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Relationship of ozone and its precursors in the West Coast Air Basin of Athens: a statistical model for the assessment of air quality in an urban area
Atmospheric Research, 28 ( 1992 ) 41-47
41
Elsevier Science Publishers B.V., Amsterdam
Relationship of ozone and its precursors in the West Coast Air Basin of Athens: a statistical model for the assessment of air quality in an urban area C. V a r o t s o s , C. C a r t a l i s , C. F e i d a s , E. G e r a s i a n d D . N . A s i m a k o p o u l o s
Laboratory of Meteorology, Department ofAppliedPhysics, UniversityofAthens, Athens, Greece (Received June 12, 1991; revised and accepted November 7, 1991 )
ABSTRACT Varotsos, C., Cartalis, C., Feidas, C., Gerasi, E. and Asimakopoulos, D.N., 1992. Relationship of ozone and its precursors in the West Coast Air Basin of Athens: a statistical model for the assessment of air quality in an urban area. Atmos. Res., 28: 41-47. Surface measurements of ozone are correlated to measurements of its precursors (NOx and hydrocarbons) for the period 1986-1989, for the West Coast Air Basin of Athens. We first find that the concentrations of total hydrocarbons (THC) and non-methane hydrocarbons (NMHC) are linearly correlated. We then attempt to express the relationship of ozone to its precursors via polynomial and multiplicative regression models, to find that in all cases ozone variation is explained by the mixing ratios of NOx and non-methane hydrocarbons in conjunction with a meteorological parameter, namely the temperature at the pressure level of 850 hPa. Depending on the regression model, the correlation coefficient increases from 0.73 to 0.98. The satisfactory performance of the regression models support their application for both the assessment of air quality in urban areas and the formulation of localized environmental protection policies. RI~SUMI~ On 6tablit des corr61ations entre les mesures de l'ozone et celles de ses pr6curseurs (NOx et hydrocarbones) sur la pattie occidentale c6ti6re du bassin d'Ath6nes pour la p6riode 1986-1989. On trouve d'abord que les concentrations en hydrocarbone total et en hydrocarbone non m6than6 sont lin6airement corr616s. On exprime ensuite la relation de l'ozone ~ ses pr6curseurs par des mod61es de r6gression polynomiaux et multiplicatifs, pour trouver que darts tous les cas les variations de I'ozone s'expliquent par les taux de m61ange de NOx et d'hydrocarbone non m6than6 en relation avec un param6tre m6t6orologique, la temp6rature au niveau 850 hPa. Scion le mod61e de r6gression utilis6, le coefficient de corr61ation varie de 0,73 ~ 0,98. La bonne performance des mod61es de r6gression permet leur application aussi bien/l la d6termination de la qualit6 de rair dans les zones urbaines qu'/l la formulation de r6gles de protection de l'environnement. Correspondence to: C. Cartalis, 33 Ippokratous Str., Laboratory of Meteorology, Athens 106 80, Greece.
0169-8095/92/$05.00
© 1992 Elsevier Science Publishers B.V. All rights reserved.
42
c. VAROTSOS ET AL.
