monoterpene, and isoprene emission rates (ngC gdw1 h1) observed in the branch emission measurements made on Norway spruce at Asa, southern Sweden, May–June, 1997. In the 4th–6th columns are given the b coefficient (Eq. (1)) and the correlation coefficient for the temperature dependences, as well as standardized emission rates (E(208C)) VOC
(n)
Temp. (8C) min–max, median
Emission range (ng C gdw1 h1)
b
R2
E(208C)
Acetone Monoterpenes Isoprene
(21) (25) (23)
4–25, 13 4–25, 13 4–25, 13
520–290 9–324 0–151
0.0654 0.0975
0.3874 0.4173
138 90 108
and visible needle growth was not observed until the last day of the measurements. Emissions were low for all the BVOC measured. As can be seen in Table 4, the correlation between acetone and temperature was not good. Neither could the isoprene algorithm (Eq. (3)), which includes PAR, explain the variations in acetone emission. However, the data are few, the values low, many below the detection limit, and thus more data are needed. The isoprene emissions were also lower than usual. The standard emission factor (308C) established by Eq. (3) was only 400 200 ng C gdw1 h1 as compared to the 1200 250 ng C gdw1 h1 observed at other Swedish sites and the values reported by Steinbrecher and Rabong (1994) for Norway spruce in Germany.
acetone from 5 specimens of boreal Scots pine are 870 480 ng C gdw1 h1 and 0.12, respectively. New monoterpene emission data for five specimens of Scots pine yield an average standard emission rate (308C) and b-coefficient of 900 640 ng C gdw1 h1 and 0.12, respectively. Acetone emissions from Norway spruce were found to be as high as those for the monoterpenes and isoprene on a carbon mass basis. The standard emission rate (308C) was found to be 265 ng C gdw1 h1, but the sparsity of data, along with the unusual weather conditions, precludes the establishment of a summertime best estimate emission rate.
Acknowledgements 4. Conclusions Acetone has been found to be a significant emission of both Scots pine (Pinus sylvestris) and Norway spruce (Picea abies). It makes up about 64% (on a carbon basis) of the light carbonyl emission from Scots pine and perhaps as much as 80–90% from Norway spruce, the other significant carbonyl compounds being formaldehyde and acetaldehyde. The diurnal pattern and correlation to temperature of the emission is similar to that of the monoterpenes. The average standard emission rate (308C) and b-coefficient for
We thank the staffs of the Mekrij.arvi Research Station and the SMEAR II station and Hyyt.ıa. l.a Research Station in Finland, the staff of the Asa Research Station in Sweden, Carlein Mak#a of the University of Utrecht, Peter Tunved of this institute for their help in the field. The HPLC analysis of the carbonyl compounds was done by Vaclav Vesely and the GC-FID and GC-MS analyses of the hydrocarbons by . Maria Ohrn, Katri Puhto and Dr. Ulla Wideqvist, all from this institute. We thank the Stockholm Environmental Agency for lending us the CO2 instrument. The projects (ENV4-CT95-0022, BIPHOREP and
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ENV4-CT97-0405, BIOFOR) were financed by the Environment and Climate Research Programme of the European Commision.
References Apel, E.C., Calvert, J.G., Riemer, D., Pos, W., Zika, R., Kleindienst, T.E., Lonneman, W.A., Fung, K., Fujita, E., Shepson, P.B., Starn, T.K., Roberts, P.T., 1998. Measurements comparison of oxygenated volatile organic compounds at a rural site during the 1995 SOS Nashville Intensive. Journal of Geophysical Research 103, 22295–22316. Fall, R., 1999. Biogenic emissions of volatile organic compounds from higher plants. In: Hewitt, C.N. (Ed.), Reactive Hydrocarbons in the Atmosphere, pp. 41–96. Gilpin, T., Apel, E., Fried, A., Wert, B., Calvert, J., Zhang, G.F., Dasgupta, P., Harder, J.W., Heikes, B., Hopkins, B., Westberg, H., Kleindienst, T., Lee, Y.N., Zhou, X.L., Lonneman, W., Sewell, S., 1997. Intercomparison of six ambient (CH2O) measurement techniques. Journal of Geophysical Research 102, 21161–21188. Goldan, P.D., Kuster, W.C., Fehsenfeld, F.C., Montzka, S.A., 1995. Hydrocarbon measurements in the southeastern United States: the Rural Oxidants in the Southern Environment (ROSE) program 1990. Journal of Geophysical Research 100, 25945–25963. Goldstein, A.H., Schade, G.W., 2000. Quantifying biogenic and anthropogenic contributions to acetone mixing ratios in a rural environment. Atmospheric Environment 34, 4997–5006. Guenther, A., Zimmerman, P., Harley, P., Monson, R., Fall, R., 1993. Isoprene and monoterpene emission rate variability: model evaluations and sensitivity analyses. Journal of Geophysical Research 98, 12609–12617. Hakola, H., Laurila, T., Rinne, J., Puhto, K., 2000. The ambient concentrations of biogenic hydrocarbons at a northern European, boreal site. Atmospheric Environment 34, 4971–4982. Helmig, D., Klinger, L., Guenther, A., Vierling, L., Geron, C., Zimmerman, P., 1999. Biogenic volatile organic compound emissions (BVOCs) 1. Identifications from three continental sites in the U.S. Chemosphere 38, 2163–2187. Isidorov, V.A., Zenkevich, I.G., Ioffe, B.V., 1985. Volatile organic compounds in the atmosphere of forests. Atmospheric Environment 19, 1–8. Janson, R.W., 1993. Monoterpene emissions from Scots pine and Norwegian spruce. Journal of Geophysical Research 98, 2839–2850. Janson, R., Rosman, K., Karlsson, A., Hansson, H.-C., 2001. Biogenic emissions and gaseous precursors to forest aerosols. Tellus (in press). Juuti, S., Arey, J., Atkinson, R., 1990. Monoterpene emissions rate measurements from a Monterey pine. Journal of Geophysical Research 95, 7515–7519. Kesselmeier, J., Staudt, M., 1999. Biogenic volatile organic compounds (VOC): an overview on emission, physiology and ecology. Journal of Atmospheric Chemistry 33, 23–88.
