fog- and rainwater chemistry in the tropical seasonal rain forest of xishuangbanna, southwest china

FOG- AND RAINWATER CHEMISTRY IN THE TROPICAL SEASONALRAIN FOREST OF XISHUANGBANNA, SOUTHWEST CHINA

WEN JIE LIU1,2,3,∗, YI PING ZHANG1, HONG MEI LI1, FAN RUI MENG4,YU HONG LIU1 and CHANG-MING WANG5

1Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla, YunnanProvince 666303, P. R. China; 2Research Centre for Eco-environmental Sciences, Chinese Academy

of Sciences, Beijing 100085, P. R. China; 3Graduate School of the Chinese Academy of Sciences,Beijing 100039, P. R. China; 4Faculty of Forestry and Environmental Management, University of

New Brunswick, Fredericton, NB, Canada E3B 6C2; 5Southwest Forestry College,Kunming 650224, P. R. China

(∗author for correspondence, e-mail: [email protected]; [email protected];Fax: +86-691-8715070)

(Received 2 December 2003; accepted 17 June 2005)

Abstract. Fogwater, fog drip and rainwater chemistry were examined at a tropical seasonal rain forestin Xishuangbanna, southwest China between November 2001 and October 2002. During the period ofobservation, 204 days with the occurrence of radiation fog were observed and the total duration of fogwas 1949 h, of which 1618 h occurred in the dry season (November to April), accounting for 37.0%of the time during the season. The mean pH of fogwater, fog drip and rainwater were 6.78, 7.30, and6.13, respectively. The ion with the highest concentration for fog- and rainwater was HCO−

3 , whichamounted to 85.2 and 37.3 µeq l−1, followed by Ca2+, Mg2+ and NH+

4 . Concentrations of NO−3 ,

HCO−3 , NH+

4 , Ca2+, and K+ in fogwater samples collected in the dry season were significantly greaterwhen compared to those collected in the rainy season. It was found that the ionic concentrations infog drip were higher than those in fogwater, except for NH+

4 and H+, which was attributed to thewashout of the soil- and ash-oriented ions deposited on the leaves and the alkaline ionic emissions bythe leaves, since biomass burns are very common in the region and nearby road was widening.

Keywords: fog chemistry, fog drip, precipitation, radiation fog, tropical seasonal rain forest, southwestChina

1. Introduction

Xishuangbanna tropical rain forest reserve (21◦09′–22◦33′ N, 99◦58′–101◦34′ E) islocated at the northern edge of the tropical zone in the South-East Asia. The mainvegetation types include tropical seasonal rain forest, tropical montane rain forest,evergreen broad-leaved forest, monsoon forest over limestone, and monsoon foreston river banks. The seasonal rain forest is primarily located in wet valleys, lowlandsand on low hills (less than 1000 m above sea level) where heavy radiation fogsfrequently occur (Cao et al., 1996). The mean annual rainfall is 1487 mm, which isconsiderably lower than rain forest in other parts of the world. The radiation fogs

Water, Air, and Soil Pollution (2005) 167: 295–309 C© Springer 2005

296 W. J. LIU ET AL.

was identified to be a critical source of water that sustains the tropical rain forestduring dry seasons (Cao et al., 1996; Liu et al., 2004).

Radiation fogs are formed during the night when cooling effect caused by long-wave radiation reduces the air temperature to or below its dew-point (LaBastilleand Pool, 1978; Liu et al., 2004). Fog droplets are first formulate on or depositon to forest canopy. When the canopy reaches its storage capacity, water will dropto ground as throughfall. However, most of the intercepted water stored in forestcanopy evaporates back to atmosphere in the following day if it a sunny clearday. As a result of the process of interception, a rain gauge in the open commonlyreceives more water during a rainfall event than throughfall gauges positioned undera canopy. If average throughfall exceeds incident precipitation, the additional watercould be considered as fog that has been intercepted by the canopy.

