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Environmental Pollution 155 (2008) 88e98 www.elsevier.com/locate/envpol

Seasonal variation of heavy metals in ambient air and precipitation at a single site in Washington, DC Samuel Melaku*, Vernon Morris, Dharmaraj Raghavan, Charles Hosten Department of Chemistry, Howard University, 525 College Street, NW Washington, DC 20059, USA Received 8 June 2007; received in revised form 23 October 2007; accepted 26 October 2007

High seasonal variability of heavy metals were observed in both ambient air and wet deposition samples. Abstract Atmospheric samples of precipitation and ambient air were collected at a single site in Washington, DC, for 7 months (for ambient air samples) and 1 year (for wet deposition samples) and analyzed for arsenic, cadmium, chromium and lead. The ranges of heavy metal concentrations for 6-day wet deposition samples collected over the 1-year period were 0.20e1.3 mg/l, 0.060e5.1 mg/l, 0.062e4.6 mg/l and 0.11e3.2 mg/l for arsenic, cadmium, chromium and lead, respectively, with a precision better than 5% for more than 95% of the measurements. The ranges of heavy metal concentrations for the 6-day ambient air samples were 0.800e15.7 ng/m3, 1.50e30.0 ng/m3, 16.8e112 ng/m3, and 2.90e137 ng/m3 for arsenic, cadmium, chromium and lead, respectively, with a precision better than 10%. The spread in the heavy metal concentration over the observation period suggests a high seasonal variability for heavy metal content in both ambient air and wet deposition samples. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Wet deposition; Ambient air; Metals

1. Introduction The urban atmosphere is subjected to large inputs of anthropogenic contaminants arising from both stationary (power plants, industries, incinerators, and residential heating) and mobile sources (road traffic) (Bilos et al., 2001; Pacyna, 1984; Sweet and Vermette, 1993; Sullivan and Woods, 2000). According to their physical and chemical properties, these pollutants are partitioned between particulate, liquid, and vapor phases and are subsequently transported to the Earth’s surface through dry and wet deposition (Lawlor and Tipping, 2003). The composition of the air pollutants can be inorganic, organic, or a complex mixture of both. Environmental sources for pollutants could include construction and demolition activities, mining and mineral processing, agricultural activities, * Corresponding author. Tel.: þ1 202 865 8536; fax: þ1 202 806 5442. E-mail address: [email protected] (S. Melaku). 0269-7491/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2007.10.038

sea spray, wind-blown dust, automobiles and transportation related activities on the road. According to the World Health Organization (WHO), 4e8% of deaths occurring annually in the world are related to air pollution associated with anthropogenic activities (Kathuria, 2002; Lopez et al., 2005). Among the many inorganic pollutants originating from anthropogenic activities, heavy metals such as arsenic, cadmium, chromium, and lead, are of a major concern due to their toxic and potentially carcinogenic characteristics. There have been several recent studies of heavy metal deposition in both urban and rural settings (Seung-Muk et al., 2006; Green and Morris, 2006; Michael and Christos, 2006; Zhong et al., 1994). To date, most of these studies have been limited to either wet or dry deposition individually. This study represents a unique combination of sampling of heavy metals in ambient air and wet deposition in the Washington, DC, watershed. The District of Columbia is one of the most densely populated areas in the US (550,521 inhabitants based on the 2005 population estimate). Although, it does not host many

S. Melaku et al. / Environmental Pollution 155 (2008) 88e98

industrial sites, it includes power plants (http://www.npr.org/ templates/story/story.php?storyId¼5673425) and experiences some of the nation’s heaviest road traffic (http://www.washing tonpost.com/wp-dyn/content/article/2005/05/09/AR20050509 00408.html). The District of Columbia consistently ranks among the nation’s cities with the worst air quality (CDC, 1999). Despite increased awareness and interest in air quality in urban zones information regarding the seasonal variation of toxic heavy metals is currently lacking. Recent work has highlighted the utility of high-resolution mapping of heavy metals in airborne aerosols on urban scales in Washington, DC, to understanding impacts on urban public health. For example, the unit risk associated with arsenic, cadmium, chromium, and lead exposure were determined to be 0.0043, 0.0018, 0.001, and 0.000012, respectively (Green and Morris, 2006). This work was limited to ambient air measurements and application of EPA PAM (Particulate Air Matter) particle analyses. In this work, we present results from heavy metal analyses of both ambient air and wet deposition samples collected at a single site with heavy pedestrian traffic, typical of downtown, Washington, DC. The period of observation for wet deposition was from January 2006 to January 2007 and that of ambient air was from July 2006 to January 2007. The primary aim of this study is to investigate the seasonal variation of heavy metals in ambient air and wet deposition samples, to evaluate the content of heavy metals with respect to meteorological parameters such as temperature and amount of precipitation and to compare observed trends in the relationship between concentrations of selected heavy metals (As, Cd, Cr and Pb) in ambient air and wet deposition samples in Washington, DC. 2. Materials and methods 2.1. Site selection and sampling procedures 2.1.1. Site selection The selection of the sampling site was based on several factors including ease of access, safety, minimizing potential for sample contamination, and representativeness. Based on the above criteria, the site chosen was alongside an internal street on the main campus of Howard University in Washington, DC (38 550 16.200 N, 77 010 10.900 W). The map of the study area is shown in Fig. 1. The Total Precipitation Collector, TPC-3000 (Yankee Environmental System, YES), and the home-built cyclone impactor were placed on a concrete platform with the sample intake at roughly 1 m above the surface. The TPC3000 is designed for fully automatic remote operation. It consists of a collection vessel (about 20 l capacity), a motor-operated lid and a precipitation sensor. As soon as the precipitation sensor indicates the beginning of a precipitation event, the sampler is opened by electric motor. The collection vessel is normally kept covered by the lid during periods of non-precipitation. The internal diameter, external diameter and height of the home-built cyclone impactor were 9.2 cm, 10 cm, and 17 cm, respectively. Ambient air samples were isolated by drawing air through a quartz fiber filter which is placed in the home-built cyclone impactor. 2.1.2. Sampling procedure All precipitation samples were collected for 12 months between January 2006 and January 2007, and the ambient air samples were collected for 7 months; July 2006 to January 2007. Wet deposition samples were collected every 6 days during the observation period. It must be noted that in the absence of precipitation during the 6-day collection event, no wet deposition sample

