Ozone in the Troposphere: Measurements, Climatology,Budget, and TrendsD.W. Tarasick1,* and R. Slater2

1Air Quality Research Division, Environment Canada, Downsview ON M3H 5T42Centre for Research in Earth and Space Science, York University, Ontario

[Original manuscript received 17 May 2007; accepted 2 November 2007]

ABSTRACT An improved understanding of the global tropospheric ozone budget has recently become of greatinterest, both in Canada and elsewhere. Improvements in both modelling and measurement have made it possi-ble for weather centres to begin to forecast air quality using numerical weather prediction models. Despite sub-stantial progress, there are many open questions regarding tropospheric ozone photochemistry, long-rangetransport and the importance of the stratospheric source; this remains an area of very active research. Sinceozone in association with particulate matter causes respiratory problems in humans, trends and forecasting offuture surface ozone levels are also of great importance. The current status of measurement and modelling, aswell as the current understanding of tropospheric ozone budgets and trends, are reviewed, with an emphasis onCanada within the global context.

RÉSUMÉ [Traduit par la rédaction] Récemment, au Canada comme ailleurs dans le monde, on a vivementressenti l’importance d’une meilleure compréhension du bilan de l’ozone troposphérique à l’échelle planétaire.De meilleures méthodes de modélisations et de mesures ont permis aux centres météorologiques de commencer àprévoir la qualité de l’air à l’aide de modèles de prévision météorologique numériques. Malgré des progrèsimportants, plusieurs questions subsistent à propos de la photochimie de l’ozone troposphérique, de son transportà grande distance et de l’importance de la source stratosphérique; ces points continuent à faire l’objet d’unerecherche très intensive. Comme l’ozone, de pair avec les particules en suspension, occasionne des problèmesrespiratoires chez les humains, il est également très important de connaître les tendances et de prévoir les niveauxfuturs de l’ozone en surface. Nous faisons un tour d’horizon de l’état actuel des méthodes de mesure et demodélisation de même que de la compréhension actuelle des bilans et des tendances de l’ozone troposphérique,en mettant l’accent sur le Canada dans le contexte mondial.

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*Corresponding author’s e-mail: david.tarasick@ec.gc.ca

1 Introduction Ozone plays a major role in the chemical and thermal balanceof the troposphere. It controls the oxidizing capacity of thelower atmosphere (it is a primary precursor to the formationof OH radicals) and thereby the capacity of the lower atmos-phere to remove other pollutants. Ozone acts as an importantinfrared absorber (greenhouse gas), particularly in the uppertroposphere, and also is an absorber of solar ultraviolet (UV)radiation. Because of multiple scattering, tropospheric ozoneis more effective in filtering UV-B than its small abundance(about 10% of the total column) would suggest. However, atground level ozone is responsible for significant damage toforests and crops. It is also a principal factor in air quality(AQ), because ozone in association with particulate matter inthe lower troposphere has adverse effects on human health.

In this paper methods of measuring tropospheric ozone arereviewed, its climatology is described, and several importantsources are discussed, particularly the role of stratosphere-troposphere exchange (STE). The essential topic of tropos-

pheric ozone chemistry is discussed elsewhere (seeMcConnell, this issue). Recent analyses of troposphericozone trends are reviewed, and a brief discussion of somerecent developments in air quality modelling is presented.

2 Systematic measurements and methods

Systematic measurements of tropospheric ozone are made bya number of different agencies. Data are typically freely avail-able for research use. Ozone data from many large fieldexperiments and other limited-duration measurement cam-paigns also exist and are sometimes included in the samearchives. However, in general these are held in individualcampaign archives and are not as easily accessed.

a Surface MeasurementsRegular measurements of ozone at the surface are made at avery large number of sites. In Canada two major networks,

the National Air Pollution Surveillance (NAPS) network of177 sites (http://www.etc-cte.ec.gc.ca/NAPS/) and theCanadian Air and Precipitation Monitoring Network(CAPMoN) of 29 sites (http://www.msc.ec.gc.ca/capmon/),make continuous measurements of ozone, particulate matterand several other chemical species involved in ozone and acidprecipitation chemistry. The NAPS network sites are primar-ily in urban areas while CAPMoN emphasizes non-urban airquality with measurement locations that are regionally repre-sentative, i.e., not affected by local sources of air pollution(Fig. 1). In the United States, the AIRNow Monitoring andInformation Network of more than 1000 sites (http://airnow.gov/) makes similar measurements. In Europe, the Co-opera-tive Programme for Monitoring and Evaluation of the Long-range Transmission of Air pollutants in Europe (EMEP)collects data at more than 150 sites (http://www.emep.int/).

Several methods for measuring ozone concentration havebeen used, including the reaction of ozone with potassiumiodide (mostly before 1980), and the chemiluminescent reac-tions of ozone with ethylene and with nitric oxide (NO). Themost common method used currently is UV absorption, andcommercially available instruments employing this methodare used at most monitoring sites. Most instruments rely onthe 253.7 nm emission line emitted from a mercury dischargelamp as the UV light source. The best UV absorption instru-ments are probably reliable for measurement of troposphericozone with uncertainties of less than 3%, although there issome evidence of positive interference from volatile organiccarbon (VOC) species in polluted areas (Kleindienst et al.,1993; Williams et al., 2006) and of large transient errorscaused by humidity changes (Wilson and Birks, 2006).

b OzonesondesOzone soundings are the major source of information on theamount of ozone in the free troposphere. While originallydeveloped for stratospheric monitoring, ozonesondes producegood measurements of tropospheric ozone as well. Mostsoundings are made with Electrochemical Concentration Cell(ECC) sondes (Komhyr, 1969), although the Brewer-Mast(BM) sonde (Brewer and Milford, 1960) is still used (at onelong-term site in Germany) and was much more common inthe past. The Indian sonde and the Japanese sonde are alsoused currently at sites in those countries. In Canada, routinesoundings have been carried out at Resolute Bay since 1966(making this record the longest in the world), and theCanadian network now consists of ten sites (Fig. 2).Soundings are made weekly, with extra flights during specialcampaigns. Since 1980 Canadian stations have used ECCsondes, with BM sondes in use before that. Worldwide, thereare about 60 ozonesonde stations making regular soundingsand reporting the data to the World Ozone and UltravioletRadiation Data Centre (WOUDC) (http://www.woudc.org/).Most are in the northern hemisphere, at mid-latitudes (Fig. 3).Until recently there was a significant lack of data in the trop-ics, but this has been greatly improved by the establishmentof 14 new stations in the tropics under the Southern

Hemisphere Additional Ozonesondes (SHADOZ) program(Thompson et al., 2003).

