nox emisiones

8/4/2019 nox emisiones

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Pergamon Atmospheric EnvironmentVol . 30, No. 22, pp. 3741-3755, 1996Cop yr ight 1996 Elsevier Science LtdPr inted in Great Br i tain. Al l r ights reservedP I I : S1352--2310(96)00104-5 1352-2310/96 $15.00 + 0.00

A E U R O P E A N I N V E N T O R Y O F S O IL N I T RI C O X I D EE M I S S IO N S A N D T H E E F F E C T O F T H E S E E M I SS IO N S O N

T H E P H O T O C H E M I C A L F O R M A T I O N O F O Z O N EA N D R E A S S T O H L, * E R I C W l L L I A M S , t G E R H A R D W O T A W A *

a n d H E L G A K R O M P - K O L B **Institute of Meteorology and Physics, University for Agriculture, Forestry and Renewable NaturalResources, TiirkenschanzstraBe 18, 1185 Wien, Austria; and fAeronomy Laboratory, National Atmo-spheric and Oceanic Administration, 325 Broadway, Boulder, CO 80303, U.S.A.

(First received 18 Auoust 1995and in final form 13 March 1996)Abstract--An inventory of soil nitric oxide (NO) emissionsfor Europe was developed. The emission of NOwas parameterized using empirical relationships with type of landuse, fertilization rate of agricultural areasand soil temperature. For the year 1994, t was estimated that annual soil NO emissions n the inventoriedarea amounted to 535 kt NO-N yr- 1, i.e. 8% of the emissions rom combustion processes. On a hot summerday, this fraction increased to 27%. The uncertainty of these emission estimates, however, is high. They areaccurate only within a factor of three to four. As the major fraction (in summer 81%) of the emissions camefrom arable ]and, soil NO emissions have to be considered as largely anthropogenic. Large regionaldifferences existed in the soil NO emission rates with the highest emission rates found in southern andwestern Europe, intermediate rates in eastern Europe and the lowest rates in Scandinavia and northernRussia. The effect of the soil NO emissions on the photochemical formation of ozone (03) was investigatedwith a one-dimensional photochemical model. When soil NO emissions were considered in the modelsimulations, computed mean daily maximum 0 3 concentrations over Europe from June to August 1994were 4 ppb (uncertainty range: 1.4-9.6 ppb) higher than without these emissions. Reductions of pyrogenicNO~ emissions were less efficient n reducing O3 concentrations when soil NO emissions were taken intoaccount. Copyright 1996 Elsevier Science LtdKey word index: Nitric oxide, nitrogen oxides, soil NO emissions, emission inventory, ozone formation,photochemisl:ry.

1. INTRODUCTIONElevated concentrations of nitrogen oxides (NOx= NO + NO2) and volatile organic compo unds

(VOCs) cause the photochemical production of03 under meteorological conditions conducive tophotochemistry. In rural areas, NOx concentrationsare much lower than in urban areas, whereas VOCconcentrations can be relatively high. This is due tothe longer residence time of some VOCs in the atmo-sphere in compari son with NOx and to the substantia lcont ribut ion of biogenic VOC emissions to total VO Cemissions. Therefo~.'e, photo chemi cal 0 3 produc tionin rural areas is generally NO~-limited (Nationa l Re-search Council, 1992). This implies that in rural areas03 concen trations are much more sensitive to cha-nges in NOx emissions than to changes in VOC emis-sions. Thus, for developing strategies to reduce cur-rent rural 03 levels it is critical to have reliable esti-mates of the sources of NO~ in rural areas.

The ma jor source of NOx are emissions from com-bus tio n processes. In recent studies, however, it wasfound that emissions of NO from microbial processes

in soils may be a significant source of NOx in ruralareas (e.g. Galba lly and Roy, 1978; Williams et al.,1992b; Valente and Thor nton , 1993). Although mo stauthors report only on soil emissions of NO, a fewstudies also indicate significant NO2 emission rates(e.g. Slemr and Seiler, 1984), but this cou ld be theresult of the reaction of NO with 03 in the top soillayer that produces NOz before NO can escape to theatmosphere (Delany et al . , 1986).

In soils, NO is produced by nitrification, the oxida-tion of NH~ to NO~ and NO3, and denitrification,the anaerobic reduction of NO r and NO3 to gaseousNO, N20 or N2 (Williams et al., 1992b). Severalenviro nment al factors cont rol the activity of themicroorganisms responsible for these processes andthus the produc tion rate of NO. In addition, some ofthese factors also modify the physical and chemicalparameters that regulate the transport of NO throughthe soil and its subsequent exchange with the atmo-sphere. The most impo rtan t controlli ng factors forsoil NO emissions are soil nitrogen availability, soiltemperature and soil water content, but many otherfactors, for instance, plowing (Sanhueza et al., 1994),

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3742 A. STO HL et a l .i n f l u e n c e t h e e m i s s i o n s . T h e g r e a t s p a t i a l a n d t e m -p o r a l v a r i a b i l i t y o f t h e c o n t r o l l i n g f a c t o rs c a u s e s a ne x t r e m e l y h i g h v a r i a b i l i t y o f t h e N O e m i s s io n s t h a t i nt u r n m a k e s i t v e r y d i ff i c u lt t o e s t i m a t e t h e m o n l a r g e rs p a t i a l a n d t e m p o r a l s c a l e s ( S h e p h e r d e t a l . , 1991).

A l l o f t h e f e w e x is t i n g c o u n t r y w i d e t o g l o b a l e m i s -s i o n e s t i m a t e s w e r e b a s e d d i r e c t l y o n p o i n t m e a s u r e -m en t s ( e . g . Sk iba e t a l . , 1992 ; W i l l i am s e t a l . , 1992a;Y i e n g e r a n d L e v y , 1 99 5 ), s o m e t i m e s t a k i n g t h e v a r i a -b i l i t y o f s o i l t e m p e r a t u r e , f e r t i l i z a t io n r a t e s a n d o t h e rf a c to r s i n to a c c o u n t t o e x t r a p o l a t e p o i n t m e a s u r e -m e n t s u n d e r t a k e n d u r i n g s p e c if ic e n v i r o n m e n t a l c o n -d i t i o n s t o t h e w h o l e y e a r a n d t o l a r g e r a r e a s . A l -t h o u g h m o d e l s w e r e d e v e l o p e d t o s i m u l a t e so i l N Oem is s ions exp l i c i t l y ( e .g . R em de e t a l . , 1993) , thei ra p p l i c a t i o n s a r e s t il l l i m i t e d t o s i n g l e c a s e s t u d i e sd o n e u n d e r w e l l - c o n tr o l l e d c o n d i t io n s .

I n t h i s s tu d y , w e a t t e m p t t o c o m b i n e l a n d u s e d a t a ,m e t e o r o l o g i c a l d a t a a n d i n f o r m a t i o n o n n i t r o g e n f e rt -i l iz a t i o n r a te s t o c r e a t e a n i n v e n t o r y o f N O e m i s s io n sf r o m s o i l s i n E u r o p e . I n a f u r t h e r s t e p , w e e x a m i n e t h ei m p a c t o f t h e se e m i s s io n s o n t h e p h o t o c h e m i c a l p r o -d u c t i o n o f 0 3 a n d p e r o x y a c e t y l n i t r a t e (P A N ) b ym e a n s o f a s i m p l e o n e - d i m e n s i o n a l p h o t o c h e m i c a lm o d e l . T h e e f fe c t o f so i l N O e m i s s i o n s o n t h e 0 3l e v e l s f o r d i f f e r e n t r e d u c t i o n s c e n a r i o s o f p y r o g e n i cN O x e m i s s io n s is d e m o n s t r a t e d .