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was recorded and hence no reporting of data. Wet deposition samples were collected immediately after precipitation events. Wet deposition sample containers were cleaned prior to sampling with distilled water and a final rinse with a 1% nitric acid solution to eliminate particle deposition or adsorption onto container walls during prior collections. Ambient air samples were collected every 6 days over the 7-month observation period using a multi-stage cyclone impactor. Particles were collected on a 9-cm diameter glass fiber filter (Fisher Scientific, USA) at a flow rate of 0.126 l/s. The average volume of air sampled over 6 days was 65,318 l. It must be noted that ambient air filter samples were only collected during non-precipitation periods.

2.2. Analytical instrumentation A Perkin-Elmer Model 800 atomic absorption spectrometer with inverse longitudinal Zeeman-effect background correction system was used for quantification of heavy metals. This instrument was equipped with a Perkin-Elmer THGA type graphite oven and an AS-70 autosampler. Measurements were made at wavelengths of 193.7, 228.8, 357.9, and 283.3 nm for arsenic, cadmium, chromium, and lead, respectively, with a slit width of 0.7 nm using a hollow cathode lamp (Cathodeon Ltd, UK).

2.3. Reagents and standard solution Analytical reagent-grade chemicals were used for all procedures, unless stated otherwise. Nitric acid (50e70%) and 37% hydrochloric acid (Fisher Scientific, USA) were used for the digestion of ambient air filter samples. Deionized water was used for washing, dilution, and digestion purposes. Arsenic, cadmium, chromium, and lead calibration standards were prepared by diluting single element atomic absorption standard solutions (Acros Organics, USA) containing 1 mg/ml of metal ions. A mixture of ammonium di-hydrogenphosphate (NH4H2PO4, Aldrich) and magnesium nitrate (Mg(NO3)2, Fisher Scientific) was used as matrix modifier.

2.4. Sample pretreatment Separate sample pretreatment methods were used for the ambient air and wet deposition samples. After wet deposition samples were brought to the laboratory, nitric acid was added to adjust the pH to less than 2.0 and to dissolve most of the particles (Colin et al., 1990; Desboeufs et al., 2001; Sandroni and Migon, 2002). The acidified sample was then preserved in a refrigerator at 4 C until further analysis. Ambient air samples collected on glass fiber filters were wrapped with aluminum foil and sealed in a zip-lock bag immediately after collection and stored at 4 C until the samples were subjected to aqua regia digestion.

2.5. Analysis procedures 2.5.1. Aqua regia digestion The heavy metal content in the certified standard reference materials (SRM 1649a) and in ambient air samples were extracted using aqua regia digestion, following the procedure recommended by International Organization for Standardization, ISO 11466 (Melaku et al., 2005). After a 16-h room temperature digestion, samples were further digested at 130 C, for 2 h under reflux conditions. Finally, the suspensions were filtered with ashless Whatman filter papers (90 mm diameter. Fischer Scientific) and the filtrates were diluted to a final volume of 100 ml with 0.5 M HNO3 prior to further analysis. 2.5.2. GF-AAS measurement The data for calibration curve plot for individual metals were obtained by injecting 20 ml of five standard solutions containing 0e15 mg/l of arsenic, cadmium, chromium, and lead, into the graphite tube and measuring the corresponding absorbances at the four respective wavelengths. This was followed by the determination of the individual heavy metal content in the standard reference material. To work in the linear range of GFAAS, the aqua regia digested SRM sample had to be diluted 100-fold with 1% nitric acid.


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Fig. 1. Map of the study area.

