Urban Dew: Composition and Influence on Dry Deposition Rates

5

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 31, 2018 | https://pubs.acs.org Publication Date: September 25, 1986 | doi: 10.1021/bk-1986-0318.ch005

Urban Dew: Composition and Influence on Dry Deposition Rates Patricia A. Mulawa, Steven H . Cadle, Frank Lipari, Carolina C. Ang, and René T. Vandervennet Environmental Science Department, General Motors Research Laboratories, Warren, MI 48090-9055

The composition of dew collected from a Teflon surface was compared to summer rainwater concentrations at a site in Warren, Michigan. This comparison showed that natural dew is similar to rainwater with the exception that dew has much higher concentrations of Ca and Cl and much lower acidity. Dry deposition rates of several species were measured to artificially-generated dew and a dry surface. It was found that deposition rates were 2 to 20 times greater to the artificial dew than to the dry surface indicating that the presence of dew enhances both the retention of dry deposited particles and the absorption of water soluble gases. Measurement of the atmospheric concentrations of the depositing species permitted the calculation of deposition velocities for particulate Cl , NO-3, SO-24, Ca , Mg , Na , andNH+4.Deposition velocities for gaseous HNO , HCl,SO andNH were also determined after correction for particle deposition. These results indicate that acid dew is not a problem at this site. However, the ability of dew to increase the deposition rate of acids and acid precursors to some surfaces suggests that dew may be more acidic at sites with lower deposition rates of basic particles. +2

-

-

+2

+2

+

3

2

3

Recently, interest in acid deposition has broadened to include s p e c i a l a c i d i c events such as dew, f r o s t , and fog. L i t t l e i s known about the frequency with which a c i d i c dew occurs, i t s composition, or i t s effect on dry deposition rates. However researchers have long recognized that surface wetness contributes to the corrosion of metal surfaces O ) and to the deterioration of stonework (2). 0097-6156/86/0318-0092S06.00/ 0 © 1986 American Chemical Society

Baboian; Materials Degradation Caused by Acid Rain ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

5.

MULAWA ET A L .

93

Urban Dew

Additionally, acid dew may also be involved in plant e f f e c t s since i t has been reported that acid r a i n can damage protective surfaces on leaves, i n t e r f e r e with guard c e l l s , and poison plant c e l l s (3). A few studies on the composition of dew have been reported. Yaalon and Ganor (4), Brimbleeombe and Todd (5), Anderson and Landsberg (6) and Smith and Friedman (7) c o l l e c t e d dew from a variety of surfaces and report median pH values in the range of 5.7 to 7.7. Wisniewski (8) reviewed the sparse acid dew l i t e r a t u r e and calculated that dew could have a pH as low as 2 based s o l e l y on the oxidation of a l l deposited S0 to ^SOjj and no subsequent n e u t r a l i z a t i o n . Recently Pierson et a l . (9) found the pH of dew samples from Alleghany Mountain in Pennsylvania ranged from 3.5 to 5.3 with a volume weighted average of 4.0.


2

Cadle and G r o b l i c k i (JO) determined the composition of dew deposited naturally on glass, Teflon, and p l a s t i c surfaces in Warren, MI. Dew composition was compared to wet and dry deposition obtained the previous year at the same s i t e . In this paper, the comparison of dew and r a i n composition i s updated and the r e s u l t s of a new study of the composition of a r t i f i c i a l l y - g e n e r a t e d dew are reported. Deposi tion v e l o c i t i e s to the dew of S0 , HNO3, HC1, NH^, C a , Mg"", Na , and K are also presented. + 2

1

2

+

2

+

Experimental S i t e . Samples were collected from a s i t e located on a 330 acre parcel of undeveloped land in Warren, MI, a suburb north of Detroit. Most of the surrounding area i s highly developed. A major surface street 300 m south of the s i t e has a t r a f f i c flow of 20 000 vehicles/day. Another s t r e e t , 800 m east of the s i t e has a t r a f f i c flow of 38 000 vehicles/day. Annual emissions of Ν 0 , S0 , and TSP for the surrounding area have been presented elsewhere (11). χ

