~rmarplvric EnviroMvnt Vol. I& No. IO. pp. 2131-2134, 1984 ooo44981/84 13.00 + 0.00

Rimed in Great Britain. 0 1984 pcr@lrmn Press Ltd.

AN UPDATE ON OXIDANT TRENDS IN THE SOUTH COAST AIR BASIN OF CALIFORNIA

SUDARSHAN KUMAR and DAVID P. CHOCK

Environmental Science Department, General Motors Research Laboratories, Warren, MI 48090, U.S.A.

(First receitied 6 December 1983 and received for publication 1 March 1984)

Abstract-The trends in oxidant air quality in the South Coast Air Basin of California are of great interest because of the high oxidant levels in the region. In this work, we have updated the trends for daily peak oxidant levels in the entire Basin and at two sites, Azusa and San Bernardino. Since the daily oxidant levels are highly dependent on the local meteorology, we have determined trends under fixed meteorological conditions. We previously applied this approach to data from the 1971 to 1979 period. In the present work, we found that, for the 1971-1981 period, the oxidant trends basinwide and in Azusa are downward while the trend in San Bernardino is flat. An additional finding of this study is that in recent years the basinwide daily peak oxidant concentration has been occurring in San Bernardino with increasing frequency. The overall decrease in and eastward movement of the basinwide maxima are discussed in relation to the trends in the emissions of reactive hydrocarbons and nitrogen oxides over the time period as well as the predictions of smog chamber experiments.

Key word index: Oxidant trends, ozone trends, southern California.

INTRODUCTION

Ozone trend analysis has frequently been used to assess the effectiveness of past emission control measures in lowering O3 levels in or near an urban area. A major difficulty encountered in such an analysis is the interference of meteorology. One must first remove the influence of meteorology to determine the underlying trend in O3 due to changes in primary pollutant emissions. In most O3 trend studies, a model relating O3 concentrations to meteorology is first constructed from one or more years of data base. The influence of meteorology in the O3 trend is then assumed to have been removed by normalizing the observed O3 concentrations by predictions from the model for identical meteorology. However, such ag proaches do not adequately remove the influence of meteorology since the models constructed are de- pendent on the meteorology of the base years. A conceptually simple approach in O3 trend analysis was proposed by Chock, Kumar and Herrmann (1982, referred to as CKH henceforth). In this approach, the O3 (or oxidant) trend is determined under fixed meteorological conditions. This approach was pre- viously applied to oxidant data from the South Coast Air Basin of California; the period covered was the high oxidant season (June-October) from 1971 to 1979. In that study, the trends for the daily peak oxidant concentrations were found to be essentially flat for the basinwide maximum and for two specific sites, Azusa and San Bernardino. With the availability of more recent data, the oxidant trends basinwide and for Azusa and San Bernardino are updated through 1981 in the present report.

DATA DRSCRIFTION

The 0, data for 1980 and 1981 were acquired from the California Air Resources Board. Since a bulk of the air quality data was available as total oxidant and not OS, we converted the 0, concentrations to oxidant concentrations by using the regression equation

Oxidant = 0.8702 OS + 0.4831 R = 0.917,

where oxidant and 0s concentrations are in parts per hundred million (pphm) and R is the correlation coefficient. This quation is based on a large data base obtained in a side- by-side comparative study between oxidant concentrations by the KI method and 0s concentrations by the U.V. method (CARB, 1978). The maxima of the converted daily peak hourly oxidant concentrations from 11 monitoring sites are referred to as the basinwide daily maximum oxidant concen- trations (MAXOX). The 11 sites are Azusa, Burbank, West Los Angeles, Lennox, Downtown Los Angeles, North Long Beach, Pomona and Whittier in Los Angeles County, Anaheim and LaHabra in Orange County and San Bernardino in San Bernardino County. In addition, the converted daily peak hourly oxidant concentrations from Azusa and San Bernardino will be referred to as AZOX and SBOX. resmivelv. The 0, data from June to October of 1980 Ad ‘1981 are available for all 11 sites except San Bernardino where the data for October 1981 are missing.

