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Transport, formation and sink processes behind surface ozone variability in north european conditions
Atmospheric Environment Vol. 25A. No. 8, pp. 1437 1447, 1991. Printed in Great Britain.
0004 6981/91 $3.00+0.00 :i~" 1991 Pergamon Press plc
TRANSPORT, FORMATION AND SINK PROCESSES BEHIND SURFACE OZONE VARIABILITY IN NORTH EUROPEAN CONDITIONS H. HAKOLA, S. JOFFRE,* H. L,~TTILA a n d P. TAALAS Finnish Meteorological Institute, Air Quality Department, Sahaajankatu 22E, SF-00810 Helsinki, Finland
(First received 19 April 1990 and in final form 11 October 1990) Abstraet Measurements of ozone, nitrogen dioxide and meteorological parameters at the two Finnish EMEP background stations of,~ht/iri (forested site) and Ut6 (an offshore island) show clear indications of the influence of the precursor source areas of Western and Eastern Europe on surface ozone behaviour at these higher latitudes. The mean ozone levels are relatively high, with maximum monthly values of 41 and 42 ppb, respectively. These values occur in April at both sites nearly irrespectively of wind direction, pointing to a global feature. Other spring, as well as summer and autumn, months have lower ozone values for wind direction sectors corresponding to clean air masses (north-westerlies and north-easterlies). Surface uptake is an important sink in the local ozone budget, especially during late spring, summer and early autumn. Chemical losses are more efticient in winter, when the ground is covered by snow.
Key word index: Background ozone, long-range transport, surface deposition, boundary layer variability, ozone precursors. I. INTRODUCTION The potentially adverse effects of ozone on forests, crops and h u m a n health (e.g. Schneider et al., 1989), as well as the increasing trend of tropospheric ozone c o n c e n t r a t i o n s (Angell a n d Korshover, 1983; Bojkov, 1986), have triggered m o n i t o r i n g efforts in various countries in order to u n d e r s t a n d a n d predict its behaviour in various environments. A l t h o u g h the main a t t e n t i o n has been paid to ozone as a secondary product from a n t h r o p o g e n i c emissions, it has also a b a c k g r o u n d c o m p o n e n t whose 'clean pre-industrialized' value depends o n various geographical, chemical a n d physical parameters a n d is still subject to debate (e.g. Kley et al., 1988). O x i d a t i o n reactions which either produce or destroy ozone, depending on the nitrogen oxide concentrations, can take place globally in the t r o p o s p h e r e by the mechanism proposed by Crutzen (1974), a n d also regionally near source areas. Photochemical smog was detected in Europe in the 1970s (Guicherit a n d van Dop, 1977). Wind patterns over n o r t h e r n Europe favour southwesterlies, especially in winter, so that in the case of Finland, high ozone episodes, mainly originating in E u r o p e a n industrial areas, are t r a n s p o r t e d over the Baltic Sea where dry deposition is weak (Joffre et al., 1988). F u r t h e r downwind, however, ozone can be efficiently deposited upon the extended Finnish forests. Finland's own sparsely distributed emissions can interfere with these air masses, so that ozone can also be built up locally. Moreover, as F i n l a n d is mainly covered by forests (70% of the country, of which 90% are coniferous; E n v i r o n m e n t Statistics, * To whom correspondence should be addressed.
1987), natural h y d r o c a r b o n emissions are likely to have an effect on ozone levels. We shall report here on m e a s u r e m e n t s of ozone performed at two Finnish b a c k g r o u n d stations, one in a maritime e n v i r o n m e n t in the south-western archipelago, a n d the other in Central F i n l a n d in a forested environment. The measurements will be interpreted in the light of c o n c o m i t a n t chemical (nitrogen dioxide) a n d meteorological m e a s u r e m e n t s (wind, temperature, radiation a n d humidity), in order to assess a n d u n d e r s t a n d the variability a n d b e h a v i o u r of ozone concentrations comparatively at these two stations. O u r data are also included in the j o i n t E u r o p e a n O E C D project O X I D A T E compiled by Grennfelt et al. (1986, 1987), which shows that the ozone diurnal curves on U t 6 are very similar in level a n d shape to the c o r r e s p o n d i n g curve at the other maritime station of Westerland (F.R.G.). 2. MEASUREMENTS AND DATA The Finnish Meteorological Institute started ozone measurements in 1985 on the island of Ut6 (59°47'N, 21°2YE; altitude 7 m), located about 80km south-west of mainland Finland. The location of this small rocky treeless island on the southern margin of the ,~land archipelago is favourable for picking up situations with air masses unaffected by anthropogenic emission for hundreds of kilometres (Fig. 1). The reported data cover the period from February 1986 to December 1988. Ozone measurements were started in summer 1986 at a site in Aht/iri in Central Finland (62°33'N, 24°13'E; altitude 162 m). The area is densely forested but with some cultivated clearings. The reported data extended from September 1986 to December 1988. Ozone measurements were performed using an ozone monitor of type 1003-RS, manufactured by Environment SA under DASIBI licence. It is based on u.v.-radiation absorp-
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tion by ozone at 240 nm. Both sensors used at Ut6 and ~.ht/iri have been intercalibrated against other sensors used in the Nordic countries, so that results are intercomparable (Oyola and Areskoug, 1988). The measurements were l-h averages with an accuracy better than 2 ppb. At Ut6, the presence of a diesel generator (at 50 m on the northern side of the monitor) introduced some perturbations into the ozone records from time to time. These effects were eliminated by deleting ozone data having a relative standard deviation (calculated from the 1-min samples within 1 h) larger than
14%. This threshold corresponded best to those situations when the generator was upwind of the sampler and when NO 2 concentrations were also very high and fluctuating. Measurements of NO 2 were performed using an automated version of the Salzman method. The resolution is 0.1 ppb for 1-h sampling periods. In the Salzman method, the air sample is bubbled through the reagent (14% acetic acid, 0.5% sulphanilic acid, 0.002% N(1-naphthyl)-ethylenediamine dihydrochloride and distilled water), in which the absorbed NO2 forms an azo dye. The amount of absorbed NO2 is determined photometrically from the transmittance of light by the reagent at a wavelength of 550 nm (Punkkinen, 1988). The Nordic intercalibration of NO2-sensors (Oyola and Areskoug, 1988) did not allow firm conclusions to be drawn, due to the large NO 2 concentrations used and the wide scatter between the compared measuring methods. The NO2 measurements cover the period from July 1986 to December 1988 at Ut6 and from February 1987 to December 1988 at ,g,ht~iri. Both sites are EMEP-stations with the standard chemical measurement protocol (CCC, 1977). Additionally, automatic wind and global radiation measurements are performed in the close vicinity of the chemical samplings, providing data every hour. Other meteorological observations (temperature and humidity) are carried out every 3 h. The marine site of Ut6 is a station of the EUROTRAC/TOR network also. The frequency distribution of ozone hourly values is shown in Fig. 2. On Ut6 the hourl.y, values are concentrated in the range 20-40 ppb, while at Aht/iri the distribution is more peaked and the concentrations are mainly between 20-30 ppb. We notice also the lack of low values on Ut6 due to low deposition to the sea surface.
3. OZONE ANNUAL VARIATIONS 3.1. M e a n concentrations
Fig. 1. Map.of Northern Europe with the two Finnish background stations and the sounding station of Sodankyl/i.
W e p r e s e n t in Fig. 3(a) the a n n u a l cycle o f global r a d i a t i o n t o g e t h e r with c o n c e n t r a t i o n s of o x i d a n t s O x = 0 3 + N O 2 a n d n i t r o g e n dioxide at o u r t w o sites: U t 6 ( m a r i n e site) a n d ~ h d i r i (forested site). In Fig. 3(b), we c o m p a r e the m e a n m o n t h l y values o f o z o n e conc e n t r a t i o n s at o u r stations with o t h e r sites. T h e a n n u a l cycle o f the m o n t h l y m e a n o x i d a n t O x level has
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Fig. 2. Frequency distribution of hourly ozone concentrations on Ut6 and at ,g,ht~iri.
