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Atmospheric Environment 89 (2014) 721e730

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Atmospheric Environment

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Signature of tropospheric ozone and nitrogen dioxide from space:A case study for Athens, Greece

C. Varotsos a,*, J. Christodoulakis a, C. Tzanis a, A.P. Cracknell b

aClimate Research Group, Division of Environmental Physics and Meteorology, Faculty of Physics, University of Athens, University Campus Bldg. Phys. V,Athens 15784, GreecebDivision of Electronic Engineering and Physics, University of Dundee, Dundee DD1 4HN, Scotland, UK

h i g h l i g h t s

� Tropospheric ozone and nitrogen dioxide association is investigated.� We examine satellite observations’ adequacy in detecting local TOR and NO2 sources.� Long-term trend in the tropospheric ozone column over Athens, Greece is examined.� Correlation between TOR and outgoing longwave radiation is examined.

a r t i c l e i n f o

Article history:Received 23 April 2013Received in revised form13 February 2014Accepted 25 February 2014Available online 26 February 2014

Keywords:Tropospheric ozoneNitrogen dioxideTropopause heightOxidation capacityAthensGreece

* Corresponding author.E-mail address: covar@phys.uoa.gr (C. Varotsos).

http://dx.doi.org/10.1016/j.atmosenv.2014.02.0591352-2310/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The aim of the present study is to investigate the variability of the tropospheric ozone and nitrogendioxide (NO2) columns over mainland Greece, by using observations carried out by satellite-borneinstrumentation and Multi Sensor Reanalysis. The results obtained show that the tropospheric ozoneresidual (TOR) dispersed farther away than the tropospheric NO2 column (TNO), due to the longer TOR’slifetime in respect to that of TNO. This results in the influence of the air quality of the nearby southernislands from the air pollution of the greater Athens basin. Furthermore, the TOR and TNO columns overAthens, for the period October 2004 to December 2011 were found to be negatively correlated with acorrelation coefficient �0.85, in contrast to recent findings which suggested strong positive correlation.Interestingly, this strong negative correlation into a slight positive correlation when the TNO concen-tration becomes higher than around 4 � 1015 molec cm�2, thus being best fitted by a quadratic rela-tionship. In addition, the temporal evolution of TOR during 1979e1993 showed a decline of 0.2% perdecade and just after 1993 it seems to obey a positive trend of 0.1% per decade, thus recovering duringthe period 1993e2011 almost 63% of the lost TOR amounts through the years 1979e1993. Finally, theassociation between TOR, the total ozone column (TOZ), the tropopause height and the outgoing long-wave radiation (OLR) is presented by analysing observations during 1979e2011. An unexpected positivecorrelation between OLR and TOR was found, which may probably be attributed to the fact that enhancedabundance in tropospheric water vapor reduces the summertime TOR maximum by destructing ozone inthe lower and middle troposphere through uptake mechanisms, thus emitting higher amounts oflongwave radiation upwards.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Atmospheric ozone is mainly produced in the tropical strato-sphere by the interaction between ultraviolet radiation (UVR) andoxygen, thus providing a protective shield from the biologically

effective UVR from the Sun that would be harmful to most forms oflife in large doses.

While ozone in the stratosphere has a beneficial role on the lifeon the Earth’s surface, the ozone in the atmospheric region knownas the troposphere, displays a destructive behavior (e.g., Efstathiouet al., 2005; Royal Society, 2008). For instance, tropospheric ozoneis a secondary pollutant of increasing concern, especially inmegacities, because it exists in high concentrations in the ambientair to which people, crops and ecosystems are exposed every day

Fig. 1. Schematic diagram of the Tropospheric Ozone Residual technique.

