Environ. Sci. Technol. 2000, 34, 2498-2500

Absorption of Ozone and Seven
Organic Pollutants by Populus
nigra and Camellia sasanqua
K E N J I O M A S A , * ,† K A Z U O T O B E , ‡MASAAKI HOSOMI,§ AND
MIZUHO KOBAYASHI§
Department of Biological and Environmental Engineering,
Graduate School of Agricultural and Life Sciences, The
University of Tokyo, Yayoi 1-1-1, Bunkyo, Tokyo 113-8657,
Japan, The National Institute for Environmental Studies, 16-2
Onogawa, Tsukuba, Ibaraki 305-0053, Japan, and Faculty ofEngineering, Tokyo University of Agriculture and Technology,
2-24-16 Nakamach, Koganei, Tokyo 184-0012, Japan

Foliar absorption of seven organic pollutants (acetone,acetonitrile, acrolein, methyl ethyl ketone (MEK), isobutyl
methyl ketone (IBMK), chloroform, and benzene), and ozone
was examined. Two woody species (Populus nigra andCamellia sasanqua ) were exposed to each pollutant at aconcentration of 0.5 or 1.0 ppmv (µmol mol-1), and gas
absorption and transpiration rates were measured
simultaneously. Ozone and acrolein were effectivelyabsorbed by both species. MEK was absorbed by C.
sasanqua only. A model analysis of gas exchange rates
revealed that foliar absorption of the three pollutants was
predominantly through the stomata, and cuticular
contribution on gas removal is, if any, very small. Theplant leaves showed no detectable absorption of the otherfive organic pollutants. We conclude that plant leaves
act as an effective sink for some organic pollutants but
not for others. The factor that determines whether plantleaves can effectively absorb pollutants in this study seems
to be whether the pollutant is effectively metabolized inthe leaf cells.

Introduction
Atmospheric pollutants such as SO2, NO2, and O3 areeffectively absorbed by plant leaves through the stomata (16). The stomatal absorption of these air pollutants proceedsin the following sequence (1, 6): (i) gas enters the leaf by
gas-phase diffusion through the stomata; (ii) gas in the air
space inside the leaf dissolves in the water on the surface ofthe plant cells; (iii) gas in the liquid-phase diffuses into thecells according to its concentration gradient; (iv) gas ismetabolized or decomposed inside the cells, thus maintaining
a concentration gradient of gas between the atmosphere andthe interior of the plant cells. Recently, the importance of O3
decomposition in the cuticle on the leaf surface in addition
to the stomatal absorption have been reported by someresearchers (7, 8). However, knowledge on the capacity andmechanism of cuticular ozone decomposition is lacking.
Among the organic pollutants, some reports indicate thatformaldehyde (9-11) and phenol (12) are continuously* Corresponding author phone: +81-3-5841-5340; fax:+81-3-58418175; e-mail: aomasa@mail.ecc.u.-tokyo.ac.jp.
† The University of Tokyo.
‡ The National Institute for Environmental Studies.
§ Tokyo University of Agriculture and Technology.
2498 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 12, 2000

absorbed by leaves through the stomata at high rates. Thesestudies suggested that the metabolism of the absorbed
pollutants maintains the concentration gradient between theatmosphere and the interior of the leaves and acts as thedriving force that allows their continuous stomatal absorption. However, some organic pollutants absorbed by leaves
remain inside them without being decomposed (13-16). In
this case, the pollutants in plant leaves would eventually
become equilibrated with those in the ambient air, andconsequently the capacity of plants to remove these pollutants from the atmosphere would be very limited.
In the present paper we have reported foliar absorbencyof seven notable atmospheric organic pollutants (acetone,acetonitrile, acrolein, methyl ethyl ketone (MEK), isobutyl
methyl ketone (IBMK), chloroform, and benzene), of which
the absorbency by plants and the absorption mechanisms
are not yet understood clearly. The data were analyzed witha simple gas-diffusion model to investigate the foliar gas
absorption mechanism in comparison with transpirationfrom stomata. Foliar absorption of O3 was examined tocompare with that of the organic pollutants. In addition, thecapacity of cuticular contribution such as ozone decomposition was estimated through the analysis using the simple
gas-diffusion model.

