Measurement Challenges in Atmospheric Chemistry - American

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for Eddy Correlation Flux Measurements A. C. Delany National Center for Atmospheric Research, P.O. Box 3000, Boulder, C O 80307 The requirements for chemical sensors suitable for use in eddy correlation direct measurements of surface fluxes are examined. The resolution of chemical sensors is examined and defined in terms of surface flux and commonly measured micrometeorological parame ters. Aspects of the design and operation of sensor systems are con sidered. In particular, the effects of the inlet ducting, the sensing volume, and the signal processing on the ability to measure surface fluxes were analyzed.

MAJOR LIMITATION RESEARCH

το o n surface-exchange a n d flux m e a surements is the lack o f sensitive, r e l i a b l e , a n d fast-response c h e m i c a l species sensors that can b e u s e d for e d d y c o r r e l a t i o n flux m e a s u r e m e n t . T h e r e f o r e w e r e c o m m e n d that c o n t i n u e d effort a n d resources b e e x p e n d e d i n d e v e l o p i n g c h e m i c a l species sensors w i t h t h e responsiveness a n d s e n s i t i v i t y r e q u i r e d for d i r e c t e d d y c o r r e l a t i o n flux m e a s u r e m e n t s . " T h i s r e c o m m e n d a t i o n (J) was assigned the first p r i o r i t y i n the report of the recent G l o b a l T r o p o s p h e r i c C h e m i s t r y w o r k s h o p j o i n t l y c o n v e n e d b y the N a t i o n a l Science F o u n d a t i o n , the N a t i o n a l A e r o n a u t i c s a n d Space A d m i n i s t r a t i o n , a n d the N a t i o n a l O c e a n i c a n d A t m o s p h e r i c A d m i n i s t r a t i o n . T h e authors o f the r e p o r t r e c o g n i z e d that the l i m i t e d availability of fast, accurate c h e m i c a l sensors is a major m e a s u r e m e n t challenge i n the field o f a t m o s p h e r i c c h e m i s t r y . O n e o f the greatest uncertainties i n the u n d e r s t a n d i n g o f t h e m e c h a n i s m s that c o n t r o l the c h e m i c a l c o m p o s i t i o n of the a t m o s p h e r e concerns t h e ex change o f trace species b e t w e e n the atmosphere a n d the surface. T h e s e surface exchanges i n c l u d e b o t h e m i s s i o n a n d d e p o s i t i o n a n d are i n t i m a t e l y 0065-2393/93/0232-0091$06.00/0 © 1993 American Chemical Society

Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

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MEASUREMENT CHALLENGES IN ATMOSPHERIC CHEMISTRY

c o n n e c t e d to processes i n v o l v i n g the b i o s p h e r e . T h e d r y d e p o s i t i o n o f atm o s p h e r i c acids a n d oxidants, the natural e m i s s i o n o f b i o g e n i c h y d r o c a r b o n s , the anthropogenically p e r t u r b e d cycles of n u t r i e n t s , a n d m a n y o t h e r i m portant aspects o f surface exchange are i n v o l v e d i n these processes. T o investigate surface exchange, measurements of e m i s s i o n a n d d e p o s i t i o n fluxes m u s t b e m a d e o v e r selected representative sites. S e v e r a l t e c h n i q u e s to measure these c h e m i c a l fluxes have b e e n d e v e l o p e d . H o w e v e r , the most d i r e c t t e c h n i q u e for m e a s u r i n g the e m i s s i o n or d e p o s i t i o n is e d d y c o r r e l a t i o n . T h i s m e t h o d is a f u n d a m e n t a l l y d i r e c t t e c h n i q u e that has the a d d e d advantage of not d i s t u r b i n g the nature of the surface. T h e concepts i n v o l v e d i n the e d d y c o r r e l a t i o n t e c h n i q u e a n d the factors i n v o l v e d i n the d e s i g n o f a p p r o p r i a t e c h e m i c a l sensors are b r i e f l y e x a m i n e d i n this chapter. T h e discussion focuses o n surface-layer m e a s u r e m e n t s b e cause i t is i n this l a y e r that the a t m o s p h e r e - b i o s p h e r e i n t e r a c t i o n is most readily examined.