C. Varotsos et al. / Atmospheric Environment 89 (2014) 721e730722

causing various adverse effects especially on human health (e.g.,Wang and Isaksen, 1995; EEA, 2007). In megacities, a major part ofit is formed in the presence of sunlight by photochemical reactionsof primary pollutants, known as precursors (nitrogen oxides: NOx,carbon monoxide: CO, volatile organic compounds: VOCs),(Varotsos et al., 2003; Worden et al., 2012). Another source of thetropospheric ozone is the stratosphere, especially when the ex-change mechanisms (e.g. tropospheric folding) between strato-sphere and troposphere are favored (Varotsos et al., 2008a,b). Forinstance, Kondratyev and Varotsos (2002) refer that the contribu-tions of stratospheric ozone to the tropospheric ozone content are35% and 50% during the summer and winter, respectively, in theSouthern Hemisphere, while Hsu and Prather (2009) estimatedstratosphereetroposphere exchange (STE) ozone fluxes of the orderof 290 Tg/yr and 225 Tg/yr in the Northern and Southern Hemi-sphere, respectively, concluded that STE drives a seasonal peak-to-peak Northern Hemisphere variability of about 7e8 DU in tropo-spheric ozone.

Tropospheric ozone is also considered as a trace gas, which af-fects the oxidation capacity or the cleansing or the self-cleaningcapacity of the atmosphere (Prinn, 2003; Hu et al., 2010; Maoet al., 2010; Li et al., 2011). In addition, there is a well-establishedrelationship between tropospheric ozone and NO2 concentrations.The latter is another trace gas that affects the oxidation capacity ofthe troposphere (Cheng et al., 2008; Kar et al., 2010; Montzka et al.,2011; Villena et al., 2011).

Recently, Ziemke and Chandra (2012) by analyzing satellite dataof tropospheric and stratospheric ozone for the period 1979e2010suggested a positive trend in the global stratospheric ozone after1996 with the strongest trend occurring in the Northern Hemi-sphere. They also concluded that ozone levels in the stratosphereapproached that of the year 1980, suggesting a faster recovery thanthat predicted by the current chemistry-climate models. Regardingtropospheric ozone, Ziemke and Chandra (2012) found slight pos-itive trends in mid-latitudes of both hemispheres in agreementwith those that were found earlier by Ziemke et al. (2005) for theperiod 1979e2003. Ziemke et al. (2006) using one year (September2004 to August 2005) data of daily tropospheric ozone derivedfrom Aura OMI and MLS measurements found lowest global valuesof tropospheric ozone (20 DU or less) over the broad tropical Pacificand in the southern polar region in summer and autumn months.Oltmans et al. (2013) suggested that the gradual decline of surfaceozone at the South Pole during the first half of the record (1975e2010) has reversed, so that overall there has been no change, whileafter 1991 in the South Pole surface record both sonde records showa small increase at most levels.

Kar et al. (2010) showed enhanced amounts of TOR and TNOover megacities by using satellite observations (Van der et al.,2008). Ozone and NO2 not only are indicators of urban pollutionlevels but also affect the oxidation capacity of the atmosphere andhence the residence time of various trace gases. We focus our studyon the greater Athens area, because it is the biggest heavilypopulated city in Greece with frequent pollution episodes. Hilbollet al. (2013) using satellite data (from GOME, SCIAMACHY, OMIand GOME-2) for the period 1996e2011 found that TNO overAthens increased until about 2004, then remained relatively con-stant for some years, followed by a sharp decrease after 2008/2009due to the economic crisis in Greece (Kanakidou et al., 2011;Vrekoussis et al., 2013).

In this study, we are focusing on the variability of TOR and TNOover the greater Athens area by employing satellite observations.For the interpretation of the results obtained we analyse the vari-ability of other atmospheric parameters (e.g. tropopause height andoutgoing longwave radiation) that are closely associated with thetropospheric ozone and nitrogen dioxide.

2. Data and analysis

In the present study, we have used the available monthly meandata of Tropospheric Ozone Residuals (i.e. TOR) for the period1979e2011 (Ziemke et al., 2006; Fishman et al., 2003). It is calledresidual because it is calculated by subtracting an estimate ofstratospheric ozone (STR) from a total ozone column (TOZ)measurement.

