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Estimation of dry deposition fluxes of particulate species to the water surface in the Qingdao area, using a model and surrogate surfaces
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Atmospheric Environment 39 (2005) 2081–2088 www.elsevier.com/locate/atmosenv
Estimation of dry deposition fluxes of particulate species to the water surface in the Qingdao area, using a model and surrogate surfaces Jianhua Qia,, Peiliang Lib, Xianguo Lic, Lijuan Fengc, Manping Zhangc a
College of Environmental Science and Engineering, Ocean University of China, Qingdao 266003, China b College of Oceanography, Ocean University of China, Qingdao 266003, China c College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266003, China Received 5 February 2004; received in revised form 22 November 2004; accepted 9 December 2004
Abstract Measurements of dry deposition flux to surrogate surfaces were made in Qingdao in July 2001 and March 2002, and airborne concentration measurements of Fe, Al, Cu, Pb, Zn and Cd were made from April 2001 to May 2002 to determine atmospheric inputs of pollutants to the coastal waters. Size-dependent particle dry deposition velocities were obtained using Williams’ model with meteorological inputs from past observations, taking into account the particle growth in the humid region near the air/sea interface. Sensitivity tests show that the model provides deposition velocities comparable with recent reference values. A comparison of the modeled dry deposition fluxes with measurements and GESAMP (Group of Experts on Scientific Aspects of Marine Pollution) suggestions demonstrated that the current Williams’ model produced reasonable results. Using the averages of measured concentrations of six metal elements, the dry deposition fluxes for four seasons in the Qingdao area were calculated. The results showed that the deposition fluxes of crustal elements Al, Fe and Mn in spring and winter account for more than 70% of the whole year’s deposition, while fluxes of Cu, Pb and Zn in autumn and winter contribute more than 70% of the whole year’s deposition. r 2005 Elsevier Ltd. All rights reserved. Keywords: Dry deposition fluxes; Water surface; Surrogate surface; Williams’ model; Metal elements
1. Introduction Trace metal elements loaded in aerosol particles are of great interest since they have profound effects on marine ecosystems and the biogeochemical cycle of metal elements. Recent research shows that atmospheric dry deposition is one of the major paths for the input of Corresponding author. Tel.: 86 532 2031949; fax: 86 532 2031755. E-mail address: [email protected] (J. Qi).
trace metals to waters (Duce et al., 1991; Jickells, 1995; Spokes et al., 2001; Herut et al., 2001). Therefore it is very important, when calculating atmospheric dry deposition flux, to be aware of the biogeochemical cycle of trace metals. It is very difficult to obtain an accurate measurement of dry deposition velocity due to the complex dependence of deposition on particle size, deposition surface, meteorological conditions and chemical species. The velocity is usually obtained by a deposition model (Ruijgrok et al., 1995; Zhang et al., 2001). Williams’
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.12.017
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model includes the growth effect of particles under high humidity, but it only takes account of three special values for relative humidity (RH). The humidity is substantial in Qingdao and the average RH is 73%. Therefore the humidity effect cannot be ignored in the Qingdao area. We calculate the growth effect of aerosol particles under high humidity using the updated equation developed by Quinn and Ondov (1998). Since there are no experimental data available to evaluate this model, measurement data published in the literature (Zufall et al., 1998; Caffrey et al., 1998) are used to evaluate the current model. Sensitivity tests show that the model provides deposition velocities comparable with recent reference values. Dry deposition fluxes of the metals were directly measured using sub-boiling water as a surrogate surface in humid (July 2001) and floating-dust (March 2002) weather conditions. At the same time, dry deposition fluxes were also calculated using modeled dry velocities and GESAMP (Group of Experts on Scientific Aspects of Marine Pollution)suggested values. Using the averages of measured concentrations of six metal elements and dry deposition velocities from the model, the dry deposition fluxes in the Qingdao coastal region are calculated for four seasons.
laboratory. The sampler was mounted and secured on a high platform during the entire sampling period, and the volume of air sampled was monitored using a flowmeter. In order to reduce the sampling error and make each sample representative, the sampler was operated for a 20-h period within each sampling period to collect about 100 m3 of air. After sampling was finished, the flowmeter reading was recorded and the membrane was put in a disposable culture dish with the sampling side upwards to be analyzed in the laboratory. A clear plastic nipper and one-off plastic gloves should be used during the operation process to avoid external pollution. Dry deposition flux measurements were made using sub-boiling water (water below boiling point redistilled by a quartz distiller) contained in four 400 mL beakers, collected concurrently with the TSP samples. Deposition particles were collected over 10-day periods to ensure an adequate mass was collected for analysis. The samples were collected at the Baguanshan site in the Qingdao area. The first one was collected in July 2001 and the second in March 2002. Two beakers with sub-boiling water sealed with sealing film as field blank substrates were used to monitor potential contamination experienced while placing surrogate surfaces in the sampling position.
2. Methods
2.1.2. Sample analysis The TSP samples loaded on films were desiccated in a desiccator until the weights were constant, and then weighed. The particulates collected using sub-boiling water as a surrogate surface were weighed after they were transferred to Teflon jars (the weight is constant). The contents of metal elements in samples were determined by means of inductively coupled plasmaatomic emission spectrometry (ICP-AES). Prior to the analysis, an aerosol sample was first acid-digested in a Teflon jar following a two-step procedure: first at 160 1C for 3–4 h using a mixture of 2 mL of nitric acid (superior grade) and 2 mL of perchloric acid (superior grade), and then at 160 1C for 2–3 h in 2 mL of hydrofluoric acid (superior grade). The solution was then heated until it was dried completely, followed by cooling for a period of 1 h. Next, the residue was dissolved with 1.0 mL ultrapure HNO3 and transferred into a 10 mL colorimetric tube. The volume of the resulting solution was fixed to 10 mL with sub-boiling water. Samples were subsequently analyzed by ICP-AES. The sample injection volume is 10 mL. The precision of this method (RSD%) for Fe, Al, Mn, Cu, Pb and Zn was 1.21, 1.03, 0.87, 3.01, 5.07 and 2.49 respectively, whereas the detection limit (mg(10 mL)1) were respectively 0.087, 0.034, 0.011, 0.054, 0.093 and 0.086. The mean recoveries for Fe, Al, Mn, Cu, Pb and Zn were 95.1%, 102.5%, 106.4%, 105.4%, 94.0% and 96.1% respectively.