Eddy Correlation T h e e d d y c o r r e l a t i o n t e c h n i q u e d i r e c t l y d e t e r m i n e s the flux o f a n a t m o s p h e r i c trace constituent t h r o u g h a p l a n e p a r a l l e l to the surface. F o r the d e t e r m i n a t i o n of surface e m i s s i o n a n d d e p o s i t i o n fluxes, the m e t h o d is r i g orous w h e n specific c r i t e r i a are met. I d e a l l y , the m e t e o r o l o g i c a l c o n d i t i o n s c o n t r o l l i n g the state of t u r b u l e n c e s h o u l d not vary o v e r the course of the m e a s u r e m e n t s . T h e surface v i e w e d b y the sensors s h o u l d b e h o r i z o n t a l l y u n i f o r m , b o t h i n its p h y s i c a l a n d c h e m i c a l - b i o l o g i c a l aspects, a n d s h o u l d stretch for a distance m u c h greater t h a n the h e i g h t at w h i c h the m e a s u r e m e n t s are m a d e . T h i s h e i g h t s h o u l d b e m u c h larger t h a n the scale of the surface roughness a n d the i n t r i n s i c scale of the sensors. T h e extent to w h i c h these c r i t e r i a can b e relaxed w i t h the m e t h o d r e m a i n i n g v a l i d is a subject o f o n g o i n g debate (2). T h e e d d y c o r r e l a t i o n t e c h n i q u e has b e e n e x a m i n e d t h o r o u g h l y , a n d a considerable l i t e r a t u r e exists that deals w i t h p o t e n t i a l errors that can result because of flow d i s t o r t i o n (3), i n a p p r o p r i a t e signal processing (4), failure to make the necessary corrections for d e n s i t y effect (5), a n d the c h e m i c a l r e a c t i v i t y o f the m e a s u r e d species (6). Stratagems have b e e n d e s i g n e d to make the most advantageous c o m p r o m i s e b e t w e e n the n e e d to accrue the best statistics a n d the desire to operate w i t h i n p e r i o d s w i t h o u t significant changes i n meteorological c o n d i t i o n s (7). A p p r o a c h e s d e a l i n g w i t h the p r o b l e m s of c o r r e l a t e d a n d u n c o r r e c t e d noise h a v e b e e n e x p l o r e d (8). H o w e v e r , the basic r e q u i r e m e n t for the m e a s u r e m e n t of the surface flux of atmospheric c h e m i c a l species involves the a b i l i t y to m a k e the appropriate c h e m i c a l measurements. Because the e d d y c o r r e l a t i o n m e t h o d m a y be c o n s i d e r e d as d e f i n i n g the instantaneous u p w a r d or d o w n w a r d transport of the constituent a n d t h e n averaging c o n t r i b u t i o n s to give the net flux, it m u s t take into account the