It should be stressed that satellite observations are oftenretrieved for tropospheric ozone and NO2 measurements in aspecial procedure, by employing several a-priori assumptions,which some times contaminate substantially the data obtained.(e.g. Richter, 2009). The TOR amount is deduced with the techniquethat is schematically explained in Fig. 1. More detailed informationabout this technique, the accuracy and the uncertainties of the datais given in Creilson et al. (2003), Fishman et al. (2005), Wozniaket al. (2005), Ziemke et al. (2009), Wang et al. (2011), Ziemke andChandra (2012) and Reed et al. (2013). In this context, Ziemkeet al. (2006) pointed out that the TOR satellite measurements donot exhibit substantial offset differences relative to ozonesondes.Biases of small absolute values in the tropospheric ozone columnderived from the Infrared Atmospheric Sounding Interferometer(on board the MetOp-A European satellite), lidar and in-situ ob-servations were also reported by Pommier et al. (2012).

The monthly means TOR values, with grid analysis of 1�

Latitude � 1.25� Longitude, were used to calculate TOR annualmeans over Athens (and also over mainland Greece), for the wholeperiod.

For the interpretation of TOR variability, we have also used thedaily observations of the total ozone (TOZ) over Athens, Greece(37.98�N, 23.73�E) made by TOMS (Total Ozone Mapping Spec-trometer) instrument onboard Nimbus-7, Meteor-3 and EarthProbe satellites for the period 1979e2004. For the period 2005e2011 the daily TOZ data were obtained from OMI (Ozone Moni-toring Instrument) instrument onboard EOS (Earth ObservingSystem) Aura (Veefkind et al., 2006). In order these data to beconsistent with TOMS data, OMI-TOMS algorithm, level 3, datawere used (Kroon et al., 2008).

C. Varotsos et al. / Atmospheric Environment 89 (2014) 721e730 723

The compatibility of the observations made by the satelliteflown instrumentation and the results of the ground-based mea-surements has previously been investigated using the Dobsonspectrophotometer No. 118 installed at Athens; we compared theirdata and the correlation coefficients between the Dobson spec-trophotometer and Nimbus-7, Meteor-3, Earth Probe, Aura TOZdata were found to be 0.95, 0.94, 0.94 and 0.93 (statistically sig-nificant at 99% confidence level), respectively (Cracknell andVarotsos, 1994, 1995, 2007; Tzanis, 2009). Especially, for theperiod 1 October 2004 to 13 December 2005, when both EarthProbe (TOMS) and Aura (OMI) were in use, the above mentionedcorrelation coefficients were found to be 0.95 and 0.96 (statisticallysignificant at 99% confidence level), respectively. In addition, thecorrelation coefficient between those two satellites’ observations,during the previous mentioned period, was found to be 0.97 (sta-tistically significant at 99% confidence level) with mean differenceof about 0.7%. These good results indicate that observations of twoinstruments, TOMS and OMI, can be concatenated in agreementwith Kroon et al. (2008) results. It should be mentioned here thatthe correlation coefficient between TOR data deduced from TOMSand OMI instruments was found to be 0.89 (statistically significantat 99% confidence level).

In the data used, a gap between July 1994 and July 1996 exists, asno satellite equippedwith TOMSwas operating. In order to producethe missing TOR monthly mean values, we made use of the dailyTOZ values obtained from the Multi Sensor Reanalysis (MSR)dataset. The MSR is a coherent total ozone dataset extending from1978 to 2008 and has been created using all the available satelliteTOZ observations in the near UV Huggins band (Van der et al.,2010). This was done by establishing the ratio between theannual mean TOMS data and the annual mean MSR data for each ofthe years for which TOMS and OMI datawere available. Themean ofthese ratios was then used to reconstruct the missing monthlymean TOZ data to use in determining the missing monthly meanvalues of TOR.

Using these monthly values, the annual mean values of TOR forthe missing years were also determined.

