Airsurface exchange of peroxyacetyl nitrate at a ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, D10310, doi:10.1029/2004JD004533, 2004

Air-surface exchange of peroxyacetyl nitrate at a grassland site Paul V. Doskey,1 V. Rao Kotamarthi,1 Yoshiko Fukui,2 David R. Cook,1 Fred W. Breitbeil III,3 and Marvin L. Wesely1,4 Received 12 January 2004; revised 21 March 2004; accepted 12 April 2004; published 29 May 2004.

[1] Direct measurements of the dry deposition velocity of peroxyacetyl nitrate (PAN)

were made during the daytime between the months of July and October above a grassland surface in northern Illinois by a modified Bowen ratio technique. Differences in the air temperature, water vapor content, and PAN concentration were measured between the heights of 3.0 m and 0.92 m. Although the measurement uncertainties were large, the cumulative data indicate a slight downward flux of PAN, with an average and standard error of 0.13 ± 0.13 cm s1 for the dry deposition velocity. Theoretical calculations showed that thermochemical decomposition of PAN on leaf and soil surfaces heated to temperatures above the ambient air levels would contribute less than 15% of the total PAN flux at the elevations of the PAN measurements. A theoretical evaluation of the transfer of PAN through leaf stomata and the plant cuticular membrane indicated that uptake of PAN by vegetation during the daytime is controlled by transfer through the leaf stomata rather than the cuticular membrane. The stomatal resistance for PAN is greater by a factor of 1.6 than the value for O3. The mesophyll resistance for O3 is also expected to be less than the value for PAN, because O3 has more reaction sites within plant cells and reacts faster than PAN with protein thiols of the cell membranes. Measurements from other studies indicate that the dry deposition velocity for PAN above a vegetated surface during the daytime is lower by a factor of 0.5–0.3 than for O3. Our measurements of the PAN deposition velocity agree with the results of previous studies and with theoretical calculations based on the physicochemical properties of PAN and the grassland surface. These measurements imply that removal of PAN from the daytime atmospheric boundary layer by thermochemical decomposition is more rapid than dry INDEX TERMS: 0315 Atmospheric Composition and Structure: deposition to a grassland surface. Biosphere/atmosphere interactions; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; KEYWORDS: PAN, air-surface exchange, dry deposition Citation: Doskey, P. V., V. R. Kotamarthi, Y. Fukui, D. R. Cook, F. W. Breitbeil III, and M. L. Wesely (2004), Air-surface exchange of peroxyacetyl nitrate at a grassland site, J. Geophys. Res., 109, D10310, doi:10.1029/2004JD004533.

1. Introduction [2] Peroxyacetyl nitrate or PAN (CH3C(O)OONO2) is formed in the lower troposphere via photochemical reactions involving nitrogen oxides (NOx) and nonmethane organic compounds (NMOCs). PAN, an important reservoir of reactive nitrogen, exists in equilibrium with NO2 and is greatly affected by small changes in temperature [Singh, 1987]. When this photochemical oxidant enters the relatively cool upper troposphere, its stability contributes to its long1 Environmental Research Division, Argonne National Laboratory, Argonne, Illinois, USA. 2 MDL Information Systems, Inc., San Leandro, California, USA. 3 Retired 1996, Chemistry Department, DePaul University, Chicago, Illinois. 4 Deceased, January 20, 2003.

Copyright 2004 by the American Geophysical Union. 0148-0227/04/2004JD004533

range transport. In remote regions PAN can return to the warmer lower troposphere, where it degrades thermally (releasing NO2) and contributes to the formation of O3. [3] Thermochemical decomposition is one of the most important removal mechanisms for PAN, while oxidation by hydroxyl radical (OH) and photolysis are relatively slow [Singh, 1987; Talukdar et al., 1995]. The rates of photolysis and oxidation of PAN by OH in the temperature range 280– 300K are smaller by 3 orders of magnitude than the thermal dissociation rate. Other removal mechanisms include air-surface exchange by wet and dry deposition. The magnitude of PAN’s Henry’s law constant [Kames et al., 1991] would eliminate wet deposition as an important removal mechanism, and the few measurements that have been made to derive it’s dry deposition velocity [Hill, 1971; Garland and Penkett, 1976; Shepson et al., 1992; Schrimpf et al., 1996] indicate that dry deposition might also be slow relative to thermochemical decomposition during the daytime.

