Measurements of ozone and peroxyacetyl nitrate (pan) in Munich

Atmospheric Environment Vol. 27B, No. 3, pp. 293 305, 1993.

0957 1272/93 S6.00+0,00 © 1993 Pergamon Press Ltd

Printed in Great Britain.

MEASUREMENTS OF OZONE AND PEROXYACETYL N I T R A T E (PAN) IN M U N I C H B. RAPPENGLOCK,*K. KOURTIDIS]"and P. FABIAN* *Lehrstuhl fiir Bioklimatologie und Angewandte Meteorologie, AmalienstraBe 52, D-8000 Mfinchen 40, FR.G. and tMax-Planck-Institut fiir Aeronomie, Postfach 20, D-3411 Katlenburg-Lindau, F.R.G. (First received 16 July 1991 and in final form 1 February 1993) Abstract--Simultaneous measurements of ozone and for the first time in Munich -of PAN were carried out at two sites in the urban area of Munich during two periods (16 June-15 September 1989 and 1 January-30 April 1990). Maximum mixing ratios reached 75 ppbv for ozone (10-min-average) and 5.6 ppbv for PAN (20-min-value), respectively. Ozone showed more pronounced diurnal variations than PAN. In both cases they were related to diurnal variations of the global radiation and the mixing ratios of NOx. Regarding ozone linear correlation analysis with meteorological parameters revealed distinct dependencies, especially on UV radiation, whereas the results for PAN reflected its more complicated formation. As far as dependencies on wind velocity are concerned, both ozone and PAN exhibited maximum mixing ratios in cases where the wind velocity was below 5 m s- 1. With increasing wind velocity both mixing ratios tended towards their natural background concentrations. Investigations concerning the influence of the wind direction did not disclose any particular local effects, but rather a relationship to the general weather situations. On the whole, PAN could be considered as a more characteristic indicator of smog conditions than ozone due to its low background concentrations and its thermal instability. Key word index: Ozone, peroxyacetyl nitrate (PAN), Munich, linear correlation analysis, meteorological parameters.

INTRODUCTION Ozone and peroxyacetyl nitrate (PAN, CH3C(O)O2NO2) are the two most important components of photochemical smog. Their formation involves reactions of NOx and hydrocarbons initiated by sunlight. Ozone and P A N are considered to be the cause of eye irritation (OAdW, 1989) and may affect the respiratory tract (Arndt, 1980) even during shortterm high concentrations. Long-term elevated concentrations may have severe impacts on plants, especially caused by P A N (e.g. Posthumus, 1977; Temple and Taylor, 1983; BSfLuU, 1987). Damage of various non-biological materials have also been reported (Arndt, 1980; O A d W , 1989). During the last decades ozone has been measured at many sites throughout the world and to some extent continuously, whereas for P A N there are only a few measurements available (e.g. Brasser et al., 1977; Becker et al., 1983; Temple and Taylor, 1983; Tsalkani et al., 1987). Due to its more complicated measurement technique there are hardly any long-term investigations. So far, the results indicate differences in the characteristics of these two compounds: it is suggested that P A N has only very low natural background concentrations (Singh and Salas, 1983) in contrast to ozone which has a relevant source in the stratosphere (Guicherit and Van Dop, 1977; Becker et al., 1983). Due to its thermal instability, the lifetime of P A N in the atmosphere strongly depends on the ambient temper-

ature (Seinfeld, 1986; Singh, 1987; Tsalkani et al., 1987). This enables P A N to persist for a longer time at low temperatures (e.g. upper troposphere, winter time). Furthermore, P A N is removed only slowly from the atmosphere through dry deposition contrary to ozone where dry deposition represents an effective destruction mechanism (Garland and Penkett, 1976; Penkett et al., 1977; Schurath et al., 1984; OAdW, 1989). Therefore, long-range transport of P A N is likely to occur (Greenfelt et al., 1982; Brice et al., 1984) and it is generally suggested that PAN, in contrast to the short-lived N O or N O 2 molecules, might constitute the largest fraction of the natural NOx reservoir (Singh and Salas, 1983). This paper reports results of simultaneous measurements of ozone and P A N in the urban area of Munich carried out at two different sites during two different measurement periods. To our knowledge there have not been any P A N measurements in Munich before. The results were subject to linear correlation anlaysis with meteorological parameters in order to determine the meteorological conditions under which photochemical smog might occur in Munich.