INTRODUCTION
It is considered that enhanced photochemical production due to increased emissions of NOx and HC is largely responsible for the increase in surface ozone concentrations in both rural and urban areas (Fehsenfeld et al., 1983; Bojkov, 1986; Crutzen, 1988; Varinou et al., 1988 ). Several researchers have attempted to express the relationship of ozone to its precursors (NOx and HC) via statistical or empirical models (Tiao et al., 1976; Capovaletti et al., 1977; EPA, 1986; Paskind and Kinosian, 1974; Kuntasal and Chang, 1987 ), as a well constructed model may compensate for the analytical description of the complex photochemical processes which control the emissions and the fate of chemical compounds within the lower atmosphere and may also allow the easier treatment of air quality studies. One noteworthy weakness of such models is that since photochemical oxidant formation involves complex spatial and temporal characteristics of emissions, meteorological variables and photochemical reactions, it is difficult to isolate realistic databases for the statistical O3-HC-NOx relationship. In addition in order to circumvent possible complications in the statistical approach and to ensure the best possible performance of the constructed model, several key factors should be observed. Meteorology is one of the key factors as it is well documented that ozone formation in an urban area depends strongly on several meteorological variables, such as the 850-hPa temperature, the inversion base height and the wind speed (Chock et al., 1982; Kumar and Chock, 1984; Finlayson-Pitts and Pitts, 1986). However, fluctuations in both the inversion height and wind speed appear complex and uncertain, which implies that the most efficient way to treat meteorology is to incorporate the 850-hPa temperature in the functional relationship expressing ozone levels. This is because the 850-hPa temperature is an indicator of air mass stability, which implies that it determines the potential of the lower troposphere to support the air circulation within a polluted urban area. It yields that in the absence of such air mass circulation, the ozone-producing potential increases substantially. A second key factor is the variation of solar ultraviolet radiation at the surface and the subsequent variation of surface ozone. To minimize the variation of ultraviolet radiation it is imperative to base the construction of the model on measurements taken in summer months and for clear skies. Finally wind characteristics (direction and speed) influence ozone levels in a substantial way. This problem is circumvented by excluding measurements for wind speed exceeding a specific value. In the present work monthly averages of daily maximum hour mixing ratios were used for regression analyses, in order to examine the broad features of basin-wide O3-HC-NOx relationships based on the monthly variation of ozone with respect to its precursors. A simple statistical analysis was per-
RELATIONSHIP OF OZONE AND ITS PRECURSORS IN THE WEST COAST AIR BASIN OF ATHENS
43
formed so that to correlate the abundance of total hydrocarbons to non methane hydrocarbons, and the performance of several functional relationships was examined to recognize the correlation of ozone to its precursors and to a meteorological parameter, namely the temperature at the 850 hPa level. DISCUSSION AND RESULTS
In this study monthly averages of daily maximum-hour mixing ratios of surface ozone, NOx, HC (total and non-methane) and of the 850-hPa temperature for the period 1986 to 1989 - - for the months of May, June, July, August and September - - were used to examine the relationship of ozone and its precursors in the West Coast Air Basin (WCA) of Athens which is located 16 km to the west of the centre of the city of Athens. Monthly averages were estimated from the daily observed levels excluding measurements taken at cloudy days or in the event that the prevailing winds exceeded speeds of 4 m/s. Cloudy days were excluded in order to minimize the variation of ultraviolet radiation, the latter strongly influencing the ozone levels. It should be also mentioned that the variation of ultraviolet radiation was further reduced by considering only months of high ozone levels. In addition data were complemented with the air temperature at the 850 hPa level, as obtained from the twice-daily radiosoundings performed by the National Meteorological Service. It should be noted that no measurements were available for May and August, 1987 and August and September 1988 due to instrumental malfunction.
Relationship of total to non-methane hydrocarbons Firstly measurements of total hydrocarbons and non-methane hydrocarbons in WCA were examined to yield a significant correlation expressed via a linear relationship: (NMHC) =0.6[ (TH¢)--235]
( 1)
The correlation coefficient of the regression is 0.629, with this correlation being significant at 0.05 level. In eqn. 1, THE and NMnC are mixing ratios in parts per hundred million of carbon (pphmC). Trijonis ( 1983 ) found that the relationship between THC and NMHC for the area of the South Coast air basin of California (SCC) is linear and is given by: (NMHC)
=0.7[
(THC)
--
150]
(2)
where the mixing ratios of NMHC, and THE are in pphmC (parts per hundred million carbon). Equations 1 and 2 show a remarkable similarity. The slopes of the equa-
44
C. VAROTSOSET AL.
tions for WCA and SCC are 0.6 and 0.7 respectively, implying a similarity of the photochemical mechanisms which drive the mixing ratio of hydrocarbons in the lower atmosphere of the two regions. From the formulation of the two equations, it is clearly seen that they represent roughly parallel lines, with the line corresponding to the WCA measurements having larger value of intersect with the y-axis. This difference in the intersect value may be attributed to a higher level of background pollution in WCA compared to that in SCC.
