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Dry deposition measurements of aerosol particles in a corn field
J. Aerosol Sci., Vol. 22, Suppl. 1, pp. $577-$580, 1991. Printed in Great Britain.
0021-8502/91 $3.00+0.00 Pergamon Press plc
DRY DEPOSITION MEASUREMENTS OF AEROSOL PARTICLES IN A CORN FIELD
G. Santachiara,
F. Prodi
and F. Vivarelli
Istituto FISBAT-CNR, Bologna, Italy • Also: Dipartimento di Fisica dell'UniversitA,
Ferrara.
ABSTRACT Aerosol size distribution was measured at a rural site at the 60 cm canopy level (Low) and at 160 cm (High) together with airborne concentration, dry deposition rates and velocity on surrogate surfaces ( Nuclepore and Petri dishes) for several water-soluble ions (CI-, NO 3- , S042-, NH4 ÷ , Na + , K ÷ , 2+ Ca , Mg2+). The airborne ion concentrations detected were t~pical Kfor rural sites and in general similar at both heights, except for Na and Upward surface deposition velocity at the higher level was generally much higher than the downward rate, whereas at canopy level both u p w a r d and downward deposition velocities were comparable for some ions (CI , NO 3 , NH4 ÷ ) and downward
velocity
was
higher
for
K+
and
SO 2-. A generally lower dry 4 deposition to filter surfaces than to filters inside Petri dishes was also detected. Some of these findings can be explained by canopy or land resuspension.
KEY WORDS Dry deposition;
aerosol size distribution;
surrogate surfaces.
INTRODUCTION Dry deposition is the removal of gases and particles from the atmosphere to the earth's surface by processes other than precipitation. Dry deposition of particles can derive from Brownian or eddy diffusion, impaction, interception, phoretic effects and thermophoresis, sedimentation. For very small particles (diameter, D < 0.1 ~m ) Brownian deposition is prevalent and increases as particle diameter decreases because the B r o w n i a n diffusivity increases. For l a r g e r particles ( D> 1 ~m), eddy diffusivity becomes important as do gravitational settling and particle inertia in causing deposition. In the 0. I-I ~m range, where phoretic effects are present but other mechanisms are negligible, the deposition velocity is minimum. Field measurements can be either direct , that is by the measured contamination of a real or artificial surface, or indirect. The latter includes micrometeorological techniques, i.e. profile method or eddy correlation. Micrometeorological measurements evaluate predominantly submicronic particles in the atmosphere, while total mass dry deposition is dominated by sedimentation of the small fraction of large airborne particles. Thus micrometeorological methods and surrogate surface techniques can be viewed as complementary (Davidson, 1985). Surrogate collectors provide the only means of routine monitoring and complete chemical analysis by obviating the problem of chemical element uptake or leaching from biological surfaces . Different types of surfaces have been used for collection, i.e. open buckets, Petri $577
$578
G. SANTACHIARAet al.
dishes, microscope slides, filter paper and so on, and a large range of collection efficiencies are reported (Sehmel, 1980). The present study evaluated the dry deposition of several water-soluble ions on Nuclepore filter and Petri dishes, by measuring atmospheric concentrations and dry deposition rates.