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[4] Estimates of the dry deposition velocity of PAN are 0.2 –0.8 cm s1. Measurements of the uptake of PAN by alfalfa in growth chambers indicated that the dry deposition velocity was 0.75 cm s1 [Hill, 1971]. Garland and Penkett [1976] measured a dry deposition velocity of 0.25 cm s1 for PAN to grass and soil in a return-flow wind tunnel. Dollard et al. [1990] measured deposition velocities of 0.09 cm s1 and 0.23 cm s1 to moorland grass (in acidic soil) and bare, wet calcareous soil, respectively, with a flowthrough chamber technique. Shepson et al. [1992] analyzed trends of PAN and O3 concentrations in the stable nocturnal boundary layer over a mixed deciduous and coniferous forest at night, when leaf stomata were closed, and concluded that the deposition velocity for PAN was at least 0.5 cm s1. Schrimpf et al. [1996] determined an average dry deposition velocity of 0.54 cm s1 for PAN in a corn field at night by measuring PAN and 222Rn concentration gradients. Sparks et al. [2003] measured the uptake of PAN by eight species of plants in a leaf chamber. The maximum PAN uptake rates at a mixing ratio of 250 pptv implied that the deposition velocities were 0.20 –0.54 cm s1. The deposition velocity of PAN to a vegetative surface during the day, when leaf stomata are open, is expected to be higher than the values determined at night, because the stomata offer another pathway for the surface uptake of PAN. However, no direct measurements of the deposition velocity of PAN during the day have been made to confirm this hypothesis. [5] We conducted our investigation to make direct measurements of the dry deposition velocity of PAN to a grassland surface during the daytime and to answer the following questions: (1) Does thermochemical decomposition of PAN alter gradients in concentration near a vegetated surface? (2) Is uptake of PAN by the plant cuticular membrane an important pathway for removing PAN from the atmosphere?

2. Experimental Aspects [6] The experiments were performed at the Argonne meteorological research site (41420N, 88000W), an area occupying 0.1 km2 in the southwest corner of Argonne National Laboratory, Argonne, Illinois [Doskey et al., 2000]. The site, formerly part of a prairie restoration area, is covered with a mixture of herbaceous vegetation that includes big and little bluestem (Andropogon gerardii and Schizachyrium scoparium, respectively), Indian grass (Sorghastrum nutans), blue grass (Poa spp.), quack grass (Agrostis alba), meadow fescue (Festuca elatior), crown vetch (genus Vicia), tall goldenrod (Solidago altissima), horse-nettle (Solanum carolinese), and wild carrot (Daucus carota). The average height of this vegetation was 15 cm above the soil surface. [7] Simultaneous measurements of air temperature, water vapor content, and chemical species concentrations at two heights above the surface are required to derive the deposition velocity of a trace gas by the modified Bowen ratio technique. An energy balance Bowen ratio system was used to measure gradients of air temperature and water vapor content in concert with routine measurements of wind speed, wind direction, net radiation, soil temperature, soil moisture, and soil heat flux [Fritschen and Simpson, 1989]. Temperature and relative humidity (RH) were measured at

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two heights with T/RH probes that contained a platinum resistance temperature detector (PRTD) and a capacitive RH element. Water vapor content was calculated from the probe measurements. Aspirated shields, located at heights of 0.92 m and 3.0 m above the surface, contained the T/RH probes and were exchanged automatically every 15 min with a motorized belt system. This exchange allowed the effect of sensor offsets to be minimized in the analysis. All of the instruments except the soil heat flux sensors were calibrated before and after the investigation. The soil heat flux was measured with a set of 3, heat flux plates 10 cm below the surface. Heat flux plate measurements were adjusted for soil heat storage above the plates by using the change of soil temperature with time as determined from a set of 3 soil temperature sensors, each of which integrated the temperature from the 10-cm depth to the surface. Latent heat flux (LE) and sensible heat flux (H) were calculated with the Bowen ratio equations. [8] Observations and calculations were made of several important micrometeorological parameters. Solar irradiances and net radiation were measured with a nearby pyranometer and net radiometer, respectively. Friction velocities (u*) were estimated via a bulk aerodynamic technique involving iterative calculations based on the locally measured mean wind speed and stability adjustments made with the Bowen ratio station’s estimates of H and LE. Estimates of the local aerodynamic surface temperature were extrapolated from air temperatures by using flux-gradient relationships [Brutsaert, 1982]. [9] Ambient air samples for PAN analysis were collected simultaneously over 30-min intervals at 0.92 and 3.0 m above the surface within 1 m of the aspirated shields of the Bowen ratio system. The samples were collected in Tedlar bags (778 cm2, 0.010 cm thick, PMC Corporation, Oak Park, IL) by an evacuated-chamber technique [Fukui and Doskey, 1998a]. They were immediately stored at 25C in a field laboratory after each sampling interval to minimize losses by thermal decomposition. Comparisons of samples analyzed immediately after collection with samples stored for 15 min, which was the maximum length of time between collection and analysis, indicated that wall losses of PAN were negligible. The samples were analyzed by a cryogenic preconcentration/high-resolution gas chromatographic technique [Fukui and Doskey, 1998a]. Briefly, a 50-cm3 ambient air sample was cryogenically concentrated in a 15-cm section of a glass-lined, U-shaped stainless steel tube (0.180 cm i.d.) packed with 60/80-mesh fused-silica beads (Alltech Associates, Inc., Deerfield, IL) that was immersed in liquid argon (180C). The analytes were desorbed by immersing the cryogenic trap in water (25C) and were then cryofocused in a 18-cm section of 0.53-mm (i.d.) Silcosteel1 tubing (Restek Corp., Bellefonte, PA) that was immersed in liquid argon (180C). The analytes were desorbed by immersing the tubing in water (25C) and then were transferred to a 30-m 0.32-mm-i.d. fused-silica capillary column coated with a 1-mm film thickness of cross-linked polydimethylsiloxane (DB-1; J&W Scientific, Folsom, CA). The analytic column was mounted in a Hewlett-Packard 5890 Series II high-resolution gas chromatograph (HRGC) with electron capture detector. A mixture of peroxyacyl nitrates, alkyl nitrates, and halocarbons was resolved in 12.5 min, allowing one pair of samples to be