2.6. Optimization of pyrolysis and atomization temperature The temperature programs for the pyrolysis and atomization steps were optimized by performing triplicate measurements using standard solutions of arsenic, cadmium, chromium, and lead in the presence of a matrix modifier (NH4H2PO4 and Mg(NO3)2). This matrix modifier has been shown to enhance absorbances and improve the peak shape of chromium (Stasinakis et al., 2002). The results of the pyrolysis and atomization temperature studies are presented in Tables 1 and 2, respectively. During the pyrolysis studies, the maximum absorbances for arsenic, cadmium, chromium, and lead were observed at 1300 C, 300 C, 1400 C, and 550 C, respectively. Similarly, during the atomization studies the maximum absorbances were recorded at 2100 C, 1400 C, 2200 C, and 1400 C for arsenic, cadmium, chromium, and lead, respectively. These optimized pyrolysis and atomization temperatures were used for all the subsequent sample analyses.

Welz et al. (1988), which is 0.02 mg/l for cadmium in water samples as determined by GF-AAS. However, the LOD value for Cr reported in this study is slightly improved compared to the data reported by Stasinakis et al. (2002), i.e. 0.39 mg/l for Cr using electro-thermal atomic absorption spectrometry (ETAAS). The LOD of lead is lower than those reported by Sun et al. (1997), which is 1.4 mg/l for lead in water as determined by derivative atom trapping flame atomic absorption spectrometry. The difference in the LOD could be due to the difference in sensitivity of the various analytical techniques.

3. Results and discussion 3.1. Validation of the method

The LOD is defined as the concentration corresponding to three times the standard deviation for ten reagent blank determinations. The LODs obtained for individual heavy metals are presented in Table 3. The LOD for arsenic, in wet deposition, is comparable to the value obtained by Zhang et al. (2001), which is 0.02 mg/l for As in natural water as determined by solid sampling atomic-absorption spectrometry. Likewise, the limit of detection for cadmium is comparable to the value obtained by

The method used in heavy metal analysis of ambient air and wet deposition was validated by determination of arsenic, cadmium, chromium, and lead in the Standard Reference Material, SRM 1649a, (Urban Dust), obtained from the National Institute of Standard and Technology (NIST). The results of the validation studies are presented in Table 4. In Table 4, the heavy metal certification as verified by NIST using instrumental neutron activation analysis (INAA) and inductively coupled plasma atomic emission spectroscopy (ICP-AES)

Table 1 Optimization of pyrolysis temperature ( C)

Table 2 Optimization of atomization temperature ( C)

2.7. Limits of detection (LOD)

As Cd Cr Pb

Temperature range

Temperature selected

Maximum absorbance

1000e1400 200e600 1100e1500 450e850

1300 300 1400 550

0.0069 0.1321 0.0639 0.0202

As Cd Cr Pb

Temperature range

Temperature selected

Maximum absorbance

1800e2200 1300e1700 2000e2400 1300e1800

2100 1400 2200 1400

0.0083 0.1107 0.0515 0.0122

S. Melaku et al. / Environmental Pollution 155 (2008) 88e98 Table 3 Limits of detection (LOD) for heavy metals Element GF-AAS analysis (this work) (mg/l)

As Cd Cr Pb

Literature (mg/l)

Ambient air (DD)

Wet deposition (WD)

WD References

0.163 0.131 0.143 0.124

0.015 0.057 0.055 0.059

0.02 0.02 0.39 1.4

Zhang et al., 2001 Welz et al., 1988 Stasinakis et al., 2002 Sun et al., 1997

techniques is also shown. These results show that our analytical procedure and method of detection provided data within an acceptable range of the certified values. For example, for all the heavy metals measured using GF-AAS, in this study, the recovery was better than 85% relative to that reported using other techniques. The precision, expressed as the percent relative standard deviation for the determinations of arsenic cadmium, chromium, and lead in the certified standard reference material by GF-AAS technique was better than 10%, except for cadmium (14.9%). 3.2. Analysis of wet deposition samples The mid-Atlantic region, in particular the Washington, DC, area, experienced below normal rainfall for 6 months of the year, namely February, March, May, July, August, and December. For the remaining 6 months, the average precipitation was higher than the normal monthly precipitation values. Results obtained during the observation period were roughly grouped by season as follows: the winter period was defined as December 15 to March 15, spring period as March 15 to June 15, summer period as June 15 to August 15, and fall period as August 15 to December 15. The total number of wet deposition samples was 50. Sample IDs were defined by the samples type W for wet deposition, followed by the date of collection in DDMMYY format. Results of heavy metals, in wet deposition, for the entire observation period are presented in Table 5. The ranges of total heavy metal concentrations were 0.20e 1.3 mg/l, 0.060e5.1 mg/l, 0.062e4.6 mg/l and 0.11e3.2 mg/l for arsenic, cadmium, chromium and lead, respectively, with a precision better than 5%, for more than 95% of the measurements. A summary of the results is presented in Table 5. The magnitudes of the peak concentration values observed for each heavy metal (especially during summer and fall periods) are significant enough to skew the variances for the entire data set. For this reason, we report only the average deviation Table 4 Results of SRM 1649a (Urban Dust), 95% CI Element

GF-AAS analysis Certified value % RSD % Recovery (this work) (SRM 1649a)

As (mg/kg) Cd (mg/kg) Cr (mg/kg) Pb (% mass fraction)

62.3 6.2 18.9 3.4 210.1 3.5 1.22 0.09

a

67 2 22a 211 6 1.24 0.04

9.7 14.9 1.4 7.4

92.5 86 99.5 98.5

Mean of the results from INAA (18.3 mg/kg) and ICP-AES (26.5 mg/kg).