Relationship of ozone and its precursors The relationship between ozone and its precursors was firstly examined by applying two models (a polynomial and a multiplicative), as developed by Kuntasal and Chang ( 1987 ). The polynomial model A is: [03] =a+bT(850) +c[NOx] +d[NMHC] where T(850) is the 850-hPa temperature in degrees Celcius, [NOx] and [NMH¢] are the mixing ratios of ( N O + N O 2 ) and non-methane hydrocarbons in pphm, and a, b, c and d coefficients that result from the regression analysis (the arithmetic values of the coefficients are given in Table 1 ). Whereas the multiplicative model B is: [03]
=eaT( 850 )b[NOx]C[NMHC ]d
Next, a modification of these models was attempted, to yield a new model (model C) which is a combination of both multiplicative and polynomial terms: [03]
=eaT( 850 )b[NOx]C[NMHC ]d+f T( 850 )
The performance of all models was examined to show that the correlation coefficient r increases from 0.73 for Model A to 0.89 for Model B, and from TABLEI Regression results for models A, B, and C ModeP
A:[O3]=a+bT+c[NOx]+d[NMriC] B: [03] =eaTb[NOx]C[NMHC]a C:[O3] = eaTb[NOx]C[NMHC ] a + f T
Coefficient a
b
c
d
.f
rz
-6.9108 -- 3.543 3.900
1.401 3.036 2.866
0.444 0.564 2.627
-0.3239 -- 1.110 -4.820
0.286
0.73 0.89 0.98
' [ 03 ], [ NOx ], [ NMHC ] are the mixing ratios o f ozone, nitrogen oxides and n o n - m e t h a n e hydrocarbons in p p h m , and T i s the temperature at 850 hPa in degrees Celcius. r2 is the correlation coefficient (significant at the 0.05 level).
45
RELATIONSHIP OF OZONE AND ITS PRECURSORS IN THE WEST COAST AIR BASIN OF ATHENS
0.89 for Model B to 0.98 for Model C. Table 1 provides the regression results and the correlation coefficients for all three models. The performance of model A is presented in Fig. 1. Measured and calculated ozone levels are given for five months per year (May to September). The average absolute difference between the measured and calculated values is 43.8%, which implies that model A can not reproduce efficiently the measured ozone values. As seen in Fig. 2 the overall performance of model B has improved com-
30 --+-- 0 3 (rheas,) 03 25 ....~.... 03 (ca[c.) pphm 2O /,//"
I
I
I
/~"'---~"
"''J"
I
I
1
i
I
I
I
I
I
I
1
5/866/867/868/869/866/877/879/875/886/887/885/896/897/898/899/89
Month/ Year
Fig. 1. Measured and calculated values (model A) for monthly ozone levels for the coastal city Elefsina. Five points per year are presented (May to September) for the period 1986 to 1989.
30 --+-- 03 (meas.) 25 " " ~ " 03(taLc')
A /"~",I
,",
15
"'~!/ ~I//' il
10
OI
1
I
I
I
I
I
I
I
I
I
I
I
I
I
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Month / Year
Fig. 2. Same as in Fig. 1 but for model B.
46
c. VAROTSOS ET AL.
3o •
.
.
.
.
.
.
.
.
.
J __+__ 03(meas.)
oL__, _~
............................
J
5/866/867/868/869/866/877/879/875/886/887/885/896/897/898/899/89 M o n t h / Yeor
Fig. 3. S a m e as in Fig. 1 b u t for m o d e l C.
