Stratospheric ozone into the troposphere over Portugal

Transcription

1 Stratospheric ozone into the troposphere over Portugal N. Barros 1, C. Borrego 2, T. Fontes 1, A. C. Carvalho 2, N. Moreira 3, P. Leitão 3 & D. Henriques 3 1 Centre of Environment Modelling and Systems Analysis, Department of Science and Technology, University Fernando Pessoa, Oporto, Portugal 2 Department of Environment and Planning, University of Aveiro, Portugal 3 Portuguese Meteorological Office, Lisbon, Portugal Abstract The main purpose of this paper is to present a preliminary study on the impact of stratospheric ozone on tropospheric ozone levels under specific atmospheric dynamical conditions. It is well accepted that stratospheric ozone can be the source of part of the tropospheric ozone. Previous studies indicate that the mechanism responsible for this ozone intrusion occurs generally in several steps or just in a single step, usually associated with strong upward motion. In the first part of this paper, the methodology used in order to identify particular short-term episodes, potentially associated to the abovementioned phenomenon, is presented. Several episodes have been studied occurring during 14 years of ozone data collection, recorded by the Portuguese ozone network. Then, an analysis of the dynamical atmospheric conditions associated to previously identified episodes have been developed in order to verify the possibility of stratospheric contribution for the observed ozone level in each episode. Two of these episodes show a relatively good relationship between synoptical patterns related to stratospheric intrusions and backward trajectories. For these cases, the possibility of stratospheric origin should not be discarded before further study is developed. Keywords: ozone episodes, stratospheric intrusions, tropopause fold.

2 316 Air Pollution XII 1 Introduction Ozone is an extremely reactive oxygen form and its concentration on the boundary layer results mainly from dynamic and chemical mechanisms, such as stratospheric ozone intrusion, synoptic scale transport with subsequent entrainment to the boundary layer and photochemical reactions. About 90 % of this compound is present in the stratosphere and the remaining 10 % is diffused into the free troposphere and the planetary boundary layer. Although stratospheric ozone is crucial for life on earth, as we know it, at the surface it has harmful effects on humans, vegetation and materials due to its oxidizing power. Due to its adverse effects, ozone is legislated in the European Union in the latest 2002/3/EC Directive [1]. In this document, the member states are obliged to inform the public, justify and develop short-term action plans in order to reduce the risk of exceedances of the alert threshold. However, these plans are meaningful when measured surface ozone is related to deep vertical dynamical processes in the atmosphere rather than to photochemical processes. The EU Directive also indicates that ozone episodes forecasts are compulsory. This fact introduces a major role in the assessment of stratospheric ozone contribution to tropospheric chemistry composition in order to improve chemical transport models forecast [2, 3]. In 1964 Danielsen developed the concept on how stratospheric air may reach low tropospheric levels (described in [4]). Stratospheric air mass initially penetrates into the upper troposphere in a sheet or veil in the southern portion of an upper-level trough and is associated with a jet stream in this region. This pattern is related to a surface low, usually associated to a cold front extending in a southerly direction leaving a surface high pressure to the west. Johnson and Viezze [4] have summarized the most likely mechanisms that control the transportation of stratospheric ozone into the troposphere and that can be all active at the same time as: (i) dissipation of the intrusion by general mixing and diffusion into the free troposphere; (ii) persistence of the intrusion down to the top of the atmospheric boundary layer, where turbulence and convection are the responsible processes for mixing the stratospheric air; (iii) coupling of the intrusion to the frontal zone associated with a cold front, where ozone rich air mass is transported directly to the ground by frontal downdrafts and (iv) similar to the previous one, but in this case there is an entrainment of stratospheric air in organized frontal and pre-frontal convection and ultimately transported to the ground by rain showers or thunderstorm downdrafts. Stratospheric air masses are characterized by high ozone concentration levels, low moisture content, high levels of 7 Be, 10 Be and potential vorticity [5]. Based on these facts it is possible to correlate surface measurements with stratospheric intrusions. Several studies indicate that mountain monitoring stations have higher probability of registering intrusion events compared with lower altitude monitoring stations [6].

