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Deposition of sulfate during stable atmospheric transport over lake michigan
Atmosphrr~ Enuironmenr Vol. 13. pp. 1717-1718. 0 Pergamon Press Ltd. 1979 Printed in Great Britain.
CMM-6981r79;1201-1717 $O2.00/0
SHORT COMMUNICATION DEPOSITION OF SULFATE DURING STABLE ATMOSPHERIC TRANSPORT OVER LAKE MICHIGAN (First received 6 February 1979 and infimdform 24 May 1979)
Abstraet - Sulfate concentration measurements were made during very stable atmospheric conditions ata Lake Michigan sampling site 55 km downwind of Chicago. Chicago sulfate concentrations are compared with the midlake concentrations. Dilution and transformation effects upon downwind sulfate concentrations are considered to be small during the very stable atmospheric conditions encountered. Depositional loss to the lake is then assumed to explain essentially all the differences between Chicago and midlake concentrations. The deposition velocity of sulfate to the lake is found to be 0.2 + 0.16 cm s- ‘ despite the very stable conditions prevailing.
Within the context of a 1977 field study on Lake Michigan an 18-20 May sampling period afforded extremely steady meteorological conditions for a ship sampling site at 87”OO’W and 42’OO’N directly downwind of the Chicago metropolitan area. Hi-volume filter samples taken in Chicago and aboard ship were analyzed by standard U.S. EPA automated procedures. Sampling and analysis details are described by Sievering et rrl. (1978). The wind direction and speed during the sampling period were 225” f 15” and 3.8 + 0.8 m s-l, respectively; the travel time was 4.0 f 0.7 h. The average air tempapture, T,, at a 5 m height at the midlake sampling point was 15.8’C ; the lake’s surface water temperature, T, (mmaured by i.r. thermometry), was 7.3”C. Extremely stable air prevailed over the lake throughout this sampling period. No precipitation occurred during the period. Analysis of the 19 May 2Ch Hi-Vol samples taken in resulted in a sulfate concentration of Chicago 12.3 f 0.6 pg rne3. This is an average of 22 sampling stations located throughout the city. Analysis of six 5-h filter sets collected at midiake resulted in a sulfate concentration of 10.0 f 2.4pg rnm3. A coarse and fine particulate sizing showed880/,ofthesulfatemasstobeinthe
May SO1 transformation to sulfate probably occurred in the immediate vicinity of SO2 sources resulting in the rather uniform city-wide mean sulfate concentration. To test this notion Chicago recorded data on sulfate, SO2, total SUSpcaded part&late (TSP) and meteorological variables from 1966-1977 were regressed for various combinations of these variables. Sulfate was the dependent variable and TSP, SOI, city air temperature and relative humidity the independent variables (Roberts and Sievering, 1977). Of these, the formulation which yielded the most statistically significant (r = 0.68) results is [Sulfate] = 5.0 + 0.07[TSP-J + O.OZ[SGJ,
(1)
with all variables in pg rnw3. This relation was found to be statistically significant by the F test for p c 0.05. Note that temperature and relative humidity do not appear to conttib ute to sulfate generation. This supports the notion that TSP and SO2 related sulfate generation is occurring in the immediate vicinity of major air pollution sources and not in the ambient air for which meteorological influences are more important. Forrest and Newman (1977) also found no correlation between sulfate and 10 < T < 25” and 32 < RH c 85%. This is in contrast to the results of Meyers and Ziegler (1978). They found a correlation of sulfate with the product of TSP and the square of RH for a network of sulfate sampling stations dispersed across thirteen eastern states. In this case long-range transport may afford an enhancement of SO1 transformation to sulfate. The Chicago sulfate source is not strongly dependent upon meteorological variables. Xhis, combined with the slowly changing meteorology during the 18-20 May period and the relative spatial constancy of the Chicago sulfate source on 19 May, argue for a relatively constant sulfate source strength for the entire 18-20 May period. Horizontal dilution over the lake itself was at a minimum during the entire sampling period. The bulk Richardson number at midlake was 0.17 + 0.10 which is equivalent to a G stability classification. Since the standard deviation in wind direction a1 the midlake sampling site was only 1s” for the entire sampling period, a back trajectory with 2a envelope reaches shore entirely within the bounds of the 100 km long Chicago source. The midlake sampling site then “sees” the Chicago source as an accumulation of line sources of relatively constant sulfate source strength (12.3 + 0.6pgm-“). Horizontal dilution is not a factor in the constant line source diffusion quation. Vertical dilution was capped at a much lower height than the mixed layer. Transport of warm air from Chicago
