Precursor relationships, ozone formation and midday meteorology in Sydney

0004-6981!83~030633416 103.00~0 Pergamon Press Ltd.

PRECURSOR

RELATIONSHIPS, OZONE FORMATION MIDDAY METEOROLOGY IN SYDNEY K. POST and N.

AND

CARRUTHERS

Department of Mechanical Engineering, The University of Sydney, NSW 2006, Australia ( First receiued 22 June 1982 and receiwdfor

publihtion 9 September)

Abstract-Precursor concentration distributions and the derived relationships which characterize the spatial distribution ofemission sources in the Sydneyairshed in f975/6/7arecompared with precursor data obtained during the 1981/Z summer. The 1981/2 data shows that: (i) NMHC point sources no longer dominate the character of Sydney’s moring time urban air parcels and (ii) morning time meteorology and/or overall 6:0&10:00 a.m. mass emission rates have changed, resulting in an increase in the frequency of occurrrence of moderate to high NHMC and NO,Vconcentration events. The absence of dominant morning time NMHC point sources balances the increased precursor concentrations and gives rise to a predicted ozone concentration distribution for 1981/2 which is similar to that reported elsewhere for 1975/6/7. The marked reduction in the monitored frequency of occurrence of high ozone events since 1975/6/7 cannot be explained by changes in the characteristics of morningtime air parcels and it is suggested that the observed reduction may be due to a change in the midday (10: 00 a.m.4: 00p.m.) meteorology since 1975/6/7.

lNTRODUCTlON

University during the October 1981 to April 1982 summer season. The 1981/82 instrumentation and calibration procedures were identical to those used to collect the original joint NHMC and NO, concentration distribution data during 1975/6/7 (Post and Bilger, 1978; Post, 1979, 1981). The 1981/82 sample site at Sydney University was located at the 1975/6/7 mobile laboratory base station. The 1981/82 data set contains 873 1/2-h averaged 6-10 a.m. NMHC and NO, observations obtained at the Sydney University site whilst the 1975/6/7 data set contains 610 Sydney University and 226 mobile field site observations. The 1975/6/7 data was obtained using an instrumented vehicle which was stationed at Sydney University and only sampled at that site on days which were not spent in the field. The vehicle was active in the field on days which were forecast to have been high ozone days and consequently, no 1975/6/7 data from the Sydney University site is available for those days. Post (1981) has shown that Sydney’s urban precursor distributions are not strongly dependent upon sample site location and the remainder of this paper

Ozone concentration data monitored in Sydney by the New South Wales State Pollution Control Commission (SPCC) has shown a marked drop in the frequency of occurrence of high ozone events since 1975/6/7. This trend is shown in Table 1 where the number of days for which [0,] exceeded 0.12 ppm, 1 h avg. has fallen from 66 per year in 1976/7 to 7 per year in 1981/2. In a previous paper (Post, 1981), it was shown that for 1975/6/7, high ozone events in the Sydney airshed could be associated with the existence of a number of dominant NMHC point emission sources. It was argued that reduction of NMHC emissions from these point sources would give a larger reduction in the frequency of occurrence of high ozone events than would be expected from an equivalent mass emission reduction of NMHC from distributed sources. In order to determine if NHMC point source emission behaviour had changed since 1975/6/7 the NSW SPCC provided support for the establishment of a NMHC and NO, monitoring site at Sydney

Table 1. Ozone concentration frequency data monitored by the SPCC Sydney network

Number of days per year on which the one hour averaged [0,] 0.12

197314

197415

1975/6

I97617

197718

1978/9

1979180

1980/l

1981/2

17

18

25

66

13

13

10

9

7

exceeded

ppm

Data from Mitchell et al. (1982), SPCC (1982). 633

K. Posr and N.

634

compares the characteristics of the complete 1975/6/7 and 1981/2 joint precursor concentration distributions and makes use of the “air parcel” concept. In this context an “air parcel” is considered to be that air mass which traversed the sample site within a half hour period. It is characterized by its joint NMHC and NO, concentration and for typical 6-10 a.m. conditions in Sydney such an air parcel would be from 2 to 6 km in scale and would be contained below a radiation inversion and have a depth of from 100 to 200m.