EXPERIMENTAL

Instruments Ozone was measured by a commercial instrument (Dasibi Model 1008-RS) using a spectroscopic method: UV radiation 293

294

B. RAPPENGLUCK et al.

of 253.7 n m which is strongly absorbed by ozone molecules makes it possible to determine the mixing ratio of ozone in a given air sample by observing the weakening of the UV rays. A measurement cycle of 10s consists of a comparison between an ambient air sample and a "zero-air" sample i.e. the same air sample purified of ozone. The flow-rate was adjusted to approximately 2 # m i n - 1. The instrument's precision was about + 2 ppbv and its estimated accuracy was 10%, N o special filter for the air samples was used in order to avoid additional surface contacts for ozone thereby causing possible lower ozone mixing ratios. Dirt effects could be eliminated by the instrument's zero-air reference procedure and by regular cleaning of its optical system. Calibrations were carried out by m e a n s of a M E C 1000 ozone-generator which was regularly checked by titration against a 0.1 N KI solution via the following reaction: 2KI + O 3 + H 2 0 ~ 2 K O H + 12 + 0 2 . Measurements of P A N were carried out by means of a gas chromatograph (GC; Model 438A Packard Instrument Co.). An electron capture detector (ECD) equipped with a Ni 63foil of 10 mCi was used and a glass tube (length 40 cm, i.d. 2 mm) packed with 10% Carbowax on Chromosorb W H P 80/100 mesh served as a column. Carrier gas was nitrogen with purity of 99.999%. An additional molecular 5 A sieve filter was m o u n t e d in the carrier gas line in order to avoid any pollution caused by humidity or hydrocarbons. The flow-rate through the column was adjusted to 20 ml m i n - '. Via a bypass, an additional flow-rate of 20 m l m i n - i supplied the E C D with a total flow-rate of 40 m l m i n - I in order to improve the E C D sensitivity. The G C operated in the constant current mode applying variable frequencies of voltage pulses. The temperature of the G C oven and the injection was kept at 40°C, whereas the detector's temperature was 100°C. No filter was used for the air samples. Possible dirt effects in the system indicated by a deterioration of the E C D sensitivity were eliminated by an approximately 3 h heating procedure, setting the E C D temperature to 250°C. This procedure was carried out about once a month. An external p u m p (flow-rate 800 m l m i n -1) supplied the G C with ambient air, and every 20 min air samples (volume of sampling loop 2 ml) were automatically injected into the G C system through a 4-port valve regulated by pressurized air. For the G C conditions described above the P A N retention time was usually about 4 min. The measurement's estimated accuracy was about + 2 0 % , its precision 2 0 - 4 0 0 p p t v or + 10%, whichever was greater. Data were recorded by a Shimadzu C h r o m a t o p a c C-R1A integrator. Calibration procedures were done as follows: a PANstandard produced according to a method developed by Nielsen et al. (1982) was provided by the Max-Planck Institute for Aeronomy (MPAE) in Katlenburg-Lindau. This standard was a solution of P A N in n-heptane (usually 1-6 #1 PAN/#I solution) whose absolute content of P A N was determined by means of iodometry. Calibration mixtures close to ambient P A N mixing ratios were obtained by dynamic dilution with synthetic air in a 29 • calibration bag. Dilution of the P A N solution was done by letting in synthetic air under a flow-rate of 5 C m i n - 1 for about 4.5 min. During this procedure the P A N solution was directly injected into the synthetic air flow passing the inlet of the calibration bag by means of a syringe. A volume of 0.2 ml of distilled water had to be introduced into the calibration bag first, in order to produce realistic humidity and to suppress any disturbances that have been reported by some authors concerning measurement of P A N at low ppbv levels (Holdren et al., 1976; Lonneman, 1977; Lrbel et al., 1980). As extensive series of calibration procedures at M P A E have shown, this dynamic dilution method ensures the most homogeneous distribution of P A N within the bag.

