Ozone and sulfur dioxide dry deposition to forests:

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. D12, PAGES 15,365-15,377, JUNE 27, 2000 Ozne and sulfur dixide dry depsitin t frests: Observatins and mdel evaluatin Peter L. Finkelstein,, 2 Thmas G. Ellestad, 3 Jhn F. Clarke,, 2 Tilden P. Meyers, 4 Dnna B. Schwede,, 2 Eric O. Hebert? and Julie A. Neal, 2 Abstract. Fluxes and depsitin velcities f 03 and SO2 were measured ver bth a deciduus and a mixed cniferus-deciduus frest fr full grwing seasns. Fluxes and depsitin velcities f 03 were measured ver a cniferus frest fr a mnth. Mean depsitin velcities f 0.35 t 8 cm/s fr 03 and t 0.72 cm/s fr SO2 were bserve during the grwing seasns f 1997 and Weekly averages f 03 depsitin velcity ranged frm 0.25 cm/s at the beginning and end f the seasn t 5 cm/s in late June. SO2 had a smaller seasnal variatin, frm 0.75 t 1.5 cm/s between the beginning and peak f the seasn. Because 03 cncentratins are higher, the flux f 03 t frests is cnsiderably greater than the flux f SO2. Daytime depsitin velcities are very similar at each site, frm 0.75 t 0.79 cm/s fr 03, and frm 1.01 t 1.04 cm/s fr SO2. Diurnal cycles fr bth gases are discussed, as are the impact f sme weather events. The peak time fr 03 depsitin velcity is in midmrning, while it is near midday fr SO2. Surface wetness is usually assciated with a small increase in depsitin velcity, but fr sme rain events a majr increase was nted. Minimum depsitin velcities usually ccur at night and increase slwly in the predawn hurs befre light. Cmparisns are made between bservatins f depsitin velcity and predictins made with the Meyers multilayer depsitin velcity mdel. While the mdel is, n average, unbiased fr 03, it tends t underpredicthe higher depsitin velcity values. The mdel is slightly biased lw (underpredicts) fr SO2 depsitin velcity. The strengths f the mdel are nted, as are pprtunities fr imprvement. 1. Intrductin In additin t their use in lcal and reginal scale air quality mdels, depsitin velcity mdels are integral t the inferential dry depsitin mnitring netwrks that perate in the United States, Canada, and Eurpe [Clarke et al. 1997]. Because f the high cst and cmplexity f direct flux measurements, peratinal dry depsitin netwrks measure cncentratins f air pllutants and infer the flux f pllutants t the surface using a mdeled depsitin velcity (flux equal t depsitin velcity times cncentratin). In an earlier paper [Meyers et al., 1998] we reprted n the frmulatin f the Multilayer Mdel (MLM) fr depsitin velcity which is being used in peratinal dry depsitin netwrks in the United States and the bservatins frm three Cpyright 2000 by the American Gephysical Unin. Paper number 2000JD /00/2000JD $09.00 study, including instrumentatin, calibratin, and quality cntrl, were presented. This wrk reprts n three new field studies which measured fluxes f 03 and SO2 ver frests, and the evaluatin f the MLM with thse data. The studies were cnducted ver a pine plantatin in central Nrth Carlina during the spring f 1996, ver a deciduus frest in nrthwestern Pennsylvania during the grwing seasn f 1997, and ver a mixed frest in the Adirndack Muntain regin f New Yrk during the grwing seasn f These studies were part f a series f field and mdeling experiments undertaken t evaluate and imprve depsitin velcity mdels used in U.S. inferential dry dep- sitin netwrks by systematically bserving flux and depsitin velcity ver a range f vegetatin types and weather cnditins. The data are als being used t evaluate depsitin field studies which were used t evaluate and imprve the velcity mdels used in reginal scale air quality mdels [Pleim MLM. Thse studies were perfrmed ver agricultural fields f et al., 1997] and t develp new mdels fr special applicatins pasture, crn, and sybeans. Details f the mdel and field [Pleim et al., 1999]. There have been several experimental studies f the dep- Atmspheric Sciences Mdeling Divisin, Air Resurces Labra- sitin f 03 and SO2 t frests. Review papers n depsitin try, NOAA, Research Triangle Park, Nrth Carlina. velcity ver all surfaces, including frests, were prepared by 2On assignment t the U.S. Envirnmental Prtectin Agency, Re- McMahn and Denisn [1979] and Sehmel [1980]. Reprts n search Triangle Park, Nrth Carlina. field bservatins f 03 depsitin velcity t deciduus fr- 3Natinal Expsure Research Labratry, Envirnmental Prtectin Agency, Research Triangle Park, Nrth Carlina. ests include wrks by Meyers and Baldcchi [1988], Padr et al. 4Atmspheric Turbulence and Diffusin Divisin, Air Resurces [1991], Fuentes et al. [1992], Gergiadis et al. [1995], Munger et Labratry, NOAA, Oak Ridge, Tennessee. 5Envirnmental Sciences and Engineering Inc., Gainesville, Flrida. 15,365 al. [1996], and Waltn et al. [1997]. The length f the field activity in these studies varies frm a few days t a seasn r mre. They reprt maximum midday 03 depsitin velcities ver deciduus frests in the range f 0.5 t cm/s. O3 depsitin velcities t bare trees in the winter have been

2 15,366 FINKELSTEIN ET AL.: OZONE AND SULFUR DIOXIDE DRY DEPOSITION TO FORESTS Table 1. Heights f Sensrs Abve the Grund at the Three Sites cnducted ver a pine plantatin in the Duke University Experimental Frest in Nrth Carlina (Duke). Height, m 2.1. Kane Sand The Kane Experimental Frest is lcated adjacent t the Instrument Duke Kane Flats Allegheny Natinal Frest in nrthwestern Pennsylvania, (41.595øN, øW). The area is n the tp f a plateau, with Rain gage and radiatin sensrs Snic anemmeter, chemical analyzer gently rlling tpgraphy and changes in elevatin f 15 t input lines, temperature, wet bulb 30 m within 1500 m f the site. There is a slight rise due east temperature f the site. Suth f the site the terrain slpes dwn gradually Temperature t Wlf Run, a creek, then rises t a ridge at abut the same Wind speed and directin elevatin as the site apprximately 2 km away. The tree canpy Temperature Wind speed and directin is nearly unifrm in height and tends t damp ut variatins in Temperature grund elevatin. There were n majr pint surces r rads Surface wetness sensr within 30 km f the site. Minimal lgging activity ccurred Canpy height 14 _ i 22 _ _ 2 spradically abut 5 km frm the site. In the vicinity f the experiment there is a mix f tree species, 38% Black Cherry (Prunu sertina), 34% Red Maple (Acer rubrum), 23% Sugar Maple (Acer saccharinurn), and 5% reprted in the range f 0.2- cm/s by Wesely [1983] and Padr et al. [1992]. Observatins f SO2 depsitin velcity t deciduus frests have been mre limited. An early study by Petit et al. [1976] fund values f 1.8 t 3.7 cm/s fr tw shrt bservatinal thers, f which a fair part are Hemlcks (Tsuga canadensis) that tend t grw adjacent the stream beds. The canpy is 22 t 23 m high. The understry in the vicinity f the experiment is quite sparse. The majrity f the grund is cvered with ferns, the predminant types being Bracken (Pteridium aqperids. Meyers and Baldcchi [1988] bserved midsummer uilinim), Hay Scented (Dennstaedita punctilbula), and New daily maximums in the range f 0.5 t 1.1 cm/s, while Padr [1993] bserved depsitin velcities f 0.25 t 0.5 cm/s during the fall seasn. Observatins f depsitin velcity t cnifers have been made in several studies, including thse reprted by Garland and Bransn [1977], Rnd6n et al. [1993], Lamaud et al. [1994], Ce et al. [1995], Pilegaard et al. [1995], Munger et al. [1996], Yrk (Thelypteris nvebraensis), with a small fractin f Interrupted fern (Osmunda claytniana). There are small patches f grasses, such as Shrt Husk (Brachyelytrum erectum), and a few small trees, Striped Maple (Acerpennsylvanicum) and Cucumber (Magnlia acuminata). The frest in this area is secnd grwth but has nt been lgged in mre than 50 years. The sil is a rich lam and frest litter fr a depth f 10 t 20 cm, and Aurela et al. [1996]. Ozne depsitin velcities bserved verlying a layer f impervius hardpan. Because the area is in in these cniferus frest studies have a wide range f values with daily maximums f frm 0.25 t 2 cm/s. Mst were in the range f 0.5 t 1 cm/s. A few nighttime bservatins were made. They ranged frm 0.2 t 0.35 cm/s. Aurela et al. [1996] reprted nighttime values f 5 cm/s ver Scts Pine in nrthern Finland. Zeller [2000] has reprted small negative depsitin velcities (psitive upward fluxes) ver a snwcvered Spruce-Fir frest. Galbally et al. [1979] cnducted the first set f measurements f SO2 depsitin ver a cniferus frest in England. The needles were ld (i.e., the stmatal a slight depressin, and there are plentiful springs at higher elevatin, the sil is mist thrughut the year. The twer and instruments were installed during April 1997 and remved at the end f Octber. Valid data were cllected frm April 29 t Octber 23. Because f a late spring, leaf bud did nt ccur until apprximately the secnd week f May. Leaves were senescent n all the majr deciduus species by the middle f Octber. Thus the experiment gathered data thrughut the full grwing seasn at the site. Instruments were munted n a 36 m guyed, walk-up, scafresistance was high), and the system they used was less sensi- fld twer that was erected fr this experiment 200 m frm the tive t turbulent fluxes than later systems, but they were able t nearest rad. Data were transmitted by fiber-ptic cable t an measure depsitin velcities f 0.2 cm/s with an assumed ffice/labratry trailer near the rad. Instruments were errr f _+50%. Lrenz and Murphy [1985] measured SO 2 dep- munted n the suth face and suthwest crner f the twer sitin velcity ver lbllly pine and bserved average daily depsitin velcities f 0.72 _+ 5 cm/s. In sectin 2 f this paper the sites f the experiments are described, and sme details f instrument deplyment unique t the frest setting are given. Sectin 3 summarizes the data. The mdel predictins are cmpared with bservatins in sectin 4, and a summary and cnclusins are presented in sectin 5. at heights as nted in Table 1. Chemical analyzers, data prcessing cmputers, and calibratin equipment were hused in shelters built n the twer belw canpy height. Wind velcity and turbulence were measured with an Applied Technlgy Inc. (ATI) snic anemmeter. 03 and SO2 were sampled frm a draft tube whse pening was immediately adjacent t the snic. Fast respnse measurements f 03 were made with a specially cnstructed analyzer that uses the chemiluminescent 2. Site Descriptin and Field Installatin reactin f 03 with esin-y dye. Fast respnse SO2 measurements were taken with a mdified Mely SA285-E ttal sulfur Three studies f trace gas fluxes t frests were cnducted. analyzer. Details n the instrumentatin, data handling, and The first majr study, f a deciduus frest, was cnducted quality assurance are given by Meyers et al. [1998]. ver a full grwing seasn in the Kane Experimental Frest in Pennsylvania (Kane). The secnd study tk place in a mixed Sil temperature and sil heat flux were measured near the base f the twer and are used as part f an energy balance deciduus and cniferus stand in the Sand Flats State Frest quality cntrl check n the flux measurements. Leaf area in New Yrk State (Sand Flats). A shrt, 1 mnth, study was index (LAI) measurements were made at several lcatins

3 FINKELSTEIN ET AL.: OZONE AND SULFUR DIOXIDE DRY DEPOSITION TO FORESTS 15,367 thrughut the area, and leaf prmeter measurements were taken n several leaves within reach f the twer, near and at the tp f the canpy. These measurements were used in the MLM, discussed belw. Turbulence parameters u/u, and rrw/u, where u is the average wind speed in the mean wind directin, u, is the frictin velcity, and 0% is the standard deviatin f the vertical cmpnent f the wind speed were pltted as a functin f wind directin. These variables are gd indicatrs f undisturbed flw. There were n ntable directinal effects be- tween the angles f 70 ø thrugh suth t 260 ø. This was chsen as the "acceptance sectr" fr the turbulence and flux data. Only data frm this sectr are reprted Sand Flats The Sand Flats site is a mixed cniferus-deciduus frest. Observatins were taken frm May 12, 1998, t Octber 20, The bservatin site was in the Sand Flats State Frest, in Lewis Cunty, New Yrk; abut 7 miles NE f Bnville, New Yrk, and n the suthwest bundary f the Adirndack Park (43.565øN, øW). The frest is secnd grwth, mst naturally seeded, but there are sme areas f planted pines frm the Civilian Cnservatin Crps era f the late 1930s. Sme thinning has ccurred, as directed by the New Yrk State Frest Service. The area had been farmed at ne time. Cm- psitin f the frest in the sectr t the west f the twer, tward which the anemmeter was pinting, includes 20% White Pine (Pinus strbus), 20% Black Cherry (Prunus sertina), 17% Sugar Maple (Acer saccharinurn), 15% Hemlck (Tsuga canadensis), 10% White Spruce (Picea glauca), 8% pen space, and a scattering f Red Maple (Acer rubrum), Elm (Ulmus sp.), Balsam Fir (Abies balsamea), and Yellw Birch (Betulus lutea syn. allegheniensis). The sil thrughuthe area is sandy and well drained. The tpgraphy f the area is quite flat within 0.5 km f the twer. Ging farther ut, the terrain remains flat t the nrth, west, and suth. T the nrthwest, a small marsh, abut 100 m wide, is in a depressin that feeds int a lake farther t the west and nrthwest. T the suthwest is the head f a valley that runs suthwest and west, becming narrw and deep a few kilmeters frm the site. T the east, the terrain is flat fr 500 m, then slwly rises abut 30 m within the next 500 m. The nearest rads were 300 m east f the twer, and 800 m nrth. Traffic n bth were light. There were a few huses alng the rads, but they were nt a surce f significant emissins. State rute 12 was 7.4 km t the west and had mderate traffic. N lgging r ther activity that wuld generate emissins f reactive gases tk place in the vicinity f the site. There were n large pint surces within 30 km f the site. The twer was 36 m tall and similar t the ne used at Kane. The flux system was munted n the tp f the twer, with prfiles f temperature and wind taken between the tp f the twer and the tp f the canpy. The heights f the instruments are given in Table 1. The distributin f tree species near the twer was smewhat hetergenus, with an average height f 26 _+ 2 m. Because f the mix f species, the vertical structure f the frest was quite different frm that at Kane, with a much higher density f branches and needles at lwer levels n the cnifers mixing with the higher leaves f the deciduus trees. Plts f u/u, and rrw/u, shwed little directinal dependance within the acceptance sectr used fr this analysis f 185 ø t 330 ø Duke The Duke field prgram had different experimental gals and was f limited scpe cmpared t the thers field prgrams reprted here. There are sme data, hwever, that are useful fr this study. Ozne fluxes were measured during a 30 day perid in the spring f 1996 (April 15 t May 15). The site was the Blackwd Divisin f Duke Frest (35.97øN, 79.13øW). Measurements were made n a 21 m walk-up twer ver a unifrmly planted 14 m tall Lbllly pine (Pinus taeda) stand that was 1000 m lng in the nrth-suth directin, and 300 t 600 m wide in the east-west directin. Tree density is 3700/ha. Understry cnsists f small trees, mstly Red Maple (Acer rubrum) and White Oak (Quercus alba), that are naturally seeded. Measured LAI near the base f the twer was =3.1. The sil near the twer is an Enn silt lam. The terrain within 1-2 km f the twer is relatively flat. There are n nearby surces f pllutin that wuld impact the zne flux measurements. The twer was lcated in the suth- ern end f the stand, and near the center in the east-west directin. The snic anemmeter was riented t the nrth, s that winds frm west thrugh nrth t east wuld be free frm twer interference. Wind and turbulence measurements were made with a Slent snic anemmeter. SO2 and ancillary meterlgical measurements were nt taken in this study, which had ther bjectives. Therefre mdel evaluatin using data frm this site is nt pssible. 