2.1. Experimental section 2.1.1. Sampling Three sampling sites, Yangkou, Baguanshan and Cangkou, are located in the Qingdao area (shown in Fig. 1). TSP sample collection was carried out using a KB-120 air sampler, and the ambient particles were separated on 0.4 mm pore size Nuclepore films, which were pretreated with acid on a clean bench in the
Jiaozhou Bay
Cangkou
Yangkou
Baguanshan
Yellow Sea
Fig. 1. Locations of the Qingdao area sampling sites.
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2.2. Modeling Williams’ (1982) model separates the atmosphere below a reference height (10 m) into two layers (see Fig. 2). Particulate transfer through the upper layer is governed by turbulent transfer and gravitational settling. In the deposition layer, transfer can follow two parallel paths in addition to gravitational settling. One path is controlled by transfer resistance from air to the smooth water surface. The other path is transfer to the fraction of the water surface that is broken with the formation of spray and bubbles. Horizontal transfer between the smooth and broken areas represents an interaction effect. In addition, gravitational settling in the lower layer is adjusted for particle growth due to high RH. The intent of Williams’ model is to examine the importance of wind speed, air/water temperature difference, RH, broken surface transfer and dry particle size on deposition velocity. Following Williams (1982), the dry deposition velocity Vd can be expressed as V d ¼ ðA=BÞ ð1 aÞðK ss þ ngw Þ K m aðK bs þ ngw Þ þ K m þ aðK ab þ K bs þ ngw Þ aðK bs þ ngw ÞaðK ab þ ngd Þ ð1Þ þ K m þ aðK ab þ K bs þ ngw Þ where A ¼ K m ½ð1 aÞK as þ aK ab þ vgd þ ð1 aÞðK ss þ vgd ÞaðK ab þ K bs þ vgw Þ
ð2Þ
F (flux)=Vdz CZ CZ (Concentration) RAS=1/KAS
RM=1/KM
CδS δ
Vg
RSS=1/KSS Co
SMOOTH SURFACE (Area=1-α)
CZ RAB=1/KAB CδB RBS=1/KBS Co
TURBULENT LAYER (Dry) DEPOSITION LAYER (Humid)
BROKEN SURFACE (Area=α)
Fig. 2. A two-layer multiple-path model for particle dry deposition to water, including the effect of wave breaking and particle growth by water vapor absorption (adapted from Williams, 1982). The terms F, C, Km, Ka, Kbs, Kss, Ra, Rbs, Rss, Rm, Vd, Vg, d are respectively pollutant dry deposition flux, pollutant concentration, lateral transfer coefficient, turbulent transfer coefficient, broken surface transfer coefficient, smooth surface transfer coefficient, turbulent transfer resistance, broken surface transfer resistance, smooth surface transfer resistance, lateral transfer resistance, deposition velocity, gravitational settling velocity and height of deposition layer.
B ¼ K m ½ð1 aÞðK as þ K ss Þ þ aðK ab þ K bs Þ þ vgw þ ð1 aÞðK as þ K ss þ vgw ÞaðK ab þ K bs þ vgw Þ.
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ð3Þ
In the turbulent transport layer the RH is generally very low so that the particles do not grow. However, within the deposition layer the RH approaches 100% and particles grow continuously with increasing RH. At saturation, the ultimate size of particles is governed by the availability of water vapor and the residence time in air. As Quinn and Ondov (1998) indicated, particles grow very little until the deliquescence point is reached in the laboratory, but the atmospheric aerosol particles often grow continuously, even at quite low RH, due to the formation of supersaturated solution even at quite low RH (Winkler, 1973). Hygroscopic growth influences particle deposition velocity to some degree, especially for particles with diameters in the deposition minimum during episodes of high RH. Williams’ model calculated the wet diameters using the work of Fitzgerald (1975) for three RH values, 0%, 99% and 100%. In the deposition layer over fresh water, the RH may approach 100%, while over salt water the activity of water vapor above a droplet containing dissolved material is governed by Raoult’s law, and the RH is limited to about 98.3%. In fact, these three cases (0%, 99% and 100%) cannot be applied in an actual region. As the water activity depends on the property and concentration of the solute, and the natural particle composition is internally and externally mixed, particles of different size and composition will lie on a different growth curve that is difficult to model. Quinn and Ondov (1998) discussed the hygroscopic growth by combining field studies with modeling and found that dry deposition based on MOI spectra generally agreed well with those calculated using the modified Koutrakis equation (Koutrakis et al., 1989), despite the conservative nature of the equation. The modified equation is used in our work to predict the humidity effect on dry deposition velocity.
3. Results and discussion 3.1. Model assessment The particle dry deposition velocities can be calculated using Eq (1). Variations of these parameters over expected natural ranges have been used to determine their effect on the model results. Fig. 3 shows the dependence of deposition velocity on wind speed and particle size (for RH ¼ 63%, Ta–Tw ¼ 0, Kbs ¼ 10 cms1, Km ¼ u). It is seen that Vd for particles has higher values at high wind speeds except for very large particles, which are gravitationally controlled. Increasing wind speed increases the turbulent transfer coefficient and broken surface area, which results in a
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10
1
0.1
RH=0.63 Ta-Tw=0 Km=Kas Kbs=10 cms-1
0.01 0.01
0.1 1 10 Dry particle diameter, µm
100
Fig. 3. Sensitivity tests for dry deposition velocity depending on wind speed for the model.
large deposition velocity and less dependence on particle size. Particles in the size range 0.2–1 mm have the smallest Vd values since the effect of Brownian diffusion, impaction or interception is not very efficient for this size range. Vd for this size range can change by two orders of magnitude when the wind speed changes from 2 to 20 m s1. The effect of RH is seen in Fig. 4 (for U ¼ 10 m s1, DT ¼ 0 1C, Kbs ¼ 10 cm s1, Km ¼ Kas). Quinn and Ondov (1998) assume that only fine particles grow, so the dry deposition velocity changes considerably for particles in the size range 0.01–2.5 mm, especially when RH approaches 100%. Vd calculated by the current model is 1–80% greater than that calculated by the original model for particles in the size range 0.025–0.25 mm, and 1–30% lower for those in the range of 0.5–2.5 mm. We discussed the relation between deposition velocity and parameters of interest. The credible values of these parameters over expected natural ranges have been used to determine dry deposition velocity. Due to the difficulty in obtaining direct measurements for size-segregated dry deposition on natural water, the published measurements are very limited. The dry deposition velocities from the current model are compared with measurements from Zufall et al. (1998) and that from the chemical mass balance deposition model (CMBD) (Caffrey et al., 1998). Zufall et al. (1998) measured the dry deposition flux over southwestern Lake Michigan using Zefluor Teflon filters coated with dimethylpolysiloxane. Dry deposition velocities for particles are found to range from 0.0062 cms1, for particles with a diameter of 0.75 mm, to 5.4 cms1 for those with a diameter of 24 mm. Caffrey et al. (1998) measured the dry deposition flux and aerosol size distribution concurrently, aboard the RV
100 RH=0 RH=0.99 RH=1
10 1
u=10 ms-1 Ta-Tw=0 Km=Kas
0.1
Kbs=10 cms-1 0.01
(a) Dry deposition velocity, cm sec-1
Dry deposition velocity, cm sec-1
u=2 u=5 u=10 ms-1 u=15 u=20
Dry deposition velocity, cm sec-1
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(b)
0.1 1 10 Dry particle diameter, µm
100
100 RH=0 RH=0.99 RH=1
10 1
u=10 ms-1 Ta-Tw=0 Km=Kas
0.1
Kbs=10 cms-1 0.01
0.1 1 10 Dry particle diameter, µm
100
Fig. 4. Sensitivity tests for dry deposition velocity depending on relative humidity for (a) Williams’ model and (b) the current model.