Transport, formation and sink processes behind surface ozone variability 7000
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Fig. 3(b). Mean monthly values of ozone concentrations at Ut6 (squares) and ~,ht/iri (triangles) together with the annual trend at the U.K. station of Bottesford (asterisks), and three annual trends reported by Logan (1985): in the Eastern U.S. (dashed line), at two elevated European and U.S. stations (dotted line), and an estimate for Canadian stations (hatched area). roughly the same annual amplitude as the ozone levels: 0.5 ppb less in Ut6 and 1.5 ppb less in Xht/iri. This indicates that the observed levels of NO2, mainly local due to its short lifetime, are not the main contributor to the relatively high ozone levels observed at our stations, but rather that ozone has been transported to the receptor points. Moreover, the seasonal trend of global radiation is not in phase with the ozone concentration curves. We observe that the maximum mean monthly concentrations occur either only in spring (March-April) at ~ht/iri or in both spring and summer (March-July) at Utr. A weak summer peak was also observed in ,~ht/iri in 1988, but not in 1987. The 1987 temperatures from June to August were about 3-5°C lower than in 1988, which could explain the lack of a summer maximum in 1987. We have also shown in Fig. 3(b) the annual trend of daily averaged surface ozone at other mid-latitude sites in the Northern Hemisphere as follows: (a) the
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mean annual trend from nine sites in the Eastern U.S.; (b) an estimate from two sites in Canada; (c) the mean of two high altitude ozone-level sites in the U.S. (Missouri) and Europe (Hohenpeissenberg); and (d) the monthly mean 0 3 concentrations at the rural site of Bottesford (Leicestershire, U.K.) taken from the U.K. Review Group's (1987) report. Curves (a), (b) and (c) are from Logan (1985). We note that the Ut6 curve best follows the Eastern U.S. and high Europe-U.S. curves with an extended spring-summer maximum (March-July), while the ~,ht~iri curve, characterized by a single spring maximum (April), is very close to the curve for the Canadian sites. The Bottesford curve is close to the Xht/iri curve except in spring, when Aht/iri's values are clearly higher due to the presence of the snow cover implying a slow deposition. The spring peak, which has also been observed elsewhere (Logan, 1985; Bojkov, 1986; Feister and Warmbt, 1985; Guicherit, 1988), has been previously attributed to intrusion of stratospheric ozone-rich air, that should be more frequent in spring (Danielsen, 1968; Johnson and Viezee, 1981). However, these stratospheric intrusions are also frequent in other seasons, and the recently re-analysed Montsouris data by Volz and Kley (1988) showed that in early industrial times the ozone annual cycle had only a weak and diffuse maximum from February to May, suggesting that the marked spring maximum has another origin. Due to the relative short lifetime of ozone in the troposphere (1 month), and since this strong maximum has been observed at various background stations of the Northern Hemisphere (Logan, 1985; Guicherit, 1988), it should have a global origin corresponding to the tropospheric production from precursor compounds accumulated in the troposphere in winter as first suggested by Penkett and Brice (1986). Peroxyacetylnitrate (PAN) concentrations, which have only a chemical origin in the troposphere, are noticed to peak in spring (Penkett, 1989), suggesting that the spring ozone maximum may be also due to chemistry, since PAN and ozone are both formed chemically from a number of hydrocarbons. Both Finnish stations can be classified as background stations, and thus have this spring maximum. When we differentiate the annual trend with respect to wind direction sectors, the ozone concentrations are noticed to be clearly higher under wind directions corresponding to European source areas from March to August-September (Fig. 4). We have used here the local wind direction although the wind direction from a higher layer or air trajectory sectors would be more recommendable. Nevertheless, there is a strong correlation between the surface wind direction and the mean boundary layer wind direction, the latter being veered by about 25 ° from the former. The ozone annual cycle, both in the atmospheric boundary layer (ABL) under the close influence of precursors' emissions (NOx, CO and volatile organic compounds), and also above the ABL is characterized
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HI HAKOLAet al.
by a summer maximum (Logan, 1985). The summer peak as observed in the Ut6 curve has been related to regional photo-chemical production under the influence of air pollution (Guicherit, 1983; Penkett, 1989). Consequently, this marine background station is within the reach of the industrialized European areas. This fact is helped by the presence of the sea, where ozone
50
AHTARI
has a low deposition rate, upwind of the station. On the other hand, since in ~htfiri (downward of Ut6) observed concentrations are clearly lower, especially in summer, this means that a large fraction of this imported ozone is deposited to vegetation in continental Finland. In wintertime, when the ground is covered by snow and the deposition velocity is just as low as to the sea surface (Garland, 1983), the differences in ozone concentrations between Ut6 and J, htfiri are not large.
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The highest absolute ozone levels were observed in spring and summer at Ut6 (7(~80 ppb). The spring concentrations in Ahtfiri also showed very high values (45-55 ppb). The monthly mean concentration maxima were similar at both stations during the study period: 42 ppb on Ut6 and 41 ppb at J, ht~iri. The annual minima were similar from 1 year to the next, with about 21 ppb on Ut6 and 15 ppb at Xhtfiri. These yield very similar yearly amplitudes at each station of about 21 and 26 ppb, respectively. In Fig. 5 we present the monthly averages of daily maximum concentrations of ozone on Ut6 and at ~,ht~iri together with the mean values of ozone in the layer 800-500 hPa for the year 1989 at the Finnish sounding station of Sodankyl~i (67.5°N, 26.5°E). The concentrations in Sodankylfi have a maximum in April. We also present in Fig. 5 the annual trends of the maximum surface value at the Alpine site of Hohenpeissenberg (1162 m), together with values for the 700 hPa level from the same site (Logan, 1985). At Finnish stations we do not observe the symmetrical curves with respect to the summer maximum of radiation intensity typical of Hohenpeissenberg. Instead, we notice that the ozone monthly maxima at Finnish stations are well correlated with the corresponding
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Fig. 5. Mean monthly values of daily maximum ozone concentrations at Ut6 (asterisks) and J,htfiri (dashed line). Layer means (500-800 hPa) from the Sodankyl~isounding station (squares), as wellas surfacemaximum concentration (solid line) and 700 hPa values from Hohenpeissenberg(dashed~totted line) are also shown.
Transport, formation and sink processes behind surface ozone variability mean monthly values (in Fig. 3(b)), being a roughly constant excess of 5 ppb on Ut6 but with a 15% increase added to a similar constant excess of 4 ppb at Xht/iri. The mean daily maximum concentrations also are lower at .~ht/iri (see Fig. 5) due to efficient deposition to the forests. The ozone daily maxima at Ut8 and ~ht/iri are well correlated when the ground is covered by snow (r = 0.89). This correlation weakens when the snow melts away, with r =0.84 when less than half of the ground is covered by snow, and r = 0.61 when there is no snow. On UtS, the difference [O3max]--[O3] is rather fluctuating with a diffuse autumn maximum (9 ppb in September and November), while at Aht/iri this difference has a clear annual trend with a single maximum in May (13 ppb). This also points to the fact that both stations are globally under the same regime, but that on the regional scale, the processes at work are different. In contrast, at Hohenpeissenberg (48°N) the difference [O3max]--[O3] has a maximum of 15 ppb in August, although there is a second maximum in April of the same intensity. When considering the dependence between monthly mean values of [O3max] (see Fig. 5) and [ 0 3 ] (see Fig. 3(b)) at our stations (Ut6, Aht/iri) and in Giles (Tennessee, 35°N; Meagher et al., 1987), we noticed that each curve tends towards an excess of 5 ppb at zero ozone concentration. An interesting feature is the apparent hysteresis cycle, at least at these sites, with the branch corresponding to the first months of the year being on the lower side of maximum ozone values compared with the branch corresponding to the second half or so of the year. At Hohenpeissenberg (data from Logan (1985)), this feature is also visible but is very weak. This would mean that during the later period there is either more excess ozone production due to enhanced photochemistry and an increased
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level of precursors, or that mixing between the ABL and the free atmosphere above is activated. The second explanation is perhaps more plausible, since the summer season is characterized by strong surface buoyant fluxes and the autumn by strong wind speeds, both stirring up and thickening the ABL.
4. OZONE DIURNAL VARIATIONS
The diurnal variation of ozone is a quite wellknown phenomenon, described by Garland and Derwent (1979) and Galbally et al. (1986). At ,~ht/iri the ozone diurnal cycle displays the usual trend, with a maximum at noon and a minimum at night. On the other hand, the diurnal cycle is very weak at the island site of Ut8 (9-14 ppb) with a maximum in the evening (1800-2000 LT) and a morning minimum (Fig. 6). At ,~ht/iri, nights are frequently characterized by low wind speeds, producing ineffective mixing. Ozone is then removed to the vegetation and reacts with other compounds faster than it is transported downwards from aloft. In summer, the diurnal variation is slightly dependent on wind speed at both measuring sites, with a stronger diurnal amplitude at low wind speeds (Fig. 7) and slightly lower concentrations as the wind speed increases, especially during the day. We shall come back to this feature in section 5. In winter, when the diurnal concentration curve is fairly fiat, the wind speed does not affect the diurnal amplitude on Ut6, but at ~htfiri ozone levels are clearly higher at high wind speeds. This is probably due to the fact that on Ut6, where the sea is partially open upwind and moderate winds are rather persistent, buoyant mixing between the lower and upper layers of the ABL is quite efficient, so that wind mixing does not bring any significant additional contribution. At ~ht/iri, on the
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Fig. 6. The mean diurnal cycle of ozone concentrations at Ut6 and .~,ht/iriin autumn/winter and spring/summer. AE(A) 25:8-C
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other hand, persistent low winter inversions are a typical feature so that an increase in wind speed will raise and weaken the inversion, bringing ozone from aloft to the ground level. On Ut6, the summer diurnal amplitude is the largest (16 ppb) during north-easterlies and northwesterlies, although concentrations are lower, while the diurnal variation is smaller for the more polluted air masses of south-west--south-east winds. This means that there is not much local formation of ozone in already ozone-rich air formed during long-range transport. At Ahtfiri, the summer diurnal variation is slightly larger for wind direction sectors corresponding to the highest ozone concentrations, i.e. south-easterlies and south-westerlies, which should correspond to longrange transport situations. This opposite behaviour in summer at Ut6 and at ~htfiri indicates that between these two locations Finnish emissions could be sufficient to provide precursors for some ozone formation. Furthermore, enhanced summer mixing over continental Finland could be also responsible for the stronger diurnal ozone amplitude. In winter, both stations have their highest ozone concentrations during north-westerly winds (see section 7), but no obvious diurnal cycle is observed.