The meteorological sounding data were acquired from Southern California Edison which sponsored the sounding measurements at El Monte in 1980 and at Ontario in 1981. From the sounding data at noon, we determined the mean temperatures between 850 and 900 mb (denoted EUT, in “C) and the mixing heights based on the relative humidity profile (denoted MH, in m). Since El Monte is 40 km east of the coast and Ontario is 40 km further east, we do not expect the influence of coastal meteorology on the soundings at either location to be signi6cant. Furthermore, the upper level temperature and MH measurements from both locations should be similar. In other words, moving the sounding site from El Monte lo Ontario is not expected to introduce a significant distortion in the sounding data

2131

2132 SWARSHAN KUMAR and DAVID P. CHOCK

Weather data for Ontario were acqmred from the California South Coast Air Quality Management District. From these data, the daily m~imum Ontario surface tem- peratures (OST, in “C) and the average relative humidities (r.h.) over the period of 1200-f 600 PST were determined. The wind data (WS, in m s- ’ ) used in CKH were the surface measurements of the noon soundings in El Monte. Since the sounding site was moved to Ontario in 198 1 and wind data for El Monte were not available. we had no choice but to use the surface measurements of the noon soundings in Ontario for 198 1. Fortunately, a statistical analysis of 9 years of data m CKH shows that EUT and OST are the two most significant parameters affecting oxidant concentrations and the maxi- mum oxidant concentrations are not particularly sensitive to WS. Again as in CKH, weekends as well as days with an east- wind component (based on the El Monte or Ontario noon soundings) were excluded from consideration here.

ANALYSIS AND RESULTS

The methodology employed in the trend analysis is the same as in CKH. The five meteorological para- meters (EUT, OST, r.h.. MH, WS) were held within fixed ranges or at fixed values. Three sets of fixed ranges and fixed values were established to correspond to days with high, moderate and low oxidant forma- tion potential. The fixed values were chosen to be the midpoints of the fixed ranges. For example, EUT, which is by far the most important parameter, has the fixed values of 27,203 and 14°C for the categories of high, moderate and low oxidant formation potential, respectively. Additional details are presented in CKH.

In determining the trends based on fixed ranges of meteorological parameters, days falling in the same set of ranges were grouped together. The trends and un~rtainti~ were then represented by the mean values and standard deviations of peak oxidant of those days within each set of ranges for each year. In 1980, there were 11, 20 and 5 days falling in the ranges for high, moderate, and low oxidant formation potential, re- spectively. In 198 1, the corresponding numbers of days were 4, 9 (7 for San Bernardino), and 5 (4 for San Bernardino), respectively. Figures 1 (a), (b) and (c)show the trends for MAXOX, AZOX and SBOX, together with their standard deviations.

In determining the trends based on fixed values of meteorological parameters, simple regression models were first constructed using the whole set of data for each of the oxidant variables. The models have the form

oItI;iiii,,lJ 1971 1973 1975 1977 1979 1981

Year

Fig. I (a). Trends for basinwide daily maximum oxidant (MAXOX) based on fixed ranges of meteorological parameters. 0, high oxidant days; q , moderate oxidant

days: 0, low oxidant days.

35 -

30 -

g 25-

5 zo-

:: N Q 15-

10 -

5-

_i ol I I I a i I I I I 1 t

1971 1973 1975 1977 1979 1981 Year

Fig. 1 (b). Trends for daily maximum oxidant at Azusa (AZOX) based on fixed ranges of meteorological

parameters.

40

35

30 -

g 25 -

L? ; 20 - 0 g 15-

10 -

5

0 I * I b 1 I I * I ,I 1971 1973 1975 1977 1979 1981

Fig. l(c). Trends for daily maximum oxidant at San Bernardino (SBOX) based on fixed ranges of meteoro-

logical parameters.