Lake Guardian 19 km east of the Chicago shoreline. The dry deposition velocities were calculated using the CMBD model for particle size range 0.049–42.7 mm. The dry deposition velocities from these measurements, Williams’ model and the current model are listed in Tables 1 and 2, respectively. The RMS error of Williams’ model and the current model is 0.60 and 0.55 respectively for Zufall’s measurement, and is 0.41 and 0.084, respectively, for Caffrey’s. By comparison, the results of the current model agree well with that of measurement. 3.2. Comparison among dry deposition fluxes from different methods 3.2.1. Dry deposition flux measurement using a surrogate surface The dry deposition flux measured in July 2001 and March 2002 using sub-boiling water is listed in Table 3. The deposition flux of Al and Fe is about two orders of magnitude higher than those of other elements. In floating-dust weather conditions the fluxes of Al, Fe, Mn, Cu, Pb and Zn are 21.9, 12.7, 16.2, 2.5, 3.2 and 3.3 times higher than those in humid weather conditions respectively. We can see that the dry deposition flux in March was higher than
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Table 1 Comparison of deposition velocities measured by Zufall et al. (1998) and the model Particle diameter (mm)
Zufall(1998) experimental(cm s1)
Williams’ model (cm s1)
Current model (cm s1)
0.25 0.80 1.00 2.00 3.00 4.00 5.00 6.00 8.00 9.00 10.00 20.00 24.00
0.040 0.0062 0.070 0.031 0.040 0.10 0.20 0.40 0.70 0.80 1.05 3.60 5.40
0.0090 0.043 0.064 0.29 0.68 0.87 0.98 1.08 1.28 1.40 1.52 3.37 4.45
0.0093 0.038 0.046 0.055 0.030 0.060 0.10 0.17 0.60 0.74 0.87 2.69 3.74
0.60
0.55
RMS error
Table 2 Comparison of deposition velocities measured by Caffrey et al. (1998) and the model Particle diameter (mm)
Caffrey (1998) experimental (cm s1)
Williams’ model (cm s1)
Current model (cm s1)
0.049 0.087 0.16 0.28 0.52 0.95 1.70 4.90 21.20 42.70
0.0054 0.0059 0.0052 0.0043 0.0073 0.018 0.023 0.14 2.67 10.98
0.0058 0.0055 0.0062 0.0091 0.020 0.056 0.16 0.78 3.44 11.66
0.0072 0.0064 0.0068 0.0095 0.020 0.049 0.13 0.16 2.93 11.04
0.41
0.084
RMS error
Table 3 Dry deposition fluxes for July 2001 and March 2002 measured using sub-boiling water as a surrogate surface
Dry deposition flux (mg m2 day1) Dry deposition velocity(cm s1)
Al
Fe
Mn
Cu
Pb
Zn
July March July March
2.97 64.99 4.96 4.36
2.49 31.68 3.66 3.81
0.042 0.68 3.44 3.49
0.024 0.061 3.61 2.51
0.034 0.11 1.22 0.84
that in July, because there were several episodes of strong dust weather conditions in March 2002 and much rainfall in July. The measurement results are consistent with the reported values (Shahin et al., 2000; Paode et al., 1998) except for the special weather conditions in March.
0.084 0.28 1.81 1.66
3.2.2. Dry deposition flux estimate with the GESAMP suggestion The dry deposition of aerosol particles is given as follows: F d ¼ V d C air ,
(4)
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Table 4 Comparison of dry deposition fluxes(mg m2 day1)of July 2001 obtained by different methods
Al Fe Mn Cu Pb Zn
Average content (ng m3)
Experimental fluxes (mg m2 day1)
Modeled fluxes (mg m2 day1)
GESAMP suggested fluxes (mg m2 day1)
693.39 788.17 14.04 7.60 32.50 53.77
2.97 2.49 0.042 0.024 0.034 0.084
3.10 3.52 0.063 0.034 0.14 0.24
0.60 0.68 0.012 0.00066 0.0028 0.0047
0.47
1.33
RMS
Table 5 Comparison of dry deposition fluxes(mg m2 day1)of March 2002 obtained by different methods
Al Fe Mn Cu Pb Zn RMS
Average content (ng m3)
Experimental fluxes (mg m2 day1)
Modeled fluxes (mg m2 day1)
GESAMP suggested fluxes (mg m2 day1)
17266.54 9634.36 226.04 28.30 149.10 193.34
64.99 31.68 0.68 0.061 0.11 0.28
84.53 47.16 1.11 0.14 0.73 0.95 11.16
14.92 8.32 0.20 0.0024 0.013 0.017 24.71
where Fd is the flux of aerosol particles due to dry deposition (mgm2s1), Cair is the concentration of that substance in the near-surface atmosphere (mgm3), Vd is the dry deposition velocity (m s1). Group of Experts on Scientific Aspects of Marine Pollution (GESAMP) (1989) suggested that the best values for the dry deposition velocities of Cu, Pb, Zn and Cd should be 0.1 cm s1, and those for Al, Fe and Mn should be 1.0 cm s1. The calculation results are listed in Tables 4 and 5. 3.2.3. Dry deposition flux estimate with the model We estimate the dry deposition fluxes for Al, Fe, Mn, Cu, Pb and Zn in March 2002 and July 2001 using the dry deposition velocities from the model, with meteorological input provided by past observations (Compiler group, 1984; Yan et al., 1993). First we predict the dry deposition velocities for particles with size range 2–100 mm in March and July using the model. Second we obtain the weighted total deposition velocity using velocities and aerosol particle distribution ratio dr for different size ranges (Zhang and Zhou, 1995) following the formula X vdz ¼ vdi dri . (5) i
Then we calculate the dry deposition fluxes for Al, Fe, Mn, Cu, Pb, Zn with the average concentration of each