3. Observatins 3.1. Leaf Area Index and Respiratin T quantify the grwth and density f actively grwing leaves, mst mdels f depsitin velcity use the bserved LAI fr the canpy. Observatins f LAI and measures f vegetatin respiratin and grwth were made at the Kane and Sand Flats sites as nted belw in sectin Chasn et al. [1991] reprt that measurements f LAI made with cmmercially available sensrs which estimate LAI frm shrtwave radiatin measurements, as were used in this experiment, frequently underestimate LAI in frests due t the uneven distributin and clumping f the leaves. They suggest a crrectin factr f 1.89 based n destructive sampling calibratin. Fassnacht et al. [1994] suggested a crrectin factr in the range f 2, but pint ut that the data are bth scattered and sparse and that n crrectin factr can be used with great cnfidence. Because destructive sampling was nt an available ptin in the frests we were studying, we have adpted Chasn et al.'s suggested calibratin factr fr all midseasn LAI measurements reprted here and used in the mdeling analysis. The calibratin factr is tapered t 1 because the dumping is reduced in early and late seasns when the leaves are small r have fallen ff LAI and stmatal resistance at Kane. Observa- tins f LAI were made at selected statins surrunding the twer fllwing the prtcl nted by Meyers et al. [1998]. These calibrated bservatins are presented in Figure la. LAI has nt been crrected fr stem and trunk mass. The figure clearly shws the seasnal cycle f leaves in a deciduus frest, frm leaf ut starting near day f the year (DOY) 128 (May 8) t leaf fall at abut DOY 290 (Octber 17). Fr purpses f later discussin, we have selected the perid f active grwth and large LAI as DOY 148 (May 28) t 283 (Octber 10). Stmatal resistance t the diffusin f water vapr and trace

4 15,368 FINKELSTEIN ET AL.: OZONE AND SULFUR DIOXIDE DRY DEPOSITION TO FORESTS KANE ß ß A ß LAI NDVI '97 6/21/97 8/20/97 10/19/97 SAND FLATS DATE ß ß ß ß ß (a) (b) 5/5/98 7/4/98 9/2/98 11/1/98 DATE Figure 1. (a) Seasnal cycle f LAI and NDVI at the Kane Experimental Frest and (b) seasnal cycle f LAI at the Sand Flats State Frest. gases is a critical part f depsitin velcity mdels. Measurements f stmatal resistance were made n fur species f trees thrughut the grwing seasn using a Licr prmeter at midday. The resistances were quite lw (200 t 400 s/m) thrughut the summer, then increased in the fall t 2000 s/m and mre. The time f increase in the fall was speciesdependent. Measurements were made at several levels in the canpy. Usually, the readings near the tp f the canpy had the lwest resistance because the leaves were sunlit. The av- erage resistance measured fr each species during the summer were as fllws: Black Cherry, 320 s/m; Sugar Maple, 647 s/m; Red Maple, 407 s/m; and Ash, 252 s/m, but the scatter was quite high. These bservatins cmbine readings at the tp f the canpy with sme frm farther dwn. The values fr the maples were higher than nrmally quted because they were all made in a shaded area belw the tp f the canpy. Fr many areas it is impractical t measure LAI directly, yet knwledge f plant grwth timing is imprtant fr lcal and reginal scale mdeling f dry depsitin. Remte sensing methds have been frequently suggested as an alternative. One f these methds uses the Nrmalized Difference Vegetatin Index (NDVI), which is derivable frm the advanced very high reslutin radimeter (AVHRR) sensr n the Natinal Oceanic and Atmspheric Administratin (NOAA) meterlgical satellites [Justicet al., 1985]. T see hw they cmpared in terms f the annual grwth cycle f deciduus leaves, and in rder t evaluate NDVI measurements as an input t future mdeling, we extracted daily 4 km NDVI data frm the NASA Gddard's EOS Distributed Active Archive Center [Kidwell, 1991]. This file is develped with an algrithm that chses the highest NDVI value available fr a given pixel frm all f the individual images fr that day and 4 km grid. These values can range between zer and ne. A plt f each day's index value was made fr the 4 km pixel which included the Kane site. On many days the data were cntaminated with clud cver, s that the index values fr thse days were small. The pints making the uter envelpe f index values were chsen as being "gd" clear-sky readings. This admittedly subjective methd prduced the NDVI pints shwn in Figure la. The vertical scale has been set t match the maximum scale f the LAI, but n ther adjustments were made. As can be seen frm the figure, the NDVI values derived in this way made a quite satisfactry depictin f the seasnal cycle at this site, indicating that it wuld be a useful methd t define green-up in the spring and leaf senescence in the fall, and a useful surce f input data fr depsitin velcity mdels. 3.. LAI and stmatal resistance at Sand Flats. The distributin f tree species arund the sampling twer was nt unifrm in either cmpsitin r density. T see if this nnunifrmity had an effect n measured LAI, bservatins were taken alng radials centered n the twer. Radials f nrth, nrthwest, west, suthwest, and suth were used. Alng each radial, five statins were used as lcatins fr measurements, starting at 50 m frm the twer, and extending t 250 m. Readings frm each statin n a radial were averaged t give an "effective" LAI that might represent the fetch cnditins and ftprint fr winds cming frm that directin. The calibrated LAI readings, using the same calibratin as was used with the Kane data, averaged ver all radials, are shwn in Figure lb. At the time bservatin started in late spring the average LAI was abut 4.5. It increased t 6.5 during the summer. It began t decrease in early September and reached a minimum bserved value f 2 by the middle f Octber. At that time, site lgs indicate that all leaves were ff the deciduus trees, s the LAI is measuring the cnifers as well as trunks and branches f the hardwds. Variatin arund the mean value was cnsiderable, ranging frm abut 0.75 t 1.0 LAI unit. Differences in radials were quite cnsistent, with the nrth radial usually the lwest and the west r suthwest usually the highest. Fr depsitin velcity mdel runs the average radial LAI crrespnding t the apprpriate wind directin was used. Stmatal resistance was measured n several species f trees. Minimum resistance fr white pine and white spruce were bth apprximately 250 s/m. Readings fr sugar maple were between 200 and 250 s/m. These readings agree with the previus literature and are equivalent the mdel inputs used in the multilayer mdel [Meyers et al., 1998]. In the fall, when the stmatal resistance increased fr the deciduus leaves that were changing clr, the cnifers seemed unaffected by the change in seasn. Their stmatal resistance remained cnstant up t the end f the field prgram Depsitin Velcity and Flux Data The bservatins at each site are presented and discussed belw, fllwed by an intercmparisn and summary. Data were edited fr errrs, utliers, and ther unreasnable values as nted by Meyers et al. [1998].