Additionally, the data of the tropopause height (hPa) which isclosely associated with TOZ variability were used. In this context,Varotsos et al. (2004) found an inverse relationship between TOZand tropopause height over eastern midlatitudes of the NorthernHemisphere, suggesting also that a vertical displacement of thetropopause by 1 km is associated with a TOZ anomaly of 10 DU.Possible mechanisms of this close association between TOZ andtropopause height are described in detail in Varotsos et al. (2004)and references therein. These were obtained from NCEP/NCARreanalysis dataset using the PSD Interactive Plotting and AnalysisPages of NOAA website (http://www.noaa.gov). In this regard,Kalnay et al. (1996) have classified the tropopause pressure data inthe “A” class (the most reliable class), because it is strongly influ-enced by the observations. This means that the results obtained bythe model used for the reanalysis do not affect this parameter.

The data of the outgoing longwave radiation (OLR) have beenconsidered to further explore the TOR seasonal variability. Thesedata were also obtained from NCEP/NCAR reanalysis dataset. Forthe purposes of this study the monthly mean OLR values have beencalculated from the daily measurements at the top of the atmo-sphere. It should be mentioned that Yang et al. (1999) suggestedthat OLR global annual mean obtained from the above-said rean-alysis is approximately 1.5% higher than that of the Earth RadiationBudget Experiment (ERBE).

Finally, the observations of the TNO were obtained from OMI/Aura L2G level data using NASA’s online visualization and analysistool, Giovanni (http://disc.sci.gsfc.nasa.gov/Aura/data-holdings/OMI/omno2g_v003.shtml, accessed 18 May 2012). These data

were produced by the OMNO2G.003 algorithm and have gridanalysis of 0.25� � 0.25� while underestimation of total andtropospheric NO2 ranges from 15 to 30% (Celarier et al., 2008).

3. Results and discussion

3.1. Tropospheric O3 as an oxidation capacity parameter; itsassociation with NO2

Fig. 2 presents the geographical distribution of the monthlymean values of TOR and TNO over mainland Greece for the centralmonths of winter and summer seasons (January and July) of theyears 2005, 2008 and 2011. It is worth noting that TOR data have agrid analysis of 1� � 1.25�, while the grid analysis of TNO data is0.25� � 0.25�. Inspection of Fig. 2 shows that the locations of whichtropospheric ozone and NO2 are emitted, may be easily defined.However, the location of the source of NO2 is more clearly defineddue to the fact NO2 grid resolution is more refined than that of thetropospheric ozone. It is therefore suggested that the instrumentsflown on a new geostationary satellite mission for the air qualitymonitoring, should have a grid analysis at least of 0.25� � 0.25�.Comparing the January and July pictures in Fig. 2 we conclude thatTOR presents greater spread than TNO. This is probably due to thefact that the lifetime of the tropospheric ozone is about 2e5 days inthe lower troposphere reaching 90 days in the middle troposphere,while the lifetime of NO2 is generally lower, ranging from hours to afew days depending upon the altitude, location and season (Karet al., 2010).

In addition, Fig. 2 shows that the TOR and TNO travel from thewider Athens area southwards thus affecting the islands nearbyAttica. This fact raises the question about the necessity of anexpansion of the monitoring sites network, operated by the Min-istry for the Environment Energy & Climate Change, at these re-gions in order to examine the exposure of the local population tothe migrated air pollution from the Athens basin as well as to give abetter insight of the migration of the air pollution.

In particular, Fig. 3 shows typical airmass forward trajectoriesfor a few of the months (January 2008 and July 2005, 2008, 2011)presented in Fig. 2, where ozone sources are not clearly identified.According to Fig. 3, trajectories coincidewith ozone transfer patternmentioned in Fig. 2.

We next move to the investigation of the potential interplaybetween TOR and TNO. In Fig. 4 the relationship between TOR andthe corresponding TNO values over Athens, for the period October2004 to December 2011 is presented. According to Spearman’s testthe correlation coefficient between these two datasets was found tobe �0.85 (statistically significant at the 99% confidence level). Ac-cording to Fig. 4 the relationship between TOR and TNO can bemodeled by the following relationship (statistically significant atthe 99%: confidence level).