All types of sondes have at least one chamber containing a(usually buffered) weak solution of potassium iodide, throughwhich air is forced by a small pump. The four types of sondesdiffer somewhat in construction: the ECC, as its name sug-gests, is an electrochemical concentration cell, with twochambers containing different concentrations of potassiumiodide, connected by an ion bridge, while the other three areall single-chambered galvanic cells. The BM and Indian son-des employ a silver anode while the Japanese sonde has ananode of activated carbon. Other differences are in pumpdesign and material. (The ECC and Indian sonde pumps aremade of Teflon, while the BM and Japanese sondes havepumps constructed of methacrylate resin.) The operating prin-ciple in all cases is the well-known reaction:

2 KI + O3 + H2O → 2 KOH + I2 + O2 (1)

followed by reduction of the iodine at the cathode

I2 + 2e– → 2I–. (2)

Thus, for each molecule of ozone two electrons flow throughthe external circuit. The measurement is therefore, in princi-ple, absolute; however there may be losses of ozone in thepump and of ozone or iodine to the walls of the sensor cham-ber, as well as iodine evaporation and possibly adsorption tothe platinum cathode (Tarasick et al., 2002). The efficiency ofthe pump is also reduced at low pressures, and a correction(measured or estimated) must be applied for this. In additionthere appear to be slow side reactions with the phosphatebuffer that can cause iodine to be produced in excess of thatpredicted by the reaction in Eq. (1) (Saltzman and Gilbert,1959; Flamm, 1977; Johnson et al., 2002). Interference fromother gases can be a problem in polluted areas: SO2 has a neg-ative effect, cancelling ozone mole for mole, while NO2 has amuch smaller, positive effect (Schenkel and Broder, 1982;Tarasick et al., 2000). In the absence of significant concen-trations of interfering gases, ECC ozonesondes have a preci-sion of 3–5% and an absolute accuracy of about 10% in thetroposphere (World Climate Research Programme, 1998;Smit et al., 2007; Kerr et al, 1994; Thompson et al., 2007a;Deshler et al., 2008), since differences in sonde manufactureand preparation introduce tropospheric biases of approxi-mately ±5%. Although in the past BM sondes showed some-what variable response in the troposphere, depending onpreparation (World Climate Research Programme, 1998; Kerret al, 1994; Tarasick et al., 2002), recent intercomparisonsshow little bias in the troposphere and a precision of about10% (Smit et al., 1996). The Indian sonde shows a precisionof about 20% (Smit et al., 1996). The Japanese KC96 sondeshows a precision of about 5%, but a low bias in the tropos-phere of about 5% (Smit and Straeter, 2004; Fujimoto et al.,2004; Deshler et al., 2008).

Response time (e–1) is about 25 seconds for ECC and BMsondes, and about 40 seconds for KC96 and Indian sondes

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Fig. 1 CAPMoN monitoring sites (from Environment Canada www.msc.ec.gc.ca/capmon/).

Environment Canada

Ozone Monitoring Network

Brewer

Ozonesonde

Lidar

Saturna Island

Kelowna

Regina(Brattís Lake)

Toronto(Downsview)

Egbert

Yarmouth

Edmonton(Stony Plain)

Churchill

Resolute

Eureka

Alert

Goose Bay

Fig. 2 The Canadian ozone monitoring network. Symbols indicate the locations of continuous Brewer spectrophotometer (total ozone) monitoring and week-ly ozonesonde launches (balloon symbols).

(Smit and Kley, 1998). This corresponds (based on a typicalballoon ascent rate of 4–5 m s–1 in the troposphere) to a ver-tical resolution of about 100–125 m and 160–200 m, respec-tively. Since data are typically recorded at intervals of10 seconds or less, such data are not independent and somedegree of averaging, and deconvolution for height-sensitiveapplications, is appropriate (e.g., De Muer and Malcorps,1984).

c LidarThe DIfferential Absorption Lidar (DIAL) technique makes itpossible to retrieve ozone vertical profile information bycomparing the atmospheric return of laser pulses at two ormore UV wavelengths. The signals received are binned insub-microsecond time intervals, giving typical altitude reso-lutions of approximately 100 m. Most systems use one or twoNd:YAG lasers operated at 266 nm, followed by either dyelasers or high-pressure hydrogen and deuterium cells to pro-duce beams at wavelengths (typically 289 and 299 nm) withsimilar aerosol backscatter and Rayleigh extinction coeffi-cients but with large differences in ozone absorption cross-section. Some systems use the 266 nm beam directly andobserve the Raman-scattered return from O2, N2 and H2O at278, 283 and 295 nm (e.g., Lazzarotto et al., 2001). These aremuch less sensitive to atmospheric aerosols, but are limited tothe lowest 2 km of the atmosphere because of the muchgreater ozone absorption at 266 nm.

Because the DIAL method is differential in wavelength andaltitude it is self-calibrating and well-suited to long-term rou-tine measurements. Observations can be made during the day,averaged over about ten minutes to produce profiles of accu-racy and precision similar to ozonesondes, that is, a precision

of about 5% and an absolute accuracy of about 10% below10 km, degrading to about 20% at higher altitudes. Errorsources include imperfect knowledge of the atmosphericaerosol distribution, other atmospheric absorbers such as SO2and NO2, and the temperature dependence of ozone absorp-tion coefficients, as well as beam alignment errors and photoncounting statistics. A number of such systems have been built,both ground-based (e.g., Ancelet et al., 1989; Baray et al.,1999; McDermid et al., 2002) and airborne (Browell et al.,2003). They are expensive both to build and operate and, as aresult, current efforts are focussed on developing systems thatare capable of autonomous operation.

d AircraftThe Measurement of OZone and water vapour by AIrbus in-service airCraft (MOZAIC) programme is a cooperativeeffort of European scientists, aircraft manufacturers and air-lines (Marenco et al., 1998). It consists of automatic, regularmeasurements of ozone, water vapour, carbon monoxide andnitrogen oxides, by five specially modified passenger airlin-ers flying all over the world. Profiles are obtained on takeoffand landing. Since 1994 more than 50,000 such profiles havebeen collected. They are unevenly distributed, however,being concentrated around major air traffic hubs. Most pro-files reach the lowermost stratosphere, since cruise altitudesare generally above the tropopause at mid-latitudes and pole-ward. The instrument package uses the aircraft power supplyand records the aircraft position, altitude, speed, and meteo-rological data, as well as the measured species data. Theozone analyzer is a UV absorption instrument, similar to theone described in Section 2a, with a special pump for pullingair through the analyzer, accomodating the varying externalpressure (150–250 hPa at cruise altitude).

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ATMOSPHERE-OCEAN 46 (1) 2008, 93–115 doi:10.3137/ao.460105La Société canadienne de météorologie et d’océanographie

Fig. 3 World ozonesonde network: stations reporting ozone sounding data to the World Ozone and Ultraviolet Radiation Data Centre in Toronto (not all arecurrently active).

A related development in Canada consists of a miniaturizedUV absorption instrument and two miniature UV-visiblespectrometers, the latter measuring the concentrations of sev-eral species by differential absorption. The package isdesigned to operate autonomously on small aircraft(Bottenheim, et al., 2005). Although such aircraft do notreach the altitudes at which commercial jets fly, they do offerthe possibility of profile data at a variety of places not servedby large planes. In Canada, MOZAIC profiles are availableonly for the cities of Montreal, Toronto, and Vancouver.Figure 4 shows some typical profiles from MOZAIC and theCanadian package, Routine Inflight Assessment of LowerTropospheric Oxidants (RIALTO).

Many limited-duration investigations of troposphericozone chemistry have also been conducted using speciallyinstrumented research aircraft, typically in combination withsurface, sonde, and satellite measurements. Data from manyof the larger campaigns are publicly available and archived athttp://acdisc.sci.gsfc.nasa.gov/ozone/ozone_aircraft.shtml.

e Satellite MeasurementsAlthough the Total Ozone Mapping Spectrometer (TOMS)was built to monitor ozone in the stratosphere, a number oftechniques have been developed to derive information abouttropospheric ozone from TOMS data. Hudson and Thompson(1998) proposed a modified residual method for the tropicswhich combined TOMS and ozonesonde measurements.Their method assumes that the longitudinal variation of totalcolumn ozone at the equator consists of a zonally (longitudi-nally) invariant stratospheric component and a troposphericcomponent consisting of a constant background, a zonalwavenumber 1† component and superimposed smaller-scalefluctuations. These fluctuations are characterized as ‘excessozone’ and attributed primarily to biomass burning. The tro-pospheric ozone residual (TOR) technique (Fishman andBalok (1999) and references therein) uses height-resolvedozone information from the Solar Backscatter Ultraviolet(SBUV/2) instrument to subtract the stratospheric ozone fieldfrom the total column ozone field measured by TOMS(Fig. 5). Stratospheric data from the Microwave LimbSounder (MLS) and the Halogen Occultation Experiment(HALOE) have also been used to deduce the stratosphericcolumn (Ziemke et al., 1998), and the TOR technique has alsobeen applied to total column ozone data from the newerOzone Monitoring Instrument (OMI), launched aboard theEarth Observing System (EOS) Aura, spacecraft in July 2004(Ziemke et al., 2006). The most recent versions of the TORtechnique use ozonesonde measurements to adjust the SBUVretrievals (Fishman et al., 2003) and forward trajectory modelcalculations with MLS data (Schoeberl et al., 2008), in bothcases to produce a product with higher horizontal resolution.