3 Air Pollution XII The STRATOZON project The main objective of the STRATOZON project (Stratospheric ozone into the troposphere over Portugal) is to justify the photochemical short-term ozone episodes that cannot be controlled by any local/regional ozone precursors emissions plans. The project is divided in four stages that consist of: the identification of high short-term ozone episodes from historical records, weather patterns analysis, high short term ozone episodes modelling and the development of guidelines to be used by Portuguese Environment Institute (IA Instituto do Ambiente) containing further scientific explanations of high short-term ozone episodes due to dynamic atmospheric transport. The first stage was developed over two phases: analyses of surface ozone data and vertical structure of atmosphere, concerning total ozone column and radiosonde data. The second stage consisted of the weather pattern analysis over a period covering at least one surface ozone episode probably related with stratospheric intrusion. 2.1 Surface ozone data Initially, the statistical analysis was done considering data from two ozone air quality networks over Portugal mainland, the Portuguese Meteorological Office (IM Instituto de Meteorologia) and the IA network. These two networks are in a sense complementary because the IA network includes a great number of air quality stations near the coast whereas the IM network provides information about the interior region of Portugal (P. Douradas, C. Branco e Beja) The IA ozone data set The IA air quality network is spread over 5 regions, which are mainly located along the coast measuring ozone surface data since 1988 (Figure 1). Data has been analysed up to Atlantic Ocean Oporto Coimbra Lisbon Alentejo Algarve Figure 1: Beja Penhas Douradas Castelo Branco LEGEND: Station types: traffic background industrial Locations of the air quality stations in mainland Portugal.

4 318 Air Pollution XII The available data was treated following the methodology identified in Figure 2. First, the data sets have been validated in accordance with the methodology proposed by the World Meteorological Organization (WMO) [7], followed by a statistical treatment. Finally the identification of episodes of interest for the project was carried out. Ozono concentration Validation using WMO methodology Doubtful ozone measurements considered by the WMO methodology: a) Greater than 300 µg.m -3 or lesser than 0 µg.m -3 ; b) Constant (+/- 1%) for over 8 hours; c) Higher than 280 µg.m -3 for more than 8 hours; d) Higher than 200 µg.m -3 for more than 10 hours; e) Less than 4 µg.m -3 for more than 6 hours. Questionable data founded by a), c) and d) have been isolated for further evaluations, whereas questionable data by b) and e) have been rejected. Years with efficiency <75% in Winter and Summer No Statistical analysis Identification of episodes with interest for STRATOZON Yes Data set rejected Criteria: 1. Ozone data validation method recommended by WMO; 2. Data of occurrence (Winter episodes between between November and February); 3. Hour of occurrence (nocturnal episodes, considering between 21:00H to 07:00H); 4. Number of stations where ozone episode with concentrations higher than 180 µg.m -3 occurs simultaneously (three or more); 5. Number of regions with simultaneous occurrence of ozone episodes (two or more). Figure 2: Applied methodology for filtering ozone episodes Statistical analysis Basic statistics and graphical analyses were applied to ozone time series on an hourly, daily, monthly, seasonal and annual basis. The maximum ozone concentrations were found between 14H00 and 15H00, during the weekend days and at summer months, respectively for hourly, daily and monthly basis. In Portugal, the analysis of ozone concentration between 1988 and 2001 shows that the 75% of Alentejo stations have an annual average higher than 50 µg.m -3. In Lisbon, more than 44% of the air quality stations present an annual average concentration that exceeds 40 µg.m -3. Moreover all the other ozone measurement sites give an annual average less then 40 µg.m -3, with an exception of 66% of the Coimbra stations and 11% of the Oporto stations. However, these results must be analysed carefully because ozone shows significant variations in accordance with station location. Considering the EU directive for air quality classification, analyses show that background or industrial stations present higher ozone concentration average, as expected.