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frequency range of the t u r b u l e n c e responsible for v e r t i c a l l y t r a n s p o r t i n g the constituent i n the atmosphere. K a i m a l et a l . (9) f o u n d that the scalar flux cospectra scaled w i t h a n o n d i m e n s i o n a l frequency, η = fz/û, w h e r e fis the m e a s u r e m e n t f r e q u e n c y , ζ is the height at w h i c h the m e a s u r e m e n t is m a d e , a n d ïï is the m e a n w i n d speed. U n d e r n e u t r a l o r unstable c o n d i t i o n s , s u c h as o c c u r d u r i n g most d a y t i m e circumstances, 9 0 % of the flux is m e a s u r e d w i t h η < 1. T h u s , for measurements o n a tower at 10 m w i t h a 5 - m / s w i n d , / n m — (1)(5)/10 = 0.5 H z . T h e highest r e q u i r e d frequency is o n l y 0.5 H z . Because of the N y q u i s t r e q u i r e m e n t the r e q u i r e d s a m p l i n g rate is 1 H z . F o r stable n i g h t t i m e conditions o r i f fluxes m u s t b e m e a s u r e d w i t h greater than 1 0 % accuracy, t h e n h i g h e r frequencies n e e d to be m e a s u r e d . K a i m a l et a l . also d e t e r m i n e d that the lowest f r e q u e n c y that needs to be i n c l u d e d is η « 0.01. T h u s , for the tower m e a s u r e m e n t s m e n t i o n e d the lowest f r e q u e n c y that needs to b e i n c l u d e d is / « (0.01)(5)/10 « 0.005 H z . T h i s calculation indicates an averaging t i m e of 200 s. U n f o r t u n a t e l y , this t i m e w o u l d not a l l o w a sufficient s a m p l i n g of the lowest f r e q u e n c y c o n t r i b u t i o n . F o r such s a m p l i n g to o c c u r a p e r i o d an o r d e r of m a g n i t u d e longer is r e q u i r e d , a n d h e n c e a p e r i o d of 30 m i n is g e n e r a l l y r e q u i r e d . T h e t e c h n i q u e r e q u i r e s simultaneous fast a n d accurate m e a s u r e m e n t s of b o t h the v e r t i c a l v e l o c i t y a n d the trace species i n q u e s t i o n . F o r t u n a t e l y the technology for the m e a s u r e m e n t of t u r b u l e n c e w i t h the necessary r e s o l u t i o n is available. Sonic anemometers can r e a d i l y y i e l d air m o t i o n data w i t h the r e q u i r e d resolution (10). L i k e w i s e , the a b i l i t y to h a n d l e the air m o t i o n a n d c h e m i c a l concentration data w i t h m o d e r n c o m p u t e r data systems is w e l l i n h a n d (11). T h u s these aspects can b e i g n o r e d , a n d the major l i m i t a t i o n can b e dealt w i t h : the availability of a p p r o p r i a t e c h e m i c a l sensors w i t h sufficient t i m e a n d c h e m i c a l r e s o l u t i o n . m i n

Chemical Resolution Required T h e effect of a surface flux o n the concentration of an a t m o s p h e r i c constituent m e a s u r e d at some h e i g h t is to impose a variance u p o n that c o n c e n t r a t i o n as t u r b u l e n c e i n t e r m i t t e n t l y transports air u p f r o m the surface, w h e r e the constituent is e i t h e r e n h a n c e d or d e p l e t e d , to the l e v e l of the sensor. T h e c h e m i c a l r e s o l u t i o n of the sensor defines the extent to w h i c h this fluctuation i n c o n c e n t r a t i o n , c\ can be assessed. T h e r e q u i r e d r e s o l u t i o n for a sensor to b e u s e d for e d d y c o r r e l a t i o n measurements of fluxes thus d e p e n d s o n the i n t e n s i t y of the flux to b e m e a s u r e d . A greater flux gives a greater c' a n d a m o r e relaxed r e q u i r e m e n t for c h e m i c a l r e s o l u t i o n . A s m a l l e r flux gives a smaller c' a n d a m o r e stringent r e q u i r e m e n t for c h e m i c a l r e s o l u t i o n . T h e relationship b e t w e e n the m i c r o m e t e o r o l o g i c a l e n v i r o n m e n t p r e v a i l i n g at the specific location a n d the c h e m i c a l r e s o l u t i o n r e q u i r e d is somewhat m o r e complex. I n g e n e r a l , a t m o s p h e r i c conditions that t e n d to dissipate or m i x out c h e m i c a l v a r i a b i l i t y w i l l impose a greater stricture o n the c h e m i c a l res-


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M E A S U R E M E N T C H A L L E N G E S IN ATMOSPHERIC CHEMISTRY

o l u t i o n , a n d conditions that a l l o w c h e m i c a l v a r i a b i l i t y to persist w i l l ease the r e q u i r e m e n t o f c h e m i c a l r e s o l u t i o n . B u s i n g e r a n d D e l a n y (12) e x a m i n e d this p r o b l e m a n d d e r i v e d a r e l a t i o n s h i p that defines the c h e m i c a l r e s o l u t i o n r e q u i r e d for sensors u s e d for e d d y c o r r e l a t i o n measurements. T h e i r approach was to specify t h e standard d e v i a t i o n o f t h e c h e m i c a l concentration o (the root m e a n square o f t h e c h e m i c a l fluctuation c') i n t e r m s o f the surface c h e m i c a l flux, F , a n d r e a d i l y m e a s u r e d m i c r o m e t e o r o l o g i c a l parameters. c