EXPERIMENTAL
APPARATUS AND PROCEDURE
/
The site used for the dry deposition measurements was a corn-field at San Pietro Capofiume, near Bologna; the surrounding area was a grassy field. Dry deposition sampling began 23 May 1989 and continued through 30 May at two heights, 60 cm canopy height (low, L) and 160 cm (high, H). Four Nuclepore filters (0.2 #m, 47 mm in diameter) facing upward were placed at the top of a square plate and four more facing downward were placed at the bottom of the same plate. The plates, with sides of 17.5 cm, thickness of 0.4 cm and edges rounded to reduce turbulence, were supported by a cylindrical stake with 1.0 cm in diameter. Dry deposition samplings were taken also with polysterene Petri of 53 mm internal diameter and a i0 mm rim. We also sampled particles suspended in air by filtration through 0.2 Nm Nuclepore filters in stainless steel holders at L and H. Sampling time was 96 h and sampled air volume 19.2 Nm 3. After sampling, filters were stored in closed plastic petri dishes and from them a l-cm-diameter plate was cut, coated with a thin film of carbon and examined at SEM for size distribution of aerosol particles. The collection filters were extracted in I0 ml of Super-Q water for 30 min in an ultrasonic bath and then washed in two 5ml aliquots of super-Q water; washing period was I h per filter. Samples were filtered through 0.4 #m Nuclepore filters and refrigerated until analysis : Na +, K +, + 2NH4 , C1 , NO3 , SO 4 were determined by a Dionex 2001i ion chromatograph; Ca 2+ and Mg 2+ were determined by spectrometer; and filter blank values conditions during dry deposition field ranged from 0.8 to 5.4 m s -z ; air relative humidity (r.h.) from 30 to 99
AEROSOL SIZE DISTRIBUTION
Perkin- Elmer 403 atomic absorption were evaluated. Daily meteorological program were also measured. Wind speed temperature from 7.4 to 29.2 °C and Z .
AND ATMOSPHERIC
CONCENTRATION
Size distribution of the aerosol particles determined from the integral filters at L and H (Filters 1 and 2) and airborne concentration of the examined ions are reported in Fig. l and Table I. AFilterl o Filter 2
Table l. Airborne concentration(~g Ion Filter I Filter 2 0.83 Cl 0.76 NO
I. 43
O. 79
7.95
7.2
2.59
1.98
Na +
O. 16
I. 57
K+
O. 19
I. i0
Ca 2÷
2.2
4.58
Mg2÷
0. I 0
0. I 0
3 2-
SO 4
o
NH + 4
~5
~
10-2
1if'
J 1o °
I
Radius, pm
Fig. l. Aerosol
size distribution
m -3)
Dry deposition measurements ofaerosolparticles
$579
The Junge distribution dN/d log R = c R -~ approximates the particles spectrum in the 0. I - I0 ~m range. The values of ~ for filters 1 and 2 are respectively 2.38 and 2.36, indicating a relatively higher ratio of large to small particles.
DRY DEPOSITION RATES AND VELOCITY The measured
fluxes are reported
Table 2. Dry deposition
(xlE-6 mg cm -2 day)
A
B
C
D
E
F
N,47
N,47
N,47
N,47
N,47
N,47
UP, H
D,H
UP, L
D,L
H, Petri
L, Petri
Ion
Cl
rates
in Table 2 by filter.
G Petri H
17
2.2
46
52
118
46
67
422
21
106
48
179
82
133
202
I0
113
235
394
93
263
12
28
7.8
9
41
19
25
Na t
"
~
4.3
"
38
25
32
Kt
"
"
"
8.6
9.4
3.1
23
Ca 2+
102
9.4
258
18
108
155
196
Mg 2+
4.7
"
9.4
"
4.1
7.2
9.5
NO 3 2SO 4 t
NH 4
":Dry deposition rates < IE-6 mg cm -2 day; N: Nuclepore filter; Up:upward deposition ; D:downward deposition; H:sampling at 160 cm; L: sampling at 60 cm ; 47: filter diameter , mm. For filters A, B, C, D upward and downward ammonium dry deposition rates are + similar. This may depend on resuspension of NH from canopy or soil. Van 4 Hove (1989) shows that NH and SO are adsorbed on leaf surface of bean and 3 2 poplar, and in presence of high r.h. produce (NH4)2SO 4. A comparison of filter E and A shows a generally lower dry deposition to filter than to filter in Petri. This may depend on the fact that the rimmed deposition plate retains aerosol particles more efficiently by decreasing wind resuspension or particle rebound. The dry deposition rates and airborne ion concentration at L and H were used to calculate dry deposition velocity (Vd) for filters C, D, F and A, B, E, G, reported in Table 3. Researchers report a wide range of V and this is due to the large number of parameters that influence this d phenomenon. Sehmel (1980), I in a review of experimental data, reports these + 2+ + range values of V (cm s- ): Na , 0.2-4.3; Mg , 0.3-3; CI , 0.2-6.3; K , d 0.6-13; Ca 2+, 0.4-1.4 . Johannes et al. (1983) repot# for nitrate and sulphate (summer 1981) V in the range of 1.6-2.4 cm s and 0.15-2.43 cm s , d respectively. Our results are in the range of reported data. A comparison of upward filters shows a greater deposition velocity at H with respect to L except for Ca 2t , Na t P Mg 2t (filters A, C, E, F) and CI- (filters A, C) Previous studies (McMahon et at., 1979; Davidson et al., 1985) have shown that atmospheric coarse particles have relatively large V (5-10 cm s -I) d -I compared to atmospheric fine particles (0.2-1.0 cm s ).
