Atmospheric turbidity at Valentia observatory

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ATMOSPHER

SUplmar y

IC TURBIDITY AT VALENTIA OBSERVATORY

V a l u e s 0.f t h e Angstrom T u r b i d i t y C o e f f i c i e n t (

P

) d e t e r m i n e d from S o l a r R a d i a t i o n o b s e r v a t i o n s a t V a l e n t i a O b s e r v a t o r y o v e r t h e p e r i o d 1955

-

1971 a r e a n a l y s e d . Mean d a i l y t u r b i d i t y most f r e q u e n t l y l i e s between 0 . 0 3 0 and 0 . 0 4 0 w i t h a median v a l u e of 0 . 0 4 7 . T u r b i d i t y i s h i g h e r i n Summer t h a n i n W i n t e r , t h e

h i g h e s t mean monthly v a l u e b e i n g i n A p r i l and t h e lowest i n December/January. The s c a v e n g i n g by r a i n f a l l p r o b a b l y h a s a c o n s i d e r a b l e a f f e c t i n d e t e r m i n i n g t h i s d i s t r i b u t i o n .

The c h i e f f a c t o r d e t e r m i n i n g t h e t u r b i d i t y i s t h e a i r mass t y p e . C o n t i n e n t a l T r o p i c a l a i r i s seldom e x p e r i e n c e d b u t f o r t h e two months f o r which means a r e a v a i l a b l e t h e t u r b i d i t i e s are h i g h e r t h a n f o r any o t h e r a i r mass. R e l a t i v e l y h i g h t u r b i d i t i e s are a s s o c i a t e d w i t h C o n t i n e n t a l P o l a r a i r and t h e lowest v a l u e s a r e found i n A r c t i c a i r . The p r e v a i l i n g a i r mass, Maritime P o l a r , shows r e l a t i v e l y low t u r b i d i t y and d o m i n a t e s t h e a n n u a l p a t t e r n .

T h e r e i s n o e v i d e n c e of any t r e n d of i n c r e a s i n g t u r b i d i t y d u r i n g t h e p e r i o d .

1 . I n t r o d u c t i o n

S o l a r r a d i a t i o n o b s e r v a t i o n s have been made a t V a l e n t i a O b s e r v a t o r y s i n c e September, 1 9 5 4 . Continuous r e c o r d s have been m a i n t a i n e d of Global and D i f f u s e S o l a r R a d i a t i o n . A Linke-Feussner t h e r m o e l e c t r i c i r o n - c l a d A c t i n o m e t e r (Kipp and Zonen) was a l s o b r o u g h t i n t o u s e at t h e same time and a s c h e d u l e of r o u t i n e o b s e r v a t i o n s on d i r e c t s u n l i g h t h a s been m a i n t a i n e d . The normal s c h e d u l e was t h r e e o b s e r v a t i o n s p e r day a t a b o u t 1 0 h r s . , 1 2 h r s . 'and 1 4 h r s . G.M.T. when w e a t h e r and sky c o n d i t i o n s p e r m i t t e d . In a d d i t i o n

t o t h e measurements of t o t a l d i r e c t s u n , measurements were a l s o made of t h e i n t e n s i t y of t h e d i r e c t s o l a r beam b e h i n d two f i l t e r s of S c h o t t g l a s s , OG1 and RG2. T h e s e f i l t e r s were r e c e i v e d from t h e R a d i a t i o n Commission of t h e

I n t e r n a t i o n a l A s s o c i a t i o n of Meteorology and c a l i b r a t e d

a t

Davos. A " f i l t e r f a c t o r " was s u p p l i e d w i t h e a c h f i l t e r s o t h a t measurements made b e h i n d t h e f i l t e r c o u l d be c o r r e c t e d f o r s u r f a c e r e f l e c t i o n and a b s o r p t i o n by t h e f i l t e r From t h e measurements of t h e i n t e n s i t y of t h e d i r e c t s o l a r beam w i t h and w i t h o u t f i l t e r s a " f a c t o r " o r c o e f f i c i e n t e x p r e s s i n g t h e t u r b i d i t y of t h e a t m o s p h e r e ( o r amount of a e r o s o l p r e s e n t ) c a n b e e v a l u a t e d .

S i n c e 1971 V a l e n t i a h a s been p a r t i c i p a t i n g as a R e g i o n a l a i r p o l l u t i o n s t a t i o n w i t h i n t h e network e s t a b l i s h e d by W.M.O. t o m o n i t o r a i r p o l l u t i o n . I t w i l l b e some t i m e b e f o r e s u f f i c i e n t d a t a c a n b e accumulated from t h i s network t o e n a b l e any c o n c l u s i o n s t o b e drawn. Thus an a n a l y s i s of t h e t u r b i d i t y d a t a a v a i l a b l e f o r V a l e n t i a s i n c e 1955 may b e of i n t e r e s t . The p u r p o s e of t h i s p a p e r i s t o p r e s e n t such an a n a l y s i s .

2. S i t e of t h e O b s e r v a t i o n s

The O b s e r v a t o r y , which is i n t h e e x t r e m e SW of I r e l a n d , i s s i t u a t e d

on t h e SE s i d e of t h e narrow e s t u a r y of V a l e n t i a r i v e r which r u n s a p p r o x i m a t e l \ NE-SW ( F i g . 1 ) . I t i s a b o u t 1 Km t o t h e SW of t h e town of C a h i r c i v e e n . The p o p u l a t i o n of C a h i r c i v e e n i s a b o u t I 8 0 0 and a p a r t from d o m e s t i c f i r e s f o r c o o k i n g and h e a t i n g t h e r e a r e no major s o u r c e s of p o l l u t i o n . The r e s t of t h e a r e a i s v e r y s p a r s e l y p o p u l a t e d . I t c o u l d be e x p e c t e d , t h e r e f o r e , t h a t r e g u l a r measurements o f t u r b i d i t y i n such a l o c a t i o n , remote from s o u r c e s of a r t i f i c i a l a t m o s p h e r i c a e r o s o l would h e l p i n e s t a b l i s h i n g a "background

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Fig.1

Map

showing t h e

s i t e

of V a l e n t i a O b s e r v a t o r y and

i t s

e n v i r o n s .