Deposition of cloud and fog water onto vegetation is an important hydrologi-cal and chemical inputs in some montane and coastal ecosystems (Azevedo andMorgan, 1974; Bruijnzeel and Proctor, 1995; Lovett et al., 1982). Fog droplets areknown to be efficient scavengers of atmospheric contaminants close to the earth’ssurface. A large number of studies showed high concentrations of organic or inor-ganic molecules in fog water (Herckes et al., 2001; Joos and Baltensperger, 1991).The chemical input from fog and cloud into forest ecosystem is significant enoughto cause negative impact on forest growth (Cox et al., 1989; Unsworth, 1984).Most investigations of cloud water as a source of chemical deposition, however,have been conducted in areas of frequent cloud cover which are expected to re-ceive polluted air masses, particularly in temperate regions of North America andEurope (Collett et al., 1999; Lovett, 1994; Schemenauer, 1986; Weathers et al.,1988). Little attention has been given to the fog-inundated tropical seasonal rainforests of Xishuangbanna, Southwest China and the impacts of radiation fog as acontributor to chemical fluxes into this forest are not clear. The objective of thisresearch is to measure and report the ion concentrations in fog water and in fogdrips. The fog water is collected at 38 meter above ground daily (except rainy days)from November 2001 to October 2002. Fog drip and rainwater collections werealso made during the same period.

2. Measurement Site

The experiment site is a permanent plot dedicated to the long-term ecologicalresearch managed by the Xishuangbanna Tropical Rainforest Ecosystem Station,Chinese Academy of Sciences. The site (21◦55′39′′ N, 101◦15′55′′ E, 750 m asl) islocated in the Menglun Forest Reserve in Mengla County, Yunnan province. Theforest is surrounded by farmlands with relatively low residential areas. A nationalroad lies about 1.5 km from the site on the southwest. The site is situated on topof a small plateau between two hills extending from east to west. The width ofthe plateau is approximately 40 m. Slopes to the south and north of the site are

FOG- AND RAINWATER CHEMISTRY IN SW CHINA 297

about 15◦. A 1-meter wide stream flows through the site. This is a typical site of thetropical seasonal rain forest in the area, which shows an intense tropical tendency inforest flora and is closely related to Malesian forests in flora (Cao et al., 1996). Theforest is differs from tropical lowland rain forest in that some of its tree species aredeciduous under the monsoon climate, although they do not shed leaves in the sameseason. The number of species close to those of Sarawak lowland rain forests, lowerthan those of Malesian rain forests, but higher than those of Australian, Africanrain forests, Barro Colorado Island and Panama (Cao et al., 1996).

Long-term climate records as measured at a weather station (600 m asl) 5 kmSE from the study site between 1959 and 2002 shows that the mean annual air tem-perature is 21.7 ◦C with a maximum monthly temperature of 25.7 ◦C for the hottestmonth (June) and a monthly minimum of 15.9 ◦C for the coldest month (January)(Liu et al., 2004). There are two distinct seasons, each with their distinctive char-acteristics. A dry season occurs between November to April, which includes a coolsub-season from November to February and a hot sub-season from March to April.The cool-dry sub-season is characterized by highest frequency of heavy radiationfog during the night and morning. The hot-dry sub-season is characterized by dryand hot weather during the afternoon and with heavy radiation fog during the morn-ing only. A rainy season occurs between May to October, and is characterized byhigh rainfall, which is mainly brought by the southwestern summer monsoon. Themean annual rainfall in the past 40 years is 1487 mm, of which 1294 mm (87%) oc-curs in the rainy season vs. 193 mm (13%) in the dry season. Radiation fog almostoccurs nearly every day from November to April and is heaviest from midnight(23:00–02:00) until mid-morning (09:00–11:00) (Liu et al., 2004). There is a broadrange of droplet sizes in these radiation fogs and, frequently, droplet diameters of8.0–13.6 µm are formed (Huang et al., 2001), which is the ideal size for producingfog drip (Grunow, 1955). The mean monthly relative humidity is 87%. The prevail-ing wind direction is SW, with mean annual wind speed of 0.7 m s−1 (Liu et al.,2004).