Other techniques use the fact that, particularly in the trop-ics, the tops of the highest clouds are essentially at the

tropopause, and so the tropospheric ozone column (TOC) canbe found from the difference in total ozone measured in adja-cent cloudy and cloud-free pixels. The Convective CloudDifferential (CCD) method (Ziemke et al., 1998) and theClear-Cloudy Pair (CCP) method (Newchurch et al., 2003)are examples of this approach. In an extension of this method,Ziemke et al. (2001, 2003) have proposed a technique called‘cloud slicing’, which uses measurements of above-cloud col-umn ozone from TOMS together with Nimbus-7 temperature-humidity and infrared radiometer (THIR) cloud-top pressuredata to derive ozone column amounts in the upper tropos-phere for the period 1979–84. By combining total tropos-pheric ozone measurements from the CCD method with 100to 400 hPa upper tropospheric column ozone amounts fromcloud slicing, they also estimate 400 to 1000 hPa lower tro-pospheric column ozone.

Contrast between total column ozone measured overcoastal mountains and the nearby ocean has also been used toinfer lower tropospheric ozone (Jiang and Yung, 1996; Kimand Newchurch, 1996; Newchurch et al., 2001). This methodevidently suffers from limited spatial coverage. The scan-angle method (Kim et al., 2001) uses the fact that the sensi-tivity of TOMS to tropospheric ozone decreases withincreasing scan angle, as the effective scattering height for thereceived photons increases with path length. However, thismethod requires the assumption that stratospheric ozone isconstant over successive orbits, since the TOMS instrumentdoes not see the same portion of the atmosphere at differentangles on the same orbital pass.

All of these techniques suffer, to some extent, from thenecessity of assuming that the stratospheric ozone column iszonally invariant, or has a simple, well-defined zonal struc-ture. For this reason their usefulness is, in general, restrictedto tropical latitudes where stratospheric variability is typical-ly small. Although the TOR method has been extended tomid-latitudes (Fig. 5c), it suffers from uncertainty in thelower-stratospheric ozone amounts resulting from limitedsampling and limited accuracy of the lower stratosphericmeasurements. These difficulties limit the accuracy possibleusing any of these techniques, given the large stratosphericozone burden that satellite instruments must look through;typical variations in the stratosphere at mid-latitudes (>10%)are larger than the entire amount of ozone in the troposphere. More recent satellite instruments have been designed toattempt to address this issue. The Global Ozone MonitoringExperiment (GOME) on board the European Remote-SensingSatellite-2 (ERS-2) satellite (Burrows et al., 1999) measuresthe back-scattered radiances from 240–790 nm in the nadir-viewing geometry. Observations with moderate spectral reso-lution of 0.2–0.4 nm and high signal-to-noise ratio in theHartley (200–320 nm), Huggins (320–350 nm), and Chappuis(400–700 nm) bands yield information on the vertical distrib-ution ozone from the effective scattering depth at differentwavelengths and also use the temperature dependence of theozone absorption cross-sections in the Huggins bands to dis-tinguish ozone in the warmer troposphere from colder

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† a wave whose wavelength is the circumference of the Earth

stratospheric ozone (Chance et al., 1997). Results from thesemethods are promising and do appear to be an improvementon the TOMS-based techniques described in this section; twoto three, not entirely independent, points are retrieved in thetroposphere with errors of 20–30% (Liu et al., 2005b, 2006a).Vertical resolution in the troposphere is approximately9–11 km in the extratropics and approximately 13–16 km inthe tropics. Outside of the tropics, however, the informationcontent of the retrievals in the lower troposphere is less (moredependence on the a priori estimate), owing to reduced pene-tration of solar photons in the troposphere at larger incidenceangles.

The same technique should be applicable to the OMIinstrument, and has been applied to the Scanning ImagingAbsorption Spectrometer for Atmospheric Chartography(SCIAMACHY) instrument on the Environmental Satellite(ENVISAT) (Liu et al., 2005a). However, the SCIAMACHYinstrument (Bovensmann et al., 1999) is designed to measurescattered sunlight both in nadir-looking mode and in limbmode, and is able to observe the same atmospheric volumefirst from the side (limb mode) and then after about sevenminutes from directly above, in the UV, visible and nearinfrared wavelength regions (240–2380 nm) at moderatespectral resolution (0.2–1.5 nm). The stratospheric profile can be determined from the limb measurements and this per-mits the tropospheric column to be derived from the nadirmeasurements of total column ozone (Schmoltner et al.,2004).

The first satellite instrument designed specifically for tro-pospheric ozone measurement, the Tropospheric EmissionSpectrometer (TES) on the EOS Aura spacecraft is aninfrared Fourier Transform Spectrometer (FTS) with bothlimb and nadir sounding capabilities (Beer et al., 2001). It

observes in the 15.4 to 3.1 µm wavelength range, a region thatincludes the 9.6 µm band of ozone. Relative to the instru-ments previously discussed it has a small footprint for nadirobservations (5×8 km), as well as better ability see throughclouds (Kulawik et al., 2006).

Initial results from TES suggest that nadir-mode retrievalshave a vertical resolution of about 6 km and errors in the tro-posphere of 10–20% (Bowman et al., 2002; Worden et al.,2007a; Nassar et al., 2008). Initial efforts to assimilate TESretrievals of tropospheric ozone with global chemical trans-port models are promising: TES was able to see a majorstratospheric intrusion, and TES retrievals improved theheight representation of the tropopause dramatically(Parrington et al., 2007). Complementary information fromother sounders may significantly improve TES profiles:model studies suggest that, when combined with OMI radi-ances, there is a 20–60% improvement in vertical resolutionand a large improvement in the ability to resolve boundarylayer ozone (Worden et al., 2007b).

Several other satellite instruments can also provide infor-mation about tropospheric ozone. The Infrared AtmosphericSounding Interferometer (IASI) instrument, launched on theMeteorological Operational-A (MetOp-A) satellite inOctober 2006, is also an FTS, operating in the 3.7 to 15.5 µmspectral range. It is expected to retrieve ozone profiles withlower resolution in the troposphere than that of TES(Blumstein et al., 2004). Although designed primarily forstratospheric observations, the MLS instrument on Aura(Jiang et al., 2007; Stajner et al., 2008), the Polar Ozone andAerosol Measurement (POAM) instrument on the Systèmepour l’observation de la terre-4 (SPOT-4) satellite (Prados etal., 2003), the Michelson Interferometer for PassiveAtmospheric Sounding (MIPAS) on ENVISAT (Cortesi et

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Ozone Concentration (ppbv)

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RIALTONova Scotia

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MOZAICMontreal

Fig. 4 Typical ozone profiles from aircraft measurements: (a) from a test flight of the Canadian small aircraft instrument package, RIALTO, (b) from MOZA-IC aircraft measurements (at Montreal). Note the different vertical and horizontal scales in (a) and (b).