5. O Z O N E
CONCENTRATION
CORRELATIONS
WITH
TRANSPORT
According to the wind direction distribution, between 50 and 60% of the wind frequency on Ut6 is in the south-eastern and south-western sectors, so that chemical measurements will be significantly under the influence of anthropogenic sources. Considering the dependence of ozone concentrations on wind direction separately for each season (Fig. 8), we observe some similarities between the two sites of Ut6 and ,g,htfiri. Expectedly, the spring curves are the highest, especially at .~ht~iri, with a systematic 9-14 ppb overshoot above the summer curve for all wind directions. The spring and summer curves at ~,ht~ri are very similar, with an ozone level maximum during south and south-westerlies, while m i n i m u m levels occur in the northern sector in spring and summer. On Ut6 the spring curve is only 2-7 ppb above the summer curve, and maximum values occur in the south-south-east-eastern sector, whereas in summer the maximum is slightly shifted to the south-south-east sector. M i n i m u m ozone levels occur in the north-western sector during both spring and summer. During these clear dry warm seasons, air masses from European industrialized areas contribute
Transport, formation and sink processes behind surface ozone variability 50
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Fig. 8. Seasonal dependence of ozone concentrations on wind direction at Ut6 (a), and .~ht/iri (b).
to the formation of ozone, whereas clean air masses are ozone-poor. The enhanced ozone formation can be explained by favourable photochemical conditions during the transport of pollutants downwind of anthropogenic sources. The wind direction dependence is much stronger on Ut6 than at ~ht/iri with an approximate doubling of the amplitude, except in winter when it is similar. Winter is characterized at both stations by a relative independence on wind direction, although at both stations there seems to be a maximum for westerlies and north-westerlies and a minimum for southeasterlies. This latter feature is perhaps due to the consumption of ozone in the oxidation of NOx and SO 2 in south-easterly winds. The wet autumn season is also interesting since there is an approximate 90 ° phase-shift compared to the dry seasons, on Ut6 especially, with a marked maximum (westerlies) and minimum (north-easterlies). At ~,ht/iri, the minimum also occurs during north-easterlies but the maximum is more diffuse in the south-west-west-north-western sectors. These characteristics indicate that during the darker cold or wet seasons ozone sinks are predominant downwind of source areas. Air masses coming from the wind direction sector 120 220 ° enhanced ozone levels by about. 2ppb during the daytime and by 5 ppb at night. At ~ht/iri,
air masses from the sector 90-210 ° bring an additional contribution of 5 ppb in the daytime and 9 ppb at night. The comparative study of the dependence of ozone concentration on wind speed at the two sites is interesting because it demonstrates the influence of local conditions on ozone climatology. This is shown in Fig. 9 with a classification according to season. Ozone concentrations at Ut6 are practically independent of wind speed although there is a slight increasing trend, except in summer. It is relevant to notice also that under strong wind conditions, the spring curve is clearly above the others by about 19 ppb, the three other seasons being within 5 ppb. This asymptotic case of large wind speeds corresponds to maximum exchange between the ABL and the free troposphere aloft. The summer curve is interesting because 0 3 concentrations decrease under higher wind speeds at both stations (Fig. 9). Van Aalst (1989) obtained similar results in The Netherlands, with an increase of ozone concentrations with wind speed in winter and a decreasing trend in summer when the wind speed is higher than 5 ms-~. He argued that this is due to increased dilution of ozone formed within the ABL. On the other hand, we note that higher summer winds are generally connected, at these latitudes, with overpassing cloudy and moist low-pressure systems, thus decreasing the photochemical ozone production
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H. HAKOLAet al.
and/or increasing the ozone consumption in clouds (Lelieveld and Crutzen, 1990). An additional explanation on Ut6 could be the enhancement of the ozone reaction in the stirred sea surface layer with, e.g. iodine (Garland et al., 1980). Galbally and Roy (1980) have also reported a halving of the surface resistance as the water surface roughens. Parallel to the increased reaction rate in the water as wind waves are formed, another reason could be the increased effective surface for uptake with direct impingement on wave slopes. At ,~ht/iri, on the other hand, the decrease of 0 3 concentration with wind speed in summer is more problematic. It may be related to the fact that higher wind conditions correspond in general to a decrease in buoyant mixing inthe ABL, thus yielding a lowering of the ABL depth and a decrease in the entrainment effect at the top of the ABL.
6. OZONE FORMATION Ozone formation is controlled by the ratio of hydrocarbons and nitrogen oxides. We lack data on hydrocarbon concentrations in Finland. On the other hand, the daytime 0 3 concentrations tend to decrease with increasing NO2 concentrations. The highest ozone concentrations are measured when the NO 2 varies from 1 to 4 ppb in the daytime on Ut6, and is below 2 ppb at ~,htfiri. When NO 2 concentrations are above the mentioned values, ozone concentrations decrease. We present in Table 1 the seasonal correlation coefficients between ozone daily maxima and daily global radiation. The correlation is good at ,~htfiri, especially in summer, suggesting the possibility of local ozone formation in the forested area. At Ut6, on the other hand, there is no correlation in summer. We selected from the database situations with ozone concentrations higher than 50 ppb. The relative contribution of different wind direction classes to high ozone concentrations is shown in Fig. 10. At Ut6, the ozone episodes correspond clearly to European industrialized sources. At .~ht/iri, the high ozone concentrations are more evenly distributed, although the southern peak is also observed there. The diurnal amplitude (daily max-daily min; Fig. 11) showed no seasonal dependence on Ut6, but at ~,ht~ri the largest amplitudes (25-30 ppb) occur in late spring and summer, indicating local formation at that time. On these bases, we can say that at Ut6 the long-range
transport is mainly responsible for the high ozone concentrations, while at Aht~iri there is also the addition of locally-formed ozone.
7. OZONEDESTRUCTION We have studied the ozone loss L N at night at both stations, defined as the difference between the concentration at sunrise and at sunset. The mean monthly values of LN are shown in Fig. 12. At ~,ht/iri, ozone is removed efficiently from May until July (about 0.9 ppb h-l), while on Ut6 the maximum ozone loss occurred in July (0.7 ppbh-1). The ozone loss was greater at Aht/iri than on Ut6 in spring and early summer. At ~,ht/iri, August was an exceptional month, with ozone concentrations increasing through the night. In winter, practically no ozone was lost during night-time. The ozone loss depends on the state of the ground at both stations, as can be seen from Fig. 13, showing how LN values in spring and autumn depend on the presence of snow cover. We chose these two seasons in order to have the occurrence of both dry ground and snow. At ,~ht~iri the snow cover melts generally in April. We observe that only very little ozone is lost when at least half of the ground is covered by snow, when LN is about the same at both stations. On the other hand, the ozone loss rate increases when the amount of snow cover decreases, with a much greater rate of increase at Aht~iri than at Ut6. When the
summer
urO 30
[] spring
25 20 15 10 5 0
t .
30
60
90
.
.
.
.
120 ~50 180 210 wind direction (deg.)
.
240
270
500
330
360
% 35 AHTARI
~ summer
30
[] spring
25
(b)
20 15
Table 1. Seasonal correlation coefficients between daily global radiation and ozone daily maxima
10 5
,~,ht~ri
Ut6
0 30
Winter Spring Summer Autumn
- 0.03 0.60 0.72 0.61
0.23 0.62 0.23 0.40
60
90
120 150 180 210 240 wind direction (deg.)
270
300
330
360
Fig. 10. Relative contribution of different wind direction sectors to high ozone concentrations (above 50 ppb) at Ut6 (a), and ,~ht/iri (b).
1445
Transport, formation and sink processes behind surface ozone variability 40
AhtOri Uto
30
-
........
o_
0
20
10
0
I F
I kl
I A
I M
I d
I d
I A
I S
I 0
I N
I D
month
Fig. 11. Monthly means of diurnal amplitudes (daily maximum~laily minimum) of ozone concentrations at Ut6 and ~,ht~iri.
12
,' ,,'
"
Ut6 ',,
//
',
"/
i
Ahtbri . . . . . . . . .
i
~" o JC
8
6 Ct. © <~ ,x-
4
0
2
-2
I
I F
I M
I A
E M
I d
I d
1 A
I S
I 0
I N
month
Fig. 12. Average monthly ozone night-time loss rate at Ut6 and ~.ht/iri.
ground is dry, during spring and autumn nights, LN increases markedly and is twice as large at ~,ht/iri compared to Ut6 (0.8 vs 0.4 ppb h - ~) due to a higher deposition to vegetation. Ozone can also be destroyed by nitrogen dioxide and nitrogen oxide. The reaction between ozone and nitrogen oxide is faster, and would cause an increase in nitrogen dioxide concentrations. We show in Table 2 the seasonal variability of the correlation coefficient between [NO2] and [O3] calculated from hourly values for night-time situations (negative solar angle). Nitrogen dioxide and ozone are significantly negatively correlated from October to April at ~ht/iri and from November to March on Ut6, i.e. when NO 2 concentrations are at a maximum. This confirms the fact that chemistry acts as a sink for ozone in winter. In
summer, no correlation between ozone and NO 2 is observed at night.