3. Methods

3.1. SAMPLE COLLECTION

Plastic funnel collectors (80 cm in diameter), each connected with a 1000-ml plasticbottle, were specially designed, fabricated, and mounted at 0.7 m above the forestfloor to collect fog dripping from the canopy. Litter fall was excluded from thebottle by a nylon mesh (1 mm square). Each 503-ml of water collected in the bottlewas equivalent to 1 mm of net precipitation. Twelve of these collectors were placedin a fixed but random pattern on the forest floor to collect fog drip. The collectorswere read in the morning after fog drip had stopped, and were leveled and cleanedof any litter present. If rain occurred in the preceding night, no data were collected


on the following morning. This may have slightly underestimated the amount ofannual fog drip. However, in Xishuangbanna, days with night rain generally donot have radiation fog the following morning (Liu et al., 2004). Thus, the missedamount of fog drip should constitute a very small proportion of total annual fogdrip. At each fog drip sampling for chemical analysis, funnels and bottles in thefield were replaced with clean ones. To reduce the number of samples for chemicalanalysis, the twelve fog drip samples for each collection period were combinedafter separate measurement of the volumes collected. Stemflow was not collectedbecause no fog water was seen to move to the forest floor as stemflow. Visibleobservations indicated that only a small proportion of the bark of the upper stemssaturated occasionally during the measurement period.

Fogwater samples were collected on an event basis, by using passive cylindricalstring collectors described by Schmitt (1987). A description of the collectors canbe found in Falconer and Falconer (1980) or in Schemenauer (1986). Collectorsconsisted of two Teflon rings fixed at a distance of 30 cm apart. The inner andouter diameter of both rings was 80 and 100 mm, respectively. A nylon line of72 m total length and 0.4 mm diameter was strung between the two rings everythree degrees, resulting in an impaction area of about 0.03 m2. The lower cut-size(50% collection efficiency) of the collector was at a radius of 6 and 3 µm at windspeeds higher than 1 and 5 m s−1, respectively, with a shift to higher collectionefficiency for smaller droplets at increased wind velocity (Schmitt, 1987). Two ofthese collectors mounted on a 72 m high meteorological tower at 38 m (0.5 m abovethe canopy) in the study site were used to collect sufficient fogwater. Each collectorwas connected to 1 cm plastic pipe that was interconnected so that all fogwaterdrained into a 1000-ml watertight plastic container. Each collector was coveredby a plastic bag when not in use and was cleaned with distilled water before eachsample collection period.

Rainwater samples were collected with a collector consisting of a stainless steelfunnel (collecting area of 314 cm2) connected to a 2000-ml plastic bottle. The funnelwas mounted on top of the tower to avoid obstacles from trees and the tower itself.The duration of each sample was 24 h. The funnel was washed twice daily in themorning and evening to avoid dry deposition. However, contamination due to drydeposition cannot be completely ruled out.

A tipping bucket raingauge (Model SL-3; Changchun Institute of Meteorolog-ical Instruments, P. R. China), mounted on top of the tower, was used to deter-mine rainfall intensity and event duration. A three-cup anemometer (Model VF-2;Changchun Institute of Meteorological Instruments, China) on top of the towerwere used to measure wind speed. Wind speed and rain were recorded hourly usingan automated data logger (DT500, Data Electronic Pty. Ltd., Australia) throughout2001–2002. A fog day is defined as a day when visibility is 1000 m or less formore than 15 min. The fog frequency and daily fog duration are calculated basedon hourly visual observation by our observer. In fact, radiation fog nearly occursevery day from November to April, and the average daily fog duration is regularly


between 7–12 h in the dry season (Liu et al., 2004). During the period of obser-vation from November 2001 to October 2002, a total number of 17 samples havebeen collected for each of fogwater and fog drip, whereas, 9 samples for rainwater.