½TOR� ¼ 2:93½TNO�2 � 27:75½TNO� þ 92:93; with R2 ¼ 0:74

where [TOR] is the tropospheric O3 column concentration and[TNO] the tropospheric NO2 column concentration. According tothis relationship, the strong negative correlation between the TORand TNO holds up to a turning point (i.e. about 25 DU,4 � 1015 molec cm�2) and then it turns into a slight positive cor-relation (for linear relationships shown in Fig. 4; Left: statisticallysignificant at the 99% confidence level, Right: not statistically sig-nificant). This turning point could be interpreted by the “optimumVOC/NOx ratio”. In this regard, Seinfeld and Pandis (2006) sug-gested that for a given amount of VOC, there is an optimum VOC/NOx ratio, which maximizes the ozone production. In particular, forratios less than the optimum, an increase in NOx leads to an O3

Fig. 2. Geographical distribution of monthly mean values of TOR and TNO over mainland Greece.

Fig. 3. Typical airmass forward trajectories for several months presented in Fig. 2, where ozone sources are not clearly identified.

C. Varotsos et al. / Atmospheric Environment 89 (2014) 721e730 725

decrease, while for ratios greater than the optimum, an increase inNOx lead to an increase in O3. The significance of this point shouldbe investigated further.

In addition, a characteristic example of the negative correlationbetween the TOR and TNO is given in Fig. 5.

It should be emphasized that this result is opposite to that ob-tained by Kar et al. (2010). In this regard, Kar et al. (2010) found thatthe correlation coefficient between TOR and TNOwas 0.6. They alsosuggested that the correlation between the tropospheric columns

Fig. 4. Correlation between TOR and the corresponding tropospheric column NO2

(TNO) values over Athens for the period October 2004eDecember 2011.

of the two species likely depends upon whether the ozone pro-duction is NOx limited or VOC limited and this can vary significantlyfrom city to city and also possibly from event to event. However,according to Seinfeld and Pandis (2006) a positive correlation be-tween TOR and TNO, as in the case of Kar et al. (2010), indicates thatthe ozone production is possibly NOx limited. On the contrary, ac-cording to former studies, ozone production at Athens is VOClimited (or NOx saturated) (Ziomas et al., 1998; Bossioli et al., 2007).

Fig. 5. Tropospheric Ozone Residual (TOR) and tropospheric column NO2 (TNO)monthly mean values over Athens during the period 2005e2011.

Fig. 6. Interannual variability of total (TOZ), stratospheric (STR) and tropospheric(TOR) ozone columns over Athens, Greece during 1979e2011.

C. Varotsos et al. / Atmospheric Environment 89 (2014) 721e730726

This is also confirmed in the present study, by the fact that thecorrelation coefficient between NOx and O3 is negative, which im-plies possible VOC limited characteristics of the site underconsideration. It may probably happens because high NOx con-centrations tend to consume more OH radicals through HNO3forming reactions and thus reducing the oxidizing capacity of VOCand NO2, which are the main O3 precursors (Seinfeld and Pandis,2006).

We may easily reach the above conclusions recalling the mainreaction cycles between O3 and NO2 in the troposphere, whichcould be briefly summarized as follows (e.g., Atkinson, 2000):

� Nitrogen monoxide reacts rapidly with O3 to form NO2 which inturn is photolyzed regenerating O3:

NO þ O3 / NO2 þ O2 (1)

NO2 þ hn / NO þ [O] (2)

O þ O2 þ M / O3 þ M (3)

M is another molecule (e.g., N2 or O2)

� NO is also converted to NO2 by reaction with hydroperoxyl andorganic peroxy radicals

HO2 þ NO / NO2 þ OH (4)

RO2 þ NO / NO2 þ RO (5)

(R ¼ H or any organic group).Additionally,NOandNO2canbeconverted toperoxyacetyl nitrate