Measurement sites

Measurements were carried out during 16 June-15 September 1989 (period I) and 1 J a n u a r y - 3 0 April 1990 (period II). During period I the measurement site was located in a residential area in the northern outskirts of Munich (Fig. 1; 48°11'N, 11°37'E) which mainly consists of concrete buildings (up to 20 floors) situated in a park-like area. Only to the north and to the west are there bigger roads with heavy traffic especially on weekdays, whereas to the east there lies an extended park. During this period air samples were taken at a height of approximately 3 m above ground using a 4 m Teflon-line (i.d. 4 mm). Residence time was about 3.4 s for the ozone system and 8.5 s for the P A N system, respectively. During period II measurements were taken on the roof platform of our institute (48°09'N, 11 °36'E) near the centre of the city, at a height of approximately 38 m above ground, which enabled measurement in a more homogeneous layer of the urban atmosphere. In the neighbourhood of the institute there are hardly any higher buildings. It is a closely built area with a lack of park areas, but with several narrow streets causing regular traffic jams on weekdays. During period II it was necessary to use a Teflon-line of a length of 20 m (i.d. 2 cm) and, furthermore, a powerful p u m p for the main air flow. This p u m p was strong enough to supply the analysers with a sufficient air sample and to minimize at the same time the residence time of ozone and P A N in the line system (approx. 20 s). Four-hour reference measurements with an additional ozone analyser on the platform did not reflect any significant ozone loss in the line system (mean difference 0.83 ppbv, standard deviation 1.29 ppbv; ozone concentration varied in the range 53-66 ppbv). In both periods meteorological data were obtained from our own weather station on the roof platform of our institute. T h o u g h the measurement sites are different regarding their location types, measurement periods and, to a certain extent, their sampling systems, it is possible to draw some conclusions from these measurement results.

RESULTS AND DISCUSSION S e a s o n a l variations

Figure 2 shows the seasonal variation of the diurnal maximum of ozone and PAN during the two periods. Statistical d a t a a r e listed in T a b l e 1. M a x i m u m m i x i n g r a t i o s r e a c h e d 75 p p b v for o z o n e ( 1 0 - r a i n - a v e r a g e ) a n d 5.6 p p b v for P A N (20-rain-value), respectively.

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Ozone and PAN measurements

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Regarding the relatively low ozone mixing ratios it is necessary to keep in mind that during period I in a few cases (late August and September) stable general weather situations with undisturbed sunshine lasted for some days. According to the figures ozone has a stronger seasonal variation, indicating a relationship to the annual variation of the solar radiation, than P A N which exhibits rather distinct smog episodes, e.g.

in late summer. Though solar radiation is not shown in the figures on account of its well-known annual variation, it is obvious that the variation of ozone reflects the effects of solar radiation exhibiting a similar characteristic variation, especially discernible in the increase of the diurnal maxima in the course of time in period II. Regarding P A N smog episodes, high P A N mixing ratios may occur even during winter

296

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time, as happened for the 10 13 January period when actually the highest PAN values in this period could be observed. In accordance with results obtained by Dollard et al. (1991) linear regression analysis between the diurnal maxima of ozone and PAN yielded higher correlation coefficients than similar calculations for their diurnal averages. In our investigations, correlation coefficients for period I were r=0.37 (for diurnal averages) and r=0.42 (for diurnal maxima), respectively, for period II r =0.37 (for diurnal averages) and r =0.57 (for diurnal maxima), respectively. Linear regression analysis between the diurnal maxima of ozone and PAN (Table 2) showed that in most cases ozone mixing ratios in urban air are 5- ! 3 times higher than PAN mixing ratios. A supposed ambient air without PAN might contain 13-35 ppbv ozone according to equation (I). Though the value for period II (approx. 13 ppbv) seems to be low, possibly caused by the effects of low inversions during winter time, this range of ozone mixing ratios can be expected to be found in natural air (Gaffney et al., 1989). Regarding a supposed ambient air without ozone, however, this air exhibits 140-310 pptv PAN (equations II). This is an amount which exceeds natural background mixing ratios of about 32 pptv (Singh and Salas, 1983) and which, therefore, is a hint to the anthropogenic pollution of urban air.