5 Air Pollution XII 319 Geographically, in the south, namely in Alentejo and Lisbon regions, larger values for this parameter are measured Identification of interesting ozone episodes for the project A set of ozone episodes with concentration values higher than the public information threshold of 180 µg.m -3 (Directive 2002/3/EC) was constructed. From it, a subset of ozone episodes was isolated for the STRATOZON project. The episodes selection was based on the fulfilment of at least one of the following criteria: 1. Ozone data out of the validation method recommended by WMO [7]; 2. Date of occurrence (Winter episodes between November and February); 3. Hour of occurrence (nocturnal episodes, considering itself of 21H00 to 7H00; 4. Number of stations where ozone episodes with concentrations higher than 180 µg.m -3 occur simultaneously (three or more); 5. Number of regions with simultaneous occurrence of ozone episodes (two or more). A set of 156 days of ozone episodes has been isolated based on the above criteria. Among the selected episodes, 59% fits only one of the criteria. Only 2% percent of the episodes fulfil four criteria and none registered the five criteria simultaneously. Out of the selected episodes 68% occurred in the years 1988, 1989, 1993 and These are also the years where the applied criteria are more diverse (Figure 3.A). Years Criterion 2 Criterion 3 Criterion 4 Criterion 5 Criterion Number of ozone episodes A) Number of occurrences B) Criteria CRITERIA: 1. Ozone data validation method recommended by WMO [6]; 2. Date of occurrence (Winter episodes between November and February); 3. Hour of occurrence (nocturnal episodes, considering itself of 21H00 to 7H00); 4. Number of stations where ozone episodes with concentrations higher than 180 µg.m -3, occurs simultaneously (three or more); 5. Number of regions with simultaneous occurrence of ozone episodes (two or more). Figure 3: Number of: A) Criteria fulfilled by the ozone episodes; B) Selected ozone episodes selected highlighting the repetition of criteria. Both, (A) and (B), for the period

6 320 Air Pollution XII Criterion 2 has been the most applied whereas criterion 4 has been the less used (Figure 3.B). In terms of percentages, criteria 2 and 3 were the most used for 1988 to 1997 and criteria 4 and 5 were the most applicable in the years of 1998 to The IM ozone data set This database comprises hourly ozone surface concentrations measured at six stations, since 1971, at Lisboa/Gago Coutinho, Castelo Branco, Beja, Penhas Douradas, Funchal (Madeira Island) and Angra do Heroísmo (Azores). Nevertheless, the historical registers are not homogeneous because till 1988, the applied method to measure ozone concentration was the Potassium Iodide (KI). The UV photometric method is now being applied since All the ozone concentration time series lack information during several periods. Identification of episodes was based on ozone concentration thresholds of 100, 180, 240 e 360 µg.m -3 [1]. Results in Table 1 reflect a great inter-annual variability of surface ozone episodes. 2.2 Vertical structure of atmosphere The adopted methodology consists of the inventorying of surface ozone measured episodes, total ozone column and multiple tropopauses. This method is based on the principle that a surface ozone episode with stratospheric origin results from a significant variation of the total ozone column amount, which may be related to a tropopause discontinuity or folds. The ozone stratospheretroposphere exchange may also occur in the neighbourhood of isolated depressions at altitude (cut-off lows). In both situations, a high gradient over the total ozone column field is verified. In these conditions, the temporal and spatial evolutions of the total ozone column values constitute important data in order to explain tropospheric ozone variations with stratospheric origin. On the other hand, the inter-daily variability of the total ozone column amount is associated with transport!processes in low stratosphere, where ozone remains in photochemical balance, thus constituting a natural tracer in the atmospheric circulation in this region. Daily average values of total ozone column measured with Dobson (Lisbon) and Brewer (Lisbon and Funchal) ozone spectrophotometric methods during the period were used in this analysis. For each pair of consecutive observation days, inter-daily variation of the total ozone column amount was calculated. The resulting series of differences for each of the data sets shows a near normal distribution. Hence, the standard deviation is an estimator for the dispersion of the samples. Standard deviations of 15.4 D (Funchal), 17.8 D (Lisbon - Dobson) and 19.9 D (Lisbon - Brewer) were found. A threshold of 30.0 D, around the smallest standard deviation, was established to specify all the consecutive pairs of days with differences of total ozone column values higher than 30.0 D or less than 30.0 D (Table 2). Tables 2 and 3 indicate data only from Funchal.