2

Dew C o l l e c t i o n . In our previous work, natural dew was c o l l e c t e d from a Teflon surface. The Teflon c o l l e c t o r consisted of a sheet of aluminum backed FEP Teflon bonded to a 1 m copper plate mounted on a plywood base. The c o l l e c t o r was t i l t e d 30° from horizontal with the centerpoint 1 m above the ground. In t h i s work the intent was to perform a more comprehensive analysis of the dew and to compare deposition v e l o c i t i e s to a wet and a dry surface. In order to bring more control to the experiment, dew was generated a r t i f i c i a l l y by attaching cooling c o i l s to the Teflon covered copper plate. The dry plate counterpart consisted of a 1 m glass c o l l e c t o r covered with Teflon and mounted in the same manner as the copper plate. A l l deposition rates are based on the actual area of the plate rather than the projected horizontal area, which was 0.87 m. 2

2

2

The natural dew c o l l e c t i o n procedure used previously consisted of washing the c o l l e c t o r with deionized water l a t e in the afternoon. Dew was collected the following morning at approximately 7:00 a.m.



94

MATERIALS DEGRADATION CAUSED BY ACID RAIN

The average elapsed time between cleaning and dew c o l l e c t i o n was 17 h. Since dew was not present the e n t i r e time, the dew concentra tions reported below include some material which was deposited on the dry c o l l e c t i o n plate and subsequently dissolved i n the dew. The samples were f i l t e r e d through 0.2 ym Gelman Acrodisc f i l t e r s prior to determining pH. The samples were then refrigerated u n t i l the remain ing analyses could be performed. Generation of a r t i f i c i a l dew started at approximately 7:30 a.m. and lasted for periods of 1.5 to 4.5 h. The average generation time was 3.1 h. A r t i f i c i a l dew generation was done only during periods with wind speeds less than 2 mph. Higher wind speeds limited our a b i l i t y to generate dew. Also, this procedure minimized turbulent mixing and thus approached more r e a l i s t i c nighttime deposition rates. The c o l l e c t o r was washed with deionized water immediately before use. Samples were processed i n the same manner as the natural dew samples. The dry c o l l e c t i o n plate was likewise cleaned with deionized water immediately prior to the onset of a r t i f i c i a l dew formation. Thus, the two c o l l e c t i o n plates were exposed for essen t i a l l y the same time periods. At the end o f an experiment the a r t i f i c i a l l y - g e n e r a t e d dew was collected and the dry plate was misted twice with deionized water. This wash water was c o l l e c t e d i n the same manner as dew samples. Contact time between the water and the plate was approximately 5 minutes. Atmospheric Concentrations. Atmospheric concentrations of the major depositing species were determined during the a r t i f i c i a l dew forma tion period. The species measured were NO, N 0 , 0o, HNOo, S 0 , HC1, NH3, and p a r t i c u l a t e NO3, SOij , CI", NH]J, C a , Mg* , Na , and K . NO, N 0 , and 0^ were monitored continuously with a Monitor Labs dual channel chemiluminescence Ν 0 analyzer and an AID portable ozone analyzer. S 0 was determined with the carbonate-glycerol impregnated f i l t e r technique. HNO^ was determined by the dénuder difference method {VZ). HC1 was determined with the Na C0o impregnated f i l t e r technique (J_3). The o x a l i c acid impregnated f i l t e r method was used to determine NHg. Total p a r t i c u l a t e was collected on 1-ym pore s i z e Ghia Tefweb f i l t e r s . Aqueous extracts of the f i l t e r s were analyzed to determine p a r t i c u l a t e NO3, SOjJ , CI", NH]J, C a , Mg , Na , and K concentrations. 2

2

+ 2

2

2

+

2

χ

2

2

2

+

+ 2

+2

+

2

Analysis. S O ^ » SOjj , NO^, and CI" were measured on a Dionex 2110i ion chromatograph. SO^ responses were attributed to the c o l l e c t i v e presence o f S(IV) species, without any attempt to i d e n t i f y the s p e c i f i c form, and hereafter w i l l be referred to as S(IV). Ca , Mg , Na , and K were determined on a Perkin-Elmer atomic absorption spectrometer. H concentrations were calculated from pH measurements obtained using an Orion combination electrode and pH meter. NHjJ was determined using a modified indophenol blue method on either a Technicon Autoanalyzer or a Lachat flow i n j e c t i o n analyzer. 2

+ 2

+2

+

+

+


Urban Dew

5. M U L A W A E T A L .

95

Results and Discussion


Natural dew samples were c o l l e c t e d between June 14, 1982 and October 14, 1982. Dew and rain event frequencies were recorded for 74 days. Dew occurred on 61? of the days and rain occurred on 15? of the days. A r t i f i c i a l dew samples were generated between June 21, 1984 and August 17, 1984. Dew and rain event frequencies were not recorded. However, t h i s period was t y p i c a l of a normal summer for the area. Dew, Rainwater and Dry Plate Wash Composition. Table I contains the average concentrations of measured ions in natural dew, a r t i f i c i a l l y generated dew and dry plate washes. Also included are the volume weighted average summer rainwater concentrations of these same species for the period June 1981 through July 1983 (J_5). Table I.