PRECURSOR

CONCENTRATION

DISTRIBUTIONS

Precursor concentration frequency distributions for Sydney were presented for 1975/6/7 by Post (1979) and Figs 1 and 2 present these data together with [NMHC] and [NO,] distributions for 1981/2. It is clear from these figures that there has been some change in the i-h averaged 610 a.m. summertime concentration distributions since 1975/6/7 and both species currently

. -a x~>~s~ 2.0-o

CARRUTHERS

exhibit a higher frequency of occurrence for moderate to high concentrations than was observed during 1975/6/7. The higher frequency of occurrence of high NMHC and NO, concentrations could be expected to produce a higher number of 1981/2 high ozone events than were recorded for 19751617. Current emission inventory estimates (Eiser et cl., 1982; Stewart et al., 1982; Mitchell et al., 1982) indicate only minor changes in NMHC and NO, emission rates since 1976 (7% increase in 24-h NO, mass emission rate and 9 7; decrease in 24-h NMHC mass emission rate). Whole day (24 h) inventories do not necessarily reflect mass emission rates applicable to the 610 a.m. period and Figs 1 and 2 are not inconsistent with the inventory data. In summary, Figs 1 and 2 suggest either a mild growth in morning time NMHC and NO, mass emission rates or a change in morning time weather conditions since l975/6/7. More frequent or shallower morning time radiation inversions, or lower average morning time wind speeds are examples of weather changes which would generate a higher frequency of occurrence of high NMHC and NO,.

0 1975 176 DATA

'Q

1.0 -

I* 0 z

05-

i

o.2-

u I 1

Ol-

%o

=% m

-

at 1976177

OATA

.

DATA

1981 I82

cu

PRECURSOR

*. 0. 0(. 0

02

0.5

PERCENT EPUALLEO

1 1

I 2

I 5

I 10

FRECIUENCY WITH OR EXCEEDED.

I M

I / I 30Lo506070

WHICH

1

STATE0

I

I 80

LEVEL

1 90

IS

Fig. 1. Summertime distribution of non-methane hydrocarbon concentration observations for the Sydney urban airshed.

0 1975 I76 DATA X 1976 / 77 DATA . 1981182 DATA

-

.lO OS .os -

0.x 09

j

.02 -

x

0.X Q

0” 5

.Ol

-

PERCENT FREQUENCY WITH EQUALLEO OR EXCEEDED

WHICH

STATED

LEVEL

IS

Fig. 2. Summertime distribution of nitrogen oxides concentration

observations for airshed.

the

Sydney

urban

SOURCE CHARACTERISTICS

Post (1981) has shown that the joint precursor concentration distribution can be used to determine the relative importance of different emission source characteristics within an airshed. The relationships existing in the joint distribution between the NMHC and NO, concentrations, the [NMHC]/[NO,] ratio and the ozone formation potential parameter [NMHC] x [NO,] give information about the relative significance of point and distributed sources. If an airshed contained only distributed sources of NMHC and NO,, with a uniform mass emission rate for each species spread over the whole region, then the [NMHC]/[NO,] ratio of individual air parcels would show no dependence upon the absolute NMHC or NO, concentrations. Individual species concentrations would vary widely due to wind speed and mixing height effects but all air parcels would exhibit the same [NMHC]/[NO,] ratio. If an airshed contained distributed sources of different [NMHC]/[NO,] ratio, individual air parcels would exhibit different ratios but there would be no relationship between the individual species concentration and the mean ratio of sets of air parcels grouped according to their absolute species concentration. In fact, the only way in which an airshed can exhibit a relationship between the [NMHC]/[NO,] ratio and the individual species concentrations is for that airshed to contain substantial point sources of either NMHC or NO,. An air parcel of “typical” [NMHC], [NO,] and ratio, if exposed to a point source of NMHC or NO,, would gain a higher species concentration and a higher or lower ratio, respectively.

635

Precursor relationships, ozone formation and midday meteorology in Sydney The relative importance of either NMHC or NO, point sources within an airshed can, therefore, be assessed by investigating the strength of the relationships between the air parcel ratios and [NMHC] and

I

1 1975 I6 I7

Wxl Figures 3 and 4 present these relationships for the 1981/2 joint precursor concentration distribution and compare them to the 1975/6/7 relationships. These figures show the mean ratio (ratio) associated with air parcels containing NMHC or NO, within the concentration elements GO.1,0.1-0.2,. . ppmC and (rl, 1-2, . . pphm, ‘respectively. {ratio = f:

i= 1

[NO,J)/n,

1

NHHC.

( [NMHC],/

where n is the number of air parcels in each particular subset) Figure 3 shows that the strong relationship between

ratio and [NMHC] which was present in the 1975/6/7 distribution does not exist for the 1981/2 data. To a first approximation the 1975/6/7 data can be described by a linear relationship

g

where [NMHC]

15.