8. C O N C L U S I O N S
Current levels of ozone at the measuring locations result either from stratospheric sources, local photochemistry in the troposphere or tropospheric chemistry elsewhere and long-range transport of products. According to Wege et al. (1989), the ozone variation coefficient (ratio of ozone standard deviation to mean concentration) is large close to the surface, very small in the middle troposphere and increasing again in the upper troposphere. This would indicate that the ozone variability near the surface would be mainly due
1446
H. HAKOLAet al. 0..9
Uto 0.8
AhtOri
0r"
o.7
........
--. 4
t~ o_ 0.6
<~
"'"-,..
0.5
"'"'-....
0.4
0.3
0.2 dry
snow
round
1 cover < 1 / 2
snow
I cover > 1 / 2
Fig. 13. Dependence of the ozone night-time loss rate on the state of the ground in spring and autumn at Ut6 and ~ht~iri.
Table 2. Monthly correlation coefficients between hourly ozone and nitrogen dioxide concentrations when the solar angle is negative (night situations) Month
Xhtfiri
Ut6
Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.
-0.47 - 0.69 - 0.47 -0.43 -0.10 -0.02 0.00 -0.04 -0.26 -0.50 -0.66 -0.78
-0.51 - 0.45 - 0.46 -0.14 -0.15 -0.16 -0.03 -0.11 -0.24 -0.22 -0.41 -0.42
to processes in the lower troposphere. Furthermore, a comprehensive search for the origin of ozone level rises at Ut6 during a 1-year period (1986-1987) showed only two, but these significant, spring episodes (15-30 ppb above the monthly mean) that could be attributed to stratospheric injections (Hakola et al., 1987). Our results from the analysis of 2-3 years' data consisting of ozone measurements at two background stations in Finland, i.e. characteristic of North European conditions, showed that this region is widely under the influence of air masses bringing along ozone formed in the downstream plumes of the main European industrialized areas. Ozone episodes are more frequent when the wind is from the main European source areas. These episodes are characterized by NO2 concentrations between 1 and 4 ppb. At ~ht/iri the daily global radiation is correlated with ozone
maxima in summer, indicating the possibility of local production there. The spring maximum is a c o m m o n feature of the annual ozone cycle, as at many other European stations. On the other hand, the summer maximum is only observed at the southernmost station which is the closest to potential precursor sources, and has an environment with little surface uptake. Deposition to the ground is an important sink for ozone, when there is no snow. However, when the ground is covered by snow, the reaction between ozone and nitrogen oxide is important. The diurnal cycle of ozone is strongly modulated by local meteorology, while background values are mainly determined by the air mass origin. This work is now continuing with the further addition of P A N and N O measurements. Our research activities will also concentrate on a more detailed analysis of the chemical reactions taking place and influencing the local ozone budget under specific meteorological conditions.
Acknowledgements--We are very grateful to our colleagues Pekka Plathan and Sisko Laurila for processing the raw data, and to Dr E. Kyr6 for providing us with his sounding data from Sodankyl/i. This work was initiated and partly funded through the Acidification project HAPRO of the Ministry of the Environment and the Ministry of Agriculture and Forestry, and partly by the Ministry of the Environment in the framework of the EUROTRAC/TOR project.
REFERENCES
van Aalst R. M. (1989) Ozone and oxidants in the planetary boundary layer. In Atmospheric Ozone Research and its Policy Implications (edited by Schneider T. et al.), pp. 573-587. Elsevier, Amsterdam.
Transport, formation and sink processes behind surface ozone variability Angell J. K. and Korshover J. (1981) Global variation in total ozone and layer mean ozone: an update through 1981. J. clim. appl. Met. 22, 1611-1626. Bojkov R. D. (1986) Surface ozone during the second half of the nineteenth century. J. clim. appl. Met. 25, 343-352. Crutzen P. (1974) Photochemical reactions initiated by and influencing ozone in unpolluted tropospheric air. Tellus 26, 47-56. Danielsen E. F. (1968) Stratospheric-tropospheric exchange based on radioactivity, ozone and potential vorticity. J. atmos. Sci. 2, 502 518. CCC (1977) EMEP manual for sampling and chemical analysis, EMEP/CHEM. 3/77, Norwegian Institute for Air Research, Lillestr6m. Environment Statistics (1987) Central Statistical Office of Finland, Helsinki. (ISBN 951-47-0920-9.) Feister U. and Warmbt W. (1985) Long-term measurements of surface ozone in the German Democractic Republic. J. atmos. Chem. 5, 1-21. Galbally I. E., Miller A. J., Hoy R. D., Ahmet S., Joynt R. C. and Attwood D. (1986) Surface ozone at rural sites in the Latrobe Valley and Cape Grim, Australia. Atmospheric Environment 20, 2403 2422. Galbally I. E. and Roy C. R. (1980) Destruction of ozone at the Earth's surface. Q. Jl R. met. Soc. 106, 599-620. Garland J. A. (1983) Principles of dry deposition: application to acidic species and ozone. VDI-Berichte 500, 83. Garland J. A. and Derwent R. G. (1979) Destruction at the ground and the diurnal cycle, of concentration of ozone and other gases. Q. Jl R. met. Soc. 105, 169-183. Garland J. A., Elzerman A. W. and Penkett S. A. (1980) The mechanism for dry deposition of ozone to seawater surfaces. J. geophys. Res. 85, 7488-7492. Grennfelt P., Hoem K., Saltbones J. and Schjoldager J. (1989) Oxidant data collection in OECD-Europe 1985-1987 (OXIDATE). Report on ozone, nitrogen dioxide and peroxyacetyl nitrate. October 1986-March 1987, AprilSeptember 1987 and October-December 1987. NILU 63/89. Grennfelt P., Saltbones J. and Schjoldager J. (1988) Oxidant data collection in OECD-Europe 1985-1987 (OXIDATE). Report on ozone, nitrogen dioxide and peroxyacetyl nitrate. October 1985-March 1986 and April-September 1986. NILU 31/88. Guicherit R. (1988) Ozone on an urban and regional scale. In Tropospheric Ozone, Regional and Global Scale Interactions (edited by Isaksen I. S. A.), NATO ASI Series C227, pp. 49-62. Reidel, Dordrecht. Guicherit R. and van Dop H. (1977) Photochemical production of ozone in Western Europe (1971-1975) and its relation to meteorology. Atmospheric Environment II, 145-155. Hakola H., Taalas P., L/ittil~i H., "Joffre S. and Plathan P. (1987) Short and long-range variations of ozone in Finland based on one-year data. Finnish Meteorological Inst.
1447
Reports 1987:6 ( in Finnish). Joffre S. M., L/ittil~i H., Hakola H., Taalas P. and Plathan P. (1988) Measurements of ozone at two background stations in Finland. In Tropospheric Ozone, Regional and Global Scale Interactions (edited by Isaksen I. S. A.), NATO ASI Series C227, pp. 137-147. Reidel, Dordrecht. Johnson W. B. and Viezee W. (1981) Stratospheric ozone in the lower troposphere. Presentation and interpretation of aircraft measurements. Atmospheric Environment 15, 13091323. Kley D., Volz A. and M/ilheims F. (1988) Ozone measurements in historic perspective. In Tropospheric Ozone, Regional and Global Scale Interactions (edited by Isaksen I. S. A.), NATO ASI Series C227, pp. 63-72. Reidel, Dordrecht. Lelieveld J. and Crutzen P. J. (1990) Influences of cloud photochemical processes on tropospheric ozone. Nature 343, 227-233. Logan J. A. (1985) Tropospheric ozone: seasonal behavior, trends, and anthropogenic influence. J. geophys. Res. 90D6, 10,463-10,482. Meagher J. F., Lee N. T., Valente R. J. and Parkhurst W, J. (1987) Rural ozone in the southeastern United States. Atmospheric Environment 21, 605-615. Oyola P. and Areskoug H. (1988) Nordic intercomparison of ozone and nitrogen oxide. National Swedish Environmental Protection Board Report 3422. (ISBN 91-6203422-7.) Penkett S. A. (1989) Trends in atmospheric trace gases. In Atmospheric Ozone Research and its Policy Implications (edited by Schneider T. et al.), pp. 159 166. Elsevier, Amsterdam. Penkett S. A. and Brice K. A. (1986) The spring maximum in photo-oxidants in the Northern Hemisphere troposphere. Nature 319, 655-657. Punkkinen R. K. (1988) Automatic colorimetric detector for traces of nitrogen dioxide in air. Rev. Sci. Instrum. 59, 163-166. Schneider T., Lee S. D. and Grant L. D. (Eds.) (1989) Atmospheric Ozone Research and its Policy Implications. Studies in Environmental Sciences 35. Elsevier, Amsterdam. U.K. Review Group (1987) Ozone in the United Kingdom. United Kingdom Photochemical oxidants Review Group interim Report. Department of the Environment. Volz A. and Kley D. (1988) Evaluation of the Montsouris series of ozone measurements made in the nineteenth century. Nature 332, 240-242. Wege K., Claude H. and Hartmannsgruber R. (1989) Several results from 20 years of ozone observations at Hohenpeissenberg. In Ozone in the Atmosphere (edited by Bojkov R. D. and Fabian P.). Proceedings of the Quadrennial Ozone Symposium, G6ttingen (F.R.G.), 8-13 August 1988, pp. 109-112. A. Deepak, Hampton, U.S.A.