3 . The T u r b i d i t y C o e f f i c i e n t (

p )

The b a s i c r e l a t i o n f o r t h e e x t i n c t i o n of s o l a r r a d i a t i o n may be e x p r e s s e d as f o l l o w s :

where

I

=

t h e r a d i a t i o n i n t e n s i t y measured by t h e a c t i n o m e t e r and e x p r e s s e d i n t h e I n t e r n a t i o n a l P y r h e l i o m e t r i c S c a l e 1956.

I (

h )

=

s o l a r r a d i a t i o n i n t e n s i t y o u t s i d e t h e atmosphere a t t h e mean 0

s u n - e a r t h d i s t a n c e , a s a f u n c t i o n of t h e wavelength

( h

) and e x p r e s s e d i n t h e same u n i t s

as

I .

S

=

t h e r e d u c t i o n f a c t o r f o r mean s o l a r d i s t a n c e . e

=

t h e b a s e of n a t u r a l l o g a r i t h m s .

A (

h

=

t h e e x t i n c t i o n exponent which i s a f u n c t i o n of wavelength (

h

1.

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t h e e x t i n c t i o n d u e t o c l e a r d r y a i r a c c o r d i n g t o R a y l e i g h ' s t h e o r y of s c a t t e r i n g by a i r m o l e c u l e s .

I t may be t a k e n as m X 0.00897h -4'09 where "m" i s t h e o p t i c a l a i r mass.

t h e e x t i n c t i o n due t o h a z e which a c c o r d i n g t o t h e t h e o r y of A . Angstrom [ 1 , 2 , ] may b e e x p r e s s e d by

where t h e wavelength exponent

" a "

v a r i e s from c a s e t o c a s e b u t r e m a i n i n g between 0 and 4. I t i s a good g u i d e t o t h e d i s t r i b u t i o n of t h e number of t h e h a z e p a r t i c l e s a s a f u n c t i o n of t h e i r s i z e . 8 ,

,,

i n c r e a s e s when t h e r a t i o of t h e s m a l l p a r t i c l e s i n t h e atmosphere i n c r e a s e s and when t h e atmosphere c o n t a i n s a h i g h p r o p o r t i o n of v e r y l a r g e a e r o s o l " a " a p p r o a c h e s z e r o . Angstrom h a s shown t h a t o

=

1 . 3 i s a v e r y good a v e r a g e v a l u e s o t h a t e x t i n c t i o n due t o h a z e ( e q u a t i o n 3 ) may b e w r i t t e n i n t h e form

"

P

'I _{i s t h e n a measure of t h e q u a n t i t y of h a z e i n t h e a t m o s p h e r e} and i s known a s t h e Angstrom T u r b i d i t y C o e f f i c i e n t .

t h e e x t i n c t i o n c a u s e d by water vapour i n t h e atmosphere.

4 . Computation of

P

The most d i r e c t method of d e t e r m i n i n g

P

i s by m e a s u r i n g t h e

i n t e n s i t y of t h e d i r e c t s o l a r beam i n t h e u l t r a - v i o l e t and v i s i b l e p a r t of t h e s p e c t r u m (

<

0.630

p

) s i n c e i n t h i s r e g i o n t h e s e l e c t i v e a b s o r p t i o n by water vapour i s n e g l i g i b l e and t h e a t t e n u a t i o n of s o l a r r a d i a t i o n i s c a u s e d o n l y by d r y a i r ( R a y l e i g h s c a t t e r i n g ) and h a z e .

The r a d i a t i o n i n t h e u l t r a - v i o l e t and v i s i b l e p a r t s of t h e s p e c t r u m ( i . e . t h e s h o r t wave r a d i a t i o n ) i s t h e d i f f e r e n c e between t h e t o t a l r a d i a t i o n a s measured w i t h o u t f i l t e r and t h e r a d i a t i o n p a s s e d by t h e r e d f i l t e r RG2. I f w e d e n o t e t h i s measured r a d i a t i o n d i f f e r e n c e a d j u s t e d by t h e f i l t e r f a c t o r by I k t h e n

where Ad(

A

) i s g i v e n by e q u a t i o n (4 ).

A l l t h e terms of t h i s e q u a t i o n a r e known e x c e p t

P

which can be e v a l u a t e d from t a b l e s such as t h o s e g i v e n i n t h e Appendix t o t h e Annals of t h e I G Y [ 3 ] .

.

5. O r d e r of Magnitude of

P

~ o l l o w i r i ~ t h e p r o c e d u r e o u t l i n e d above

p

was computed f o r a l l o c c a s i o n s when w e a t h e r and sky c o n d i t i o n s p e r m i t t e d o b s e r v a t i o n s t o b e made. Mean d a i l y v a l u e s were t h e n o b t a i n e d .

Of t h e 1162 d a i l y means a v a i l a b l e t h e most f r e q u e n t v a l u e was between - 0 3 0 and - 0 4 0 which r a n g e a c c o u n t e d f o r 16.4% of a l l d a y s . Values l e s s t h a n - 0 1 0 were o b s e r v e d o n 1.2% of t h e d a y s and v a l u e s g r e a t e r than - 2 0 0 o c c u r r e d on 2 . 3 % of t h e o b s e r v a t i o n d a y s . T a b l e 1 shows t h e p e r c e n t a g e

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TABLE I PWCENTAGE FREQUENCY OF DAILY MEANS OF

!