(PAN) e formed from NOx and VOCs, particulate nitrate, HNO2,HO2NO2, NO3, N2O5, and organic nitrates, thus providing temporaryreservoirs for NOx. The afore-mentioned set of reactions (1)e(3)implies a mechanism for O3 production but cannot interpret theobserved O3 amount in the troposphere, as the O3 produced in (3) isalmost totally destroyed in (1). Reactions (4) and (5) suggest alter-native ways for NO2 formation without destroying O3. As VOCamount is greater than the amount of HO2 in the troposphere, (5)becomes more important than (4) in O3 lifecycle. Summarizing, wecould say that O3 production is prescribed by the NO2 formationprocedure.When troposphere is rich inVOCsubstances, i.e. the valueof VOC/NOx ratio is higher than the optimum,NO2 formation followsthe reaction (5) and as a result the O3 production is limited by theavailable amount of NOx (NOx limited situation). Under this situation,an NOx increase results in an O3 increase. On the contrary, whentroposphere is not rich in VOC substances, i.e. the value of VOC/NOx

ratio is less than the optimum, NO2 formation mainly follows reac-tion (1) and O3 production is limited by the available amount of VOCfor reaction (5) (VOC limited situation). Under this situation, an NOx

increase is followed by an O3 decrease. From theoretical estimationsOLR depends on surface temperature, clouds, aerosols and otherfactors also (Alexopoulos and Varotsos, 1981; Varotsos, 2007; Tzaniset al., 2009). Interestingly, isolating one component, TOR, then astrong correlation between them is found.

3.2. Long-term trend in the tropospheric ozone column over Athens,Greece

As mentioned in the Introduction the ozone budget in thetroposphere is determined by both chemical and dynamical

mechanisms. For example, as it was said above, the ozone exchangebetween the troposphere and stratosphere through the tropopauseis linked with mechanisms of dynamical origin (e.g. tropopausefolding) (Junge, 1962; Holton et al., 1995; Varotsos et al., 2008a). Onthe other hand, increasing human activities (i.e., enhanced emis-sions of primary air pollutants) result in the increased photo-chemical production of tropospheric ozone. It is therefore expectedto observe a positive long-term trend in the temporal evolution ofTOR. In parallel, the amount of STR is decreasing with time due tothe release into the stratosphere of ozone depleting substances (e.g.chlorofluorocarbons e CFCs). Thus, the conventional pattern of thetemporal evolution of the ozone content in the atmosphere (due tohuman activities) is often a positive trend in TOR and a negativetrend in STR (Kondratyev and Varotsos, 2002; WMO, 2010).

Fig. 6 shows the interannual variability of TOR, STR and TOZ overAthens for the period 1979e2011. In Fig. 6 the linear fit lines,calculated by the least squares method, to the above mentionedozone values, for the periods 1979e1993 and 1993e2011, are alsopresented. It should be mentioned that based on the results ob-tained from the statistical tests the year 1993was the first year after1990, when the correlation coefficient of the trend line before theturning point is statistically significant at 99% confidence level. Dataanalysis also revealed that TOZ over Athenswas depleted from 1979to 1993 at a rate of almost �1.3 DU per year (statistically significant3s), while during the next few years, from 1993 to 2011, it wasincreased at a rate of about þ0.18 DU per year (not statisticallysignificant). Stratospheric and tropospheric ozone follow similarbehavior, i.e. during the first period, STR and TOR declined at a rateof �1.28 DU and �0.02 DU per year (statistically significant 3s),respectively. After 1993, STR increased at a rate of almost þ0.17 DUper year (not statistically significant) and TOR at a rate of þ0.01 DUper year (not statistically significant). It is worth noticing that ac-cording to the above mentioned increasing and declining rates ofTOR almost 63% of the tropospheric ozone, which was lost duringthe years 1979e1993, was replenished during the period 1993e2011. On the other hand, the recovery and decline rates of STR showthat the stratospheric ozone needs more time to be replenished.According to the above mentioned recovery rate of STR, strato-spheric ozone over Athens is expected to recover to the amount of1979 at about 2120. As mentioned above the decline of the

Fig. 8. Annual mean time series of TOR and the individual contributions of QBO, ENSOand 11-year solar cycle (in DU) to the interannual variability, during 1979e2011 overAthens, Greece.