0004 6981/91 $3.00+0.00 :i~" 1991 Pergamon Press plc
TRANSPORT, FORMATION AND SINK PROCESSES BEHIND SURFACE OZONE VARIABILITY IN NORTH EUROPEAN CONDITIONS H. HAKOLA, S. JOFFRE,* H. L,~TTILA a n d P. TAALAS Finnish Meteorological Institute, Air Quality Department, Sahaajankatu 22E, SF-00810 Helsinki, Finland
(First received 19 April 1990 and in final form 11 October 1990) Abstraet Measurements of ozone, nitrogen dioxide and meteorological parameters at the two Finnish EMEP background stations of,~ht/iri (forested site) and Ut6 (an offshore island) show clear indications of the influence of the precursor source areas of Western and Eastern Europe on surface ozone behaviour at these higher latitudes. The mean ozone levels are relatively high, with maximum monthly values of 41 and 42 ppb, respectively. These values occur in April at both sites nearly irrespectively of wind direction, pointing to a global feature. Other spring, as well as summer and autumn, months have lower ozone values for wind direction sectors corresponding to clean air masses (north-westerlies and north-easterlies). Surface uptake is an important sink in the local ozone budget, especially during late spring, summer and early autumn. Chemical losses are more efticient in winter, when the ground is covered by snow.
Key word index: Background ozone, long-range transport, surface deposition, boundary layer variability, ozone precursors. I. INTRODUCTION The potentially adverse effects of ozone on forests, crops and h u m a n health (e.g. Schneider et al., 1989), as well as the increasing trend of tropospheric ozone c o n c e n t r a t i o n s (Angell a n d Korshover, 1983; Bojkov, 1986), have triggered m o n i t o r i n g efforts in various countries in order to u n d e r s t a n d a n d predict its behaviour in various environments. A l t h o u g h the main a t t e n t i o n has been paid to ozone as a secondary product from a n t h r o p o g e n i c emissions, it has also a b a c k g r o u n d c o m p o n e n t whose 'clean pre-industrialized' value depends o n various geographical, chemical a n d physical parameters a n d is still subject to debate (e.g. Kley et al., 1988). O x i d a t i o n reactions which either produce or destroy ozone, depending on the nitrogen oxide concentrations, can take place globally in the t r o p o s p h e r e by the mechanism proposed by Crutzen (1974), a n d also regionally near source areas. Photochemical smog was detected in Europe in the 1970s (Guicherit a n d van Dop, 1977). Wind patterns over n o r t h e r n Europe favour southwesterlies, especially in winter, so that in the case of Finland, high ozone episodes, mainly originating in E u r o p e a n industrial areas, are t r a n s p o r t e d over the Baltic Sea where dry deposition is weak (Joffre et al., 1988). F u r t h e r downwind, however, ozone can be efficiently deposited upon the extended Finnish forests. Finland's own sparsely distributed emissions can interfere with these air masses, so that ozone can also be built up locally. Moreover, as F i n l a n d is mainly covered by forests (70% of the country, of which 90% are coniferous; E n v i r o n m e n t Statistics, * To whom correspondence should be addressed.
1987), natural h y d r o c a r b o n emissions are likely to have an effect on ozone levels. We shall report here on m e a s u r e m e n t s of ozone performed at two Finnish b a c k g r o u n d stations, one in a maritime e n v i r o n m e n t in the south-western archipelago, a n d the other in Central F i n l a n d in a forested environment. The measurements will be interpreted in the light of c o n c o m i t a n t chemical (nitrogen dioxide) a n d meteorological m e a s u r e m e n t s (wind, temperature, radiation a n d humidity), in order to assess a n d u n d e r s t a n d the variability a n d b e h a v i o u r of ozone concentrations comparatively at these two stations. O u r data are also included in the j o i n t E u r o p e a n O E C D project O X I D A T E compiled by Grennfelt et al. (1986, 1987), which shows that the ozone diurnal curves on U t 6 are very similar in level a n d shape to the c o r r e s p o n d i n g curve at the other maritime station of Westerland (F.R.G.). 2. MEASUREMENTS AND DATA The Finnish Meteorological Institute started ozone measurements in 1985 on the island of Ut6 (59°47'N, 21°2YE; altitude 7 m), located about 80km south-west of mainland Finland. The location of this small rocky treeless island on the southern margin of the ,~land archipelago is favourable for picking up situations with air masses unaffected by anthropogenic emission for hundreds of kilometres (Fig. 1). The reported data cover the period from February 1986 to December 1988. Ozone measurements were started in summer 1986 at a site in Aht/iri in Central Finland (62°33'N, 24°13'E; altitude 162 m). The area is densely forested but with some cultivated clearings. The reported data extended from September 1986 to December 1988. Ozone measurements were performed using an ozone monitor of type 1003-RS, manufactured by Environment SA under DASIBI licence. It is based on u.v.-radiation absorp-
1437
1438
H. HAKOLA et al.
tion by ozone at 240 nm. Both sensors used at Ut6 and ~.ht/iri have been intercalibrated against other sensors used in the Nordic countries, so that results are intercomparable (Oyola and Areskoug, 1988). The measurements were l-h averages with an accuracy better than 2 ppb. At Ut6, the presence of a diesel generator (at 50 m on the northern side of the monitor) introduced some perturbations into the ozone records from time to time. These effects were eliminated by deleting ozone data having a relative standard deviation (calculated from the 1-min samples within 1 h) larger than
14%. This threshold corresponded best to those situations when the generator was upwind of the sampler and when NO 2 concentrations were also very high and fluctuating. Measurements of NO 2 were performed using an automated version of the Salzman method. The resolution is 0.1 ppb for 1-h sampling periods. In the Salzman method, the air sample is bubbled through the reagent (14% acetic acid, 0.5% sulphanilic acid, 0.002% N(1-naphthyl)-ethylenediamine dihydrochloride and distilled water), in which the absorbed NO2 forms an azo dye. The amount of absorbed NO2 is determined photometrically from the transmittance of light by the reagent at a wavelength of 550 nm (Punkkinen, 1988). The Nordic intercalibration of NO2-sensors (Oyola and Areskoug, 1988) did not allow firm conclusions to be drawn, due to the large NO 2 concentrations used and the wide scatter between the compared measuring methods. The NO2 measurements cover the period from July 1986 to December 1988 at Ut6 and from February 1987 to December 1988 at ,g,ht~iri. Both sites are EMEP-stations with the standard chemical measurement protocol (CCC, 1977). Additionally, automatic wind and global radiation measurements are performed in the close vicinity of the chemical samplings, providing data every hour. Other meteorological observations (temperature and humidity) are carried out every 3 h. The marine site of Ut6 is a station of the EUROTRAC/TOR network also. The frequency distribution of ozone hourly values is shown in Fig. 2. On Ut6 the hourl.y, values are concentrated in the range 20-40 ppb, while at Aht/iri the distribution is more peaked and the concentrations are mainly between 20-30 ppb. We notice also the lack of low values on Ut6 due to low deposition to the sea surface.
3. OZONE ANNUAL VARIATIONS 3.1. M e a n concentrations
Fig. 1. Map.of Northern Europe with the two Finnish background stations and the sounding station of Sodankyl/i.
W e p r e s e n t in Fig. 3(a) the a n n u a l cycle o f global r a d i a t i o n t o g e t h e r with c o n c e n t r a t i o n s of o x i d a n t s O x = 0 3 + N O 2 a n d n i t r o g e n dioxide at o u r t w o sites: U t 6 ( m a r i n e site) a n d ~ h d i r i (forested site). In Fig. 3(b), we c o m p a r e the m e a n m o n t h l y values o f o z o n e conc e n t r a t i o n s at o u r stations with o t h e r sites. T h e a n n u a l cycle o f the m o n t h l y m e a n o x i d a n t O x level has
%
20 15 I
0
~i Ute ~] Aht~ri
5
10 15 20 2.5 30 35 40 45 50 55 60 65 70 75 80 85 0 3 (ppb)
Fig. 2. Frequency distribution of hourly ozone concentrations on Ut6 and at ,g,ht~iri.
Transport, formation and sink processes behind surface ozone variability 7000
50
6ooo 40
~7 E
5ooo
~ 30
4000
°-
3000
~
a.
E2o [2:1.. .. :.
.. .. '..~ ....
zX
lo
2000 -~ O
0
F
M
A
M
J
d
A
S
0
N
O
month
Fig. 3(a). Mean monthly values of oxidant levels Ox=O 3 +NO 2 (solid line), daily global radiation (dotted line), and NO2 concentrations (dashed line) on Ut6 (squares) and at ,~ht/iri (triangles).