pared to the performance of model A. The correlation coefficient has increased to 0.89, and the model has the capability to follow the variation of ozone depending on the 850-hPa temperature, and the mixing ratios of NOx and NMHC even in the event of severe air pollution episodes (6/88 and 7/ 88 ). The average absolute difference between the measured and calculated ozone mixing ratios is now 32.3%. In Fig. 3 the improved excellent performance of model C is clearly demonstrated. The correlation coefficient is 0.98 and the average absolute difference between the measured and calculated ozone mixing ratios has dropped to 13.8%. The improved performance of model C is explained largely by the new term added to model B which depends on the 850-hPa temperature. Therefore it may be concluded that the monthly ozone variation in WCA depends strongly on meteorology. The reasoning for the strong dependency of the performance of the described models on the temperature at 850 hPa may be related to the fact that the 850-hPa temperature is a strong indicator of air mass stability as it determines the potential of the atmospheric layer that extends from the ground to the altitude of 850 hPa to support air mass circulation, Such a potential is very important for polluted areas. CONCLUSIONS
The findings of this study demonstrate the capability to use regression models in order to simulate the monthly variation of surface ozone in relation to its precursors within an urban polluted area. Such a perspective is of great importance as it allows to overcome the complex - - temporally and spatially dependent - - photochemical processes which are responsible for ozone for-
RELATIONSHIP OF OZONE AND ITS PRECURSORS IN THE WEST COAST AIR BASIN OF ATHENS
47
m a t i o n a n d v a r i a t i o n a n d w h i c h are usually difficult to t r e a t analytically. A critical q u e s t i o n relates t o the a p p l i c a b i l i t y o f the m o d e l in o t h e r areas. Bearing in m i n d the close a g r e e m e n t in the p e r f o r m a n c e o f m o d e l s A a n d B a n d o f Eqs. 1 a n d 2 for the areas o f W C A a n d SCC, it m a y be c o n c l u d e d t h a t m o d e l C w o u l d be valid for coastal areas t h a t r e s e m b l e the characteristics o f W C A a n d SCC, i.e. coastal areas w i t h w a r m c l i m a t e a n d high s u n s h i n e d u r a t i o n , a n d air p o l l u t i o n o f t h e p h o t o c h e m i c a l type. F i n a l l y the excellent p e r f o r m a n c e o f the d e v e l o p e d m o d e l s s u p p o r t s its app l i c a t i o n for t h e a s s e s s m e n t o f air q u a l i t y in the u r b a n area o f the West C o a s t Air Basin o f Athens, a n d suggests its use as a d i a g n o s t i c t o o l for the f o r m u l a t i o n o f the strategy for e n v i r o n m e n t a l p r o t e c t i o n w h i c h is m o s t a p p r o p r i a t e for the local c o n d i t i o n s o f the area.
REFERENCES Bojkov, R.D., 1986. Ozone changes at the surface and in the free troposphere. In: Tropospheric Ozone. Reidel, Hingham, MS, pp. 83-96. Capovaletti, J.M., Myrabo, L.N., Schleifer, P., Stanonik, A. and Wilson, K.R., 1977. Statistical oxidant air quality prediction model for land use and transportation planning. Atmos. Environ., I l: 449-458. Chock, D.P., Kumar, S. and Herrmann, R., 1982. An analysis of trends in oxidant air quality in the South Coast Air Basin of California. Atmos. Environ., 16:2615-2624. Crutzen, P.J., 1988. Tropospheric ozone: A review. In: Tropospheric Ozone. Reidel, Hingham, MS, pp. 3-32. Environmental Protection Agency (EPA), 1986. Air quality criteria for ozone and other photochemical oxidants, Vol. II, EPA/600/8/8-84/020bF. Fehsenfeld, F.C., Bollinger, M.J., Liu, S.C., Parrish, D.D., McFarland, M., Trainer, M., Kley, D., Murphy, P.C., Albritton, D.L. and Lenschow, D.H., 1983. A study of ozone in the Colorado Mountains. J. Atmos. Chem., 1: 87-93. Finlayson-Pitts, B.J. and Pitts, J.N., 1986. Atmospheric Chemistry: Fundamental and Experimental Techniques. Wiley, New York, 1098 pp. Kumar, S. and Chock, D.P., 1984. An update on oxidant trends in the South Coast Air Basin of California. Atmos. Environ., 18: 2131-2134. Kuntasal, G. and Chang, T.Y., 1987. Trends and Relationships ofO, NOx and HC in the South Coast Air Basin of California. J. Air Pollut. Control Assoc., 37:1158-1163. Paskind, J. and Kinosian, J.R., 1974. Hydrocarbons, Oxides of Nitrogen and Oxidant Pollutant Relationships in the Atmosphere over California Cities. APCA, Pittsburg, PA. Tiao, G.C., Phadke, M.S. and Box, G.E.P., 1976. Some empirical models for the Los Angeles photochemical smog data. J. Air Pollut. Control Assoc., 26: 485-490. Trijonis, J., 1983. Analysis of historical ozone trends in Los Angeles sorted out by the NMHC/ NOx ratio. Final Rep. Calif. Air Sources Board, Contract No. AI-056-32, Sacramento, CA. Varinou, M., Loizidou, M. and Varotsos, C., 1988. Study of the correlation of surface ozone in urban areas to the ratio [NOx]/[THC]. In: Karapiperis. Volume pp. 49-56. (edited by Univ. Athens).