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analyzed while another pair was collected. The precision for the PAN measurements was about ±3.2% for PAN concentrations greater than 60 pptv. The PAN standards were prepared in tridecane according to the procedure of Gaffney et al. [1984]. A 2-mL aliquot of tridecane was transferred to a capillary diffusion tube, which was immersed in ice water, and a gaseous PAN standard was generated in a two-step dilution process [Fukui and Doskey, 1998a]. The concentration of PAN was verified with a chemiluminescence NO2 monitor (Monitor Labs, Inc., Englewood, CO). [10] The dry deposition rate (FPAN) of PAN was estimated with the modified Bowen ratio technique as follows: 1 FPAN ¼ H rCp

D½PAN

DT

ð1Þ

Here r is the air density, Cp is the specific heat capacity of air at constant pressure, T is the air temperature, and D signifies the difference between the lower and upper observations. The deposition velocity (vd) was found as vd ¼

FPAN ; ½PAN

ð2Þ

where the mean of the measured PAN concentration at a height of 3 m was used.

3. Results and Discussion [11] The flux of PAN to the canopy is influenced by surface uptake through leaf stomata and cuticles, thermochemical decomposition of PAN on leaf and soil surfaces, and production of PAN by photochemical oxidation of NMOCs emitted from the grassland surface in the presence of NOx. Uptake of PAN by vegetation would reduce concentrations of PAN near the surface, while photochemical oxidation of PAN precursors might increase concentrations of PAN near the surface relative to those measured at higher elevations. Removal of PAN near the surface might also be accelerated by thermochemical decomposition on dry leaf and soil surfaces, because the absoprtion of solar radiation can elevate surface temperatures to several degrees above ambient air values. Measurements of the dry deposition velocity of PAN are compared here to expectations based on the physicochemical properties of PAN and the surface. 3.1. Measurements of the Dry Deposition Velocity [12] Ambient PAN concentrations were typically less than 500 pptv during the field experiment. Concentrations were low in the morning, increased toward midday, and then decreased in the afternoon. Decreasing concentrations toward the evening are caused by thermal dissociation, a lower abundance of radical precursors, and air-surface exchange processes. The PAN concentration gradient is affected by atmospheric turbulence near the surface. Increasing wind velocities in the afternoon tend to decrease the PAN concentration gradient. A typical variation of the PAN concentration at two heights above the surface during the daytime is shown in Figure 1. Although the gradient of PAN was extremely small during most measurement periods, the cumulative data taken from July

Figure 1. Typical diurnal variation of the peroxyacetyl nitrate (PAN) concentrations at 3.0 and 0.92 m above the grassland surface on two consecutive days in September. through October exhibit more frequent occurrences of PAN concentration gradients with higher levels of PAN at 3 m than at 0.92 m, which is typical of a substance that is being deposited to the surface (Figure 2). However, during some measurement periods, particularly in the afternoon on sunny days, PAN concentrations near the surface were greater than those at 3 m. Fukui and Doskey [1998b] observed emissions of PAN precursors from the grassland surface at this site, which could promote the formation of PAN above the plant canopy and elevate concentrations near the surface. The largest PAN concentration gradients were observed on two consecutive sunny days in September (Figure 1), when the average dry deposition velocity and standard error of the mean (standard deviation divided by the square root of the number of data points) was 0.23 ± 0.14 cm s1. The average dry deposition velocity for all of the measurements was 0.13 ± 0.13 cm s1. [13] The uncertainties in the deposition velocities of PAN derived from our measurements are large. Deposition velocities were derived from meteorological data, the gradients of temperature and water vapor content, and the chemical species concentration. During nearly all of the measurement

