NATURAL DUST AND ACID R A I N
ERHARD M. WINKLER, Departmerit of E a r t h S c i e n c e s , U n i v e r s i t y
of Notro Dale, Notre Dame, I n d i a n a .
ntmospheric d u s t o r i g i n a t e s from t h r e e s o u r c e s , t e r r e s t r i a l
a i r b o r n m a t t e r , v o l c a n i c , and cosmic. T e r r e s t r i a l n a t u r a l d u s t
makes up t h e main b u l k r e f l e c t i n g t h e s o i l . composition t o
150 m i l e s away, S o i l e r o s i o n from f l o o d p l a i n s , plowed f i e l d s
and c o n s t r u c t i o n s i t e s a r c t h e main s o u r c e , Q u a r t z , f e l d s p a r ,
f:he c a r b o n a t e s c a l c i t e and dolomite, and c l a y m i n e r a l s a r e
t h e components i n d e c r e a s i n g o r d e r of frequency. N a t u r a l
d u s t i n t h e atmosphere i n t e r a c t s w i t h r a i n w a t e r c o n v e r t i n g
t h e c a r b o n a t e s t o benign gypsum (CaS04.2 NZO). N a t u r a l l y
leached s o i l s produce l e s s c a l c i t e t h a n unweathered s e d i m e n t s
on f l o o d p l a i n s and c o n s t r u c t i o n sl.tcs, and i n g r a n i t i c and
c r y s t a l l i n e r o c k s l o s s t h a n i n l i m e e tone a r e a s .
Heavy i n d u s t r i a l i z a t i o n a s s o c j . a t e d w i t h high emission o f C02
and $02 on t h e one hand, and e x c e s s productiorr of d u s t on t h e
o t h e r appear t o c o u n t e r a c t man's i n t e r f e r e n c e w i t h n a t u r a l
ecosystems i n o p p o s i t e d i r e c t i o n .
Acid r a i n i s made r e s p ~ n s i b l ef o r t h e a c c e l e r a t i o n of t h e
n a t u r a l decay of s t o n e , f o r t h e a c c e l e r a t e d Xeac't~ingr a t e of s o i l s and
t h e change of growth r a t e of v e g e t a t i o n , Likens, Bomann and Johnson
(1972) c l a i m t h a t tile a v e r a g e pH of r a i n w a t e r o f t h e Rhine
Ruhr
i n d u s t r i a l a r e a of West Cemany h a s dropped from a b o u t pfr=5 t o l e s s
t h a n pN=4 i n t h e y e a r s between 1956 and 1966, The n e a r exponent,ial
r a t e change of weathering of s t o n e monuments f o r t h e Rhine-Ruhr area
was doctunentad by Winkler (1973) w i t h t i m e - l a p s e p h o t o s of Herten
C a s t l e i n W e s t f a l i a , Germany ( F i g . 1) c a r v e d from c a l c i t e - c e m e n t e d
Barnberg sandstone: one p h o t o was t a k e n i n 1908, t h e second i n 1968,
F i g u r e 2, shows t h e s e m i - q u a n t i t a t i v e p l o t f o r t h e fierten C a s t l e s i n c e
i t s c o n s t r u c t i o n i n 17021 t h i s c u r v e i s b e l i e v e d t o b e v a l i d a l s o f o r
many o t h e r monxunents i n i n d u s t r i a l i z e d a r e a s of t h e world (Fig. 3 ) .
Likens and Bormann (1974) a s c r i b e t h e d e c r e a s e o f r a i n w a t e r p H i n t h e
l a s t 20 y e a r s i n New England and o t h e r a r e a s of t h e world t o t h e
-
F i g u r e 1. Time-lapse p h o t o s of s t a t u e s a t H e r t e n C a s t l e
n e a r Reckliny-hausen, W e s t f a l i a , Germany, b u i l t i n l 7 0 2 ,
The l e f t s i d e p i c t u r e was t a k e n i n 3.308, t h e r i g h t sich
p i c t u r e 1969, showing a l m o s t c o m p l e t e d e s t r u c t i o n o f t h e
s u r f a c e . P o r o u s i3atunberg s a n d s t o n e i s composed o f m o s t l y
q u a r t z g r a i n s which a r c cemented w i t h solub1.e c a l c i t e ,
P h o t o s and i n f o r m a t i o n were s u p p l i e d by M r . Schmidt-Tho~nsen,
I,andesdenkmalamt Westfalen-l;ippe, Muenster, Germany.
