high temporal resolution measurements of ozone precursors in a rural background station. a two-year...

High temporal resolution measurements of ozone precursorsin a rural background station. A two-year study

M. Navazo & N. Durana & L. Alonso &

M. C. Gómez & J. A. García & J. L. Ilardia &

G. Gangoiti & J. Iza

Received: 21 December 2005 /Accepted: 31 October 2006 / Published online: 11 May 2007# Springer Science + Business Media B.V. 2007

Abstract We present a very complete database ofindividual non-methane hydrocarbon (NMHC) mea-surements with high temporal resolution (hourly) in arural background atmosphere. We show their use tocharacterize the biogenic NMHC as well as to identifythe transport and impact of anthropogenic NMHC onrural areas. In January 2003 an automatic GC-FIDanalyzer of volatile organic compounds between 2and 10 carbon atoms (C2–C10 VOCs) was placed inthe centre of the Valderejo Natural Park in northernIberia (42.87°N, 3.22°W), far away from importantcities. The system operated continuously until De-cember 2004. Data coverage was higher than 70% fora total of 59 VOC of both anthropogenic and biogenicorigin, with detection limits in the range of pptv. Ourresults allow for the description of the behaviour ofthese compounds, in order to identify external impactsarriving to the sampling site which has been recog-nized to be highly representative of a rural backgroundatmosphere. Biogenic VOC concentrations have beencompared also with the calculated emissions, usingGuenther’s algorithm, and the discrepancies inter-

preted in terms of the different reactivity of suchcompounds.

Keywords Volatileorganiccompounds . Non-methanehydrocarbons . Isoprene .Monoterpenes .

Ozone precursors

Introduction

Volatile organic compounds (VOC) have a key rolein the formation of tropospheric ozone, and otheroxidizing agents of the atmosphere (Crutzen et al.1999). Ozone directly acts as a greenhouse gas, andindirectly as a controller of other greenhouse gaseslifetime (Intergovernmental Panel on Climate Change(IPCC) 2002).

VOC may be found in measurable amounts inremote areas (Blake et al. 1999), and in significantamounts in both rural areas (Goldan et al. 1995) andurban environments (Navazo et al. 2003). They areintroduced into the atmosphere through fossil fuel use(evaporation and combustion processes), emissionfrom vegetation on land and in the sea, biomass burn-ing, and geochemical processes. Although there arefewer species of biogenic origin, this group representsby far the largest source of VOC to the atmosphere,isoprene being the most important single globalsource at approximately 500 Tg/a (Guenther et al.1995, 2000).

Environ Monit Assess (2008) 136:53–68DOI 10.1007/s10661-007-9720-4

M. Navazo (*) :N. Durana : L. Alonso :M. C. Gómez :J. A. García : J. L. Ilardia :G. Gangoiti : J. IzaEscuela Técnica Superior de Ingeniería, UPV-EHU,Alameda de Urquijo s/n,48013 Bilbao, Spaine-mail: [email protected]

Early ozone management strategies emphasizedreductions of anthropogenic emissions of VOC, suchas those emitted by automobile exhausts. These strat-egies, in cities where anthropogenic emissions dom-inate the ambient VOC mixture, have successfullyreduced peak ozone concentrations over time.

In some locations, however, large emissions ofvery reactive VOC from biogenic sources have beenresponsible to substantially contribute to ozone for-mation in both rural and urban areas. Combinedcontrol of both anthropogenic NOx and VOC is nowthought to be needed to reduce ozone in regions char-acterized by strong biogenic VOC sources (Simpson1995; Simpson et al. 1995; Fuentes et al. 2000;Jiménez and Baldasano 2004).

In order to characterize VOC of biogenic originand the transport and impact of anthropogenic VOCon remote areas, the analyzer was operated in a ruralarea, far away from important cities, in the centre ofthe Valderejo Natural Park (Northern Spain) sinceJanuary 2003 until December 2004.

Nowadays due to the scarcity of data, the currentknowledge on the composition of VOC of naturalorigin at rural background areas is very limited withrespect to urban and industrial areas (Kang et al.2001; Mohamed et al. 2002). That lack of data wasvery relevant at the Northern and Western areas of theIberian Peninsula. Therefore, one of the objectives ofour study is to cover (part of) that gap, providing witha quite complete database of individual VOC mea-surements, with high temporal resolution (hourlymeasurements). In this paper we present and describethis database and a selection of examples to showsome interesting applications in air quality studies.

Methodology

Sampling site description

The sampling site is located in the geographic centreof the Valderejo Natural Park, near the Western borderof the Basque country (Fig. 1). It is located nearby asampling station from the Regional Air QualityMonitoring Network from the Basque Government,and, thus, fulfils all micro- and macro-sitting criteria.