3.2. CHEMICAL ANALYSIS

Fogwater, fog drip, and rainwater samples were collected in the 250-ml precleanedpolyethylene bottles, and transported to the laboratory. The pH of samples wasmeasured immediately with a digital pH meter. Thereafter, these samples werefiltered (0.45-µm) and refrigerated at 4 ◦C until chemical analysis could be con-ducted, approximately seven days later. Chemical analysis was performed by theXishuangbanna Water Extension Laboratory, Water Environment Monitoring Cen-ter of Yunnan, China. The samples were analyzed for major ionic components. TheNa+, K+, Ca2+ and Mg2+ concentrations were analyzed using atomic absorptionspectrometry. The CI−, SO2−

4 and NO−3 concentrations were determined using ion

chromatography. The HCO−3 was also measured by ion chromatography, using dis-

tilled, deionized water as the eluent. This eluent, and the Na2CO3 standard used togenerate calibration curves, were kept closed to the atmosphere. The HCO−

3 con-centration estimated from the standard curve was multiplied by the ratio of HCO−

3to total carbonate species plus dissolved CO2 at the sample pH (which was alwaystoo low for any significant CO2−

3 to exist) (Foster et al., 1990). NH+4 concentration

was determined using a spectrophotometer (655 nm) after chemical reactions (fordetails see Xie and Wang, 1998).

3.3. DATA ANALYSIS

Volume-weighted mean ion concentrations were calculated for fogwater, fog dripand rainwater events. Since very large variances are common, ranges rather thanstandard variations are reported for ion concentrations. Wilcoxon two-sample non-parametric tests were used to detect differences in ion concentrations in fogwater,fog drip and rainwater between early dry season and middle to end dry seasonsampling periods. SPSS 10.0 for Windows was used for statistical calculations.

4. Results

4.1. FOG FREQUENCY

The frequency of fog events was particularly high at the study site. In Figure 1,the number of days with fog and the average daily fog duration per month duringthe period of observation from November 2001 to October 2002 are presented.Fog events were more frequent and longer during the cool-dry season (Novemberto February) and hot-dry season (March to April) months than during the rainy


Figure 1. Monthly distribution of foggy days (�) and average daily fog duration (�) in the tropicalseasonal rain forest at Xishuangbanna, SW China during 2001–2002. The error bars represent standarddeviation.

season months (May to October). During the period of observation, 204 days withthe occurrence of fog were observed and the total duration of fog was 1949 h,about 22% of the total time, of which 1618 h occurred in the dry season (Novemberto April), accounting for 37% of the time during the season. The cool-dry seasonmonths and the hot-dry season months had average daily fog duration of 11 and 9 h,respectively, while the rainy season months had 6 h. However, on many occasionsfog only occurred at the very summit of the study site in rainy season.

In Figure 2, an analysis of the frequency of the wind directions during fogis presented. It becomes evidence that fog occurred with an enhanced relativefrequency during southwesterly winds.

4.2. FOG- AND RAINWATER CHEMISTRY

Seventeen fog episodes were sampled, eleven in cool-dry season, four in hot-dryseason and two in rainy season. The fog episodes are heaviest from midnight (23:00–02:00) until mid-morning (09:00–11:00). Meanwhile, nine rainwater samples werecollected, two in cool-dry season, two in hot-dry season and five in rainy season.

