Environ. Sci. Technol. 2006, 40, 135-141
Field Measurements of Aerosol Particle Dry Deposition on Tropical Foliage at an Urban Site RANJIT KUMAR, K. MAHARAJ KUMARI, AND S. S. SRIVASTAVA* Department of Chemistry, Faculty of Science, Dayalbagh Educational Institute, Dayalbagh, Agra 282 005, India
This paper presents dry deposition of major ions on tropical foliage (leaves of Ashok (Polyalthia longifolia) and Cassia (Cassia siamea)) at St. John’s, Agra, an urban site of tropical India on nonrainy, nondewy, and nonfoggy days. The deposition flux was higher on Cassia leaf than Ashok leaf probably due to a rougher surface as shown by scanning electron microscopy. Dry deposition of cations varies from 0.46 to 12.16 mg m-2 day-1 while anions vary from 0.04 to 3.24 mg m-2 day-1. The percentage contribution of alkaline components is greater than that of acidic components, indicating the alkaline nature of dry deposition. Two-way analysis of variance results reveal significant seasonal variation only for K+, SO42-, and F-; however, values varied season to season for Na+, Ca2+, Mg2+, Cl-, NO3-, and NH4+ also. The large seasonal variation in deposition flux may be due to meteorological conditions, diameter of particles, and variation in atmospheric level. SO42- and NO3- show significant correlation, indicating their origin from similar sources while significant correlation between Ca2+ and Mg2+ implies their origin from soil. Poor correlation between Ca2+ and SO42-, Ca2+ and NO3-, and Mg2+ and SO42- indicates that in addition to soil other sources also contribute to dry deposition. Low dry deposition fluxes of SO42- and NO3compared to Ca2+ and Mg2+ may be due to low mass medium diameters of SO42- and NO3- and may be due to uptake through the stomatal pores abundant on leaf surfaces. Factor analysis was employed to identify the sources. F-, Cl-, SO42-, NO3-, and K+ are grouped together in the first factor, indicating their probable contribution from combustion, Ca2+, Mg2+, and NH4+ are grouped in factor II, which may be attributed to road dust and soil, and factor III includes mainly Na+ and F-, probably contributed from brick-kiln industries. Atmospheric concentrations of F-, Cl-, NO3-, SO42-, Na+, K+, Ca2+, Mg2+, and NH4+ were found to be 0.38, 2.28, 1.31, 2.74, 0.44, 0.59, 1.21, 1.2, and 2.29 µg m-3, respectively.
1. Introduction Dry deposition is the process by which atmospheric trace chemicals are transferred by air motions to the surface of the earth, and it can account for a large portion of the removal of trace chemicals from the troposphere (1). The dry deposition process involves a close interaction between the * Corresponding author fax: +91-562-281226; e-mail: sssdei@ yahoo.com. 10.1021/es048761f CCC: $33.50 Published on Web 11/18/2005
2006 American Chemical Society
atmosphere and the surface. The characteristics of individual underlying surfaces determine the mass-transfer rate, and it has been reported that dry deposition plays a major role in the deposition of polluting species from the atmosphere to natural surfaces. Vegetation is an important sink for airborne materials originating from natural and anthropogenic sources (2). Its large surface area serves as a very effective receptor for airborne substances. Foliage also provides a surface of interaction for particulate substances that may range from biologically toxic to those essential for life processes. Atmospheric deposition includes both particles and gases. Special attention is needed for particles, because field data of their deposition velocities is limited (3). SO42- and NO3particles are important due to their detrimental effects, while data for Na+, K+, Ca2+, Mg2+, and NH4+ can be important for nutrient cycling in soils and ecosystems, they can also neutralize acid input (4) and are of particular interest for the evaluation of critical loads. Despite knowing the importance of dry deposition, limited studies on deposition of atmospheric particles have been reported. Dry deposition studies have revealed that deposition may depend on the size, shape, roughness, and composition of the surface (5-8), meteorology (9-10), particle size (11), and seasonal variations (1215). Meteorological conditions, such as wind speed, wind direction, solar radiation, relative humidity, etc., play a significant role in deposition. Wind plays a very important role in increasing the deposition rate, which