c

w h e r e u* = (-u'w ) is t h e f r i c t i o n v e l o c i t y , Θ* = -w'Q'/u* is t h e charac teristic p o t e n t i a l t e m p e r a t u r e , σ = ( θ ' θ ' ) is t h e standard d e v i a t i o n o f the p o t e n t i a l t e m p e r a t u r e , u is t h e h o r i z o n t a l v e l o c i t y , w is t h e v e r t i c a l v e l o c i t y , a n d θ is t h e p o t e n t i a l t e m p e r a t u r e . It was t h e n a r g u e d that i f the flux is to b e k n o w n to ± 1 0 % t h e r e is a r e s o l u t i o n r e q u i r e m e n t that R — 0. l o for t h e worst-case scenario. f

m

172

θ

c

R

c

=

0.1 a

c

c

=

0.1|F,|-^T U*\O*\

(2)

T h e value o f σ /«*|θ*|, w h i c h B u s i n g e r a n d D e l a n y t e r m e d t h e a t m o s p h e r i c p a r a m e t e r , is p l o t t e d i n F i g u r e 1 as a f u n c t i o n o f a t m o s p h e r i c stability, z / L , a n d f r i c t i o n v e l o c i t y , u*. θ


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T h i s a t m o s p h e r i c stability versus f r i c t i o n v e l o c i t y p l o t is capable of r e p r e s e n t i n g any a t m o s p h e r i c c o n d i t i o n except that w h e n the stability is n e u t r a l . T h e i n f o r m a t i o n c o n t a i n e d i n F i g u r e 1 a n d e q u a t i o n 2 allows the r e l a t i o n s h i p b e t w e e n the v a l u e o f the surface flux a n d the r e q u i r e d c h e m i c a l sensor r e s o l u t i o n to b e e s t i m a t e d for a full range o f a t m o s p h e r i c c o n d i t i o n s .

Chemical Sensors T h e factors that influence the c h e m i c a l r e s o l u t i o n o f sensors are w e l l u n d e r stood a n d are not discussed h e r e . T h i s section r e v i e w s the factors that c o n t r o l the t e m p o r a l r e s o l u t i o n o f sensors to be u s e d for e d d y c o r r e l a t i o n . I n t h e analysis o f the d e s i g n of c h e m i c a l sensors to b e u s e d for e d d y c o r r e l a t i o n it is i n s t r u c t i v e to c o n s i d e r the different c o m p o n e n t s o f c h e m i c a l sensor systems separately to d e t e r m i n e the influences that t h e y have o n the t e m p o r a l r e sponse to variations i n the a t m o s p h e r i c c o n c e n t r a t i o n o f a trace c o n s t i t u e n t . O f course this analysis is an o v e r s i m p l i f i c a t i o n because the total systems operate i n a m o r e c o m p l e x fashion, b u t it is a useful exercise.