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Diurnal variations of ozone and PAN display similar features, especially in period I (Fig. 3). Period II, however, shows a more pronounced diurnal variation for ozone than for PAN. This different characteristic might be caused by PAN's low deposition velocity as well as its thermal instability. Altogether this leads to a much slower destruction mechanism during the colder nighttime and a possibility of thermal decomposition during the daytime when the temperature reaches its maximum. The diurnal variation of global radiation shows a clear relationship to both photooxidants. As shown in Fig. 3, the global radiation peaks between 1200 and 1300 h MET (Middle European Time) thereby stimulating formation of ozone and PAN. Ozone reaches its diurnal maximum between 1400 and 1500h. Subsequently, the destruction mechanism gains importance and ozone is removed effectively from the atmosphere through dry deposition, whereas PAN's lower deposition velocity and its increasing stability at lower temperatures might contribute to a later maximum between 1600 and 1700 h. In some cases, however, especially on warmer days the PAN peak occurs before the ozone maximum (Fig. 4). Often these were days when high PAN mixing ratios could be found. This may contradict the results of investigations carried out by Dollard et al. (1991) who often detected low PAN concentrations on very warm days. The explanation for this behaviour, therefore, is not yet clear. Probably the increasing temperature in the

Ozone and PAN measurements

course of the day plays a certain role. Moreover, it may be possible that first NO2 is bound into the PAN molecule during the cooler morning hours and then is released later when the temperature rises and leads to an enhanced thermal decay of PAN. At this time this might stimulate ozone production via the photolysis of the NO2 molecule, as might be indicated by the ozone peaks that often follow the PAN peaks according to Fig. 4. Above all, this is a hint to a special air mass quality with a strong photochemical activity and a supposed high burden of precursors. Ozone exhibits a distinct diurnal minimum at approximately 600 h in period I and 700 h in period II. PAN reaches its minimum mixing ratios at about 500 h in period I and between 800 and 900 h in period II. Splitting the data set into weekday and Sunday measurements, these phenomena can be explained as follows (Fig. 5): On Sundays ozone does not show this distinct early minimum. Moreover, it has lower mixing ratios at night and higher values during day time as compared to weekdays. Diurnal maxima were about 11.6% (period I) and 13.7% (period II), respectively, higher than on weekdays. Similar results have been reported elsewhere (Guicherit, 1987; Bower et al., 1989). PAN, however, does not display great differences between weekdays and Sundays except for the fact that in both periods slightly lower mixing ratios could be observed on Sunday afternoons. Measurements of NOx at various sites in Munich that were analysed by Brindl et al. (1987) imply a strong relation with the phenomena described above. On weekdays NO maxima usually occur during the rush hours in the morning. On Sundays this early rush hour does not exist, but there is a NO maximum appearing in the late afternoon indicating the return of the excursionists that have been out of town during the day. On the whole, Sunday NO levels are lower than on weekdays. On Sundays, the lack of N O leads to higher ozone levels during daytime hours whereas on weekdays the abundance of NO during the rush hour causes the early morning minimum. Another hint to this effect is the different hour of this minimum probably caused by the change of time between these two measurement periods ( M E T + 1 h in period I) shifting the rush hour by 1 h. PAN seems to be influenced by the Sunday afternoon NO maximum leading to lower PAN values, whereas on weekdays low temperature in the early morning keeps the PAN molecule stable enough to resist NO attack.

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To determine meteorological influences on the formation of ozone and PAN, linear correlation analysis was applied using the parameters listed in Table 3. Best correlation coefficients could be calculated taking the diurnal sum of parameters describing solar radiation or the diurnal maximum of temperature.

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Fig. 3. Mean diurnal variation of ozone, PAN and global radiation. The error bars shown are representative for all times (ozone, global radiation: 10-min-average; PAN: 1-h-average). T h e dirunal sum was determined by a d d i t i o n of all m e a n values o b t a i n e d o n one day for one p a r a m e t e r multiplied by the time of averaging. T h e dependence o n wind velocity p r o v e d to be m o r e complex. The results of these investigations are given in Tables 4 a n d 5. In m a n y cases diurnal m a x i m a r a t h e r t h a n diurnal m e a n values of ozone a n d P A N showed better relationships to these meteorological parameters.