50
..... ..................
40
~ 30 E 20 10
I F
I M
I A
I I M J
I
I J
I A
I S
I O
N
D
month
Fig. 3(b). Mean monthly values of ozone concentrations at Ut6 (squares) and ~,ht/iri (triangles) together with the annual trend at the U.K. station of Bottesford (asterisks), and three annual trends reported by Logan (1985): in the Eastern U.S. (dashed line), at two elevated European and U.S. stations (dotted line), and an estimate for Canadian stations (hatched area). roughly the same annual amplitude as the ozone levels: 0.5 ppb less in Ut6 and 1.5 ppb less in Xht/iri. This indicates that the observed levels of NO2, mainly local due to its short lifetime, are not the main contributor to the relatively high ozone levels observed at our stations, but rather that ozone has been transported to the receptor points. Moreover, the seasonal trend of global radiation is not in phase with the ozone concentration curves. We observe that the maximum mean monthly concentrations occur either only in spring (March-April) at ~ht/iri or in both spring and summer (March-July) at Utr. A weak summer peak was also observed in ,~ht/iri in 1988, but not in 1987. The 1987 temperatures from June to August were about 3-5°C lower than in 1988, which could explain the lack of a summer maximum in 1987. We have also shown in Fig. 3(b) the annual trend of daily averaged surface ozone at other mid-latitude sites in the Northern Hemisphere as follows: (a) the
1439
mean annual trend from nine sites in the Eastern U.S.; (b) an estimate from two sites in Canada; (c) the mean of two high altitude ozone-level sites in the U.S. (Missouri) and Europe (Hohenpeissenberg); and (d) the monthly mean 0 3 concentrations at the rural site of Bottesford (Leicestershire, U.K.) taken from the U.K. Review Group's (1987) report. Curves (a), (b) and (c) are from Logan (1985). We note that the Ut6 curve best follows the Eastern U.S. and high Europe-U.S. curves with an extended spring-summer maximum (March-July), while the ~,ht~iri curve, characterized by a single spring maximum (April), is very close to the curve for the Canadian sites. The Bottesford curve is close to the Xht/iri curve except in spring, when Aht/iri's values are clearly higher due to the presence of the snow cover implying a slow deposition. The spring peak, which has also been observed elsewhere (Logan, 1985; Bojkov, 1986; Feister and Warmbt, 1985; Guicherit, 1988), has been previously attributed to intrusion of stratospheric ozone-rich air, that should be more frequent in spring (Danielsen, 1968; Johnson and Viezee, 1981). However, these stratospheric intrusions are also frequent in other seasons, and the recently re-analysed Montsouris data by Volz and Kley (1988) showed that in early industrial times the ozone annual cycle had only a weak and diffuse maximum from February to May, suggesting that the marked spring maximum has another origin. Due to the relative short lifetime of ozone in the troposphere (1 month), and since this strong maximum has been observed at various background stations of the Northern Hemisphere (Logan, 1985; Guicherit, 1988), it should have a global origin corresponding to the tropospheric production from precursor compounds accumulated in the troposphere in winter as first suggested by Penkett and Brice (1986). Peroxyacetylnitrate (PAN) concentrations, which have only a chemical origin in the troposphere, are noticed to peak in spring (Penkett, 1989), suggesting that the spring ozone maximum may be also due to chemistry, since PAN and ozone are both formed chemically from a number of hydrocarbons. Both Finnish stations can be classified as background stations, and thus have this spring maximum. When we differentiate the annual trend with respect to wind direction sectors, the ozone concentrations are noticed to be clearly higher under wind directions corresponding to European source areas from March to August-September (Fig. 4). We have used here the local wind direction although the wind direction from a higher layer or air trajectory sectors would be more recommendable. Nevertheless, there is a strong correlation between the surface wind direction and the mean boundary layer wind direction, the latter being veered by about 25 ° from the former. The ozone annual cycle, both in the atmospheric boundary layer (ABL) under the close influence of precursors' emissions (NOx, CO and volatile organic compounds), and also above the ABL is characterized
1440
HI HAKOLAet al.
by a summer maximum (Logan, 1985). The summer peak as observed in the Ut6 curve has been related to regional photo-chemical production under the influence of air pollution (Guicherit, 1983; Penkett, 1989). Consequently, this marine background station is within the reach of the industrialized European areas. This fact is helped by the presence of the sea, where ozone
50
AHTARI
has a low deposition rate, upwind of the station. On the other hand, since in ~htfiri (downward of Ut6) observed concentrations are clearly lower, especially in summer, this means that a large fraction of this imported ozone is deposited to vegetation in continental Finland. In wintertime, when the ground is covered by snow and the deposition velocity is just as low as to the sea surface (Garland, 1983), the differences in ozone concentrations between Ut6 and J, htfiri are not large.
(a) II
~
~
3.2. Average maximum concentrations
III~
4o
/ ,
The highest absolute ozone levels were observed in spring and summer at Ut6 (7(~80 ppb). The spring concentrations in Ahtfiri also showed very high values (45-55 ppb). The monthly mean concentration maxima were similar at both stations during the study period: 42 ppb on Ut6 and 41 ppb at J, ht~iri. The annual minima were similar from 1 year to the next, with about 21 ppb on Ut6 and 15 ppb at Xhtfiri. These yield very similar yearly amplitudes at each station of about 21 and 26 ppb, respectively. In Fig. 5 we present the monthly averages of daily maximum concentrations of ozone on Ut6 and at ~,ht~iri together with the mean values of ozone in the layer 800-500 hPa for the year 1989 at the Finnish sounding station of Sodankyl~i (67.5°N, 26.5°E). The concentrations in Sodankylfi have a maximum in April. We also present in Fig. 5 the annual trends of the maximum surface value at the Alpine site of Hohenpeissenberg (1162 m), together with values for the 700 hPa level from the same site (Logan, 1985). At Finnish stations we do not observe the symmetrical curves with respect to the summer maximum of radiation intensity typical of Hohenpeissenberg. Instead, we notice that the ozone monthly maxima at Finnish stations are well correlated with the corresponding
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/1
deg ..........
270-360 deg. _ _ I I I I M A M d
I J
I A
I S
I 0
I N
| t ] I/ O
S
0
N
D
month
50
UTO ,,,.;,
.~ ,,.
,o 20 O 9 0 - 1 8 0 de 9 . . . . . . . 10
180 270 de 9. 2 7 0 - 3 6 0 deg. d
F
M
A
M
_
_
d
J
A
month
Fig. 4. Mean monthly values of ozone concentrations at (a) ,~ht/iri, and (b) Ut6 according to wind direction sectors.
80
7O .I.. • ,,
60
..J/ I
i
I..
m..
-.•l
~ - ~ .
-..
U~
........
ao
..............................
0
J
F
2
I
I
I
I
I
I
I
I
I
I
M
A
M
d
d
A
S
O
N
D
Month
Fig. 5. Mean monthly values of daily maximum ozone concentrations at Ut6 (asterisks) and J,htfiri (dashed line). Layer means (500-800 hPa) from the Sodankyl~isounding station (squares), as wellas surfacemaximum concentration (solid line) and 700 hPa values from Hohenpeissenberg(dashed~totted line) are also shown.
Transport, formation and sink processes behind surface ozone variability mean monthly values (in Fig. 3(b)), being a roughly constant excess of 5 ppb on Ut6 but with a 15% increase added to a similar constant excess of 4 ppb at Xht/iri. The mean daily maximum concentrations also are lower at .~ht/iri (see Fig. 5) due to efficient deposition to the forests. The ozone daily maxima at Ut8 and ~ht/iri are well correlated when the ground is covered by snow (r = 0.89). This correlation weakens when the snow melts away, with r =0.84 when less than half of the ground is covered by snow, and r = 0.61 when there is no snow. On UtS, the difference [O3max]--[O3] is rather fluctuating with a diffuse autumn maximum (9 ppb in September and November), while at Aht/iri this difference has a clear annual trend with a single maximum in May (13 ppb). This also points to the fact that both stations are globally under the same regime, but that on the regional scale, the processes at work are different. In contrast, at Hohenpeissenberg (48°N) the difference [O3max]--[O3] has a maximum of 15 ppb in August, although there is a second maximum in April of the same intensity. When considering the dependence between monthly mean values of [O3max] (see Fig. 5) and [ 0 3 ] (see Fig. 3(b)) at our stations (Ut6, Aht/iri) and in Giles (Tennessee, 35°N; Meagher et al., 1987), we noticed that each curve tends towards an excess of 5 ppb at zero ozone concentration. An interesting feature is the apparent hysteresis cycle, at least at these sites, with the branch corresponding to the first months of the year being on the lower side of maximum ozone values compared with the branch corresponding to the second half or so of the year. At Hohenpeissenberg (data from Logan (1985)), this feature is also visible but is very weak. This would mean that during the later period there is either more excess ozone production due to enhanced photochemistry and an increased
1441
level of precursors, or that mixing between the ABL and the free atmosphere above is activated. The second explanation is perhaps more plausible, since the summer season is characterized by strong surface buoyant fluxes and the autumn by strong wind speeds, both stirring up and thickening the ABL.