Experimental Section
Seedlings of Populus nigra (deciduous tree) and Camellia
sasanqua (evergreen tree) of heights 0.4-0.7 m were used asplant materials. These seedlings were grown under ca. 25 °C
air temperature and nonlimiting soil moisture conditions in
a controlled greenhouse.
The gas exposure system used in this study is essentiallythe same as previously reported (10). Briefly, a potted plant
was confined in a transparent acrylic box (45 cm × 40 cm
in sides of the bottom × 80 cm in height), which was ventilatedwith air containing a pollutant. Air inside the acrylic box wasmixed by two fans set on the bottom of the box. The pot wascovered with a plastic film to avoid gas or water vapor transfer
between soil/pot and the air inside the acrylic box. To
measure transpiration rate, the potted plant was put on anelectric balance installed in the acrylic box; transpiration
rate was evaluated from the rate of decrease in the weightof the pot. The pollutant gas concentration at the air inlet
made in a side near the bottom of the acrylic box wasmaintained at about 1 ppmv (organic pollutants) or 0.5 ppmv
(O3). Gas concentration of the pollutant at the inlet and theoutlet made in a side near the top of the acrylic box wasmeasured with a flame ionization detection gas chromatograph (organic pollutants) or a UV-absorption O3 analyzer(O3). Air temperature and relative humidity were kept at ca.
25 °C and 65% during the experiments. Light intensity wasmaintained at 500 µmol photons m-2 s-1 except an additional
environment, for obtaining Figure 1c, of MEK exposure of C.
sasanqua.
Foliar gas-phase conductance of water vapor (gW; mmolm-2 s-1) was calculated from transpiration rate, leaf temperature, humidity, and air temperature in the acrylic box.
Foliar gas absorption rate on unit leaf area basis (Q) wasdetermined from the difference in gas concentrations at theinlet and outlet of the box. There were no detectabledifferences in gas concentrations at the inlet and outlet ofthe empty box. Molar fraction of pollutant gas on the leafsurface boundary layer (co) was assumed equal to the molar
fraction of pollutant gas at the outlet of the box, and Q was
normalized by dividing it by co to obtain q (mmol m-2 s-1)
(i.e., q ) Q/co). The data were analyzed by a simple gas10.1021/es991285m CCC: $19.00

 2000 American Chemical Society
Published on Web 05/04/2000

TABLE 1. Total Foliar Gas Sorption in 4 h (from 1 to 5 h afterthe Gas Exposure) by Unit Leaf Area Surface

pollutantO3acetoneacetonitrile
MEK
acrolein
IBMK
chloroform
benzenecumulative foliar gas uptake(mol m-1)
solubility in watera-1(mol kg )Populus nigra Camellia sasanqua
0.012
∞
∞
4.95
4.63
0.017
0.00690.0023

1.29
ndb
ndb
ndb
1.05
ndb
ndb
ndb

0.77
ndb
ndb
0.05
0.15
ndb
ndb
ndba From refs 6 and 17 and other literatures (at either 20, 22, or 25 °C).
b nd indicates that gas sorption was not detected by this method andis less than 0.01 mmol m-1.

FIGURE 1. Relationships between gw and q in Populus nigra (O)and Camellia sasanqua (×) when plants were exposed to 0.5 ppmv
O3 (a) or 1 ppmv acrolein (b) or MEK (c). Regression lines are shownby solid lines, and lines q ) kbgW and q ) ksgW are indicated bybroken and dotted lines, respectively. Equations of the regression
lines are as follows: O3, y ) 0.746x + 0.035 (r ) 0.981); acrolein,y ) 0.312x + 1.479 (r ) 0.997); and MEK, y ) 0.134x - 0.478 (r )0.938). Note that the scales of the axes in c differ from those in aand b.
diffusion model previously published in other literatures (2,3, 11, 12). Based on this model, q is related to gW by thefollowing equationq ) k(1 - ci /co)gW + R/co(1)

where ci and R are the molar fraction of the pollutant gas in
the air space inside a leaf and nonstomatal pollutant-gas
sorption/decomposition rate on unit leaf area basis (mmolm-2 s-1), respectively, and k is a constant, of which the range
isks < k < kb(2)where ks and kb are constants calculated from molecularweights of water and the pollutant gas. For analysis of theexperimental data, q was plotted against gW, and R and ci/cowere evaluated from the y-intercept and slope of theregression line of this plot.