O n the whole, ozone exhibited a distinct dependence o n solar r a d i a t i o n reflecting a certain order of these parameters: the most i m p o r t a n t one is U V r a d i a t i o n (actually the best correlation coefficient r = 0 . 8 2 ) followed by global radiation. The least imp o r t a n t factor is the d u r a t i o n of sunshine. Since cloudiness has less impact o n U V r a d i a t i o n t h a n o n global r a d i a t i o n (Dirmhirn, 1964) it c a n be concluded

Ozone and PAN measurements ozone

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that photochemical ozone formation is not confined to sunny days, but also occurs on cloudy or at least hazy days. A certain dependence on the diurnal maximum temperature is discernible, too. This effect, however, might rather be an indicator of a different air-mass than a cause for high ozone mixing ratios. These results concerning the influence of temperature and solar radiation may be confirmed by the investigations of Dollard et al. (1991): in their analysis of a smog episode, ozone and PAN maxima were found at the end of this period on a day with reduced solar radiation and temperature. The results for PAN are different: overall lower correlation coefficients (Ir1<0.55) often reaching no levels of significance at all indicate a far more complex formation mechanism as compared to ozone. Probably non-linear dependencies might play an important role concerning PAN. Additionally carried out partial correlation analysis (e.g. global radiation set constant) gave no further information. As can be seen from Tables 4 and 5 (especially in period II) wind velocity controls mixing ratios of ozone and P A N in a more complex way. Further investigations were done by calculating the mixing ratios for different classes of wind velocity (Fig. 6). It is necessary to keep in mind that period I is mostly marked by summertime situations. Convective wind circulations that exhibit diurnal variations as well as

regular inversions during nighttime which rapidly break up in the morning hours are typical for this period. Period II, however, often has either advective situations, even during nighttime, without special diurnal variations or long-persisting inversions. In both cases maximum mixing ratios of ozone and PAN were measured only when wind velocities were below approximately 5 m s-~, i.e. preferably during high pressure situations. At wind velocities above 5 m s-1 there is a general decrease in the maximum values of ozone as well as of PAN. Additional results are obtained regarding the median and the so-called central 68%-mass, a statistical measure to determine the majority of measurements of a certain parameter that does not follow a Gaussian distribution as is the case for ozone and PAN. During period I there is a considerable increase in the mixing ratios within the lowest classes of wind velocity (up to 1-2 m s - l ) possibly indicating the effect of the inversion's breaking up during the morning hours. Especially regarding high wind velocities as seen in period II, the majority of the measurements of ozone and PAN tend towards their natural background concentrations. Since strong winds are usually westerly winds in the Munich area (Briindl et al., 1987) and, therefore, do not transport polluted air as can be concluded from measurements of other important pollutants (BLfU, 1990), there must also be an ozone exchange with the free troposphere

300

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Ozone and PAN measurements

301

Table 3. Parameters used for linear correlation analysis Parameters

Diurnal mean

Diurnal maximum

X X

X X

Ozone mixing ratio* PAN mixing ratiot Global radiation* UV radiation* Duration of sunshines Temperature* Wind velocity*

Diurnal sum

X X

X X

X X X

X X

* 10-min-averages. t20-min-values. ++Duration of sunshine in hours per day.

Table 4. Linear correlation analysis between the diurnal maximum and the mean diurnal mixing ratio of ozone with meteorological parameters (ozone and meteorological parameters except for the duration of sunshine: 10-min-average)

Meteorological parameter

Linear correlation with the mean diurnal Linear correlation with the diurnal mixing ratio of ozone maximum mixing ratio of ozone Number of data r r' max* r' min* Number of data r r' max* r' min*

Period I Diurnal maximum of global radiation Diurnal sum of global radiation Diurnal maximum of UV radiation Diurnal sum of UV radiation Diurnal duration of sunshine Mean diurnal temperature Diurnal maximum of temperature Mean diurnal wind velocity Diurnal maximum of wind velocity