Species Ca+ Mg

2

+2

K+

Average Dew,

31 ±

±

CI"

106 ±

252

NO3

166 ±

282

SOiJ

2

242 ±

1.5

19 ±

29

9.5

5.1

37 ±

75

52 ±

39

40

52 ±

38

ND

50 ±

57

96

112 ±

69

6.7

312

±

4.4

±

6.5

18 ± 10 6.9

140 ± 108

31

ND*

S(IV)

62 ± 43

16

3 6

Average Dry Plate Cone. yeq/L

26 ±

6.7

26

Average A r t i f i c i a l Dew Concentration yeq/L

6.9

11

65 ±

NHj

Concentrations

155 ± 121

24

690 ± 935

20 ±

Na

Average Rainwater Concentration yeq/L

Average Natural Dew Concentration yeq/L

4.1 +

Rainwater and Dry Plate

±

6.6

24 ± 17 17 ± 13 8.2

± 5

16 ± 12

* not determined The pH of natural dew samples ranged from 3.62 to 8.20 with a median of 6.5 (0.3 yeq H /L). The average natural dew volume was 82.4 mL. The average pH of the rainwater samples was 4.14 (73 yeq H"7L). Natural dew concentrations of NH]j, Mg , Na , K , NO3, and SOjJ were 2 to 4.6 times higher than t h e i r concentration in r a i n water, while C a and C l ~ were 29 and 16 times more concentrated, respectively, in the dew. Ca concentrations are elevated because Ca i s present in predominately large p a r t i c l e s at t h i s s i t e (16) and large amounts of dry deposited C a are incorporated into the dew. Some large p a r t i c l e chloride i s also present at t h i s s i t e during the summer which may account for the higher dew chloride concentrations. S(IV) and N0 were frequently present in natural dew +

+2

+

+

2

+ 2

+ 2

+ 2

2


96


samples. S e m i - q u a n t i t a t i v e a n a l y s i s by i o n chromatography i n d i c a t e d t h a t S ( I V ) s p e c i e s can be a major component i n t h e dew a t t h i s s i t e , but t h a t Ν 0 was a minor component. A r t i f i c i a l dew and d r y p l a t e washes were c o l l e c t e d on 2 4 morn i n g s . The average dew volume was 8 2 mL and t h e average d r y p l a t e wash volume was 7 6 mL. The pH o f a r t i f i c i a l dew ranged from 4 . 4 2 t o 8 . 1 6 w i t h a median o f 5 . 3 0 ( 5 . 0 yeq H " V L ) . The pH o f d r y p l a t e washes ranged from 5 . 2 8 t o 9 . 0 4 w i t h a median o f 6 . 3 ( 0 . 5 yeq H / L ) . Thus t h e a r t i f i c i a l dew samples were more a c i d i c than e i t h e r t h e n a t u r a l dew o r t h e d r y p l a t e washes. I n s p e c t i o n o f T a b l e I shows t h a t t h e h i g h e r a c i d i t y o f t h e a r t i f i c i a l dew samples can be e x p l a i n e d by t h e r e l a t i v e l y lower C a concentrations i n the a r t i f i c i a l dew. The average r a t i o s o f e q u i v a l e n t s o f C a to the equiv a l e n t s o f SOJj i n a r t i f i c i a l dew, n a t u r a l dew, and d r y p l a t e samples were 1 . 5 9 , 3 . 5 5 , and 4 . 6 6 , r e s p e c t i v e l y . 2