Inlet System. T h e simplest i n l e t system is that o f the o p e n - p a t h i n situ sensor. T h i s system is the i d e a l ; o n l y subtle effects are e x p e c t e d . T h u s , flow d i s t o r t i o n can p r o d u c e a i r d e n s i t y changes that interfere w i t h the m e a s u r e m e n t o f m o l e c u l a r d e n s i t y b y o p t i c a l a b s o r p t i o n t e c h n i q u e s , a n d size fractionation of large aerosol particles can result because o f airflow a r o u n d the b o d y o f the sensor. F o r m a n y c h e m i c a l sensors this o p e n p a t h cannot b e a c h i e v e d , a n d a t m o s p h e r i c air m u s t b e d u c t e d to the sensor. E v e n w h e n optical absorption t e c h n i q u e s are u s e d , the n e e d to r e d u c e the p r e s s u r e b r o a d e n i n g of absorption lines can necessitate the a t m o s p h e r i c s a m p l e b e i n g d r a w n into a r e d u c e d - p r e s s u r e c e l l for analysis. F o r analysis t e c h n i q u e s i n v o l v i n g c h e m i l u m i n e s c e n c e , flame p h o t o m e t r y , e l e c t r o n c a p t u r e , a n d o t h e r t e c h n i q u e s , it is often i m p e r a t i v e to enclose the s e n s i n g v o l u m e , h e n c e the a i r m u s t be d u c t e d to the sensor. T h e distance that the s a m p l e m u s t b e d u c t e d d e p e n d s o n several factors. I f the p h y s i c a l size o f the c h e m i c a l sensor system is large o r i f it r e q u i r e s special e n v i r o n m e n t a l h o u s i n g , t h e n i t cannot be p l a c e d near the t u r b u l e n c e sensor or the flow d i s t o r t i o n w o u l d b e too severe. H o w e v e r , too great a separation can i m p o s e l i n e loss a n d o t h e r penalties. C o n s i d e r a t i o n s of c h e m i s t r y a n d t u r b u l e n c e can l e a d to a c o m p r o m i s e b a l a n c i n g the disadvantages. T h e most obvious result of d u c t i n g the a t m o s p h e r i c sample f r o m the v i c i n i t y o f the sonic a n e m o m e t e r to the c h e m i c a l sensor is the i n t r o d u c t i o n of a t i m e delay. T h i s t i m e lag m u s t b e e l i m i n a t e d before the c o r r e l a t i o n b e t w e e n c h e m i c a l concentration a n d the v e r t i c a l air m o t i o n variances is m a d e to y i e l d the covariance. S e v e r a l different approaches h a v e b e e n t a k e n to d e t e r m i n e the l e n g t h of the delay. O n e s i m p l e m e t h o d i n v o l v e s s p i k i n g a b a l l o o n w i t h the c o m p o u n d i n v o l v e d , a n d t h e n inflating a n d b u r s t i n g i t i n such a m a n n e r that the sonic a n e m o m e t e r p a t h is i n t e r r u p t e d at the same Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

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instant the sample is released at the c h e m i c a l sensor i n t a k e . A fast s o l e n o i d valve m a y b e u s e d to enable the r a p i d i n j e c t i o n of the trace c o m p o u n d i n t o the sample intake. A m o r e sophisticated approach u t i l i z e s the fact that the c h e m i c a l concentration a n d the t e m p e r a t u r e are c o r r e l a t e d i n the a t m o sphere. B y adjusting the t i m e lag of the c h e m i c a l sensor data s t r e a m , one can d e t e r m i n e the m a x i m u m possible c o r r e l a t i o n (or anticorrelation), a n d the correct value can b e o b t a i n e d . A m o r e serious difficulty arises i f the c h e m i c a l c o m p o u n d interacts w i t h the m a t e r i a l of the d u c t i n g . A l t h o u g h an appropriate c a l i b r a t i o n p r o c e d u r e , one that i n v o l v e s the e n t i r e i n l e t system, w i l l a l l o w the effect o n the m e a n concentration of the trace constituent to b e c o r r e c t e d for, the c a l i b r a t i o n m a y not compensate for the effect o n the fluctuation. F o r the effect of c h e m i s o r p t i o n of a c o m p o u n d o n the w a l l of the d u c t , for example, elevated concentrations w o u l d cause a d s o r p t i o n of molecules that w o u l d be released w h e n the c o n c e n t r a t i o n decreased.

The

net effect of this s m o o t h i n g w o u l d be to d i m i n i s h the fluctuation a n d h e n c e the p e r c e i v e d flux. E v e n w i t h o u t c h e m i c a l i n t e r a c t i o n the effect of a v e l o c i t y profile across the section of the duct leads to a s i m i l a r effect. T h e result, again, is an attenuation of variance. L e n s c h o w a n d R a u p a c h (13) investigated this aspect and p r e s e n t e d t h e i r results i n terms of the h a l f - p o w e r f r e q u e n c y , f , 05

the

f r e q u e n c y at w h i c h the variance has b e e n d i m i n i s h e d b y 5 0 % . T h i s halfpower

frequency

is g i v e n b y the expression / o s = %.5 X v e l o c i t y / ( r a d i u s X l e n g t h )

where n Sc)

m

0 5

is a dimensionless

frequency

for l a m i n a r flow a n d a value of n

w i t h a value of n 0 5

= 0*066Re

1/16

(3)