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Kleindienst, T.E., Shepson, P.B., Nero, C.M., Arnts, R.R., Tejada, S.B., Mackay, G.I., Mayne, L.K., Schiff, H.I., Lind, J.A., Kok, G.L., Lazrus, A.L., Dasgupta, P.K., Dong, S., 1988. An intercomparison of formaldehyde measurement techniques at ambient concentration. Atmospheric Environment 22, 1931–1939. Kleindienst, T.E., Corse, E.W., Blanchard, F.T., Lonneman, W.A., 1998. Evaluation of the performance of DNPH-coated silica gel and C18 cartridges in the measurement of formaldehyde in the presence and absence of ozone. Environmental Science and Technology 32, 124–130. Kulmala, M., H.ameri, K., Aalto, P., M.akel.a, J., Pirjola, L., . Dal Maso, D., Seidl, Nilsson, D., Buzorius, G., Rannik, U., W., Hoffmann, T., Janson, R., Hansson, H.C., Viisanen, Y., Laaksonen, A., O’Dowd, C., 2001. Overview of the international project on Biogenic aerosol formation in the boreal forest (BIOFOR). Tellus (in press). Laurila, T., Lindfors, V. (Eds.), 1999. Biogenic VOC emissions and photochemistry in the boreal regions of Europe. Air Pollution Research Report No. 70, Commission of the European Communities, Luxembourg, 158pp, ISBN 92828-6990-3. MacDonald, R.C., Fall, R., 1993. Acetone emission from conifer buds. Phytochemistry 34, 991–994. Martin, R., Villanueva, I., Zhang, J., Popp, C., 1999. Nonmethane hydrocarbon, monocarboxylic acid, and low molecular weight aldehyde and ketone emissions from vegetation in Central New Mexico. Environmental Science and Technology 33, 2186–2192. Reimer, D., Pos, W., Milne, P., Farmer, C., Zika, R., Apel, E., Olszyna, K., Kliendienst, T., Lonneman, W., Bertman, S., Shepson, P., Starn, T., 1998. Observations of nonmethane hydrocarbons and oxygenated volatile organic compounds at a rural site in the southeastern United States. Journal of Geophysical Research 103, 28111–28128. Rinne, J., Hakola, H., Laurila, T., Rannik, U., 2000. Canopy scale monoterpene emissions of Pinus sylvestris dominated forests. Atmospheric Environment 34, 1099–1107. . Simpson, D., Winiwarter, W., Borjesson, G., Cinderby, S., Ferreiro, A., Guenther, A., Hewitt, C.N., Janson, R., Aslam, M., Khalil, K., Owen, S., Pierce, T.E., Puxbaum, H., Shearer, . M., Skiba, U., Steinbrecher, R., Tarrason, L., Oquist, M.G., 1999. Inventorying emissions from nature in Europe. Journal of Geophysical Research 104, 8113–8152. Singh, H.B., Kanakidou, M., Crutzen, P.J., Jacob, D.J., 1995. High concentrations and photochemical fate of oxygenated hydrocarbons in the global troposphere. Nature 378, 50–54. Singh, H.B., Ohara, D., Herlth, D., Sachse, W., Blake, D.R., Bradshaw, J.D., Kanakidou, M., Crutzen, P.J., 1994. Acetone in the atmosphere distribution, sources, and sinks. Journal of Geophysical Research 99, 1805–1819. Steinbrecher, R., Rabong, R., 1994. Emission of terpenoids and C2–C6 hydrocarbons by Norway Spruce: phytotron experiments and field studies. In: Borrell, P.M. et al. (Eds.), Proceedings of EUROTRAC Symposium ’94. SPB Academic Publ., The Hague, The Netherlands, pp. 502–505.