1717
Short Communication
1718
over the cold lake surface in spring and early summer creates a thermal internal boundary layer (TIBL) which sharply reduces mixing in the lowest few hundred meters above the surface and allows almost no mixing above the TIBL (Lyons, 1975).An apprdximate equation for the height of the TIBL is (Raynor et al., 1974) H TlEL
ut
FV” - T,)
= ; J
-AT/AZ
’
(2)
where F is the fetch in m, AT/AZ is the lapse rate in ‘C m _ * and II+ is the friction velocity. HTraL estimated in this way gives a value of 150 m for the conditions encountered 18-20 May. Mixing over the warmer city environment distributed sulfate fairly evenly to this 150 m depth before transport over the lake began. Themafter, almost no mixing occurred in the over lake G stability class conditions. Several studies show a subatantiaily smaller than l”,‘,h- ’ SO2 to sulfate transformation downwind of sourcea (Samson, 1978; Meagher et ol., 1978; Forrest et al., 1979). It can safely be stated that transformation will enhance sulfate concentrations at midlake due to the 4-h travel time from Chicago to midlake, but by only 2-3x or even less. This is within the precision of the sulfate analytical procedure. It appears then that the sulfate concentration difference between Chicago and midlake can be attributed to &positional loss. Dilution and transformation dfects inaigni&antiy contribute to midlake st&te concentrationa, esp&ally considering the first adds to and the sacond subtracts from measured concentratioils. The expected depositional loss of sulfate during transport may be atimated by assuming the deposition velocity, vd, to represent acroaol vertical motion within a well mixed layer of depth H over the lake. The kpcdon of aerosol maas removed from this well mixed layer by dry deposition is ud x Ar/H. The midlake concentration, C,,, is then a fraction of the Chicago source region concentration, Cal, given by Cm, 2: Ccn[l - (ud x h/H)].
(3)
When considering long range transport, H is usually chosen as the average mixed layer height of approx 1000 m. Here, the TIBL height of 150 m has already bacn shown to be appropriate. The mean of the six measured midlake sulfate concentrations with the Chicago source directly upwind is c,, = 10.0 i 2.4pgmm3. cc,,, = 12.3 f 0.61cgmd3 and At =I 4.0 f 0.7 h. Equation (3) gives v,, = 0.2 f 0.16 cm s - ’ with extreme values of 0 and 0.51 cm s-‘. The deposition velocity for sulfate has been estimated as ranging from less than 0.1 ems-’ to as much as 1 ems-I. If mass flux is assumed proportional to momentum flux, v, can be simply estimated by the mean horizontal wind speed times the diabatic drag coeflicient (Sievering, 1979). On this basis the 18-20 May mean deposition velocity, 17~= 0.25 + O.lEcms-‘. This compares well with the 0.2 f 0.16 cm s- ’ determined by Equation (3)and suggests that the Reynoldsanalogy between mass and momentum transfer may hold under these very stable atmospheric conditions. For Pb, v,, by Equation (3) using its measured t?,, and &,, is 0.13 cm s-l, for Fe, 0.65 cm s- ’ and for Mn, 0.55 cm s-l. Although all simple estimates, these v,, compare favorably with those found by Prahm et al. (1976) for sulfate of 0.4 rt 0.2 cm s- ’ and by Cawse (1974) for Pb, Mn and Fe of 0.3, 0.6 and 1.0 cm s-l. respectively. This is especially true when one considers that Cawse’s MMDs were 0.5, 1.3 and 2.5 p for Pb, Mn and Fe, whereas it was earlier noted that 82,49 and 47% of Pb, Mn and Fe, respectively. were found in the < 1 pm size over Lake Michigan.
These favorable comparisons for trace metal deposition further substantiate the calculation of a sulfate deposition velocity by Equation (3). The resulting 0.2 + 0.16 cm s- ’ is unexpectedly large for the extremely stable conditions over the lake. Even larger values of v,, may be encountered in neutral stability and higher wind speed conditions over the lake. College of
Environmental Sciences, Governors Stare University. Park Forest South. IL60466.
U.S.A.
