CHEMICAL PROPERTIES OF SNOW IN THE NORTHEASTERN UNITED STATES

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CHEMICAL PROPERTIES OF SNOW IN THE NORTHEASTERN UNITED STATES

M. Kumai

To cite this version:

M. Kumai. CHEMICAL PROPERTIES OF SNOW IN THE NORTHEASTERN UNITED STATES.

Journal de Physique Colloques, 1987, 48 (C1), pp.C1-625-C1-630. �10.1051/jphyscol:1987186�. �jpa-

00226452�

JOURNAL DE PHYSIQUE

C o l l o q u e C 1 , s u p p l e m e n t au n o 3, Tome 48, mars 1987

CHEMICAL PROPERTIES OF SNOW IN THE NORTHEASTERN UNITED STATES

M. KUMAI

U.S. Army C o l d R e g i o n s R e s e a r c h and E n g i n e e r i n g L a b o r a t o r y , H a n o v e r , NH 03755-1290, U.S.A.

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Abstract. Samples of fresh snow from Hanover, N.H., were found t o be slightly acidic, with pH ranging from 3.56 t o 5.63, and had electrolytic conductivities in t h e range 2.52 t o 80.0 pS cm-l. Snowfalls accompanied by southerly winds from densely populated a r e a s averaged about 3 t i m e s higher in hydrogen ion concentration and electrolytic conductivity than snowfalls ac- companied by northerly winds from less populated areas. Particles found in fresh snow exam- ined with a scanning electron microscope and a n energy dispersive X-ray analyzer were most frequently soil minerals, with some fly ash particles, and occasionally diatoms and pollen. Sul- fur-rich black particles were presumed t o be from local oil-fired heating and e l e c t r i c power plants, while silicon-rich fly ash particles were assumed t o have originated at distant coal-fired e l e c t r i c power plants.

1. Introduction

The chemical properties of snow and of aerosols in snow have been reported on by many work- ers. The study of t h e chemical properties of particles from ancient polar atmospheres is based on relationships linking t h e chemical composition of t h e precipitation t o t h a t of t h e atmospheric aerosols. Chemical studies on d e e p ice cores recovered from Greenland and Antarctica have provided valuable information concerning global atmospheric history (1, 2, 3).

Firn samples from t h e Antarctic Peninsula covering t h e years 1975-79 (4) were found t o be slightly acidic, with a pH of 5-6, and showed seasonal variations in H2SO4 deposition and marine background aerosols. The pH of snowfall in Sapporo, Hokkaido, Japan, was frequently lower than 5.6. The mean pH weighted by t h e amount of precipitation was 4'.69 f o r 49 snowfalls in t h e winter of 1980-81. Areas of high H2SO4 deposition in t h e snow were located on t h e lee side of t h e c i t y r a t h e r t h a n in t h e city center, indicating t h a t these a r e a s a r e a sink f o r a i r pollutants produced by human activities in t h e city (5).

This paper presents 1) t h e results of measurements of pH and electrolytic conductivity of snow- falls at Hanover. N.H.. in t h e winter of 1984-85; 2) t h e e f f e c t of wind direction on pH and elec- trolytic conductivity; 3) t h e ,identification, and determination of t h e origin, of fly ash and other aerosols in fresh snow using t h e scanning electron microscope (SEM) and energy dispersive X-ray analyzer (EDXA); and 4) t h e concentration and types of precipitated particles in t h e fresh snow.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987186

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0 snow Rain

0 1

6 5 10 1 0 0

E i e c t r o l ~ t i c Conductivity (pS ern-')

Fig. 1. Daily precipitation (water Fig. 2. pH vs electrolytic conductiv- equivalent) and depth of snow ity of snow and rain in winter 1984- cover in winter 1984-85 a t Han- 8 5 a t Hanover, N.H.

over, N.H.

2. Experimental Procedure

Fresh snow samples w e r e collected following snowfalls in a field in a rural a r e a of t h e north- e a s t e r n United S t a t e s about 3 km north of Dartmouth College, Hanover, N.H. T h e mean month- ly a i r t e m p e r a t u r e at Hanover remains below freezing f r o m December t o March, s o snow gener- ally accumulates o n t h e ground during those months. During t h e winter of 1984-85. 22 periods of snowfall (189.4 mm t o t a l water equivalent) and 4 periods of rainfall (46.3 mm t o t a l water equivalent) w e r e recorded. The daily precipitation and t h e depth of t h e snow cover a r e shown in Figure 1. About one-third of t h e annual precipitation occurs a s snow, and t h e ten-year aver- a g e maximum depth of t h e snow cover in Hanover is 47 cm.