2 . A E U R O P E A N I N V E N T O R Y O F S O I L N I T R I C O X I D EE M I S S I O N S

2.1. M e t h o d a n d d a t a b a s eT h e m e t h o d u s e d in t h i s p a p e r t o d e v e l o p a E u r o -p e a n i n v e n t o r y f o r N O e m i s s i o n s f ro m s o i l s i s b a s e do n t h e w o r k o f W i l l i a m s e t a l . ( 1 9 92 a ) w h o e s t a b l i s h e da n i n v e n t o r y f o r t h e U n i t e d S t a t e s . T h e y p a r a -m e t e r i z e d t h e s o il N O e m i s s i o n w i t h tw o v a r i a b l e s ,n a m e l y s o i l t e m p e r a t u r e a n d l a n d u s e c l a s s . S o i l t e m -p e r a t u r e i s a n i m p o r t a n t c o n t r o l l i n g e l e m e n t b e c a u s ei t c a n i n f l u e n c e b i o l o g i c a l r e a c t i o n r a t e s a n d s o i l g a sd i f f u s io n r a te s . O n t h e b a s i s o f p r e v i o u s m e a s u r e -m e n t s w i t h t h e e n c l o s u r e t e c h n i q u e ( W i l l i a m s e t a l . ,1987, 1988; W i l l i am s an d F ehsen fe ld , 1991) W i l l i am se t a l . ( 1 9 92 a ) e s t a b l i s h e d t h e f u n c t i o n a l d e p e n d e n c e

R = A exp[ (0 . 071 _+ 0 . 00 7)T ] (1 )

w h e r e R i s t h e N O e m i ss i on ra t e ( n g N m - 2 s - 1 ) ,A a f a c t o r a s s o c i a t e d w i t h t h e l a n d u s e c l a s s( n g N m - 2 s - 1 ) a n d T t h e s o i l t e m p e r a t u r e ( C ) .

W i l l i a m s e t a l . ( 1 9 9 2 a ) c o m p u t e d s o i l t e m p e r a t u r e sw i t h a l i n e a r f u n c ti o n a l d e p e n d e n c e o n a i r t e m p e r -a t u r e s f o r w h i c h t h e y h a d c l i m a t o l o g i c a l d a t a a v a i l -a b l e . I n t h i s s t u d y w e u s e d h o u r l y s o i l t e m p e r a t u r e st h a t w e c o m p u t e d f r o m d a t a f r o m m o r e t h a n 5 0 0s y n o p t i c s t a t i o n s i n E u r o p e w i t h t h e f o r c e - r e s t o r em e t h o d d e s c r i b e d i n S t u l l ( 19 8 8) . T h i s m e t h o d i s

a t w o - l a y e r a p p r o x i m a t i o n w h e r e a s h a l l o w s l a b o fs o i l ( in o u r c a s e 5 c m ) i s b o u n d e d b e l o w b y a t h i c kc o n s t a n t - t e m p e r a t u r e s l ab . G i v e n a n i n i ti a l c o n d i t i o n ,t h e t e m p o r a l d e v e l o p m e n t o f th e s o i l t e m p e r a t u r eT c a n b e d e t e r m i n e d b y t h e p r o g n o s t i c e q u a t i o n

2 ~d T Q + [ T M - T ] - a [ T - T ~ ] , (2 )

w h e r e Q i s t h e n e t r a d i a t i o n c a l c u l a t e d w i t h t h e m e -t e o r o l o g i c a l p r e p r o c e s s o r O M L ( O l e se n a n d B r o w n ,1 9 88 ), a b o u n d a r y l a y e r m o d e l b a s e d o n r o u t i n e m e -t e o r o l o g i c a l o b s e r v a t i o n s , C i s t h e h e a t c a p a c i t y o f t h es o i l p e r u n i t a r e a , ~ i s t h e p e r i o d o f t h e d a i l y c y c le(86 , 400 s ) , TM i s t he t em pera tu r e o f t he l ow er s l ab ,T a is th e a i r t e m p e r a t u r e a n d a i s a f o r m o f c o n d u c t i v -i t y b e t w e e n t h e g r o u n d a n d t h e a i r . T h r o u g h e q u a t i o n( 2) t h e t e m p e r a t u r e o f th e t o p s l a b r e s p o n d s t o t h e n e tr a d i a t i v e f o rc i n g , t o c o n d u c t i o n f r o m t h e d e e p e r s l a ba n d t o t u r b u l e n t h e a t t r a n s p o r t w i t h t h e a i r . W ec o m p a r e d s o i l t e m p e r a t u r e s c o m p u t e d w i t h e q u a t i o n( 2) w i t h m e a s u r e m e n t s o f s o i l t e m p e r a t u r e s a t 2 c md e p t h a t a g r a s s - c o v e r e d s it e i n V i e n n a a n d f o u n da g o o d c o r r e s p o n d e n c e ( F i g . 1 ) . T h e c o m p u t e d m a x -i m u m t e m p e r a t u r e s f o r b a r e s o i l s w e r e p r o b a b l ys o m e w h a t l o w e r t h a n a c t u a l v a l u e s g i v i n g a c o n s e r v a -t i v e e s t i m a t e f o r th e N O e m i s s i o n r a t e .

A l t h o u g h a m o r e s i m p l e a p p r o a c h w o u l d h a v e b e e ns u f f i c i e n t f o r c o m p u t i n g a v e r a g e e m i s s i o n r a t e s , a na c c u r a t e d e s c r i p t i o n o f s o i l t e m p e r a t u r e b e h a v i o rw a s e s s e n t i a l f o r m o d e l i n g t h e e f fe c t o f s o i l N O e m i s -s i o n s o n p h o t o c h e m i s t r y . S o i l t e m p e r a t u r e p e a k se a r l ie r d u r i n g t h e d a y t h a n a i r te m p e r a t u r e b e c a u s ei t r e s p o n d s f a s t t o s o l a r r a d i a t i o n , w h e r e a s t h e a i rt e m p e r a t u r e m a x i m u m l a g s t h e r a d i a t io n m a x i m u mb y s e v e r a l h o u r s . T h us , t h e d a i l y m a x i m u m N O e m i s -s i o n ta k e s p l a c e a t a t i m e w h e n t h e s o l a r r a d i a t i o n a n dh e n c e t h e p o t e n t i a l f o r 0 3 f o r m a t i o n i s g r e a t e s t .

E q u a t i o n (1 ) w a s b a s e d o n d a t a t h a t h a d b e e nm e a s u r e d b e t w e e n 5 a n d 3 5 C a n d t h u s t h e t e m p e r -a t u r e r a n g e w a s t r u n c a t e d a t 3 5 C . H o w e v e r , t h el o w e r t e m p e r a t u r e r a n g e f o r w h i c h e q u a t i o n ( 1 ) w a sa p p l i e d , w a s e x t e n d e d t o 0 C . A s e m i s s i o n r a t e s a tt h e s e t e m p e r a t u r e s a r e l o w , t h e u n c e r t a i n t y i n t r o -d u c e d b y t h i s e x t e n s i o n i s s m a l l . F o r s o i l te m p e r a t u r e sb e l o w 0 C o r s n o w - c o v e r e d g r o u n d , t h e e m i s s i o n r a tew a s s e t t o z e r o , a l t h o u g h t h e r e e x i s ts e x p e r i m e n t a le v i d en c e t h a t N O e m i s s io n s c a n e v e n p e n e t r a t et h r o u g h a s n o w p a c k ( S t o c k e r e t a l . , 1995) . The so i lt e m p e r a t u r e s i n t h e g r i d c e l l s o f o u r l a n d u s e i n v e n t o r yw e r e c a l c u l a t e d u s i n g l i n e a r i n t e r p o l a t i o n o f t h e s o i lt e m p e r a t u r e s a t t h e c l o s e s t t h r e e s y n o p t i c s t a t i o n s .S u b s e q u e n t l y , t h e s o i l t e m p e r a t u r e s w e r e r o u g h l y c o r -r e c t e d f o r t h e e f f e ct o f a lt i t u d e , a s s u m i n g a d r ya d i a b a t i c l a p s e r at e . I t w o u l d h a v e b e e n p h y s i c a l l ym o r e c o r r e c t t o a p p l y t h e c o r r e c t i o n t o t h e a i r t e m p e r -a t u r e s . T h i s , h o w e v e r , w o u l d h a v e r e q u i r e d t o r u n t h es o i l t e m p e r a t u r e m o d e l f o r e a c h g r i d c e ll o f th e l a n d -u s e i n v e n t o r y a n d n o t o n l y f o r t h e s y n o p t i c s ta t i o n s a sw e d i d .