HERMANSIE~ERINC MEHUL DAVC PATRIC MCCOY NELL SUTTON
REFERENCES Cawse P. A. (1974) A survey of trace elements in the U.K., 1972-73. U.K. At. Energy Auth. Rep. AERE-R7669. Harwell, U.K. Forrest J. and Newman L. (1977) Further studies on the oxidation of sulfur dioxide in coal-fired power plant plumes. Atmospheric Enoironwtent 11.465-474. Forrest J., Schwartx S. E. and Nuwman L. (1979) Conversion of sulfur dioxide to sulfate during the DaVinci fights. Atmospheric Envirwunent 13, 157-167. Lyons W. (1975) Turbulent di&+ion and pollutant transport in shoreline enviromneots, in Lactwes on Air Pollution and Environmental Imprret Analysis, pp. 136-208. Am. Met. Sot. Pt.&l.. Boston. Meagher J., Stockburger L., Bailey E. and Huff 0. (1978) The oxidation of sulfur dioxide to sulfate aerosols in the plume of a coal-fired power plant. Atmospherfc Environmerv 12, 2197-2203. Meyers R. E. and Ziegler E. N. (1978) Relationship ofambient sulfate concentration to meteorological and chemical variables. En&. Sci. Technol. 12, 302-308. Newman L., Forrest J. and Manowitz B. (1975) The appiication of isotopic ratio technique to a study of the atmospheric oxidation of sulfur dioxide from a coal-fired power plant. Atmospheric Environment 9, W-977. Prahm L., Tarp U. and Stem M. (1976) Deposition and tranaformatidn of sulfur oxides during atmoiphtic transwrt over the Atlantic. Tcllus XXVIIL 355-372. Riynor G., Michael P., Brown R. and Sat&&man S. (1974) A research program on atmoapheric diITuaion from an oceanic site. Am. Ma. Sec. Symp. on Atmos. 08 and Air Pollut., pp. 289-295, Santa Barbara, CaL, Sept. 9-13. 1974. Roberts H. and Sievering H. (1977) A guide to Environmental Benefits Assessment in Economic impuct Studies. Ill. Inst. for Natural Resources, Chicago. Samson P. (1978) Ensemble trajectory analysis of summertime sulfate concentrations in New York State. AmmospheriE Enuironment 12, 1889-1893. Sievering H., Dave M., McCoy P. and Walther K. (1978) Cellulose filter high-volume cascade impactor aerosol collection &ciency. A technical note. Envir. Sci. Tech&. lf 1435-1437. Sievering H. (1979) Dry Deposition of Atmospheric Aerosols to Lake Michigan as a Function of Meteorology and Aerosol Size. Am. Met. Sot. Symp. on Atmos. fMjI and Air Pollut., pp. 518-521, Rena, Ne., 15-18 Jan., 1979.Am. Met. Sot. Publ., Boston.
CMM-6981r79;1201-1717 $O2.00/0
SHORT COMMUNICATION DEPOSITION OF SULFATE DURING STABLE ATMOSPHERIC TRANSPORT OVER LAKE MICHIGAN (First received 6 February 1979 and infimdform 24 May 1979)
Abstraet - Sulfate concentration measurements were made during very stable atmospheric conditions ata Lake Michigan sampling site 55 km downwind of Chicago. Chicago sulfate concentrations are compared with the midlake concentrations. Dilution and transformation effects upon downwind sulfate concentrations are considered to be small during the very stable atmospheric conditions encountered. Depositional loss to the lake is then assumed to explain essentially all the differences between Chicago and midlake concentrations. The deposition velocity of sulfate to the lake is found to be 0.2 + 0.16 cm s- ‘ despite the very stable conditions prevailing.