The Park has a size of about 3,500 ha and islocated more than 50 km away of the main urban sitesof the area. The site elevation is 900 m (bottom of thevalley), surrounded by mountains ranging from 1,040to 1,235 m of elevation. The whole valley ispractically inhabited.

On the mountains surrounding the site there is amixture of Atlantic and Mediterranean forest, poten-tially emitter of Biogenic VOC, with large extensionsof Pinus sylvestris, Fagus sylvatica, Quercus ilexsubsp. rotundifolia, and Quercus Faginea trees (Ruizde la Torre 2002). On clear areas, Erica lusitanicabushes are frequent, and, at the bottom of the valleythere are some grasslands and cereal cultures.

Analytical equipment

For the experimental setup, the ozone precursorautomated measurement system, previously optimizedin the Bilbao urban area during the period 1997–2001, was used (Durana et al. 2002; Navazo et al.2003; Gómez et al. 2004). The system is based on anautomated Perkin Elmer GC-FID analyzer fitted witha pre-concentration trap, operating at low temperature,

Fig. 1 Geographic locationof the sampling site

54 Environ Monit Assess (2008) 136:53–68

and double column with two FID. Operation wasoptimized for systematic, unattended measurement ofmore than 60 C2–C10 NMHC every hour, withsensitivities better than 0.1 ppbv from most of thecompounds.

The very low concentration values expected at therural background site obliged us to further optimizethe sensitivity of the system, by refining the gas de-livery auxiliary subsystems and enhancing the signal-to-noise ratio of the detector (reducing the noisescaused by switching valves and controlling thehumidity of the gases and the flame characteristicsof the FID).

For all the measured compounds, detection limits,calculated as per the Code of Federal Regulations 40,Part 136, Appendix B (Code of Federal Regulations(CFR) 1993), are always lower than 0.11 ppbv. Inpractice, the system actually in use at the Valderejosite reaches detection limits lower for all the measuredcompounds, respect to the previously reached in the

urban area of Bilbao (Durana et al. 2002), especiallyfor propene, acetylene, i-pentane and n-heptane.

The described system gives an adequate answer toall the requirements posed by the remote sitemeasurement: FID detection offers sub-ppbv sensitiv-ity, bidimensional chromatography allows for anhourly resolution of the measurements, and theancillary equipment enables for an unattended, auto-mated measuring process during long periods.

The system was operated uninterruptedly for twoyears, making a daily remote diagnostic and “in situ”servicing when required (every two to three weeks).Due to the fact that the amount of data generated bythe system is quite large, and that some of the peaksare very close to their detection limits, making moredifficult their identification and quantification, thedata validation procedure is very complex and timeconsuming, as much as the chromatographic tech-niques themselves. Using the acquired knowledgewhile the system was operating at an urban site, the

Table 1 Summary of relevant statistical data for C2–C6 VOC concentrations (ppbv) at the Valderejo rural background site during2003–2004

Compound Min Max Median Mean Std. dev. N of valid data

Ethane 0.37 7.25 1.42 1.44 0.53 12,031Ethene <DL 12.44 0.13 0.24 0.42 12,031Propane 0.02 6.41 0.42 0.47 0.29 12,031Propene <DL 3.71 0.07 0.09 0.11 11,975i-Butane <DL 13.97 0.10 0.12 0.19 12,031n-Butane <DL 2.78 0.17 0.20 0.17 12,031Acetylene <DL 4.82 0.22 0.25 0.17 12,031Trans-2-butene <DL 0.21 0.04 0.05 0.02 11,9741-Butene <DL 0.65 <DL 0.01 0.02 12,030i-Butene 0.02 0.76 0.08 0.09 0.03 10,959Cis-2-Butene <DL 0.12 0.01 0.01 0.01 11,974Cyclopentane <DL 0.07 <DL <DL <DL 12,030i-Pentane <DL 1.29 0.08 0.09 0.07 12,026n-Pentane <DL 0.78 0.05 0.06 0.05 12,0251,3-Butadiene <DL 0.67 0.03 0.03 0.03 12,0253-Methyl,1-butene <DL 0.06 <DL <DL <DL 12,006Trans-2-pentene <DL 0.12 <DL <DL <DL 12,0262-Methyl,2-butene <DL 0.15 <DL 0.01 0.01 12,0271-Pentene <DL 0.19 <DL <DL 0.01 12,027Cis-2-pentene <DL 0.33 <DL <DL <DL 12,0292,2-Dimethylbutane <DL 0.03 <DL <DL <DL 12,0292,3-Dimethylbutane <DL 0.12 <DL <DL 0.01 12,0292-Methylpentane <DL 0.41 0.02 0.02 0.02 12,0293-Methylpentane <DL 0.52 0.01 0.01 0.02 12,023Isoprene <DL 1.77 <DL 0.05 0.12 12,024

<DL Below detection limit

Environ Monit Assess (2008) 136:53–68 55

original measurement and data quality control proto-cols were adapted to the measurement at remote areas(Durana et al. 2002; Gómez et al. 2004).