W I l E I N DIFFERENT RANGES

'

. ' '

j ~ x d

I

Y e a r

I

1 - 2 9.5 24.0 4 0 - 6 3 4 63-1 70.9 7 6 - 5 80.8 84.5 87.5 8 9 - 0 91.1 9 3 - 0 9 4 - 3 9 5 - 3 9 5 - 9 9 6 - 6 97-0 9 7 - 7 1

'000 010 020 030 040 050 080 070 080 090 100 I 1 0 120 130 140 150 160 170 180 190 200

: t o t o t o t o t o t o

t i

t o t o t o t o t o t o t o t o t o t o t o t o t o 6r

009 019 029 039 049 059 069 079 089 099 1 0 9 1 1 9 1 2 9 1 3 9 '149 1 5 9 169 179 189 1 9 9 more

I

TABLE l a CUMULATIVE PWCENTAGE FREQUENCY OF DAILY MEANS OF

P

I

Winter

I

2 - 0 16.4 34.9 I 6 1 - 6 69-0 7 6 - 4 83.0 87-1 90.6 92.6 9 3 - 4 9 5 - 4 96.7 9 7 - 6 9 7 - 8 97-S 9 8 - 3 9 8 - 7 98.71

1

p

x i o 3

'

-

- -

I

~ummer

1

0 . 7 5.0 1 6 - 9 3 3 - 5 48.2 89.2 6 7 - 3 72-3 76.7 80.5 8 4 - 2 8 6 - 1 88.4 90.6 92.2 93.6 94.6 95.5 95.9 97.0

1

Less L e s s Less L e s s L e s s

L e y Less ~ e s s L e s s L e s s Less L e s s L e s s L e s s L e s s Less L e s s L e s s L e s s L e s s '

t h a n t h a n t h a n t h a n t h a n t h a n t h a n t h a n t h a n than t h a n t h a n t h a n than t h a n than than t h a n than t h a n

010 020 030 040 050 060 070 080 090 100 110 120 130 140 150 160 170 180 190 200

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Range of

P

x

lo3

F i g . 2 Frequency of

p

w i t h i n v a r i o u s r a n g e s

I t must b e b o r n e i n mind when i n t e r p r e t i n g t h e s e v a l u e s t h a t s i n c e o b a e r v a t i o n s a r e p o s s i b l e o n l y when c l e a r d i r e c t sun r a d i a t i o n i s a v a i l a b l e t h e d a t a are a l r e a d y "typed" a s a p p r o p r i a t e t o f i n e o r f a i r w e a t h e r

c o n d i t i o n s . Such c o n d i t i o n s at V a l e n t i a are moat f r e q u e n t l y a s s o c i a t e d w i t h a n t i c y c l o n i c o r r i d g e of h i g h p r e s s u r e s i t u a t i o n s .

F i g . 3 shows t h e c u m u l a t i v e f r e q u e n c y of d a i l y mean v a l u e s of

P

.

The median v a l u e i s - 0 4 7 and t h e lower and u p p e r q u a r t i l e s a r e a t

P

=

-030 and

p

=

no77 r e s p e c t i v e l y . F l o w e r s , McCormick and K u r f i s [ 4 ] g i v e s i m i l a r c u m u l a t i v e f r e q u e n c y c u r v e s f o r r u r a l and i n d u s t r i a l s i t e s i n t h e U . S . A . Huron i s g i v e n as a t y p i c a l r u r a l s i t e and t h e c u r v e f o r t h i s

a t e

i s a l s o [image:6.605.114.482.473.859.2]

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s h o w on F i g . 3 as c l o s e l y as _it_{c o u l d b e t r a n s f e r r e d from t h e graph i n}[ 4 ] .

I t i s clear t h a t t h e t u r b i d i t y c o n d i t i o n s . a t Huron and a t V a l e n t i a a r e v e r y s i m i l a r . S i g n i f i c a n t p o i n t s on t h e c u r v e s g i v e t h e f o l l o w i n g :

V a l e n t i a

-

Huron

Median 0.047 0.057

Lower q u a r t i l e 0.030 0.038

Upper q u a r t i l e 0.077 0.080

These v a l u e s a r e d r a m c l o s e r t o g e t h e r when it i s n o t e d t h a t t h e Huron f i g u r e s are f o r t h e Volz t u r b i d i t y c o e f f i c i e n t (B) whereas t h e V a l e n t i a f i g u r e s are f o r t h e Angstrom t u r b i d i t y c o e f f i c i e n t

( $

). In t h e o r y , a t any rate, t h e Volz c o e f f i c i e n t

(B)

may b e t a k e n a s a b o u t 7% h i g h e r t h a n t h e Angstrom c o u n t e r p a r t

( $

1.

V a r i a t i o n of

$

w i t h Wind D i r e c t i o n

Mean v a l u e s of

$

were computed f o r e a c h wind d i r e c t i o n on an 8-point compass. They a r e g i v e n i n T a b l e 2 and shown g r a p h i c a l l y on F i g . 4.

Summer and w i n t e r are t a k e n a s t h e p e r i o d s A p r i l

-

September and October

-

March r e s p e c t i v e l y . Summer t u r b i d i t i e s are h i g h e r t h a n w i n t e r t u r b i d i t i e s f o r e a c h wind d i r e c t i o n . The v a r i a t i o n of

$

i n b o t h s e a s o n s is v e r y s i m i l a r w i t h h i g h e s t t u r b i d i t i e s b e i n g a s s o c i a t e d w i t h East and South-East winds. T h i s i s i n a c c o r d a n c e w i t h e x p e c t a t i o n s s i n c e winds from t h i s s e c t o r would be e x p e c t e d t o be more p o l l u t e d a s a r e s u l t of t h e i r passa- o v e r t h e i n d u s t r i a l c e n t r e s t o t h e E a s t .

T a b l e 2 Mean v a l u e s o f 1 0 3

$

f o r v a r i o u s Wind D i r e c t i o n s

The p r e s e n t a u t h o r a n a l y s e d t h e v a r i a t i o n of Atmospheric Nuclei c o n t e n t of t h e a i r o v e r V a l e n t i a [ 5 ] and found a p a t t e r n v e r y similar t o t h a t now found f o r

$

.