Acetone and monoterpene emissions from the boreal forest in northern Europe Robert Janson*, Claes de Serves Air Pollution Laboratory, Institute for Applied Environmental Research, Stockholm University, S-106 91 Stockholm, Sweden Received 21 September 2000; received in revised form 21 February 2001; accepted 7 March 2001
Abstract Acetone is a ubiquitous component of the atmosphere which, by its photolysis, can play an important role in photochemical reactions in the free troposphere. This paper investigates the biogenic source of acetone from Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) in the Scandinavian boreal zone. Branch emission measurements of acetone, monoterpenes, and isoprene were made with an all-Teflon flow-through branch chamber from five specimens of Scots pine at three sites in Sweden and Finland, and from one specimen of Norway spruce at one site in Sweden. Acetone samples were taken with SepPakTM DNPH cartridges, monoterpenes with Tenax TA, and isoprene with 3 l electropolished canisters. Acetone was found to dominate the carbonyl emission of both Scots pine and Norway spruce, as large as the monoterpene emissions and for Norway spruce, as the isoprene emission. The average standard emission rate (308C) and average b-coefficient for the temperature correlation for 5 specimens of Scots pine were 870 ng C gdw1 h1 (gdw=gram dry weight) and 0.12, respectively. For the monoterpenes the values were 900 ng C gdw1 h1 and 0.12, respectively. The standard emission rate (308C) for acetone from Norway spruce was 265 ng C gdw1 h1, but the sparsity of data, along with the unusual weather conditions at the time of the measurements, precludes the establishment of a summertime best estimate emission factor. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Carbonyls; BVOC; Biogenic emissions; Scots pine; Norway spruce
1. Introduction Atmospheric acetone has been found not only to be a ubiquitous component of the atmosphere, but also at relatively high concentrations compared to other nonmethane organic species (Singh et al., 1994, 1995). Model results suggest that acetone concentrations in the free troposphere are sufficient to render its photolysis an important source for peroxyacetylnitrate (PAN), which removes NOx from further photochemical activity, as well as an important source of free radicals in upper and dry regions (Singh et al., 1995). Thus, it is an important component of photochemical cycles in the upper troposphere, affecting the tropospheric ozone budget. *Corresponding author. E-mail address: [email protected] (R. Janson).
The sources of acetone are many, both primary and secondary, natural and anthropogenic (e.g. Singh et al., 1994). Estimates of the secondary production by the atmospheric oxidation of mainly anthropogenic, and of those mainly propane, but also natural hydrocarbons show it to be the largest source on a global basis. Primary anthropogenic sources include evaporative loss during its use as a solvent and as an intermediate in the chemical industry, automobile exhaust, and biomass burning, where biomass burning is thought to be the most important. Direct emissions from land vegetation, notably evergreens, contribute 9 Tg to the estimated global source of 40 Tg yr1. The uncertainties for most of the sources are large, in particular those for the biogenic sources, for which source strength estimates range from 4 to 18 Tg yr1 (Singh et al., 1994). Very few direct measurements of foliar emissions have been made
1352-2310/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 1 6 0 - 1
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(e.g. Simpson et al., 1999; Kesselmeier and Staudt, 1999; Fall, 1999). Isidorov et al. (1985) identified acetone in the emissions of a number of tree species and MacDonald and Fall (1993) reported small emissions from the buds of conifers. Martin et al. (1999) investigated emissions from 3 deciduous and 5 coniferous trees in New Mexico, USA, and reported the overall average emission of acetone (all species) to be 8% by mass. They do not report specific emission rates for acetone. In a screening experiment at three sites in the USA, in which one or two samples were taken from each of 63 species, Helmig et al. (1999) report acetone emission rates for 14 species. In this paper, we present results from the first field emission measurements of light carbonyl compounds made on Scots pine and Norway spruce and summarize those results as standard emission factors. Results from the complementary measurements of monoterpenes and isoprene are also presented and emission factors given.