F i g u r e 2. G r a p h i c p r e s e n t a t i o n o f
approximate weatheriny rate f o r p i c t u r e set of s t a t u e a t i l e r t e n Castle,
W e s t f a l i a , Germany, i l l u s t r a t e d i n
F i g u r e l.
Figure 3 . Rapid decay towards t o t a l destruction of r e l i e f on
tympanon of parish church i n Opherdicke, near Unna, Westfalia.
Destruction was severe i n 1893, a f t e r 600 years of exposure,
but catastrophic within the l a s t 80 years. The r a t e of weathering of Figure 2. appears t o apply here too. Stone i s Soest
Greensandstone w i t h 65% c a l c i t e . Photos and information were
supplied by M r . ScImidt-'l'homsen, Landcsdenkmalamt WestfaliaXdppe, Germany,
increased use of Eucl o i l and natural gas, a l s o t o the reduction of
neutralizing a l k a l i c reacting soot by mandakory i n d u s t r i a l dust
p r e c i p i t a t o r s , The proscnee ancl thc i n t e r a c t i o n of natural airborn
dust i s nob mentioned. Winkler (l.9'73) gives three compor~entsf o r
natural dust:
I. Windblown dust, from dry r i v e r beds, flood p l a i n s and
beaches, but a l s o from plowed f ielcis and constxuctislr s i t e s lacking
a protecti-ve vegetatj.vc cover. Quartz, carbonates, feldspars, atld
clay minerals a r e the most i.mportant mineral components.
2. Volcanic dust: Tho explosive eruption of volcanoes has s e n t
d u s t i n t o the upper a t n ~ s p h c r e ,e s s e n t i a l l y as s i l i c a g l a s s , where it
has travelled for several years before it could s e t t l e back t o the
e a r t h ' s surface.
3 . Extratcsrrestri.al dust o r cosmj.c dust I.s found i n marine
sediments of most geological ages a s small p e l l e t s of iron and
s i l i c a t e glass. 9'hci.r chemically inactive character and the minute
w a n t i t i e s present make &heir e f f e c t i n s i g z ~ i f i c a n t ~
The following t & l e sumariees natural dust, i t s mineral content
and r e l a t i v e r e a c t i v i t y w i t h a i r pollutantst
COMPOSPTLON OF WATUWL BUST
Mineral Composition:
l.
Approx. Reactivi%y:
T e r r e s t r i a l Dusk, the most e s s e n t i a l componeng sf windblown
silt:
a)
dust from carbonate rocks (limestone, dolomite):
agr9culturaE s o i l s , , , clay, quartz
little
reactive
,, c a l c i t e , some clay
eafcite
flood plains
most reactive
construckion s i t e s
c a l c i t e , clay
calcite
most reactive
a,s,,,,,
.,
...,,,
.,,
b)
dust from c r y s t a l l i n e rocks (qranite, gneiss, slake, e t c , ) :
agricultural s o i l s
quartz, clap
little
reactive
flood plains
quartz, clay, s i l i c a t e s , , l i t t l e
reactiv";
construction sikes
quartz, clay, s i l i c a t e s , , l i t t l e
reactive
.,,,,,,,
......,,,
...
c)
dusk from g l a c i a l till (nmrafnes) :
agricultural s o i l s
quartz, clay, ( s i l i c a t e s ) , ,
L i t t l e reactive
flood plains
quartz, clay, c a l c i t e , . .
r e a c t i ve ( c a l c i t e )
constructiow s i t e s ,,, q u a r t z , clay, c a l c i t e , , ,
reactive (calcike)
.,,
.,,,,,,,,
dl
dusk from glacial outwash
a g ~ i c u J t u r a ls o i l s
reactive
f l ~ s dplains
reactive
construckion s i t e s
reactive
[sand and gravel):
silicates,(calcite),
... quartz,
....,,,,, quartz,
... quartz,
silicates, calcite,,
silicates, calcite,.