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Soil nitric oxide emissions 3743

Ud

35-30-25-20-151050

I I I Im

; l ' O 1 ' 5 2 ' 0 2 ' 5 3 ' 0 3 ' 5MODELED SOIL TEMPERA TURE [C]

Fig. 1. Comparison of hourly values of soil temperatures measured at 2 cm depth at a grass-covered site inVienna with soil temperatures computed using the force restore method from January to October 1994. Thelines are the linear regression curve fit through the data, and the ideal relationship. The observed meantemperature was 14.7C, he calculated mean temperature was 14.4C. The correlation coefficient was 0.98.

Table 1. The landuse classes, their fraction of the total in-ventoried area (%) and their associated A factorsLanduse Area Acategory (%) (ng N m- 2s 1 )Grassland 15.6 0.70Arable land 26.1 See textPermanent crops 2.1 0.70Forests 30.2 0.05Inland water 1.8 0.00Urban areas 1.2 0.05Others 23.0 0.40

The A factors in equati on (1) depend o n the type oflanduse. We used the recently published database ofvan de Velde e t a l . (1994) for infor mation on landuse.It has a resolution of 10 min an d conta ins the areafractions of seven landus e classes (Table 1). Each classwas attribu ted an i ndividua l A factor (see Table 1),except for the category "arable land". Emissions fromagricultural areas depend on fertilization. Shepherd e ta l. (1991) and Skiba e t a l . (1992) reported linear rela-tionships between NO emission rates and the am oun tof nitrogen fertilizer applied. Therefore, we used theempirical relationship

A = 0.05 F, (3)with F being the amo unt of nitrog en fertilizer appliedand given in kg N ha- 1 yr - 1, indep endent of the typeof fertilizer. Fertili zer usage varies cons idera bly inEurope. We calculated the fertilization amounts andthe associated A factor for each coun try from countrystatistics on total nitrogen fertilizer consumption in

1992/93 an d total arable land in 1993 that were pub-lished by the Food and Agriculture Association(FAO, 1994a, b). Dividing the fertilizer consumptionby the area of arable land yielded the fertilizationrates presented in Table 2. As in Williams e t a l .(1992a), these A factors were used throughout thegrowing season from May to August. Duri ng the restof the year, A for arable land was set to 0.5.

Valente and Thornton (1993) also reported an ex-ponential dependence of NO emissions from corn,forest and pasture on te mperature, but the increase ofthe emission rates with tempera ture was steeper thanthat given by Williams e t a l . (1992a). For corn, theirrelationship gave almost the same emission rates asWilliams and coworkers formula at high temper-atures, but significantly smaller emission rates at lowtemperatures. On the other hand , their emission ratesfrom forest soits were higher than those of Willi ams e ta l . (1992a) and they found very high values for pas-ture.

Stocker e t a l . (1993) used the eddy correla tion tech-nique to measure fluxes of NO2 and NOr at 6 mheight at the same grassland site as Williams andFehsenfeld (1991). Their fluxes corresponded to NOemission rates that were on ly slightly lower than thoseof Williams and coworkers. The agreement was verygood, taking into account that the soil temperaturesinside the enclosures of Williams and Fehsenfeld(1991) were probably somewhat higher than thoseoutside of the enclosures. In addition, eddy correla-tion measu rements integrate over a larger spatial scalethan enclosure measurements and thus the results ofthe two methods are not directly comparable. Thefinding of this good correspondence is important,


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3744 A . STO H L e t a l .Tab le 2. Nitro gen ferti lization ra tes (F) for arable lan d in E urop e (1993) used in this study(kg N ha - 1 y r - 1)

F FC o u n t ry ( k g N h a - I y r - 1 ) C o u n tr y ( k g N h a - I y r - 1 )Alban ia 26 Aust r ia 86Belgium + Luxembourg 226 Bulgaria 56Czech Republic 47 Denm ark 13lFinland 68 Form er Yugoslavia 41Form er Soviet Union 37 France 119Ge rma ny 147 Greece 115Hung ary 26 I re land 365I ta ly 100 The Nether lands 425Norw ay 124 Polan d 48Por tugal 57 Rom ania 28Slovak Republic 47 Spain 50Sweden 69 Switzerland 157U.K. 203

b e c a u s e i t s h o w s t h a t a l a r g e f r a c t i o n o f t h e e m i t t e dN O i s n o t l o c a l l y d e p o s i t e d o n p l a n t s u rf a ce s , b u ta c t u a l l y r e a c h e s t h e a t m o s p h e r e , a t l e a s t i n l o c a t i o n sw i t h r a t h e r s p a r s e v e g e t a t i o n .

T h e A f a c t o r s a n d t h e i r d e p e n d e n c e o n n i t r o g e nf e r t i li z a t i o n u s e d i n t h i s s t u d y w e r e e s s e n t i a l l y ta k e nf r o m W i l l i a m s e t a l . (1992a ) , bu t w e re r educed bya p p r o x i m a t e l y 3 0 % t o a c c o u n t f o r t h e s o m e w h a tl o w e r e m i s s io n r a t e s f o u n d b y S t o c k e r e t a l . (1993) forg r a s s l a n d a n d V a l e n t e a n d T h o r n t o n ( 1 9 9 3 ) f o r c o r n .F o r t h o s e l a n d u s e c l a s s es fo r w h i c h d a t a w e r e l a c k i n g( " p e r m a n e n t c r o p s " , " u r b a n a r e a s " , " o th e r s " ) , c o n s e r -v a t i v e e s t im a t e s w e r e m a d e . S e e th e d i s c u s s i o n o n t h eu n c e r t a i n t i e s o f t h e i n v e n t o r y h o w t h e s e A f a c t o r sw e r e d e t e r m i n e d .

T h e e m i s s io n i n v e n t o r y w a s d e v e l o p e d f o r t h e a r e a2 4.6 W t o 4 1 . 9 E a n d 3 4 . 9 N t o 7 2 . 1 N w h i c h c o v e r sm o s t o f E u r o p e . W e i n v e s t i g a t e d t h r e e c a se s :

1. T h e a n n u a l e m i s si o n s b a s e d o n m e t e o r o l o g i c a ld a t a f o r t h e y e a r 1 9 9 4 .

2 . T h e s u m m e r t i m e ( J u n e - A u g u s t ) e m i s si o n s i n1994.