S580
G. SANTACHIARAetal.
T a b l e 3. D e p o s i t i o n
(cm s -1)
Ion
A
B
C
D
E
F
G
CI-
0.24
0.030
0.71
0.77
1.64
0.66
0.93
NO
5.4
0.30
I.I
0.39
2.6
0.66
1.9
0.32
0.02
0.16
0.34
0.63
0.14
0.42
0.07
0.15
0.035
0.039
0.24
0.085
0.14
Na +
•
"
0.25
i
0.28
1.81
0.24
K÷
•
•
m
0.53
0. I0
0.19
0.25
Ca 2+
0.2
0.024
1.36
0. I0
0.27
0.81
0.50
Mg2+
0.39
0.34
0.99
0.79
3 SO 24 NH4 ÷
"" The
velocity
V
d
low V
particles
<0.01 cm s
d
"
i.I
"
-I
for NH ÷ confirms 4 , while the higher
that
these
ions
are mostly
on
the
submicronic
V
for nitric ion is due to high mass median d diameter ( Servant et al., 1984). Nitric ion, when temperature is high or there is appreciable H SO , will tend to evaporate from smaller particles 2 4 onto larger particles, such as those that are soil derived (Sisterson, 1989). While V for filters A and B (H) is much higher on upward than downward d surfaces ( but NH ÷ ) , at canopy level(Filters C and D) upward and downward 4 deposition velocities are comparable for some ions (CI-, NO - , NH ÷ ) and 3 4 the downward V d is higher for K + and SO 42- . At this level it is probable that resuspension phenomena, temperature, r.h., complex flow around a plant surface and concentration fields will affect the rate of transport to surface. Comparison of collectors E and G shows in general higher V d for most ions in Petri with inner filter
(E), except for K + , Ca2÷ , Mg 2+ .
Acknowledgements -The cooperation of Mrs Laura Santoli and Mr Gabriele Tigli (PMP- USL 28, Sezione chimica, Bologna ) in performing the chemical analysis and the assistance of Marcello Tercon are gratefully acknowledged. REFERENCES Davidson , C.I. ,S.E. Lindberg, J.A. Schmidt, L.G. Cartwright, and L.R. Landy (1985). Dry deposition of sulphate onto surrogate surfaces, J. Geophys. Res. , 90, 2123-2130. Johannes, A.H., E.R. Altwicker (1983). Relationship between dry d e p o s i t i o n as measured via collection with a dry bucket vs. ambient air concentration. In:Precipitation scavenging, Dry deposition and Resuspension (edited by Pruppacher H.R., Semonin R.G. and Slinn W.G.N.) pp.903-912. McMahon T.A. and P.J. Denison (1979). Empirical atmospheric deposition parameters - a survey . Atmos. Environ., 13, 571-585. Sehmel, G.A. (1980). Particle and gas dry deposition : A Review. Atmos. Environ., 14, 983-1011. Servant, J., R. Delmas, J. Rancher and M. Rodriguez (1984). J. of Atmos. Chem.,
!,
391-401.