is associated with the transport and dispersion of pollutants from one place to other. Atmospheric stability depends on wind speed. Unstable conditions represent wind speeds >1 m s-1, which influences the deposition rate (10). The metabolic activity of the plant also plays a significant role in dry deposition to vegetation. The extent of pollutant uptake onto/into plant tissue depends on a combination of physical, chemical, and biological factors. It has been reported that approximately 30-70% of the total dry deposited SO2 may be retained in the canopy, while 50% of nitrogen deposition is irreversibly retained within the canopy. Deposition of gases and particles are governed to a large extent by the nature and chemistry of leaf surfaces. Wet surfaces, stomatal aperture, relative humidity, total leaf surface area, arrangement of leaf in the space, surface roughness, and density of stomatal openings would also affect the uptake (1, 2, 4-6). The techniques available for measuring fluxes of atmospheric particles to ecosystems range from micrometeorological methods to chamber methods (16-17). Micrometeorological methods are strongly advocated (18) but are difficult to apply routinely (19-20) and do not result in a direct sampling of the surface. Surrogate surfaces provide accurate measurements for rapidly falling particles (21) and also gain credibility as particle size increases (13), but the performance of these devices to measure the deposition of small particles, such as sulfate, which lies in the submicron range (22), is often questioned (23, 13). Surrogate surfaces are also unable to simulate the processes of gas exchange between the atmosphere and biological systems (18). Wyers and Duyzers (24) estimated the dry deposition velocity of particulate to a coniferous forest using a gradient technique. Ferm and Hultberg (25) have calculated deposition from the ratio between the deposition of an ion and sodium on the surrogate surfaces and the net throughfall of sodium to the forest. Parametrization (26-28) and modeling (29-31) have been done extensively, but they need further improvement (1). There are no widely accepted methods for routine measurements of dry deposition of particles to forest VOL. 40, NO. 1, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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canopies, and it has been underestimated until now (32). It is conceded that existing methods for estimating dry deposition to sensitive receptor areas are scarce and that an expanded research program is needed to improve these estimates. Our understanding of dry deposition in the tropics is restricted to the uptake velocities (33), and atmospheric concentrations of the particles and gases involved remain unknown. Although it is probable that dry deposition is by far the most dominant deposition process in India, because dry conditions prevail for most parts of the year, while rains are confined to a short monsoon period, relatively little research has been conducted using surrogate surfaces (8, 10, 14, 15, 34-43). No study on vegetation at an urban site has been reported from India. In this study the dry deposition of atmospheric particles at an urban site on tropical foliage (leaves of Ashok (Polyalthia longifolia) and Cassia (Cassia siamea) (natural surfaces)) is investigated under field conditions in tropical India. The objective of the present study is to characterize the dry deposition of atmospheric particles to leaves at an urban site.
2. Methodology and Theory 2.1. Site Description. Sampling was performed at St. John’s, Agra, India (27°10′ N, 78°05′ E), which is about 200 km southeast of Delhi. Agra is surrounded by the Thar Desert of Rajasthan on two-thirds of its periphery. In this area, soil is calcareous and sandy in nature. Agra, which is about 169 m above the mean sea level (msl), has a semiarid climate with an atmospheric temperature ranging between 11-48 °C (max) and 0.7-30 °C (min), 25-95% relative humidity, light intensity 0.7-5.6 oktas (cloudiness), and annual rainfall of 650 mm. Unexpectedly for 2000 and 2001, annual rainfall reduced to 377 and 385 mm, respectively. The climate of Agra is broadly divided into three seasons: winter (November to February), summer (March to June), and monsoon (July to October). At Agra, except in the summer season, calm conditions prevail with a maximum frequency in post monsoon followed