2,
Volcanic Bust: glassy s i l i c a t e s ; about one %ankhs sf terrest r i a l dust:
quartz, s i l i c a t e s , , , very l i t t l e reactive
3,
Cosmic Bust: glassy s i l i c a t e and glassy metallic; amount i s
minimal :
no reaction expected
Quartz, the carbonate minerals calcite and dolomite, the feldspars, ferro-magnesian silicates and various clay minerals are the
mineral components of importance.. Calcite in dust readily reacts with
the acids in the rainwater despite its brief contact between the cloud
base and the surface of the terrain. The slow sinking velocity of
silt-sized particles in still air ranges from 1 mm to 1 cm per second
(fig.4); calcite has thus enough time to react with the acids in the
I
I
k-
I
I
I
dre
I <-Thin Smokes
-
I
I-
-
!
k-
I
I
Cloud pticles
l
---A- Atmospheric Dust &Fag
I
~rt.4
Lv-
DUS~
sand -I
I
I
I
I
SEE OF A9RTlCLES fmm)
Figure 4. Sinking velocities of
particles in still air. The size of
natural dust is marked.
air whereby high relative humidity can accelerate the reaction. The
sulfate ion is not eliminated from rain but converts from sulfurous or
sulfuric acid to relatively harmless gypsum, CaS04. The sulfate ion
tied up in gypsum is not noxious to man. Waters rich in sulfate, however, are known to have severely attacked alkalic cement and concrete.
The presence of water-soluble gypsum appears to explain the frequent
high calcium content of rainwaters. Sumi et al. (1959) describe gypsum
as the chief water-soluble component in the atmosphere of almost all
industrialized areas.
The mineral quartz is inert and non-reactive; clay crystals,
however, can absorb ingredients and hold them tightly on their crystal
surfaces which may help to neutralize some acid components in the air
temporarily untilthey can be released again to the soil. Feldspars and
the ferro-magnesian silicates react with the tiny H+ cation in acid
rain producing less acid conditions of the rain when tied up during
the weathering process. The time of interaction in the atmosphere
appears to be sufficient for the reaction.
The mineral composition of natural dusts depends primarily upon
the a r e a of o r i g i n , a s well a s whether t h e d u s t i s derived from deeply
weathered a g r i c u l t u r a l s o i l s , from f r e s h o r weathered g l a c i a l d r i f t
m a t e r i a l s , o r from flood p l a i n s and c o n s t r u c t i o n s i t e s . Despite l o c a l
accumulations of windblown d e b r i s of considerable magnitude i n t h e geol o g i c a l p a s t , t h e r a t e of d u s t deposition i s believed t o have p o s s i b l y
t r i p l e d s i n c e man has s t a r t e d a g r i c u l t u r a l a c t i v i t y and construction by
s c a r r i n g the p r o t e c t i v e sod cover of t h e American p l a i n s . Davitaya
(1969) has noticed considerable i n c r e a s e of d u s t i n t h e f i r n of t h e remote mountains of t h e Caucasus caused by both n a t u r a l and human r e c e n t
events, l i k e t h e e r u p t i o n of M t . Krakatoa i n 1883, and o t h e r s . Yaalon
and Ganor (1973) discuss i n d e t a i l d u s t t r a n s p o r t from d e s e r t f l o o r s ,
dry flood p l a i n s and v a l l e y t r a i n s of g l a c i a l outwash. Dust can be
l i f t e d upward from bare ground t o a h e i g h t of 1500 t o 2000 m, when t u r bulent surface winds p r e v a i l , common near high-pressure f r o n t s .
T h e o r e t i c a l l y , wind does not l i f t s i l t - s i z e d p a r t i c l e s o f f t h e ground
a s shown i n Figure 5., a s t h e dead a i r l a y e r a f f e c t s t h e
.L
+
0)
-";-C
Figure 5. Sand and S i l t g r a i n s ,
schematized. S i l t g r a i n s within
t h e dead a i r l a y e r a r e not
r e a d i l y picked up by wind.
s i l t s i z e . Splashing and bouncing impacts only can mobilize t h e
grains. S o i l aggregates smaller than 0.84 mrn diameter a r e s u b j e c t t o
s i n d erosion with a non-linear i n c r e a s e with decreasing g r a i n s i z e s ,
whereby a i r f r i c t i o n can break up t h e s o i l grains. Grain diameters
l a r g e r than 20 pm s e t t l e t o t h e ground r e l a t i v e l y f a s t . The plowing
and t i l l i n g of dry f i e l d s , t h e a c t i v i t y of road graders and s o i l
moving devices on major construction s i t e s s t i r up clouds of d u s t
eliminating t h e need f o r s t r o n g a i r turbulence a t the ground surface
l e v e l . The d u s t production i s a l s o increased by over-grazing and
t r a f f i c by c a t t l e , l i k e i n the Sahel drought b e l t of Africa today.