3 . T h e d a i l y e m i s s i o n s o n 5 A u g u s t 1 9 94 w h i c h w a so n e o f t h e h o t t e s t d a y s o f t h e y e a r , e s p e c i a l l y i nC e n t r a l E u r o p e .T h e i n v e n t o r y o f p y r o g e n i c e m i s s io n s f o r t h e y e a r1 99 1, d e v e l o p e d b y t h e C o - O p e r a t i v e P r o g r a m m e f o rM o n i t o r i n g a n d E v a l u a t i o n o f t h e L o n g - R a n g eT r a n s m i s s i o n o f A i r P o l l u t a n t s in E u r o p e ( E M E P )( S a n d n e s , 1 9 9 3) , w a s u s e d f o r c o m p a r i s o n . T h e r e s o l u -t i o n o f t h is i n v e n t o r y w a s 1 50 k m , b u t f o r t h e O E C Dc o u n t r i e s i t w a s r e f i n e d t o 5 0 k m r e s o l u t i o n u s i n g t h er e l a t i v e e m i s s i o n i n te n s i t i e s o f t h e O E C D i n v e n t o r y( L / i b k e r t a n d d e T i l l y , 1 9 8 9 ). F o r t h e c o m p a r i s o n i tw a s a s s u m e d t h a t t h e p y r o g e n i c N O ~ e m i s s i o n s h a v en o a n n u a l v a r i a t i o n , a r e a so n a b l e a s s u m p t i o n a s t h em a j o r p y r o g e n i c N O x s o u r c e a r e t ra f fi c a n d p o w e rp l a n t s w h i c h i n d e e d h a v e l i t t l e a n n u a l v a r i a t i o n . B o t ht h e p y r o g e n i c a s w e l l a s t h e s o i l e m i s s i o n s w e r e a p p o r -

t i o n e d t o i n d i v i d u a l c o u n t r i e s u s in g a c o u n t r y - c o d ei n v e n t o r y w i t h 7 5 k m r e s o l u t i o n . T h i s i n v e n t o r y g iv e st h e c o u n t r y w i t h t h e d o m i n a t i n g a r e a f r a c t io n w i t h i ne a c h g r i d e l e m e n t . D u e t o i t s c o a r s e r e s o l u t i o n , h o w -e v e r, c o u n t r y t o t a l s m u s t b e i n t e r p r e t e d w i t h c a u t i o n ,e s p e c i a l l y fo r s m a l l e r c o u n t r i e s .2.2. R e s u l t s

O v e r t h e w h o l e i n v e n t o r i e d a r e a , t o t a l s o i l e m i s -s i o n s w e r e 5 3 5 k t N O - N y r - 1 ( T a b l e 3 ). T h i s r e s u l t i si n g o o d a g r e e m e n t w i t h t h e g l o b a l s o i l N O x i n v e n t o r yo f Y i e n g e r a n d L e v y (1 9 95 ). F o r E u r o p e a n d R u s s i at h e y c a l c u l a t e d a t o t a l e m i s s i o n o f 7 4 0 k t N O x - Ny r - 1 . A s em i s s i o n r at e s in E u r o p e w e r e m u c h h i g h e rt h a n i n t h e A s i a n p a r t o f R u s s i a , m o s t o f t h e s o i le m i s s i o n s s t e m s f r o m E u r o p e . T h u s , t h e a g r e e m e n tw i t h o u r t o t a l e m i s s i o n e s t i m a t e i s e x c e l le n t . F i g u r e 2s h o w s t h e a n n u a l s o i l N O e m i s s io n s in E u r o p e . F r o ma v i s u a l c o m p a r i s o n i t a p p e a r e d t h a t t h e s p a t i a l d i s t ri -b u t i o n o f th e e m i s s i o n w a s a l s o in g o o d a g r e e m e n tw i t h t h e r e s u l t s o f Y i e n g e r a n d L e v y ( 1 99 5) . Y i e n g e ra n d L e v y ( 1 99 5 ) a c c o u n t e d e x p l i c i t ly f o r t h e a t t e n u -a t i o n o f s o i l e m i s s i o n s b y v e g e t a t i v e c a n o p i e s . W ea c c o u n t e d f o r t h i s e ff e ct o n l y b y r e d u c i n g t h e o r i g i n a lA f a c t o r s o f W i l l i a m s e t a l . ( 1 9 9 2a ) b y 3 0 % . P r o b a b l y ,t h i s r e d u c t i o n w a s s u f f ic i e n t t o c o m p e n s a t e f o r t h el o s s t h r o u g h d e p o s i t i o n o n t h e f o l i a g e .

B a s i c a l l y , t h r e e l a r g e a r e a s w i t h s i g n i f i c a n t ly d i ff e r -e n t e m i s s i o n l e v e ls c o u l d b e i d e n t i f i e d :

1. T h e l o w e st e m i s si o n r a te s ( < 2 x l 0 - S k g Nm - 2 y r - 1 ) w e r e f o u n d i n S c a n d i n a v i a a n d n o r t h e r nR u s s i a . T h i s w a s d u e t o " f o r e s t s " b e i n g t h e d o m i n a n tt y p e o f l a n d u s e a n d r e l a ti v e l y l o w t e m p e r a t u r e s p r e -v a i li n g t h r o u g h o u t t h e y e a r. F o r t h e s a m e r e a s o n s ,e m i s s io n m i n i m a w e r e e s t im a t e d f o r t h e A l p s a n d t h eP y r e n e e s .

2 . M e d i u m e m i s s i o n r a t e s (2 x 1 0 - 5 t o 1 x 1 0 - 4 k gN m - 2 y r - 1) p r e d o m i n a t e d i n E a s t e r n E u r o p e . H e r e ,s u m m e r t e m p e r a t u r e s w e r e h i g h , a n d " a r a b l e l a n d "a n d " g r a s s l a n d s " w e re t h e d o m i n a t i n g l a n d u s e c l a s se s .


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Soi l n i t r ic ox ide em iss ions 3745Table 3 . Com par ison o f the NO em iss ions f rom so il s and pyrogen ic sources fo r the year 1994 , fo r sum m er1994 (June-Augus t ) , an d fo r 5 A ugus t 1994 fo r ind iv idua l coun tr ies . S g ives the ann ua l so i l em iss ions(kt N y r- 1), p t i le annua l py rogen ic emissions (kt N yr - 1), S/P (year) their ratio (%), S /P (summer) gives thera t io (%) o f so il and pyrogen ic em iss ions dur ing sum m er (June-Augus t ) , and S /P (day) the ra t io (%) o f soi land pyrogen ic em iss ions on 5 A ugus t 1994. The sum of the coun try em iss ions is s l igh tly less than the to ta lem iss ion whic lz i s m os t ly due to the e r roneous c lass if ica t ion o f land a reas as sea in the coun try -cod einven tory .

S P S/P (year) S/P (summer) S/P (day)C o u n tr y ( k t N y r - ' ) ( k t N y r - ' ) ( % ) ( % ) ( % )Al ban ia 1.0 1.4 70.2 138.5 154.8Au stria 4.4 74.2 6.0 15.4 22.0Be lgiu m 3.5 125.2 2.8 8.2 11.3Bu lgar ia 8.2 56.9 14.4 36.6 41.4Czec h Republic: + Slova k Repu blic 8.5 243.4 3.5 9.0 12.7De nm ark 4.8 32.8 14.8 43.9 63.8Fi nla nd 2.9 58.6 5.0 11.4 15.7Fr an ce 61.6 549.2 l 1.2 31.4 40.5G erm an y 45.3 972.1 4.7 13.7 20.9Gr eec e 12.5 198.0 6.3 15.8 17.7Hu ng ary 5.4 62.6 8.7 20.4 27.3Ice lan d 1.4 1.0 135.9 191.4 209.2Ire lan d 9.8 38.4 25.5 69.4 77.6Ita ly 31.5 362.4 8.7 23.8 29.4The Neth erlan ds 8.8 120.6 7.3 22.4 32.0N orw ay 4.4 28.9 15.2 28.1 37.3Po lan d 19.7 340.7 5.8 15.4 22.3Po rtu ga l 6.9 22.6 30.3 74.4 80.6Ro m an ia 14.1 121.9 11.5 25.9 30.6Fo rm er Sovie t Un ion 120.1 825.8 14.5 34.7 44.6Spa in 35.6 151.8 23.5 57.9 64.7Swe den 6.5 88.8 7.3 17.4 24.0Sw itzer land 1.5 49.1 3.0 7.2 10.1U.K . 25.4 770.5 3.3 9.2 11.2Fo rm er Yug oslavia 17.3 148.7 11.6 27.8 34.5To tal 535.4 6437.3 8.3 21.4 27.2

T h e f e r t i l i z a t i o n o f t h e " a r a b l e l a n d " , h o w e v e r , w a sm o d e r a t e ( < 5 0 k g N h a - 1 y r - 1).3 . T h e h i g h e s t e m i s s i o n r a t e s ( 1 1 0 - 4 t o 8 1 0 - 4

k g N m - 2 y r - 1 ) w e r e l o c a t e d i n s o u t h e r n a n d w e s t e r nE u r o p e , w h e r e t h e c o n d i t i o n s w e r e g e n e r a l l y s im i l a rt o e a s t e r n E u r o p e , b u t w h e r e a h i g h n i t r o g e n i n p u t( u p t o 4 0 0 k g N h a - ~ y r - 1 ) t o a g r i c u l t u r a l a r e a s w a sr e s p o n s i b l e fo r h i g h s o il N O e m i s s i o ns . F o r t h e U . K . ,t h e t o t a l e m i s s i o n f r o m s o i l s i n 1 9 9 4 w a s 2 5 . 4 k t N O -N . T h i s is i n g o o d a g r e e m e n t w i t h a n a n n u a l e m i s s i o ne s t i m a t e o f 1 3 - 3 8 k t N O - N b y S k i b a e t a l . (1992).