41
Elsevier Science Publishers B.V., Amsterdam
Relationship of ozone and its precursors in the West Coast Air Basin of Athens: a statistical model for the assessment of air quality in an urban area C. V a r o t s o s , C. C a r t a l i s , C. F e i d a s , E. G e r a s i a n d D . N . A s i m a k o p o u l o s
Laboratory of Meteorology, Department ofAppliedPhysics, UniversityofAthens, Athens, Greece (Received June 12, 1991; revised and accepted November 7, 1991 )
ABSTRACT Varotsos, C., Cartalis, C., Feidas, C., Gerasi, E. and Asimakopoulos, D.N., 1992. Relationship of ozone and its precursors in the West Coast Air Basin of Athens: a statistical model for the assessment of air quality in an urban area. Atmos. Res., 28: 41-47. Surface measurements of ozone are correlated to measurements of its precursors (NOx and hydrocarbons) for the period 1986-1989, for the West Coast Air Basin of Athens. We first find that the concentrations of total hydrocarbons (THC) and non-methane hydrocarbons (NMHC) are linearly correlated. We then attempt to express the relationship of ozone to its precursors via polynomial and multiplicative regression models, to find that in all cases ozone variation is explained by the mixing ratios of NOx and non-methane hydrocarbons in conjunction with a meteorological parameter, namely the temperature at the pressure level of 850 hPa. Depending on the regression model, the correlation coefficient increases from 0.73 to 0.98. The satisfactory performance of the regression models support their application for both the assessment of air quality in urban areas and the formulation of localized environmental protection policies. RI~SUMI~ On 6tablit des corr61ations entre les mesures de l'ozone et celles de ses pr6curseurs (NOx et hydrocarbones) sur la pattie occidentale c6ti6re du bassin d'Ath6nes pour la p6riode 1986-1989. On trouve d'abord que les concentrations en hydrocarbone total et en hydrocarbone non m6than6 sont lin6airement corr616s. On exprime ensuite la relation de l'ozone ~ ses pr6curseurs par des mod61es de r6gression polynomiaux et multiplicatifs, pour trouver que darts tous les cas les variations de I'ozone s'expliquent par les taux de m61ange de NOx et d'hydrocarbone non m6than6 en relation avec un param6tre m6t6orologique, la temp6rature au niveau 850 hPa. Scion le mod61e de r6gression utilis6, le coefficient de corr61ation varie de 0,73 ~ 0,98. La bonne performance des mod61es de r6gression permet leur application aussi bien/l la d6termination de la qualit6 de rair dans les zones urbaines qu'/l la formulation de r6gles de protection de l'environnement. Correspondence to: C. Cartalis, 33 Ippokratous Str., Laboratory of Meteorology, Athens 106 80, Greece.
0169-8095/92/$05.00
© 1992 Elsevier Science Publishers B.V. All rights reserved.
42
c. VAROTSOS ET AL.
INTRODUCTION
It is considered that enhanced photochemical production due to increased emissions of NOx and HC is largely responsible for the increase in surface ozone concentrations in both rural and urban areas (Fehsenfeld et al., 1983; Bojkov, 1986; Crutzen, 1988; Varinou et al., 1988 ). Several researchers have attempted to express the relationship of ozone to its precursors (NOx and HC) via statistical or empirical models (Tiao et al., 1976; Capovaletti et al., 1977; EPA, 1986; Paskind and Kinosian, 1974; Kuntasal and Chang, 1987 ), as a well constructed model may compensate for the analytical description of the complex photochemical processes which control the emissions and the fate of chemical compounds within the lower atmosphere and may also allow the easier treatment of air quality studies. One noteworthy weakness of such models is that since photochemical oxidant formation involves complex spatial and temporal characteristics of emissions, meteorological variables and photochemical reactions, it is difficult to isolate realistic databases for the statistical O3-HC-NOx relationship. In addition in order to circumvent possible complications in the statistical approach and to ensure the best possible performance of the constructed model, several key factors should be observed. Meteorology is one of the key factors as it is well documented that ozone formation in an urban area depends strongly on several meteorological variables, such as the 850-hPa temperature, the inversion base height and the wind speed (Chock et al., 