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and standard deviation for all seasonal periods and not for the full year. A comparison of the 6-day mean temperature versus sample concentration, over the common observation period, showed strong correlations for arsenic, chromium, and lead in wet deposition samples and an anti-correlated behavior for arsenic in dry deposition (Fig. 2). The concentration of chromium and lead in ambient air samples also showed strong correlation with the mean temperature for the samples collected between the beginning of October 2006 and the end of January 2007. In contrast to the other metals, the concentration of cadmium in both dry and wet deposition samples showed no correlation with the mean temperature. We note the presence of another striking feature in each of the time series of the heavy metal concentrations in the precipitation (wet) and ambient air (dry) samples in Fig. 2. One can clearly discern a decrease in heavy metal concentrations between the fifth and fifteenth weeks in the study. Every sample type decreases during this time period. Analysis of the precipitation record reveals that about 50% of the total precipitation events during the 7-month study occurred during this period. These data provide strong evidence that rainfall impacts the heavy metal concentrations of either type of sample. The average concentration of arsenic peaked during the summer period with highest concentration near the end of July (Fig. 3). This maximum value was about four times higher than the mean value of the entire observation period. Sharp transitions in the mean concentration of arsenic were observed from the spring period to the summer period, by 73.5%, and from the summer period to the fall period, by 59.3% (Table 5). The general trends observed for arsenic in this study agree qualitatively with previous observations of arsenic in wet deposition in the eastern shore of Maryland; with maximum values reported during the summer period (Zhong et al., 1994). The seasonal trends observed for chromium and lead in wet deposition samples follow a similar pattern as that of arsenic with a sharp decline in the mean concentration from summer to fall. The mean concentration of chromium increased by 42% from winter to spring and then sharply decreased from summer to fall by 53%. The temporal profiles for chromium and lead show an apparent bimodal concentration profile with peak values in spring and late summer that were factors of five and six times, respectively, greater than the annual mean concentration. Chromium exhibited comparable or more variability than lead throughout the observation period. Although the mean concentration of lead remained essentially constant over the 9 months of the observation period the mean value is not reflective of the individual sample concentrations. High sample-to-sample variability was observed for lead concentrations throughout the entire 12 months. For example, there were only five precipitation events sampled during the 3-month winter period with concentration values ranging from 0.13 mg/l to 3.0 mg/l. Thus, the annual mean is less useful for evaluating deposition to the area watersheds than the seasonal mean because the latter is better able to capture the behavior of the concentrations in episodic precipitation events.


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Table 5 Seasonal averages of heavy metal concentration for ambient air (Air) and wet deposition (WD) samples Element

As Cd Cr Pb

Winter

Spring

Summer

Fall

WD (mg/l)

Air (ng/m3)

WD (mg/l)

Air (ng/m3)

WD (mg/l)

Air (ng/m3)

WD (mg/l)

Air (ng/m3)

0.33 0.22 (0.26) 0.40 0.20 (0.24) 0.69 0.57 (0.78) 1.12 0.70 (1.16)

5.50 0.46 (0.69) 4.61 0.27 (0.31) 60.6 2.1 (2.9) 5.7 1.5 (2.2)

0.34 0.11 (0.13) 0.27 0.26 (0.33) 0.98 0.74 (1.0) 1.41 0.92 (1.07)

e

0.60 0.5 (0.58) 0.32 0.39 (0.47) 1.12 1.0 (1.58) 1.20 0.79 (1.09)

11.1 6.7 (8.3) 6.25 3.53 (4.2) 51.0 20.5 (26.9) 103.4 55.3 (65.5)

0.25 0.16 (0.18) 1.1 1.2 (1.5) 0.52 0.45 (0.70) 0.33 0.25 (0.36)

4.2 2.0 (2.4) 7.1 2.8 (4.26) 50.3 18.3 (20.9) 30.9 19.6 (23.6)

e e e

Annual mean, WD, mg/l

7-Month mean, Air, ng/m3

0.35 0.21

5.7 2.9

0.59 0.68

6.5 2.6

0.80 0.74

52.3 16.7

0.89 0.75

39.5 32.4

Values in parentheses indicates the variance (s2).