‘i 0” gi.

lo-

? Y

= 4.7 [NMHC] + 5.8 ppmC/ppm; (R* = 0.8)

5INMHC/NO~I*

FINMHCI

(1) 2

1

is in ppmC.

NMHC, ppmC

The 1981/2 data shows no dependence of ratio with [NMHC] and an attempt to describe the data by a relationship of the same form as Equation (1) gave a correlation coefficient, R’, of 0.04. The data of Fig. 3 therefore indicates that the NMHC point sources which strongly influenced the character of any given air parcel in 1975/6/7 are no longer dominant. In fact the 1981/2 data in Fig. 3 shows that NMHC morning time emissions in the Sydney airshed come predominantly from distributed sources. Such a marked change in the [NMHC] vs ratio relationship since 1975/6/7 can only be the result of a reduction in the mass emission rate, or a change in the timing of “large lump” emissions, from a number of large NMHC point sources. Such changes need only involve a small part of the total mass of all NMHC emissions and would not necessarily alter the overall NMHC concentration distribution. Figure 4 shows the [NO,] vs ratio relationships for 1975/6/7 and 1981/2 and it is clear that both data sets exhibit the same dependence of ratio on [NO,] 1975/6/7 [NMHCJ/[NO,]

ppmC

1981 / 2

-i

f

[NMHC]/[NO,]

2

1

Fig. 3. Mean [NMHC]/[NO,] ratio of air parcels grouped according to their NMHC content.

(9751617

01

02 No,.ppm

= -21 [NO,] + 10 ppmC/ppm; (R* = 0.48)

1981/2 [NMHC]/[NO,]

(2)

= - 18 [NO,] + 9.9 ppmC/ppm; (R’ = 0.30)

(3)

where [NO,] is m ppm. The data of Fig. 4 therefore indicates that the relative importance of NO, emissions from both point

01

(12 No,.

Fig.

PP~

4. Mean [NMHC]/[NO,J ratio grouped according to their NO,

of air content.

parcels

K. POST and N. CARRUTHERS

636

“A”. There are three ways in which this, or any other air parcel can be further polluted by NMHC or NO,: (1) by exposure to an NMHC point source; (2) by exposure to an NO, point source; (3) by exposure to a combination of NHMC and NO, point or distributed sources. Each event shifts point A in a different direction as shown by points 1,2 and 3 in Fig. 5. All three kinds of exposure event will raise the value of the ozone formation potential parameter, [NMHC] [NO,], but I

I

1

02

0.L I NO,Ii

QB

0.6

event 1 will raise the ratio, event 2 will lower the ratio

1.0

and event 3 will tend to leave the ratio unchanged. In 19751617, air parcels with higher than “typical” ozone formation potentials were those which had

NMI-CI. PP~C PP~

Fig. 5. Mean [NMHC]/[NO,] ratio of air parcels grouped according to their ozone formation potential as defined by the parameter [NMHC] x [NO,] (see text for discussion of point A).

and distributed sources has not changed in the Sydney airshed since 19751617. Figure 5 shows the ozone formation potential parameter [NMHC] [NO,] against the ratio for the air parcels of 1975/6/7 and 1981/2. The ozone formation potential parameter [NMHC] [NO,] is a reliable indicator of ozone formation for Sydney’s 19751617 precursor conditions and it is part of the simple ozone formation model of Equation (4) (Post, 1979). Potential Afternoon Downwind Ozone = 0.39 ( [NMHC]

[No,] )“.36 ppm

(4)

and where ~[No,] < [NMHC] < 20 [NO,] [NMHC] and [NO,] are morning upwind precursor concentrations, ppmC and ppm, respectively. The relationship is a simplification of an isopleth relationship derived from air mass trajectory experiments which experimentally linked morning precursor concentrations with afternoon ozone concentrations (see Post and Bilger, 1978; Post, 1979). The 19751617 data of Fig. 5 shows an increasing trend in ratio as [NMHC] [NMHC]/[NO,]

[NO,] increases

= 17 [NMHC]

[NO,] + 7.2;

(R2 = 0.62),

(5)

whereas the 1981/2 data exhibits a trend in the opposite sense [NMHC]/[NO,]

= - 2.3 [NMHC]

[NO,] + 8.4;

(R2 = 0.59).