Peak daify oxidant concentration (pphm) = [exp(a)] (EUT)~(OST)c(r.h.)d(MH~(WS)f.

The exponents were estimated by linear regression are shown in Table 1. The relative importance of the

after converting the above equation to logarithmic parameters is similar to that found in CKH. For

space. These models were then used to interpolate the example, EUT again stands out as the most important oxidant concentrations for fixed values of the meteoro- parameter influencing oxidant levels. Figures 2(a), (b)

logical parameters. The exponents of the models and and (c) show the trends for MAXOX, AZOX and

themultiplecorrelationcoefficient R for 1980and 1981 SBOX, respectively. The standard error for each indi-

An update on oxidant trends in the South Coast Air Basin of California

Table 1. Regression models

2133

Year

MAXOX 1980

1981

Parameter estimates [see Equation (l)] No. of

obs. a b C d e f R

81 - 0.0792 0.9017’ 0.4199 0.0314 - 0.2057’ - 0.0305 0.902 (1.2582)? (~~~~) (0.4210) (0.0564) (0.0673) (0.0497)

80 - 3.4634 (0.9804) (id:281 1;

1.3507” -0.0146 -0.0757 -0.0971 0.911 (0.4525) (0.0754) (0.0607) (0.0636)

Azox 1980 87 -0.3958 0.7589* 0.7854 0.0305 -0.3012*

(1.6321) (0.2868) (0.5461) (0.0732) 1981 80 -2.1534 1.0334 0.9600 -0.1903 _Gp;;)

(2.0098) (0.5763) (0.9275) (0.1546) (k244)

SBOX 1980 87 0.9865 0.9142* 0.1375 0.2843” - 0.3893

(2.2699) (0.7595) (0.10x8) (0.1214) 1981 6% -0.2001 (f;;;;) 0.0518 -0.1518

(1.7787) (O&O; -(z& (0.1209) (0.0980)

* Parameters which are signikant at 5% Ievel or less. t Standard errors of the &mated pram&en are shown in parentheses. *Ozone data in San Barnardino are missing for Oetoher 1981.

1971 1973 1975 1977 1979 1981 Year

Fig. 2(a). Trends for ha&wide daily maximum oxidant Fig. 2(c). Trends for daily maximum oxidant at San (MAXOX) based on fixed vtiues of met~roio~f Bamardino (SBOX) based on fixed values of metaoro-

parameters. logical parameters.

1971 1973 1975 1977 1979 1981 Year

Fig. 2(b). Trends for daily maximum oxidant at Aausa (AZOX) based on fixed values of meteorological

parameters.

- 0.0471 0.875 (0.0644)

- 0.2593 0.767 (0.1305)

0.1251 0.797 (0.08%)

-0.1680 0.579 (0.1213)

viduai point is defined as the square-root of the second moment relative to the median of the log normal distribution.

We have remarked previously (CKH) that the trends determined with fixed parameter values, in coutrast to the fixed range approach, are not susceptible either to bias due to a noncentral distribution of data points in each parameter range or to the fluctuations in the number of days fahing in each set of parameter ranges. Consequently, our discussion will be based on the fixed value approach. Qualitatively, the results from both approaches are in agreement.

From Fig. 2(a), it is evident that, basinwide, the daily peak oxidant concentrations are decreasing on the average. This downward trend would have been evident in CKH had it not been for the upward spike of 1978. A linear regression fit to the points with high oxidant formation potential shows a decline in the daily peak oxidant level at a rate of 0.46pphm y-l.

SL’DARSHAN KUMAR and DAVID P. CHOCK

73 74 75 76 77 78 79 80 El1

Fig. 3. Frequency of the daily basinwide maxImum occurring at Azusa, San Bernardino, or any other station.

This downward trend is significant at the 5 P,, level. The downward trend is evident also for days with moderate and low oxidant formation potential, but to a lesser degree. Figure 2(b) shows a similar pattern for the trends in Azusa. A straight line fit for the points with high oxidant formation potential gives a decline rate of 0.63 pphm y - ‘. The oxidant trends in San Bernardino. on the other hand, remain flat [cf. Fig. 2(c)]. In fact, one cannot reject the flat-trend assumption at the 5 “” significance level.