5 FINKELSTEIN ET AL.: OZONE AND SULFUR DIOXIDE DRY DEPOSITION TO FORESTS 15, KANE (a) ß 03 4/30 5/28 6/25 7/23 8/20 9/17 10/15 WEEK 1.50 SAND FLATS 5 (b) KANE -e HOUR (a) -- SAND FLATS [[ O3 (b)..:.. s O.5O /13 6/3 6/24 7/15 8/5 8/26 9/16 10/7 WEEK Figure 2. Seasnal cycle f weekly average daytime depsitin velcity at (a) Kane fr 03 and SO2 and (b) Sand Flats fr 03. Errr bars are plus r minus ne standard errr HOUR DUKE (c) Ozne and sulfur dixide data frm the Kane site. Over the curse f the field experiment, and 1625 SO2 half-hur depsitin velcity bservatins were taken. The lwer number f SO2 bservatins is a cnsequence f the lw cncentratins, which were frequently belw the detectable limit f the fast analyzers (5 ppb fr 03 and 2 ppb fr SO2). Data were als rejected during the ccasinal shrt-term variatins in SO2 cncentratin caused by plumes frm remte industrial surces. In Figure 2 the seasnal distributin f depsitin velcity is given as weekly averages f daytime hurs fr all the weeks f the Kane and Sand Flats field prgrams. At Kane (Figure 2a) the seasnal signal, and variatins caused by changes in weather, are clearly apparent fr 03 depsitin velcity. Seasnal changes fr SO2 are apparent, but less striking. The upswing the last 2 weeks may be an artifact f a small sample size during thse weeks. There was n seasnal effect n nighttime depsitin velcity fr either gas. Average diurnal cycles f depsitin velcity fr bth 03 and SO2, taken when the trees were in full leaf, are given in Figure 3. The tw bservatins taken during each hur perid are averaged. 03 depsitin velcity bserved at the Kane site (Figure 3a) is very lw thrugh the night, starting t increase a bit in the hurs after midnight. This increase may be due t increased surface misture, which is mre frequent in the early mrning hurs. The stmata f sme plants are mst sensitive HOUR Figure 3. Average diurnal cycles f depsitin velcity at (a) Kane, (b) Sand Flats, and (c) Duke. t blue light and pen very early in the predawn hurs [Musselman and Minnick, 2000] which als wuld reduce the canpy resistance at these times. Depsitin velcity ges up rapidly in the hur beginning at 0600; peaking in the 0900 t 1000 hur. It then decreaseslwly t a minimum starting at This cycle, with a peak in midmrning, crrespnds t the nrmal time fr stmata t fully pen, s that the stmatal resistance is a minimum [Tan and Black, 1976; Davies, 1995; Duyzer et al., 1995; Kesselmeier and Bde, 1997]. The diurnal cycle f 03 mixing rati (Figure 4) culd nt explain the depsitin velcity cycle. The cncentratin cycle is typical f a rural site. It has a minimum near daybreak, peaks in early mrning, and re-

6 15,370 FINKELSTEIN ET AL.- OZONE AND SULFUR DIOXIDE DRY DEPOSITION TO FORESTS ,--MR Hur,-e-- DV '' :AN :... (b)' SO 2,.ß.ß -2[]'[]'[] x.. ' ß /.:\b "Z : ' : e'& ' :,,, l, " ' [ --A-- M DV Hur $ E Figure 4. Average diurnal cycles f cncentratin, depsitin velcity, and flux f (a) O 3 and (b) SO2 at Kane. Nte that the rdinates in Figures 4a and 4b are nt the same. 0.2 mains at that level well int the evening, when it starts t slwly decrease. SO2 depsitin velcity (Figure 3a) is at a minimum shrtly after dark, then increases gradually until dawn, when it ges up quickly t a peak between 1000 and It stays high during the afternn, then decreases t a minimum at the same time as the O3 minimum. Higher SO2 depsitin velcity during the early mrning hurs are prbably due t surface wetness increasing in the predawn hurs alng with early stmatal pening. The SO2 diurnal cycle clearly reflects atmspheric resistance, which is a minimum in the early afternn, as well as stmatal cntrl, as discussed belw. Variatins in depsitin velcity f trace gases are cntrlled predminantly by the mixing capability f the atmsphere, the rate f phtsynthetic activity f the vegetatin, and the amunt f vegetatin. Phtsynthetic activity is in turn regulated t a large degree by bth the diurnal and seasnal cycles, while LAI is cntrlled by the seasnal cycle. A third independent factr that frequently impacts depsitin vel )city is surface chemistry, which is influenced by surface wetness. T examine the impact f these factrs, the bserved depsitin velcity and flux are categrized by time, grwth rate, and wetness in Table 2. The table is read by nting that the majr gruping is n the left, with subsets f that grup t the right. The data distributins, regardless f categry, are skewed tward high values. The standard errrs, reprted in the table, tend t be lw (less than 10%), but because the individual readings are frequently aut-crrelated, this measure f scatter may be misleadingly lw. Over all cnditins the mean SO2 depsitin velcity was 0 cm/s, and the mean O 3 depsitin velcity 0.35 cm/s, r a rati f almst 2:1. Cmparing day and night perids, ne can see that the differences in depsitin velcity between the tw chemical species are much strnger at night than during the day. Alternatively, while neither 03 nr SO2 is taken up t a Table 2. Summary f Observatins at the Kane Experimental Frest O, DV O 3 Flux O 3 N SO2 DV SO2 Flux SO 2 N All data _ _ _ Day _ _ Night 9 _ _ _ _ High LAI _ _ _ Day _ _ _ Night 0.1 _ _ _ Lw LAI 0.14 _ _ _ Day 0.24 _ _ _ _ Night 8 _ _ _ _ Dry 1 _ _ _ _ Day 0.75 _ _ _ _ High LAI 4 _ _ _ Lw LAI 0.24 _ _ _ Night 7 _ _ _ High LAI 7 _ _ _ Lw LAI 7 +_ _ _ Wet 0.24 _ _ _ Day 0.73 _ _ _ _ High LAI 0.76 _ _ _ _ Lw LAI 0.17 _ _ _ Night _ _ _ High LAI 0.12 _ _ _ Lw LAI 9 _ _ _ Mean depsitin velcities (DV) (cm/s) and fluxes (ppb m/s) f O3 and SO2, with _+ 1 standard errr and number f bservatins N, under specified cnditins. Day is defined as all hurs ->0900 and -<1500. Sampling perids are dented frm the beginning time. Fr each hur the tw half-hur samples are gruped. Therefre 1500 includes the half-hur samples frm 1500 and Night is defined as all hurs ->2000 and -<0400."High LAI" and "Lw LAI" perids refer t the develpment and mass f leaves n the trees. The High LAI perid is defined as all days between DOY 148 (May 28, 1997) and 283 (Octber 10, 1997). All ther days befre and after the perid are defined as Lw LAI.

7 FINKELSTEIN ET AL.: OZONE AND SULFUR DIOXIDE DRY DEPOSITION TO FORESTS 15,371 Table and SO 2 Depsitin Velcity, Standard Errr f Depsitin Velcity, and Number f Observatins fr Dry, Rain, and Dew Perids During the Day and at Night at the Kane Experimental Frest 03 DV SO 2 DV 0 3 DV N SO 2 DV N Day Dry 0.75 _ _ Rain 0.71 _ Dew 7 _ _ Night Dry 7 _ _ Rain _ Dew 7 _ _ significant degree when the stmata are clsed, SO 2 seems t be depsited mre readily t ther surfaces. Granat and Richter [1995], in a reprt n studies f SO2 depsitin t pines using flux chambers, als nted that a significant fractin f the SO2 depsitin is t surfaces rather than thrugh the stmatal pathway. This difference is again apparent when cmparing average depsitin velcity between high and lw LAI perids. There is almst n difference between high and lw LAI perids fr SO2 (1 versus 0.57), but there is a cnsiderable difference fr 03 depsitin velcity (0.38 versus 0.14), again shwing the imprtance f the stmatal pathway fr zne, and the pssibility f an alternative pathways fr sulfur dixide. Cmparing dry and wet cnditins (Table 2) as measured by the surface wetness sensr [Meyers et al., 1998] placed at the tp f the canpy, the rati f SO2 t 03 depsitin velcities is smewhat larger fr wet cnditins. During the day there is little difference in either the 03 r SO2 depsitin velcity between wet and dry cnditins. This may be due t the rain events that wuld be the predminant surce f wet surfaces during the day. During the night, 03 depsitin velcity is apprximately twice as high when surfaces are wet. Mst f this difference is seen during the summer when LAI is greatest. SO2 als has a larger depsitin velcity during wet cnditins at night, but this difference is greater during