(6)

This change in the character of the joint NMHC and NO, distribution data since 1975/6/7 is important and its significance is best illustrated with a simple example. Consider an air parcel with a given combination of NMHC and NO, concentrations. It will have some ratio value, and an associated ozone formation potential so that it can be represented in Fig. 5 by a point,

higher than “typical” ratio. 1975/6/7 air parcels could only attain those characteristics by having been exposed to NMHC point sources. The 1981/2 data do not exhibit any increasing trend in ratio as the ozone formation potential increases and in fact shows that air parcels with higher than “typical” ozone formation potential tend to have lower than “typical” ratio. This characteristic is due to the mild influence of NO, point sources and the 19751617dominance of NMHC point sources is no longer apparent.

OZONE FORMATION AND SOURCE CHARACTERISTICS

The precursor data discussed above have shown that the 1981/Z pre-10 a.m. air parcels exhibited high NMHC or NO, concentrations more frequently than was observed during 1975/6/7. The data have also shown that the 1981/2 air parcels did not exhibit any of the strong NMHC point source features which were evident in 19751617.The significance of these changes, in terms of ozone formation, can now be assessed. The Sydney ozone formation isopleths (Post and Bilger, 1978) have been used to generate an ozone concentration distribution for 1975/6/7 from the 1975/6/7 joint precursor concentration distribution and the predicted ozone distribution compares very well with that monitored independently by the SPCC (Post, 1979). The isopleths are based upon experimental air parcel trajectory experiments and model the chemical reaction and dispersion of air parcels as they were irradiated and advected throughout the Sydney airshed from 10 a.m. to mid-afternoon during 19751617. These isopleths may be used to predict an ozone concentration distribution from the 1981/2 precursor data and such an application requires that the overall joint precursor concentration domain, hydrocarbon reactivity and the distribution of midday weather conditions for 1981/2 were similar to the 1975/6/7 situation. Figure 6 presents the 1981/2 joint precursor concentration distribution together with the 1975/6/7 data for comparison and it is clear that there has been no

637

Precursor relationships, ozone formation and midday meteorology in Sydney

&I

~.05-

4

/,,,,,/l/,//,,//“l~l7//1

0”

’ 02.

CLEAN AIR E*cKGROJND ///,/11,//1////,/1f,l/l///

LEVELS

, 1

NUMBER DF DAYS PER YEAR THAT MMIMUM READING EXCEEDENG STATED VALUE

Fig. 7. Predicted ozone concentration distributions from Sydney’s 19751617and 1981/2 precursor data.

assumes that midday (10 a.m.4 p.m.) weather conditions during 1981/2 were comparable to those prevailing during 1975/6/7. Figure 7 shows the resulting ozone distributions predicted for 1981/2 and 19751617and it can be seen that there is little difference between the predicted ozone distributions for each period. It would appear that although high precursor concentrations occurred more frequently during the 1981/2 summer (see Figs 1 and 2), the absence of strong morning time NMHC point sources (see Figs 3 and 5) has had an opposing effect upon the predicted ozone distribution. The scale of each of these effects cannot be determined from the available data although a preliminary review published elsewhere (Post, 1982) based upon the 1981/2 summer data up to mid-March may give some indication.

OZONE 1.0

FORMATION

AND MIDDAY

METEOROLOGY

1.5

NMHC

;

ppm as cH4

Fig. 6. Joint distribution of non-methane hydrocarbon and nitrogen oxides observations for (a) 1975/6/7 and (b) 1981/2. Numbers shown in individual elements indicate the number

of $-h averaged air parcel observations which fell within that cell’s given concentration limits.

substantial change in the NMHC and NO, field occupied by the data over the 5-year period. It is not known whether the reactivity of Sydney’s ambient NMHC mix has altered since 1975/6/7 but current emission inventory estimates compiled by the SPCC indicate that no major change is expected to have occurred since 1975/6/7 in the overall composition of NMHC emissions. An ozone concentration distribution can now be predicted using the 1981/2 joint precursor distribution and the Sydney isopleths. The procedure is straightforward and is outlined elsewhere (Post, 1979). Application of the Sydney isopleths to the 1981/2 data

The 198112 predicted ozone concentration distribution of Fig. 7 does not match the 1981/2 SPCC monitored ozone concentration distribution. Table 1 shows that the Sydney network recorded ozone concentrations in excess of 0.12 ppm on only 7 days during the 1981/2 summer period whereas Fig. 7 predicts approximately 60 days in excess of this level. The discrepancy between the predicted and monitored 1981/2 ozone concentration data indicates that the Sydney isopleths together with the assumption of common 1975/6/7, 1981/2 midday meteorology, do not adequately describe the ozone formation conditions prevailing during 1981/2. It would appear that there has been a change in the frequency of occurrence of suitable midday ozone formation conditions since 1975/6/7. Pre- 10 a.m. weather conditions and overall emission rates for 1981/2 gave individual species concentration distributions for NMHC and NO, which exhibited higher frequencies of occurrence. of high precursor levels. Figures 3 and 5 indicated that NMHC emission source characteristics for 1981/2 were different from