T o i n v e s t i g a te t h e i m p o r t a n c e o f s o i l e m i s s i o n s r e l a -t i v e t o p y r o g e n i c e m i s s io n s , t h e f o r m e r w e r e e x p r e s s e da s t h e f r a c t io n o f t h e t o t a l p y r o g e n i c e m i s s i o n s t a k e nf r o m t h e E M E P i n v e n t o r y . F o r t h e w h o l e in v e s t i g a -t i o n a r ea , t h e a n n u a l s o i l N O e m i s s i o n s w e r e 8 .3 % o ft h e a n n u a l p y r o g e n i c N O ~ e m i s s i o n s , b u t l a r g e r e -g i o n a l d i f fe r e n c e s w e .r e o b s e r v e d t h r o u g h o u t E u r o p e( F i g . 3) , r e f l e c t i n g t h e l a r g e v a r i a b i l i t y o f b o t h , p y r o -g e n i c a n d s o i l e m i s s i o n s. F o r i n d i v i d u a l c o u n t r i e s( T a b l e 3 ) , s o i l N O e m i s s i o n s c o n t r i b u t e d b e t w e e n2 . 8 % ( B e l gi u m ) a n d 7 0 . 2 % ( A l b a n ia ) o f t h e a n n u a lp y r o g e n i c e m i s s i o n s. T h e e x t r e m e l y h i g h f r a c t i o n f o rI c e l a n d ( 1 3 6 % ) s h o u l d b e i n t e r p r e t e d w i t h c a u t i o n .N o N O f l ux m e a s u r e m e n t s e x i st f o r th e l a n d u s e st y p i c a l l y f o u n d i n I c e l a n d . A l a r g e f r a c t i o n o f I c e l a n d s

l a n d u s e w a s c l a s s i f i e d a s " o t h e r s " , t h e c l a s s w i t h t h em o s t u n c e r t a i n A f a c t o r s .I n s u m m e r ( J u n e - A u g u s t ) , s o il N O e m i s s i o n r a te s

w e r e m u c h h i g h e r t h a n t h e a n n u a l a v e r a g e e m i s s i o nr a t e s . I n t h i s p e r i o d , t h e s o i l N O e m i s s i o n s f o r t h ew h o l e i n v e s t i g a t io n a r e a a m o u n t e d t o 2 1 % o f t h ep y r o g e n i c N O x e m i s s i o n s ( T a b l e 3 ), b u t a g a i n , l a r g er e g i o n a l v a r i a t i o n s w e r e fo u n d , r a n g i n g f r o m 7 . 2 % f o rS w i t z e r l a n d t o 1 3 9 % f o r A l b a n i a .

E v e n h i g h e r t h a n t h e a v e r a g e s u m m e r t i m e s o il N Oe m i s s i o n r a t e s w e r e t h o s e e s t i m a t e d f o r a h o t s u m m e rd a y ( 5 A u g u s t 1 9 94 ) ( F i g . 4 ). T h e f r a c t i o n o f t h e s o i lN O e m i s s i o n s o f t h e p y r o g e n i c N O x e m i s s i o n s , a v e r -a g e d o v e r t h e w h o l e i n v e s t i g a t i o n a r e a, i n c r e a s e d f r o m8 . 3 % f o r t h e w h o l e y e a r a n d 2 1 % f o r s u m m e r t o 2 7 %o n 5 A u g u s t ( T a b l e 3 ). F o r s o m e c o u n t r i e s , t h e m a g n i -t u d e o f th e s o i l e m i s s i o n s w a s c o m p a r a b l e t o t h a t o ft h e p y r o g e n i c e m i s s io n s a n d i n a ll c o u n t r ie s s o i l N Oe m i s s i o n s c o n t r i b u t e d t o m o r e t h a n 1 0 % o f t h e p y r o -g e n i c N O x e m i s s io n s .

E u r o p e a n s o il N O e m i s s io n s w e r e d o m i n a t e d b ye m i s s i o n s f r o m " a r a b l e l a n d " t h a t a c c o u n t e d f o r 7 2 %o f t h e a n n u a l s o i l e m i s s i o n s a n d 8 1 % o f t h e s o i le m i s s i o n s i n s u m m e r a n d o n 5 A u g u s t ( T a b l e 4 ) .S i m i l a r f i g u r e s w e r e c a l c u l a t e d b y W i l l i a m s e t a l .( 1 9 92 a ) f o r t h e U n i t e d S t a t e s . T h u s , s o i l N O e m i s s i o n sa r e s t r o n g l y i n f l u e n c e d b y a g r i c u l t u r a l a c t i v i t i e s .


6/15

3746 A. STOHL e t a l .

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7/15

Soil nitric oxide emissions

DATA SET: EMTOT.yeor

3747

LIA

5

65"N

55ON -

45ON -

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RATIO

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SOILS/PYROGENIC

iiiii!i!i~i~i l i ~ i ! ~ : !

Fig. 3. The ratio of annual soi l NO emissions and annual pyrogenic NOx emissions. In some areas,pyrogenic NOx emissions are zero and the ratio is thus infinte. For practical reasons, however, ratios werelimited to a maximum value of 5.

0.5

0.2

0.1

0.05

0.02

2.3. D i s c u s s i o n o f t h e u n c e r t a i n t i e sThere were several sources of error that produced

a relatively large uncert ainty for the whole inventory.One error source wa.s the co mputa tion of soil temper-atures. We used the same soil temperatures--whichwe found to agree well with measurements at a grass-covered site---for all land use classes. For "forests", thecompute d temperatures were too high, especially dur-ing daytime, because forest soils are experiencinga weaker radiative fi)rcing. As the emission rates from"forests" were much smaller than those from otherlanduses, this effect was of minor importan ce for thetotal emission, but could be important in forestedregions. On the other hand, for bare soils the dailymaximum soil temperatures were probably under-estimated. This was of importance for "arable land"for certain periods of the year and for the landuseclass "others". Nevertheless, the uncertainty rangedwithin a few degrees, y ielding an uncerta inty for theemission rate of les.,; tha n 50%.

The maj or source, of error arose from the form ula-tion of the temperature dependence of the emissionsand from the preexpone ntial factor in e quat ion (1) (seealso Williams et al . , 1992a). No measurements wereavailable to derive preexponential factors for the land -

use classes "permanent crops", "urban areas" and"others". The area fractions of the first two classestogether were only 3.3% and thus their contributionto the total emission was of minor importance. Theclass "urban areas" only covered city centers wheremost of the ground is sealed and thus a very lowA factor was chosen. Recently, Tho rnt on et al . (1996)presented measurements from Nashville that supportthis choice of the A factor. Fo r "perm anent crops", wetook the A factor for "grasslands" which should givea m ini mum estimate. To define an A factor for theland use class "others" was more difficult because of itsheterogeneity. In addition, emissions from this la nd-use class had a larger impact as it contrib uted 23% ofthe total area. A large, but not explicitly specifiedfraction of class "others" are mo unt ain ous areas(rocks, ice, etc.) where hardly any emissions can beexpected. Therefore, we took a factor tha t was onlyabout half of that for grasslands.