by winter. Calm conditions represent the wind velocity Cl> NO3- > Ca2+ > Mg2+ > K+ > Na+ > F-. The MMDs of F-, Cl-, NO3-, SO42-, Na+, K+, Ca2+, Mg2+, and NH4+ are 4.9, 4.8, 3.4, 4.4, 5.4, 3.8, 5.0, 5.6, and 3.8 µm, respectively. Unlike other studies, the MMDs of NO3- and SO42- (3.44 and 4.4 µm, respectively) are very large; this might be due to association with alkaline particles of Ca2+ and Mg2+. In earlier reported studies also, NO3- and SO42- are associated with Ca2+ and Mg2+ and are present in supermicron particles (4647). Table 1 also shows atmospheric concentrations of gaseous SO2, HNO3, and NH3. The concentration of gaseous NH3 is higher than those of HNO3 and SO2. Seasonally, SO2 showed the highest concentration in the winter and the lowest concentration in the summer, and HNO3 showed the highest concentration in the summer and the lowest concentration in the winter, while NH3 showed the highest values in the summer and the lowest concentration in the winter. 3.2. Deposition Flux. The deposition of all major ions to the leaves of Ashok and Cassia are presented in Table 2. The table shows that the dry deposition flux of Ca2+ was the highest VOL. 40, NO. 1, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Average Dry Deposition Flux (mg m-2 day-1) of Major Ions on Natural Surfaces (Ashok and Cassia Leaves)a Ashok leaf species F-
ClNO3SO4Na+ K+ Ca2+ Mg2+ NH4+
Cassia leaf
AM ( SD
GM ( SD
AM ( SD
GM ( SD
0.31 ( 0.27 0.75 ( 0.37 0.89 ( 0.39 0.91 ( 0.58 0.37 ( 0.36 1.17 ( 0.71 3.25 ( 2.72 2.80 ( 2.04 1.84 ( 1.03
0.22 ( 0.29 0.66 ( 0.38 0.81 ( 0.40 0.74 ( 0.60 0.27 ( 0.37 0.91 ( 0.75 2.45 ( 2.81 2.24 ( 2.09 1.62 ( 1.02
0.48 ( 0.41 0.96 ( 0.63 1.23 ( 0.79 1.58 ( 1.48 0.51 ( 0.49 2.39 ( 1.67 4.31 ( 3.05 3.78 ( 2.26 2.86 ( 1.50
0.33 ( 0.43 0.78 ( 0.64 1.05 ( 0.90 1.12 ( 1.50 0.35 ( 0.50 1.91 ( 1.68 3.63 ( 3.59 3.24 ( 2.80 2.52 ( 1.32
a AM ) arithmetic mean; SD ) standard deviation; GM ) geometric mean.
FIGURE 2. Scanning electron micrographs of a Cassia leaf.
TABLE 3. Ionic Balance (in µequiv m-2 day-1) species F-
ClNO3SO4∑ANa+ K+ Ca2+ Mg2+ NH4+ ∑C+
FIGURE 1. Scanning electron micrographs of an Ashok leaf. followed by Mg2+, NH4+, K+, SO42-, NO3-, Cl-, Na+, and F-. Higher deposition was observed on Cassia leaf, which might be due to a rougher surface as shown in scanning electron micrographs of the leaves (Figures 1 and 2). It is seen (in the micrographs) that the leaf hairs and stomatal pores are more prevalent on the Cassia leaf than on the Ashok leaf and that the leaf surface of Cassia was rougher than that of the Ashok leaf. The relatively rougher surface represented by the Cassia 138
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Ashok leaves
Cassia leaves
16.3 21.1 14.3 18.9 70.6 16.0 29.9 162.5 233.3 102.2 543.9
25.2 27.0 19.8 32.9 104.9 22.2 61.1 215.5 315 158.8 772.6
leaf prevented the re-entrainment of deposited particles and therefore has a higher deposition flux. It has been reported that the deposition flux is a function of the collector surface and a smooth surface provides low deposition flux (7, 13). The ion balance (µequiv m-2 day-1) between the sum of cations (ΣC+) and the sum of anions (ΣA-) is presented in Table 3. The higher value of the cationic components may be due to unmeasured anions (CO32-, HCO3-, and organic ions (HCOO- and CH3COO)). Similar results have been reported earlier also (8, 10, 15).
TABLE 4. Seasonal Dry Deposition Flux of Major Ions on Ashok and Cassia Leavesa Ashok leaf variables FClNO3SO42Na+ K+ Ca2+ Mg2+ NH4+
TABLE 5. Comparison of the Present Study and Earlier Reported Annual Fluxes (mol hectare-1 annum-1)
Cassia leaf
M (16)
S (19)
W (10)
M (16)
S (19)
W (10)
0.35 (0.19) 0.78 (0.35) 1.05 (0.37) 1.15 (0.60) 0.32 (0.19) 1.35 (0.65) 4.74 (3.49) 3.96 (2.19) 2.24 (1.19)
0.31 (0.37) 0.74 (0.46) 0.78 (0.35) 0.66 (0.52) 0.44 (0.50) 1.20 (0.74) 2.53 (1.18) 2.13 (1.58) 1.55 (0.86)
0.23 (0.14) 0.64 (0.26) 0.79 (0.39) 0.94 (0.47) 0.34 (0.19) 0.58 (0.43) 1.41 (0.62) 1.71 (0.75) 1.12 (0.30)
0.65 (0.29) 1.03 (0.50) 1.19 (0.54) 1.57 (1.36) 0.56 (0.37) 2.96 (2.36) 4.77 (2.18) 4.62 (2.02) 3.55 (1.55)
0.49 (0.55) 0.74 (0.55) 0.99 (0.54) 1.31 (1.11) 0.50 (0.47) 2.53 (0.89) 4.56 (3.01) 3.87 (2.68) 2.89 (1.15)
0.28 (0.21) 1.13 (0.82) 1.55 (1.16) 1.91 (1.98) 0.45 (0.67) 1.55 (1.04) 3.49 (3.96) 2.68 (1.63) 2.15 (0.95)
a The values given in parentheses are standard deviations: M ) monsoon; S ) summer; W ) winter.