metal in March and July. The results are listed in Tables 4 and 5. From these tables, we can see that the modeled fluxes are higher than the experimental ones, the GESAMP suggested values being the lowest, but the modeled values are close to experimental values compared with those of the GESAMP suggested velocities. The possible reason is that the direct measurement of dry deposition fluxes using a surrogate surface has some errors due to the complexity of a natural deposition surface and the dry deposition process. The dry deposition velocities suggested by GESAMP are mainly applicable to a fine mode, resulting in the low values. In fact, the dry deposition process is very complicated and updating the model depends on the accurate measurement of dry deposition velocities. In any case, dry deposition fluxes calculated from the current model agree with experimental values especially for crustal elements, and the model is useful for predicting dry deposition fluxes of an actual region. 3.3. Dry deposition fluxes for the Qingdao area Dry deposition fluxes of six metal elements in the Qingdao area are calculated using average concentrations of each element and dry deposition velocities predicted from the model. Considering a surface area of 1 km2 as the study area along the Qingdao shore, then
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Fe 6000 5000 8000 4000 6000 55.7% 3000 50.2% 4000 26.7% 2000 23.4% 15.6% 14.3% 2000 1000 7.5% 6.5% 0 0 spring summer autumn winter the whole year spring summer autumn winter the whole year Seasons Seasons Mass (kg)
Mass (kg)
Mn 180 160 140 120 100 46.5% 80 27.5% 60 18.2% 40 7.7% 20 0 spring summer autumn winter the whole year Seasons
35 30 25 20 15 10 5 0
Cu
22.6%
32.1% 32.9% 12.4%
spring summer autumn winter the whole year Seasons
Pb Zn 180 160 200 140 120 150 100 40.0% 37.0% 80 27.8% 100 24.7% 25.1% 60 22.9% 40 50 12.0% 10.6% 20 0 0 spring summer autumn winter the whole year spring summer autumn winter the whole year Seasons Seasons Mass (kg)
Mass (kg)
Mass (kg)
Mass (kg)
Al
Fig. 5. Dry deposition fluxes of Al, Fe, Mn, Cu, Pb and Zn for four seasons in the Qingdao area.
the dry deposition fluxes of Al, Fe, Mn, Cu, Pb and Zn for four seasons in the Qingdao area are shown in Fig. 5. The dry deposition masses of Al, Fe, Mn, Cu, Pb and Zn in every square kilometer area of the Qingdao shore for the whole year are 9215.5, 5743.4, 169.2, 34.0, 179.8 and 230.4 kg, respectively. The input masses for crustal elements Al, Fe and Mn are largest in spring, accounting for 46–56% of the whole year, and the masses in summer are the lowest, contributing to up to 10%. As for Cu, Pb and Zn, the deposition masses in autumn and winter are higher, making up 61–65%, whereas the masses are the lowest in summer, contributing less than 13%. Overall, for the crustal and the anthropogenic elements, the lowest contents of the whole year are in summer due to the high rainfall and humidity of Qingdao. Wet deposition considerably reduces the contents of all the metal elements by mechanisms such as washing out and cleaning out by rain. The contents of crustal and anthropogenic elements are much increased in winter, caused by an increase in anthropogenic emission sources such as coal burning for heating. There is a maximum for Fe, Al and Mn in spring, due to the
increase of floating-dust weather, which is influenced by the windy weather occurring in spring at Qingdao and several dust-storms from the northwest. But the values for Cu, Pb and Zn are not the highest in spring, showing that these three anthropogenic elements are less affected by dust-storm and floating-dust weather in spring compared with crustal elements.
4. Conclusion Dry deposition fluxes of the metals were directly measured using sub-boiling water as a surrogate surface in humid (July 2001) and floating-dust (March 2002) weather conditions. The deposition flux of Al and Fe is about two orders of magnitude higher than that of other elements. In floating-dust weather conditions the fluxes of Al, Fe, Mn, Cu, Pb and Zn are 21.9, 12.7, 16.2, 2.5, 3.2 and 3.3 times higher than those in humid weather conditions, respectively. The dry deposition velocity is calculated as a function of particle size using Williams’ model, taking into
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account the particle growth in the humid regions. A comparison of the modeled dry deposition fluxes with measurements and GESAMP suggestions demonstrated that current Williams’ models produced reasonable results. The model is applied to compute dry deposition fluxes in the Qingdao region. The results show that the deposition fluxes of crustal elements Al, Fe and Mn in spring and winter account for more than 70% of the whole year’s deposition, while the deposition fluxes of Cu, Pb and Zn in autumn and winter contribute more than 70% of the whole year’s deposition. Overall, for the crustal and the anthropogenic elements, the lowest contents of the whole year are in summer, due to the high rainfall and humidity of Qingdao. However, due to limited knowledge on particle dry deposition, the current model with empirical and simplified formulae needs further improvements.
Acknowledgments This project was supported by NSFC under Grant no. 49976020. The authors gratefully acknowledge very helpful discussions with Dr. Leiming Zhang of the Meteorological Service of Canada.