lw LAI cnditins. The reasn fr these differences are nt clear, but may invlve surface chemistry and the cmpsitin f trace cnstituents in the thin water layer n the plant. The abve analysi separated data int wet and dry surface cnditins but did nt distinguish between wet surfaces caused by precipitatin as ppsed t wet surfaces caused by dew. Sme differences between the tw culd be caused by changes in atmspheric resistance R a, as the atmsphere is usually quite stable during dew frmatin. Wesely [1989], hwever, suggests that there shuld be differences caused by the cn- tant as well. The chemistry f the surface is als effected by the length f time between precipitatin events that may cause "wash ff" f sme chemical species. A mre directed study is needed t understand these alternative pathways f the direct remval f trace gases frm the atmsphere t plant and ther surfaces fllwing precipitatin. The flux t the surface depends n bth the depsitin velcity and the mixing rati. Figure 4 cntains average diurnal plts f depsitin velcity, mixing rati, and flux fr bth 03 (Figure 4a) and SO2 (Figure 4b). The mixing rati f 03 decreases during the night t a minimum that ccurs between 0600 and 0800, and then increases t a maximum near nn. It decreases nly slwly after dark. This diurnal pattern is much less dramatic than it wuld be in a mre urban setting with nearby surces f NO, which scavenge 03. The peak in the depsitin velcity cmes well befre the peak in mixing rati. As a result, the diurnal cycle fr the flux is quite smth and symmetric, with a maximum dwnward flux (negative values dente flux tward the grund) between 0900 and 1300, and a minimum (near zer) during the evening hurs. Similar curves fr SO2 are shwn in Figure 4b. The mixing rati f SO2 is relatively cnstant, with a small minimum in the afternn, perhaps related t the grwth f the mixing layer. The time variatins in flux, then, reasnably reflect the variatins in the average depsitin velcity cycle, althugh the maximum is earlier due t the decrease in average mixing rati as the day prgresses. The patterns f the diurnal cycles f flux and mixing rati at Sand Flats and Duke (03 nly) were remarkably similar t thse at the Kane site Ozne and sulfur dixide bservatins frm Sand Flats. Ambient levels f SO2 were very lw at Sand Flats, usually belw the minimum detectable level f the fast SO2 analyzer; resulting in nly 191 valid bservatins frm the ttal pssible 7776 half-hur perids. These bservatins tended t ccur n nly a few days, s seasnal trends are difficult t define and are mitted frm Figure 2b. 03 mixing ratis were usually well abve the minimum detectable level, s that valid readings culd be taken whenever the winds were frm the preferred sampling directin, allwing a ttal f 2494 flux and depsitin velcity bservatins t be made. The seasnal cycle f zne depsitin velcity is shwn in Figure 2b. Each pint in the figure is the weekly average f all daytime (as defined in Table 2) depsitin velcity bservatins during that week. With the usual variatins caused by changes in weather, a seasnal cycle, crrespnding t the seasnal cycle f vegetatin, is apparent. Nte that the difference between early seasn and peak seasn depsitin velcity is nt as large as it was at Kane. The diurnal cycle f depsitin velcity fr SO2 and 03 are shwn in Figure 3b. It shws depsitin velcity fr SO2 in the taminants usually disslved in the rainwater, as ppsed t the vicinity f 0.2 t cm/s during the night, which quickly rises dew which shuld be relatively pure if the surfaces are clean. t a peak f 1.4 cm/s by During the day the depsitin Data summaries in these categries are shwn in Table 3. velcity remains fairly steady at 1.0 t cm/s, and then starts During the day, rain has little effect n 03 r SO2 depsitin t decrease at 1600 t the nighttime levels. Average ncturnal velcity, while mrning dew may increase depsitin velcity values f 03 depsitin velcity are in the range f 0.2 t 0.3 slightly. At night, dew has little effect, while depsitin velcity cm/s. As at Kane, it increases in the early mrning, peaking increases substantially during and after a rainfall. Changes in between 0800 and 0900; then remains fairly cnstant at abut atmspheric resistance are certainly imprtant, but increases cm/s until early afternn (between 1300 and 1400), when in wetted surfaces beynd what is likely during dew (e.g., it slwly starts t fall back t the ncturnal values again. branches, trunks, lwer leaves, understry, and sil surface), as Summary statistics f flux and depsitin velcity measurewell as changes in surface chemistry, are undubtably impr- ments at Sand Flats are shwn in Table 4. Frm the summary

8 15,372 FINKELSTEIN ET AL.: OZONE AND SULFUR DIOXIDE DRY DEPOSITION TO FORESTS Table 4. Summary Statistics fr Sand Flats State Frest 0 3 DV 03 Flux 03 N SO2 DV SO2 Flux SO2 N All data 8 _ _ _+ 02 Day _ Night 0.24 _ _ _ 59-1 _+ 03 High LAI 0.53 _ _ _ _ 02 Day 2 _ _ _+ 03 Night 0.26 _ _ _ _+ 02 Lw LAI 0.36 _ _ _ 04 Day 0.55 _ _ _ Night 0.20 _ _ _ _+ 05 Dry _ _ 42-3 _+ 02 Day _ _ 51-4 _+ 03 High LAI 2 _ _ _ _+ 03 Lw LAI 0.54 _ _ _+ 06 Night 0.16 _ 1-5 _ _ _+ 03 High LAI 0.16 _ _ _ _ 021 Lw LAI _ 18-4 _ _ 72-1 _+ 06 Wet 4 +_ _ _ Day 0 _ High LAI _ Lw LAI _ _ Night 0.32 _ _ _ _+ 07 High LAI _ 19-9 _ Lw LAI _ 22-6 _ _ _ The High LAI perid is frm June 3, 1998, t September 21, Other definitins are as in Table 2. table it is clear that SO2 depsitin velcity is significantly might be expected in an urban site where NO titratin adds t greater than 03 depsitin velcity (0.72 cm/s fr SO2 versus the ncturnal depletin f 03. The magnitudes f the flux and 8 cm/s fr 03), althugh fluxes are smaller (-3 ppb m/s fr SO2 versus ppb m/s fr 03). As expected, the depsitin velcities are higher during the day than at night, and abut the same fr wet perids as fr dry perids, althugh the small number f wet samples fr SO2 limits the cnclusins that shuld be drawn. Cmparisns f depsitin velcity fr wet and dry cnditins at night shw a large difference. Much f this difference, hwever, can be explained by the atmdepsitin velcity are similar t thse at ther sites. The average depsitin velcity fr zne ver all cases was 4 cm/s, with an average during the day f 0.79 cm/s, and 0.16 cm/s at night Between site cmparisn. Cnsidering the data in Tables 2, 4, and 5, mean zne depsitin velcity is rughly the same at Sand Flats and Duke, and slightly lwer at Kane. The difference in lwer verall mean at Kane is caused by its spheric resistance R a, which was much lwer fr the wet cases. lwer nighttime depsitin velcities since mean depsitin Interestingly, the day-t-night rati fr depsitin velcities velcities during the day are equivalent at all three sites. The was larger fr SO2 than fr 03. This is reversed frm what was seen at Kane. The high LAI t lw LAI rati is larger fr 03 than fr SO2, again shwing the imprtance f the stmatal pathway fr 03. As nted fr Kane, the dry t wet rati fr day-night difference is als cnsistent when cnsidering the data segregated by either dry r wet perids. Daytime depsitin velcities are the same, but ncturnal values are lwer at Kane than at the ther tw sites, and lwer than at Sand Flats bth is nt t different frm 1. fr bth dry and wet perids. The mean sulfur dixide dep- Fr 03 the enhanced depsitin velcity at night during wet cnditins des nt seem t be caused by a difference in the atmspheric resistance, as they are abut the same; 353 s/m fr dry cnditins at night, versus 416 s/m fr wet cnditins at night. Resistances during the day are quite different fr wet and dry cnditins (41 and 20 s/m), but bth are small enugh sitin velcities shw strng similarities between Kane and Sand Flats. Day-night and dry-wet differences are als cnsistent between sites. The difference between the tw sites fr wet cnditins at night prbably results frm the small number