7 Air Pollution XII 321 Table 1: Number of occurrences found with the KI method ( ) and UV photometric method ( ) for each established ozone threshold at Lisbon station. Year O 3 Concentration Limit (µg.m -3 ) Year O 3 Concentration Limit (µg.m -3 ) > 100 > 180 > 240 > 360 > 100 > 180 > 240 > Table 2: Consecutive pairs of days with differences in the total ozone column amount higher than 30.0 D or lesser than 30.0 D in Funchal. Month Grand Total Year The tropopause normally is characterized by a sudden variation of the temperature gradient [8]. Concerning the multiple tropopause situations the aerological soundings in Lisbon, Lajes and Funchal, available between 1995 and 2002, were used to make the inventory (Table 3). 2.3 Weather pattern analysis The purpose of the present work is to evaluate if surface ozone results from intrusions of stratospheric air through the tropopause during the days before the

8 322 Air Pollution XII ozone event. In that sense, meteorological episodes for each event are defined here as follows: a meteorological episode starts 8 days before the ozone episode, includes the day of the ozone episode and ends 2 days after the ozone episode. In case the ozone episode extends for several days, the meteorological episode includes all this period. This procedure results in 100 meteorological episodes, from which 69 have 11 days (that corresponds to a one-day event), the other 31 meteorological episodes are 12 to 20 days long (two-days to ten-days events), when applied to the set of ozone episodes isolated in Table 3: Number of days with multiple tropopauses (trp) recorded in Funchal. Years Data Month Grand Total Sum of 2 trp Sum of 3 trp Sum of 2 trp Sum of 3 trp Sum of 2 trp Sum of 3 trp Sum of 2 trp Sum of 3 trp Sum of 2 trp Sum of 3 trp Sum of 2 trp Sum of 3 trp Sum of 2 trp Sum of 3 trp Total Sum of 2 trp Total Sum of 3 trp To perform the meteorological analysis, Numerical Weather Prediction (NWP) fields from European Centre for Medium-range Weather Forecasting (ECMWF) ERA-40 data and particle trajectory data from the Dutch Meteorological Office model (KNMI), running at ECMWF, are used [9]. The potential vorticity (PV) field, usually used to make the distinction between stratospheric and tropospheric air [10], is a key parameter on the current analysis. A map of PV = 2 x 10-6 m 2 s -1 kg -1 K is available on ECMWF analysis and is referred to as a tropopause map. If the tropopause reference value is used for several pressure surfaces, then it is possible to plot tropopause maps with the superposition of isopleths, which depict tropopause folds [11]. Additionally, global images from Meteosat satellites, available since 1995 on the IM database, are also used. Water vapour (WV) images are of special interest as they are related to the upper levels of the troposphere and to tropopause features [12]. Briefly, the performed analysis consists of three main features: 1) general characterisation of synoptic patterns at the surface and at lower/upper troposphere; 2) study of backward trajectories of the particles arriving at the

9 Air Pollution XII 323 location of the ozone episode; 3) study of the dynamic tropopause and its relationship with backward trajectories. A) Tropopause maximum: 553 hpa (50ºN; 20ºW) B) Tropopause maximum: >700 hpa (53ºN; 23ºW) Figure 4: Tropopause map in isobaric coordinates for 10/01/1996, using the superposition of isopleths technique, showing tropopause pressure maximum (hpa): A) at 00H00 UTC, located close to a tropopause fold; and B) at 06H00 UTC, located on a stratospheric air pool. The analyses of two ozone episodes with high interest to the project are described hereunder. The first relevant ozone episode occurred on 17 th January 1996 in Coimbra at 14H00 UTC. Backward trajectories reach levels around 700 hpa on 11 th January 1996 (therefore 6 days before) north of the Azores Islands (near 50ºN 30ºW). On the other-hand, from 9 th at 12H00 UTC until 10 th at 00H00 UTC, a tropopause fold is found on the tropopause maps using the superposition of isopleths technique (Figure 4.A), leading in the following hours to the development of a pool of stratospheric air around 700 hpa (Figure 4.B). In this instant, the backward trajectory (reaching Coimbra at 12H00 UTC on 17 th ) passed at 877 hpa at 51ºN 27ºW. There is a vertical separation of around 1600 m and there is no complete horizontal coincidence. However, it must be stressed that tropopause maps have a temporal resolution of 6 hours, which raises the possibility that the trajectory may pass through a pool of stratospheric air during the interval between NWP analyses. On the other hand, no isobars are drawn between 500 and 700 hpa, therefore, some pressure levels are not resolved by tropopause maps using the superposition of isopleths technique. The other ozone episode also occurred in Coimbra on the 27 th December A weak cold front crosses the country on 26 th, after several days of western flow. During this period, high cyclogenetic activity and the generation of some tropopause folds over North Atlantic were observed. Four days before the ozone event, backward trajectory levels and tropopause levels were similar and close in time near Newfoundland, but there was a great horizontal separation. Nevertheless, this episode deserves further study. 3 Conclusions The applied methodology for surface ozone episodes filtering gathered 156 episodes of interest. Nevertheless, there was a need of tuning in the