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Fig. 5 Tropospheric ozone column (TOC) measurements in DU from TOMS, OMI and GOME: a) TOC maps for March–May 2006 produced by combiningOMI total column ozone measurements with Aura MLS stratospheric column ozone measurements (reproduced from Ziemke et al. (2006) by permis-sion of the American Geophysical Union); b) a three-day composite (22–24 October 1997) global map of tropospheric column ozone, from GOME UVspectral measurements (reproduced from Liu at al. (2005b) by permission of the American Geophysical Union ); c) TOC over eastern Canada and theUnited States, from TOMS and SBUV data (Fishman et al. (2003), European Geophysical Union).

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al., 2007) and both the Atmospheric Chemistry ExperimentFourier Transform Spectrometer (ACE-FTS) and theMeasurements of Aerosol Extinction in the Stratosphere andTroposphere Retrieved by Occultation (ACE-MAESTRO)instruments on the Canadian SCISAT-1 (Hegglin et al., 2007;Kar et al., 2007) have some sensitivity to upper troposphericozone and can provide useful information in the importantupper troposphere and lower stratosphere (UT/LS) region.These observations may have the most impact, as notedabove, when combined with complementary informationfrom other instruments.

f UNH Mini-O3 SensorThe University of New Hampshire (UNH) Mini-O3 sensor(Talbot et al., 2004) is a lightweight, low-power instrumentusing the UV absorption method at 253.7 nm. It weighsapproximately 295 g and uses 2.5 W of electrical power. Thisis a new device with interesting possibilities, since it is almostas light as an ozonesonde but can operate for a much longerperiod. It has been used on long-duration small balloon flightsto study the chemical evolution of pollution plumes over theAtlantic ocean (Mao et al., 2006).

g Ground-based Passive Optical MethodsIt is possible to retrieve a limited amount of information aboutthe amount of ozone in the troposphere and its vertical distri-bution from differential absorption measurements of scatteredsunlight in the UV-visible spectrum. Measurements at lowelevation angles, for moderate photon mean free path lengths,will have a greater proportion of the light path from the scat-

tering point to the instrument in the lower layers of the atmos-phere. As the vertical sensitivity is a function of elevationangle, the combination of all measurements can be used toretrieve vertical profiles of absorber concentrations. The ver-tical resolution of such profiles depends on solar zenith angle,aerosol loading and surface albedo, but it is on the order of3–5 layers for the lower troposphere. Good estimates of theaerosol optical depth and vertical distribution are needed,although these can sometimes be retrieved simultaneouslywith the ozone profile. The multi-axis differential opticalabsorption spectroscopy (MAXDOAS) method (Hönninger etal. (2004) and references therein) has been used successfullyin a number of experimental campaigns. Methods have alsobeen proposed (Liu et al., 2006b; A. Cede, personal commu-nication, 2007) that could be used with some of the existingozone spectrophotometers in the Brewer network (e.g.,Fig. 2), of which there are more than 200 worldwide).

3 ClimatologyAt the surface, ozone is observed to have typical concentra-tions between about 20 and 45 ppbv at ‘clean’ sites, althoughmuch higher values are observed in areas subject to veryactive photochemistry. Values as high as 200 ppbv are oftenseen near Houston, Texas (Pinto et al., 2007), a centre for thepetrochemical industry, and Mexico City has often recordedvalues greater than 300 ppbv (e.g., Vazquez et al., 2005).Even higher values were common in Los Angeles during the1960s. A large diurnal variation is typical: where a nocturnalsurface inversion layer develops, ozone is depleted at nightand replenished by ozone from the free troposphere in the

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DU

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Fig. 5 (Concluded)

morning. The nocturnal depletion occurs through surfacedeposition and, at urban locations, NO titration. In summer,photochemistry typically produces a maximum in the earlyafternoon. Ozone values also vary with season, showing aminimum in the late fall or winter and a maximum in the latespring or early summer.

Above the surface layer, in the free troposphere, there ismuch less diurnal variation but a similar annual cycle. In gen-eral, a broad summer maximum in ozone is found from thesurface to the tropopause (Logan, 1999). At Canadian sitesthis maximum generally occurs in May, or sometimes June(Fig. 6), while in Europe, the eastern United States and Japanthe maximum is slightly later (June or even July), reflectingthe greater influence of photochemistry on ozone levels.Typical concentrations in the lower troposphere are also10–20 ppbv higher there than at northern Canadian sites. Atthe tropopause, a strong annual cycle is evident, peaking inMay or June, with ozone levels of about 120–200 ppbv(depending less on location than on the definition of thetropopause), with winter levels being as much as 50% lower(Logan, 1999; Thouret et al., 2006). This cycle is quite dis-tinct from the annual variation in the lower stratosphere,which has a maximum in winter and a minimum in late sum-mer (Figs 6 and 7),

Ozonesonde data from the northern subtropics show aspring maximum, with higher values in the middle tropos-phere than above and below (Logan, 1999). However, in thetropics (12.5°N to 12.5°S) satellite-derived TOC measure-ments have a maximum in October or November and a mini-mum in February–April (Ziemke et al., 2001; Newchurch etal., 2003). The maximum is much larger over the Atlanticregion, and is often described as having a zonal wavenumber1 pattern, larger in the southern tropics, with an amplitude ofabout 20–30 DU there. Although largest in October, thewavenumber 1 pattern persists year-round and extends to thesouthern subtropics (Fishman et al., 2003). It is present inboth lower tropospheric (1000–400 hPa) and upper tropos-pheric (400-100 hPa) ozone (Ziemke et al., 2001) and is alsoseen in MOZAIC data for the upper troposphere (Bortz et al.,2006). First identified by Fishman et al. (1990), it has beenthe object of intense study (Thompson et al., 1996, 2001;Jonquières et al., 1998) and is believed to be caused by emis-sions from tropical biomass burning augmented by upper tro-pospheric ozone production from lightning NOx (Edwards etal., 2003; Martin et al., 2002).

Near the surface, individual sites show departures from thegeneral pattern due to the competing influence of pollutionsources (including biomass burning) and seasonal variationsin prevailing winds; this can lead to large variations in sum-mer concentrations while ozone in the middle troposphere ismore uniform. For example, sites in southern Japan show asummer minimum in lower tropospheric ozone caused byflow from the Pacific ocean during the summer monsoon; inwinter the prevailing flow is from the Asian continent (Logan,1999). At remote sites surface ozone generally shows a win-ter maximum and a summer minimum (Logan, 1999; see alsoFig. 6a).

At some high latitude sites the seasonal maximum in springis replaced at the surface by a minimum (Fig. 7). This mini-mum is in fact a result of frequent surface boundary layerozone depletion episodes that occur in spring after polar sun-rise (Fig. 8). Observations of anomalously low ozone at thesurface in the Arctic spring have been reported by severalauthors (Bottenheim et al., 1986, 2002; Oltmans and Kohmyr,1986). These events are also seen in the Antarctic (Taalas etal., 1993; Wessel et al., 1998) and are thought to be caused bycatalytic reactions involving reactive halogen atoms, espe-cially bromine, originating in the polar ocean (Barrie et al.,1988; Barrie and Platt, 1997). The record at Resolute showsdepletions from the beginning of the routine ozonesonde pro-gram in 1966, with an increase in their frequency of occur-rence over the period 1966–2000 of greater than 0.6% peryear (Tarasick and Bottenheim, 2002). These observations areconsistent with current understanding that the phenomenon isnatural, and the observed increase in frequency may be due toan increase in open water (less polar ice) leading to enhancedavailability of sea-salt halides.