100 80

Temp Vs. As (dry)

18 16 14 12 10 8 6 4 2 0

Temp., °C As, wet 3 per. Mov. Avg. (As, wet)

60 40 20 0

As noted above, we report monomodal distributions for arsenic and cadmium and a bimodal distribution for chromium and lead over the 12-month observation period. However, the occurrence of the modes for a particular season appears to be metal-specific. For example, a distinctive maximum for arsenic occurred in summer, for cadmium in fall, and lead

Temperature, °C

Temperature, °C

The concentration of cadmium showed less variability than lead (s2Cd ¼ 0.06 vs s2Pb ¼ 1.08) throughout the 9 months of the year. In the fall we observed cadmium concentrations to increase sharply from a mean value of 0.31 mg/l to nearly 1.1 mg/l (279%). Cadmium exhibited its peak value in fall unlike in summer as observed for all other heavy metals.

100 80 60 40 20 0

60 40 20

100 80

Temperature, °C

1.4 1.2 3 per. Mov. Avg. (As, wet) 1 0.8 0.6 0.4 0.2 0 -0.2 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

100

3 per. Mov. Avg. (Cr, dry)

120 100 80

60

60 40

40

20

20

0

3 40

3 per. Mov. Avg. (Cr,(wet))

2 1.5

60 1 40 0.5

20

0

0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

2

20

1

0

0

100

Temperature Vs. Pb (dry) Temperature, °C Pb, dry

80

3 per. Mov. Avg. (Pb, dry)

60 40 20

160 140 120 100 80 60 40 20 0 -20

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

Temperature, °C

Temperature, °C

80

Temperature, °C Cr, (wet)

5 4

0

0

Temperature Vs. Cr (wet)

3 per. Mov. Avg. (Cd, wet)

60

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

100

6

Temperature, °C Cd, wet

80

Temperature Vs. Cr (dry) Temperature, °C Cr, dry

Temperature Vs. Cd (wet)

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

Temperature, °C

Temperature, °C

80

0

Temperature, °C

Temperature Vs. As (wet)

Temp., °C As, wet

35 30 3 per. Mov. Avg. (Cd, dry) 25 20 15 10 5 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 Temperature, °C Cd, dry

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

100

Temperature Vs. Cd (dry)

100

Temperature Vs. Pb (wet) Temperature, °C Pb, wet

80

3 per. Mov. Avg. (Pb,wet)

60 40 20 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

Fig. 2. Temperature vs. sample concentration.

1.7 1.5 1.3 1.1 0.9 0.7 0.5 0.3 0.1 -0.1

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Arsenic

2 1.5

Mean = 0.35 µg/l

1 0.5 0 W012206 W012806 W020306 W020906 W021506 W022106 W022706 W030506 W031106 W031706 W032306 W032906 W040406 W041006 W041606 W042206 W042806 W050406 W051006 W051606 W052206 W052806 W060306 W060906 W061506 W062106 W062706 W070306 W070906 W071506 W072106 W072706 W080206 W080806 W081406 W082006 W082606 W090106 W090706 W091306 W091906 W092506 W100106 W100706 W101306 W101906 W102506 W103106 W110606 W111206 W111806 W112406 W113006 W120606 W121206 W121806 W122406 W123006 W010506 W011107 W011706 W012307

Conc., µg/l


6 5 4 3 2 1 0 -1

Cadmium Mean = 0.59 µg/l

W012206 W012806 W020306 W020906 W021506 W022106 W022706 W030506 W031106 W031706 W032306 W032906 W040406 W041006 W041606 W042206 W042806 W050406 W051006 W051606 W052206 W052806 W060306 W060906 W061506 W062106 W062706 W070306 W070906 W071506 W072106 W072706 W080206 W080806 W081406 W082006 W082606 W090106 W090706 W091306 W091906 W092506 W100106 W100706 W101306 W101906 W102506 W103106 W110606 W111206 W111806 W112406 W113006 W120606 W121206 W121806 W122406 W123006 W010506 W011107 W011706 W012307

Conc., µg/l

Sample ID

Chromium

5 4 3 2 1 0

Mean = 0.80 µg/l


Conc., µg/l

Sample ID

Lead

3.5 3 2.5 2 1.5 1 0.5 0

Mean = 0.89 µg/l


Conc., µg/l

Sample ID

Sample ID Fig. 3. Trends for heavy metal concentrations in wet deposition (mg/l) samples.

and chromium in the summer. Indeed, correlations between heavy metal concentrations within wet deposition samples showed no significant correlations, with all correlation coefficients being less than 0.13. These underscore the importance of long-term and simultaneous, seasonally resolved monitoring of multiple heavy metals in the urban environment. Applying blanket seasonal averages for all heavy metals may tend to mask out distinctions between intra-seasonal behaviors of individual heavy metals. In the Washington, DC, metropolitan region, the presence of As in wet deposition is likely due to its use in the Maryland timber industry and from nearby coal-fired power plants (Alexandria, VA and Capitol Hill, District of Columbia) (Pacyna and Graedel, 1995). Other minor sources may include regional agricultural practices and waste incinerators. The observation of greater arsenic concentrations in wet deposition during the summer months may be a result of higher