2. Experimental 2.1. Branch emissions Scots pine (Pinus sylvestris) emission measurements were made in the spring and summer of 1997, at Asa Research Park (578N, 128E) in southern Sweden, and at Mekrij.arvi Research Station (638N, 318E), in southeastern Finland, respectively, as part of the BIPHOREP (Biogenic VOC emissions and photochemistry in the boreal regions of Europe) project (Laurila and Lindfors, 1999). A few measurements were also made on Norway spruce (Picea abies) at Asa. Asa is a mixed pine and spruce forest in the southern boreal zone and Mekrij.arvi a pine forest at the border of the southern and middle boreal zones. Measurements from Scots pine were also made during the summer of 1998 and spring of 1999, at the Hyyt.ıa. l.a Research Station in southern Finland (628N, 248E) as part of the BIOFOR (BIOgenic aerosol formation in the boreal FORest) project (Kulmala et al., 2001). The forest at Hyyt.ıa. l.a is predominantly pine with some Norway spruce and birch. The monoterpene data from Hyyt.ıa. l.a are reported in Janson et al. (2001) but are included here for comparison and calculation of the emission factor. The measurements were made with a 25 cm diameter, 18 liter all-Teflon chamber made of 0.05 mm transparent FEP-Teflon film enclosing a 20–30 cm branch segment. A fan, with the motor externally mounted, ensured mixing of the air inside the chamber. Care was taken to avoid contact between the branch and the walls of the chamber in order to avoid mechanical abrasion which can temporarily affect emission rates (Juuti et al., 1990). The chamber was continuously flushed with 9 NL min1 (NL=normal liters) ambient air (Bronkhorst HI-TEC massflow controller), purified of ozone with a KI
scrubber, and of particles with a 47 mm Teflon filter (2 mm). The water content of the inlet airflow was reduced by cooling with a Peltier element, regulated manually to keep the chamber humidity at about the same as ambient humidity. Ambient, inflow, and chamber temperatures and relative humidity were measured continuously during all experiments with Rotronic MP-100 sensors. Photosynthetic active radiation (PAR) was measured with a LiCOR LI-190SA quantum sensor mounted above the chamber. During the Hyyt.ıa. l.a experiments, the CO2 flux was recorded with a Model 41H Ambient Gasfilter Correlation CO2 Instrument (Thermal Environmental Instrument Inc.). Simultaneous samples were taken from a constant flow tapped from the chamber inlet and outlet lines via 8-way Teflon ports and 20 cm 0.0300 i.d. Teflon tubing. In this way, the flow through the chamber was not disturbed every time sampling commenced (0.8 l min1 for DNPH cartridges). Sampling times and volumes were controlled by a system of timers, solenoid valves, and Honeywell mass flow sensors. Sample flow, meteorological and CO2 data were continuously collected on a PC via ADAM1 Data Acquisition Modules (ADAM 4017 and 4520, Advantech Co., Ltd.). The branch emission was determined from the concentration difference between the outlet and inlet, the airflow through the chamber, and the branch needle dry weight. Rates are given as mass carbon, ng C gdw1 h1 (gdw=gram dry weight). The all-Teflon chamber was found to have small and variable artifacts for formaldehyde and 2-butanone, which may have been due to adsorption and desorption effects. However, they were at all times below the detection limits in ng C gdw1 h1 for these gases. No such effects were seen for acetaldehyde or acetone. The uncertainty in the chamber emission data for acetone, as determined by the blank variability, see below, was 4 ng C gdw1 h1. 2.2. Ambient air concentrations Daily concentration measurements were made inside the forest at Hyyt.ıa. l.a during the BIOFOR campaigns in the summer of 1998 and spring of 1999. Samples were taken automatically with a system of timers and solenoid valves, and the flows monitored with Honeywell air mass sensors. 2.3. Sampling and analysis The light carbonyl compounds were sampled on SepPakTM DNPH cartridges at a flow rate of 0.8 l min1 for 2 h (4 h for concentration measurements). All cartridges were stored at 8–108C. In the laboratory, the compounds were eluted with acetonitrile, and analyzed by HPLC. Liquid standards included formaldehyde, acetaldehyde, acetone, propanal, and 2-butanone.
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SepPakTM DNPH cartridges are known to have high and variable blank values. Blank values must therefore be carefully determined from every batch. In order to test the variability, 2 boxes were purchased and analyzed for their blank values. The average ( 1 S.D.) blank values were 13 11 ng cartridge1 for formaldehyde, 73 49 ng cartridge1 for acetaldehyde, 158 74 ng cartridge1 for acetone, and 50 29 ng cartridge1 for 2-butanone (n=35 cartridges). Cartridges stored at room temperature, at 8–108C, and at 208C for 3, 8, and in some cases 12 weeks, showed no significant differences in their blank values. In any given experiment, the detection limits for the carbonyls are a function of the blank value of the particular batch DNPH cartridges, as well as the experimental setup as regards sample volume and for emission measurements chamber flow and needle dry weight. In the experiments reported here, detection limits for concentration and emission measurements varied from 50 to 80 pptv and 3 to 24 ng C gdw1 h1, respectively, for formaldehyde, 40 to 200 pptv and 9 to 140 ng C gdw1 h1 for acetaldehyde, 40 to 60 pptv and 11 to 370 ng C gdw1 h1 for acetone, and 100 to 200 pptv and 24 to 230 ng C gdw1 h1 for 2-butanone. Ozone is known to create artifacts when sampling with silica gel DNPH cartridges (e.g. Kleindienst et al., 1988, 1998; Gilpin et al., 1997; Apel et al., 1998). We conducted parallel sampling with and without KI ozone scrubbers in front of the cartridges. Ozone levels were generally below 50 ppbv and carbonyl mixing ratios quite low. The samples without an ozone scrubber all showed lower values for formaldehyde, acetaldehyde, and acetone, but the differences were generally below the level of significance set by the variability of the DNPH cartridge blanks. Nevertheless, all concentration sampling was done through ozone scrubbers (KI coated annular denuders) placed at the air inlet and immediately in front of the cartridges. KI scrubbers take up water at high relative humidities and therefore no sampling was done under rainy conditions. For the chamber emission measurements, the O3-scrubber was placed at the chamber air inlet. Thus, the chamber air was free of ozone, which serves the twofold purpose of avoiding sampling artifacts and avoiding ozone reactions in the chamber. From 10 experiments in which two cartridges were placed in series, we estimate sampling efficiency for acetaldehyde and acetone to be better than 95% and 90%, respectively, while that for formaldehyde was variable and two trials gave values as low as 62%. These results are in agreement with those of the intercomparison reported by Gilpin et al. (1997), which indicated that silica gel DNPH cartridges trap formaldehyde at only about 75% efficiency, even without interfering gases such as ozone. Kleindienst et al. (1998), on the other hand, reported 95% recovery.