Within the context of a 1977 field study on Lake Michigan an 18-20 May sampling period afforded extremely steady meteorological conditions for a ship sampling site at 87”OO’W and 42’OO’N directly downwind of the Chicago metropolitan area. Hi-volume filter samples taken in Chicago and aboard ship were analyzed by standard U.S. EPA automated procedures. Sampling and analysis details are described by Sievering et rrl. (1978). The wind direction and speed during the sampling period were 225” f 15” and 3.8 + 0.8 m s-l, respectively; the travel time was 4.0 f 0.7 h. The average air tempapture, T,, at a 5 m height at the midlake sampling point was 15.8’C ; the lake’s surface water temperature, T, (mmaured by i.r. thermometry), was 7.3”C. Extremely stable air prevailed over the lake throughout this sampling period. No precipitation occurred during the period. Analysis of the 19 May 2Ch Hi-Vol samples taken in resulted in a sulfate concentration of Chicago 12.3 f 0.6 pg rne3. This is an average of 22 sampling stations located throughout the city. Analysis of six 5-h filter sets collected at midiake resulted in a sulfate concentration of 10.0 f 2.4pg rnm3. A coarse and fine particulate sizing showed880/,ofthesulfatemasstobeinthe
May SO1 transformation to sulfate probably occurred in the immediate vicinity of SO2 sources resulting in the rather uniform city-wide mean sulfate concentration. To test this notion Chicago recorded data on sulfate, SO2, total SUSpcaded part&late (TSP) and meteorological variables from 1966-1977 were regressed for various combinations of these variables. Sulfate was the dependent variable and TSP, SOI, city air temperature and relative humidity the independent variables (Roberts and Sievering, 1977). Of these, the formulation which yielded the most statistically significant (r = 0.68) results is [Sulfate] = 5.0 + 0.07[TSP-J + O.OZ[SGJ,
(1)
with all variables in pg rnw3. This relation was found to be statistically significant by the F test for p c 0.05. Note that temperature and relative humidity do not appear to conttib ute to sulfate generation. This supports the notion that TSP and SO2 related sulfate generation is occurring in the immediate vicinity of major air pollution sources and not in the ambient air for which meteorological influences are more important. Forrest and Newman (1977) also found no correlation between sulfate and 10 < T < 25” and 32 < RH c 85%. This is in contrast to the results of Meyers and Ziegler (1978). They found a correlation of sulfate with the product of TSP and the square of RH for a network of sulfate sampling stations dispersed across thirteen eastern states. In this case long-range transport may afford an enhancement of SO1 transformation to sulfate. The Chicago sulfate source is not strongly dependent upon meteorological variables. Xhis, combined with the slowly changing meteorology during the 18-20 May period and the relative spatial constancy of the Chicago sulfate source on 19 May, argue for a relatively constant sulfate source strength for the entire 18-20 May period. Horizontal dilution over the lake itself was at a minimum during the entire sampling period. The bulk Richardson number at midlake was 0.17 + 0.10 which is equivalent to a G stability classification. Since the standard deviation in wind direction a1 the midlake sampling site was only 1s” for the entire sampling period, a back trajectory with 2a envelope reaches shore entirely within the bounds of the 100 km long Chicago source. The midlake sampling site then “sees” the Chicago source as an accumulation of line sources of relatively constant sulfate source strength (12.3 + 0.6pgm-“). Horizontal dilution is not a factor in the constant line source diffusion quation. Vertical dilution was capped at a much lower height than the mixed layer. Transport of warm air from Chicago
1717
Short Communication
1718
over the cold lake surface in spring and early summer creates a thermal internal boundary layer (TIBL) which sharply reduces mixing in the lowest few hundred meters above the surface and allows almost no mixing above the TIBL (Lyons, 1975).An apprdximate equation for the height of the TIBL is (Raynor et al., 1974) H TlEL
ut
FV” - T,)
= ; J
-AT/AZ
’
(2)
where F is the fetch in m, AT/AZ is the lapse rate in ‘C m _ * and II+ is the friction velocity. HTraL estimated in this way gives a value of 150 m for the conditions encountered 18-20 May. Mixing over the warmer city environment distributed sulfate fairly evenly to this 150 m depth before transport over the lake began. Themafter, almost no mixing occurred in the over lake G stability class conditions. Several studies show a subatantiaily smaller than l”,‘,h- ’ SO2 to sulfate transformation downwind of sourcea (Samson, 1978; Meagher et ol., 1978; Forrest et al., 1979). It can safely be stated that transformation will enhance sulfate concentrations at midlake due to the 4-h travel time from Chicago to midlake, but by only 2-3x or even less. This is within the precision of the sulfate analytical procedure. It appears then that the sulfate concentration difference between Chicago and midlake can be attributed to &positional loss. Dilution and transformation dfects inaigni&antiy contribute to midlake st&te concentrationa, esp&ally considering the first adds to and the sacond subtracts from measured concentratioils. The expected depositional loss of sulfate during transport may be atimated by assuming the deposition velocity, vd, to represent acroaol vertical motion within a well mixed layer of depth H over the lake. The kpcdon of aerosol maas removed from this well mixed layer by dry deposition is ud x Ar/H. The midlake concentration, C,,, is then a fraction of the Chicago source region concentration, Cal, given by Cm, 2: Ccn[l - (ud x h/H)].