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position of PAN at a heated surface can contribute to the gradient in PAN concentration above the canopy. We evaluated the effect of elevated surface temperatures on the PAN concentration gradient by using the theoretical approach described below. [15] The PAN flux (F(z)) within the atmospheric layer from the surface to the height of the measurement, z, includes components due to surface uptake (Fu) and thermochemical decomposition (Fc) and can be expressed as follows: F ð zÞ ¼ Fu þ Fc

ð3Þ

The following reactions can be used to evaluate Fc: ðO2 Þ

CH3 CHO þ OH ! CH3 C ðOÞOO þ H2 O k4 ¼ 1:6 1011 ð4Þ k5 ¼ 6 1012 CH3 C ðOÞOO þ NO2 $ CH3 C ðOÞOONO2

13; 330 k5 ¼ 1:12 1016 exp T

ð5Þ

Figure 2. Cumulative PAN gradient (Dz = 2.08 m) data taken during the daytime from 9 July to 19 October. periods, we observed very small gradients in the PAN concentration that were near the measurement precision of the technique. Most of the gradients were distributed within ranges of ±5% and ±10% (Figure 2). The uncertainty in the deposition velocity is derived from the combination of errors associated with measurements of temperature, water vapor content, and chemical species concentration. The uncertainty in the PAN concentration gradient can be estimated from the sum of the squares of the error ranges for the measured concentrations of PAN at each height. For a measurement precision of ±3.2%, the corresponding uncertainty in concentration difference measurements is ±4.5%. This value, when divided by the concentration difference and multiplied by the deposition velocity, yields the minimal uncertainty in deposition velocity. A spread in distribution of concentration differences within approximately ±5% (Figure 2) corresponds to a wide variation in possible deposition velocities. For the heights of the measurements and typical conditions encountered, use of micrometeorological relationships indicates that a deposition velocity of 1 cm s1 corresponds to a concentration difference of 1 – 10% of the mean and that the concentration difference in any given set of micrometeorological conditions is proportional to the deposition velocity. Hence, the uncertainty in the deposition velocity associated with the precision of the concentration varies from 450% to 45% for any given data collection period. The uncertainty in estimating H(rCp)1/DT in equation (2) is considerably smaller, typically ±10%. 3.2. Thermochemical Decomposition [14] Absorption of solar radiation by dry plant leaves and soil may increase their surface temperature to several degrees above ambient values. The thermochemical decom-

CH3 C ðOÞOONO2 ! CH3 ONO2 þ CO2 12; 525 12 k6 ¼ 2:1 10 exp T

ð6Þ

Here the rate constants for each reaction are in units of cm3 molecule1 s1 [Singh, 1987; Gaffney et al., 1989]. Reaction (5) is sensitive to temperature. Reaction (6) in which methyl nitrate is produced by the autodissociation of PAN is much slower than the reverse reaction (5), which represents the thermal decomposition of PAN. Thus reaction (6) is neglected in the analysis. When NOx levels are less than 200 pptv, peroxyacetyl radicals are oxidized by HO2 and RO2, essentially removing PAN from the atmosphere. Under high NOx conditions and where the levels of NO are much greater than those of NO2, the following reaction accelerates the removal of PAN [Parrish et al., 1993]: CH3 C ðOÞOO þ NO $ CH3 C ðOÞO þ NO2

k7 ¼ 1:4 1011 ð7Þ

here k7 has units of cm3 molecule1 s1 at 298K and 1 atm pressure [Finlayson-Pitts and Pitts, 2000]. Thus the thermal decomposition of PAN depends on the ratio [NO]/ [NO2]. At night the ratio can be quite small (e.g., 0.05) and suppress the thermal decomposition of PAN [Shepson et al., 1992]. However, during the daytime, large NO concentrations relative to NO2 concentrations may accelerate the thermal decomposition of PAN. Therefore to thoroughly evaluate the contribution of the thermal decomposition of PAN to the concentration gradient above the surface, the profiles of the NO2 and NO concentrations above the surface must be known.