The landuse class "forests" covered large areas andwas thus potentiall y importan t. Ou r A factors for"forests" were based o n the measuremen ts of Williamsand Fehsenfeld (1991), althoug h some authors re-ported much higher emission rates (e.g. Valente andThornton, 1993). However, as it was not clear what


8/15

3748 A. STOHL e t a l .DATA SET: EMTOT.doy

6 5 N

IL l

5 5 N -I - -I---


9/15

Soil nitric oxide emissions 3749Tab le 5. Nitro gen ferti l izer loss rates as NOx. This tab le wasorigina lly compiled b y Yienger an d Levy (1995). TheYamulki et a l . (1995) reference was add ed b y the authorsReference Perio d % lossHutchinson and Brams (1992) 9 weeks 3.2Wil l iams e t a l . (1992b) 4 weeks 2.8Valente and Th orn ton (1993) 15 weeks 2.5Shepherd et a l . (1991) 26 week s 11.0Slemr an d Seiler (1984', 4 week s 2.7Slemr and Sei ler (1991) 3 weeks 0 .0 64 ) .2 1Anderson and Levine (1987) 52 weeks 0.8Johansson e t a l . (1988) 30 h 0.5Yamulki e t a l . (1995) 52 week s 0.5

f r o m 0 . 0 6 t o 1 1 % o f th e f e r t i l iz e r n i t r o g e n a p p l i e d , b u tw i t h m o s t v a l u e s fr o m 2 t o 3 % ( T a b l e 5 ). A n n u a l l o s sr a t e s a r e l i k e l y h i g h e r t h a n t h a t . S i g n i f i c a n t l y l o w e rf i gu r e s w e re r e c e n t l y f o u n d b y Y a m u l k i e t a l . (1995)f o r a f e r t i li z e d w h e a t f i e l d i n t h e U . K . T h e y e s t i m a t e dt h a t a p p r o x i m a t e l y 0 . 9 3 % o f t h e f e r t il i z e r n i t r o g e nw a s e m i t te d a s N 2 0 a n d o n l y 0 .5 3 % w a s e m i t t e d a sN O . T h e s e r e s u lt s a t e u n u s u a l a s t h e y i n d i c a t e h i g h e rN 2 0 t h a n N O e m i s ,d o n ra t es . M o r e t y p i c a l i s a p r o -d u c t i o n r a t i o o f N O / N 2 0 o f 2 : 1 ( B o u w m a n , 1 9 90 ).O u r e s t i m a t e o f t h e f r a ct i o n o f th e a n n u a l n i t r o g e nf e r ti li z e r a p p l i c a t i o n l o s t a s N O i s l o w e r t h a n t h a t o fS h e p h e r d e t a l . b y a f a c t o r o f th r e e , b u t h i g h e r t h a nt h a t o f Y a m u l k i e t a l . b y a f a c t o r o f e i g h t. I t a g r e e sr e a s o n a b l y w e l l w i t h t h e o t h e r v a l u e s g i v e n i n T a b l e 5 .

T h e A f a c t o r s fo r ; ' a r a b l e l a n d " d e p e n d e d o n f e r ti l -i z a t i o n r a t e s a n d t h u s w e r e d i ff e r e n t f o r e a c h c o u n t r y .O u r c o u n t r y - c o d e i n v e n t o r y h a d a r a t h e r r o u g h r e s -o l u t i o n o f 7 5 k m w h i c h i n t r o d u c e d a n e r r o r t h a t w ee s t i m a t e w a s l e s s t h a n 1 0 % o f th e t o t a l e m i s s i o n , b u tm u c h h i g h e r f o r i n d i v i d u a l , e s p e c i a l l y s m a l l , c o u n -t r ies .

A f u r t h e r u n c e r t a i n t y o f th e i n v e n t o r y r e s u l t e d f ro mt h e d e p o s i t i o n o f r e d u c e d a n d o x i d i z e d n i t r o g e n f r o mt h e a t m o s p h e r e . S a n d n e s ( 19 9 3) r e p o r t e d t h a t i n 1 9 9 1t h e a n n u a l d e p o s i t i o n e x c e ed e d 1 5 k g N h a - 1 i n l a rg ep a r ts o f E u r o p e a n d 2 5 k g N h a - 1 i n w e s te r n a n dc e n t r a l E u r o p e . T h e r e, b o t h n a t u r a l a n d a g r i c u l t u r a la r e a s e x p e r i e n c e d a d d i t i o n a l n i t r o g e n i n p u t w h i c hn e a r l y e q u a l l e d t h e f e r t i l i z a t i o n r a t e s o f s o m e e a s t e r nE u r o p e a n c o u n t r i e s .

F r o m t h e d i s c u s si o n a b o v e , w e e s ti m a t e t h e u n c e r -t a i n t y o f th e w h o l e i n v e n t o r y t o b e a f a c t o r o f 3 - 4 . F o rt h e d e v e l o p m e n t o f t h e A f a c t o r s w e e x c l u d e d r e p o r t so f e x tr e m e l y h i g h N O e m i s s io n s fr o m o u r c o n s i d e r -a t i o n s , a n d w e r e d u c e d t h e A f a c t o r s o f W i l l i a m s e t a l .( 19 9 2a ) b y a p p r o x i m a t e l y 3 0 % . T h u s , o u r i n v e n t o r y i sl i k e l y t o g i v e a c o n s e r v a t i v e e s t i m a t e o f a c t u a l e m i s -s i ons .

3. EFFECTSOF SOIL NO EMISSIONSON PHOTOOXIDANTCONCENTRATIONS

W h e r e a s b i o g e n i c V O C s h a v e a l r e a d y b e e n g e n e r -a l ly r ec o g n i z e d a s g i v in g a n i m p o r t a n t c o n t r i b u t i o n t o

t o t a l V O C e m i s s i o n s , n e a r l y a l l s t u d i e s i n v e s t i g a t i n gp h o t o c h e m i c a l p o l l u t i o n i n E u r o p e n e g l e ct t h e e f fe c to f s o i l N O e m i s s io n s . W e m a d e a f i r st a t t e m p t t oa s s e s s t h e s e n s i t i v i ty o f o z o n e t o s o i l N O e m i s s i o n su s i n g a p h o t o c h e m i c a l m o d e l .3.1. M o d e l d e s c r i p t i o n

W e u s e d a o n e - d i m e n s i o n a l p h o t o c h e m i c a l m o d e lw i t h e i g h t l a y e r s a n d t e s t e d i t s s e n s i ti v i t y t o s o i l N Oe m i s s i o n s . T h e m o d e l i s a m o d i f i e d v e r s i o n o f t h eL a g r a n g i a n p h o t o c h em i c a l m o d e l P O P d e s cr i be d b yS t o h l e t a l . ( 1 9 9 5 ) a n d W o t a w a e t a l . (1996) . Here , i tw a s u s e d t o m o d e l a n o n - m o v i n g c o l u m n o f a i rc e n t e r e d o v e r E u r o p e . T h e c h e m i c a l m e c h a n i s m w eu s e d is a n u p d a t e d v e r s i o n o f t h e c a r b o n b o n d m e c h a -n i s m ( C B M - I V ) f ir s t d e s c r i b e d b y G e r y e t a l . (1989).P h o t o l y s i s r a t e s r e q u i r e d b y t h e c h e m i s t r y m o d u l ew e r e c o m p u t e d f r o m s o l a r e l e v a ti o n a n d c l o u d c o v e r ,r e a c t i o n r a t e s d e p e n d e d o n t e m p e r a t u r e a n d r e a c t a n tc o n c e n t r a t i o n s .