The reasons for the spring maximum in tropospheric ozonehave been the subject of considerable debate. The spring max-imum at higher latitudes and remote sites suggests a connectionwith the late winter maximum in stratospheric ozone, via STE.The summer maximum at middle and lower latitudes suggeststhe increasing importance of photochemical production closerto the equator where solar radiation is stronger. Indeed, thespring maximum is observed in the earliest records of tropos-pheric ozone, from the late nineteenth century (Marenco et al.,1994). Observations of radioactive tracers originating in thestratosphere from nuclear bomb testing indicate a spring maxi-mum in STE (Danielson and Mohnen, 1977), and this is con-firmed by more recent observations and modelling (Holton etal., 1995; Stohl et al., 2003; Wernli and Bourqui, 2002).Nevertheless, there is considerable evidence that photochem-istry may be important in the formation of the maximum evenin remote regions. Monks (2000), in an extensive review, notesthat a number of tropospheric species without stratosphericsources have similar annual cycles and that several studies havesuggested that the buildup of ozone precursors during winter atnorthern latitudes might lead to enhanced ozone production inspring. The long photochemical lifetime of ozone in wintermay also contribute to its accumulation. Similarly, Atlas et al.(2003), reporting on the results of the Tropospheric OzoneProduction about the Spring Equinox (TOPSE) experiment,found that STE was relatively constant during the spring of2000 (Allen et al., 2003) and concluded that in situ photo-chemical production drives the spring increase in ozone atnorthern mid-latitudes. However, dynamical studies indicatethat the residence time of (polluted) air masses is at most 1–2weeks in winter, making such a buildup of precursors unlikely(Stohl, 2006). Thus, more than 100 years after its discovery thisphenomenon is still not completely understood.

4 BudgetInitially, injection from the stratosphere was thought to dom-inate the tropospheric ozone budget, at least in non-urban

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areas (e.g., Junge, 1962; Danielsen, 1968). The stratosphere isevidently a large reservoir of ozone with concentrations (mea-sured in mixing ratio, or parts per unit volume) typically 100times larger than tropospheric mixing ratios, and the typicalvertical profile of ozone shows a monotonic decline of ozonemixing ratio from the tropopause to the planetary boundarylayer, strongly suggesting a stratospheric source and a surfacesink. However, improved understanding of troposphericchemistry (Crutzen, 1973; Chameides and Walker, 1973; Liuet al., 1987) led to a recognition that rates of photochemicalproduction of ozone were much larger than estimates ofstratospheric input, so that, in general, the tropospheric con-

centration should be, to first order, the difference betweenmuch larger chemical production and loss terms (e.g., Wanget al., 1998; WMO, 1999; von Kuhlmann et al., 2003).

Current estimates by chemical transport models ofglobal chemical production and destruction ratesrange from 2495–6920 Tg(O3) yr-1 (production) and2510–6617 Tg(O3) yr-1 (loss), with typical values in the rangeof 3500–5000 Tg(O3) yr-1 (WMO, 1999; Brasseur et al.,2003; von Kuhlmann et al., 2003; Stevenson et al., 2006).These are large with respect to the total tropospheric burdenof approximately 350 Tg(O3), and much larger than thecross-tropopause flux of ozone from the stratosphere,

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400–1400 Tg(O3) yr-1, estimated by the same global chemicaltransport models. However, as noted by von Kuhlmann et al.(2003), these large production and loss terms are not inde-pendent of each other, but are coupled via the OH radical.Net production rates are much smaller, of the order of1000 Tg(O3) yr-1 mostly over continental areas, with losses ofsimilar magnitude, primarily over the oceans. One additionalmajor loss term brings the budget into rough balance: surfacedeposition of ozone is estimated at 150–1253 Tg(O3) yr–1,with typical values of about 550 Tg(O3) yr–1.

In addition to the principal terms just discussed, there areseveral other important terms in the ozone budget. In the

upper troposphere, NOx (total concentration of NO plus NO2)generated by lightning is believed to lead to significant pho-tochemical production (Li et al., 2005; Huntreiser et al., 1998;Cooper et al., 2006, 2007). This source is of course seasonal,with a maximum during the summer lightning season, andappears to be quite important in the upper troposphere overthe southern United States and Mexico, generating about11–13 ppbv of ozone there (Cooper et al., 2006).

Owing to its relatively long lifetime in the troposphere,long-range transport of ozone from distant sources can bequite important for regional budgets. The chemical precursorsof ozone can also be transported long distances and under

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appropriate conditions (other reactants, sunlight) produceozone. For example, very high levels of ozone seen overHouston, Texas in July 2004 have been shown to be a resultof a smoke plume from forest fires in Alaska (Morris et al.,2006). Calculations constrained by observations suggest thatthe relatively high emissions of ozone precursors from Asiacontribute typically 4–9 ppbv to the North American budget,with occasional events (e.g., Siberian forest fires) addingamounts as much as four times greater (Fiore et al., 2002;Jaffe et al., 2004). Transport of ozone and ozone precursorsfrom North America to Europe may also significantly affectozone amounts there (Li et al., 2002b; Creilson et al., 2003;Naja et al., 2003; Mao et al, 2007); this was the subject of the2004 Intercontinental Chemical Transport Experiment-NorthAmerica (INTEX-A) (Singh et al., 2007 and references there-in). Transport from Europe to Asia has been less studied butappears to affect mainly near-surface ozone concentrations ineast Asia (Akimoto, 2003; Naja and Akimoto, 2004).Emissions from South America and Africa, particularly frombiomass burning, but also from other anthropogenic sources,contribute to the ozone maximum over the tropical Atlantic(Thompson et al., 1996; 2001; Jonquières et al., 1998;Lelieveld et al., 2004). Significant interhemispheric transportalso occurs: ozone from biomass burning in South Americaand Africa reaches the upper troposphere over Japan, theNorth Pacific, and North America, as well as being distrib-uted widely in the southern hemisphere (Sudo and Akimoto,2007). Similarly, surface ozone from Asia is distributedthroughout the northern hemisphere, and ozone from theAsian free troposphere is transported to the southern Pacific.Overall, transport from polluted source regions generallyaccounts for more than 40% of ozone abundance even inremote locations (Sudo and Akimoto, 2007).

The polar sunrise phenomenon of surface boundary layerozone depletion episodes illustrated in Figs 7 and 8 has beendescribed in Section 3. The surface can also be a source ofsignificant amounts of ozone, at least on the Antarcticplateau, apparently due to the release of NOx from the snow-pack leading to photochemical production in the stable sur-face boundary layer in summer (Crawford et al., 2001;Helmig et al., 2007).

5 Stratosphere-Troposphere Exchange Observational studies on stratosphere-troposphere exchange(STE) of ozone comprise a large literature (e.g., Danielsen,1968; Dutkiewicz and Husain, 1985; Oltmans et al., 1989;Wakamatsu et al., 1989; Davies and Schuepbach, 1994; Choet al., 1999; Beekmann et al., 1997). By the early 1990s suchobservations, combined with modelling of the global trans-port of chemical species, had provided a much clearer globalpicture of cross-tropopause transport, described in the land-mark review by Holton et al. (1995). In this picture, injectionsof stratospheric air into the troposphere at mid- and higher lat-itudes are small-scale manifestations of the global, wave-dri-ven (Brewer-Dobson) circulation that transports ozone andother chemical species from equator to pole. This circulation

is driven by waves generated in the troposphere that dissipatein the stratosphere and induce a poleward mass flux, drawingair up at the equator into the stratosphere and therefore caus-ing it to move down in the extratropics. Thus, the global con-tribution of STE to the tropospheric ozone budget can beestimated from models of the global circulation. This is anextremely important result, since it has a bearing not only onthe flux of ozone to the troposphere but also on the residencetime of air in the stratosphere and the lifetime of chemicalssuch as chloroflourocarbons (CFCs).