f samples fr that cmparisn. Differences between nighttime depsitin velcities at the t nt be limiting. It wuld seem that at night the wet surfaces deciduus (Kane) versus the mixed (Sand Flats) and pine reduce the surface resistance while the stmata are clsed and the stmatal resistance is high. (Duke) sites culd be due t systematic differences in the atmspheric resistance R a r ncturnal stmatal resistance Re Ozne data frm the Duke site. Because the winds T check, a simple mdel f R = u/u,2 was cmputed fr all were mre frequently frm the suth than anticipated, and the anemmeter was riented tward the nrth, the number f valid half-hur flux measurements is limited. An average diurnal cycle f depsitin velcity is shwn in Figure 3c. Nte that Table 5. Ozne Cncentratin, Flux, and Depsitin Velcity Summary fr Duke Frest Observatins the peak f the depsitin velcity again ccurs in the late Cncentratin, Depsitin mrning ( ) as at the ther frest sites. The mean ppb Velcity Flux N value f the zne flux and depsitin velcity bservatins, All data 4 _ _ _ ver all cases and fr daytime and nighttime hurs, are given in Day 47 _ _ _ Table 5. The differences in 03 mixing rati between day and Night 37 +_ _ night during gd sampling perids are small, smaller than

9 FINKELSTEIN ET AL.: OZONE AND SULFUR DIOXIDE DRY DEPOSITION TO FORESTS 15,373 Table 6. Average Atmspheric Cnductance (u/u,2)- fr the Three Sites Kane Sand Flats Duke Day Night All Table 7. Bias (Observed Minus Predicted) f the Multilayer Mdel (MLM) fr 0 3 and SO 2 Depsitin Velcity at Kane and Sand Flats 0 3 MLM Bias, cm/s SO2 MLM Bias, cm/s Kane Sand Flats Kane Sand Flats Units are in m/s. All data Day Night High LAI sites and bservatin times. The results are given in Table 6 as Day Night cnductance (R - ). (Nte that while fr trace gases this Lw LAI mdel is strictly nly valid fr neutral cnditins, it is useful Day when examining differences between sites.) It is clear frm the Night table that the atmspheric resistance is nt the reasn fr the Dry differences in bserved depsitin velcity. Day High LAI Pssibly the difference may be due t lwer stmatal resis- Lw LAI tance. Musselman and Massman [1999] explain that stmata usually clse at night in mst trees, that the stmatal resistance Night High LAI Lw LAI is very large, and that the subsequent depsitin velcity is very Wet lw. It has recently been reprted, hwever, that the stmata f Day sme species may remain partially pen at night t facilitate High LAI transpiratin. (See Wieser and Havranek [1993] fr Nrway Lw LAI spruce; see Wieser and Havranek [1995] fr Eurpean larch; Night see Giinthardt-Gerg et al. [1997] fr birch; see Zhang (unpub- High LAI Lw LAI lished data, 1999) fr Pndersa pine; and see Musselman and Minnick [2000] fr a review that lists 127 species f plants in which ncturnal transpiratin has been bserved.) While there Cnditins are as defined in Table 2. Missing values ccur when the number f samples is less than five. are n bservatins f the diurnal cycle f stmatal cnductance that we are aware f fr the frest types seen at Duke r Sand Flats, it is nt unreasnable t surmise that the larger than expected bserved depsitin velcity during the night assess the accuracy f certain cmpnents f the mdel. In the peratinal mde the mdel cmputes LAI frm prespecified may indeed be due t sme degree f stmatal pening during minimum and maximum values f LAI and the date. Sil thse hurs. An hypthesis f lwer stmatal resistance wuld misture is cmputed frm rainfall amunts. Fr this exercise explain the higher ncturnal depsitin velcity at the cnifer the LAI and sil misture were determined frm bservatins. sites. Further studies f ncturnal transpiratin and its relatinship t depsitin rates are clearly warranted. The midmrning peak in O3 depsitin velcity, as ppsed t a nn t early afternn peak seen ver crps, may be related t earlier clsure f stmata during the day. This culd result frm the vapr pressure deficit between the leaf and the atmsphere which increases as a result f changes in air temperatures, r it culd als relate t the lwered value f CO2 which may ccur if the leaf verheats [Bannister, 1976]. This midmrning peak is nt, hwever, evident in average diurnal cycles f depsitin velcity bserved ver crps [Meyers et al., 1998]. The reasns fr these differences between frests and Therefre the mdel's ability t cmpute these input variables is nt included. Anther change frm nrmal peratinal prcedure is that wind speed and turbulence inputs were taken frm the snic anemmeter n the twer abve the canpy, while in netwrk peratin they wuld be taken frm a 10 m twer set in a frest clearing r ther nearby site where cncentratin measurements are made. In netwrk peratins the mdel is run fr each hur f a week, and the results are averaged ver that week t get an average depsitin velcity. It is multiplied by the integrated cncentratin measurement frm a filter-pack r cntinuus crps are nt bvius, but seem t be related t the sensitivity instrument t estimate the weekly ttal flux [Clarke et al., f different species t vapr pressure deficit, sunlight, and phtsynthetic activity. Observatins f the mrning peak ver frests and similar reasning abut its cause were als presented by Aurela et al. [1996] and Lamaud et al. [1994]. 1997]. Because ur measurements are made using a snic anemmeter with an acceptance angle f ø, we d nt have a valid cmplete set f inputs fr the whle week with which t run the mdel and cmpute weekly depsitin velcities as are dne in an peratinal sense. 4. Mdel Evaluatin Mdel perfrmance is evaluated using the mean bias (bserved depsitin velcities (DV) minus predicted DV) and is The NOAA Multilayer Depsitin Velcity Mdel (MLM), described by Meyers et al. [1998], was run using input data bserved at bth the Kane and Sand Flats sites, s that it culd be cmpared with bservatins and its strengths and weaknesses culd be nted. Data necessary t run the mdel were presented in Table 7 fr varius categries as defined in Table 2. Fr SO2 the mean bias fr all data was 0.21 cm/s at Kane and 0.25 cm/s at Sand Flats, which imply a significant underpredictin by the mdel. The bias was higher during the day (0.29 and 0.36) than at night (0.14 and 7). The biggest differences nt available frm the Duke site. between sites are the tree species. The different biases may Sme variatins in applicatin f the mdel frm that dne in nrmal netwrk peratinal prcedures were made t better reflect the way thse species are parameterized by the mdel. Bias was als higher during wet perids (0.29 and 0.35) than

10 15,374 FINKELSTEIN ET AL.' OZONE AND SULFUR DIOXIDE DRY DEPOSITION TO FORESTS 1.4 KANE (a) SAND FLATS (b) E > O +% + 0 % ø. O. + uj ß + Day Night Trans ø + + : Day ß Night + Trans E 1.0 > O O.6 ua OBSERVED 0 3 DV (cm/s) - (c) KANE SAND FLATS ø < % %% O. jy O ;y;tj. % t. % ø ß * ß Night Trans OBSERVED 0 3 DV (cm/s) 2.4 (d) Day ß Night + Trans OBSERVED SO 2 DV (cm/s) OBSERVED SO 2 DV (cm/s) Figure 5. Mdeled and bservedepsitin velcity. Day is defined as , night is defined as , and transitin is defined as the remaining times. (a) 03 at Kane, (b) 03 at Sand Flats, (c) SO2 at Kane, and (d) SO2 at Sand Flats. dry (0.17 and 0.24). This held bth during the day and at night. During lw LAI perids the difference between wet and dry perids was even greater than it was during high LAI perids with plentiful leaves. The differences between wet and dry perids, bth in general, and as separated between high and lw LAI, wuld imply that depsitin t wet surfaces, especially the nnleaf surfaces (sil, trunk, and stem, etc.) are greater than are accunted fr in the mdel. Mdel results fr 03 depsitin velcity shwed better agreement with the bserve data. Average bias fr all valid bservatins was -04 cm/s at Kane and 0.10 cm/s at Sand Flats. At night the average bias was -2 and 0.10, while during