76 76 76 76 76 76 76 76 76

0.62 0.63 0.64 0.69 0.51 0.73 0.76 -0.01 0.33

0.78 0.78 0.79 0.82 0.70 0.84 0.86 n.s. 0.57

0.41 0.41 0.43 0.49 0.25 0.55 0.61 n.s. 0.04

90 90 90 90 89 90 90 90 90

Period II Diurnal maximum of global radiation Diurnal sum of global radiation Diurnal maximum of UV radiation Diurnal sum of UV radiation Diurnal duration of sunshine Mean diurnal temperature Diurnal maximum of temperature Mean diurnal wind velocity Diurnal maximum of wind velocity

91 87 91 87 96 87 91 87 91

0.63 0.63 0.70 0.71 0.18 0.48 0.41 0.43 0.35

0.77 0.77 0.82 0.82 n.s. 0.66 0.61 0.63 0.57

0.43 0.42 0.54 0.54 n.s. 0.23 0.16 0.17 0.10

106 102 106 102 110 102 106 102 106

0.62 0.76 0.65 0.78 0.60 0.75 0.66 0.79 0.55 0.71 0.68 0.80 0.72 0.83 -0.28 -0.01 0.04 n.s.

0.72 0.79 0.75 0.82 0.49 0.61 0.63 0.12 0.12

0.82 0.87 0.84 0.89 0.66 0.75 0.76 n.s. n.s.

0.42 0.46 0.40 0.47 0.33 0.50 0.56 -0.51 n.s.

0.57 0.67 0.62 0.71 0.28 0.43 0.45 n.s. n.s.

*r' max and r' min, respectively, stand for the upper and the lower limits of the range of confidence of 99% for the total sample correlation coefficient r' (calculated by the means of the Fisher's Z-transformation). n.s. = non-significant, i.e. r ' = 0 cannot be excluded.

enabling ozone to maintain its relatively high concentrations at g r o u n d level. Similar effects have been observed elsewhere (Nieboer and Van Ham, 1976; Guicherit, 1987). In this situation ozone is rather an indicator of unpolluted air. Figure 7 depicts s t o r m y - d a y situations where wind velocity obviously controls the a m o u n t of b a c k g r o u n d ozone and P A N . The ozone peak on 4 February, however, must be attributed to the n o r m a l diurnal variation of global radiation.

Influences of wind direction are difficult to take into account, because the data-base is rather limited. In our investigations local effects, i.e. caused by roads or park areas in the n e i g h b o u r h o o d of the m e a s u r e m e n t sites, could not clearly be identified. The median and the central 6 8 % - m a s s showed the following trend: the majority of the lowest mixing ratios were usually observed with southerly winds, whereas highest ozone and P A N values mostly occurred when the wind came from northeasterly directions. These p h e n o m e n a may

302

B. RAPPENGLOCK et al.

Table 5. Linear correlation analysis between the diurnal maximum and the mean diurnal mixing ration of PAN with meteorological parameters (PAN: 20-min-value; meteorological parameters except for the duration of sunshine: 10-minaverage)

Meteorological parameter

Linear correlation with the mean diurnal Linear correlation with the diurnal mixing ratio of PAN maximum mixing ratio of PAN Number of data r r' max* r' min* Number of data r r' max* r' min*

Period I Diurnal maximum of global radiation Diurnal sum of global radiation Diurnal maximum of UV radiation Diurnal sum of UV radiation Diurnal duration of sunshine Mean diurnal temperature Diurnal maximum of temperature Mean diurnal wind velocity Diurnal maximum of wind velocity

67 67 67 67 67 67 67 67 67

0.05 0.26 -0.10 0.13 0.27 0.18 0.21 -0.23 -0.15

Period II Diurnal maximum of global radiation Diurnal sum of global radiation Diurnal maximum of UV radiation Diurnal sum of UV radiation Diurnal duration of sunshine Mean diurnal temperature Diurnal maximum of temperature Mean diurnal wind velocity Diurnal maximum of wind velocity

92 88 92 88 97 88 92 88 92

0.50 0.68 0.55 0.71 0.50 0.68 0.55 0.72 0.41 0.61 0.25 n.s. 0.30 0.53 -0.37 -0.11 -0.43 -0.18

n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s.

n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s.