Estimates of the cross-tropopause flux of ozone from thestratosphere vary from about 400 to 1400 Tg(O3) yr–1 (WMO,1999; Brasseur et al., 2003; Stevenson et al., 2006). Lelieveldand Dentener (2000) have estimated it to be 565 Tg(O3) yr–1,based on a model study using the European Centre forMedium-Range Weather Forecasts (ECMWF) meteorologi-cal reanalyses and ozonesonde data. However, Olsen et al.(2002), using the observed relationship between ozone andpotential vorticity, estimate the flux to be 500 Tg(O3) yr–1

just in the northern hemisphere from 30–60°N. The globalflux has also been estimated from measurements of N2O and ozone, based on the observed correlation of these species, at 400 Tg(O3) yr–1 (Murphy and Fahey, 1994), and at475 Tg(O3) yr–1 (McLinden et al., 2000), from measurementsof N2O, NOy (total concentration of NOx and its oxidationproducts, primarily nitric acid and peroxyacetyl nitrate) andozone, based on the observed correlations between N2O andNOy and between NOy and ozone. These fluxes are compara-ble to the total tropospheric burden of approximately350 Tg(O3). They are also, as noted earlier, an order of mag-nitude smaller than typical chemical production and destruc-tion rates predicted by the same global models, but onlyslightly smaller than net chemical production rates, of about1000 Tg(O3) yr–1.

Another mechanism that has received less attention is hor-izontal exchange along isentropes. The tropopause in thetropics is at a higher potential temperature (approximately380 K) than at mid-latitudes (approximately 330 K). Air canmove adiabatically along isentropes between about330–380 K both into and out of the extratropical lowermoststratosphere and the tropical troposphere. The amount ofozone thus entering the tropical troposphere is about 5–10times smaller than the diabatic flux estimates above but maybe important to the ozone budget in the upper tropospherethere (Jing et al., 2004, 2005). This is also an important path-way for tropospheric air to enter the mid-latitude stratosphere(Schoeberl, 2004).

Despite the fact that the cross-tropopause flux of ozone isapparently dwarfed by photochemical production rates, thereis ample evidence that it is an important component of the tro-pospheric budget. Stratospheric ozone intrusions are occa-sionally observed to reach the ground (e.g., Lefohn et al.,2001; Elbern et al., 1997; Davies and Schuepbach, 1994;Wakamatsu et al., 1989; Oltmans et al., 1989). Much morefrequently, intrusion events are observed to reach the upper ormiddle troposphere, where they appear to dissipate and

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contribute to the ‘background’ ozone, generally defined astropospheric ozone that is more than seven days old andtherefore of uncertain origin, and estimated at about20–45 ppbv (Naja et al., 2003; Altshuller and Lefohn, 1996;Hirsch et al., 1996; Lin et al., 2000). This ambiguity in thefate of ozone injected into the upper and middle tropospheremay help explain differences in the conclusions of past obser-vational studies. A number of these studies have suggestedthat STE is quite important to the tropospheric ozone budget(e.g., Bachmeier et al., 1994; Browell et al., 1994; Mauzerallet al., 1996; Dibb et al., 1997; 2003; Allen et al., 2003), whileothers have concluded that it is a minor source (e.g., Dibb etal., 1994; Bazhanov and Rodhe, 1997; Elbern et al., 1997; Liet al., 2002; Browell et al., 2003). In general, the former stud-ies dealt with the upper troposphere while the latter conclud-ed that stratospheric ozone was a minor source at the surface.Lelieveld and Dentener (2000) estimate STE to be 30% of thebudget at 4 km, rising to 80% in the upper troposphere, for a42% contribution to the total column.

Ozonesonde-model comparison studies (Hoff et al., 1995;Mauzerall et al., 1996; Tarasick et al., 2007) have also con-cluded that a stratospheric ozone source is indicated in orderto reproduce the observed vertical distribution of ozone.Indeed, the observed monotonic decline of ozone mixing ratiofrom the tropopause to the planetary boundary layer at anyCanadian ozonesonde site cannot readily be explained bymeans of only tropospheric sources.

Most recently, several observational studies using the largeset of profile data from the INTEX Ozonesonde NetworkStudy, 2004 (IONS-04) have concluded that the stratosphereis an important source of free tropospheric ozone. Thompsonet al. (2007b, 2007c), using a number of observational crite-ria to classify portions of ozone soundings, calculate total tro-pospheric column contributions between 16 and 34% atIONS-04 sites. Cooper et al. (2006), using the potential vor-ticity based FLEXPART retroplume technique, estimate thatbetween 13 and 27% of ozone in the upper troposphere atIONS-04 sites is of recent stratospheric origin. These sites inthe vicinity of the mid-latitude jet stream are probably moreinfluenced by stratospheric air than are locations at lower lat-itudes (e.g., Fig. 9).

It must also be noted that none of these observations, withthe exception of the infrequent intrusion events that reach theground, is definite evidence that STE is important to the sur-face ozone budget. However, it is notable that studies of long-term ozonesonde records find statistically significant (95%confidence) correlations between ozone mixing ratio in thelower stratosphere and in the troposphere, right down to thesurface, at mid- and high latitude sites far from major anthro-pogenic pollution sources (Tarasick et al., 2005; Wardle et al.,2005; Taalas et al., 1997).

Although the flux of ozone from the stratosphere is con-trolled non-locally, by the global circulation, it is observed tobe sporadic, being associated with tropospheric weather sys-tems. Numerous case studies have attempted to characterizeand model individual events (e.g., Danielsen, 1968; Shapiro,

1980; Davies and Schuepbach, 1994; Lamarque and Hess,1994; Gouget et al., 2000; Reid and Vaughan, 2004). Currentinterest in studying STE events is motivated not only by thevariation noted above in model estimates of the global cross-tropopause flux and the need to test global models of thelarge-scale circulation of ozone and other chemical speciesbut also, in part, by improvements in tropospheric air qualitymodels, and the desire to forecast these small-scale eventsaccurately (e.g., Hocking et al., 2007). Moreover, it is nowrecognized that the importance of STE to the troposphericbudget depends critically on the distribution, with geographyand season, of STE events, and on the fate of the exchangedair parcels, particularly their vertical penetration and resi-dence time (Stohl et al., 2003). Several recent studies there-fore distinguish ‘deep’ and ‘shallow’ STE events (Bourqui,2006; Wernli and Bourqui, 2002; Sprenger et al., 2003; Stohl,2001). The vast majority of events are ‘shallow’, i.e., returnto the stratosphere or troposphere within a few days.However, a climatology of ‘deep’ STE shows much geo-graphic variability and maxima at mid-latitudes in winter(Fig. 9) (Wernli and Bourqui, 2002). Such geographical vari-ation in STE adds another degree of complexity to the globaltropospheric ozone budget.

6 TrendsMeasurements from the early stages of industrializationin Europe appear to indicate that surface ozone levels at thattime were about 10 ppbv (Volz and Kley, 1988; Marenco etal., 1994; Pavelin et al., 1999). There are many problems withthese early measurements, most of which were made withpaper strips containing a mixture of potassium iodide andstarch that changed colour when exposed to ozone. However,a quantitative method involving the oxidation of arsenite wasused at the Observatoire de Montsouris (near Paris) from1876–1910, and these measurements show mean concentra-tions between 5 and 16 ppbv (Volz and Kley, 1988). The ear-liest spectroscopic measurements of ozone, in the 1930s,indicate levels of about 20 ppbv (Marenco et al., 1994). Suchlevels are close to those now observed at remote sites inCanada in winter, when photochemical ozone production isminimal.