1/2

0 5

= 0.92/(Re X

for t u r b u l e n t flow,

R e is the R e y n o l d s n u m b e r (Re = d i a m e t e r X v e l o c i t y X d e n s i t y / v i s c o s i t y ) , a n d Sc is the S c h m i d t n u m b e r (Sc = m o l e c u l a r v i s c o s i t y / m o l e c u l a r d i f f u sivity; Sc is a p p r o x i m a t e l y 0.5 for c o m m o n gases). T h e best single system to use to d u c t a t m o s p h e r i c air to c h e m i c a l sensors u s e d cannot b e d e f i n e d for e d d y c o r r e l a t i o n m e a s u r e m e n t s because t h e r e are m a n y parameters capable of adjustment a n d o t h e r considerations o f cost, available e l e c t r i c a l p o w e r , a n d so forth. H o w e v e r , the p r o b l e m c a n b e i l l u s t r a t e d b y a n analysis of a realistic e x a m p l e . A fast c h e m i c a l sensor that operates at a r e d u c e d pressure o f 50 t o r r (6700 Pa) a n d w i t h a flow of 1 standard l i t e r p e r m i n u t e m u s t b e m a i n t a i n e d i n an i n s t r u m e n t shelter. C o n s i d e r a t i o n s of flow d i s t o r t i o n r e q u i r e that 10 m separate the sensor from its intake o n the tower near the sonic a n e m o m eter. T h r e e d u c t i n g arrangements can b e c o n s i d e r e d . T h e first w o u l d i n v o l v e d r a w i n g air along 10 m of 1 / 4 - i n c h t u b i n g (0.2 c m i n t e r n a l radius) a n d c o n t r o l l i n g the flow a n d pressure at the sensor itself. T h e second o p t i o n w o u l d place the p r e s s u r e - f l o w c o n t r o l l e r at the i n l e t a n d allow the i n l e t intake to flow at the r e d u c e d pressure. T h e t h i r d course w o u l d b e to use a


DELANY

3.

Fast-Response

Chemical Sensors

97

h i g h - f l o w m a n i f o l d of 2 - i n c h p i p e w i t h a flow of 10 c u b i c feet p e r m i n u t e (2.5-cm i n t e r n a l radius a n d 4.7 liters p e r second) to b r i n g the air to the sensor, a n d t h e n s a m p l i n g w o u l d b e d o n e f r o m the m a n i f o l d . T a b l e I gives the relevant d i m e n s i o n s a n d calculated parameters. T h e results o f u s i n g each of the t h r e e options are s h o w n schematically i n F i g u r e 2.

Table I. Dimensions and Calculated Parameters for the Three Different Inlet Configurations Inlet Parameter Length (cm) Radius (cm) Density (g/cm ) Ambient flow (cm Time delay (s) Re 3

-3

s ) _1

fo.5 (S" ) 1

no.5

( ) s_1

1.0 2.0 1.2 1.7 7.4 3.5 7.0 6.6

3

10 x ΙΟ" x 10x 10

X

Configuration Reduced Pressure

Ambient Pressure 1 3

1

2

x 10 x ΙΟ" x ΙΟ"

2

1

1.0 2.0 7.9 2.6 4.8 3.5 7.0 1.0

Manifold 3

10 x ΙΟ" x 10x 10 x ΙΟ" x 10 Χ ΙΟ" X 10 X

1 5

2

1

2

2

1

1.0 2.5 1.2 4.7 4.2 7.8 1.2 5.5

X

3

x 10x 10 3

3

x 10 Χ ΙΟ" x ΙΟ"

1

1

DELAY TIME * 75 SECONDS SENSOR

HALF POWER FREQUENCY • 0.66 HERTZ HALF POWER TIME • IS SECONDS

DELAY TIME « 0.48 SECONDS

JFL SENSOR

HALF POWER FREQUENCY = 10 HERTZ HALF POWER TIME « 0.1 SECONDS

DELAY TIME s 42 SECONDS

SENSOR

tft

HALF POWER FREQUENCY * 055 HERTZ HALF POWER TIME s IS SECONDS

PUMP

Figure 2. Three possible options for ducting atmospheric air from the inlet on the tower to the sensor in the instrument shelter.