consumption of coal to support energy requirements for cooling and greater precipitation events during this period (http:// www.erh.noaa.gov/lwx/). However, it is important not to rule out the possibility of the precipitation pattern biasing this result. Ongoing studies will further investigate the interplay between precipitation amount and source variability. One of the most important sources of cadmium, chromium and lead in the urban environment is road traffic (Zhong et al., 1994; Kowalczyk et al., 1982; Ayras and Kashulina, 2000; Sweet et al., 1998; Chandler, 1996; Dianne and Baker, 1994). Other contributors include waste incinerators, coal fired power plants, geogenic dust, and construction debris (Ayras and Kashulina, 2000; Chandler, 1996). The observed bimodal distribution and high sample-tosample variability for chromium is reflective of multiple sources whose contributions vary throughout the calendar year. The summer peak value is most likely due to a combination


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of elevated power plant energy consumption and construction debris (Dianne and Baker, 1994). We did not observe the pattern of cadmium concentrations in wet deposition to be consistent with the limited traffic or with any of the other heavy metals characterized in this study. We did observe significantly higher concentrations of cadmium in wet deposition during the fall months than in any other season. These peak values are consistent with a similar elevation in cadmium concentrations reported in ambient air in both this study and in an air deposition study conducted in eastern Maryland (Zhong et al., 1994). It is interesting to note that the peak values in both studies happen during the fall season. At present, we are not able to specify the source of cadmium but it appears that its origin is not closely correlated to the sources of the other metals (As, Cr, and Pb) in this study. Further studies are needed to confirm this phenomenon and identify the source of cadmium. The concentration of heavy metals in the wet deposition has been observed in various climate regions to be related to the amount of precipitation. Recent work (http://www.erh.noaa. gov/lwx/; Azimi et al., 2005) has reported a direct relation between the amount of precipitation and the heavy metal concentration in the wet deposition. In general, a greater volume per precipitation event resulted in higher concentration of heavy metals in wet deposition. There were a total of 50 precipitation events during the observation period: 29 events below 12.7 mm of rain; 21, 14, and 10 events above 12.7, 25.4, 38.1 mm of rain, respectively. In this study, we observe that when the amount of precipitation exceeds a threshold value of >38.1 mm of rain over the 6-day observation period (Table 6), very strong correlations exist between the heavy metal concentration in wet deposition and the amount of precipitation with a correlation coefficients of 0.56 (for As) and 0.61 (for Cd). However, the peak values of heavy metal concentration in any given seasonal period did not correspond directly to a peak value in precipitation. The number of events meeting or exceeding the threshold represented about 20% of the total number of precipitation events during the observation period. The precipitation (rainfall) during the defined summer and fall periods were well above the climatological precipitation amounts by 80% and 60%, respectively, while the mean precipitation for the winter and the fall periods fell below the normal. In general, the measured heavy metal concentrations in wet deposition were found to be in exceedance of the recommended guideline values of national and international regulatory bodies. For example, the guideline value for lead is 0.0015 mg/l

based on the National Ambient Air Quality Standards (US EPA, 1997) and for cadmium is 0.000005 mg/l and for As is 0.001 mg/l, based on the World Health Organization recommendations (WHO, 2000). This suggests that heavy rainfall combined with significant atmospheric loading of certain heavy metals (cadmium and arsenic in this study) can lead to significant and potentially hazardous enhancements in heavy metals in the run-off to the local watershed. When the heavy metal concentration results are compared with similar measurements in wet precipitation in the Kaynaklar Campus of the Dokuz Eylul University in a suburban area away from specific local sources of heavy metals (Muezzinoglu and Cizmecioglu, 2006), the lead concentration in Washington, DC, in all seasons, was found to be rather low by a factor of 3. However, the concentration of cadmium in this study was found to be higher than the value reported by Takeda et al. (2000) for fall and spring seasons in Higashi, Hiroshima, Japan. 3.3. Analysis of ambient air samples Fig. 4 shows the heavy metal concentrations of arsenic, cadmium, chromium, and lead, in the ambient air samples collected over a period of 7 months. The total number of ambient air samples was 22. Concentrations ranged from 0.800 to 15.7 ng/m3 for arsenic, 1.50e30.0 ng/m3 for cadmium, 16.8e 112 ng/m3 for chromium, and 2.90e137 ng/m3 for lead. As observed for the wet deposition, the peak concentrations of arsenic occurred in summer and the minimum values were observed in early fall. The highest single concentration was observed in July 2006, at about a factor of three times the 7-month mean value. However, the average summer concentration even without this large value was an average of 56% greater than the winter, spring, and summer means. Similar to arsenic, we observed a singular event peak value for Cd that was five times higher than the 7-month mean concentration. We note that this peak concentration of cadmium occurred during the fall period. A similar peak value was observed in our deposition studies. This observation is in agreement with the Chesapeake Bay study by Zhong et al. (1994) for dry deposition samples. Obviously, this may simply be a case of serendipity but it warrants further attention in follow-up work. The annual mean concentration of Cr is the highest of the four metals tested in this study. It also consistently exhibits one of the greater mean variances from season to season than the other heavy metals over the full observation period.