The v a r i a t i o n of t h e Atmospheric Nuclei c o n c e n t r a t i o n w i t h wind d i r e c t i o n i s a l s o shown on F i g . 4 f o r comparison. The p a t t e r n f o r t h e v a r i a t i o n of

$

f o l l o w s c l o s e l y t h a t f o r t h e v a r i a t i o n of Nuclei c o n t e n t e x c e p t f o r o n e n o t i c e a b l e wind d i r e c t i o n i . e . North-Past. The N u c l e i c o n c e n t r a t i o n i s h i g h w i t h winds from t h i s d i r e c t i o n . In f a c t t h e h i g h e s t v a l u e s o f Nuclei c o n c e n t r a t i o n were found i n NE winds i n w i n t e r . The t u r b i d i t y , however, shows no similar i n c r e a s e . P e r h a p s t h e e x p l a n a t i o n

l i e s i n t h e f a c t t h a t winds from t h i s d i r e c t i o n p a s s d i r e c t l y o v e r t h e town of C a h i r c i v e e n which

is

o n l y 1 K m from t h e O b s e r v a t o r y . Though t h e r e i s no great i n d u s t r i a l s o u r c e of p o l l u t i o n t h e smoke and grime from d o m e s t i c f i r e s

c o u l d be e x p e c t e d t o c o n t r i b u t e c o n s i d e r a b l y t o t h e N u c l e i c o n t e n t b u t s i n c e I

F u l l y e a r Winter Summer

t h e a e r o s o l would be c o n f i n e d t o a v e r y Bhallow l a y e r n e a r t h e s u r f a c e t h e

e f f e c t on t h e t u r b i d i t y would o n l y b e m a r g i n a l .

1

North North- East South- S o u t h South- West North-

East E a s t West West

057 05 7 0 76 078 064 059 063 059

038 04 7 071 054 049 045 04 9 038

066 074 088 104 0 73 06 7 066 063

When a n a l y s i n g t h e Atmospheric Nuclei d a t a McWilliams [ 5 ] s u g g e s t e d I t h a t t h e c h i e f s o u r c e of a e r o s o l i n winds from an E e s t e r l y d i r e c t i o n was

t h e i n d u s t r i a l a r e a s of c e n t r a l England. The t u r b i d i t y d a t a would c o n f i r m I

t h i s f o r t h e w i n t e r s e a s o n . In summer however, t h e most t u r b i d a i r a r r i v e s I

[image:7.595.41.506.425.525.2]

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P

z

-

Winter

----

F i g . 4 V a r i a t i o n o f

P

and Z ( ~ t m . N u c l e i C o u n t ) w i t h Wind D i r e c t i o n ( D e v i a t i o n f r o m Annual Mean V a l u e s )

f r o m t h e E u r o p e a n C o n t i n e n t a f t e r a f a i r l y l o n g p a s s a g e o v e r t h e sea. L o v e l o c k [ 6 ] a l s o f o u n d t h a t w i n d s f r o m C o n t i n e n t a 1 Ehrope a r r i v i n g a t B o w e r c h a l k e i n S o u t h e r n Ehgland d u r i n g Summertime were much more t u r b i d t h a n t h e a i r f r o m t h e i n d u s t r i a l r e g i o n s closer a t hand a n d t o t h e N o r t h .

I f w e e l i m i n a t e t h e w i n d s w i t h a n E a s t e r l y component t h e r e m a i n i n g w i n d s a r r i v e a t V a l e n t i a e i t h e r d i r e c t l y f r o m t h e A t l a n t i c o r a f t e r o n l y a v e r y b r i e f p a s s a g e o v e r s p a r s e l y p o p u l a t e d c o u n t r y s i d e . T h e s e w i n d s i n t h e s e c t o r S o u t h t h r o u g h West t o N o r t h h a v e b e e n c l a s s i f i e d i n [ 5 ] a s "sea w i n d s " . They show a u n i f o r m v a l u e o f t u r b i d i t y s l i g h t l y a b o v e t h e o v e r a l l l o n g period a n n u a l mean d u r i n g t h e summer a n d g e n e r a l l y v e r y c l e a r a i r , c o n s i d e r a b l y below t h e o v e r a l l a n n u a l mean, d u r i n g t h e W i n t e r s e a s o n .

!

i

7. V a r i a t i o n o f

P

w i t h S y n o p t i c A i r M a s s e s

i

-

Wind d i r e c t i o n c a n o n l y i n d i c a t e i n a g e n e r a l way t h e o r i g i n of t h e

a e r o s o l i n t h e a i r . A b e t t e r c r i t e r i o n i s p r o b a b l y t h e a i r mass c l a s s i f i c a t i o n . 1

-

_wereThe t y p e o f _made _{w a s}a i r _{s u p p l i e d by t h e C e n t r a l A n a l y s i s a n d F o r e c a s t i n g O f f i c e of t h e}mass o v e r V a l e n t i a a t t h e t i m e when t u r b i d i t y o b s e r v a t i o n s i I r i s h M e t e o r o l o g i c a l S e r v i c e . T h e mean t u r b i d i t y a s s o c i a t e d w i t h e a c h a i r

mass f o r e a c h month o f t h e y e a r was computed a n d t h e r e s u l t i s shown on

i

F ~ K . 5 .

I

I l i K h e s t t u r h i d i t i e s a r e f o u n d i n C o n t i n e n t a l T r o p i c a l ( c T ) a i r . T h i s t y p e o f a i r mass i s however v e r y r a r e l y e x p e r i e n c e d a t V a l e n t i a a n d v a l u e s of t u r b i d i t y a r e a v a i l a b l e o n l y f o r two m o n t h s i n t h e y e a r i . e . May and O c t o b e r . The v a l u e s a r e e n t e r e d f o r t h e s e m o n t h s on F i g . 5 ( c T ) and c a n b e s e e n t o b e h i g h e r t h a n t h e c o r r e s p o n d i n g m o n t h l y mean f o r a n y o t h e r a i r m a s s . I t i s c l e a r , however. t h a t t h e c h i e f c o n t r i b u t o r t o h i g h t t u h i d i t y i n a l l m o n t h s o f t h e y e a r i s a i r o f C o n t i n e n t a l P o l a r (cP) o r i g i n .