Sisterson, D.L. (1989). A method for evaluation of acidic sulfate and nitrate 43, 61-72. in precipitation, Water, Air Soil Pollution, Van Hove, L.W.A., E.H. Adema, ~.J. Vredenberg and G.A. Pieters (1989). A study of the adsorption of NH and SO on the leaf 3 2 surfaces. Atmos. Environ., 23, 1479-1486
0021-8502/91 $3.00+0.00 Pergamon Press plc
DRY DEPOSITION MEASUREMENTS OF AEROSOL PARTICLES IN A CORN FIELD
G. Santachiara,
F. Prodi
and F. Vivarelli
Istituto FISBAT-CNR, Bologna, Italy • Also: Dipartimento di Fisica dell'UniversitA,
Ferrara.
ABSTRACT Aerosol size distribution was measured at a rural site at the 60 cm canopy level (Low) and at 160 cm (High) together with airborne concentration, dry deposition rates and velocity on surrogate surfaces ( Nuclepore and Petri dishes) for several water-soluble ions (CI-, NO 3- , S042-, NH4 ÷ , Na + , K ÷ , 2+ Ca , Mg2+). The airborne ion concentrations detected were t~pical Kfor rural sites and in general similar at both heights, except for Na and Upward surface deposition velocity at the higher level was generally much higher than the downward rate, whereas at canopy level both u p w a r d and downward deposition velocities were comparable for some ions (CI , NO 3 , NH4 ÷ ) and downward
velocity
was
higher
for
K+
and
SO 2-. A generally lower dry 4 deposition to filter surfaces than to filters inside Petri dishes was also detected. Some of these findings can be explained by canopy or land resuspension.
KEY WORDS Dry deposition;
aerosol size distribution;
surrogate surfaces.
INTRODUCTION Dry deposition is the removal of gases and particles from the atmosphere to the earth's surface by processes other than precipitation. Dry deposition of particles can derive from Brownian or eddy diffusion, impaction, interception, phoretic effects and thermophoresis, sedimentation. For very small particles (diameter, D < 0.1 ~m ) Brownian deposition is prevalent and increases as particle diameter decreases because the B r o w n i a n diffusivity increases. For l a r g e r particles ( D> 1 ~m), eddy diffusivity becomes important as do gravitational settling and particle inertia in causing deposition. In the 0. I-I ~m range, where phoretic effects are present but other mechanisms are negligible, the deposition velocity is minimum. Field measurements can be either direct , that is by the measured contamination of a real or artificial surface, or indirect. The latter includes micrometeorological techniques, i.e. profile method or eddy correlation. Micrometeorological measurements evaluate predominantly submicronic particles in the atmosphere, while total mass dry deposition is dominated by sedimentation of the small fraction of large airborne particles. Thus micrometeorological methods and surrogate surface techniques can be viewed as complementary (Davidson, 1985). Surrogate collectors provide the only means of routine monitoring and complete chemical analysis by obviating the problem of chemical element uptake or leaching from biological surfaces . Different types of surfaces have been used for collection, i.e. open buckets, Petri $577
$578
G. SANTACHIARAet al.
dishes, microscope slides, filter paper and so on, and a large range of collection efficiencies are reported (Sehmel, 1980). The present study evaluated the dry deposition of several water-soluble ions on Nuclepore filter and Petri dishes, by measuring atmospheric concentrations and dry deposition rates.