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[16] Other reactions pertinent to the analysis are the photolysis of PAN and the oxidation of PAN by OH: PAN þ hu ! products jPAN ¼ 1:2 107 s1

[17] The change in concentration of PAN above the surface is then given by the following: d½PAN k4 k5 ½OH ½CH3 CHO ½PAN ¼ k7 ½NO dt tPAN k5 þ ½NO2

at a height of 1 –2 m, under which condition the air temperature at height z in the surface layer can be expressed as

ð8Þ

651 PAN þ OH ! products k9 ¼ 1:23 1012 exp ð9Þ T

T ¼ Ts

Zz Fc ¼ z0

Here tPAN is the lifetime of PAN, defined as follows: Here l =

=k7 þ½NO =½NO2

tPAN ¼

ðk5 þ k9 ½OH þ jPAN Þ½NO=½NO2 þ ðk5=k7 Þðk9 ½OH þ jPAN Þ ð11Þ

If we neglect the photolysis of PAN [reaction (8)] because it is so slow and ignore the loss of PAN via reaction with OH [reaction (9)], we can calculate the lifetime of PAN from the following: =k7 þ½NO =½NO2 k5 ½NO=½NO2

k5

tPAN ¼

ð12Þ

ð13Þ

At steady state, the change in flux with height due to thermochemical decomposition is then @Fz ½PAN : ¼ @z tPAN

ð14Þ

To include effects of thermochemical decomposition within the layer between z0 and z, where z0 is the aerodynamic roughness length, the following integral must be evaluated: Zz Fc ¼

½PAN ðzÞ tðzÞ

zo

Assuming Fcjzo = 0.0 and if

½NO

dz

½PAN ðzÞ dz 13; 330 2:806 1016 exp Ts l lnðz=zo Þ

ð18Þ

H rCp ku

½PAN z ¼

Fu lnðz=zo Þ: ku

ð19Þ

Substituting equation (19) into (18) and setting k = 0.4 and u* = 20 cm allows us to evaluate the ratio of the flux due to thermochemical decomposition to the surface uptake as follows: Fc ¼ 4:45 1014 Fu

Zz z0

2

3 13; 330 5dz lnð =zo Þ exp4 Ts l ln z=zo z

ð20Þ

Equation (20) was solved numerically with z0 = 10 cm and at z = 100 cm and 10 m. The ratio of Fc/Fu is below 20% for [NO]/[NO2] = 0.2 and below 40% for [NO]/[NO2] = 1.0 at 10 m. For z = 100 cm the flux ratio is less than 5% for both cases. [18] To evaluate a wider range of parameters, we also solved the coupled differential equations (13) and (14) by using an explicit centered finite difference scheme. For these calculations we explored the effects of different temperature profiles and parameterizations for surface layer properties. Steady state is not assumed, and equations (13) and (14) are rewritten and evaluated as follows [Gao et al., 1991]:

ð15Þ

=½NO2 = 0.2, then

13; 330 16 t ¼ 2:806 10 exp : T

ð17Þ

. * For the case in which thermochemical decomposition is not considered, the conserved concentration profile is given by

Here we assume a steady state level of CH3C(O)OO. The change in concentration of PAN at the height z is then given by the following: @C ð zÞ Fz ¼ þ thermochemical decomposition @z @t

H z : ln rCp ku zo *

here k is the von Karman constant (0.38), u* is the turbulence friction velocity, and Ts is the surface temperature. Substituting equation (17) into (16) and equation (16) into (15) allows us to evaluate the flux due to thermochemical decomposition as follows:

ð10Þ

k5

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@CPAN @Fz ½ PAN ¼ ¼ @t @z tPAN

ð21Þ

@F @CPAN F ¼ hwCPAN i @z @t tt þ RR

ð22Þ

hwCPAN i ¼ 1:75u2x ½1 16ðz=L Þ

ð23Þ

ffi tt ¼ 0:4*z= 1:75 u pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 116ðz=LÞ

ð24Þ

ð16Þ

To solve equation (15), we need to express the lifetime of PAN as a function of height by using the temperature profile. For simplicity here we consider nearly neutral atmospheric stability within the measured atmospheric layer 5 of 9

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temperatures lead to larger contributions of thermochemical decomposition to the PAN flux. At the measurement height of 1 m thermal decomposition contributes less than 10% of the total flux in all the cases.

Figure 3. Contribution of thermal decomposition and [NO]/[NO2] ratio to the flux of PAN at various heights above the surface.

T ð zÞ ¼ Ts þ

H z lnð =zo Þ exp Atop Btop ½exp Ax Bx rCp ku* ð25Þ Atop ¼ 0:598 þ 0:39 lnðztop=L Þ