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Mediterranean coast of Israel (SE Mediterranean): sources and fluxes. Atmospheric Environment 35, 803–813. Jickells, T., 1995. Atmospheric inputs of metals and nutrients to the oceans: their magnitude and effects. Marine Chemistry 48, 199–214. Koutrakis, P., Wolfson, J.M., Spengler, J.D., Stern, B., Franklin, C.A., 1989. Equilibrium size of atmospheric aerosol sulfates as a function of the relative humidity. Journal of Geophysical Research 88, 5101–5108. Paode, R.D., Sofuoglu, S.C., Sivadechathep, J., Noll, K.E., Holsen, T.M., Keeler, G.J., 1998. Dry deposition flux and mass size distribution of Pb, Cu and Zn measured in southern Lake Michigan during Aeolos. Environmental Science and Technology 32, 1629–1635. Quinn, T.L., Ondov, J.M., 1998. Influence of temporal changes in relative humidity on dry deposition velocities and fluxes of aerosol particles bearing trace elements. Atmospheric Environment 32, 3467–3479. Ruijgrok, W., Davidson, C.J., Nicholson, K.W., 1995. Dry deposition of particles—implications and recommendations for mapping of deposition over Europe. Tellus 47B, 587–601. Shahin, U., Yi, S.M., Paode, R.D., Holsen, T.M., 2000. Longterm elemental dry deposition fluxes measured around Lake Michigan with an automated dry deposition sampler. Environmental Science and Technology 34, 1887–1892. Spokes, L., Jickells, T., Jarvis, K., 2001. Atmospheric inputs of trace metals to the northeast Atlantic Ocean: the importance of southeasterly flow. Marine Chemistry 76, 319–330. Williams, R.W., 1982. A model for the dry deposition of particles to natural water surface. Atmospheric Environment 16, 1933–1938. Winkler, P., 1973. The growth of atmospheric aerosol particles as a function of the relative humidity—II. An improved concept of mixed nuclei. Aerosol Science 4, 373–387. Yan, J.Y., Chen, Q.J., Zhang, X.Z., Huang, A.F., 1993. Climate in near China Sea. Science Press, Beijing, pp. 109–335. Zhang, C.C., Zhou, W.X., 1995. Study course on atmospheric aerosol. China Meteorological Press, Beijing, pp. 13–14. Zhang, L.M., Gong, S.L., Padro, J., Barrie, L., 2001. A sizesegregated particle dry deposition scheme for an atmospheric aerosol module. Atmospheric Environment 35, 549–560. Zufall, M.J., Davidson, C.I., Caffrey, P.F., Ondov, J.M., 1998. Airborne concentrations and dry deposition fluxes of particulate species to surrogate surface deployed in southern Lake Michigan. Environmental Science and Technology 32 (11), 1623–1628.
Atmospheric Environment 39 (2005) 2081–2088 www.elsevier.com/locate/atmosenv
Estimation of dry deposition fluxes of particulate species to the water surface in the Qingdao area, using a model and surrogate surfaces Jianhua Qia,, Peiliang Lib, Xianguo Lic, Lijuan Fengc, Manping Zhangc a
College of Environmental Science and Engineering, Ocean University of China, Qingdao 266003, China b College of Oceanography, Ocean University of China, Qingdao 266003, China c College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266003, China Received 5 February 2004; received in revised form 22 November 2004; accepted 9 December 2004
Abstract Measurements of dry deposition flux to surrogate surfaces were made in Qingdao in July 2001 and March 2002, and airborne concentration measurements of Fe, Al, Cu, Pb, Zn and Cd were made from April 2001 to May 2002 to determine atmospheric inputs of pollutants to the coastal waters. Size-dependent particle dry deposition velocities were obtained using Williams’ model with meteorological inputs from past observations, taking into account the particle growth in the humid region near the air/sea interface. Sensitivity tests show that the model provides deposition velocities comparable with recent reference values. A comparison of the modeled dry deposition fluxes with measurements and GESAMP (Group of Experts on Scientific Aspects of Marine Pollution) suggestions demonstrated that the current Williams’ model produced reasonable results. Using the averages of measured concentrations of six metal elements, the dry deposition fluxes for four seasons in the Qingdao area were calculated. The results showed that the deposition fluxes of crustal elements Al, Fe and Mn in spring and winter account for more than 70% of the whole year’s deposition, while fluxes of Cu, Pb and Zn in autumn and winter contribute more than 70% of the whole year’s deposition. r 2005 Elsevier Ltd. All rights reserved. Keywords: Dry deposition fluxes; Water surface; Surrogate surface; Williams’ model; Metal elements
1. Introduction Trace metal elements loaded in aerosol particles are of great interest since they have profound effects on marine ecosystems and the biogeochemical cycle of metal elements. Recent research shows that atmospheric dry deposition is one of the major paths for the input of Corresponding author. Tel.: 86 532 2031949; fax: 86 532 2031755. E-mail address: [email protected] (J. Qi).
trace metals to waters (Duce et al., 1991; Jickells, 1995; Spokes et al., 2001; Herut et al., 2001). Therefore it is very important, when calculating atmospheric dry deposition flux, to be aware of the biogeochemical cycle of trace metals. It is very difficult to obtain an accurate measurement of dry deposition velocity due to the complex dependence of deposition on particle size, deposition surface, meteorological conditions and chemical species. The velocity is usually obtained by a deposition model (Ruijgrok et al., 1995; Zhang et al., 2001). Williams’
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.12.017
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model includes the growth effect of particles under high humidity, but it only takes account of three special values for relative humidity (RH). The humidity is substantial in Qingdao and the average RH is 73%. Therefore the humidity effect cannot be ignored in the Qingdao area. We calculate the growth effect of aerosol particles under high humidity using the updated equation developed by Quinn and Ondov (1998). Since there are no experimental data available to evaluate this model, measurement data published in the literature (Zufall et al., 1998; Caffrey et al., 1998) are used to evaluate the current model. Sensitivity tests show that the model provides deposition velocities comparable with recent reference values. Dry deposition fluxes of the metals were directly measured using sub-boiling water as a surrogate surface in humid (July 2001) and floating-dust (March 2002) weather conditions. At the same time, dry deposition fluxes were also calculated using modeled dry velocities and GESAMP (Group of Experts on Scientific Aspects of Marine Pollution)suggested values. Using the averages of measured concentrations of six metal elements and dry deposition velocities from the model, the dry deposition fluxes in the Qingdao coastal region are calculated for four seasons.
laboratory. The sampler was mounted and secured on a high platform during the entire sampling period, and the volume of air sampled was monitored using a flowmeter. In order to reduce the sampling error and make each sample representative, the sampler was operated for a 20-h period within each sampling period to collect about 100 m3 of air. After sampling was finished, the flowmeter reading was recorded and the membrane was put in a disposable culture dish with the sampling side upwards to be analyzed in the laboratory. A clear plastic nipper and one-off plastic gloves should be used during the operation process to avoid external pollution. Dry deposition flux measurements were made using sub-boiling water (water below boiling point redistilled by a quartz distiller) contained in four 400 mL beakers, collected concurrently with the TSP samples. Deposition particles were collected over 10-day periods to ensure an adequate mass was collected for analysis. The samples were collected at the Baguanshan site in the Qingdao area. The first one was collected in July 2001 and the second in March 2002. Two beakers with sub-boiling water sealed with sealing film as field blank substrates were used to monitor potential contamination experienced while placing surrogate surfaces in the sampling position.