the day it was 03 and The mdel did quite well during bth the lw LAI perid (-6 and 7) and the high LAI perid (6 and 0.11), suggesting that the respnse t plant activity is reasnable. The mdel underpredicted t a greater degree during wet perids during the day (0.16 and 0.3) than fr cmparable dry perids (-2 xk0.12). As nted abve, the effect f surface wetness n zne depsitin seems t increase slightly the depsitin velcity, especially when wetness is due t rain. This is nt recgnized in the mdel. As ppsed t the SO2 results, the mdel did better fr 03 depsitin velcity at Kane than at Sand Flats under mst cnditins, althugh smetimes verpredicting during the night at Kane. Certainly during the night, the nted transpiratin f sme species that seems t be ccurring at Sand Flats has an effect n these results. Figures 5a-5d are scatterplts f mdeled versus bserved depsitin velcity fr 03 and SO2 at Kane and Sand Flats fr all valid data. Daytime, nighttime, and transitinal (dawn and dusk) perids (as defined in Table 2) are indicated. The figures make clear that while there is, n average, fairly lw bias between the mdel and bservatins, there is significant scatter, with mre high bservatins that are underpredicted than lw bservatins that are verpredicted. The patterns fr daytime and night are similar in that there seems t be a ppulatin f rather extreme underpredictin distinct frm the majrity f the mdel predictins which are in reasnable agreement with the data. This is especially true fr SO2, which has higher bserve depsitin velcities than des 03, while the maximum mdeled depsitin velcities are abut the same fr bth gases. The range f bserved SO2 depsitin velcities is als much greater than predicted by the mdel, bth during the day and at night; implying that the mdel is nt respnsivenugh t changes in envirnmental factrs influencing depsitin. The mdeled seasnal cycle f depsitin velcity at the Kane Frest, as illustrated by the weekly daytime averages fr 03 and SO2 and the crrespnding bservatins, is shwn in Figure 6. Fr 03 the mdel replicates the seasnal cycle quite

11 FINKELSTEIN ET AL.: OZONE AND SULFUR DIOXIDE DRY DEPOSITION TO FORESTS 15, e Md 3/31/97 6/19/97 9/7/97 11/26/97 DATE Figure 6. Seasnal cycle f bserved and mdeled 03 and SO2 at Kane. Each pint is a weekly average f daytime values, as in Figure 2. well, matching the spring increase and fall decrease. Hwever, it des underpredicthe summer maximum. The bserved seasnal cycle fr SO2 depsitin velcity is much less prnunced. The mdel utput fr SO2, hwever, is similar t that fr 03 and, as a cnsequence, des nt match the seasnal prgressin well. The degree f daytime underpredictin fr SO2 is als evident in this figure. The mdeled seasnal cycle fr 03 at Sand Flats is similar t that at Kane. This suggests that the mdel is veremphasizing the imprtance f the stmatal pathway fr SO2. Average diurnal cycles f mdeled and bserved depsitin velcity fr bth gases and sites are shwn in Figure 7. The 03 cycle matches quite well at Kane (Figure 7a) in the early mrning, afternn, and evening, but the peak is nt high enugh and cmes at the middle f the day instead f at the time f bserved peak in midmrning. The crrespnding diurnal cycle at Sand Flats (Figure 7c) has the same prblem with the shape and magnitude f the peak. The mdel als des nt capture the substantial depsitin velcity at night which was discussed abve. Fr SO2 (Figures 7b and 7d) the shape f mdeled and bserved depsitin velcity are similar. The differences are greatest during midday, when the mdel underestimates the maximum. The SO2 diurnal cycle des nt shw the strng influence f the stmatal resistance and the midmrning peak seen with 03, perhaps because f the lwer resistance t surface depsitin. 5. Summary and Cnclusin Full grwing seasn bservatins f depsitin velcity and flux f SO2 and 03 have been made ver deciduus and mixed frests, as well as a 1 mnth set f 03 bservatins ver a pine plantatin. SO2 depsitin velcity is, n average, greater than 1.5 KANE 03 [ '"' " Obs Mdel (a) 1.5 KANE (b) 802 l ß '" " Obs Mdel ' 1'0' 1 '2' 1'4' 1 '6' 1'8'2' Hur HOUR 1.5 SAND FLATS (C) l ,',-- Obs Mdel SAND FLATS SO 2 Obs l---, - - _ Md Hur HOUR Figure 7. Diurnal cycles f predicted and bservedepsitin velcity. (a) Kane 03, (b) Kane SO2, (c) Sand Flats 03, and (d) Sand Flats SO2.

12 15,376 FINKELSTEIN ET AL.: OZONE AND SULFUR DIOXIDE DRY DEPOSITION TO FORESTS that fr 03 depsitin velcity, especially during the daytime. Average daily depsitin velcities are quite similar amng sites. Fr 03 they are 0.75, 0.75, and 0.79 cm/s at Kane, Sand Flats, and Duke, respectively. Fr SO2 they are 1.04 and 1.01 cm/s at Kane and Sand Flats, respectively. The seasnal cycles f 03 depsitin velcity are prnunced, and clsely fllw the green-up and senescence f the fliage. The seasnal cycle fr SO2 depsitin velcity has less variatin between spring, summer, and fall, but still shws the effects f vegetative uptake superimpsed n year-rund surface uptake. Because the cncentratins f SO2 in remte areas are usually substantially less than thse fr 03, the fluxes f 03 are usually higher. Acknwledgments. The authrs wish t thank Susan Stut and Steve Steele f the Kane Experimental Frest, U.S. Frest Service, Pat Putman and Bernie Davis f the Sand Flats State Frest, New Yrk State Department f Envirnmental Cnservatin, and Gabriel Katul and the staff f the Duke Frest, Duke University, fr making it pssible fr us t cnduct these studies in their frests. We wuld als like t thank Ellen Cter and Brian Eder, and the annymus reviewers fr their very helpful cmments f this paper. Our research was funded by the U.S. Envirnmental Prtectin Agency. This paper has been reviewed and apprved fr publicatin by bth the Envirnmental Prtectin Agency and the Natinal Oceanic and Atmspheric Administratin. Mentin f specific prducts des nt cnstitute endrsement by either agency. References Average daytime peak values fr 03 depsitin velcity ver frests peak in midmrning when stmatal activity is greatest, Aurela, M., T. Laurila, and J.-P. Tuvinen, Measurements f 03, CO2, and nt near nn as it des ver crps, r as SO2 depsitin and H20 fluxes ver a Scts Pine stand in eastern Finland by the micr-meterlgical eddy cvariance methd, Silva Fennica, 30, velcity des ver crps and frests. Minimum values f dep , sitin velcity fr bth SO2 and 03 ccur early in the night Bannister, P., Intrductin t Physilgical Plant Eclgy, 274 pp., and then tend t slwly increase, perhaps due t surface ab- Blackwell Sci., Malden, Mass., srptin caused by increases in wetness and/r t early pening Chasn, J. W., D. D. Baldcchi, and M. A. Hustn, A cmparisn f direct and indirect methds fr estimating frest canpy leaf area, f the stmata. Even befre any light has reached the trees, Agric. Fr. Meterl., 57, , stmata may be pen in sme species, resulting in sme uptake Clarke, J. F., E. S. Edgertn, and B. E. Martin, Dry depsitin calcuthrughuthe night. This was bserved at the mixed and pine latins fr the clean air status and trends netwrk, Atms. Envirn., frests, but nt at the deciduus frest. 