EXPERIMENTAL
APPARATUS AND PROCEDURE
/
The site used for the dry deposition measurements was a corn-field at San Pietro Capofiume, near Bologna; the surrounding area was a grassy field. Dry deposition sampling began 23 May 1989 and continued through 30 May at two heights, 60 cm canopy height (low, L) and 160 cm (high, H). Four Nuclepore filters (0.2 #m, 47 mm in diameter) facing upward were placed at the top of a square plate and four more facing downward were placed at the bottom of the same plate. The plates, with sides of 17.5 cm, thickness of 0.4 cm and edges rounded to reduce turbulence, were supported by a cylindrical stake with 1.0 cm in diameter. Dry deposition samplings were taken also with polysterene Petri of 53 mm internal diameter and a i0 mm rim. We also sampled particles suspended in air by filtration through 0.2 Nm Nuclepore filters in stainless steel holders at L and H. Sampling time was 96 h and sampled air volume 19.2 Nm 3. After sampling, filters were stored in closed plastic petri dishes and from them a l-cm-diameter plate was cut, coated with a thin film of carbon and examined at SEM for size distribution of aerosol particles. The collection filters were extracted in I0 ml of Super-Q water for 30 min in an ultrasonic bath and then washed in two 5ml aliquots of super-Q water; washing period was I h per filter. Samples were filtered through 0.4 #m Nuclepore filters and refrigerated until analysis : Na +, K +, + 2NH4 , C1 , NO3 , SO 4 were determined by a Dionex 2001i ion chromatograph; Ca 2+ and Mg 2+ were determined by spectrometer; and filter blank values conditions during dry deposition field ranged from 0.8 to 5.4 m s -z ; air relative humidity (r.h.) from 30 to 99
AEROSOL SIZE DISTRIBUTION
Perkin- Elmer 403 atomic absorption were evaluated. Daily meteorological program were also measured. Wind speed temperature from 7.4 to 29.2 °C and Z .
AND ATMOSPHERIC
CONCENTRATION
Size distribution of the aerosol particles determined from the integral filters at L and H (Filters 1 and 2) and airborne concentration of the examined ions are reported in Fig. l and Table I. AFilterl o Filter 2
Table l. Airborne concentration(~g Ion Filter I Filter 2 0.83 Cl 0.76 NO
I. 43
O. 79
7.95
7.2
2.59
1.98
Na +
O. 16
I. 57
K+
O. 19
I. i0
Ca 2÷
2.2
4.58
Mg2÷
0. I 0
0. I 0
3 2-
SO 4
o
NH + 4
~5
~
10-2
1if'
J 1o °
I
Radius, pm
Fig. l. Aerosol
size distribution
m -3)
Dry deposition measurements ofaerosolparticles
$579
The Junge distribution dN/d log R = c R -~ approximates the particles spectrum in the 0. I - I0 ~m range. The values of ~ for filters 1 and 2 are respectively 2.38 and 2.36, indicating a relatively higher ratio of large to small particles.
DRY DEPOSITION RATES AND VELOCITY The measured
fluxes are reported
Table 2. Dry deposition
(xlE-6 mg cm -2 day)
A
B
C
D
E
F
N,47
N,47
N,47
N,47
N,47
N,47
UP, H
D,H
UP, L
D,L
H, Petri
L, Petri
Ion
Cl
rates
in Table 2 by filter.
G Petri H
17
2.2
46
52
118
46
67
422
21
106
48
179
82
133
202
I0
113
235
394
93
263
12
28
7.8
9
41
19
25
Na t
"
~
4.3
"
38
25
32
Kt
"
"
"
8.6
9.4
3.1
23
Ca 2+
102
9.4
258
18
108
155
196
Mg 2+
4.7
"
9.4
"
4.1
7.2
9.5
NO 3 2SO 4 t
NH 4
":Dry deposition rates < IE-6 mg cm -2 day; N: Nuclepore filter; Up:upward deposition ; D:downward deposition; H:sampling at 160 cm; L: sampling at 60 cm ; 47: filter diameter , mm. For filters A, B, C, D upward and downward ammonium dry deposition rates are + similar. This may depend on resuspension of NH from canopy or soil. Van 4 Hove (1989) shows that NH and SO are adsorbed on leaf surface of bean and 3 2 poplar, and in presence of high r.h. produce (NH4)2SO 4. A comparison of filter E and A shows a generally lower dry deposition to filter than to filter in Petri. This may depend on the fact that the rimmed deposition plate retains aerosol particles more efficiently by decreasing wind resuspension or particle rebound. The dry deposition rates and airborne ion concentration at L and H were used to calculate dry deposition velocity (Vd) for filters C, D, F and A, B, E, G, reported in Table 3. Researchers report a wide range of V and this is due to the large number of parameters that influence this d phenomenon. Sehmel (1980), I in a review of experimental data, reports these + 2+ + range values of V (cm s- ): Na , 0.2-4.3; Mg , 0.3-3; CI , 0.2-6.3; K , d 0.6-13; Ca 2+, 0.4-1.4 . Johannes et al. (1983) repot# for nitrate and sulphate (summer 1981) V in the range of 1.6-2.4 cm s and 0.15-2.43 cm s , d respectively. Our results are in the range of reported data. A comparison of upward filters shows a greater deposition velocity at H with respect to L except for Ca 2t , Na t P Mg 2t (filters A, C, E, F) and CI- (filters A, C) Previous studies (McMahon et at., 1979; Davidson et al., 1985) have shown that atmospheric coarse particles have relatively large V (5-10 cm s -I) d -I compared to atmospheric fine particles (0.2-1.0 cm s ).