Fresh snow samples collected in I-liter beakers w e r e melted in a microwave oven and kept at about 25OC during measurements of pH and electrolytic conductivity. A pH m e t e r (Solution Analyzer, Model 4505A) was calibrated with t w o buffer solutions of pH 4.00 and 7.00 a t 25OC f o r e a c h examination. Two m e t e r s f o r electrolytic conductivity measurements (Solution Ana- lyzer, Model 4505A, and Conductance Bridge, RCM 1581) were calibrated with a standard solu- tion of 0.005 N KC1 (720

+

3. pH a n d Electrolytic Conductivity

Measurements of pH and electrolytic conductivity w e r e performed on 22 snow samples and 4 rain samples collected during t h e winter. The pH of t h e snow and rain ranged f r o m 3.56 t o 5.63 and t h e electrolytic conductivity f r o m 2.5 t o 80.0 pS cm-l. T h e relationship between t h e s e measurements is shown in Figure 2. The acidity of t h e snow and rain increased with t h e electro- lytic conductivity, indicating t h a t t h e atmospheric pollutants in t h e acid precipitation derive from t h e exhaust from fossil fuel combustion in densely populated a r e a s (6). The values of pH and e l e c t r o l y t i c conductivity of t h e snow and rain varied considerably among samples. The variation is considered t o b e caused by several factors, such a s wind direction, atmospheric pollution, and amount of precipitation (5, 6).

4. pH of Snow With Northerly Wind

The relationship between snow pH and amount of precipitation was examined f o r nine snowfalls accompanied by northerly winds a t Hanover during t h e 1984-85 winter. The pH of t h e snow

ranged from 5.63 t o 4.06. Precipitation ranged from 2 t o 22.5 mm w a t e r equivalent (w.e.), and totaled 109.5 mm. The mean snow pH, weighted f o r t h e amount of precipitation, was 4.77.

Fig. 3. Surface weather map f o r 0700 EST on 6 D e c 84. A t Hanover a snow- storm was accompanied by northerly wind; snow accumulation was 21 cm (21 mm w a t e r equivalent), and t h e snow pH was 5.63.

I ~ o r t h e r l ; Wind I

Fig. 4. pH vs amount of snow (water equivalent) falling during s t o r m s with northerly wind in winter 1984- 85 at Hanover, N.H.

The acidity and electrolytic conductivity of t h e s e snowfalls w e r e relatively low. A s u r f a c e weather m a p f o r a typical snowstorm t h a t produced northerly winds in Hanover (Fig. 3) shows a low pressure system located over Long Island, N.Y. The w a t e r vapor of t h e snow-producing clouds over New Hampshire is being supplied by northerly winds off t h e Atlantic Ocean. This snowstorm deposited 21 c m of snow (21 mm w.e.). The pH of t h e snow was 5.63, t h e least acid f o r t h e season. The electrolytic conductivity of t h e snow was 2.5 yS c m - l , also t h e lowest value of t h e season.

The relationship between pH and amount of snowfall is shown in Figure 4. The acidity decreased with increasing precipitation, indicating t h a t atmospheric pollutants over New Hampshire w e r e being diluted by t h e w a t e r vapor supplied by northerly winds off t h e Atlantic Ocean.

5. pH of Snow With Southerly Wind

The relationship between pH of snow associated with southerly winds and amount of precipita- tion was examined during t h e s a m e winter (1984-85). Nine snowfalls and t h r e e rainfalls w e r e recorded. Precipitation from t h e s e storms ranged f r o m 1.2 t o 30.0 mm w.e., and t h e t o t a l was 101.4 mm. The pH of t h e precipitation ranged from 3.66 t o 4.70. T h e mean pH weighted by t h e amount of precipitation was calculated t o be 4.14 f o r a l l precipitation combined.

The acidity and electrolytic conductivity of t h e s e snowfalls w e r e relatively high. A weather m a p for a typical snowstorm t h a t produced southerly winds at Hanover (Fig. 5) shows a low pressure system located over Lake Erie. The w a t e r vapor and air pollution in t h e snow-produc- ing clouds over New Hampshire a r e being supplied by southerly winds f r o m densely populated areas. This storm s t a r t e d a s snow at 1700 EST on 12 February, changed t o rain, and ended at 0900 EST on 1 3 February. The t o t a l precipitation was 17.8 mm w.e. The pH of t h e snow and rain was 3.90, more acid t h a n t h e annual average. The electrolytic conductivity was 34.3 yS cm-l, higher than t h e average. T h e insoluble particle concentration in t h e snow and rain was 2.4 m g kg-l.

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100' 90' so' 7 oe

Fig. 5. Surface weather m a p for 0100 EST on 13 F e b 1985. At Hanover, snow followed by rain was accompanied by a southerly wind; precipitation totaled

17.8 mm w.e. and had a bulk sample pH of 3.90 at Hanover.

'5.5t

Fig. 6. pH vs amount of snow o r rain (water equivalent) falling during storms with southerly wind in winter

1984-85 at Hanover, N.H.

The relationship between pH and amount of snow and rain is shown in Figure 6. The pH of t h e snow and rain was almost constant within a narrow range, regardless of t h e amount of precipita- tion. This indicates t h a t atmospheric pollution over New Hampshire was not being diluted by t h e amount of precipitation during southerly winds from densely populated areas.

The mean pH was 4.14 f o r precipitation during southerly winds a s opposed t o 4.77 f o r precipita- tion during northerly winds. The mean hydrogen ion concentration of t h e snow and rain occur- ring during southerly winds was calculated t o be 3.20 t i m e s t h a t occurring during northerly winds in winter 1984-85. (In winter 1983-84 i t was 3.84 times a s much).