88 88 88 88 88 88 88 88 88

0.04 n.s. 0.22 n.s. -0.10 n.s. 0.09 n.s. 0.28 0.51 0.19 n.s. 0.22 n.s. -0.36 -0.10 -0.25 n.s.

n.s. n.s. n.s. n.s. 0.01 n.s. n.s. -0.58 n.s.

0.27 0.32 0.27 0.33 0.17 n.s. 0.04 -0.58 -0.62

107 103 107 103 Ill 103 107 103 107

0.42 0.61 0.45 0.63 0.40 0.59 0.43 0.61 0.41 0.60 0.13 n.s. 0.22 n.s. -0.40 -0.16 -0.44 -0.21

0.20 0.23 0.17 0.20 0.19 n.s. n.s. -0.59 -0.62

*r' max and r' min, respectively, stand for the upper and the lower limits of the range of confidence of 99% for the total sample correlation coefficient r' (calculated by the means of the Fisher's Z-transformation). n.s. = non-significant, i.e. r' = 0 cannot be excluded.

be explained by the presence of a local wind circulation t h a t can develop in the M u n i c h area due to its location near the Alps d u r i n g high pressure situations with wind velocities below 5 m s - ~ (Brfindl et al., 1987). Westerly w i n d s - - m o s t l y a c c o m p a n i e d with high wind velocity--led to almost b a c k g r o u n d P A N a n d ozone values. F u r t h e r investigations are necessary to prove these mechanisms, but nevertheless, a certain impact of general weather situations on the f o r m a t i o n of b o t h p h o t o o x i d a n t s can be deduced from these results. High mixing ratios of ozone a n d PAN, therefore, can be expected if there is a high pressure area to the n o r t h w e s t of s o u t h e r n G e r m a n y .

CONCLUSIONS 1. O z o n e a n d P A N reached m a x i m u m mixing ratios of 75 p p b v (10-min-average) a n d 5.6 p p b v (20min-value), respectively, in the u r b a n air of Munich. C o n t r a r y to ozone, P A N exhibits distinct smog episodes, even in S e p t e m b e r a n d d u r i n g winter time. 2. The diurnal variations of ozone a n d P A N depend o n the diurnal variations of the global r a d i a t i o n a n d NOx. D u e to its lower deposition velocity a n d its t h e r m a l instability, P A N can have s m o o t h e r diurnal variations t h a n ozone. In c o n t r a s t to the n o r m a l day

situation when ozone reaches its diurnal m a x i m u m earlier t h a n PAN, the P A N peak may occur before the ozone peak on w a r m e r days. 3. Linear correlation analysis with meteorological p a r a m e t e r s revealed distinct dependencies regarding ozone, especially on U V radiation, whereas the results for P A N yielding only low correlation coefficients indicated a more complicated formation. High correlation coefficients between diurnal m a x i m a of ozone a n d t e m p e r a t u r e are considered to be rather a n effect of different air-masses t h a n a cause for ozone formation. 4. M a x i m a of ozone a n d P A N mixing ratios occurred only at wind velocities below 5 m s 1. Above 5 m s t b o t h p h o t o o x i d a n t s tended towards their natural b a c k g r o u n d concentrations. 5. High mixing ratios of ozone a n d P A N were observed with northeasterly winds, whereas southerly winds were a c c o m p a n i e d with low values of ozone a n d PAN. G e n e r a l weather situations (e.g. high pressure area to the n o r t h w e s t of s o u t h e r n G e r m a n y ) m a y c o n t r i b u t e to an e n h a n c e d f o r m a t i o n of b o t h compounds. 6. O n the whole, P A N can be considered a more characteristic indicator of smog conditions t h a n ozone due to its low n a t u r a l b a c k g r o u n d c o n c e n t r a t i o n s a n d its thermal instability.

Ozone and PAN measurements

303

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Fig. 6. Distribution of the ozone and PAN mixing ratios among different classes of wind velocity. The bars indicate the area of the central 68%-mass. (ozone: 10-min-average; PAN: 20-min-value).

304

B. RAPPENGLOCK et al.

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Ozone and PAN measurements

Acknowledgements--This work has been partly funded within the Bavarian climate research program BayFORKLIM. Funds provided by the Bayerische Staatsministerium fiir Wissenschaft und Kunst are gratefully acknowledged. REFERENCES

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