Results and Discussion
Acrolein and O3 showed considerable absorption by P. nigraand relatively lower absorption by C. sasanqua; absorptionof MEK was detected only in C. sasanqua (Table 1). Absorption
by plants of any of the other five pollutants was not detected(Table 1). Both plant species exposed to acrolein or O3 showedmarked decreases in both q and gW, indicating that exposure
to either pollutant caused stomatal closure. MEK also caused
decreases in both q and gW in C. sasanqua but to a lesser
extent than did acrolein or O3. No detectable decreases in gW
resulted from exposure of P. nigra to MEK or from exposureof either species to acetone, acetonitrile, IBMK, chloroform,or benzene.
Changes in stomatal opening are reported for leaves
exposed to SO2 and O3 (2, 3, 18, 19). The concentration ofpollutants to which the plants were exposed in this study
was considerably higher (0.5 or 1 ppmv) than those in theambient air. If plants were exposed to lower concentrationsof O3, acrolein, or MEK, the effects on stomatal opening would
be less, thus a high gas absorption rate would be maintained
for longer periods.
From the above measurements, q at various intervals wasplotted against gW for O3 and acrolein (Figure 1a,b). The data
show that the maximum gW obtained under a constant lightintensity (500 µmol photons m-2 s-1 ) was considerably
smaller in C. sasanqua than in P. nigra. For MEK, gW in P.
nigra kept at ca. 400 mmol m-2 s-1 during the exposure under
the light intensity but q was not detected, and the variationof q and gW in C. sasanqua was too small to permit
investigation of the relationship between q and gW. Thus, anadditional experiment for MEK with C. sasanqua was carriedout, in which light intensity was changed from 0 to 500 µmol
photons m-2 s-1 to vary the stomatal opening, while the plant
was exposed to MEK (Figure 1c).
In all the data (Figure 1a-c), q was found to increaselinearly with increasing gW, and the y-intercept was almostzero. The results indicate that absorption of all thesepollutants by plants was predominantly through the stomata,and cuticular contribution on gas removal such as ozone
decomposition is, if any, very small. For O3, the slope of theregression line almost equaled the kb value (Figure 1a). Fromeqs 1 and 2, it would be reasonable to interpret ci as being
close to zero for O3. For acrolein and MEK, the slope of theregression line was considerably smaller than ks. The rangeof ci/co calculated from eqs 1 and 2 is 0.45 < ci /co < 0.54 foracrolein and 0.73 < ci /co < 0.79 for MEK.
In other reports (2-5, 11, 12), almost the same model wasapplied for the analysis of foliar absorption of SO2, NO2, O3,formaldehyde, and phenol, and it was concluded that foliar
absorption of these pollutants is predominantly through thestomata. For SO2, NO2, O3, and formaldehyde, these reports
state that ci Z 0 even at a pollutant concentration of 1 ppmv.
Compared with these results, ci/co of acrolein and MEK islarger, indicating that the capacity to remove acrolein and
MEK from the interior of leaves is lower than is the case for
SO2, NO2, O3, or formaldehyde.
Among the eight tested pollutants, no conspicuousrelationship was found between solubility of a pollutant inwater and its foliar absorbency (Table 1). Although thesolubility of O3 in water is relatively low, O3 showed thesmallest ci value among the eight pollutants, suggesting that
O3 is most quickly scavenged inside the leaves. The ci/co values
VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2499

calculated for acrolein and MEK are both considerably larger
than that estimated for O3, although their solubility in wateris 2 orders of magnitude higher than that of O3. Furthermore,
foliar absorption was not detected for acetone or acetonitrile,
each of which is highly soluble in water (Table 1). Theseresults show that solubility in water is not a critical factor in
determining the foliar absorbency of a pollutant. The factor
that determines whether plant leaves can effectively absorb
pollutants in this study seems to be whether the pollutant
is effectively metabolized in the leaf cells.

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Received for review November 16, 1999. Revised manuscript
received March 20, 2000. Accepted March 21, 2000.
ES991285M