Since those early measurements, ozone in the lower tro-posphere has increased and typical surface ozone values inEurope and much of the northern hemisphere are about twiceas high. In general, trends were upward until about 1980. Inthe decade after 1980 the pattern is mixed, with sites inCanada showing significant downward trends (Tarasick et al.,1995, 2005) and in the United States zero or slightly negativetrends, while in Europe and to a lesser extent in Japan, ozoneamounts continued to increase until the late 1980s beforebeginning to decline (Oltmans et al., 1998, 2006; Logan etal., 1999). Interestingly, trends in Europe are almost the mir-ror image of those in Canada (compare Figs 3 and 5 ofOltmans et al. (2006)).

Analyses of ozonesonde data from 1980 show decreases inthe troposphere over Canada at all sites (53°–80ºN) up to

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about 1993, followed by increases up to the present, withozone values by the mid-2000s similar to those in the early1980s (Fig. 10), both in the troposphere and in the lowerstratosphere below 100 hPa (Tarasick et al., 2005; Wardle etal., 2005). Over Canada, tropospheric trends appear to followthose in the lower stratosphere, and indeed, annual averageozone anomalies in the troposphere are highly correlated withthose in the lower stratosphere (Table 1).

In the eastern United States (Wallops Island) the overallchange from 1970 to the present is small (approximately2.5%), although high values are observed in the lowermosttroposphere in the late 1980s. The record for Boulder,Colorado shows a small decline since 1985 (Oltmans et al.,2006). In contrast, an analysis of seven years of MOZAICdata finds an increase of 8% in the middle troposphere overNew York from late 1994 to 2001; the increase is primarily inwinter and spring (Zbinden et al., 2006).

Over Europe, the sonde data from Hohenpeissenberg showa steady increase from 1967 to the late 1980s, followed by amore modest decline to the present. As in Canada, presentvalues are similar to those in the early 1980s. There is someconsistency between the Hohenpeissenberg sonde record andthose from alpine surface sites; Wank shows a similar decline(in summer) but Zugspitze shows increases, particularly inwinter (Oltmans et al., 2006). The MOZAIC data at Frankfurtand Paris show an increase of 5% and 11% for 1994–2001,again primarily in winter (Zbinden et al., 2006). Model stud-ies suggest that these apparently conflicting observations areexplained by decreasing levels of ozone precursor emissions,reducing photochemical production in summer but alsoreducing the removal of ozone by NO titration in winter(Jonson et al., 2006).

Long-term ozone measurements over Japan from threesonde stations (32°–43°N) show an overall increase from

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TABLE 1. Correlations between annual average ozone mixing ratio anomalies in the troposphere and in the lower stratosphere, for northern mid-latitude andArctic stations in Canada. Data are from 1980–2005. Shaded boxes indicate statistically significant (95% confidence) correlations (adapted fromTarasick et al., 2005 by permission of the American Geophysical Union).

Northern Mid-latitudes Arctic

Ground Ground–630 hPa 630–400 hPa 400–500 hPa Ground Ground–630 hPa 630–400 hPa 400–250 hPa

Ground – – – – – – – –Ground–630 hPa 0.79 – – – 0.82 – – –630–400 hPa 0.58 0.87 – – 0.75 0.95 – –400–250 hPa 0.62 0.79 0.93 – 0.70 0.80 0.84 –250–158 hPa 0.67 0.68 0.70 0.68 0.50 0.64 0.71 0.57158–100 hPa 0.56 0.61 0.66 0.64 0.34 0.62 0.64 0.44100–63 hPa 0.43 0.57 0.70 0.65 0.18 0.51 0.55 0.3563–40 hPa 0.44 0.67 0.75 0.68 0.02 0.23 0.28 0.12

Fig. 9 Climatology of rapid exchange (less than four days) deep stratosphere-troposphere transport (STT) events between the stratosphere (as defined by thelayer above the dynamical 2PVU-tropopause) and the boundary layer (defined as the layer below 700 hPa): a) cross-tropopause mass flux in kg km–2 s–1 of rapid exchange, deep STT events during winter (DJF); b) frequency of such events, where the scale represents the probability that a par-cel in the boundary layer came from the lower stratosphere in the last four days. Estimates were performed by an objective analysis of trajectories start-ed from a dense, global and regular grid, driven by the ERA-15 data set (Sprenger and Wernli, 2003).

1970 to the late 1980s, but, as in Europe, the increases arelarger before 1980. A downward trend in ozone began duringthe 1990s (Oltmans et al., 2006). MOZAIC data for Japanshow an increase of 6% for 1994–2001, which is apparentlyinconsistent with the sonde data; the increase is in fall, winterand spring (Zbinden et al., 2006). Naja and Akimoto (2004),using back-trajectories, attribute the ozone increase from the1970s to the 1990s to increasing NOx emissions from Chinaand admit that the reasons for the subsequent downturn inozone, as reflected in the sonde data, are not clear. Moreover,observations at Hong Kong show large increases up to 1999(Chan et al., 2003) which the authors also attribute to NOxemissions from east Asia. Ozonesonde observations in Indiaalso show large increases in the lower troposphere from 1970to the late 1980s and smaller or no increases after that (Sarafand Beig, 2004).

In the tropics the trend picture appears to show importantregional differences. Shipboard measurements over the tropi-cal Atlantic show trends of about 0.5 ppbv yr–1, or about 20%per decade, resulting from NOx emissions associated withenergy use in Africa (Lelieveld et al., 2004). Surface datafrom Cape Point, South Africa (32°S) show a similar increase(Oltmans et al., 2006). An analysis of MOZAIC data (which

are also primarily from the Atlantic region) indicates thattropical ozone at cruise altitudes (7.7–11.3 km) has increasedby about 1 ppbv yr–1, or 20%, from 1994 to 2003 (Bortz et al.,2006). In absolute concentration, this is twice as large as theincrease reported by Lelieveld et al. for surface ozone, but thesame in percent (since upper tropospheric concentrations aretwice as large).

This contrasts with the Pacific region, where Ziemke et al.(2005), using the cloud-slicing method to derive the tropos-pheric column of ozone from TOMS data, find no trend in thetropics from 1979–2003. Surface and sonde data up to themid-troposphere from Samoa (14°S) show a small decreasefrom the 1980s to the most recent decade, primarily in theaustral late winter. A long-term increase in ozone over Hawaiiof 3.5±1.5% per decade, observed in surface data at 3.4 kmand sonde data at 3–6 km, has also occurred primarily duringautumn and winter but seems to be caused by a shift in long-range transport to different source regions (Oltmans et al.,2006). Analysis of low ozone events near 12 km shows that thefrequency of such events has increased at Samoa and Hawaiifrom the 1980s to the period after 1995 (Solomon et al., 2005).