1982; Kumar and Chock, 1984; Finlayson-Pitts and Pitts, 1986). However, fluctuations in both the inversion height and wind speed appear complex and uncertain, which implies that the most efficient way to treat meteorology is to incorporate the 850-hPa temperature in the functional relationship expressing ozone levels. This is because the 850-hPa temperature is an indicator of air mass stability, which implies that it determines the potential of the lower troposphere to support the air circulation within a polluted urban area. It yields that in the absence of such air mass circulation, the ozone-producing potential increases substantially. A second key factor is the variation of solar ultraviolet radiation at the surface and the subsequent variation of surface ozone. To minimize the variation of ultraviolet radiation it is imperative to base the construction of the model on measurements taken in summer months and for clear skies. Finally wind characteristics (direction and speed) influence ozone levels in a substantial way. This problem is circumvented by excluding measurements for wind speed exceeding a specific value. In the present work monthly averages of daily maximum hour mixing ratios were used for regression analyses, in order to examine the broad features of basin-wide O3-HC-NOx relationships based on the monthly variation of ozone with respect to its precursors. A simple statistical analysis was per-
RELATIONSHIP OF OZONE AND ITS PRECURSORS IN THE WEST COAST AIR BASIN OF ATHENS
43
formed so that to correlate the abundance of total hydrocarbons to non methane hydrocarbons, and the performance of several functional relationships was examined to recognize the correlation of ozone to its precursors and to a meteorological parameter, namely the temperature at the 850 hPa level. DISCUSSION AND RESULTS
In this study monthly averages of daily maximum-hour mixing ratios of surface ozone, NOx, HC (total and non-methane) and of the 850-hPa temperature for the period 1986 to 1989 - - for the months of May, June, July, August and September - - were used to examine the relationship of ozone and its precursors in the West Coast Air Basin (WCA) of Athens which is located 16 km to the west of the centre of the city of Athens. Monthly averages were estimated from the daily observed levels excluding measurements taken at cloudy days or in the event that the prevailing winds exceeded speeds of 4 m/s. Cloudy days were excluded in order to minimize the variation of ultraviolet radiation, the latter strongly influencing the ozone levels. It should be also mentioned that the variation of ultraviolet radiation was further reduced by considering only months of high ozone levels. In addition data were complemented with the air temperature at the 850 hPa level, as obtained from the twice-daily radiosoundings performed by the National Meteorological Service. It should be noted that no measurements were available for May and August, 1987 and August and September 1988 due to instrumental malfunction.
Relationship of total to non-methane hydrocarbons Firstly measurements of total hydrocarbons and non-methane hydrocarbons in WCA were examined to yield a significant correlation expressed via a linear relationship: (NMHC) =0.6[ (TH¢)--235]
( 1)
The correlation coefficient of the regression is 0.629, with this correlation being significant at 0.05 level. In eqn. 1, THE and NMnC are mixing ratios in parts per hundred million of carbon (pphmC). Trijonis ( 1983 ) found that the relationship between THC and NMHC for the area of the South Coast air basin of California (SCC) is linear and is given by: (NMHC)
=0.7[
(THC)
--
150]
(2)
where the mixing ratios of NMHC, and THE are in pphmC (parts per hundred million carbon). Equations 1 and 2 show a remarkable similarity. The slopes of the equa-
44
C. VAROTSOSET AL.
tions for WCA and SCC are 0.6 and 0.7 respectively, implying a similarity of the photochemical mechanisms which drive the mixing ratio of hydrocarbons in the lower atmosphere of the two regions. From the formulation of the two equations, it is clearly seen that they represent roughly parallel lines, with the line corresponding to the WCA measurements having larger value of intersect with the y-axis. This difference in the intersect value may be attributed to a higher level of background pollution in WCA compared to that in SCC.