E x c h a n g e o f a s p e c i e s b e t w e e n t h e e i g h t l a y e r s o ft h e m o d e l w a s d e s c r i b e d b y t h e d i f f u s i o n e q u a t i o nu s i n g K - t h e o r y ( S t u ll , 1 98 8 )

- O z \ a z } ( 4 )w h e r e C i s t h e c o n c e n t r a t i o n o f t h e s p e c i e s, t i s th et i m e , a n d K ~ i s t h e e d d y d i f f u s iv i t y c o e f f ic i e n t a th e i g h t z a b o v e t h e g r o u n d . T h e e d d y - d i f f u s i v i t y c o e ff i -c i e n t s w e r e c a l c u l a t e d a s la y e r a v e r a g e s a c c o r d i n g t oB y u n a n d D e n n i s ( 1 9 9 5 ) .

D r y d e p o s i t i o n v e l o c i t i e s Vd w e r e p a r a m e t e r i z e du s i n g t h e r e s i s t a n c e m e t h o d ( W e s e l y a n d H i c k s , 1 9 77 ),w i t h t h e s u r f a c e r e s is t a n c e b e i n g c a l c u l a t e d f o r e a c hl a n d u s e c l as s s e p a r a t e l y fo l l o w i n g th e p r o c e d u r e o fW e s e l y (1 9 89 ). T h e d r y d e p o s i t i o n f l u x V d C s e r v e d a st h e l o w e r b o u n d a r y c o n d i t i o n f o r e q u a t i o n ( 4 ) a sfo l l ow s :

K ~ a c = v d c . ( 5 )O zT h e m o d e l h a d a r e l a t iv e l y h i g h r e s o l u t io n n e a r t h eg r o u n d w i t h t h e l o w e s t l a y e r e x te n d i n g u p t o 3 0 ma b o v e t h e g r o u n d . T h i s a l l o w e d a n a d e q u a t e t r e a t -m e n t o f t h e d r y d e p o s i t i o n a n d e m i s s i o n p ro c e s se s .

W e t d e p o s i t i o n w a s c a l c u l a t e d u s i n g t h e s c a v e n g i n gc o e f f ic i e n t s o f M c M a h o n a n d D e n i s o n ( 1 97 9 ). I n i t i a la n d b o u n d a r y c o n d i t i o n s f o r s e v e r a l s p e c i e s w e r et a k e n f r o m r e s u lt s o f a t w o - d i m e n s i o n a l g l o b a l m o d e lb y I s a k s e n a n d H o v (1 98 7) a n d d e p e n d e d o n l a t i t u d e ,a l t i tu d e a n d t i m e o f t h e y e a r. T h e s e b a c k g r o u n d c o n -c e n t r a t i o n s w e r e a d v e c t e d i n t o t h e m o d e l d o m a i nw i t h a c o n s t a n t s p e e d o f 5 m s - 1 , a v a l u e t y p i c a l f o rs u m m e r c o n d i t i o n s i n t h e m i d l a t i tu d e s .

T h e m o d e l d o m a i n e x t e n d ed f ro m 1 0 W t o 4 0 E a n df r o m 3 6 N t o 6 0 N . A l l m o d e l i n p u t d a t a w e r e a v e r -a g e d o v e r t h i s d o m a i n . A l t h o u g h i t w o u l d h a v e b e e ni n t e r e st i n g t o i n v e s t i g a t e a l s o m u c h s m a l l e r m o d e l i n gd o m a i n s , t h i s w a s n o t p o s s i b l e w i t h t h e s i m p l e m o d e l


10/15

3750 A. STO HL e t al .c o n c e p t . W i t h a n a d v e c t i o n s p e e d o f 5 m s - 1 , t h e r e s i -d e n c e t i m e o f t h e a i r i n t h e m o d e l i n g d o m a i n w a sa p p r o x i m a t e l y 8 d a y s . R e d u c i n g th e m o d e l d o m a i nw o u l d h a v e r e d u c e d t h i s r e s i d e n c e t i m e . W i t h s u c ha r e d u c t i o n , t h e m o d e l r e s u l t s w o u l d h a v e b e e n t o os e n s it iv e to t h e a d v e c t e d b a c k g r o u n d c o n c e n t r a t io n s .

M e t e o r o l o g i c a l i n p u t d a t a w e r e c o m p i l e d f r o mm o r e t h a n 5 0 0 s y n o p t i c a n d 1 00 r a d i o s o n d e s t a t io n si n E u r o p e . T h e s e d a t a i n c l u d e d t e m p e r a t u r e , d e wp o i n t t e m p e r a t u r e , t h e i r v e r t i c a l g r a d i e n t s b e t w e e nt h e m o d e l l a y e r s , c l o u d c o v e r , p r e c i p i t a t i o n r a t e , se n -s i b l e h e a t f lu x , f r i c t io n v e l o c i t y , c o n v e c t i v e v e l o c i t ys c a le a n d b o u n d a r y l a y e r he i gh t . T h e b o u n d a r y l a y e rp a r a m e t e r s w e r e c a l c u l a t e d w i t h t h e O M L m e t e o r o -l o g i c a l p r e p r o c e s s o r ( O l e s e n a n d B r o w n , 1 9 88 ).

A n t h r o p o g e n i c N O ~ a n d V O C e m i s s i o n s w e r et a k e n f r o m t h e E M E P e m i s s io n i n v e n t o r y f o r 1 9 91( S a n d n e s , 1 9 93 ). T h e y w e r e t e m p o r a l l y d i s a g g r e g a t e du s i n g f u n c t i o n s f o r t h e d a i l y , w e e k l y a n d y e a r l y v a r i -a t i o n s o f e m i s s io n s t r en g t h . C a r b o n m o n o x i d e ( C O )e m i s s i o n s w e r e a s s u m e d t o b e f i v e t i m e s t h e N O xe m i s s i o n s . T h e e m i s s i o n s o f l o w - a n d h i g h - l e v e l s o u r -c e s w e r e a ss u m e d t o t a k e p l a c e fr o m g r o u n d u p t o 1 00m h e i g h t a n d f r o m 1 0 0 t o 4 0 0 m h e i g h t , r e s p e c t iv e l y .B i o g e n i c i s o p r e n e e m i s s i o n s f r o m f o r e s t s w e r e c a l -c u l a t e d a s f u n c t i o n s o f r a d i a t i o n a n d t e m p e r a t u r e ,w h e r e a s t e r p e n e e m i s s io n s d e p e n d e d s o l el y o n t e m -p e r a t u r e ( G u e n t h e r e t a l . , 1993).

N O e m i s s io n s f ro m s o il s w e r e i m p l e m e n t e d a s d e -s c r i b e d i n S e c t i o n 2 , b u t w e r e s p l i t t o 9 0 % o N O a n d1 0% o N O 2 , r e s p e c t iv e l y , t o a c c o u n t f o r t h e p a r t i a lc o n v e r s i o n o f N O t o N O 2 b y r e a c t io n w i th 0 3 t h a to c c u r s i n t h e t o p s o i l. T ~ e f a c t o r o f t h i s s p l i t is h i g h l yu n c e r t a i n . W e c h e c k e d i t s ef f ec t o n t h e m o d e l e d o z o n ec o n c e n t r a t i o n b y c a l c u l a ti n g a n a d d i t i o n a l s c e n a r i ow i t h e m is s i o ns s p l it e q u a l l y b et w e e n N O a n d N O z .I n t h i s s c e n a ri o , m o d e l e d o z o n e c o n c e n t r a t io n s , f o ri n s ta n c e , w e re h i g h e r b y o n l y 0 .2 p p b a s c o m p a r e dt o t h e 9 0 % N O a n d 1 0 % N O 2 s p li t. T h u s , t h e u nc e r -t a i n t y o f t h e m o d e l r e s u l t s d u e t o t h i s s p l i t i s r a t h e rl o w . S o i l e m i s s i o n s w e r e r e l e a s e d i n t o t h e l o w e s t l a y e ro f t h e m o d e l w h i c h i s 3 0 m h i g h .