What may b e c o n s i d e r e d a s t h e n o r m a l b a c k g r o u n d t n l u e o f turbidity a t V a l e n t i a i s r e p r e s e n t e d by t h r curv.e f o r M a r i t i m e P o l a r (mP) a i r . T h i s i s

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I

I I I I

I

I I

J F M A M J J A S O N D

Month

F i g . 5 V a r i a t i o n of

P

w i t h S y n o p t i c A i r Mass.

M a r i t i m e T r o p i c a l (mT) a i r i n i t s t r u e s e n s e i s n o t c o n d u c i v e t o t u r b i d i t y measurements at V a l e n t i a . I t i s g e n e r a l l y accompanied by low c l o u d s and d r i z z l e and seldom s u i t a b l e f o r r a d i a t i o n measurements. The few t u r b i d i t y measurements o b t a i n e d when t h e a i r w a s c l a s s i f i e d a s m a r i t i m e t r o p i c a l a r e i n s u f f i c i e n t t o p r o v i d e r e l i a b l e monthly means. The means o b t a i n e d h a v e , however, been e n t e r e d on F i g . 5 , t h e c i r c l e d f i g u r e b e s i d e e a c h mean showing t h e number of d a y s on which t h e mean was based. T h e s e r a t h e r d o u b t f u l v a l u e s would s u g g e s t t h a t t h e a i r mass i n q u e s t i o n was l i t t l e d i f f e r e n t from Maritime Polar.

The l o w e s t v a l u e s o b t a i n e d were i n A r c t i c a i r (Ak). The monthly d i f f e r e n c e s were s m a l l i n d i c a t i n g t h a t t u r b i d i t i e s -- a r e a l w a y s 1

36

r.

.

. \

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K)

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Range of

P

x lo3

F i g 7 Cumulative f r e q u e n c y of

P

w i t h v a r i o u s A i r Masses T h e r e are no v a l u e s f o r t h e months June

-

S e p t . s i n c e A r c t i c a i r d o e s n o t

p e n e t r a t e s o f a r S o u t h d u r i n g t h e s e months.

C o n t i n e n t a l P o l a r a i r o v e r V a l e n t i a i s u s u a l l y a s s o c i a t e d w i t h an a n t i c y c l o n e c e n t r e d o v e r & a ~ v l o r North Europe, t h e a i r sweeping a a c r o s s Europe and S o u t h e r n F r a n c e t o I r e l a n d . The h i g h t u r b i d i t i e s a s s o c i a t e d w i t h C o n t i n e n t a l P o l a r a i r would a g a i n a p p e a r t o c o n f i r m L o v e l o c k ' s c o n c l u s i o n t h a t a i r from C o n t i n e n t a l m r o p e i s more h e a v i l y

l a d e n w i t h a e r o s o l t h a n t h a t from t h e i n d u s t r i a l r e g i o n s of England. F i a . 6 shows t h e p e r c e n t a g e f r e q u e n c y d i s t r i b u t i o n of P f o r e a c h of . t h e t h r e e main a l r masses and F i g . 7 shows t h e c o r r e s p o n d i n g c u m u l a t i v e

f r e q u e n c y c u r v e s . T r o p i c a l a i r i s n o t i n c l u d e d due

t o

t h e p a u c i t y of o b s e r v e d d a t a .

The c u m u l a t i v e c u r v e f o r Ak shows t h e smallest s p r e a d of t u r b i d i t i e s between t h e lower and upper q u a r t i l e s a t

P

=

0.017 and 0.033 r e s p e c t i v e l y . The median v a l u e of

P

i s 0-024.

Maritime P o l a r , t h e p r e v a i l i n g a i r mass a t V a l e n t i a , shows lower and u p p e r q u a r t i l e s a t

P

=

0.030 @nd 0.069 r e s p e c t i v e l y w i t h a median v a l u e a t 0.044.

The f r e q u e n c y d i s t r i b u t i o n f o r C o n t i n e n t a l P o l a r a i r i s t y p i c a l of a i r which h a s been p o l l u t e d by p a s s a g e o v e r i n d u s t r i a l z o n e s . The d i s t r i b u t i o n

i s f a i r l y u n i f o r m o v e r t h e r a n g e of t u r b i d i t i e s o b s e r v e d , w i t h lower and upper q u a r t i l e s a t

$

: 0.050 and 0.125 r e s p e c t i v e l y . The median v a l u e

is a t

P

=

0.077.

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8 . Seasonal V a r i a t i o n

Monthly and annual mean v a l u e s a r e given i n Table 3 and t h e average annual v a r i a t i o n f o r t h e f u l l 1 7 year p e r i o d i s shown on Fig. 8. S i m i l a r

I I I I I 1 I 1 I I I

J F M A M J J A S O N

Month

Fig. 8 Annual Variat-ion of T u r b i d i t y .

annual v a r i a t i o n c u r v e s f o r Davos, Adrigole (about 45 Km from Y a l e n t i a ) end Bowerchalke i n Southern England a r e a l s o shown. The Davos v a l u e s a r e taken from [ 7 ] and t h e Adrigole and Bowerchalke v a l u e s from [6]. They

a l l

show t h e now f a m i l i a r h i g h e r v-alues of t u r b i d i t y d u r i n g t h e summer months and r e l a t i v e l y low v a l u e s d u r i n g t h e w i n t e r month* Apart from t h e d i f f e r e n c e s

i n a b s o l u t e v a l u e t h e most n o t i c e a b l e f e a t u r e i s t h e occurrence of t h e

peak v a l u e f o r V a l e n t i a i n A p r i l , whereas t h e o t h e r s , a p a r t from Adrigole have a peak i n June/July. The Adrigole c u r v e does show an A p r i l p a k b u t haa

i t s h i g h e s t v a l u e i n September. However, t h e A d r i g o l e v a l u e s are based on only two y e a r s d a t a (1970 and 1 9 7 1 ) a s compared w i t h t h e ~ a l e n t i a 1 7

year p e r i o d . W y m i t e r s a t t r i b u t e t h e g e n e r a l h i g h summer v a l u e s t o t h e i n c r e a s e d i n s t a b i l i t y d u r i n g s p r i n g and e a r l y summer r e s u l t i n g i n