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Monoterpenes were sampled with stainless steel tubes containing 200 mg Tenax TA at an air flow rate of 100 ml min1 for 12–40 min. After sampling, the tubes were stored dry until analysed by GC-MS analysis at the ITM Air Pollution Laboratory. Compound identification and quantification was accomplished with terpene standards in methanol. The standards included tricyclene, a-pinene, camphene, sabinene, b-pinene, myrcene, D3-carene, limonene, and ocimene. Isoprene was sampled from air samples collected in 3 l electropolished stainless steel canisters. The canisters were evacuated to 106 bar at 1508C for 6 h. The sampling flow rate was regulated to 0.8 l min1 and the canister filled to a final overpressure of 2 bars with a TFE-Teflon membrane pump (BRC, model FC-1121). Sampling took 10– 15 min. The samples were stored at room temperature and analyzed by GC-FID. Identification was done with authentic standards and quantification with a propane standard.
3. Results and discussion 3.1. Emissions from Scots pine (Pinus sylvestris) Acetone was found to dominate the light carbonyl emission from Scots pine, accounting for between 64% and 94% (by carbon mass) of the light carbonyl emission. As most of the emission data for formaldehyde and acetaldehyde were below the detection limit for these compounds, they will not be discussed in any detail here. The values from the spring-99 data from Hyyt.ıa. l.a, being the most reliable (i.e. with the largest number of values above detection limits), were 64 13% acetone, 23 13% acetaldehyde, and 12 7% formaldehyde (n=42). The corresponding mole fractions are 26 13%, 26 14%, and 48 14%, respectively. 2butanone was also identified, but always below the detection limit. The few nighttime samples from Hyyt.ıa. l.a with values above the detection limits posed by the cartridges, indicate a significant shift towards a slightly higher percentage of acetone in the emission during the night, 70 18% (n=8), as compared to the daytime, 60 9% (n=34). These results are in contrast to those of Martin et al. (1999), who measured carbonyl emissions from 3 deciduous and 5 coniferous trees in New Mexico, USA. While they do not report the distribution for individual species, they found the emission of formaldehyde and acetaldehyde always to be greatest. On average, acetone accounted for only 8% of the carbonyl emissions in that study. Acetone emission rates ranged from below 20 to more than 1000 ng gdw1 h1, (Table 1). The diurnal variation and absolute values were similar to those of a-pinene, with maximum values around noontime, (Fig. 1). The mechanism of acetone production and emission, as well
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Table 1 Emission rates of acetone (ngC gdw1 h1) from branches of Scots pine at Asa, southern Sweden, June, 1997; Mekrij.arvi, southeastern Finland, August, 1997; and Hyyti.al.a, southern central Finland, 1998 and 1999. Minimum, maximum, and median temperatures for each data set are given in column 2. In the 4th–6th columns are given the b coefficient (Eq. (1)) and the correlation coefficient for the temperature dependences, as well as standardized emission rates (E(208C)) for each period Site
(n)
Temp. (8C) min–max, median
Acetone (ngC gdw1 h1)
b
R2
E (208C)
Asa, June 1997 Mekri, Aug 1997 Hyyti.al.a May 1998 August 1998 Mar–Apr 11 1999
(18) (22)
9–29, 20 11–28, 22
520–640 520–760
0.1566 0.097
0.6329 0.6413
113 214
(09) (31) (40)
2–18, 10 5–25, 17 0–23, 9
520–240 520–1400 520–780
0.0799 0.1058 0.1625
0.7399 0.2398 0.7569
270a 340 330 250 90
Grand average a
Few data above detection limit during the spring-98 campaign.
Fig. 1. (a) Branch emissions of acetone and a-pinene from Scots pine, and (b) chamber air temperature (8C) and photosynthetic active radiation (PAR: mmol m2s1). Hyyt.ıa. l.a, March 29–April 5, 1999.
as the factors which regulate that mechanism, are still largely unknown (e.g. Fall, 1999). With the exception of the summer data from Hyyt.ıa. l.a, the acetone emissions from Scots pine were found to correlate fairly well with chamber temperature according to ln E ¼ T þ c;
ð1Þ 1
1
where E is the emission rate (ng C gdw h ) at temperature T, b is the slope of the correlation curve, and c is a constant (Fig. 2). b-coefficients were between 0.10 and 0.16 and R2 values between 0.63 and 0.76
(Table 1). Considerable scatter was seen in the few data when temperatures were below 08C. Reasons for poor correlation in the summer data could not be found in the temperature, relative humidity, PAR, or H2O flux data, but it can be noted that the monoterpene data also showed an abnormally poor correlation. Emission rates did not correlate to the H2O flux, as determined from the temperature and relative humidity data from the inlet and chamber air. Neither did they correlate to CO2 assimilation, (Fig. 3), which is contrary to some laboratory studies in which acetone emissions
R. Janson, C. de Serves / Atmospheric Environment 35 (2001) 4629–4637
4633
(Fig. 2). The standard rate by the emission algorithm for isoprene is defined by Es* ¼
Fig. 2. The natural logarithm of the acetone emission (ngC gdw1 h1) from Scots pine plotted against air temperature inside the branch chamber. T>08C. Hyyt.ıa. l.a, March– April, 1999.