Results and discussion

Anthropogenic VOC

We have measured on an hourly basis 59 NMHC atthis site over a period of two years (2003–2004) and,from January to June 2004, simultaneously in the

middle of the city of Bilbao, in the Basque Country(Northern Spain; see Fig. 1). Analytical data gatheredwith the instrument for 59 VOC on an hourly basishave been completed with some air quality parameters(PM10, O3, NO, NO2, CO and SO2) as well as surfacemeteorological data [wind direction, wind speed,temperature, relative humidity, radiation (total andPAR) and pressure].

As we can see in Tables 1 and 2, in Valderejoduring 2003 and 2004, all measured concentrationsranged from the detection limit of each compound upto a maximum absolute value of 13.97 ppbv. In 2003,

Table 2 Summary of relevant statistical data for C6–C10 VOC concentrations (ppbv) at the Valderejo rural background site during2003–2004

Compound Min Max Median Mean Std. dev. N of valid data

n-Hexane <DL 0.81 <DL 0.01 0.03 11,783Methylcyclopentane <DL 0.22 <DL <DL 0.01 11,7832,4-Dimethylpentane <DL 0.09 <DL <DL <DL 11,783Benzene <DL 1.55 0.07 0.075 0.07 11,783Cyclohexane <DL 0.85 <DL 0.015 0.05 11,7832-Methylhexane <DL 0.12 <DL <DL 0.01 11,7832,3-Dimethylpentane <DL 0.06 <DL <DL <DL 11,7833-Methylpentane <DL 0.18 <DL <DL 0.01 11,783Trichloroetene <DL 0.16 <DL <DL 0.01 11,7832,2,4-Trimethylpentane <DL 0.34 <DL <DL 0.01 11,783n-Heptane <DL 0.13 <DL <DL 0.01 11,783Methylcyclohexane <DL 0.13 <DL <DL 0.01 11,7832,3,4-Trimethylpentane <DL 0.10 <DL <DL <DL 11,783Toluene <DL 6.47 0.08 0.1 0.14 11,7832-Methylheptane <DL 0.04 <DL <DL <DL 11,7823-Methylheptane <DL 0.05 <DL <DL <DL 11,782n-Octane <DL 0.10 <DL <DL 0.01 11,309Tetrachloroetene <DL 0.12 <DL <DL 0.01 11,309Ethylbenzene <DL 0.63 0.01 0.015 0.02 11,782m and p-xylene <DL 2.05 0.02 0.03 0.05 11,782Styrene <DL 0.14 <DL <DL 0.01 11,298o-Xylene <DL 0.29 <DL 0.01 0.01 11,782n-Nonane <DL 0.53 <DL <DL 0.01 11,782i-Propylbenzene <DL 0.04 <DL <DL <DL 11,748n-Propylbenzene <DL 0.17 <DL <DL <DL 11,548p-Ethyltoluene <DL 0.36 <DL <DL 0.01 11,7481,3,5-Trimethylbenzene <DL 0.73 <DL <DL 0.01 11,559o-Ethyltoluene <DL 0.34 <DL <DL 0.01 11,7471,2,4-Trimethylbenzene <DL 1.63 <DL 0.01 0.03 11,744n-Decane <DL 1.49 <DL 0.01 0.03 11,7471,2,3-Trimethylbenzene <DL 0.34 <DL <DL 0.01 11,748m-Diethylbenzene <DL 0.38 <DL <DL 0.03 11,748p-Diethylbenzene <DL 0.25 <DL <DL 0.01 11,748Monoterpenes <DL 2.94 0.03 0.10 0.08 10,040

<DL Below detection limit


only a few cases showed concentrations higher than10 ppbv, whereas in 2004 no values were found to beover 5 ppbv, with the exception of ethene, with a5.08 ppbv maximum on November 27, 2004 at17:00 UTC.

In general, as VOC lifetimes in the atmosphere arerelatively short with respect to their mixing time, thereis no theoretical lower limit for their concentration and,thus, it is not possible to establish some absolute rangefor the ambient air VOC concentrations. The onlyexception is ethane, with a photochemical lifetime ofabout 10 days, whose measurements in the atmosphereof the Northern Hemisphere indicate a minimalconcentration of 0.3 ppbv, a value which could betaken as its theoretical lower limit. At the Valderejosampling site, the ethane minimum concentration valuerecorded for an hourly sample was of 0.37 ppbv.