+

+ 2

+ 2

2

Average a r t i f i c i a l dew c o n c e n t r a t i o n s were 2 t o 2 0 t i m e s h i g h e r than average d r y p l a t e c o n c e n t r a t i o n s f o r t h e measured i o n s . The l a r g e s t d i f f e r e n c e s appear f o r NHjJ, SOJJ , and S ( I V ) . There a r e two e x p l a n a t i o n s f o r these d i f f e r e n c e s . The most l i k e l y i s t h a t t h e p r e s e n c e o f m o i s t u r e on t h e s u r f a c e o f t h e c o l l e c t o r i n c r e a s e s t h e r e t e n t i o n o f d r y d e p o s i t e d p a r t i c l e s and enhances t h e c a p t u r e o f gases. T h i s p o s s i b i l i t y w i l l be e x p l o r e d i n g r e a t e r d e t a i l below. The second p o s s i b i l i t y i s t h a t t h e d r y d e p o s i t e d m a t e r i a l on t h e d r y p l a t e was n o t c o m p l e t e l y d i s s o l v e d by t h e wash p r o c e d u r e which c o n s i s t e d o f m i s t i n g t w i c e w i t h a s p r a y o f f i n e d r o p l e t s and p h y s i c a l l y sweeping t h e water around t h e p l a t e d u r i n g c o l l e c t i o n . We do not b e l i e v e t h a t i n c o m p l e t e d i s s o l u t i o n was a problem s i n c e c o l l e c t i o n o f second washes were v e r y c l e a n . 2

D e p o s i t i o n Rates. D e p o s i t i o n r a t e s which were c a l c u l a t e d from t h e sample c o n c e n t r a t i o n s , volumes, and t h e sample times a r e t a b u l a t e d i n T a b l e I I . D e p o s i t i o n r a t e s t o t h e a r t i f i c i a l dew samples were 1 . 4 t o 7 . 2 times g r e a t e r than t h e r a t e s t o t h e n a t u r a l dew samples f o r a l l s p e c i e s e x c e p t K and Wa . The K and N a were d e p o s i t e d a t r a t e s 1 9 and 1 6 t i m e s g r e a t e r t o t h e a r t i f i c i a l dew than t o t h e n a t u r a l dew, r e s p e c t i v e l y . There i s c o n s i d e r a b l e u n c e r t a i n t y i n t h e s e v a l u e s because t h e r e were r e l a t i v e l y few measurements o f t h e s e s p e c i e s made i n t h e n a t u r a l dew samples and because t h e v a l u e s were f r e q u e n t l y c l o s e t o the blank c o n c e n t r a t i o n s . Three f a c t o r s c o n t r i b u t e t o t h e h i g h e r d e p o s i t i o n r a t e s t o t h e a r t i f i c i a l dew samples. F i r s t , t h e a r t i f i c i a l dew samples were wet t h e e n t i r e sampling time whereas t h e n a t u r a l dew samples were wet o n l y p a r t o f t h e average 1 7 hour c o l l e c t i o n p e r i o d . F o r many s p e c i e s a wet s u r f a c e i s e x p e c t e d t o be a b e t t e r c o l l e c t o r than t h e d r y s u r f a c e . F o r example, Dasch ( j _ 7 ) found t h a t a l l t h e s e s p e c i e s had h i g h e r d e p o s i t i o n r a t e s t o water than t o a Teflon f i l t e r . T h e r e f o r e , t h e d e p o s i t i o n r a t e s t o n a t u r a l dew r e p o r t e d i n T a b l e I I s h o u l d be taken a s l o w e r l i m i t s . Second, meteorological conditions i n f l u e n c e the d e p o s i t i o n r a t e s . Greater a t m o s p h e r i c t u r b u l e n c e i n t h e morning compared t o t h e n i g h t would +

+

+

+


Table I I .

Species Ca

+ 2

2

Mg+ K

+

Average Deposition Rates

Nat. Dew Deposition Rate yeq/m /h

Number of Samples

Art. Dew Deposition Rate yeq/m /h

Number of Samples

2

Number of Samples

2

Dry P l a t e Deposition Rate yeq/m /h 2

2.06

± 1.09

24

2.87

± 1.65

24

1.64

9

0.14

± 0.12

24

0.54

± 0.31

24

0.17

± 0.13

± 0.02

24

0.32

± 0.26

24

0.25

± 0.18

0.62

± 0.66

24

0.47

± 0.34

24

0.20

± 0.24

31

± 1.34

9

0.02

9

0.04

± 0.05

24

NHJ

14

0.34

± 0.21

24

2.45

± 1.31

CI"

39

0.23

± 0.23

24

0.86

± 0.44

24

0.69

± 0.62

N 0

39

0.45

± 0.33

24

1.06

± 0.64

24

0.46

± 0.50

ND*

23

0.92

± 0.83

5

0.19

± 0.13

± 0.88

24

2.26

± 1.28

24

0.45

± 0.47

Na


97

Urban Dew

MULAWAETAL.