(3)
When considering long range transport, H is usually chosen as the average mixed layer height of approx 1000 m. Here, the TIBL height of 150 m has already bacn shown to be appropriate. The mean of the six measured midlake sulfate concentrations with the Chicago source directly upwind is c,, = 10.0 i 2.4pgmm3. cc,,, = 12.3 f 0.61cgmd3 and At =I 4.0 f 0.7 h. Equation (3) gives v,, = 0.2 f 0.16 cm s - ’ with extreme values of 0 and 0.51 cm s-‘. The deposition velocity for sulfate has been estimated as ranging from less than 0.1 ems-’ to as much as 1 ems-I. If mass flux is assumed proportional to momentum flux, v, can be simply estimated by the mean horizontal wind speed times the diabatic drag coeflicient (Sievering, 1979). On this basis the 18-20 May mean deposition velocity, 17~= 0.25 + O.lEcms-‘. This compares well with the 0.2 f 0.16 cm s- ’ determined by Equation (3)and suggests that the Reynoldsanalogy between mass and momentum transfer may hold under these very stable atmospheric conditions. For Pb, v,, by Equation (3) using its measured t?,, and &,, is 0.13 cm s-l, for Fe, 0.65 cm s- ’ and for Mn, 0.55 cm s-l. Although all simple estimates, these v,, compare favorably with those found by Prahm et al. (1976) for sulfate of 0.4 rt 0.2 cm s- ’ and by Cawse (1974) for Pb, Mn and Fe of 0.3, 0.6 and 1.0 cm s-l. respectively. This is especially true when one considers that Cawse’s MMDs were 0.5, 1.3 and 2.5 p for Pb, Mn and Fe, whereas it was earlier noted that 82,49 and 47% of Pb, Mn and Fe, respectively. were found in the < 1 pm size over Lake Michigan.
These favorable comparisons for trace metal deposition further substantiate the calculation of a sulfate deposition velocity by Equation (3). The resulting 0.2 + 0.16 cm s- ’ is unexpectedly large for the extremely stable conditions over the lake. Even larger values of v,, may be encountered in neutral stability and higher wind speed conditions over the lake. College of
Environmental Sciences, Governors Stare University. Park Forest South. IL60466.
U.S.A.
HERMANSIE~ERINC MEHUL DAVC PATRIC MCCOY NELL SUTTON
REFERENCES Cawse P. A. (1974) A survey of trace elements in the U.K., 1972-73. U.K. At. Energy Auth. Rep. AERE-R7669. Harwell, U.K. Forrest J. and Newman L. (1977) Further studies on the oxidation of sulfur dioxide in coal-fired power plant plumes. Atmospheric Enoironwtent 11.465-474. Forrest J., Schwartx S. E. and Nuwman L. (1979) Conversion of sulfur dioxide to sulfate during the DaVinci fights. Atmospheric Envirwunent 13, 157-167. Lyons W. (1975) Turbulent di&+ion and pollutant transport in shoreline enviromneots, in Lactwes on Air Pollution and Environmental Imprret Analysis, pp. 136-208. Am. Met. Sot. Pt.&l.. Boston. Meagher J., Stockburger L., Bailey E. and Huff 0. (1978) The oxidation of sulfur dioxide to sulfate aerosols in the plume of a coal-fired power plant. Atmospherfc Environmerv 12, 2197-2203. Meyers R. E. and Ziegler E. N. (1978) Relationship ofambient sulfate concentration to meteorological and chemical variables. En&. Sci. Technol. 12, 302-308. Newman L., Forrest J. and Manowitz B. (1975) The appiication of isotopic ratio technique to a study of the atmospheric oxidation of sulfur dioxide from a coal-fired power plant. Atmospheric Environment 9, W-977. Prahm L., Tarp U. and Stem M. (1976) Deposition and tranaformatidn of sulfur oxides during atmoiphtic transwrt over the Atlantic. Tcllus XXVIIL 355-372. Riynor G., Michael P., Brown R. and Sat&&man S. (1974) A research program on atmoapheric diITuaion from an oceanic site. Am. Ma. Sec. Symp. on Atmos. 08 and Air Pollut., pp. 289-295, Santa Barbara, CaL, Sept. 9-13. 1974. Roberts H. and Sievering H. (1977) A guide to Environmental Benefits Assessment in Economic impuct Studies. Ill. Inst. for Natural Resources, Chicago. Samson P. (1978) Ensemble trajectory analysis of summertime sulfate concentrations in New York State. AmmospheriE Enuironment 12, 1889-1893. Sievering H., Dave M., McCoy P. and Walther K. (1978) Cellulose filter high-volume cascade impactor aerosol collection &ciency. A technical note. Envir. Sci. Tech&. lf 1435-1437. Sievering H. (1979) Dry Deposition of Atmospheric Aerosols to Lake Michigan as a Function of Meteorology and Aerosol Size. Am. Met. Sot. Symp. on Atmos. fMjI and Air Pollut., pp. 518-521, Rena, Ne., 15-18 Jan., 1979.Am. Met. Sot. Publ., Boston.