3

10

98


S e n s i n g V o l u m e . T h e sensing v o l u m e of a sensor is the v o l u m e w h e r e the a i r is actually m o n i t o r e d . T h e sensing v o l u m e is the reaction c h a m b e r of a flame p h o t o m e t r i c detector or a c h e m i l u m i n e s c e n c e d e v i c e , the field of v i e w o f an o p e n - p a t h sensor, o r the W h i t e c e l l of a r e d u c e d - p r e s s u r e o p t i c a l system. T h e residence t i m e of the sample w i t h i n the sensing v o l u m e u l t i m a t e l y l i m i t s the t e m p o r a l r e s o l u t i o n of most c h e m i c a l sensors. F o r an i n situ sensor the residence t i m e for the c h e m i c a l constituent is d e f i n e d b y the sensing p a t h of the p a r t i c u l a r t e c h n i q u e . I n the best possible case this p a t h is the same as for the sonic a n e m o m e t e r , w h i c h measures air m o t i o n , a n d h e n c e t h e r e is a correspondence w h e n the two factors, c a n d w, are c o r r e l a t e d . F o r most c h e m i c a l o r o p t i c a l sensors the size a n d flow rate of t h e reactor or o p t i c a l c e l l defines the residence t i m e . T h e t i m e r e s o l u t i o n cannot b e b e t t e r than the residence t i m e τ, a l t h o u g h it can be worse. τ = volume/ambient

flow

(4)

T h u s , for a n i t r i c oxide sensor d e p e n d e n t o n ozone c h e m i l u m i n e s c e n c e , a 1.5-liter reaction c h a m b e r o p e r a t i n g at 60 torr w i t h a flow of 1.0 standard l i t e r p e r second has a residence t i m e of a p p r o x i m a t e l y τ = (1.5/1.0) X (60/ 760) = 0.12 s. Signal Processing. A l t h o u g h the t e m p o r a l r e s o l u t i o n of a sensor is u l t i m a t e l y l i m i t e d b y the residence t i m e of the sensing v o l u m e , r e s o l u t i o n can be f u r t h e r d e g r a d e d b y the necessity to integrate the signal for some l o n g e r p e r i o d i n o r d e r to accumulate sufficient data for adequate statistics. T h i s scenario can best be i l l u s t r a t e d b y c o n s i d e r i n g a sensor that i n v o l v e s a statistically c h a r a c t e r i z e d o u t p u t s u c h as p h o t o n c o u n t i n g . F o r s u c h a sensor the smallest change i n concentration that can b e r e l i a b l y d e t e c t e d is one that generates a change of o u t p u t greater t h a n or e q u a l to the statistical u n c e r t a i n t y associated w i t h the total n u m b e r of counts. T h i s v a l u e m a y be t a k e n as the square root of the total n u m b e r . Because the n u m b e r o f counts d e p e n d s o n the integration t i m e , the statistical u n c e r t a i n t y d e p e n d s o n some specific t i m e i n t e r v a l . F o r a sensor i n w h i c h S is the c h e m i c a l s e n s i t i v i t y of a sensor, the count rate generated b y some c o n c e n t r a t i o n of the specific c h e m i c a l species, a n d Β is the b a c k g r o u n d of a sensor, the count rate that is i n d e p e n d e n t of the c h e m i c a l c o n c e n t r a t i o n , the c h e m i c a l r e s o l u t i o n of this sensor, R , is the smallest change i n c h e m i c a l concentration that can b e seen against the m e a n c h e m i c a l c o n c e n t r a t i o n , c, w h e n the m e a s u r e m e n t is p e r f o r m e d for t i m e τ. T

R

T

=

(BT +

STC) ' ^)1

2

(5)

1

F o r the case w h e n the b a c k g r o u n d is n e g l i g i b l e R

T

=

(STC)

1 / 2

(ST)-

1

=

(C/ST) ' 1

2


(6)

3.