5.

+

3

S(IV)

so;j

0

2

* not

39

0.82

determined

increase the deposition rates to the a r t i f i c i a l dew samples. Third, diurnal variations i n atmospheric concentrations have not been taken into account. For example, since the a r t i f i c i a l dew samples were generated during the morning rush hour they were exposed to higher average Ν 0 l e v e l s than the natural dew samples. The greatest differences between the deposition rates to the 2 a r t i f i c i a l dew and to the dry plate were for NHjJ, S(IV), and SOjj . The r a t i o s o f the a r t i f i c i a l dew and the dry plate rates for these species were 12.3, 4.8, and 5.0, respectively. This difference i n rates i s most l i k e l y due to the enhanced retention of water soluble gases. Therefore, these r e s u l t s suggest that dry deposited NH^ and are important contributors to the dew composition. As noted above, soluble oxidant gases such as 0^ and f^C^ may also play an important role in determining the concentration of SOjj . χ

2

Deposition V e l o c i t i e s . Deposition v e l o c i t i e s , v ' s , were calculated from the a r t i f i c i a l dew deposition rates discussed above and the atmospheric concentrations of the depositing species measured during dew generation. The average atmospheric concentrations and t h e i r ranges appear in Table I I I . Average v ' s are shown i n Table IV. The c a l c u l a t i o n of V 's for Ca"*", M g , Na , and K i s straightforward since there i s only one dry deposition source for these speciesatmospheric p a r t i c l e s . The average v ' s to the Teflon plate were 0.69, 0.33, 0.23 and 0.15 cm/s for K , Na , C a , and Mg , respec t i v e l y . V 's to the dew were higher with average values of 0.88, 0.42, 0.46 and 0.41 cm/s for K , Na , C a and Mg , respectively. Since p a r t i c l e s i z e d i s t r i b u t i o n s were not measured during t h i s study, i t i s not possible to determine i f the differences i n v ' s are due to the differences i n the p a r t i c l e s i z e s . d

d

2

+2

+

d

d

+

+

+ 2

+2

d

+

+

+ 2

+2

d

The v calculations for HNOg, S0 , HC1 and NH^ become more complex due to the l i k e l i h o o d that more than one species contributed to the observed a r t i f i c i a l dew deposition rates. I f we assume that d

2


98


Table I I I .

Species

Average Concentration yg/rn^

Wo. of Samples

sof


Average Ambient Concentrations

24

6 .51

+

6.74

+

3.24

N03

24

2 .97

CI"