Volume-weighted mean ion concentrations in fogwater, fog drip and rainwaterwith minimum and maximum values are presented in Table I along with theirpH values. Great variability was found. The mean pH of fogwater, fog drip andrainwater during the study period were 6.78, 7.30, and 6.13, respectively. Therefore,the fog drip was alkaline, since a pH of 7 represents pure water neutrality. Both the


TABLE I

Volume-weighted mean ion concentrations (µeq l−1) with minimum and maximum values alongwith their pH values in fogwater, fog drip and rainwater in the tropical seasonal rain forest atXishuangbanna, SW China during 2001–2002

Fog water (n = 17) Fog drip (n = 17) Fog drip/fog water Rainwater (n = 9)

Ion Mean Min. Max. Mean Min. Max. Mean Min. Max. Mean Min. Max.

pH 6.78 5.71 7.92 7.30 6.14 8.18 – – – 6.13 5.69 7.25

H+ 0.39 0.01 2.00 0.12 0.01 0.79 0.31 0.14 1.08 0.80 0.12 2.07

CI− 22.7 n.d. 59.4 35.4 5.8 85.1 1.56 1.02 4.26 5.5 n.d. 55.5

SO2−4 27.2 n.d. 59.3 31.9 9.3 81.6 1.17 0.52 7.50 6.1 5.2 48.1

NO−3 30.7 6.7 68.7 79.1 6.6 98.4 2.58 1.51 4.67 8.3 6.3 62.5

HCO−3 85.2 22.3 172.7 149.9 26.9 192.7 1.76 1.01 7.44 37.3 16.4 79.6

Na+ 16.8 11.9 57.4 40.7 14.1 112.6 2.42 0.95 8.53 9.3 3.6 45.2

K+ 29.7 8.2 69.8 118.2 16.3 154.3 3.98 1.33 10.27 7.5 2.1 58.2

Ca2+ 66.4 9.4 107.1 97.2 12.4 136.4 1.46 1.06 7.70 18.6 4.4 113.7

Mg2+ 54.8 7.5 97.6 126.8 8.2 201.8 2.31 0.79 9.19 8.3 3.8 92.5

NH+4 52.7 n.d. 74.2 27.2 n.d. 59.0 0.52 0.17 1.35 13.2 n.d. 54.8

n.d.: not detectable.

Figure 2. Frequency of main wind directions associated with foggy conditions above canopy at thetropical seasonal rain forest in Xishuangbanna, SW China during 2001–2002.

fog- and rainwater were less acidic than pure water in equilibrium with atmosphericcarbon dioxide (pH = 5.65). The pH values in fog drip on 5 occasions in 2001 andon 12 occasions in 2002 and their corresponding values in fogwater are given inFigure 3. It can be seen from this figure that pH values in fog drip samples were, byand large, higher than those in fogwater and rainwater samples. The pH was highlyvariable in both of the sample types, and both lowest in January, which was the


Figure 3. pH values in fogwater (o), fog drip (•) and rainwater (�) in the tropical seasonal rain forestat Xishuangbanna, SW China during 2001–2002.

foggiest month in which the average fog duration reached 13 h per day (Figure 1).Fog- and rainwater were considerably more acidic during the middle of the dryseason (January) compared to the early dry season (November) and rainy season(P < 0.05).

The ion concentrations in fog- and rainwater were generally very low. The ionwith the highest concentration for fog- and rainwater was HCO−

3 , which amounted to85.2 and 37.3 µeq l−1, respectively. The HCO−

3 has been shown to be a very impor-tant ion in some tropical fogs, particularly at high pHs (Schemenauer and Cereceda,1992; Subramanian and Saxena, 1980). The HCO−

3 concentration for fogwater was2.3 times that for the rainwater. The other anions, NO−

3 , SO2−4 and CI−, were less

concentrated in fog- and rainwater than HCO−3 . The NO−

3 concentration in fogwaterwas 3.7 times that in rainwater, the SO2−

4 concentration 4.5 times, and the CI− con-centration 4.1 times. The dominant cation in fogwater was Ca2+ followed by Mg2+,NH+

4 , K+, Na+ and H+. The cation in rainwater generally had the same order exceptthat NH+

4 had the second highest concentration. Like the anions, the cation concen-trations in the fogwater were usually higher than those in the rainwater. There is abetter chemical balance between cations and anions in the fogwater and in the rain-water, with the ratio of the sum of the cations to the sum of the anions 1.35 and 1.10,respectively. It further appears that there may be some other missing anions such asformic and acetic, which were not analysed in fog- and rainwater at our study site.