ð26Þ

Ax ¼ 0:598 þ 0:39 lnðzx=L Þ

ð27Þ

3.3. Uptake by Vegetation [21] Taylor et al. [1961] observed that the uptake of PAN through leaf stomata is strongly affected by photosynthetic activity. Mudd [1975] suggested that proteins in leaves would be most susceptible to reactions with PAN when the leaves are illuminated and photosynthesis has been occurring. Because of the suspected high solubility of PAN in lipids [Gaffney et al., 1987], another pathway to proteins within the interior of leaves-in addition to the more direct route through the stomatal openings-could be across the waxy leaf cuticle. The plant cuticle is composed of the biopolymer cutin and waxlike lipids [Caldicott and Eglinton, 1973; Kolattukudy, 1980; Kolattukudy et al., 1981]. The cutin is a mixture of hydroxyalkanoic acid monomers while the lipids are a complex mixture of longchain (>C21) fatty acids, alcohols, esters, and alkanes. [22] We evaluated the uptake of PAN by the leaf cuticle through a theoretical approach. The cuticle permeability depends on the concentration of a substance in the cuticle and its mobility [Kerler and Scho¨nherr, 1988b; Riederer, 1990; Paterson et al., 1991; Welke et al., 1998]. The concentration of PAN in the cuticle can be estimated from partition coefficients, while its mobility can be derived from diffusion coefficients. Kerler and Scho¨ nherr [1988a] performed experiments in aqueous media with cuticular membranes isolated from several plant species to derive a relationship between the cuticle-water partition coefficient (KCMw) and the 1-octanol-water partition coefficient [Kow; Leo et al., 1971; Chiou et al., 1982]. For several different semivolatile organic chemicals with saturation vapor pressures (p) less than 125 Pa, they derived the following correlation: log KCMw ¼ 0:973 log Kow þ 0:045

ztop

Btop ¼ 0:09 lnð =L Þ

ð28Þ

Bx ¼ 0:09 lnðzx=LÞ

ð29Þ

Equations (21) and (22) are solved to steady state by using the explicit finite difference scheme. The parameters in equations (21), (22), (23) are as follows: k = 0.4, u* = 20 cm s1, H = 100 w m2, and rCp = 12. [19] In an iterative calculation, the surface resistance (rc) is set to 10 s cm1, which allows us to calculate the deposition velocity as r1 c . The boundary condition for the flux at z = 0 is defined as FPAN ¼ vd ½PAN x

ð30Þ

Here x is the layer closest to the surface. The initial mixing ratio of PAN is set to 1 ppb, and the mixing ratio is held constant at the upper boundary of the model. A log normal distribution is used to establish the vertical grid points [Gao et al., 1991]. [20] Figure 3 shows the effect of the [NO]/[NO2] ratio on the calculated fluxes. Larger [NO]/[NO2] ratios and higher

ð31Þ

Welke et al. [1998] evaluated the uptake of volatile organic compounds (p > 125 Pa) by plant cuticles in the laboratory and developed the following relationship between the cuticle-air partition coefficient and p: log KCMa ¼ 6:176 0:892 log p

ð32Þ

Physicochemical properties of PAN, tetrachloroethylene, 4-nitrophenol, and hexachlorobenzene are presented in Table 1. We have included organic chemicals other than PAN in the discussion of cuticle uptake, because some of their properties are similar to those of PAN. For example, the aqueous solubility (Cw) of 4-nitrophenol is similar to the value for PAN, the Henry’s law constant (H) of hexachlorobenzene is about one-third of the value for PAN, and the saturation vapor pressure of tetrachloroethylene is similar to the value for PAN. Partition coefficients for the cuticlewater and cuticle-air systems are presented in Table 2. Equations (31) and (32) were used to estimate cuticle partition coefficients for chemicals with saturation vapor pressures less than 125 Pa and greater than 125 Pa, respectively. It is evident that hexachlorobenzene has a

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Table 1. Physicochemical Properties Used in Discussion of Cuticle Uptake Chemical

Mm, g mol1

Vm, cm3 mol1a

p, Pab

Cw, mol m3b

H, Pa m3 mol1c

PAN Tetrachloroethylene 4-Nitrophenol Hexachlorobenzene

121.05 165.83 139.11 284.78

90.9 128.0 126.8 221.4

3070d 1880 0.0054 0.001

123e 1.28 106 0.00014

24.9f 1470 5.09 105 7.14

a

Molar volumes (Vm) estimated by the Le Bas method [Reid et al., 1987]. Saturation vapor pressure (p) and aqueous solubility (Cw) at 20C [Riederer, 1990; Mackay et al., 1993]. c Henry’s law constant estimated according to H = p C1 w . d Estimated according to ln p = (4586 T1) + 18.78, where p is in torr and T is in deg K [Kacmarek et al., 1978]. e 1 Aqueous solubility estimated according to Cw = p H . f Holdren et al. [1984], Lee [1984], and Kames et al. [1991]. b

stronger tendency than PAN, tetrachloroethylene, or 4-nitrophenol to partition into the cuticular membrane in a cuticle-water system. In the cuticle-air system, 4-nitrophenol and hexachlorobenzene have the strongest tendencies to partition into the leaf cuticle; partition coefficients for PAN and tetrachloroethylene are similar in magnitude. The Kow for PAN is very small compared to those for organic chemicals such as polychlorinated biphenyls, polynuclear aromatic compounds, and organochlorine pesticides, which have values in the range of 107 and have been demonstrated to accumulate in plant cuticles [Nash and Beall, 1970; Buckley, 1982; Bacci and Gaggi, 1985, 1986; Bacci et al., 1990; Simonich and Hites, 1994, 1995; McLachlan and Horstmann, 1998; Wania and McLachlan, 2001]. [23] Kerler and Scho¨nherr [1988b] investigated the permeability of plant cuticles further and developed the following relationship between the permeance coefficient (PCMw) and KCMw: log PCMw ¼ ð238 log KCMw ÞVm1 12:48