31, , A cmparisn f the average daily maximum depsitin Ce, H., M. W. Gallager, T. W. Chulartn, and C. Dre, Canpy scale measurements f stmatal and cuticular 03 uptake by Sitka velcity during fast grwth perids frm Figure 3 with similar spruce, Atms. Envirn., 29, , results ver crps frm Figure 4 fmeyers et al. [1998] indicates Davies, W. J., Plant hrmnes and ecphysilgy f cnifers, in Ecthat maximum 03 depsitin velcity is higher t frests than physilgy f Cniferus Frests, edited by W. K. Smith and T. M. crn r pasture, but abut the same t the mre dense sy- Hinckley, pp , Academic, San Dieg, Calif., Duyzer, J., J. Westrate, and S. Waltn, Exchange f zne and nitrbeans. SO2 depsitin velcity t frests is nly slightly higher gen xides between the atmsphere and cniferus frest, Water Air than it was t crn r pasture, but less than t sybeans. The Sil Pllut., 85, , rati f SO2 depsitin velcity t 03 depsitin velcity is Fassnacht, K. S., S. T. Gwer, J. M. Nrman, and R. E. McMurtrie, A greater ver crps than ver frests. cmparisn f ptical and direct methds fr estimating fliage While the effect is usually small, wetness tends t increase surface area index in frests, Agric. Fr. Meterl., 71, , Fuentes, J. D., T. J. Gillespie, G. den Hartg, and H. H. Neumann, depsitin velcity. Variatins in the effect f wetness may Ozne depsitin nt a deciduus frest during dry and wet cndepend n surface chemistry, as reflected in whether the wet- ditins, Agric. Fr. Meterl., 62, 1-18, ness is due t dew r rain. Surface chemistry and the depsi- Galbally, I. E., J. A. Garland, and M. J. G. Wilsn, Sulphur uptake tin t surfaces (e.g., leaf, stem, trunk, and the grund) seem frm the atmsphere by frest and farmland, Nature, 280, 49-50, t be quite imprtant fr SO2, but less s fr 03, as can be seen Garland, J. A., and J. R. Bransn, The depsitin f sulphur dixide t by cmparing the differences in daily maxima and minima as pine frest assessed by a radiactive tracer methd, Tellus, 29, 445- well as preleaf and midseasn differences. 454, Frm cmparisns between the Multilayer Mdel predic- Gergiadis, T., F. Rssi, and F. Nerzzi, Inferring zne depsitin n tins and bservatins f depsitin velcity, it is nted that agricultural surfaces: An applicatin t herbaceus and tree canpies, Water Air Sil Pllut., 84, , the mdel is relatively unbiased, n average, fr 03, but un- Granat, L., and A. Richter, Dry depsitin t pine f sulphur dixide derpredicts SO2 depsitin velcity. The mdel underpredicts and zne at lw cncentratin, Atms. Envirn., 29, , the higher values f depsitin velcity fr bth pllutants, bth during the day and at night. The mdel reprduces the Gtinthardt-Gerg, M. S., C. J. McQuattie, C. Scheidegger, C. Rhiner, and R. Matyssek, Ozne-induced cytchemical and ultrastructural seasnal and diurnal cycles fr 03 quite well, but misses the changes in leaf mesphyll cell walls, Can. J. Fr. Res., 27, , times and magnitude f the average daily peak. The mdel underpredicts the spring and fall depsitin velcity f SO2, Justice, C. O., J. R. G. Twnshend, B. N. Hlben, and C. J. Tucker, perhaps because f an underestimate f surface as ppsed t Analysis f the phenlgy f glbal vegetatin using meterlgical satellite data, Int. J. Remte Sens., 6, , stmatal pathways. Kesselmeier, J., and K. Bde, Bilgical knwledge needed fr the The mdel weaknesses nted abve are areas f pprtunity measurements and interpretatin f exchange prcesses between fr further research int depsitin velcity mdeling. Specific plants and the atmsphere, in Bigenic Vlatile Organic Carbn areas needing attentin include the dynamic range f the Cmpunds in the Atmsphere, edited by G. Helas, J. Slanina, and R. mdel, s that it can respnd t the full range f bservatins, Steinbrecher, pp. 9-25, SPB Acad., Amsterdam, Netherlands, Kidwell, K., NOAA plar rbiter data user's guide, Natl. Clim. Data better understanding and mdeling f ncturnal atmspheric Cent., Washingtn, D.C., stability as well as ncturnal plant respiratin and stmatal Lamaud, E., Y. Brunet, A. Labatut, A. Lpez, J. Fntan, and A. pening, and better understanding and representatin f sur- Druihet, Landes experiment: Bisphere-atmspherexchanges f face chemistry, including the rle f surface water in the up- zne and aersl particles abve a pine frest, J. Gephys. Res., 99, take f SO2 and ther gases. The ultimate fate f gases taken 16,511-16,521, Lrenz, R., and C. E. Murphy Jr., The dry depsitin f sulfur dixide up by the surface is als an area fr further study. We hpe t n a lbllly pine plantatin, Atms. Envirn., 19, , reprt n prgress in at least sme f these areas in the near McMahn, T. A., and P. J. Denisn, Empirical atmspheric depsitin future. parameters--a survey, Atms. Envirn., 13, , 1979.

13 FINKELSTEIN ET AL.: OZONE AND SULFUR DIOXIDE DRY DEPOSITION TO FORESTS 15,377 Meyers, T. P., and D. D. Baldcchi, A cmparisn f mdels fr technique fr estimating dry depsitin velcities based n similarity deriving dry depsitin fluxes f 03 and SO2 t a frest canpy, with latent heat flux. Atms. Envirn., 33, , Tellus, Ser. B, 40, , Rnd6n, A., C. Jhanssn, and L. Granat, Dry depsitin f nitrgen Meyers, T. P., P. L. Finkelstein, J. Clarke, T. G. Ellestad, and P. F. dixide and zne t cniferus frests, J. Gephys. Res., 98, Sims, A multilayer mdel fr inferring dry depsitin using standard 5172, meterlgical measurements, J. Gephys. Res., 103, 22,645-22,661, Sehmel, G. A., Particle and gas dry depsitin: A review, Atms Envirn., 14, , Munger, J. W., S.C. Wfsy, P.S. Bakwin, S.-M. Fan, M. L. Gulden, Tan, C. S., and T. A. Black, Factrs affecting the canpy resistance f B.C. Daube, A. H. Gldstein, K. E. Mre, and D. R. Fitzjarrald, a Duglas-Fir frest, Bundary Layer Meterl., 10, , Atmspheric depsitin f reactive nitrgen xides and zne in a Waltn, S. M., W. Gallagher, and T. W. Chulartn, Ozne and NO2 temperate deciduus frest and subarctic wdland, 1, Measure- exchange t fruit rchards, Atms. Envirn., 31, , ments and mechanisms, J. Gephys. Res., 101, 12,639-12,657, Wesely, M. L., Turbulent transprt f zne t surfaces cmmn in the Musselman, R. C., and W. J. Massman, Ozne flux t vegetatin and eastern half f the United States, in Trace Atmspheric Cnstituents: its relatinship t plant respnse and ambient air quality standards, Prperties, Transfrmatins, and Fates, edited by S. E. Schwartz, pp. Atms. Envirn., 33, 65-73, , Jhn Wiley, New Yrk, Musselman, R. C., and T. J. Minnick, Ncturnal stmatal cnductance Wesely, M. L., Parameterizatin f surface resistances t gaseus dry and ambient air quality standards fr zne, Atms. Envirn., 34, depsitin in reginal-scale numerical mdels, Atms. Envirn., 23, , , Padr, J., Seasnal cntrasts in mdeled and bserved dry depsitin Wieser, G., and W. M. Havranek, Ozne uptake in the Sun and shade velcities f 03, SO2, and NO2 ver three surfaces, Atms. Envirn., crwn f spruce: Quantifying the physilgical effects f zne ex- 27, , psure, Trees, 7, , Padr, J., G. denhartg, and H. H. Neumann, An investigatin f the Wieser, G., and W. M. Havranek, Envirnmental cntrl f zne ADOM dry depsitin mdule using summertime 03 measurements uptake in Larix decidua Mill: A cmparisn between different altiabve a deciduus frest, Atms. Envirn., 25, , tudes, Tree Physil., 15, , Padr, J., H. H. Neumann, and G. denhartg, Mdeled and bserved Zeller, K., Wintertime zne fluxes and prfiles ver a subalpine dry depsitin velcity f 03 abve a deciduus frest in the winter, Spruce-Fir frest, J. Appl. Meterl., 39, , Atms. Envirn., 26, , Petit, C., M. Trinite, and P. Valentin, Study f turbulent diffusin J. F. Clarke, P. L. Finkelstein (crrespnding authr), J. A. Neal, abve and within a frest--applicatin in the case f SO2, Atms. and D. B. Schwede, U.S. EPA, MD-80, Research Triangle Park, NC Envirn., 10, , (finkelstein'peter@epamail'epa'gøv) Pilegaard, K., N. O. Jensen, and P. Hummelsh0j, Seasnal and diurnal T. G. Ellestad, Natinal Expsure Research Labratry, EPA, Revariatin in the depsitin velcity f zne ver a spruce frest in search Triangle Park, NC Denmark, Water Air Sil Pllut., 85, , E. O. Hebert, ESE Inc., Gainesville, FL Pleim, J. E., A. Xiu, P. L. Finkelstein, and J. F. Clarke, Evaluatin f T. P. Meyers, Atmspheric Turbulence and Diffusin Divisin, Air a cupled land-surface and dry depsitin mdel thrugh cmpar- Resurces Labratry, NOAA, Oak Ridge, TN isn t field measurement f surface heat, misture, and zne fluxes, paper presented at 12 th Sympsium n Bundary Layers and Turbulence, Am. Meterl. Sc., Vancuver, B.C., Canada, July 28 t Aug. 1, (Received Nvember 1, 1999; revised March 14, 2000; Pleim, J. E., P. L. Finkelstein, J. F. Clarke, and T. G. Ellestad, A accepted March 15, 2000.)