S580
G. SANTACHIARAetal.
T a b l e 3. D e p o s i t i o n
(cm s -1)
Ion
A
B
C
D
E
F
G
CI-
0.24
0.030
0.71
0.77
1.64
0.66
0.93
NO
5.4
0.30
I.I
0.39
2.6
0.66
1.9
0.32
0.02
0.16
0.34
0.63
0.14
0.42
0.07
0.15
0.035
0.039
0.24
0.085
0.14
Na +
•
"
0.25
i
0.28
1.81
0.24
K÷
•
•
m
0.53
0. I0
0.19
0.25
Ca 2+
0.2
0.024
1.36
0. I0
0.27
0.81
0.50
Mg2+
0.39
0.34
0.99
0.79
3 SO 24 NH4 ÷
"" The
velocity
V
d
low V
particles
<0.01 cm s
d
"
i.I
"
-I
for NH ÷ confirms 4 , while the higher
that
these
ions
are mostly
on
the
submicronic
V
for nitric ion is due to high mass median d diameter ( Servant et al., 1984). Nitric ion, when temperature is high or there is appreciable H SO , will tend to evaporate from smaller particles 2 4 onto larger particles, such as those that are soil derived (Sisterson, 1989). While V for filters A and B (H) is much higher on upward than downward d surfaces ( but NH ÷ ) , at canopy level(Filters C and D) upward and downward 4 deposition velocities are comparable for some ions (CI-, NO - , NH ÷ ) and 3 4 the downward V d is higher for K + and SO 42- . At this level it is probable that resuspension phenomena, temperature, r.h., complex flow around a plant surface and concentration fields will affect the rate of transport to surface. Comparison of collectors E and G shows in general higher V d for most ions in Petri with inner filter
(E), except for K + , Ca2÷ , Mg 2+ .
Acknowledgements -The cooperation of Mrs Laura Santoli and Mr Gabriele Tigli (PMP- USL 28, Sezione chimica, Bologna ) in performing the chemical analysis and the assistance of Marcello Tercon are gratefully acknowledged. REFERENCES Davidson , C.I. ,S.E. Lindberg, J.A. Schmidt, L.G. Cartwright, and L.R. Landy (1985). Dry deposition of sulphate onto surrogate surfaces, J. Geophys. Res. , 90, 2123-2130. Johannes, A.H., E.R. Altwicker (1983). Relationship between dry d e p o s i t i o n as measured via collection with a dry bucket vs. ambient air concentration. In:Precipitation scavenging, Dry deposition and Resuspension (edited by Pruppacher H.R., Semonin R.G. and Slinn W.G.N.) pp.903-912. McMahon T.A. and P.J. Denison (1979). Empirical atmospheric deposition parameters - a survey . Atmos. Environ., 13, 571-585. Sehmel, G.A. (1980). Particle and gas dry deposition : A Review. Atmos. Environ., 14, 983-1011. Servant, J., R. Delmas, J. Rancher and M. Rodriguez (1984). J. of Atmos. Chem.,
!,
391-401.
Sisterson, D.L. (1989). A method for evaluation of acidic sulfate and nitrate 43, 61-72. in precipitation, Water, Air Soil Pollution, Van Hove, L.W.A., E.H. Adema, ~.J. Vredenberg and G.A. Pieters (1989). A study of the adsorption of NH and SO on the leaf 3 2 surfaces. Atmos. Environ., 23, 1479-1486