K. POST and N.

638

those of 19751617 and Fig. 7 demonstrated that the 1981/2 joint precursor concentration distribution would have generated similar ozone concentration data had the post-10 a.m. conditions been comparable to those of the 19751617 airshed. The NSW SPCC is attempting to determine whether post 10 a.m. weather conditions have altered since 19751617 but it is a difficult task since judged by different criteria, either summer could be shown to have had a more frequent occurrence of “suitable conditions”. High insolation, high temperatures, low wind speeds, low mixing heights, seabreeze or no seabreeze, those are all factors which may be important for post-10 a.m. ozone formation. Criteria to define “high” and “low” for these parameters are not known and it is not clear what combinations of any, or all, of these and other parameters would be sufficient. In addition it is known that different precursor mixtures require different conditions for maximum ozone formation. The low monitored frequency of occurrence of high ozone events during 1981/2 can only be explained by a change in post-10 a.m. irradiation and advection conditions or a substantial change in ambient NMHC reactivity. How such changes may be quantified is unclear but the absence of high ozone events during 1981/2 cannot be attributed to any lack of suitable precursor concentration levels in the 610 a.m. air parcels.

CONCLUSIONS

Sydney’s 1981/2 summer morning time joint NMHC and NO, concentration distribution has been measured and compared to that reported elsewhere for 19751617. It has been found that: (1) Morning time (pre-10 a.m.) mass emission rates and/or morning time weather conditions have altered since 19751617in such a way as to have increased the frequency of occurrence of high precursor concentrations. (2) The effects of NMHC point emission sources, which strongly influenced the character of Sydney’s 1975/6/7 morning time air parcels were absent from the 1981/2 precursor data. (3) The apparent removal of strong morning time NMHC point sources from the urban airshed has

CARRUTHERS

balanced the higher precursor levels in terms of the predicted occurrence of high ozone events. (4) The marked reduction in the monitored frequency of occurrence of high ozone events since 1975/6/7 cannot be attributed to changes in mass emission rates or altered emission source characteristics. As a consequence of the above observations, the data presented here suggest that the substantial drop in the monitored frequency of occurrence of high ozone events since 1975/6/7 is most likely due to a reduction in the frequency of occurrence of suitable ozone forming midday (10 a.m.4 p.m.) weather conditions. If this is so, then Sydney’s current emission rates and

source characteristics will generate high ozone events with unacceptable frequency when and if midday weather conditions similar to 1975/6/7 recur.

Acknowledgements-Equipment and support for this work has been provided by the New South Wales State Pollution Control Commission.

REFERENCES

Eiser C. R., Koo F., Court J. D. (1982) Emission trends in hydrocarbons and NO, from stationary sources in Sydney for the years 1976, 1980 and 1986. Proc. The Urban Atmosphere--Sydney, A Case Study. CSIRO, May 1982. Mitchell A. D., Court J. D. and Ferrari L. M. (1982) Sydney ozone trends 197551981. Proc. The Urban Atmosvhere. Sydney, A Case Study. CSIRO, May 1982. Post K. and Biluer R. W. (1978) Ozone nrecursor relationships in the Sidney airshed. ktmospherk Environment 12, 1857-1865.

Post K. (1979) Precursor distributions, ozone formation and control strategy options for Sydney. Atmospheric Environment 13, 783-790.

Post K. (1981) Ozone formation and the spatial distribution of precursor emissions in Sydney. Atmospheric Environment 15, 743-747. Post K. (1982) Precursor source characteristics and ozone formations in Sydney. Proc. The Urban AtmosphereSydney, A Case Studv. CSIRO. Mav 1982. SPCC (1982) State Pollution Co&i Commission of New South Wales Annual Report. Stewart A. C., Pengilly M. R., Brain R., Haley J. J. and Mowle M. Cl. (1982) Motor vehicle emissions into the Sydney Air Basin. Proc. The Urban Atmosphere-Sydney, A Case Study. CSIRO, May 1982. Note: CSIRO, Commonwealth Scientific and Industrial Research Organization (Australia).