Table 4 presents seasonal mean values of the dry deposition flux of major cations (Na+, K+, Ca2+, and Mg2+), NH4+, and major anions (F-, Cl-, NO3-, and SO42-) along with standard deviations on Ashok and Cassia leaves. It is evident from the table that the deposition flux of most of the cations is higher in the monsoon season, while that of most of the anions is in the winter. The absence of systematic seasonal variation may be due to the fact that anions are contributed from both gaseous and particulate sources and seasonal variation in gases and meteorological conditions influence the deposition (10). Further emissions form vegetation may also contribute to this variation. It is also in contrast to the earlier reported study (15) on a surrogate surface for 15 days of collection where deposition fluxes were at a minimum in the monsoon season and maximum in the winter. It may be due to two reasons; first the monsoon and summer conditions in the present study were practically the same due to low rain depth in both the sampling years (20002001), and second the collections in the winter were confined to nondewy and nonfoggy days. Two-way analysis of variance (ANOVA) was employed (48) to test the statistical significance of seasonal variations. Significant seasonal variation has not been seen for Na+, Ca2+, Mg2+, Cl-, NO3-, and NH4+; however values varied between season to season. Results revealed significant seasonal variation only for K+, SO42-, and F-. K+ showed significant seasonal variations with the highest values in the monsoon season, probably due to high biogenic emissions. This further finds support from the fact that K+ shows dominance in the fine fraction (61%). Fine mode K+ is normally assumed to be of a vegetative origin. Particles of sizes 2.5 µm are known as coarse particles (49). Elevated concentrations in the fine mode have been observed in the presence of burning plant material (50). SO42- showed significant seasonal variation with maximum deposition flux in the winter/monsoon. An earlier study on surrogate surfaces has also shown maximum deposition of SO42- in the winter (15). This has been attributed to more wetness of the surface due to high RH. High RH (>80% in the winter) favors aqueous heterogeneous oxidation of SO2 to H2SO4, which finds support from the high MMDs of particulate SO42- (Table 1). Unlike previous studies (10, 15), the flux of NO3- was the highest in the monsoon/winter season, because the sampling of the
a
species
Cassia leavesa
Speulder forestb
FClNO3SO4Na+ K+ Ca2+ Mg2+ NH4+
92.2 98.7 72 60 80.9 223 393.3 574.9 579.9
790 910 600 35 100 120 2185
Present study.
b
Erisman et al. (32).
dry deposition during the monsoon season was done only on days without any rain. NOx is photooxidized in intense sunlight and forms HNO3 in the atmosphere by reaction of NO2 with OH radical (51). Table 5 presents a comparison with published annual fluxes in the Speulder forest in The Netherlands (32) for available ions and the present study. It has been observed that annual fluxes of SO42-, NO3-, Na+, and NH4+ are higher on Speulder forest, while annual fluxes of Mg2+, Ca2+, and K+ are higher at the present site. Higher annual fluxes of SO42- and NO3- may be due to high levels of anthropogenic sources in The Netherlands, and the higher flux of Na+ may be due to its proximity to coastal areas. Ca2+ and Mg2+ are higher at the present site because the present site is subtropical and calcareous soil dominates in the deposition. 3.3. Source Interpretation. To explore the origin and possible sources of ions contributing toward dry deposition, data were subjected to correlation analysis and factor analysis using the principle component method using the SPSS, version 3.0, software package. A correlation matrix is a common way of hypothesizing potential precursors of ions. The correlation between variables of dry deposition on Ashok and Cassia leaves are presented in Table 6. The table shows good correlation between Ca2+ and Mg2+ (r ) 0.63), indicating their origin from the soil. In this semiarid region, soil affects the atmospheric dry deposition and aerosol constituents (15, 46). NH4+ dry deposition shows a correlation of 0.59 with Ca2+ and 0.83 with Mg2+. SO42- and NO3- are highly correlated (r ) 0.71). The high correlation between SO42- and NO3indicates their origin from similar