2. Methods
2.1.2. Sample analysis The TSP samples loaded on films were desiccated in a desiccator until the weights were constant, and then weighed. The particulates collected using sub-boiling water as a surrogate surface were weighed after they were transferred to Teflon jars (the weight is constant). The contents of metal elements in samples were determined by means of inductively coupled plasmaatomic emission spectrometry (ICP-AES). Prior to the analysis, an aerosol sample was first acid-digested in a Teflon jar following a two-step procedure: first at 160 1C for 3–4 h using a mixture of 2 mL of nitric acid (superior grade) and 2 mL of perchloric acid (superior grade), and then at 160 1C for 2–3 h in 2 mL of hydrofluoric acid (superior grade). The solution was then heated until it was dried completely, followed by cooling for a period of 1 h. Next, the residue was dissolved with 1.0 mL ultrapure HNO3 and transferred into a 10 mL colorimetric tube. The volume of the resulting solution was fixed to 10 mL with sub-boiling water. Samples were subsequently analyzed by ICP-AES. The sample injection volume is 10 mL. The precision of this method (RSD%) for Fe, Al, Mn, Cu, Pb and Zn was 1.21, 1.03, 0.87, 3.01, 5.07 and 2.49 respectively, whereas the detection limit (mg(10 mL)1) were respectively 0.087, 0.034, 0.011, 0.054, 0.093 and 0.086. The mean recoveries for Fe, Al, Mn, Cu, Pb and Zn were 95.1%, 102.5%, 106.4%, 105.4%, 94.0% and 96.1% respectively.
2.1. Experimental section 2.1.1. Sampling Three sampling sites, Yangkou, Baguanshan and Cangkou, are located in the Qingdao area (shown in Fig. 1). TSP sample collection was carried out using a KB-120 air sampler, and the ambient particles were separated on 0.4 mm pore size Nuclepore films, which were pretreated with acid on a clean bench in the
Jiaozhou Bay
Cangkou
Yangkou
Baguanshan
Yellow Sea
Fig. 1. Locations of the Qingdao area sampling sites.
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2.2. Modeling Williams’ (1982) model separates the atmosphere below a reference height (10 m) into two layers (see Fig. 2). Particulate transfer through the upper layer is governed by turbulent transfer and gravitational settling. In the deposition layer, transfer can follow two parallel paths in addition to gravitational settling. One path is controlled by transfer resistance from air to the smooth water surface. The other path is transfer to the fraction of the water surface that is broken with the formation of spray and bubbles. Horizontal transfer between the smooth and broken areas represents an interaction effect. In addition, gravitational settling in the lower layer is adjusted for particle growth due to high RH. The intent of Williams’ model is to examine the importance of wind speed, air/water temperature difference, RH, broken surface transfer and dry particle size on deposition velocity. Following Williams (1982), the dry deposition velocity Vd can be expressed as V d ¼ ðA=BÞ ð1 aÞðK ss þ ngw Þ K m aðK bs þ ngw Þ þ K m þ aðK ab þ K bs þ ngw Þ aðK bs þ ngw ÞaðK ab þ ngd Þ ð1Þ þ K m þ aðK ab þ K bs þ ngw Þ where A ¼ K m ½ð1 aÞK as þ aK ab þ vgd þ ð1 aÞðK ss þ vgd ÞaðK ab þ K bs þ vgw Þ
ð2Þ
F (flux)=Vdz CZ CZ (Concentration) RAS=1/KAS
RM=1/KM
CδS δ
Vg
RSS=1/KSS Co
SMOOTH SURFACE (Area=1-α)
CZ RAB=1/KAB CδB RBS=1/KBS Co
TURBULENT LAYER (Dry) DEPOSITION LAYER (Humid)
BROKEN SURFACE (Area=α)
Fig. 2. A two-layer multiple-path model for particle dry deposition to water, including the effect of wave breaking and particle growth by water vapor absorption (adapted from Williams, 1982). The terms F, C, Km, Ka, Kbs, Kss, Ra, Rbs, Rss, Rm, Vd, Vg, d are respectively pollutant dry deposition flux, pollutant concentration, lateral transfer coefficient, turbulent transfer coefficient, broken surface transfer coefficient, smooth surface transfer coefficient, turbulent transfer resistance, broken surface transfer resistance, smooth surface transfer resistance, lateral transfer resistance, deposition velocity, gravitational settling velocity and height of deposition layer.
B ¼ K m ½ð1 aÞðK as þ K ss Þ þ aðK ab þ K bs Þ þ vgw þ ð1 aÞðK as þ K ss þ vgw ÞaðK ab þ K bs þ vgw Þ.
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ð3Þ
In the turbulent transport layer the RH is generally very low so that the particles do not grow. However, within the deposition layer the RH approaches 100% and particles grow continuously with increasing RH. At saturation, the ultimate size of particles is governed by the availability of water vapor and the residence time in air. As Quinn and Ondov (1998) indicated, particles grow very little until the deliquescence point is reached in the laboratory, but the atmospheric aerosol particles often grow continuously, even at quite low RH, due to the formation of supersaturated solution even at quite low RH (Winkler, 1973). Hygroscopic growth influences particle deposition velocity to some degree, especially for particles with diameters in the deposition minimum during episodes of high RH. Williams’ model calculated the wet diameters using the work of Fitzgerald (1975) for three RH values, 0%, 99% and 100%. In the deposition layer over fresh water, the RH may approach 100%, while over salt water the activity of water vapor above a droplet containing dissolved material is governed by Raoult’s law, and the RH is limited to about 98.3%. In fact, these three cases (0%, 99% and 100%) cannot be applied in an actual region. As the water activity depends on the property and concentration of the solute, and the natural particle composition is internally and externally mixed, particles of different size and composition will lie on a different growth curve that is difficult to model. Quinn and Ondov (1998) discussed the hygroscopic growth by combining field studies with modeling and found that dry deposition based on MOI spectra generally agreed well with those calculated using the modified Koutrakis equation (Koutrakis et al., 1989), despite the conservative nature of the equation. The modified equation is used in our work to predict the humidity effect on dry deposition velocity.