Fig. 3. The natural logarithm of the acetone emission (ngC gdw1 h1) from Scots pine plotted against the CO2 flux (mmol m2s1). Hyyt.ıa. l.a, March–April, 1999.
were determined in relation to CO2 uptake (Singh et al., 1994). We have also tested the ability of current emission rate algorithms to reproduce the results of the spring-99 data from Hyyt.ıa. l.a. We calculated two emission factors, one by averaging standard rates determined by the temperature algorithm and one by the ‘‘isoprene’’ algorithm introduced by Guenther et al. (1993), which includes both temperature and PAR. The standard emission rate, Es , by the temperature algorithm is defined by Es ¼
E ; expðbðT Ts ÞÞ
ð2Þ
where Es is the standard emission rate at a reference temperature Ts (308C), E is the emission at temperature T, and b is the slope of the temperature correlation
E ; ðCL CT Þ
ð3Þ
where Es is the standard rate at a reference temperature and PAR flux (308C and 1000 mmol m2 s1), CL is the light dependence factor and CT is the temperature dependence factor (Guenther et al., 1993). We found that including PAR in the emission rate algorithm did not improve the agreement between modelled and measured values. The average error introduced into the spring-99 data by the ‘‘isoprene’’ algorithm was 15( 60)% (n=28) for PAR>100 mmol m2 s1, while the error for the temperature algorithm was 7( 70)% (n=42). However, it should be remembered that PAR fluxes are difficult to define under field conditions. Although the thin Teflon film is transparent to PAR, sun conditions are never the same on all needles of even a small branch segment as that enclosed in the branch chamber. Temporary shadow effects from adjacent needles, the experimental scaffold, and adjacent trees are unavoidable during the course of a day. In the light of the relatively good correlation to temperature and in the absence of more information, we will use the temperature algorithm to determine an emission factor for acetone. We would prefer to call it a best estimate emission factor, considering all the uncertainties. The 208C standard emission rates for acetone varied between 110 and 350 ng C gdw1 h1 with an average of 250 90 ng C gdw1 h1 for the five specimens of Scots pine (Table 1). Monoterpene emissions from Scots pine are generally dominated by a-pinene and D3-carene, with smaller rates of camphene, limonene, myrcene, b-pinene, and tricyclene, (Table 2) (Janson, 1993). Interestingly, very little D3-carene was observed in the emission of the Scots pine tree at Mekrij.arvi and that of the tree at Hyyt.ıa. l.a measured in the spring of 1999. In both cases, the emissions from these trees were abnormal in the sense that concentration measurements at both sites showed D3-carene to be a predominant compound of the forest emission (Hakola et al., 2000; Rinne et al., 2000; Janson et al., 2001). The higher percentage of D3-carene in the spring-98 emission at Hyyt.ıa. l.a as compared to the august-98 emission, taken from different branches of the same tree, is probably due to the seasonal variation of the relative composition (Janson, 1993; Hakola et al., 2000). As was the case for the carbonyl compounds, the monoterpene emissions showed good correlation to temperatures above 08C and much poorer correlation to temperatures below. Table 3 lists the data for the temperature correlation (T>08C) for Asa, Mekrij.arvi, and Hyyt.ıa. l.a. With the exception of the summer-98 data, correlation coefficients ranged from 0.63 to 0.76
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Table 2 Relative composition (%) of the monoterpene emission from Scots pine at Asa, Mekrij.arvi, and Hyyti.al.a. Number of samples are indicated in parentheses in the first columna
Asa: June, 1997 Mekri: August 1997 Hyyti.al.ab April–May, 1998 August 1998 March–April 1999
tri.
a-pin.
cam.
b-pin.
myr.
D3-c.
lim.