The bar chart in Fig. 3 indicates the frequency with which the daily basinwide oxidant maximum occurred at Azusa, San Bernardino, or any of the other nine stations. In earlier years, the basinwide maximum oxidant concentration occurred most frequently at Azusa while in later years San Bernardino recorded the basinwide maximum most frequently. The location of the basinwide oxidant maximum thus seems to have moved eastward from 1971 to 1981. During this period, in spite of a substantial population growth, the basinwide reactive HC emissions have been reduced significantly, whereas the basinwide NO, emissions have not declined (Trijonis er al., 1978; CARB, 1979; Hoggan et al., 1982; SCAQMD, 1983). Smog chamber studies (Glasson, 1981; Whitten er al., 1980) in general have shown that not only does the maximum 0, concentration decrease, but the time to reach the maximum concentration also increases when reactive HC concentrations are reduced for constant NO,. These experiments would, therefore, help explain both the eastward movement or delay of the oxidant peaks as well as a decrease in the maximum value. In addition to the overall decrease in reactive HC emissions, the reactivity of the emissions in the South Coast Air Basin has also changed as a result of control measures. For example. an analysis of morning ambient HC concen-

trations (Calvert, 1976; Grosjean et al., 1982) indicates that the fraction of alkenes. which are more reactive, decreased from 1973 to 1982 while the fraction of alkanes, which are less reactive, increased. This shift to less reactive HCs also contributes to the decrease in the magnitude and the delay of the 0, maximum, thus resulting in the movement of the oxidant peak further inland.

In conclusion, the daily peak oxidant concentrations basinwide and at Azusa have declined over the past decade while those at San Bernardino have remained essentially flat. Furthermore, the emission control measures of the 1970s have yieldet a result in O3 au quality which is qualitatively consistent with expec- tations based on smog chamber experiments.

Acknowledgemenr-The authors wish to thank Mrs. K. W. Wu for carrying out the data management and numerical computation.

REFERENCES

CARB (1978) California Air Quality Data: July-~ August-September, 1978, Vol. 10, No. 3, Technical Services Division. Air Resources Board, State of California.

CARB (1979) Emission Inventory 1976. Technical Services Division, Data Processing and Emissions Branch. Air Resources Board, State of California.

Calvert J. G. (1976) Hydrocarbon involvement in photo- chemical smog formation in Los Angeles atmosphere. Enoir. Sri. Technol. 10. 256262. -

Chock D. P.. Kumar S. and Herrmann R. W. (1982) An analysis of trends in oxidant air quality in the South Coast Air Basin of California. Atmospheric Encironmenr 11, 26 152624.

Glasson W. A. (1981) Effect of hydrocarbon and NO, on photochemical smog formation under simulated transport conditions. J. Air Pollur. Conrrol Ass. 31, 1169-l 172.

Grosjean D., Lloyd A., Countess R. J., Lurmann F. and Fung K. (1982) Captive air experiments in support of photo- chemical kinetic model evaluation-Phase I. Environ- mental Research&Technology, No. P-A764-500, Westlake Village, California.

Hoggan M., Davidson A., Shikiya D. C. and Lau W. (1982) Air quality trends in California South Coast Air Basin. South Coast Air Quality Management District, El Monte. California.

SCAQMD (1983) A progress report, 1977-1983. South Coast Air-Quality Man&e&t District, El Monte, California.

Triionis J.. Penn T.. McRae G. and Lees L. (1978) Oxidant and precursor trends in the Metropolitan Los Angeles region. Atmospheric Enl;ironment 12, 14131420.

Whitten G. 2.. Killus J. P. and Hogo H. (1980) Modeling of simulated photochemical smog with kinetic mechanisms. Vol. 1. EPA-600/3-80-028a. Systems Applications Inc., San Rafael, California.