No r e l i a b l e d a t a a r e a v a i l a b l e f o r man's p a r t i c i p a t i o n of n a t u r a l d u s t
i n the a i r .
Likens and Bormann (1974) r e p o r t a c i d rainwaters a t near pH = 4
i n New England which were a l s o recorded a t Geneva and I t h a c a , New York;
t h e a c i d i t y has increased s i n c e about 20 years ago and i s believed t o
have been associated with t h e increase of n a t u r a l gas and o i l consumption, b u t a l s o with t h e compulsory i n s t a l l a t i o n of particle-removing
devices from f l u e gasses. The e x t r a p o l a t i o n of a c i d i t y of r a i n i n t o
t h e near f u t u r e may r a i s e t h e question whether o r not man's i n t e r ference i n possibly two opposite d i r e c t i o n s can r e - e s t a b l i s h
original pre-interference conditions in the acidity of rainwaters:
high acidity of the rainwater by air pollution on the one hand and
higher production of neutralizing natural dust and soot on the other.
The question is thus arising why abnormal acidity arises only in some
places. e.g. in New England though the sulfates and other acid ingredients from industrial sources are spreading about equally through
atmospheric dispersion throughout the industrial part of the United
States. This writer believes that the dense and continuous forest
cover of the New England states produces much less natural dust than
the agricultural prairie states (Fig. 6 ) ; in addition, the area is
Figure 6. Distribution of forested areas of the United
States.
almost entirely underlain by igneous and metamorphic rocks which do
not produce acid-neutralizing calcite. Figure 7. shows the distribution of major areas with crystalline rocks exposed at the surface. A
tentative range of about 240km (150 miles) from carbonate areas should
be allowed, necessary to supply the major bulk of carbonate dust.
This author is also convinced that the degree of calcite leaching of
the upper soil layer has a strong bearing on the supply of calcite to
the dust.
It is difficult to evaluate the true neutralizing strength of
both natural and industrial dusts. A closer investigation could give
directives where precipitators for industrial dust removal should be
employed in order to avoid damage by acid rain in the future. Thus
the requirements for future installations of industrial scrubbers
should be based upon regional, geological-ecological considerations if
we want to eliminate further damage by acid rains to the environment,
and finally to man himself.
Figure 7.
Distribution of non-calcareous crystalline rocks
of the United States.
REFERENCES CITED
Davitaya, F. F. Atmospheric dust content as a factor affecting
glaciation and climatic change. ANNALS OF THE ASSOC. OF
AMERICAN GEOGRAPHERS, V. 59 (3), pp. 552-560, 1969.
Jackson, M. L., D. A. Gillette, E. F. Danielson, I. H. Blifford,
R. A. Bryson, J. K. Syers. Global dustfall during the Quaternary
as related to environments. SOIL SCIENCE, v. 116 (3), pp. 135145, 1973.
Likens, G. E., F. H. Bormann. Acid Rain. A serious regional environmental problem. SCIENCE, v. 184, 14 June, pp. 1176-1179, 1974.
Likens, G. E., F. H. Bormann, N. M. Johnson.
v. 14 (2) pp. 33-40, 1972.
Acid Rain.
ENVIRONMENT,
Sumi, L., A Corkery, J. I. Monkman. Calcium sulfate content of urban
air. GEOPHYSICAL MONOGRAPH NUMBER 3, Publication 652, ed.
J. P. Lodge; pp. 69-80, 1959.
Winkler, E. M. Stone. Properties, ~urabilityin Man's Environment.
Springer-Verlag, New York-Vienna, 250 p., 1973.
YaaLon, D. H., E. Ganor. The influence of dust on soil during the
Quaternary. SOIL SCIENCE, v. 116 ( 3 1 , pp. 146-155, 1973.