24

0 .28 +

0.25

NHjJ

24

4 .09 +

3.54

24

8,.02 +

9.70

24

0,.50 + 0,.82 +

0.29

20 20

1..59 +

1.09

24

0..57 +

0.36

HN0

20

2..55 ±

2.36

NH

23

0.,40 ±

0.47

20

11.. 1 ±

4.08

Ca

+ 2

Mg

+2

K

+

Na

+

HC1 3

3

N0 * 2

NO*

24

13. 4

±

12.1

21

28. 4

±

18.5

so *

23

* concentration

9. 19

±

0 .17

28.5

0 .01 0 .06 1,.13 0,.07 0..16 0..33 0..07 0.,0 0.,02 4.8 1. 2 0.0 0.0 0 .09

1.18

O3* 2

Range

9.12

14.4 0.74 15.5 37.9 1.63

5.97 4.15 1.37

7.08 2.22 22.9 43.7 70 32.5

i n ppb

adsorption of these gases onto Teflon i s minimal then the observed dry plate deposition of N O 3 , SOjj , CI" and NHjJ can be attributed to p a r t i c u l a t e deposition. Using t h i s approach the average v ' s f o r p a r t i c u l a t e N O 3 , SOJJ , CI" and NHjJ were estimated to be 0.33, 0.10, 2.36 and 0.06 cm/s, respectively. Since most of the SOjj and NHjJ has been shown to be i n the submicron s i z e range at t h i s s i t e (16,18), their v^'s appear to be reasonable. S i m i l a r l y , the higher v^'s f o r N O 3 and CI" are consistent with the fact that they are present as larger p a r t i c l e s . These v ' s are then used to correct the observed a r t i f i c i a l dew H N O 3 , S0 , HC1 and NH3 deposition rates f o r the corresponding p a r t i c u l a t e dry deposition contribution. The corrected v ' s for HNO3 and S 0 averaged 0.39 and 0.15 cm/s, respectively, r e f l e c t i n g a 47? and a M% decrease i n the v a f t e r correction f o r p a r t i c u l a t e N O 3 and SOjj input. Likewise, WH3 v and HC1 v decreased 36 and 54? a f t e r correction for p a r t i c u l a t e NHjJ and CI" to 1.90 and 0.73 cm/s, respectively. 2

d

2

2

d

2

d

2

d

2

d

d

Conclusions Concentrations of a l l measured species were higher i n natural dew than i n r a i n . The biggest differences were much higher C a and CI" + 2


5.

Urban Dew

MULAWAETAL.

Table IV.

Estimated Deposition to A r t i f i c i a l Dew

Species


99 Velocities

V , cm/s

Surface

d

HNO3

dew

0.39 ± 0.31

so

dew

0.15 ± 0.11

HC1

dew

0.73 ± 0.11

NH3 NO3 SO4

dew

1.9

2

± 1.55

Teflon

0.33 ± 0.22

Teflon

0.10 ±

NHjj

Teflon

0.06 ± 0.09

CI"

Teflon

2.36

Ca

2

+ 2

Ca+ Mg

+2

Mg

+2

Na

+

Na

+

K

+

K

+

Teflon

2

±

0.09

1.77

0.23 ± 0.18

dew

0.46 ± 0.36

Teflon

0.15 ± 0.12 0.41 ± 0.20

dew Teflon

0.33 ± 0.34

dew

0.42 ± 0.67

Teflon

0.69 ± 0.72

dew

0.88 ± 0.89

+

concentrations i n the dew than the rain and much lower H concentrations. Thus, i t was concluded that the a c i d i t y of dew at this s i t e i s controlled more by the deposition of large basic p a r t i c l e s than by the deposition of acids and acid precursors. This suggests a potent i a l for higher dew a c i d i t y i n areas where C a deposition i s lower or under conditions which minimize the deposition of large p a r t i c l e s . This effect was demonstrated previously when i t was shown that the a c i d i t y of dew was much higher on downward-facing surfaces than upward-facing surfaces (J_0). I t i s also seen i n this study with the a r t i f i c i a l dew samples which had much lower C a concentrations than the natural dew samples and thus more H . Deposition rates of a l l species except K and Na were 1.4 to 7.2 times greater to the a r t i f i c i a l dew than to the natural dew. Apparently the moisture increased the retention of p a r t i c l e s and permitted the deposition of water soluble gases. This l a t t e r effect was p a r t i c u l a r l y important f o r the deposition o f N0Ô, NHjJ, S(IV) and + 2

+ 2

+

+

+

The deposition v e l o c i t i e s to the a r t i f i c i a l dew and the dry plates appear reasonable under the experimental conditions of this study. The particulate SOJJ v of 0.10 cm/s i s somewhat lower than the weekly summertime average SOjj v of 0.29 cm/s to a dry deposition bucket reported by Dasch and Cadle (J_5) for t h i s s i t e . This 2