C. Varotsos et al. / Atmospheric Environment 89 (2014) 721e730 727

stratospheric ozone is well-tied to human activity which hasincreased dramatically the burden of chlorine and bromine in theatmosphere previous years and the consequences are observeduntil nowadays (WMO, 2007, 2010).

Many studies have estimated that a 1% decrease in TOZ cancause about 1.25 � 0.20% increase in the UV-B that reaches thesurface, causing harmful effects on human health and the envi-ronment (WMO, 1994). On the other hand, TOR is actually moreeffective at absorbing UVR than STR because UVR is scattered muchmore in the troposphere and therefore it makes a longer pathlength (Brühl and Crutzen,1989). The increase in TOR is expected toreduce the rising UVR fluxes that reach the tropopause because ofthe decrease in STR. Until recently, surface measurements of theUVR did not confirm this expectation (Feretis et al., 2002; Tzaniset al., 2008). It is not clear yet if the increased concentration ofTOR can substitute the decreased concentration of STR. Finally, thetemporal periodicities or the quasi-periodicities that TOR exhibitsinduce noise to the derivation of the long-term trend and they willbe investigated in the next sub-section.

3.3. Cyclic variations of the tropospheric ozone column over Athens,Greece

Keeping in mind the above mentioned findings about the long-term temporal evolution of TOR and the strong negative correlationbetween TOR and TNO values it should be interesting to investigatein depth the TOR monthly, seasonal and interannual variability, thesignature of which may be seen in Fig. 6.

In Fig. 7, the time series (1979e2011) of the monthly meanvariability of TOR over Athens is presented. This time series is non-stationary (e.g. its mean, standard deviation and higher moments,as well as the correlation functions, are not invariant under timetranslation) due to non-linear variations (e.g. the variations do notfollow a linear relationship) of tropospheric ozone. Spectral anal-ysis was applied to the TOR time series in order to detect the pe-riodicities that contribute to its interannual variability. To this end,the Wiener spectral analysis (Wiener, 1949) was applied to theannual mean values of TOR. The results obtained revealed thepresence in TOR time series of the quasi-biennial oscillation (QBO),the El Niño-Southern Oscillation (ENSO) and the 11-year solar cyclesignals. The percentage contributions of QBO, ENSO and 11-yearsolar cycle to the TOR interannual variability were estimated at9.61%, 21.86% and 23.13%, respectively. Fig. 8 presents the individualcomponents’ contributions as well as the time series of the TORannual mean.

Fig. 7. Time series of the monthly mean variability of TOR during 1979e2011 overAthens, Greece.

3.4. Induced TOR variability from other atmospheric parameters

As mentioned above one crucial mechanism for the tropo-spheric ozone budget is the stratosphereetroposphere exchange(STE). This mechanism is closely associated with the properties ofthe tropopause through which this exchange occurs.

Thus, we are focusing here on the study of the tropopauseheight which depends on the latitude and the season. In Fig. 9 weplot the monthly mean values of the tropopause height (hPa) alongwith the corresponding TOR values for the case of Athens. Ac-cording to Fig. 9, the maximum tropopause height occurs in the endof the summer season, while its lowest values are observed duringthe winter season. The largest values of the tropopause heightduring summer suggest a strong link of it with the troposphericozone because of the deep convection which continuously pushesthe tropopause upwards. Convective air masses penetrating intothe lower stratosphere enforce subsidence, pushing the strato-spheric air into the troposphere, not far from the regions of con-vection. Deep convection is shown to be extensive over the NH

Fig. 9. Variation of monthly means of tropopause height (hPa) and TOR over Athens,Greece.

C. Varotsos et al. / Atmospheric Environment 89 (2014) 721e730728

continents in summer (Tang et al., 2011). For instance, Logan (1999)found that the O3 maximum in the upper troposphere overnorthern midlatitudes continents occurs in June, and is consistentto the deep convective STE flux.