D E L A NY

Fast-Response

Chemical Sensors

99

T h e d e t e c t i o n l i m i t of a sensor, D , is a special case of c h e m i c a l reso l u t i o n . T h i s l i m i t is the smallest increase o f c o n c e n t r a t i o n that can b e seen against the b a c k g r o u n d w h e n the m e a s u r e m e n t is p e r f o r m e d for t i m e τ. B o t h the c h e m i c a l r e s o l u t i o n a n d the d e t e c t i o n l i m i t are d e p e n d e n t u p o n m e a s u r e m e n t t i m e . T h e longer the i n t e g r a t i o n p e r i o d , the s m a l l e r the c h e m i c a l r e s o l u t i o n a n d the d e t e c t i o n l i m i t . T h u s T

D

T

= D

( 1 5 )

(1.0/τΓ

(7)

where D is the 1-s integration d e t e c t i o n l i m i t . H o w e v e r , the d e t e c t i o n l i m i t (and the c h e m i c a l resolution) do not decrease i n d e f i n i t e l y . A l i m i t is r e a c h e d w h e n nonstatistical v a r i a t i o n , i n c l u d i n g drift a n d artifact response, b e c o m e d o m i n a n t . L i k e w i s e τ (and the t e m p o r a l resolution) cannot b e d e creased i n d e f i n i t e l y . A detector has a m i n i m u m response t i m e that is as sociated w i t h the clearance o f the sample of air from the s e n s i n g v o l u m e . { 1 s)

Conclusion M o r e i n f o r m a t i o n is n e e d e d about the surface e m i s s i o n a n d d e p o s i t i o n o f trace a t m o s p h e r i c species. T h e s e fluxes can often b e best m e a s u r e d b y the e d d y c o r r e l a t i o n t e c h n i q u e w i t h fast c h e m i c a l sensors i n c o n j u n c t i o n w i t h m i c r o m e t e o r o l o g i c a l i n s t r u m e n t a t i o n . A s analytical t e c h n i q u e s for trace spe cies progress, fast a n d sensitive sensors are b e c o m i n g available for field research. C o n s i d e r a t i o n m u s t b e g i v e n to m a t c h i n g the c h e m i c a l sensors to the e d d y c o r r e l a t i o n t e c h n i q u e .

Acknowledgment T h e N a t i o n a l C e n t e r for A t m o s p h e r i c R e s e a r c h is sponsored b y the N a t i o n a l Science F o u n d a t i o n .

References 1. Global Tropospheric Chemistry; Chemical Fluxes in the Global Atmosphere; Lenschow, D . H.; Hicks, Β. B . , E d s . ; National Center for Atmospheric Research: Boulder, C O , 1989. 2. Businger, J. A . J. Clim. Appl. Meteorol. 1986, 25, 1100-1124. 3. Wyngaard, J. C . Boundary Layer Meteorol. 1988, 42, 19-26. 4. Shaw, W. J.; Tillman, J . E. J. Appl. Meteorol. 1980, 19, 90-97. 5. Webb, E . K.; Pearman, G . I.; Leuning, R. Q. J. R. Meteorol. Soc. 1980, 106, 85-100. 6. Lenschow, D . H.; Delany, A . C . J. Atmos. Chem. 1987, 5, 301-309. 7. Arya, S. P. Introduction to Micrometeorohgy; Academic: San Diego, 1988; 307 pp; International Geophysics Series; Vol. 42. 8. Lenschow, D . H.; Kristensen, L. J. Atmos. Oceanic Technol. 1985, 2, 6 8 - 8 1 .


100

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CHALLENGES

IN ATMOSPHERIC

CHEMISTRY

9. Kaimal, J . C . ; Wyngaard, J. C . ; Izumi, Y.; Cote, O. R. Q. J. R. Meteorol. Soc. 1972, 95, 563-589. 10. Wyngaard, J. C . Annu. Rev. Fluid Mech. 1981, 13, 399-423. 11. Businger, J . Α.; Dabberdt, D . W.; Delany, A. C.; Horst, T. W.; Martin, C . L.; Oncley, S. P.; Semmer, S. R. Bull. Am. Meteorol. Soc. 1990, 71, 1006-1011. 12. Businger, J . Α.; Delany, A. C . J. Atmos. Chem. 1990, 10, 399-410. 13. Lenschow, D . H.; Raupach, M. M. J. Geophys. Res. 1991, 96, 15259-15268. RECEIVED 1992.

for review March 20, 1991.

ACCEPTED

revised manuscript September 15,


Measurement Challenges in Atmospheric Chemistry - American

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