T h e d i f f e r e n ti a l e q u a t i o n s d e s c r i b i n g b o t h p h y s i c a la n d c h e m i c a l p r o c e s s e s w e r e i n t e g r a t e d t o g e t h e r w i t ha f l e x ib l e t i m e s t e p a s s u m i n g t h e q u a s i - s t e a d y - s t a t ea p p r o x i m a t i o n ( Q S S A ) ( D e L e e u w , 1 9 8 8) .3.2. M o d e l r e s u l t s

W e c o m p u t e d t h r e e N O ~ e m i s s io n s c e n a r i o s f o r th ep e r i o d M a r c h - O c t o b e r 1 99 4:

1. P y r o g e n i c N O x a n d s o i l N O e m i s s io n s2 . P y r o g e n i c N O x e m i s s io n s o n l y3 . S o i l N O e m i s s i o n s o n ly .F i g u r e 5 s h o w s t h e t i m e s e r i e s o f t h e d a i l y m e a n

c o n c e n t r a t i o n s o f s e le c t e d s p e c i e s i n t h e s e c o n d b o x( 3 0 - 1 0 0 m a b o v e t h e g r o u n d ) f o r t h e t h r e e e m i s s i o ns c e n a r i o s . T h e r e s u l t s f o r t h e l o w e s t b o x a r e n o t p r e -s e n t ed h e r e , b e c au s e W o t a w a e t a l . (1996) hav e show n

t h a t n i g h t t i m e c o n c e n t r a t i o n s i n t h a t b o x w e r e v e r ys e n s it iv e t o t h e c h o i c e o f t h e b o u n d a r y - l a y e r p a r a -m e t e r i z a ti o n . N O x c o n c e n t r a t i o n s w e r e c o n s i d e r a b l yl o w e r w h e n p y r o g e n i c N O x e m i s s io n s w e r e o m i t t e d( F i g . 5 a ) . T h i s w a s e x p e c t e d a s i t w a s e s t i m a t e d i nS e c t i o n 2. 2 t h a t e v e n o n a h o t d a y s o i l N O e m i s s i o n sa c c o u n t e d f o r le s s t h a n 3 0 % o f t h e p y r o g e n i c N O xe m i s s i o n s . N e v e r t h e l e s s , s o il e m i s s i o n s d o h a v e a c l e a re f f e c t o n s u m m e r t i m e N O x c o n c e n t r a t i o n s a s i n -d i c a t e d b y t h e r e d u c t i o n i n e s t i m a t e d m e a n d a i l y N O xc o n c e n t r a t i o n w h e n s o il N O e m i s s io n s w e r e o m i t t e d .D u e t o t h e e p i s o d i c n a t u r e o f s o il N O e m i s si o n s ,s h o r t - t e r m c o n c e n t r a t i o n f l u c tu a t i o n s w e re m u c h l a r -g e r w h e n s o i l N O e m i s s i o n s w e r e in c l u d e d .

0 3 c o n c e n t r a t i o n s w e r e v e r y s e n s i ti v e t o c h a n g e s i nN O x e m i s s i o n s ( F ig . 5b ) a s 0 3 p r o d u c t i o n o n a r e -g i o n a l t o c o n t i n e n t a l s c a l e is N O x - l i m i te d ( N a t i o n a lR e s e a r c h C o u n c i l , 1 9 9 2 ) . W i t h s o i l N O e m i s s i o n sa l o n e , 0 3 c o n c e n t r a t io n s v a r i e d b et w e e n 2 0 a n d 3 0 p p b .T h i s w a s m u c h l es s t h a n t h e i n i t i al a n d b o u n d a r yc o n d i t i o n s fo r 0 3 u s e d i n t h e m o d e l r u n s. A d v e c t e d0 3 c o n c e n t r a t i o n s v a r i e d w i t h a l t i tu d e a n d s e a s o n ,b u t a t y p i c a l v a lu e w a s 4 5 p p b ( I s a k se n a n d H o v ,1 9 87 ). T h u s , a l l o w i n g f o r s o i l N O e m i s s i o n s o n l yr e s u l te d i n a n e t 0 3 d e s t r u c t i o n a b o v e E u r o p e .

C o n t r a r y , t h e o t h e r t w o s c e n a r io s w h i c h i n c lu d e dt h e p y r o g e n i c N O x e m i s s i o n s , r e s u l t e d i n a n e t0 3 p r o d u c t i o n . I n b o t h s c e n a ri o s , m e a n 0 3 c o n c e n -t r a t i o n s f r o m J u n e t o A u g u s t w e r e a b o v e 5 0 p p b . T h em e a n 0 3 c o n c e n t r a t i o n in t h e m o d e l r u n w i t h p y r o -g e n i c a n d s o i l e m i s s i o n s w a s h i g h e r t h a n i n t h e r u nw i t h p y r o g e n i c e m i ss i o n s o n l y . T h e a v e r a g e 0 3 c o n -c e n t r a t i o n d i f f er e n ce fr o m J u n e t o A u g u s t w a s 3 .0 p p b .F o r t h e m e a n s o f t h e d a i ly m a x i m u m 0 3 c o n c e n t r a -t i o n s t h i s d i f fe r e n c e w a s 4 . 0 p p b .

T h e m a j o r s o u r c e o f n i tr i c a ci d , H N O 3 , i s t h e r e a c -t i o n o f O H w i t h N O 2 . A s b o t h N O 2 a n d O H ( se e F i g .5 e ) c o n c e n t r a t i o n s i n c r e a s e d w i t h i n c r e a s i n g N O ~e m i s s i o n s , H N O 3 c o n c e n t r a t i o n s w e r e v e r y s e n s i ti v et o c h a n g e s i n N O ~ e m i s s i o n s ( F i g . 5 c ) . S u m m e r t i m eH N O 3 c o n c e n t r a t i o n s i n th e s c e n a r io w i th b o t h p y r o -g e n i c a n d s o i l N O ~ e m i s s i o n s w e r e h i g h e r t h a n i n t h eo t h e r t w o s c e n a r io s w h i c h e x c l u d e d e i t h e r p y r o g e n i co r s o i l e m i s s i o n s . A s H N O 3 r e a c t e d v e r y s e n s it i v e t os o i l N O e m i s s i o n s , i t c a n p o s s i b l y b e u s e d a s a ni n d i c a t o r to w h a t e x t e n t s o i l N O e m i s s i o n s in f l u e n c ep h o t o c h e m i s t r y .P e r o x y a c e t y l n i t ra t e ( P A N ) i s fo r m e d b y t h e r e -a c t i o n o f t h e p e r o x y a c e t y l ra d i c a l w i t h N O 2 . I t isi n t h e r m a l e q u i l i b r i u m w i t h i t s p r e c u r s o r s a n d c a nt h e r m a l l y d e c o m p o s e , t h e r e b y r e c y c l i n g N O 2 , w h e nt e m p e r a t u r e s a r e h i g h e n o u g h . T h i s e x p l a i n s th e p r o -n o u n c e d m i n i m u m o f P A N c o n c e n t r a t i o n s in J u l ya n d A u g u s t ( F i g . 5d ). O m i t t i n g p y r o g e n i c N O x e m i s -s i o n s r e s u l t e d i n m u c h l o w e r P A N c o n c e n t r a t i o n st h a n i n t h e o t h e r t w o s c e n a r i o s . C o n t r a r y , o m i t t i n gs o il N O e m i s s io n s h a d o n l y a m i n o r e ff ec t o n P A Nc o n c e n t r a t io n s . T h u s , P A N c o n c e n t r a t i o n s a p p e a r t ob e m u c h m o r e s e n s it iv e t o c h a n g e s i n p y r o g e n i c N O xe m i s s i o n s t h a n t o c h a n g e s i n s o i l N O e m i s s i o n s .


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