The fog droplets are not falling on the forest but are rather intercepted by forestleaves by inertial impaction, resulting in a transfer of additional mineral matter tothe forest floor. The mean concentrations of all the ionic components in fog dripwere usually higher than those in fogwater except NH+

4 and H+. Among all ions,K+ and Mg2+ were substantially increased in fog drip (Table I).


Figure 4. Ion concentrations (mean ± 1 SD) in fogwater collected in the dry season (November–April,closed bars, n = 11) and rainy season (May–October, open bars, n = 2) in the tropical seasonal rainforest at Xishuangbanna, SW China during 2001–2002. ∗represent P < 0.05, Wilcoxon two-samplenonparametric tests.

Fogwater samples collected during the dry season had greater concentrations ofNO−

3 , HCO−3 , NH+

4 , Ca2+, and K+ when compared to those collected during therainy season (P < 0.05; Figure 4). Ion concentration in fog drip and rainwateralso showed similar pattern during the two sampling periods mentioned above,coinciding with obvious atmospheric haze layers, presumably due to agriculturalactivities and biomass burning (Yang et al., 2001).

5. Discussion

5.1. CHEMICAL COMPOSITION OF FOG- AND RAINWATER

Compared with literature data (Anderson et al., 1999; Eckardt and Schemenauer,1998; Wrzesinsky and Klemm, 2000), our study site was found to be clean sitewith low ion concentration and high pH values in fog- and rainwater. MeasuredpH values can be largely explained by the balance of acidic and alkaline inputsto the fog- and rainwater. Acidic pH reveals the presence of strong acids in fog-or rainwater while neutral or alkaline pH indicates neutralization of acid by soildust and ammonia (Kumar et al., 2002). The pH of fogwater at Xishuangbannaranged between 5.71 and 7.92, and rainwater between 5.69 and 7.25 indicatingan alkaline nature as compared to the reference level of 5.65. In southwest China,pH of fogwater between 5.4 and 7.5 has been reported in rural area (Huang et al.,1992). Higher pH values (average pH exceeded 6.0) were also observed in the San


Joaquin Valley and Sacramento Valley radiation fogs in central California (Collettet al., 2002; Zhang and Anastasio, 2001). Kapoor et al. (1992) and Schemenauerand Cereceda (1992) found the pH of fogwater varied between 6.2 and 6.9 at Delhi,north India, and between 7.0 and 7.9 on the Arabian Peninsula, respectively. Incontrast, in temperate areas much lower pH values have been reported, 2.0–4.0at the Los Angeles basin, 2.3–5.7 at Riverside, California (Munger et al., 1990),3.7–5.8 at the Lageren, Switzerland (Burkard et al., 2003).

The high pH values of fog- and rainwater in our study site may be attributedto high concentrations of NH+

4 , Ca2+ and Mg2+ compared to low concentrationsof SO2−

4 and NO−3 (Table I). In general, NH+

4 is attributed mostly to bacterialproduction from agricultural activities, and is thought to neutralize the cloud orfog water acidity (Saxena and Lin, 1990). NH+

4 concentrations are higher than theNO−

3 and SO2−4 concentrations at our study site (Table I), resulting in sufficient acid

neutralization and pH values typically much higher than found in unpolluted regions(Collett et al., 2002). As pointed out by Fuzzi et al. (1992), NH3 is the only importantalkaline gaseous compound present in the atmosphere, and dissolves within thedroplets depending on its air concentration and on the pH of the droplets themselves,thus modifying their NH+

4 concentration and acid content. Sources of NH3 may beanthropogenic, from agricultural activities (livestock and fertilizing) (Thalmannet al., 2002) and large amounts of smallholder industry of rubber processing usingNH3 within the Xishuangbanna region. In addition, during the dry season, NH3

may be emitted through biomass burning, which very commonly occurs in theXishuangbanna area. As NH3 is very soluble in water, its absorption have suppliedsome of the fog- and rainwater NH+