3. Results and discussion 3.1. Model assessment The particle dry deposition velocities can be calculated using Eq (1). Variations of these parameters over expected natural ranges have been used to determine their effect on the model results. Fig. 3 shows the dependence of deposition velocity on wind speed and particle size (for RH ¼ 63%, Ta–Tw ¼ 0, Kbs ¼ 10 cms1, Km ¼ u). It is seen that Vd for particles has higher values at high wind speeds except for very large particles, which are gravitationally controlled. Increasing wind speed increases the turbulent transfer coefficient and broken surface area, which results in a
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10
1
0.1
RH=0.63 Ta-Tw=0 Km=Kas Kbs=10 cms-1
0.01 0.01
0.1 1 10 Dry particle diameter, µm
100
Fig. 3. Sensitivity tests for dry deposition velocity depending on wind speed for the model.
large deposition velocity and less dependence on particle size. Particles in the size range 0.2–1 mm have the smallest Vd values since the effect of Brownian diffusion, impaction or interception is not very efficient for this size range. Vd for this size range can change by two orders of magnitude when the wind speed changes from 2 to 20 m s1. The effect of RH is seen in Fig. 4 (for U ¼ 10 m s1, DT ¼ 0 1C, Kbs ¼ 10 cm s1, Km ¼ Kas). Quinn and Ondov (1998) assume that only fine particles grow, so the dry deposition velocity changes considerably for particles in the size range 0.01–2.5 mm, especially when RH approaches 100%. Vd calculated by the current model is 1–80% greater than that calculated by the original model for particles in the size range 0.025–0.25 mm, and 1–30% lower for those in the range of 0.5–2.5 mm. We discussed the relation between deposition velocity and parameters of interest. The credible values of these parameters over expected natural ranges have been used to determine dry deposition velocity. Due to the difficulty in obtaining direct measurements for size-segregated dry deposition on natural water, the published measurements are very limited. The dry deposition velocities from the current model are compared with measurements from Zufall et al. (1998) and that from the chemical mass balance deposition model (CMBD) (Caffrey et al., 1998). Zufall et al. (1998) measured the dry deposition flux over southwestern Lake Michigan using Zefluor Teflon filters coated with dimethylpolysiloxane. Dry deposition velocities for particles are found to range from 0.0062 cms1, for particles with a diameter of 0.75 mm, to 5.4 cms1 for those with a diameter of 24 mm. Caffrey et al. (1998) measured the dry deposition flux and aerosol size distribution concurrently, aboard the RV
100 RH=0 RH=0.99 RH=1
10 1
u=10 ms-1 Ta-Tw=0 Km=Kas
0.1
Kbs=10 cms-1 0.01
(a) Dry deposition velocity, cm sec-1
Dry deposition velocity, cm sec-1
u=2 u=5 u=10 ms-1 u=15 u=20
Dry deposition velocity, cm sec-1
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(b)
0.1 1 10 Dry particle diameter, µm
100
100 RH=0 RH=0.99 RH=1
10 1
u=10 ms-1 Ta-Tw=0 Km=Kas
0.1
Kbs=10 cms-1 0.01
0.1 1 10 Dry particle diameter, µm
100
Fig. 4. Sensitivity tests for dry deposition velocity depending on relative humidity for (a) Williams’ model and (b) the current model.
Lake Guardian 19 km east of the Chicago shoreline. The dry deposition velocities were calculated using the CMBD model for particle size range 0.049–42.7 mm. The dry deposition velocities from these measurements, Williams’ model and the current model are listed in Tables 1 and 2, respectively. The RMS error of Williams’ model and the current model is 0.60 and 0.55 respectively for Zufall’s measurement, and is 0.41 and 0.084, respectively, for Caffrey’s. By comparison, the results of the current model agree well with that of measurement. 3.2. Comparison among dry deposition fluxes from different methods 3.2.1. Dry deposition flux measurement using a surrogate surface The dry deposition flux measured in July 2001 and March 2002 using sub-boiling water is listed in Table 3. The deposition flux of Al and Fe is about two orders of magnitude higher than those of other elements. In floating-dust weather conditions the fluxes of Al, Fe, Mn, Cu, Pb and Zn are 21.9, 12.7, 16.2, 2.5, 3.2 and 3.3 times higher than those in humid weather conditions respectively. We can see that the dry deposition flux in March was higher than
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Table 1 Comparison of deposition velocities measured by Zufall et al. (1998) and the model Particle diameter (mm)
Zufall(1998) experimental(cm s1)
Williams’ model (cm s1)
Current model (cm s1)
0.25 0.80 1.00 2.00 3.00 4.00 5.00 6.00 8.00 9.00 10.00 20.00 24.00
0.040 0.0062 0.070 0.031 0.040 0.10 0.20 0.40 0.70 0.80 1.05 3.60 5.40
0.0090 0.043 0.064 0.29 0.68 0.87 0.98 1.08 1.28 1.40 1.52 3.37 4.45
0.0093 0.038 0.046 0.055 0.030 0.060 0.10 0.17 0.60 0.74 0.87 2.69 3.74
0.60
0.55
RMS error
Table 2 Comparison of deposition velocities measured by Caffrey et al. (1998) and the model Particle diameter (mm)
Caffrey (1998) experimental (cm s1)
Williams’ model (cm s1)
Current model (cm s1)
0.049 0.087 0.16 0.28 0.52 0.95 1.70 4.90 21.20 42.70
0.0054 0.0059 0.0052 0.0043 0.0073 0.018 0.023 0.14 2.67 10.98
0.0058 0.0055 0.0062 0.0091 0.020 0.056 0.16 0.78 3.44 11.66
0.0072 0.0064 0.0068 0.0095 0.020 0.049 0.13 0.16 2.93 11.04
0.41
0.084
RMS error
Table 3 Dry deposition fluxes for July 2001 and March 2002 measured using sub-boiling water as a surrogate surface
Dry deposition flux (mg m2 day1) Dry deposition velocity(cm s1)
Al
Fe
Mn
Cu
Pb
Zn
July March July March
2.97 64.99 4.96 4.36
2.49 31.68 3.66 3.81
0.042 0.68 3.44 3.49
0.024 0.061 3.61 2.51
0.034 0.11 1.22 0.84
that in July, because there were several episodes of strong dust weather conditions in March 2002 and much rainfall in July. The measurement results are consistent with the reported values (Shahin et al., 2000; Paode et al., 1998) except for the special weather conditions in March.