convection c u r r e n t s c a r r y i n g q u a n t i t i e s .of a e r o s o l i n t o the- h i g h e r

atmosphere, t h e g e n e r a l atmospheric subsidence i n w i n t e r months produciqg an o p p o s i t e c l e a n s i n g e f f e c t . Iovelock s u g g e s t s t h a t t h e &nae_r summertime a e r o s o l i s probably an end product of t h e atmospheric photochemistry of a r p o l l u t a n t s under summer c o n d i t i o n s and t h a t C o n t i n e n t a l mr-ope i s

t h e p r i n c i p a l s o u r c e . If we r e f e r a g a i n t o F i g . 5 we s e e t h e annual v a r i a t i o n of t h e V a l e n t i a d a t a w i t h r e s p e c t t o t h e p r i n c i p a l air masses. The curve f o r mP a i r i s almost i d e n t i c a l w i t h t h e o v e r a l l annual

[image:11.599.121.563.199.583.2]

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In t h e c a s e of mP a i r , which p r e v a i l s a t V a l e n t i a , t h e range between summer and w i n t e r t u r b i d i t y v a l u e s i s c o n s i d e r a b l y compressed a s compared w i t h a i r of c o n t i n e n t a l o r i g i n . This could w e l l be due t o t h e f a c t t h a t

t h e atmospheric photochemical and b i o l o g i c a l sources of summer a e r o s o l would not be such major c o n t r i b u t o r s i n t h e c a s e of maritime a i r a s

i n

t h e c a s e of a i r a r r i v i n g from c o n t i n e n t a l a r e a s .

Another f a c t o r which probably p l a y s a p a r t i n t h e annual v a r i a t i o n of t h e t u r b i d i t y i s t h e r a i n f a l l . McWilliams [ 5 ] found i n t h e c a s e of

atmospheric n u c l e i t h a t r a i n caused a r e d u c t i o n i n t h e n u c l e i c o n t e n t

amounting t o about 30%. Other m i t e r s found a similar "washing o u t " e f f e c t

200 000

1

I

I I I i I I I 1 I

J F M A M J J A S O N

Month

Fig. 9 Annual V a r i a t i o n of

$

and R a i n f a l l .

In Fig. 9 t h e t u r b i d i t y annual v a r i a t i o n c u r v e f o r a l l o b s e r v a t i o n s i s shorn w i t h t h e curve of normal monthly r a i n f a l l amounts a s measured a t V a l e n t i a . The r a i n f a l l n o r m a l e . a r e based on a 30 year average and a r e p l o t t e d i n v e r s e l y t o f a c i l i t a t e comparison w i t h t h e t u r b i d i t y curve. The p a r a l l e l t r e n d of t h e two c u r v e s i s very s t r i k i n g . The h i @ e a t t u r b i d i t y

and t h e minimum r a i n f a l l c o i n c i d e i n t h e month of A p r i l . Wetter months show reduced t u r b i d i t y and t h e c l e a r e s t a i r i s found i n t h e months Dec.

-

Jan. which have t h e maximum r a i n f a l l . I t would appear, t h e r e f o r e , t h a t t h e scavenging e f f e c t of r a i n f a l l playa a c o n s i d e r a b l e r o l e i n t h e c o n t r o l of t h e a e r o s o l c o n t e n t of t h e a i r a t V a l e n t i a

9. T u r b i d i t y Trends

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a t m o s p h e r i c warming which c o u l d be e x p e c t e d from t h e r e p o r t e d s t e a d y i n c r e a s e i n t h e C02 c o n t e n t of t h e atmosphere h a s n o t been o b s e r v e d . In f a c t t h e o p p o s i t e h a s b e e n n o t e d i n [ 1 0 , 1 1 ] . The w o r l d wide r e d u c t i o n i n mean a n n u a l a i r t e m p e r a t u r e r e p o r t e d t o have s t a r t e d a b o u t 1940 h a s been a t t r i b u t e d by a number of w r i t e r s t o a u n i v e r s a l i n c r e a s e i n a t m o s p h e r i c t u r b i d i t y . V a l u e s g i v e n by McCormick and Ludwig [ I 2 1 s u g g e s t t h a t a t Washington (U.S.A.) t h e r e h a s been a t o t a l i n c r e a s e of a b o u t 57% between 1905 and 1964 and a t Davos ( S w i t z e r l a n d ) an i n c r e a s e amounting t o 88'

between 1920 and 1958. P e t e r s o n and Bryson [ 1 3 ] i n a s t u d y of t h e o b s e r v a t i o n s a t Mauna Loa O b s e r v a t o r y , Hawaii show an i n c r e a s e i n t u r b i d i t y of a b o u t 30% i n t h e t e n y e a r p e r i o d 1958

-

68. The t r e n d was masked by a v e r y - r a p i d i n c r e a s e i n t u r b i d i t y i n 1 9 6 3 r e s u l t i n g from t h e e r u p t i o n of Mount Agung i n t h e E a s t I n d i e s i n March, 1 9 6 3 , b u t t h e a u t h o r s concluded t h a t t h e e f f e c t of t h e Mount Agung e r u p t i o n w a s of a t e m p o r a r y ( t h o u g h p r o l o n g e d ) n a t u r e and t h a t most of t h e i n c r e a s e ' w a s t h e r e s u l t of a l o n g term world wide turbidity i n c r e a s e .

Though n o o b s e r v a t i o n s a r e a v a i l a b l e a t V a l e n t i a f o r t h e earlier d a t e s r e f e r r e d t o i n t h e c a s e of Washington and Davos, t h e c o n t i n u o u s s e r i e s of o b s e r v a t i o n s from 1955 t o 1 9 7 1 , u s i n g t h e same i n s t r u m e n t s and f i l t e r s , p r o v i d e s an o p p o r t u n i t y of c o n f i r m i n g , o r o t h e r w i s e , t h e r e p o r t e d t r e n d a s f a r as V a l e n t i a i s c o n c e r n e d .