Another principal parameter for the vertical tropospheric extentis the longwave radiation leaving the top of the atmosphere. Fromtheoretical estimations OLR depends on surface temperature,clouds, aerosols and other factors also. Interestingly, isolating onecomponent, TOR, then a strong correlation between them is found.Fig. 10 shows the seasonal variability of OLR in conjunctionwith theseasonal variability of TOR. It is evident from Fig. 10 that the OLRincreases during summer reaching its highest values in July andAugust (Ardanuy and Kyle, 1986; Weare and Soong, 1990; Yanget al., 1999). This result was expected, as warm surfaces radiatemore in the longwave spectrum.

Interestingly, Fig. 10 indicates a positive correlation betweenOLR and tropospheric ozone values, unlike what was expected,notably: tropospheric ozone, as a greenhouse gas, was expected toabsorb more longwave radiation thus decreasing the total amountof the longwave energy emitted to space (Bowman et al., 2013). Inother words, the reduction in OLR due to absorption by tropo-spheric ozone was expected to lead to a negative correlation be-tween OLR and TOR. However, that is not the case, becauseaccording toWorden et al. (2011) OLR shows only a slight variabilityin the tropospheric ozone changes (due to clouds), except in theupper troposphere, where the ozone abundance there, is just asmall fraction of the columnar tropospheric ozone.

It should be stressed that the positive correlation that we havefound between OLR and TOR cannot be attributed to the seasonalvariability of the tropopause height (shown in Fig. 9) because theOLR does not depend on the changes in tropopause height (Seideland Randel, 2007).

In addition, it should be recalled that ozone is not the mostimportant greenhouse gas in the troposphere. The OLR is mainlyabsorbed by water vapor and carbon dioxide. In the case, where theabundance of water droplets or clouds was increased in the lowerand middle troposphere, then the ozone uptake that is taking placeon the water droplets or clouds would deplete ozone in thesetropospheric regions. Consequently, less ozone in the lower andmiddle troposphere (indicated as a reduced TOR maximum insummer) may be associated with enhanced content in troposphericwater content, or clouds, which emit higher amounts of longwaveradiation upwards, thus contributing to the high level of OLR.

Fig. 10. Variation of monthly mean values of OLR and TOR over Athens, Greece.

Additionally, monthly variations of TOR indicate that themaximum is observed in June and July, while peak values con-cerning OLR appear in July and August. The highest value of thesetwo atmospheric parameters seems to be time-shifted. This may bedue to the fact that TOR acts as an absorber of OLR and therefore themaximization of TOR delays the maximization of OLR, while whenTOR starts to decrease then OLR reaches its highest value.

4. Conclusions

From the discussion above the following conclusions can bedrawn:

1) The existing satellite observations, even with grid analysis of1� �1.25�, may be employed for the detection of the location ofthe tropospheric ozone and NO2 sources in the scale of a city likeAthens. However, higher grid analysis (0.25� � 0.25�) couldprovide more reliable results.

2) The correlation coefficient between TOR and TNO datasets wasfound to be �0.85 despite the recent findings which suggestedstrong positive correlation. This strong negative correlationturns out to a slight positive correlation when the TNO con-centration becomes higher than around 4 � 1015 molec cm�2.The geographical distribution of trace gases generated in widerAthens area reveals that they travel southward affecting islandsnearby Attica state (e.g. Salamis, Aegina, Angistri, Poros).

3) The satellite observations over Athens for the period 1979e2011show a decrease in total, stratospheric and tropospheric ozonevalues until 1993 with rates�13,�12.8 and�0.2 DU per decade,respectively and an increase of them until 2011 withrates þ1.8, þ1.7 and þ0.1 DU per decade. These rates suggestthat the tropospheric ozone has replenished its lost amount,while stratospheric ozone needs almost 120 years in order toreturn back to the level of 1979.

4) The tropopause height reaches its highest values during sum-mer, suggesting a strong association with tropospheric ozonebecause of the deep convection which continuously pushes thetropopause upwards.

5) An unexpected positive correlation between OLR and TOR wasfound, which may probably be explained by the reduction of theTOR summertime maximum due to the uptake mechanisms ofozone on the enhanced content of water droplets or clouds inthe lower and middle troposphere, thus resulted in higheramounts of longwave radiation propagated upwards.

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