0.084 0.28 1.81 1.66
3.2.2. Dry deposition flux estimate with the GESAMP suggestion The dry deposition of aerosol particles is given as follows: F d ¼ V d C air ,
(4)
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Table 4 Comparison of dry deposition fluxes(mg m2 day1)of July 2001 obtained by different methods
Al Fe Mn Cu Pb Zn
Average content (ng m3)
Experimental fluxes (mg m2 day1)
Modeled fluxes (mg m2 day1)
GESAMP suggested fluxes (mg m2 day1)
693.39 788.17 14.04 7.60 32.50 53.77
2.97 2.49 0.042 0.024 0.034 0.084
3.10 3.52 0.063 0.034 0.14 0.24
0.60 0.68 0.012 0.00066 0.0028 0.0047
0.47
1.33
RMS
Table 5 Comparison of dry deposition fluxes(mg m2 day1)of March 2002 obtained by different methods
Al Fe Mn Cu Pb Zn RMS
Average content (ng m3)
Experimental fluxes (mg m2 day1)
Modeled fluxes (mg m2 day1)
GESAMP suggested fluxes (mg m2 day1)
17266.54 9634.36 226.04 28.30 149.10 193.34
64.99 31.68 0.68 0.061 0.11 0.28
84.53 47.16 1.11 0.14 0.73 0.95 11.16
14.92 8.32 0.20 0.0024 0.013 0.017 24.71
where Fd is the flux of aerosol particles due to dry deposition (mgm2s1), Cair is the concentration of that substance in the near-surface atmosphere (mgm3), Vd is the dry deposition velocity (m s1). Group of Experts on Scientific Aspects of Marine Pollution (GESAMP) (1989) suggested that the best values for the dry deposition velocities of Cu, Pb, Zn and Cd should be 0.1 cm s1, and those for Al, Fe and Mn should be 1.0 cm s1. The calculation results are listed in Tables 4 and 5. 3.2.3. Dry deposition flux estimate with the model We estimate the dry deposition fluxes for Al, Fe, Mn, Cu, Pb and Zn in March 2002 and July 2001 using the dry deposition velocities from the model, with meteorological input provided by past observations (Compiler group, 1984; Yan et al., 1993). First we predict the dry deposition velocities for particles with size range 2–100 mm in March and July using the model. Second we obtain the weighted total deposition velocity using velocities and aerosol particle distribution ratio dr for different size ranges (Zhang and Zhou, 1995) following the formula X vdz ¼ vdi dri . (5) i
Then we calculate the dry deposition fluxes for Al, Fe, Mn, Cu, Pb, Zn with the average concentration of each
metal in March and July. The results are listed in Tables 4 and 5. From these tables, we can see that the modeled fluxes are higher than the experimental ones, the GESAMP suggested values being the lowest, but the modeled values are close to experimental values compared with those of the GESAMP suggested velocities. The possible reason is that the direct measurement of dry deposition fluxes using a surrogate surface has some errors due to the complexity of a natural deposition surface and the dry deposition process. The dry deposition velocities suggested by GESAMP are mainly applicable to a fine mode, resulting in the low values. In fact, the dry deposition process is very complicated and updating the model depends on the accurate measurement of dry deposition velocities. In any case, dry deposition fluxes calculated from the current model agree with experimental values especially for crustal elements, and the model is useful for predicting dry deposition fluxes of an actual region. 3.3. Dry deposition fluxes for the Qingdao area Dry deposition fluxes of six metal elements in the Qingdao area are calculated using average concentrations of each element and dry deposition velocities predicted from the model. Considering a surface area of 1 km2 as the study area along the Qingdao shore, then
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Fe 6000 5000 8000 4000 6000 55.7% 3000 50.2% 4000 26.7% 2000 23.4% 15.6% 14.3% 2000 1000 7.5% 6.5% 0 0 spring summer autumn winter the whole year spring summer autumn winter the whole year Seasons Seasons Mass (kg)
Mass (kg)
Mn 180 160 140 120 100 46.5% 80 27.5% 60 18.2% 40 7.7% 20 0 spring summer autumn winter the whole year Seasons
35 30 25 20 15 10 5 0
Cu
22.6%
32.1% 32.9% 12.4%
spring summer autumn winter the whole year Seasons
Pb Zn 180 160 200 140 120 150 100 40.0% 37.0% 80 27.8% 100 24.7% 25.1% 60 22.9% 40 50 12.0% 10.6% 20 0 0 spring summer autumn winter the whole year spring summer autumn winter the whole year Seasons Seasons Mass (kg)
Mass (kg)
Mass (kg)
Mass (kg)
Al
Fig. 5. Dry deposition fluxes of Al, Fe, Mn, Cu, Pb and Zn for four seasons in the Qingdao area.
the dry deposition fluxes of Al, Fe, Mn, Cu, Pb and Zn for four seasons in the Qingdao area are shown in Fig. 5. The dry deposition masses of Al, Fe, Mn, Cu, Pb and Zn in every square kilometer area of the Qingdao shore for the whole year are 9215.5, 5743.4, 169.2, 34.0, 179.8 and 230.4 kg, respectively. The input masses for crustal elements Al, Fe and Mn are largest in spring, accounting for 46–56% of the whole year, and the masses in summer are the lowest, contributing to up to 10%. As for Cu, Pb and Zn, the deposition masses in autumn and winter are higher, making up 61–65%, whereas the masses are the lowest in summer, contributing less than 13%. Overall, for the crustal and the anthropogenic elements, the lowest contents of the whole year are in summer due to the high rainfall and humidity of Qingdao. Wet deposition considerably reduces the contents of all the metal elements by mechanisms such as washing out and cleaning out by rain. The contents of crustal and anthropogenic elements are much increased in winter, caused by an increase in anthropogenic emission sources such as coal burning for heating. There is a maximum for Fe, Al and Mn in spring, due to the
increase of floating-dust weather, which is influenced by the windy weather occurring in spring at Qingdao and several dust-storms from the northwest. But the values for Cu, Pb and Zn are not the highest in spring, showing that these three anthropogenic elements are less affected by dust-storm and floating-dust weather in spring compared with crustal elements.
4. Conclusion Dry deposition fluxes of the metals were directly measured using sub-boiling water as a surrogate surface in humid (July 2001) and floating-dust (March 2002) weather conditions. The deposition flux of Al and Fe is about two orders of magnitude higher than that of other elements. In floating-dust weather conditions the fluxes of Al, Fe, Mn, Cu, Pb and Zn are 21.9, 12.7, 16.2, 2.5, 3.2 and 3.3 times higher than those in humid weather conditions, respectively. The dry deposition velocity is calculated as a function of particle size using Williams’ model, taking into
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account the particle growth in the humid regions. A comparison of the modeled dry deposition fluxes with measurements and GESAMP suggestions demonstrated that current Williams’ models produced reasonable results. The model is applied to compute dry deposition fluxes in the Qingdao region. The results show that the deposition fluxes of crustal elements Al, Fe and Mn in spring and winter account for more than 70% of the whole year’s deposition, while the deposition fluxes of Cu, Pb and Zn in autumn and winter contribute more than 70% of the whole year’s deposition. Overall, for the crustal and the anthropogenic elements, the lowest contents of the whole year are in summer, due to the high rainfall and humidity of Qingdao. However, due to limited knowledge on particle dry deposition, the current model with empirical and simplified formulae needs further improvements.
Acknowledgments This project was supported by NSFC under Grant no. 49976020. The authors gratefully acknowledge very helpful discussions with Dr. Leiming Zhang of the Meteorological Service of Canada.
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