sources or sources of similar strengths. However, correlation of Ca2+ and Mg2+ with SO42and NO3- has not been found in the present study. Earlier studies at this site using surrogate surfaces report good correlation between SO42-, NO3-, Ca2+, and Mg2+ (15). It may be due to the fact that the surrogate surface was positioned horizontally, which may enhance the deposition of coarse particles. The present study is on leaves, which were naturally arranged in space at a certain angle. On comparison of SO42-/ Ca2+ and NO3-/Ca2+ ratios in soil and in dry deposition to leaf surfaces, it has been found that only 31-32% of SO42-/ Ca2+ and 50-55% of NO3-/Ca2+ ratios match soil ratios. It indicates that in addition to soil other sources also contribute to dry deposition to leaves. The deposition of SO42- and NO3is lower than the deposition of Ca2+ and Mg2+ probably due to the lower MMDs of SO42- and NO3- in comparison to those of Ca2+ and Mg2+. In addition, the low deposition of SO4-2 and NO3- may be due to the uptake of sulfur and nitrogen compounds through the stomatal pores abundant on leaf surfaces. Gay and Murphy (52) have reported that approximately 30-70% of the total dry deposited SO2 may be retained in the canopy. Approximately 50% of nitrogen (NOy) deposition is irreversibly retained within the canopy (53). VOL. 40, NO. 1, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 6. Correlation Matrix of Dry Deposition Fluxes of Major Ions on Ashok and Cassia Leaves species
F-
Cl-
NO3-
SO42-
Na+
K+
Ca2+
Mg2+
NH4+
FClNO3SO42Na+ K+ Ca2+ Mg2+ NH4+
1.00 0.87b 0.36 0.52 0.63a 0.60a 0.26 0.01 -0.14
1.00 0.58a 0.58 0.23 0.72b 0.15 0.21 -0.28
1.00 0.71b -0.25 0.41 0.01 0.12 -0.01
1.00 -0.05 0.23 0.05 0.13 0.05
1.00 0.23 0.43 -0.19 -0.21
1.00 0.15 -0.03 -0.10
1.00 0.63b 0.59a
1.00 0.83
1.00
a
Significance p ) 0.05.
b
Significance p ) 0.01.
TABLE 7. Factor Matrix of Dry Deposition Fluxes of Major Ions on Ashok and Cassia Leaves loading factors I
II
III
communalitya
FClNO3SO42Na+ K+ Ca2+ Mg2+ NH4+
0.65 0.85 0.88 0.83 -0.10 0.62 -0.01 0.14 -0.04
-0.02 0.07 0.02 0.07 -0.06 -0.06 0.79 0.93 0.93
0.71 0.39 -0.22 -0.08 0.95 0.44 0.48 -0.11 -0.17
0.93 0.88 0.82 0.69 0.92 0.58 0.85 0.90 0.89
eigenvalueb proportion of variance (%)c cumulative %d
3.41 37.9
2.35 26.1
1.70 18.9
37.9
64.0
82.9
variables
a
Communality shows how much of each variable is accounted for by the underlying factor taken together. b The eigenvalue indicates the relative importance of each factor in a particular set of variables. c The proportion of variance indicates the variance represented by that individual factor. d The cumulative percentage represents the combined variance percentage.
Factor analysis was conducted on the data in an attempt to detect common variability and to identify the sources of the observed ions. The factors with eigenvalues greater than 1 have been considered for varimax rotation to obtain the final factor matrix, and loadings in excess of 0.35 on all variables have been considered significant. Table 7 presents the factor matrix for the dry deposition of major ions. Three factors have been extracted. Factor I accounts for 37.9% of the total variance, including mainly F-, Cl-, NO3-, SO42-, and K+, and has been attributed to combustion. The main source of K+ is biogenic combustion and emissions from vegetation (54). F- is mainly contributed from brick-kiln industries. Factor II accounts for 26.1% of the total variance, including mainly Ca2+, Mg2+, and NH4+, and may be contributed by road dust. Factor III accounts for 18.9% of the total variance and includes Na+ and F+. It may be attributed to the brick-kiln industries.
Acknowledgments We thank Professor L. D. Khemani, Head, Department of Chemistry, Faculty of Science, Dayalbagh Educational Institute, and Dr. Ashok Kumar, Head, Department of Chemistry of St. John’s College, Agra, India, for providing necessary facilities. Sampling assistance by Mr. Rakesh is greatly appreciated. R.K. acknowledges the CSIR, New Delhi, India, for a Senior Research Fellowship. Financial assistance from the DST (Grant No. ESS/63/166/98) is gratefully acknowledged. 140
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Received for review August 9, 2004. Revised manuscript received June 7, 2005. Accepted September 3, 2005. ES048761F
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