10 324 Air Pollution XII filtering methodology. Hence, the weather pattern analysis permits to identify surface ozone episodes that may have a stratospheric origin. In order to do it, three stages must be completed during an ozone event analysis: general characterisation of synoptic patterns at the surface and at lower/upper troposphere; study of backward trajectories of the particles arriving at the location of the ozone episode and finally the study of the dynamic tropopause and its relationship with backward trajectories. During this work, there was a need to start meteorological field analysis 8 days before high ozone surface measurements. Back trajectories were found to be a valuable tool to track particles position jointly with tropopause maps in pressure surfaces. Computation of cross-sections over tropopause slopes and pools of stratospheric air for the relevant episodes as well as forward trajectories from these regions will be carried out in the future. Simulations of these ozone events with mesoscale numerical systems are in progress. Vertical structure description based on radiosonde measurements and ozone total column show a great inter-annual variability and a higher number of multiple tropopause occurrences during Spring and Winter. The statistical analysis was also extended to ozone surface data collected by EMEP over Spain and preliminary results show that in general all the stations present higher average ozone concentration levels. Acknowledgements The authors wish to acknowledge the Science and Technology Foundation for financing the project STRATOZON (Ref POCTI/CTA/42702/2001), and for the grant of Ana Cristina Carvalho (Ref. PRAXIS XXI/BD/21474/99). Special thanks go to Environmental Institute, for ozone concentration data provision and also for all the complementary information. References [1] Directive 2002/3/EC of the European Parliament and of the Council of 12 February 2002 relating to ozone in ambient air [2] Elbern, H., Kowol, J., Sládkovic, R., Ebel, A. (1997) Deep stratospheric intrusions: a statistical assessment with model guided analysis. Atmos. Env, 31: No.10, [3] Bian, X. and Fast, J. D. (1999) Correlation between potential vorticity and ozone in the troposphere and lower stratosphere over eastern North America during the summer of Preprints, Symposium on Interdisciplinary Issues in Atmospheric Chemistry, Jan 10-15, Dallas Texas, pp AMS, Boston, Massachusetts. [4] Johnson, W. B. and Viezze, W. (1981) Stratospheric ozone in the lower troposphere I. Presentation and interpretation of aircraft measurements. Atmospheric Environment, 15: No

11 Air Pollution XII 325 [5] Tropospheric Ozone Research. Tropospheric Ozone in the Regional and Sub-regional Context Series: Transport!and Chemical Transformation of Pollutants in the Troposphere, Vol. 6 Hov, Östein (Ed.) [6] [7] WMO (1994) Global Atmosphere Watch - GAW-Quality Assurance/Science Activity Centre, Quality assurance project plan (QAPjP) for continuous ground based ozone measurements. [8] WMO No. 306 (1995) Manual on Codes - International Codes, Volume A, Alphanumeric Codes, [9] ECMWF (2003) Support for trajectories applications. OD/C/TAC/10/UM/DM/HM/228, 27th March [10] Hoskins, B.J., McIntyre M.E., Robertson A.W. (1985) On the use and significance of isentropic potential vorticity maps. Quart. J. Roy. Meteo. Soc., 111, [11] Morgan, M. and Nielson-Gammon, J. (1998) Using tropopause maps to diagnose midlatitude weather systems. Mon. Wea. Rev., 126: [12] Moreira, N. (2002) High Troposphere Analysis with Potential Vorticity and Water Vapour Imagery - tools for Nowcasting and Short-Range Forecasting. Proceedings of Second Technical Conference on Meteorology China-Macao-Portugal. Lisbon, Portugal, 8-10 May.