Table 6 Correlation between the amount of precipitation and heavy metal concentration in wet deposition Amount of precipitation, mm of rain, over the 6-day observation period

As (r2), 0.005 < p < 0.1

Cd (r2), 0.10 < p < 0.40

Cr (r2), 0.05 < p < 0.30

Pb (r2), 0.05 < p < 0.30

>12.7 (21)a >25.4 (14)a >38.1 (10)a

0.31 0.47 0.56

0.11 0.12 0.61

0.03 0.08 0.04

0.04 0.06 0.10

a

Number of precipitation events.

95

Arsenic

20

Mean = 5.7 ng/m3

15 10 5

D020207

D012707

D012107

D011506

D010906

D010307

D122806

D122206

D121606

D121006

D120406

D112806

D112006

D111406

D110806

D110206

D102706

D102106

D101506

D100906

D100306

D092706

D092106

D091506

D090906

D090306

D082806

D082206

D081606

D081006

D080406

D072906

0 D072306

Conc., ng/m3


Cadmium

35 30 25 20 15 10 5 0

D020207

D012707

D012107

D011506

D010906

D010307

D122806

D122206

D121606

D121006

D120406

D112806

D112006

D111406

D110806

D110206

D102706

D102106

D101506

D100906

D100306

D092706

D092106

D091506

D090906

D090306

D082806

D082206

D081606

D081006

D080406

D072306

Mean = 6.5 ng/m3

D072906

Conc., ng/m3

Sample ID

D012107

D012707

D020207

D012107

D012707

D020207

D011506

D010906

D010307

D122806

D122206

D121606

D121006

D120406

D112806

D112006

Mean = 52.3 ng/m3

D111406

D110806

D110206

D102706

D101506

D100906

D100306

D092706

D092106

D091506

D090906

D090306

D082806

D082206

D081606

D081006

D080406

D072906

D102106

Chromium

120 100 80 60 40 20 0 D072306

Conc., ng/m3

Sample ID

Lead

160 140 120 100 80 60 40 20 0

D011506

D010906

D010307

D122806

D122206

D121606

D121006

D120406

D112806

D112006

D111406

D110806

D110206

D102706

D102106

D101506

D100906

D100306

D092706

D092106

D091506

D090906

D090306

D082806

D082206

D081606

D081006

D080406

D072906

Mean = 39.5 ng/m3

D072306

Conc., ng/m3

Sample ID

Sample ID Fig. 4. Trend of heavy metal concentrations in ambient air (ng/m3) samples.

However, on closer inspection one can discern a monomodal distribution with peak values in the early fall. In addition, we observe a significant correlation between cadmium and chromium concentrations in ambient air samples (r2 ¼ 0.64, p ¼ 0.005) over the entire observation period (Fig. 4) and a slightly stronger correlation for the period JulyeDecember (r2 ¼ 0.68, p ¼ 0.001). The chromium concentration is also observed to exhibit a peak value coincident with that observed for cadmium. This is suggestive of a common source or dependence of cadmium and chromium in ambient air. The variability observed in the wet deposition may be reflective of the solubility or the metal speciation in the urban airshed rather than source strength variability. Arsenic and lead follow a similar trend from summer to fall with each exhibiting a peak value in July but they differ

slightly in winter. In the winter period, the concentration of lead showed a declining trend whereas the arsenic concentration remains relatively constant. The variability in the ambient air concentration data makes it difficult to identify clear distinctions in seasonal variation over the shortened observation period. However, our overall means indicate a slight enhancement in the summer and fall concentrations relative to the other observation periods for three of the four metals (Cd, Cr, and Pb). This seasonal variation of heavy metals is partially confounded by combination of increased emissions from power plants corresponding to peak air conditioning usage combined with low precipitation scavenging, and differences in transport times (private communication). Statistical analysis on the ambient air samples showed a strong correlation between arsenic and lead from summer


96

Correlation between Cr and Pb in ambient air

150

150

y =0.2973x + 15.519 R2 =0.0723

100

Correlation between Cd and Pb in ambient air y =2.5673x + 14.197 R2 =0.1583

100 50

50

0

0 0

20

40

60

80

100

5

0

120

Correlation between Cd and Cr in ambient air

200 100

15

20

25

30

35

Correlation between As and Cr in ambient air

150

y =4.6682x + 14.082 R2 =0.6397

150

10

y =4.3171x + 19.902 R2 =0.2696

100 50

50

0

0 0

5

10

15

20

25

30

Correlation between As and Cd in ambient air

40

10

15

20

Correlation between As and Pb in ambient air

100

y =0.6086x + 2.4174 R2 =0.1825

10

5

150

30 20

0

35

y =7.2624x - 0.7002 R2 =0.6245

50 0

0 0

5

10

15

20

0

5

10

15

20

Fig. 5. Correlation of heavy metal concentrations in ambient air: number of samples (n ¼ 22; 0.001 < p < 0.05).