ð33Þ

Here PCMw has units of m s1, and Vm is the molar volume of the liquid chemical at the normal boiling point (cm3 mol1), which is derived by the Le Bas estimation method [Reid et al., 1987]. The permeance coefficient is essentially the cuticle conductance. The inverse of the permeance coefficient is the resistance of the chemical to transfer through the cuticle (cuticular resistance). Permeance coefficients for the cuticle-water and cuticle-air systems are presented in Table 3. Tetrachloroethylene and hexachlorobenzene have the highest conductances in the cuticle-

water system, while PAN and 4-nitrophenol have lower but similar conductances. In the cuticle-air system, 4-nitrophenol and hexaclorobenzene have the highest conductances. Comparing the cuticular resistances in the cuticle-air system (rCMa) with values of the stomatal resistance (rs) indicates that during the daytime the uptake of PAN and tetrachloroethylene by the leaf is controlled by transport through the stomata, and transfer of these two chemicals through the cuticular membrane may not be an important pathway, in agreement with the results obtained in this study. However, PAN is known to react rapidly with protein thiols [Mudd, 1982]. Thus chemical reactions beneath the cuticle could produce a steep fugacity gradient across the cuticular membrane and accelerate the transfer of PAN within the plant cuticle. This mechanism has not been evaluated. [24] Data from the very few studies that simultaneously measured the uptake of PAN and O3 by vegetation indicate that the deposition velocity of PAN during the daytime is 0.3– 0.5 of the velocity for O3 [Hill, 1971; Garland and Penkett, 1976]. A ratio of 0.5– 0.6 can be computed with the model of Wesely [1989], which considers all pathways to uptake except that afforded by solubility in plant lipids. A number of field measurements of O3 deposition indicate that the midday O3 deposition velocity is 0.4 – 1.0 cm s1 for grasses, where the larger value is usually associated with removal at the ground surface when the soil and senescent plant material are partially exposed under a vigorously growing, short canopy. For the dry conditions that were

Table 3. Permeance Coefficients for the Cuticle-Water and Cuticle-Air Systems With Stomatal and Cuticular Resistances

Table 2. Partition Coefficients Used in Discussion of Cuticle Uptake Chemical PAN Tetrachloroethylene 4-Nitrophenol Hexachlorobenzene

log Kawa log Kow log Koab log KCMw log KCMa 1.99 0.220 7.68 2.53

1.50c 3.21c 1.92f 5.47f

3.49 3.43 9.60 8.00

1.50d 3.17d 1.91g 5.37g

3.49e 3.39e 9.59d 7.90d

a Dimensionless Henry’s law constant calculated according to Kaw = (H) (RT)1, where RT = 2.437 103 Pa m3 mol1 at 20C. b Calculated by using the relationship Koa = Kow K1 aw [Riederer, 1990; Paterson et al., 1991]. c Estimated by using the correlation log Kow = (0.862 log Cw) + 3.30 from Chiou et al. [1982]. d Calculated by using the relationship KCMw = KCMa Kaw [Riederer, 1990; Paterson et al., 1991]. e Calculated using equation (32) [Welke et al., 1998]. f From Kerler and Scho¨nherr [1988a]. g Calculated by using equation (31) [Kerler and Scho¨nherr, 1988a].

Chemical PAN Tetrachloroethylene 4-Nitrophenol Hexachlorobenzene

PCMw, cm s1a 1.75 1.62 7.96 1.23

PCMa, cm s1b

106 1.71 104 104 2.69 104 107 3.81 101 104 4.17 102

rs, s cm1c

rCMa, s cm1d

25.9 30.4 27.8 39.8

5.85 103 3.72 103 2.62 102 2.40 101

a Permeance coefficients in units of m s1 were estimated by using equation (33) [Kerler and Scho¨nherr, 1988b], which was derived from experiments with a cuticular membrane that was 2.5 mm thick. Values in the table were adjusted to a cuticular membrane thickness of 1 mm, which is representative of plant leaves, by multiplying the permeance coefficient by (2.5/1.0)2. b PCMa = PCMw K1 aw [Riederer, 1990]. c The stomatal resistance (rs) is calculated as the reciprocal of the stomatal 1 1 0.5 [Riederer, 1990]. Pw is a conductance (Ps ), where Ps Pw (Mw m Mm ) representative value for the water vapor conductance of leaf surfaces with open stomata, estimated as 1 103 m s1 [Nobel, 1983]. Mw m and Mm are the molecular weight of water and the compound, respectively. d 1 Cuticular resistance, calculated as PCMa.