For t h i s p u r p o s e F i g . 1 0 shows t h e d e p a r t u r e s from t h e monthly a v e r a g e t u r b i d i t y f o r t h e f u l l 17-year p e r i o d . S i n c e t h e monthly v a l u e s of

P

showed

a

d i s t i n c t a n n u a l v a r i a t i o n t h e p l o t t e d v a l u e f o r e a c h i n d i v i d u a l month i s t h e d e p a r t u r e from t h e normal v a l u e f o r t h a t month, t h e normal v a l u e f o r e a c h month b e i n g b a s e d on t h e d a t a f o r t h e f u l l p e r i o d . There i s no e v i d e n c e of any l o n g term t r e n d as r e p o r t e d by t h e r e s e a r c h e r s mentioned above.

I n d i v i d u a l months show c o n s i d e r a b l e d e p a r t u r e s b u t t h e background l e v e l a t t h e end of t h e p e r i o d r e m a i n s unchanged from t h a t o b s e r v e d a t t h e b e g i n n i n g of t h e p e r i o d . The a n n u a l mean v a l u e s f o r e a c h y e a r a r e a l s o shown on t h e diagram. T h i s c u r v e i n d i c a t e s a f a i r l y l o n g p e r i o d , small a m p l i t u d e

o s c i l l a t i o n i n t h e mean a n n u a l t u r b i d i t y background on which are superimposed t h e i r r e g u l a r monthly d e p a r t u r e s , n o d o u b t a s s o c i a t e d w i t h t h e f r e q u e n t

c h a n g e s i n wind d i r e c t i o n and a i r masses which a r e t y p i c a l of t h e V a l e n t i a l a t i t u d e and g e o g r a p h i c a l p o s i t i o n .

The h i g h e r monthly d e p a r t u r e s are u s u a l l y a s s o c i a t e d w i t h an e r u p t i o n of C o n t i n e n t a l P o l a r a i r from N o r t h e r n and C e n t r a l Europe. The h i g h e s t peak o c c u r r e d i n O c t o b e r , 1964. However, t h i s p a r t i c u l a r monthly mean was based o n o n l y f o u r o b s e r v a t i o n s , t h r e e of which r e s u l t e d i n a b n o r m a l l y h i g h v a l u e s . The a i r was C o n t i n e n t a l P o l a r a s s o c i a t e d w i t h an a n t i c y c l o n e c e n t r e d o v e r N o r t h e r n Germany, t h e East/S.East wind b r i n g i n g t h e a i r a c r o s s c e n t r a l Europe and o v e r Fhgland t 6 I r e l a n d .

In p e r a . 7 above it was s u g g e s t e d t h a t t h e t u r b i d i t y i n mP a i r may be c o n s i d e r e d a s r e p r e s e n t a t i v e of t h e normal background t u r b i d i t y a t

V a l e n t i a . F i g . 1 1 shows t h e s e c u l a r v a r i a t i o n of

P

i n mP a i r (shown a s d e v i a t i o n s from t h e o v e r a l l a n n u a l mean v a l u e ) . The main f e a t u r e of t h e v a r i a t i o n i s t h e d r o p i n t u r b i d i t y from 1956 t o 1961 f o l l o w e d by a

g r a d u a l r e c o v e r y t o normal v a l u e . In t h e c a s e of t h e s e a s o n a l v a r i a t i o n i t was found ( p a r a . 8 a b o v e ) t h a t t h e s e a s o n a l p a t t e r n f o l l o w e d c l o s e l y t h e

s e a s o n a l p a t t e r n i n r a i n f a l l . Hence t h e s e c u l a r v a r i a t i o n i n t o t a l

r a i n f a l l a s measured a t V a l e n t i a i s a l s o shown on F i g . 11 ( p l o t t e d i n v e r s e l y ) . There i s a c o n s i d e r a b l e s i m i l a r i t y i n t r e n d s u g g e s t i n g a g a i n , a s i n t h e

c a s e of t h e s e a s o n a l v a r i a t i o n t h a t t h e washing o u t e f f e c t of r a i n f a l l p l a y s a c o n s i d e r a b l e p a r t i n t h e c o n t r o l of t h e a e r o s o l c o n t e n t of t h e a i r a t V a l e n t ia.

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-010

-

-020

-

\ I

---

Rainfall -030

-

V

F i g . 11 S e c u l a r V a r i a t i o n of

p

and R a i n f a l l (As LJeviations from 1 7 Yeax Meam).

c o n s i d e r a b l e improvement t h e r e must b e some d o u b t am t o t h e a b s o l u t e v a l u e and c o m p a r a b i l i t y of o b s e r v a t i o n s s e p a r a t e d by

a

s p a n o f u p

t o

5 0 y e a r s . Moreover, F i g . 1.1 shows

f i a t ,

a t l e a s t a t V a l e n t i a , v a r y G g d e g r e e s o f

i n c r e a s i n g o r d e c r e a s i n g t r e n d c o u l d be c a l c u l a t e d d e p e n d i n g on t h e t e r m i n a l y e a r s used i n t h e c o m p u t a t i o n .

I t i s i n t e r e s t i n g t o n o t e t h a t McCormick, who w i t h Ludwig [ 1 2 ] u s e d t h e WashinHon and Davos d a t a t o show a l a r g e w o r l d wide i n c r e a s e i n t h e l a s t h a l f c e n t u r y l a t e r found [ 4 ] when a n a l y s i n g , w i t h F l o w e r s and K u r f i s , d a t a o b s e r v e d a t a v a r i e t y o f s t a t i o n s i n t h e U . S . A . f o r t h e p e r i o d 1960

-

66 t h a t t h e r e was no e v i d e n c e o f such a t r e n d .