Relationship of ozone and its precursors The relationship between ozone and its precursors was firstly examined by applying two models (a polynomial and a multiplicative), as developed by Kuntasal and Chang ( 1987 ). The polynomial model A is: [03] =a+bT(850) +c[NOx] +d[NMHC] where T(850) is the 850-hPa temperature in degrees Celcius, [NOx] and [NMH¢] are the mixing ratios of ( N O + N O 2 ) and non-methane hydrocarbons in pphm, and a, b, c and d coefficients that result from the regression analysis (the arithmetic values of the coefficients are given in Table 1 ). Whereas the multiplicative model B is: [03]
=eaT( 850 )b[NOx]C[NMHC ]d
Next, a modification of these models was attempted, to yield a new model (model C) which is a combination of both multiplicative and polynomial terms: [03]
=eaT( 850 )b[NOx]C[NMHC ]d+f T( 850 )
The performance of all models was examined to show that the correlation coefficient r increases from 0.73 for Model A to 0.89 for Model B, and from TABLEI Regression results for models A, B, and C ModeP
A:[O3]=a+bT+c[NOx]+d[NMriC] B: [03] =eaTb[NOx]C[NMHC]a C:[O3] = eaTb[NOx]C[NMHC ] a + f T
Coefficient a
b
c
d
.f
rz
-6.9108 -- 3.543 3.900
1.401 3.036 2.866
0.444 0.564 2.627
-0.3239 -- 1.110 -4.820
0.286
0.73 0.89 0.98
' [ 03 ], [ NOx ], [ NMHC ] are the mixing ratios o f ozone, nitrogen oxides and n o n - m e t h a n e hydrocarbons in p p h m , and T i s the temperature at 850 hPa in degrees Celcius. r2 is the correlation coefficient (significant at the 0.05 level).
45
RELATIONSHIP OF OZONE AND ITS PRECURSORS IN THE WEST COAST AIR BASIN OF ATHENS
0.89 for Model B to 0.98 for Model C. Table 1 provides the regression results and the correlation coefficients for all three models. The performance of model A is presented in Fig. 1. Measured and calculated ozone levels are given for five months per year (May to September). The average absolute difference between the measured and calculated values is 43.8%, which implies that model A can not reproduce efficiently the measured ozone values. As seen in Fig. 2 the overall performance of model B has improved com-
30 --+-- 0 3 (rheas,) 03 25 ....~.... 03 (ca[c.) pphm 2O /,//"
I
I
I
/~"'---~"
"''J"
I
I
1
i
I
I
I
I
I
I
1
5/866/867/868/869/866/877/879/875/886/887/885/896/897/898/899/89
Month/ Year
Fig. 1. Measured and calculated values (model A) for monthly ozone levels for the coastal city Elefsina. Five points per year are presented (May to September) for the period 1986 to 1989.
30 --+-- 03 (meas.) 25 " " ~ " 03(taLc')
A /"~",I
,",
15
"'~!/ ~I//' il
10
OI
1
I
I
I
I
I
I
I
I
I
I
I
I
I
5/866/867/868/869/866/877/879/875/886/887/885/896/897/898/899/89
Month / Year
Fig. 2. Same as in Fig. 1 but for model B.
46
c. VAROTSOS ET AL.
3o •
.
.
.
.
.
.
.
.
.
J __+__ 03(meas.)
oL__, _~
............................
J
5/866/867/868/869/866/877/879/875/886/887/885/896/897/898/899/89 M o n t h / Yeor
Fig. 3. S a m e as in Fig. 1 b u t for m o d e l C.