4. OZONE DIURNAL VARIATIONS
The diurnal variation of ozone is a quite wellknown phenomenon, described by Garland and Derwent (1979) and Galbally et al. (1986). At ,~ht/iri the ozone diurnal cycle displays the usual trend, with a maximum at noon and a minimum at night. On the other hand, the diurnal cycle is very weak at the island site of Ut8 (9-14 ppb) with a maximum in the evening (1800-2000 LT) and a morning minimum (Fig. 6). At ,~ht/iri, nights are frequently characterized by low wind speeds, producing ineffective mixing. Ozone is then removed to the vegetation and reacts with other compounds faster than it is transported downwards from aloft. In summer, the diurnal variation is slightly dependent on wind speed at both measuring sites, with a stronger diurnal amplitude at low wind speeds (Fig. 7) and slightly lower concentrations as the wind speed increases, especially during the day. We shall come back to this feature in section 5. In winter, when the diurnal concentration curve is fairly fiat, the wind speed does not affect the diurnal amplitude on Ut6, but at ~htfiri ozone levels are clearly higher at high wind speeds. This is probably due to the fact that on Ut6, where the sea is partially open upwind and moderate winds are rather persistent, buoyant mixing between the lower and upper layers of the ABL is quite efficient, so that wind mixing does not bring any significant additional contribution. At ~ht/iri, on the
60
50 Uto, sprin g - s u mmer, k ~ ~ " 4O o_ Q_
........................................................................
..... .........................................................
"
~
.......
o ~ 30
............................ ' ~ K " Uto, a u t u m n - w i n t e r
20
Ahtari, a u t u m n - w l n t e r 10
t
I
3
I
1
I
6
I
I
1
9
1
I
I
I
12 hours
I
I
15
I
I
1
18
I
I
I
21
I
I
24-
Fig. 6. The mean diurnal cycle of ozone concentrations at Ut6 and .~,ht/iriin autumn/winter and spring/summer. AE(A) 25:8-C
1442
H. HAKOLAet al.
50
50 AhtSri,
summer
r/
F
(a)
Ut6, s u m m e r
(c)
40
..........- . - - < 7 ~- - . ,
30
3O
20
2o
o_ v
wind speed < 2 m / s 10
2 m/s
< wind speed <4- m / s
wind speed > 4 m / s 0
I ,.~I I
I 61 t
I 91 I
I i[2 I
I 1[5 I
wind speed < 5 m / s - -
3rn/s
10
...... ....... [ 181 I
I 211 I
I
I
0
i 31 L i
6J i
, 9; t
--
< wind speed < 6 m / s
....
wind speed > 6 m / s
....
i 121 i
I 115 I
I 118 t
I 2/1 I
I
I
hours
hours
40
5O Ut6. winter
AhtOri, winter
(d)
(b) 4O
3O
/..
vo_
~ 3o 20
o~ 2o wind speed < 2 m / s 2 m/s 0
I
[ [ .3
< wind speed < 4 m / s I
wind speed < 3 m / s
- -
10
......
wind speed > 4 m / s .... i i I I I I /1 Z I 115 [ 6 9 2 hours
3m/s
< wind speed < 6 r n / s
--....
wind speed > 6 m / s I
i i 18
i
i i 21
i
0
131
i
I 61 I
I 91 I
1112 I I 115 I
I.~118
I 2J1 I
I
I
hours
Fig. 7. Diurnal variation of ozone at ,~ht~triin: (a) summer and (b) winter, as well as at Ut6 in (c) summer and in (d) winter, for different wind speeds.
other hand, persistent low winter inversions are a typical feature so that an increase in wind speed will raise and weaken the inversion, bringing ozone from aloft to the ground level. On Ut6, the summer diurnal amplitude is the largest (16 ppb) during north-easterlies and northwesterlies, although concentrations are lower, while the diurnal variation is smaller for the more polluted air masses of south-west--south-east winds. This means that there is not much local formation of ozone in already ozone-rich air formed during long-range transport. At Ahtfiri, the summer diurnal variation is slightly larger for wind direction sectors corresponding to the highest ozone concentrations, i.e. south-easterlies and south-westerlies, which should correspond to longrange transport situations. This opposite behaviour in summer at Ut6 and at ~htfiri indicates that between these two locations Finnish emissions could be sufficient to provide precursors for some ozone formation. Furthermore, enhanced summer mixing over continental Finland could be also responsible for the stronger diurnal ozone amplitude. In winter, both stations have their highest ozone concentrations during north-westerly winds (see section 7), but no obvious diurnal cycle is observed.
5. O Z O N E
CONCENTRATION
CORRELATIONS
WITH
TRANSPORT
According to the wind direction distribution, between 50 and 60% of the wind frequency on Ut6 is in the south-eastern and south-western sectors, so that chemical measurements will be significantly under the influence of anthropogenic sources. Considering the dependence of ozone concentrations on wind direction separately for each season (Fig. 8), we observe some similarities between the two sites of Ut6 and ,g,htfiri. Expectedly, the spring curves are the highest, especially at .~ht~iri, with a systematic 9-14 ppb overshoot above the summer curve for all wind directions. The spring and summer curves at ~,ht~ri are very similar, with an ozone level maximum during south and south-westerlies, while m i n i m u m levels occur in the northern sector in spring and summer. On Ut6 the spring curve is only 2-7 ppb above the summer curve, and maximum values occur in the south-south-east-eastern sector, whereas in summer the maximum is slightly shifted to the south-south-east sector. M i n i m u m ozone levels occur in the north-western sector during both spring and summer. During these clear dry warm seasons, air masses from European industrialized areas contribute
Transport, formation and sink processes behind surface ozone variability 50
1443
60
UTO
(a)
uro
(a)
50
4O
30 r
.....
/. . . . . . . . . . . . .
40
....-.....
30
O-
0
r.~:.~/i ~ ..........................................
20
20
winter 10
spring summ~ I
I
I
155-180 wind
0
I
902135 1801225 2701515
45-90
10
...... ........ -- --
autumn
00-45
winter spring summer autumn
- -
i
i
3
4
i
5
i
i
6
7
wind
i
i
8
9
110 l l l
- -
...... ........ -- --
112 113 tl4
115
(m/s)
speed
315-360
225-270
dVection 6O
50
5O
(b)
40
[3_ [3_
................... "'""-
30
............ " " " I
I
& 4-0
...............""%
o~
~----
--winter
~ ~ .
00-45
/
i
~-90
i
q
spring summer autumn
- -
spring summer
...... ........
autumn
-
-i
~0"~5 ;80'-22~ 270'-315 135-180 225-270 315-~60 wind
3o
2O
1
20 10
(b) t
AHTARI
AHTARI
direction
10 0
1
I
I
2
3
4-
wind
speed
__.
I
5
]
---
.... 6
I 7
(m/s)
Fig. 9. Seasonal dependence of ozone concentrations on wind speed at Ut6 (a), and A.ht/iri(b).
Fig. 8. Seasonal dependence of ozone concentrations on wind direction at Ut6 (a), and .~ht/iri (b).
to the formation of ozone, whereas clean air masses are ozone-poor. The enhanced ozone formation can be explained by favourable photochemical conditions during the transport of pollutants downwind of anthropogenic sources. The wind direction dependence is much stronger on Ut6 than at ~ht/iri with an approximate doubling of the amplitude, except in winter when it is similar. Winter is characterized at both stations by a relative independence on wind direction, although at both stations there seems to be a maximum for westerlies and north-westerlies and a minimum for southeasterlies. This latter feature is perhaps due to the consumption of ozone in the oxidation of NOx and SO 2 in south-easterly winds. The wet autumn season is also interesting since there is an approximate 90 ° phase-shift compared to the dry seasons, on Ut6 especially, with a marked maximum (westerlies) and minimum (north-easterlies). At ~,ht/iri, the minimum also occurs during north-easterlies but the maximum is more diffuse in the south-west-west-north-western sectors. These characteristics indicate that during the darker cold or wet seasons ozone sinks are predominant downwind of source areas. Air masses coming from the wind direction sector 120 220 ° enhanced ozone levels by about. 2ppb during the daytime and by 5 ppb at night. At ~ht/iri,
air masses from the sector 90-210 ° bring an additional contribution of 5 ppb in the daytime and 9 ppb at night. The comparative study of the dependence of ozone concentration on wind speed at the two sites is interesting because it demonstrates the influence of local conditions on ozone climatology. This is shown in Fig. 9 with a classification according to season. Ozone concentrations at Ut6 are practically independent of wind speed although there is a slight increasing trend, except in summer. It is relevant to notice also that under strong wind conditions, the spring curve is clearly above the others by about 19 ppb, the three other seasons being within 5 ppb. This asymptotic case of large wind speeds corresponds to maximum exchange between the ABL and the free troposphere aloft. The summer curve is interesting because 0 3 concentrations decrease under higher wind speeds at both stations (Fig. 9). Van Aalst (1989) obtained similar results in The Netherlands, with an increase of ozone concentrations with wind speed in winter and a decreasing trend in summer when the wind speed is higher than 5 ms-~. He argued that this is due to increased dilution of ozone formed within the ABL. On the other hand, we note that higher summer winds are generally connected, at these latitudes, with overpassing cloudy and moist low-pressure systems, thus decreasing the photochemical ozone production
1444
H. HAKOLAet al.
and/or increasing the ozone consumption in clouds (Lelieveld and Crutzen, 1990). An additional explanation on Ut6 could be the enhancement of the ozone reaction in the stirred sea surface layer with, e.g. iodine (Garland et al., 1980). Galbally and Roy (1980) have also reported a halving of the surface resistance as the water surface roughens. Parallel to the increased reaction rate in the water as wind waves are formed, another reason could be the increased effective surface for uptake with direct impingement on wave slopes. At ,~ht/iri, on the other hand, the decrease of 0 3 concentration with wind speed in summer is more problematic. It may be related to the fact that higher wind conditions correspond in general to a decrease in buoyant mixing inthe ABL, thus yielding a lowering of the ABL depth and a decrease in the entrainment effect at the top of the ABL.