Dopniaux and S n e y e r s [ 1 4 ] a n a l y s e d

a

20 y e a r p e r i o d of t u r b i d i t y d a t a a t U c c l e , B r u s s e l s , which i n c l u d e d t h e 1 7 y e a r p e r i o d c o v e r e d by t h e V a l e n t i a d a t a , and found o n l y a v e r y s m a l l b u t i n s i g n i f i c a n t i n c r e a s e i n t h e

t u r b i d i t y . Though some i n c r e a s e i n l o c a l t u r b i d i t y must b e e x p e c t e d a t s i t e s c l o s e t o i n d u s t r i a l development a r e a s t h e V a l e n t i a d a t a would a p p e a r t o

i n d i c a t e t h a t t h e r e h a s been no g e n e r a l w o r l d wide i n c r e a s e i n t u r b i d i t y e s p e c i a l l y i n r u r a l a r e a s r e m o t e from t h e s o u r c e s of man made p o l l u t a n t s .

10. C o n c l u s i o n

The a n a l y s i s of 1 7 y e a r s o f measurements o f t h e Angstrom t u r b i d i t y c o e f f i c i e n t a t V a l e n t i a shows:

1 0 . 1 . Mean d a i l y t u r b i d i t y most f r e q u e n t l y l i e s between - 0 3 0 and - 0 4 0 w i t h a median v a l u e of - 0 4 7 .

1 0 . 2 . H i g b e s t t u r b i d i t i e s a r e o b s e r v e d i n winds from a n East and South East d i r e c t i o n .

1 0 . 3 . For a l l wind d i r e c t i o n s t u r b i d i t i e s a r e h i g h e r i n summer t h a n i n w i n t e r .

[image:15.595.67.458.68.380.2]

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1 0 . 5 .

me

c h i e f f a c t o r d e t e r m i n i n g t h e t u r b i d i t y i s t h e a i r mass t y p e . C o n t i n e n t a l T r o p i c a l a i r was e x p e r i e n c e d i n o n l y two months o f t h e y e a r b u t t h e o b s e r v e d t u r b i d i t y . w a s h i g h e r t h a n t h a t f o r any o t h e r a i r mass f o r t h e c o r r e s p o n d i n g months.

O t h e r w i s e , C o n t i n e n t a l P o l a r a i r was a s s o c i a t e d w i t h h i g h e s t t u r b i d i t i e s i n e a c h month of t h e y e a r .

M a r i t i m e p o l a r a i r , t h e p r e v a i l i n g air mass, showed r e l a t i v e l y low t u r b i d i t y and seems t o dominate t h e o v e r a l l a n n u a l t u r b i d i t y p a t t e r n .

1 0 . 6 . The s c a v e n g i n g e f f e c t of r a i n f a l l would a p p e a r t o have a c o n s i d e r a b l e

i n f l u e n c e on t h e a n n u a l and s e c u l a r v a r i a t i o n .

1 0 . 7 . T h e r e i s n o e v i d e n c e of any t r e n d of i n c r e a s i n g t u r b i d i t y d u r i n g t h e

D e r i o d

.

Thanks a r e due t o M r . F. E. Dixon, O/C C e n t r a l A n a l y s i s and F o r e c a s t i n g O f f i c e f o r t h e prov.ision of t h e A i r Mass d a t a used i n t h i s a n a l y s i s and t o

M r . D. N. Reidy a t V a l e n t i a O b s e r v a t o r y who performed much of t h e l a b o r i o u s c o m p u t a t i o n a l work i n v o l v e d .

R e f e r e n c e s

[I]

Angstrom, A.

On t h e Atmospheric T r a n s m i s s i o n of Sun R a d i a t i o n and on Dust i n t h e A i r . Geogr. Ann. No. 2 , 1929.

[ 2 ] Angstrom, A .

On t h e ~ t m o s p h e r i c T r a n s m i s s i o n of Sun R a d i a t i o n Geogr. Ann. No. 2

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3 , 1930.

[3.] ICY I n s t r u c t i o n Manual

P a r t R a d i a t i o n I n s t r u m e n t s and Measurements.

Pergamon Press 1958.

[ 4 ] F l o w e r s , E.C., McCormick, R . A . and Kurf i s , K.R.

Atmospheric R r b i d i t y o v e r t h e United S t a t e s 1961

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6 6 .

J1. Appl. Met. Vol. 8 Dec., 1969.

[ 5 ] M c W i l l i a m s , S.

The C o n c e n t r a t i o n of Atmospheric N u c l e i a t V a l e n t i a O b s e r v a t o r y . I r i s h M e t . S e r v i c e T e c h n i c a l Note No. 3 3 , 1969.

[ 6 ] Lovelock, J.E.

Atmospheric T u r b i d i t y and CC13F C o n c e n t r a t i o n i n R u r a l S o u t h e r n England and S o u t h e r n I r e l a n d .

Atmospheric Environment, Pergamon Press, Vol. 6 , 1972.

[ 7 ] Valko, P.

~ n t e r s u c h u n g ' u i e r d i e v e r t i k a l e ~ ~ u ' b u n g s s c h i c h ~ u n g d e r AtmosphXre. Arch. Meteor., Geophysik und B i o c l i m a t o l o g i e .

S e r i e s B, 1 1 , 1961.

[ 8 ] Weather and C l i m a t e M o d i f i c a t i o n , Problems and P r o s p e c t s .

N a t . Acad. S c i . N e t . Res. C o u n c i l P u b l . 1350 _Vol. _{2 , 1966.}

[ 9 ] Bryson, R . A .

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[YO] Hurray Mitchell Jr.

Ann. N.Y. Acad. Sc., Vol. 90, 1961.

[ l l ] Hurray Mitchell Jr. Changes of Climate

Arid Zone Research XX (UNESCO, Paris) 1963.

[12] HcCormick. R.A. and Ludwig. J.H.

Climate Uodification by Atmospheric Aerosols. Science, Vo1. 156, June. 1967.

I

[13] Peterson, J.T., Bryson, R.A.

Atmospheric

Aerosols:

Increased Concentrations during the Laat Decade. Science. Vol. 162, October. 1968.

I [14] Dogniaux, R. and Sneyers, R.

1

Sur La Stabilite Du Trouble Atmospherique a Uccle au aours de la period 1951

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1970.