6. Particulate Matter in Snow

Particles found in snow a r e of two types: t h e c e n t r a l nucleus around which t h e snow crystal forms, o r aerosols captured during c r y s t a l growth and precipitation. Atmospheric aerosols c a n b e t e r r e s t r i a l materials, a i r pollutants f r o m combustion o r mining, o r even e x t r a t e r r e s t r i a l materials. They c a n be soluble o r insoluble.

The particle concentrations found in 22 snowfalls and 4 rainfalls in t h e winter of 1984-85 varied from 0.6 t o 12.6 mg kg-l, with t h e possible e r r o r estimated a t 10%. No correlation was ob- tained between particle concentration and wind direction during precipitation. The mean parti- c l e concentration of t h e fresh snow and rain samples was 4.0 mg k g 1 , which is about t h r e e times t h a t of snow pit samples taken in Greenland in 1980. However, t h e particle concentration in a n old (28 March) snow cover contaminated by black fly ash near a n oil-fired heating and e l e c t r i c power plant in Hanover was 265.5 mg k g 1 at t h e surface and 25.6 mg kg-l at 10 c m depth.

The particles in t h e fresh snow and rain samples were examined with t h e SEM and EDXA. They ranged from 0.1 t o 3 5 pm in diameter, t h e number of particles exponentially increasing with decrease in diameter. The meteorological conditions and t h e composition of t h e aerosols dif- fered f r o m o n e precipitation period t o another. But t h e particles were generally soil miner- als and fly ash, and occasionally diatoms and pollen (in decreasing order of frequency).

A typical SEM image of a black spherule with porous s t r u c t u r e found in t h e snow is shown in Figure 7a. The particle was composed of S(30.0% by weight), Si(18.7 %), Al(7.3 %), P(7.5 %),

K(6.7 %), Ca(6.4 Oh), Cl(5.6 %), Ti(5.6 Olo), Mg(4.5 Yo), V(3.8 010) and Na(3.7 %), a s shown in t h e elemental composition derived f r o m EDXA analysis (Fig. 7b). The spherules ranged from 0.2 t o 35 pm in d i a m e t e r with a mean diameter of 20 pm. These sulfur-rich black fly ash spher- ules c a m e f r o m local oil-fired heating and electrical power plants (6). They a r e t h e most com- mon f l y ash found in snow and rain collected in Hanover.

Fig. 7. SEM image (a) of a fly ash particle found in fresh snow a t Hanover, N.H. I t s porous s t r u c t u r e and elemental analysis (b) indicate t h a t i t originated from a n oil-fired heating and e l e c t r i c power plant.

Fig. 8. SEM image (a) of a fly ash particle found in fresh snow a t Hanover, N.H. I t s s t r u c t u r e and elemental analysis (b) indicate t h a t i t originated from a coal-fired e l e c t r i c power plant.

O t h e r kinds of spherules found in t h e snow and rain samples included Si-rich, Fe-rich and AI-rich particles. A typical SEM image of a Si-rich spherule is shown in Fig. 8a. Elemental analysis using t h e EDXA showed t h a t t h e particle was composed of Si(41.2 Oh), Al(25.8 Yo), Fe(10.9 %), K(8.2 YO), Cl(4.6 I), Mg(3.1 %), Ca(2.1 Oh), V(2.0 Yo) and Na(0.5 I ) , a s shown in Fig. 8b. The Si-rich, Fe-rich and A1-rich spherules found in t h e snow and rain were 0.2 t o 5 0 pm in d i a m e t e r and were similar t o those found in fly ash collected f r o m coal-fired e l e c t r i c power plants (6, 7). They w e r e found a t Hanover in snowstorms acompanied by southerly winds from densely populated areas.

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Fig. 9. SEM images of soil minerals (a) and a diatom (b) found in fresh snow, Hanover, N.H.

An SEM image of t h e soil particles most often found in t h e fresh snow is shown in Pig. 9a. Dia- toms (Fig. 9b) were occasionally found following snowstorms with northerly winds, possibly derived from s e a spray in t h e Atlantic Ocean.

7. Conclusion

Samples of fresh snow a t Hanover, N.H., were acidic, with pH ranging from 3.56 t o 5.63, and had electrolytic conductivity in t h e range 2.52 t o 80.0 pS cm-l. Snow accompanied by southerly winds from densely populated a r e a s averaged t h r e e t i m e s higher in hydrogen ion concentration than snow accompanied by northerly winds, whose w a t e r vapor was supplied from t h e Atlantic Ocean. Values of snow pH, electrolytic conductivity, and fly ash concentration c a n reveal t h e atmospheric conditions of snow formation and precipitation.

ACKNOWLEDGMENTS

This work was supported by DA Project 4A161102AT24/A/006 at t h e U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire. The author wishes t o acknow- ledge Dr. A. Hogan and Mr. H. O'Brien of CRREL f o r their constructive comments on t h e manu- script.

REFERENCES

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