At southern mid-latitudes, Lauder (45°S) shows anincrease from 1986 to 2003 of approximately 5% per decade

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Fig. 10 Time series of percentage deviations in average ozone mixing ratio for three mid-latitude stations and three Arctic stations in Canada. The individualtime series for these sites have similar features at all levels, and so a composite time series is shown here. The troposphere and stratosphere have beenexplicitly separated, that is, integration for the 400–250 hPa layer is from 400 hPa to 250 hPa or the tropopause, whichever comes first. Similarly, inte-gration of the 250–158 hPa layer starts either at 250 hPa or at the tropopause, if the latter is found above 250 hPa. The trend line is shown for refer-ence. There is essentially no trend in ozone in the lower stratosphere. The reason for this is a strong increase in ozone in the troposphere and lowerstratosphere during the latter part of the 1990s. The increases appear to be dynamically driven in the stratosphere (Tarasick et al., 2005), and also to beindirectly affecting tropospheric levels, as ozone mixing ratios in the lower stratosphere and in the troposphere are strongly correlated (see Table 1).

below 3 km, and no trend above that. In Antarctica, sondedata at Syowa (69°S) show little change over the long record(since 1966), while data from the South Pole shows adecrease at the surface from 1975 to 1995 and an increasesince then in late spring and summer, apparently related tophotochemical production due to release of NOx from thesnowpack (Section 4) (Oltmans et al., 2006).

7 Air quality models The recognition that air pollution causes respiratory problemsin humans (including premature deaths) has led to a desire toforecast air pollution episodes. Air quality forecast modelsare typically chemical transport models (CTMs), with a lim-ited (regional) model domain, which are driven by meteoro-logical fields from numerical weather prediction (NWP)models. In some cases, boundary conditions are specified byclimatology or by a global chemistry model. Most include afairly extensive set of gas-phase chemical reactions (>100)and chemical species (approximately 50); models differ intheir treatment of particulates, aqueous-phase chemistry, drydeposition, and boundary conditions (especially the upperboundary). Species concentrations are obtained from initialvalues set by climatology and inventories of anthropogenicand biogenic emissions. Results are critically dependent onthe accuracy of emissions inventories and considerable effortgoes into refining and updating them. Some experiments havebeen carried out with assimilation of actual measured chem-istry data and have shown potential to improve model forecastskill (accuracy) significantly but these have been of limitedduration.

The Canadian models, CHRONOS and AURAMS, and theUS model CMAQ, have recently been evaluated againstozonesonde profiles (Tarasick et al., 2007; Tong andMauzerall, 2006). All these models show some skill at fore-casting boundary-layer ozone, although mean errors are large(<35% for CMAQ and approximately 50% for CHRONOSand AURAMS). Such errors are typical and may reflect thesensitivity of the chemistry to small errors in meteorologicalfields; a recent model intercomparison found that the averageof an ensemble of models outperforms any of the models indi-vidually (McKeen et al., 2005). Differences between actualday-to-day emissions and emissions inventories may, ofcourse, also be contributing factors. In the future, the assimi-lation of real-time chemical species measurements shouldimprove forecast accuracy.

Model biases near the surface are small (typically <5–6 ppbvor 15%), and so for climatological studies these forecasterrors may be less important. The EMEP EulerianPhotochemistry model, with a climatological ozone field inthe stratosphere and using emissions inventories, shows goodagreement with monthly averaged surface measurements ofozone and NO2 and has been used to study ozone trends inEurope (Jonson et al., 2006).

CHRONOS, AURAMS and CMAQ have serious low bias-es in the middle and upper troposphere. Tarasick et al. (2007)find this bias to be considerably reduced by using a climato-

logical profile as a lateral boundary condition, over a zero-gradient boundary condition, while Tong and Mauzerall(2006) find further improvement by using a global full-chem-istry model to specify species concentration at lateral bound-aries. Both studies conclude that adding a stratospheric ozoneflux term would greatly improve agreement in the upper tro-posphere. In fact, good agreement with ozone soundings inthe upper troposphere has been shown by the US modelRAQMS (Pierce et al., 2003), with a realistic stratosphere andconstrained by satellite total ozone and stratospheric profilemeasurements (Al-Saadi et al., 2005).

Interesting progress has also been shown by the Canadianoperational weather forecast model, with the addition ofstratospheric chemistry: the model is able to reproduce anobserved STE event with qualitative agreement with ozoneand humidity observations (A. Robichaud, personal commu-nication, 2006). A parallel development using the samemodel, with both stratospheric and tropospheric chemistry,with the addition of emissions inventories, shows good agree-ment with a seasonal climatology derived from ozonesondes,with (in contrast to the AURAMS and CHRONOS resultsnoted above) a positive bias of approximately 10% throughthe middle and upper troposphere (Kaminski et al., 2007).

8 ConclusionAdvances in measurement and in modelling are likely to yieldimportant advances in our understanding of global tropos-pheric ozone over the next ten years. While observations fromsurface monitors, aircraft, and ozonesondes have providedmuch of our current understanding of tropospheric ozone,such observations are unevenly distributed. Satellite instru-ments, although possessing limited vertical resolution, withtheir high horizontal resolution can provide important addi-tional information on the global distribution and small-scalegeographic variation of tropospheric ozone. Recently theyhave been able to do so with impressive detail (e.g., Fig. 5c).New instruments (satellite and ground-based), and improvedtechniques for sampling tropospheric ozone from existinginstruments have recently become available, and the new datasources are beginning to be compared with and used to ini-tialize chemical models. Recently, such comparisons havehighlighted the importance of processes such as lightning-generated NOx chemistry and long-range transport of ozoneand its precursors and research into these processes is ongo-ing. A number of centres are incorporating chemistry intoNWP models, which appears to offer advantages over the off-line CTM approach, especially if constrained by total ozonecolumn measurements. NWP models are able to resolve themeteorological processes responsible for at least the largerscales of STE, so this important process is naturally includedin such models. Further comparison with measurement cam-paign data will reveal whether parameterization of small-scale STE is also required. While the assimilation of real-timechemical species measurements is probably some years away,a very useful intermediate step would be the integration ofexisting tropospheric ozone measurements, from satellites,

108 / D. W. Tarasick and R. Slater

ATMOSPHERE-OCEAN 46 (1) 2008, 93–115 doi:10.3137/ao.460105La Société canadienne de météorologie et d’océanographie

aircraft, lidar and the ozonesonde and surface measurementnetworks, into a single product suitable for climatological andmodel evaluation studies. Such a product would combine thehigh vertical resolution of ozonesondes and aircraft with theglobal coverage of satellites to provide an interpolated ‘bestguess’ of the daily global three-dimensional troposphericozone field.

AcknowledgementsWe thank J.C McConnell, T.G. Shepherd, and two anony-mous referees for a careful reading of the manuscript and

helpful comments. MOZAIC data are used here courtesy ofJ.-P. Cammas and the MOZAIC supporting agencies: theEuropean Commission, Airbus, Lufthansa, Austrian AirLines, Air France, Centre National de la RechercheScientifique (France), Météo-France, and ForschungszentrumJülich (Germany). RIALTO data were kindly provided by J.Bottenheim. Ozonesonde data were obtained from the WorldOzone and Ultraviolet Radiation Data Centre (WOUDC)operated by Environment Canada, Toronto, Ontario, Canadaunder the auspices of the World Meteorological Organization.

Ozone in the Troposphere: Measurements, Climatology, Budget, and Trends / 109

ATMOSPHERE-OCEAN 46 (1) 2008, 93–115 doi:10.3137/ao.460105Canadian Meteorological and Oceanographic Society

APPENDIX A: Partial list of AcronymsAURAMS A Unified Regional Air-quality Modelling SystemCMAQ Community Multiscale Air Quality ModelCHRONOS Canadian Hemispheric and Regional Ozone and NOx SystemEMEP Co-operative Programme for Monitoring and Evaluation of the Long-range Transmission of

Air pollutants in EuropeENVISAT Environmental SatelliteERA-15 ECMWF Re-Analysis (15 year)ERS-2 European Remote Sensing satellite-2FLEXPART Lagrangian transport particle dispersion modelRAQMS Regional Air Quality Modelling System

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