(15) (19)
21 10
35 4 63 4
13 6 10 3
23 41
43 31
30 8 12
96 13 4
(44) (36) (77)
11 11 21
23 9 33 6 64 11
32 62 10 4
73 52 75
21 51 44
60 12 44 7 13
35 0 12 13
a
tri.=tricyclene, a-pin.=a-pinene, cam.=camphene, b-pin.=b-pinene, myr.=myrcene, D3-c.=D3-carene, lim.=limonene, bph.=b-phellandrene. b Janson et al. (2001). Table 3 Monoterpene emission rates (ngC gdw1 h1) observed in the branch emission measurements made on Scots pine at Asa, southern Sweden, June, 1997; Mekrij.arvi, southeastern Finland, August, 1997; and Hyyti.al.a in 1998 and 1999. In the 4th–6th columns are given the b coefficient (Eq. 1) and the correlation coefficient for the temperature dependences, as well as standardized emission rates (E(208C)) for each period Site
(n)
Temp. (8C) min–max, median
Terpenes (ng C gdw1 h1)
b
R2
E(208C)
Asa, June 1997 Mekri, Aug 1997 Hyyti.al.aa May 1998 August 1998 March–April 1999
(19) (23)
9–29, 20 11–28, 22
10–450 05–600
0.0943 0.1488
0.6336 0.8542
128 135
(28) (36) (93)
2–25, 11 6–22, 16 –3–23, 6
10–1193 60–450 0–1554
0.1365 0.0676 0.1448
0.7933 0.5458 0.6723
384 204 392
Grand average a
250 130
Janson et al. (2001).
and b-coefficients from 0.08 to 0.16. The b-coefficients fall into the range of most monoterpene emission studies (Guenther et al., 1993). Interestingly, a stronger temperature dependence for D3-carene was seen in May, which is consistent with its higher fraction of the total monoterpene emission (Table 2) and with the seasonal variation of the composition of the emission previously reported by Janson (1993). The standard emission rates (Eq. (2)) varied between 130 and 390 ng C gdw1 h1, and their average, the emission factor at 208C, was 250 130 ng C gdw1 h1, which is consistent with earlier results from central Sweden (608N) (Janson, 1993). The canister samples taken at Asa, Mekrij.arvi, and Hyyt.ıa. l.a have confirmed that Scots pine is not an isoprene emitter. Monoterpenes are known to be stored in special storage cells in the needles of conifers. Their emission is primarily a function of the effect of needle temperature on their vapor pressures and diffusion pathways, although other factors are also important (e.g. Fall, 1999). For acetone and other carbonyls, plant physiological factors are expected to be more important, since these compounds, like isoprene, are not stored in the
needles. Still, acetone emission rates did not correlate to CO2 assimilation or H2O flux, nor did the isoprene emission algorithm describe the emissions better than a simple temperature algorithm. The b-coefficient is also seen to lie in the same range and have the same average as that for the monoterpenes for the five trees measured, 0.12. However, while the June b-coefficient at Asa was low for the monoterpenes, it was high for acetone, an anti-correlation which holds for several of the other experiments (Tables 1 and 3). Midday acetone mixing ratios at Hyyt.ıa. l.a were always found to be higher than monoterpene mixing ratios, as seen in Fig. 4 for the spring-99 campaign. Also, while monoterpene mixing ratios decrease rapidly above the forest, no significant difference was observed for acetone between daytime mixing ratios inside the forest and 10 m above. The relatively high acetone mixing ratios reflect the longer lifetime of acetone as well as the larger diversity of sources. Acetone has biogenic, anthropogenic, and secondary sources and a lifetime of the order of weeks, while the terpenes have only biogenic sources and lifetimes of the order of hours or less. The few data available for comparison of concentrations and fluxes
R. Janson, C. de Serves / Atmospheric Environment 35 (2001) 4629–4637
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Fig. 4. Mid-day (10:00–15:00) mixing ratios of acetone and monoterpenes (pptv) and ambient air temperatures at a coniferous forest in southern Finland. Hyyt.ıa. l.a, March–April, 1999.
indicate that the mid-day molar ratio of concentrations, acetone/monoterpenes, was considerably enhanced in relation to that of the emissions when winds were in the southern sector, the sector more likely to contain pollutants, but not when winds were from the clean air sector in the NW. In a study at a coniferous forest site (not Scots pine) in the Sierra Nevada mountains (USA), Goldstein and Schade (2000) attributed 45% of the acetone mixing ratios to biogenic sources, of which 35% were attributed to direct forest emissions, and the remaining 65% to secondary production from biogenic precursors. During the spring campaign, the daytime acetone mixing ratios at Hyyt.ıa. l.a averaged 490 330 pptv (n=23 days), while in August they were 1400 480 pptv (n=20 days). Corresponding monoterpene mixing ratios were 200 180 pptv in the spring and 450 160 pptv in August (Janson et al., 2001). The acetone concentrations in August were similar to those found at a remote forest site in southeastern Finland (Janson, unpublished data) and to those reported by Singh et al. (1994) for August values in the lower troposphere (0–6 km) over eastern Canada (1140 413 pptv), while they were generally lower than summertime concentrations in rural areas of the USA (Goldstein and Schade, 2000; Reimer et al., 1998; Goldan et al., 1995). In Tables 1 and 2, we have given standard 208C emission rates as calculated from Eq. (2) using the temperature correlation observed in each experiment, and the average standard rates for the five experiments, 250 90 ng C gdw1 h1 for acetone and 250 130 ng C gdw1 h1 for the monoterpenes. Together with the average b-coefficient, 0.120 ( 0.04) and 0.118 ( 0.04), respectively, these values should be the best estimate emission factors for acetone and monoterpenes from boreal Scots pine. We have chosen to use 208C as