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MATERA ILS DEGRADATO IN CAUSED BY ACID RAIN

difference i s reasonable since their estimate did not account for any SO2 deposition which may have occurred when the bucket was damp. Also, average atmospheric s t a b i l i t y conditions may have been d i f f e r ent. The p a r t i c u l a t e NHjJ v to the Teflon plate, 0.06 cm/s, was in good agreement with the SOjj v . Both these species are present as small p a r t i c l e s at this s i t e . The p a r t i c u l a t e NO3, CI", and K v 's were i n good agreement with those reported by Dasch and Cadle (15), while the Ca and M g v ' s to the dry plate were lower by factors of 9 and 7, respectively. The v ' s of gaseous species to the a r t i f i c i a l dew were calculated a f t e r correcting the t o t a l deposition rates f o r the deposition of p a r t i c l e s . Particulate deposition rates were assumed to be close to those observed to the dry plate. The estimated S 0 v was 0.15 cm/s as compared to the -0.04 cm/s recently reported by Pierson et a l . (9). The difference between these r e s u l t s r e f l e c t s the l e s s turbulent nighttime conditions during their study and the decreased S 0 s o l u b i l i t y due to the low pH of their samples. A higher estimate for S 0 v of 0.69 cm/s to a deionized water sur face has been reported by Dasch and Cadle (j_9) f o r spring days at this s i t e . The HNO^ v , 0.39 cm/s, was also higher than the 0.24 cm/s reported by Pierson et a l . (9). The HC1 v , 0.73 cm/s, i s in reasonable agreement with the HNO^ v . However, the NH^ v , 1.9 cm/s, appears to be inconsistent with our other r e s u l t s . Overall, i t i s concluded that acid dew i s not a s i g n i f i c a n t environmental concern at t h i s s i t e . However, the a b i l i t y of dew to increase the deposition rate of water soluble gases to some surfaces, and thus increase the a c i d i t y of the dew, may be important at other locations. The deposition v e l o c i t i e s reported above can be used to estimate dew concentrations at other s i t e s as long as differences i n p a r t i c l e s i z e s and atmospheric conditions are taken into consideration. d

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Acknowledgments The authors wish to acknowledge Kenneth Kennedy of the Environmental Science Department f o r the a n a l y t i c a l assistance he provided and Sudarshan Kumar for his h e l p f u l discussions.

Literature Cited 1. Anderson, E. A. In "Atmospheric Corrosion of Non-ferrous Metals"; ASTM STP 175, American Society for Testing and Materials, Philadelphia, PA, 1955. 2. Fassina, V. Atmos. Environ. 1978, 12, 2205-2211. 3. Bangay, G. E.; Riordan C. United States-Canada Memorandum of Intent on Transboundary Air Pollution, Impact Assessment. 4-39, 1983. 4. Yaalon, D. H.; Ganor, Ε. Nature 1968, 217, 1139-1140. 5. Brimblecombe, P.; Todd, I. J. Atmos. Environ. 1977, 11, 649-650.



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Urban Dew

6. Anderson, Ε. Α.; Landsberg, Η. Ε. Environ. Sci. Technol. 1979, 13, 992-994. 7. Smith, Β. E.; Friedman, E. J. "The Chemistry of Dew as Influenced by Dry Deposition: Results of Sterling, Virginia and Champaign, Illinois Experiments," Mitre Corporation Working Paper WP82W00141, 1982. 8. Wisniewski, J. Water, Air, Soil Pollution 1982, 17, 361-377. 9. Pierson, W. R.; Brachaczek, W. W.; Gorse, R. Α., Jr.; Japar, S. M.; Norbeck, J. M. "On the Acidity of Dew," Presented at the 78th Annual Meeting of the Air Pollution Control Association, Detroit, MI, 1985, Paper No. 85-7.4. 10. Cadle, S. H.; Groblicki, P. J. In "Transactions of the APCA Specialty Conference, The Meteorology of Acid Deposition"; Samson, P. J., Ed., 1983, pp. 17-29. 11. Dasch, J. M.; Cadle, S. H. Atmos. Environ. 1984, 18, 1009-1015. 12. Shaw, R. W.; Bowermaster, J.; Tesch, J. W.; Tew, E. Atmos. Environ. 1982, 16, 845-853. 13. Okita, T.; Kaneda, K.; Yanaka, T.; Sugai, R. Atmos. Environ. 1974, 8, 927-936. 14. Shendrikar, A. D.; Lodge, J. P., Jr. Atmos. Environ. 1975, 9, 431-435. 15. Dasch, J. M.; Cadle, S. H. Atmos. Environ., in press. 16. Dasch, J. M. "Measurement of Dry Deposition to a Deciduous Canopy," General Motors Research Publication GMR-5019, 1985. 17. Dasch, J. M. In "Precipitation Scavenging, Dry Deposition, and Resuspension; Pruppacher, H. R.; Semonin, R. G.; Slinn, W. G. N., Eds.; Elsevier Publishing: New York, Amsterdam, Oxford, 1983, Vol. 2, 883-902. 18. Cadle, S. H. Atmos. Environ. 1985, 19, 181-188. 19. Dasch, J. M.; Cadle, S. H. "Dry Deposition to Snow in an Urban Area," Presented at the 78th Annual Meeting of the Air Pollution Control Association, Detroit, MI, 1985, Paper No. 85-6B.3. RECEIVED

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