Correlation of As in dry and wet

20

y = 7.6799x + 3.5187 R2 = 0.6224

15 10 5 0 0

0.2

0.4

0.6

0.8

1

1.2

1.4

ambient air samples were found to be much lower than the guideline values set forth by the World Health Organization and the National Ambient Air Quality Standards (US EPA, 1997; WHO, 2000) and represent less risk to human health and the environment. In contrast, the cadmium concentration was found to be higher than the air quality standards. This implies a negative effect on human and environmental health. 3.4. Comparison between ambient air and wet deposition of heavy metals The heavy metal concentrations of wet deposition and ambient air samples were compared over the common observation period. This study supports other reported observations that precipitation events can strongly influence variability in Conc. (dry), ng/m3

Conc. (dry), ng/m3

to fall (r2 ¼ 0.73, p ¼ 0.001) compared to that of summer through winter (r2 ¼ 0.61, p ¼ 0.005) (Fig. 5). This correlation indicates the possibility of a common set of emission sources and/or similar sequestering in the atmosphere before they were brought down by dry deposition. The concentrations of the four heavy metals, in the ambient air samples, were not strongly correlated, except for arsenic and lead (r2 ¼ 0.61, p ¼ 0.001) and also cadmium and chromium (r2 ¼ 0.64, p ¼ 0.001). The distribution of heavy metals in this study for similar seasons is qualitatively consistent with the findings for heavy metal concentrations in dry deposition PM2.5 for the fall and summer 2003 by Green and Morris (2006). Both studies, point to the presence of lead in abundant amount in air deposit during fall and summer seasons. The content of arsenic and lead in the

Correlation of Cd in dry and wet

35 30 25 20 15 10 5 0

y = 1.2926x + 8.6878 R2 = 0.0766

0

1

2

y = -4.4185x + 61.463 R2 = 0.0312

0

1

2

3

Conc. (wet), µg/l

4

5

Conc. (dry), ng/m3

Conc. (dry), ng/m3

Correlation of Cr in dry and wet

120 100 80 60 40 20 0

3

4

5

6

Conc. (wet), µg/l

Conc. (wet), µg/l

Correlation of Pb in dry and wet

160 140 120 100 80 60 40 20 0

y = 13.727x + 35.675 R2 = 0.1089

0

0.5

1

1.5

2

2.5

3

3.5

Conc. (wet), µg/l

Fig. 6. Correlation of heavy metal concentrations in dry and wet samples (As, p ¼ 0.001; Cd, p ¼ 0.005; Cr, p ¼ 0.000; Pb, p ¼ 0.05).


concentrations of both wet and dry samples. The heavy metal concentrations in the wet deposition, in this study, were much higher than the concentrations observed for the ambient air samples, in agreement with the results reported by Muezzinoglu and Cizmecioglu (2006), despite the difference in sampling methodology. Correlation analyses between the heavy metal concentrations in the wet and dry samples was performed in order to determine if there was any discernible statistical relationship between the two types of samples. The only statistically significant relationship that has been reported for ambient air and wet deposition samples was observed between content of As with a correlation coefficient (r2) of 0.62, p ¼ 0.001. On the other hand cadmium, chromium, and lead for the same season showed weaker correlation which is 0.08, 0.03 and 0.11, respectively (Fig. 6). 4. Conclusions We have reported long-term measurements of four heavy metals in wet deposition and ambient air samples collected in an urban site in Washington, DC. The heavy metal concentrations in ambient air and in wet deposition sample showed the following trends Cr z Pb > Cd > As and Pb > Cr > As > Cd, respectively. The peak values in the concentrations of the heavy metals were typically observed during the summer months, except for cadmium (in ambient air and wet) and chromium (in ambient air), regardless of the sample type. When the heavy metals data were compared to meteorological variables temperature and precipitation, the results indicated a strong dependence on mean temperature and amount of precipitation. The concentration of the four heavy metals in wet deposition were not strongly correlated to each other, but the ambient air samples showed a strong correlation between arsenic and lead (r2 ¼ 0.61, p ¼ 0.005) and also cadmium and chromium (r2 ¼ 0.64, p ¼ 0.005). Except in the case of As, no statistically significant relationship between precipitation amount and concentration of heavy metal was observed. However, the concentration of arsenic and cadmium in wet deposition samples showed a strong correlation with the amount of precipitation once the amount of rain exceeded a threshold value of 38.1 mm of rain, with a correlation coefficient of 0.56 (for As) and 0.61 (for Cd). Several of our observations also support the need for extended observations and analysis of these heavy metals in the urban environment. The spread in the heavy metal concentration over the observation period suggest a high seasonal variability for heavy metal content in both ambient air and wet deposition samples. Acknowledgements We would like to thank the District of Columbia Water and Sewer Authority, Contract No. 05g-05-WQD04 and the NOAA EPP Program via Cooperative Agreement NA17AE1623 for financial support.

97

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