4 (Crutzen and Andreae, 1990).It is recognized that ion concentrations typically decrease when LWC increases

(Elbert et al., 2000) due to a dilution effect. Unfortunately, no LWC measurementscould be made during the period of this work. A significant difference in pH valuesbetween the large and small radiation fog drop samples was also observed, withlarge drop pH values typically several tenths of a pH unit higher than small dropvalues (Reilly et al., 2001). As pointed out by Burkard et al. (2003), radiation fogevents are associated with a significantly greater median LWC due to the widerdroplet spectrum and a larger droplet volume-mean diameter. The high pH valuesand low ion concentration in fogwater in our study site may be attributed to fogwatercollections in dense fog episodes (high LWC).

Studies on atmospheric aerosols were performed in the boundary layer ofXishuangbanna, which showed that there is an excess of soil-oriented aerosols(Ca2+ 7592 µg m−3, Mg2+ 825 µg m−3 and K+ 3092 µg m−3) in the region (Yanget al., 2001). Primarily, Ca2+ is a component of carbonate and silicate soil mineralsblown into the atmosphere after cultivation (Gorham et al., 1984). Secondarily,it may be derived from a nearby dirt road located upwind about 1.5 km from ourstudy site on the southwest (Figure 2), which was widening. Ca2+, which domi-nates the particles in the air, can neutralize the acidic effects of SO2−

4 and NO−3

(Khemani et al., 1985; Schemenauer and Cereceda, 1992). Low concentrations of


SO2−4 and NO−

3 ions were observed, which indicated that anthropogenic sourcesdid not dominate the natural particulate matter at our study site.

The ion with the highest concentration for fog- and rainwater in our study sitewas HCO−

3 . Eckardt and Schemenauer (1998) and Schemenauer and Cereceda(1992) also found that the HCO−

3 is a very important ion in some tropical fogs. Thisresult is further strengthened as HCO−

3 anion in rainwater has been identified as oneof the major anions in the Delhi region, North India, its concentration being foundvarying between 270 and 1032 µeq l−1 (Subramanian and Saxena, 1980). Similarresults were reported for fogs and dews in Indianapolis, Indiana, where HCO−

3accounted for, on average, 61% of total anion equivalents (Foster et al., 1990).Presumably, the balance of the HCO−

3 was derived from absorption of atmosphericCO2 or sedimentation of other types of carbonates, such as Na2CO3 or K2CO3,derived from fertilizers. Since our study site is surrounded by agricultural landsunderlain by carbonate bedrock, local soil-derived dust containing fertilizer saltsand carbonate, and absorption of atmospheric CO2 could have been the source ofHCO−

3 .Compared to rainwater in our study site, the fogwater showed higher ionic

concentrations (Table I). This agrees well with earlier studies showing that ionicconcentrations in fogwater are consistently higher than in rainwater (Collettet al., 2002). During the middle of the dry season, the orography of our studyarea favours, in high pressure of strong temperature inversions with low windspeeds and the formation of widespread dense radiation fog (Liu et al., 2004).When fog forms, aerosol particles are trapped within the stable inversion layerwith high residence times, and gas and particles are incorporated within thedroplets.

5.2. FOG COMPOSITION UNDER THE INFLUENCE OF FOREST CANOPY

The pH rose significantly from fogwater to fog drip in the tropical seasonal rainforest. The increase in pH could be caused by the washout of the soil- and ash-oriented ions deposited on the leaves and the alkaline ionic emissions by the leaves.H+ ion loss and pH increase in throughfall is generally believed to be due to ionexchange of H+ with other cations on canopy exchange sites (Rao et al., 1995).Also, higher pH was reported in throughfall than in bulk precipitation (Dewalleet al., 1988). Bredemeier (1988) reported that during the leafless period bufferingrates are very low compared to the leafy period when elevated pH values wereobserved in throughfall samples. The depletion of NH+