6. OZONE FORMATION Ozone formation is controlled by the ratio of hydrocarbons and nitrogen oxides. We lack data on hydrocarbon concentrations in Finland. On the other hand, the daytime 0 3 concentrations tend to decrease with increasing NO2 concentrations. The highest ozone concentrations are measured when the NO 2 varies from 1 to 4 ppb in the daytime on Ut6, and is below 2 ppb at ~,htfiri. When NO 2 concentrations are above the mentioned values, ozone concentrations decrease. We present in Table 1 the seasonal correlation coefficients between ozone daily maxima and daily global radiation. The correlation is good at ,~htfiri, especially in summer, suggesting the possibility of local ozone formation in the forested area. At Ut6, on the other hand, there is no correlation in summer. We selected from the database situations with ozone concentrations higher than 50 ppb. The relative contribution of different wind direction classes to high ozone concentrations is shown in Fig. 10. At Ut6, the ozone episodes correspond clearly to European industrialized sources. At .~ht/iri, the high ozone concentrations are more evenly distributed, although the southern peak is also observed there. The diurnal amplitude (daily max-daily min; Fig. 11) showed no seasonal dependence on Ut6, but at ~,ht~ri the largest amplitudes (25-30 ppb) occur in late spring and summer, indicating local formation at that time. On these bases, we can say that at Ut6 the long-range
transport is mainly responsible for the high ozone concentrations, while at Aht~iri there is also the addition of locally-formed ozone.
7. OZONEDESTRUCTION We have studied the ozone loss L N at night at both stations, defined as the difference between the concentration at sunrise and at sunset. The mean monthly values of LN are shown in Fig. 12. At ~,ht/iri, ozone is removed efficiently from May until July (about 0.9 ppb h-l), while on Ut6 the maximum ozone loss occurred in July (0.7 ppbh-1). The ozone loss was greater at Aht/iri than on Ut6 in spring and early summer. At ~,ht/iri, August was an exceptional month, with ozone concentrations increasing through the night. In winter, practically no ozone was lost during night-time. The ozone loss depends on the state of the ground at both stations, as can be seen from Fig. 13, showing how LN values in spring and autumn depend on the presence of snow cover. We chose these two seasons in order to have the occurrence of both dry ground and snow. At ,~ht~iri the snow cover melts generally in April. We observe that only very little ozone is lost when at least half of the ground is covered by snow, when LN is about the same at both stations. On the other hand, the ozone loss rate increases when the amount of snow cover decreases, with a much greater rate of increase at Aht~iri than at Ut6. When the
summer
urO 30
[] spring
25 20 15 10 5 0
t .
30
60
90
.
.
.
.
120 ~50 180 210 wind direction (deg.)
.
240
270
500
330
360
% 35 AHTARI
~ summer
30
[] spring
25
(b)
20 15
Table 1. Seasonal correlation coefficients between daily global radiation and ozone daily maxima
10 5
,~,ht~ri
Ut6
0 30
Winter Spring Summer Autumn
- 0.03 0.60 0.72 0.61
0.23 0.62 0.23 0.40
60
90
120 150 180 210 240 wind direction (deg.)
270
300
330
360
Fig. 10. Relative contribution of different wind direction sectors to high ozone concentrations (above 50 ppb) at Ut6 (a), and ,~ht/iri (b).
1445
Transport, formation and sink processes behind surface ozone variability 40
AhtOri Uto
30
-
........
o_
0
20
10
0
I F
I kl
I A
I M
I d
I d
I A
I S
I 0
I N
I D
month
Fig. 11. Monthly means of diurnal amplitudes (daily maximum~laily minimum) of ozone concentrations at Ut6 and ~,ht~iri.
12
,' ,,'
"
Ut6 ',,
//
',
"/
i
Ahtbri . . . . . . . . .
i
~" o JC
8
6 Ct. © <~ ,x-
4
0
2
-2
I
I F
I M
I A
E M
I d
I d
1 A
I S
I 0
I N
month
Fig. 12. Average monthly ozone night-time loss rate at Ut6 and ~.ht/iri.
ground is dry, during spring and autumn nights, LN increases markedly and is twice as large at ~,ht/iri compared to Ut6 (0.8 vs 0.4 ppb h - ~) due to a higher deposition to vegetation. Ozone can also be destroyed by nitrogen dioxide and nitrogen oxide. The reaction between ozone and nitrogen oxide is faster, and would cause an increase in nitrogen dioxide concentrations. We show in Table 2 the seasonal variability of the correlation coefficient between [NO2] and [O3] calculated from hourly values for night-time situations (negative solar angle). Nitrogen dioxide and ozone are significantly negatively correlated from October to April at ~ht/iri and from November to March on Ut6, i.e. when NO 2 concentrations are at a maximum. This confirms the fact that chemistry acts as a sink for ozone in winter. In
summer, no correlation between ozone and NO 2 is observed at night.
8. C O N C L U S I O N S
Current levels of ozone at the measuring locations result either from stratospheric sources, local photochemistry in the troposphere or tropospheric chemistry elsewhere and long-range transport of products. According to Wege et al. (1989), the ozone variation coefficient (ratio of ozone standard deviation to mean concentration) is large close to the surface, very small in the middle troposphere and increasing again in the upper troposphere. This would indicate that the ozone variability near the surface would be mainly due
1446
H. HAKOLAet al. 0..9
Uto 0.8
AhtOri
0r"
o.7
........
--. 4
t~ o_ 0.6
<~
"'"-,..
0.5
"'"'-....
0.4
0.3
0.2 dry
snow
round
1 cover < 1 / 2
snow
I cover > 1 / 2
Fig. 13. Dependence of the ozone night-time loss rate on the state of the ground in spring and autumn at Ut6 and ~ht~iri.
Table 2. Monthly correlation coefficients between hourly ozone and nitrogen dioxide concentrations when the solar angle is negative (night situations) Month
Xhtfiri
Ut6
Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.
-0.47 - 0.69 - 0.47 -0.43 -0.10 -0.02 0.00 -0.04 -0.26 -0.50 -0.66 -0.78
-0.51 - 0.45 - 0.46 -0.14 -0.15 -0.16 -0.03 -0.11 -0.24 -0.22 -0.41 -0.42
to processes in the lower troposphere. Furthermore, a comprehensive search for the origin of ozone level rises at Ut6 during a 1-year period (1986-1987) showed only two, but these significant, spring episodes (15-30 ppb above the monthly mean) that could be attributed to stratospheric injections (Hakola et al., 1987). Our results from the analysis of 2-3 years' data consisting of ozone measurements at two background stations in Finland, i.e. characteristic of North European conditions, showed that this region is widely under the influence of air masses bringing along ozone formed in the downstream plumes of the main European industrialized areas. Ozone episodes are more frequent when the wind is from the main European source areas. These episodes are characterized by NO2 concentrations between 1 and 4 ppb. At ~ht/iri the daily global radiation is correlated with ozone
maxima in summer, indicating the possibility of local production there. The spring maximum is a c o m m o n feature of the annual ozone cycle, as at many other European stations. On the other hand, the summer maximum is only observed at the southernmost station which is the closest to potential precursor sources, and has an environment with little surface uptake. Deposition to the ground is an important sink for ozone, when there is no snow. However, when the ground is covered by snow, the reaction between ozone and nitrogen oxide is important. The diurnal cycle of ozone is strongly modulated by local meteorology, while background values are mainly determined by the air mass origin. This work is now continuing with the further addition of P A N and N O measurements. Our research activities will also concentrate on a more detailed analysis of the chemical reactions taking place and influencing the local ozone budget under specific meteorological conditions.
Acknowledgements--We are very grateful to our colleagues Pekka Plathan and Sisko Laurila for processing the raw data, and to Dr E. Kyr6 for providing us with his sounding data from Sodankyl/i. This work was initiated and partly funded through the Acidification project HAPRO of the Ministry of the Environment and the Ministry of Agriculture and Forestry, and partly by the Ministry of the Environment in the framework of the EUROTRAC/TOR project.
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