the standard temperature emission factor as it is closer to summertime temperatures in the boreal zone than the 308C which is usually used as the reference temperature. For the sake of comparison, we can redo the calculation of emission rates using 308C as the reference temperature. Averaging over the five experiments gives us 308C emission factors of 870 480 and 900 640 ng gdw1 h1 for acetone and monoterpenes, respectively. The monoterpene factor is lower than the 1300 ng C gdw1 h1 used by Simpson et al. (1999) in a recent emission inventory. It is 25% lower than the results obtained by Rinne et al. (2000), who used the micrometeorological gradient method at Hyyt.ıa. l.a and at a site near Mekrij.arvi in eastern Finland. Considering the difficulties and uncertainties involved in both chamber and micrometeorological techniques, we consider the agreement to be good. 3.2. Emissions from Norway spruce (Picea abies) A few branch emission measurements were also made on Norway spruce (Picea abies) at Asa in southern Sweden, in the beginning of June. As with Scots pine, the carbonyl emission was dominated by acetone, which generally accounted for more than 80% by carbon mass. We also found the acetone emission to be as large as or larger than the terpene and isoprene emissions (Fig. 5 and Table 4). The monoterpene emissions were dominated by a-pinene (64( 23)% by mass, n=25) and limonene (14( 11)%), respectively, which is in good agreement with earlier findings (Janson, 1993). The standard 208C emission rate listed in Table 4 is lower than previous results, but probably not representative of summertime rates as the entire spring period up to the days of the experiments was unusually cold for that time of the year, the weather for the most part being lousy,
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R. Janson, C. de Serves / Atmospheric Environment 35 (2001) 4629–4637
Fig. 5. Branch emissions of acetone, a-pinene, and isoprene (ng C gdw1 h1) from Norway spruce in southern Sweden, May–June, 1997.
Table 4 Acetone, monoterpene, and isoprene emission rates (ngC gdw1 h1) observed in the branch emission measurements made on Norway spruce at Asa, southern Sweden, May–June, 1997. In the 4th–6th columns are given the b coefficient (Eq. (1)) and the correlation coefficient for the temperature dependences, as well as standardized emission rates (E(208C)) VOC
(n)
Temp. (8C) min–max, median
Emission range (ng C gdw1 h1)
b
R2
E(208C)
Acetone Monoterpenes Isoprene
(21) (25) (23)
4–25, 13 4–25, 13 4–25, 13
520–290 9–324 0–151
0.0654 0.0975
0.3874 0.4173
138 90 108
and visible needle growth was not observed until the last day of the measurements. Emissions were low for all the BVOC measured. As can be seen in Table 4, the correlation between acetone and temperature was not good. Neither could the isoprene algorithm (Eq. (3)), which includes PAR, explain the variations in acetone emission. However, the data are few, the values low, many below the detection limit, and thus more data are needed. The isoprene emissions were also lower than usual. The standard emission factor (308C) established by Eq. (3) was only 400 200 ng C gdw1 h1 as compared to the 1200 250 ng C gdw1 h1 observed at other Swedish sites and the values reported by Steinbrecher and Rabong (1994) for Norway spruce in Germany.
acetone from 5 specimens of boreal Scots pine are 870 480 ng C gdw1 h1 and 0.12, respectively. New monoterpene emission data for five specimens of Scots pine yield an average standard emission rate (308C) and b-coefficient of 900 640 ng C gdw1 h1 and 0.12, respectively. Acetone emissions from Norway spruce were found to be as high as those for the monoterpenes and isoprene on a carbon mass basis. The standard emission rate (308C) was found to be 265 ng C gdw1 h1, but the sparsity of data, along with the unusual weather conditions, precludes the establishment of a summertime best estimate emission rate.
Acknowledgements 4. Conclusions Acetone has been found to be a significant emission of both Scots pine (Pinus sylvestris) and Norway spruce (Picea abies). It makes up about 64% (on a carbon basis) of the light carbonyl emission from Scots pine and perhaps as much as 80–90% from Norway spruce, the other significant carbonyl compounds being formaldehyde and acetaldehyde. The diurnal pattern and correlation to temperature of the emission is similar to that of the monoterpenes. The average standard emission rate (308C) and b-coefficient for
We thank the staffs of the Mekrij.arvi Research Station and the SMEAR II station and Hyyt.ıa. l.a Research Station in Finland, the staff of the Asa Research Station in Sweden, Carlein Mak#a of the University of Utrecht, Peter Tunved of this institute for their help in the field. The HPLC analysis of the carbonyl compounds was done by Vaclav Vesely and the GC-FID and GC-MS analyses of the hydrocarbons by . Maria Ohrn, Katri Puhto and Dr. Ulla Wideqvist, all from this institute. We thank the Stockholm Environmental Agency for lending us the CO2 instrument. The projects (ENV4-CT95-0022, BIPHOREP and
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ENV4-CT97-0405, BIOFOR) were financed by the Environment and Climate Research Programme of the European Commision.
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