4 in fog drip in our study wasconsistent with the results of previous studies of throughfall (Joslin et al., 1988;Parker, 1983). Draaijers et al. (1997) pointed out that canopy uptake of H+ andNH+

4 could be countered by leaching of K+, Ca2+ and Mg2+. As biomass burns arevery common in the region and nearby road was widening, the deposition of ashand dust in the dry season, particularly in the middle to end dry season, is seen as


the main cause for the observed high concentrations of Ca2+, Mg2+ and K+ in thefog drip (Table I). Moreover, a wet canopy during dry period may facilitate canopyuptake of H+ and NH+

4 and canopy leaching (Draaijers et al., 1997). GenerallyCa2+, Mg2+ and K+ are subject to canopy leaching (Parker, 1983). Leaching maycontribute significantly to the throughfall flux of Ca2+ and Mg2+, especially in areawhere input of NH+

4 is substantial, leading to exchange between base cations andNH+

4 (Beier et al., 1992). Crutzen and Andreae (1990) also pointed out that biomassburning, particularly in the dry season can play a major role in tropical atmosphericchemistry, which resulted in ash deposition on the canopy and subsequent wash-offand solution of ash particles.

K+ is a well-recognized atmospheric tracer for biomass burning, and was ele-vated significantly during burning period (Gorham et al., 1984). K+ is usually foundto be relatively more susceptible to canopy leaching compared to Mg2+ and Ca2+

because it is not so tightly bound in structural tissues or enzyme complexes (Woodand Bormann, 1975). Also at the tropical seasonal rain forest in Xishuangbannaleaching of K+ is considerable.

6. Conclusions

This study is the first to describe the fogwater and fog drip chemistry in the trop-ical seasonal rain forest of Xishuangbanna. Higher radiation fog occurrence wasrecorded during 2001–2002, with January being the month with the overall highestfog frequency.

We observed wide variation in ion concentrations of individual events. The meanpH of fogwater, fog drip and rainwater during the study period were 6.78, 7.30,and 6.13, respectively. The pH of fogwater ranged between 5.71 and 7.92, andrainwater between 5.69 and 7.25 indicating an alkaline nature as compared to thereference level of 5.65. Fog- and rainwater were considerably more acidic duringthe middle of the dry season compared to the early dry season and rainy season.The high pH values of fog- and rainwater in our study site may be attributed to highconcentrations of NH+

4 , Ca2+ and Mg2+ compared to low concentrations of SO2−4

and NO−3 . Fogwater consistently showed much higher ion concentrations compared

with rainwater.The pH and most of ions rose significantly from fogwater to fog drip in the

tropical seasonal rain forest, which could be caused by the washout of the soil-and ash-oriented ions deposited on the leaves and the alkaline ionic emissionsby the leaves, since biomass burns are very common in the region and nearbyroad was widening. We realize that the fogwater deposition on the forest canopymay not contribute to the total hydrological inputs as large as precipitation duringcertain periods of time. However, the accumulation of fogwater deposition due tohigh frequency and long fog immersion time during dry season can increase thechemical inputs delivered to the forest to a great extent. The findings from this site


should only be considered preliminary since samples were very limited, and furtherverification will be essential after obtaining the more extensive data sets.

Acknowledgements

The financial assistance of the National Natural Sciences Foundation (30100019),the Chinese National Key Project of Basic Research (2003CB415100), and theNatural Sciences Foundation of Yunnan Province, P. R. China (2001C0023Q and2003C0009Z) is greatly acknowledged. We give special thanks to Drs Min Cao,Kun-Fang Cao and Li-Qing Sha for their very useful comments and helps at thebeginning of this project. Our thanks also go to Mr Wen-Ping Duan and Mr Meng-Nan Liu for their assistance with the fieldwork.

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fog- and rainwater chemistry in the tropical seasonal rain forest of xishuangbanna, southwest china

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