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prevalent at the experimental site, a reasonable estimate of the O3 deposition velocity is 0.4 –0.6 cm s1. Our average value of 0.13 cm s1 for the deposition velocity of PAN was thus 0.22– 0.33 of the expected value for O3. [25] An evaluation of the roles of plant stomatal and mesophyll resistances for PAN and O3 indicates that the deposition velocity of PAN is less than that for O3. Gases are transferred through the leaf stomata into the substomatal cavity and the plant mesophyll cells beneath the cavity. A cell wall composed of cellulose encloses the cell membrane (plasmalemma) and the cell contents. Gas transfer proceeds from the cell wall inward to the cell contents through an aqueous environment. Stomatal resistances can be correlated with the molecular diffusivity of the gas in air, while the mesophyll resistance can be correlated with (1) the Henry’s law constant (essentially an air-water partition coefficient) and (2) the reactivity of the chemical species. The stomatal resistance for PAN is greater by a factor of 1.6 than the value for O3. Data from numerous field studies indicate that the mesophyll resistance for O3 is close to zero. The rapid removal is probably caused by rapid reaction of O3 with plant cells, rather than rapid transfer through the aqueous environment of the cell contents, because the magnitude of the Henry’s law constant indicates that O3 is sparingly soluble in water. The plasmalemma is composed of polyunsaturated hydrocarbons and is considered a likely site of oxidation by O3 [Hewitt and Terry, 1992]. Other potential reaction sites might include proteins on the exterior and interior surfaces of the plasmalemma [Mudd, 1982]. PAN reacts readily with protein thiols, but it does so at a rate slower than that for the reaction with O3 [Mudd, 1982]. Thus it is reasonable to assume that the mesophyll resistance for PAN is greater than that for O3, because (1) there are more reaction sites for O3 within the plant cell and (2) the reaction of O3 with proteins is more rapid than the reactions involving PAN.

4. Conclusions [26] A dry deposition velocity of 0.13 ± 0.13 cm s1 for PAN above a grassland surface during the daytime was directly determined by a modified Bowen ratio technique. A theoretical approach used to evaluate the contribution of thermochemical decomposition to the PAN flux as a function of height above the surface indicated that thermochemical decomposition contributed less than 15% of the PAN flux at the elevations of the PAN measurements. A theoretical evaluation of the transfer of PAN through leaf stomata and the plant cuticular membrane indicated that uptake of PAN by vegetation during the daytime is controlled by transfer through the leaf stomata rather than the cuticular membrane. The stomatal resistance for PAN is greater by a factor of 1.6 than the value for O3. The mesophyll resistance for O3 is also expected to be less than the value for PAN, because O3 has more reaction sites within plant cells and reacts with protein thiols of the cell membranes at a faster rate than does PAN. Consequently, if the transfer of PAN through the cuticular membrane was rapid, deposition velocities for PAN should be greater than those for O3; however, measurements from other studies indicate that the dry deposition velocity of PAN above a vegetated surface during the daytime is lower by a factor of 0.5– 0.3 than that

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of O3. Our measurements of the PAN deposition velocity agree with results of previous studies and with theoretical calculations based on the physicochemical properties of PAN and the grassland surface. Within a surface layer of the atmosphere of 50 m and at an ambient temperature of 25C and [NO]/[NO2] of 1, the lifetimes of PAN due to dry deposition and thermochemical decomposition would be 11 hr and 1 hr, respectively, indicating that removal of PAN from the daytime atmospheric boundary layer by thermochemical decomposition is more rapid than dry deposition to a grassland surface. [27] Acknowledgments. Fred W. Breitbeil, III, was a faculty participant from DePaul University, Chicago, IL, during the investigation. Karen Haugen’s assistance with editing is greatly appreciated. This work was supported by the U.S. Department of Energy under contract W-31-109-Eng38, as part of the Atmospheric Chemistry Program (ACP) of the Office of Science, Office of Biological and Environmental Research, Climate Change Research Division.

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F. W. Breitbeil III, 531 Cocoa Ln., Orlando, FL 32804, USA. D. R. Cook, P. V. Doskey, and V. R. Kotamarthi, Environmental Research Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA. ([email protected]) Y. Fukui, MDL Information Systems, Inc., 14600 Catalina Street, San Leandro, CA 94577, USA.

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