pared to the performance of model A. The correlation coefficient has increased to 0.89, and the model has the capability to follow the variation of ozone depending on the 850-hPa temperature, and the mixing ratios of NOx and NMHC even in the event of severe air pollution episodes (6/88 and 7/ 88 ). The average absolute difference between the measured and calculated ozone mixing ratios is now 32.3%. In Fig. 3 the improved excellent performance of model C is clearly demonstrated. The correlation coefficient is 0.98 and the average absolute difference between the measured and calculated ozone mixing ratios has dropped to 13.8%. The improved performance of model C is explained largely by the new term added to model B which depends on the 850-hPa temperature. Therefore it may be concluded that the monthly ozone variation in WCA depends strongly on meteorology. The reasoning for the strong dependency of the performance of the described models on the temperature at 850 hPa may be related to the fact that the 850-hPa temperature is a strong indicator of air mass stability as it determines the potential of the atmospheric layer that extends from the ground to the altitude of 850 hPa to support air mass circulation, Such a potential is very important for polluted areas. CONCLUSIONS
The findings of this study demonstrate the capability to use regression models in order to simulate the monthly variation of surface ozone in relation to its precursors within an urban polluted area. Such a perspective is of great importance as it allows to overcome the complex - - temporally and spatially dependent - - photochemical processes which are responsible for ozone for-
RELATIONSHIP OF OZONE AND ITS PRECURSORS IN THE WEST COAST AIR BASIN OF ATHENS
47
m a t i o n a n d v a r i a t i o n a n d w h i c h are usually difficult to t r e a t analytically. A critical q u e s t i o n relates t o the a p p l i c a b i l i t y o f the m o d e l in o t h e r areas. Bearing in m i n d the close a g r e e m e n t in the p e r f o r m a n c e o f m o d e l s A a n d B a n d o f Eqs. 1 a n d 2 for the areas o f W C A a n d SCC, it m a y be c o n c l u d e d t h a t m o d e l C w o u l d be valid for coastal areas t h a t r e s e m b l e the characteristics o f W C A a n d SCC, i.e. coastal areas w i t h w a r m c l i m a t e a n d high s u n s h i n e d u r a t i o n , a n d air p o l l u t i o n o f t h e p h o t o c h e m i c a l type. F i n a l l y the excellent p e r f o r m a n c e o f the d e v e l o p e d m o d e l s s u p p o r t s its app l i c a t i o n for t h e a s s e s s m e n t o f air q u a l i t y in the u r b a n area o f the West C o a s t Air Basin o f Athens, a n d suggests its use as a d i a g n o s t i c t o o l for the f o r m u l a t i o n o f the strategy for e n v i r o n m e n t a l p r o t e c t i o n w h i c h is m o s t a p p r o p r i a t e for the local c o n d i t i o n s o f the area.
REFERENCES Bojkov, R.D., 1986. Ozone changes at the surface and in the free troposphere. In: Tropospheric Ozone. Reidel, Hingham, MS, pp. 83-96. Capovaletti, J.M., Myrabo, L.N., Schleifer, P., Stanonik, A. and Wilson, K.R., 1977. Statistical oxidant air quality prediction model for land use and transportation planning. Atmos. Environ., I l: 449-458. Chock, D.P., Kumar, S. and Herrmann, R., 1982. An analysis of trends in oxidant air quality in the South Coast Air Basin of California. Atmos. Environ., 16:2615-2624. Crutzen, P.J., 1988. Tropospheric ozone: A review. In: Tropospheric Ozone. Reidel, Hingham, MS, pp. 3-32. Environmental Protection Agency (EPA), 1986. Air quality criteria for ozone and other photochemical oxidants, Vol. II, EPA/600/8/8-84/020bF. Fehsenfeld, F.C., Bollinger, M.J., Liu, S.C., Parrish, D.D., McFarland, M., Trainer, M., Kley, D., Murphy, P.C., Albritton, D.L. and Lenschow, D.H., 1983. A study of ozone in the Colorado Mountains. J. Atmos. Chem., 1: 87-93. Finlayson-Pitts, B.J. and Pitts, J.N., 1986. Atmospheric Chemistry: Fundamental and Experimental Techniques. Wiley, New York, 1098 pp. Kumar, S. and Chock, D.P., 1984. An update on oxidant trends in the South Coast Air Basin of California. Atmos. Environ., 18: 2131-2134. Kuntasal, G. and Chang, T.Y., 1987. Trends and Relationships ofO, NOx and HC in the South Coast Air Basin of California. J. Air Pollut. Control Assoc., 37:1158-1163. Paskind, J. and Kinosian, J.R., 1974. Hydrocarbons, Oxides of Nitrogen and Oxidant Pollutant Relationships in the Atmosphere over California Cities. APCA, Pittsburg, PA. Tiao, G.C., Phadke, M.S. and Box, G.E.P., 1976. Some empirical models for the Los Angeles photochemical smog data. J. Air Pollut. Control Assoc., 26: 485-490. Trijonis, J., 1983. Analysis of historical ozone trends in Los Angeles sorted out by the NMHC/ NOx ratio. Final Rep. Calif. Air Sources Board, Contract No. AI-056-32, Sacramento, CA. Varinou, M., Loizidou, M. and Varotsos, C., 1988. Study of the correlation of surface ozone in urban areas to the ratio [NOx]/[THC]. In: Karapiperis. Volume pp. 49-56. (edited by Univ. Athens).