Yearly average concentration values are below1 ppbv, with the exception of ethane, whose averagewas of 1.50 ppbv in 2003 and 1.37 ppbv in 2004. It isimportant to notice that more than 30 compoundshave yearly averages below their detection limits.

Among VOC with more than five carbon atoms onlybenzene, toluene and m and p-xylene had yearlyaverages over their respective detection limit.

It has been observed that the main monoterpenes(summing up more than 95% of the total identifiedmonoterpenes) coelute on the chromatogram as twoseparated peaks. All of those compounds having thesame empirical formula (C10H16), they have beenquantified using response factors related to benzene.Results have been named as generic “monoterpenes”,and appear at the last row of Table 2.

As mentioned above, during the period betweenJanuary and June 2004 a similar system was operatedin a previously characterized as a urban site, Bilbao,65 km away from the rural site. Results from bothsites are presented on Fig. 2, where logarithmic scaleshave been chosen in order to ease the comparison.As a first result, it can be seen that only ethane hascomparable mixing ratios. The differences for theremaining compounds are about one order of mag-nitude, lower for the rural site, which indicates thatthe Valderejo sampling site is highly representative

Fig. 2 Comparison of hour-ly averages for NMHCobtained at a urban envi-ronment and a rural one,from January to June 2004


of a rural background atmosphere. The isoprene is aspecial case because its both anthropogenic andbiogenic origin. In the period showed in Fig. 2(January–June) the biogenic emissions are low (onlyJune shows significant concentrations of isoprene ascan be observed in Fig. 7). However, the anthropo-genic isoprene during this period is important in oururban area (Durana et al. 2006).

Compound families

Table 3 summarizes the results from 2003 and 2004,grouped as compound families, and expressed asvolume percentages.

It can be seen that the relative relevance of eachfamily does not change drastically from one year tothe other. Paraffins are the most abundant group, closeto 70% of the total volume of identified VOC,whereas olefins are in the 24–27% range, andaromatics in the 6–7% range. When comparing thoseresults with the ones obtained at the Bilbao urban site

(last column), it can be seen that the rural backgroundatmosphere is richer in paraffins (less reactive com-pounds) and far less abundant in aromatics than theurban atmosphere. Aromatics, being more reactive,are oxidized during the transport process.

It was noticed that some compounds showed aquite high background concentration, i.e. ethane andpropane. Ethane, which may be considered as a tracecomponent of the natural atmosphere composition,and propane showed minimum concentration valuesduring spring and summer, and maxima during falland winter (Fig. 3). This general trend observed isalso found in the literature (Rudoph 1995), and itseems that these changes are due to the presence inthe atmosphere of oxidant, mainly �OH, O3 and NO�

3

(Atkinson and Arey 2003).

Comparison weekend/weekday

In the Natural Park, as the presence of visitors and theirvehicles is noticeable during holidays and weekends, itwill be possible to forecast a higher influence of traffic-sourced VOC during those periods. Only some isolatedmeasurements confirmed this influence, and it wasimportant to assess if differences between both periodswere significant or not. All data from each year wassubjected to a comparison procedure, and results forthe most abundant compounds are shown on Table 4.

There are little differences between both periods –maybe due to the fact that the number of data used forthe averages is different – but they are not statistically

Table 3 Distribution of the relative relevance of eachhydrocarbon family at the rural background site (2003–2004)and at the urban site (2004)

Group 2003 2004 2004 (Bi)

Paraffins 66.4 69.6 48.0Olefins 26.5 24.1 23.0Aromatics 7.1 6.3 27.3Chlorinated 0.0 0.0 1.7

Fig. 3 Evolution of dailyand monthly mean mixingratios of ethane and propaneat the Valderejo station,years 2003 and 2004


significant, and they hold from one year to the other.Some biogenic compounds, such as monoterpenes(fully biogenic origin) and isoprene (mainly of bio-genic origin), show similar trends, confirming the factthat the influence on the air quality of the touristactivity (especially vehicle traffic) at the park ispractically negligible.

Comparison with other rural backgroundsampling sites

Data from the Valderejo site has been compared withthe Campisábalos (Guadalajara) station, being theonly sampling site of the EMEP-CAMP network (ES9)

which measures VOC at a rural background site inSpain. Data were taken from two reports from the SpanishMinistry of the Environment (MIMAM 2005a,b).

As their measuring routine only uses samples takenbetween 12:00 and 12:20 UTC every four to five days,to calculate the monthly average, we have selectedfrom our archives the data from the same periods inorder to compare them correctly. Results can be seenon Fig. 4.

It can be seen that there is a good concordance formost of the compounds, especially when consideringthe very low concentration values involved, being theexception the ethane values, a difference which maybe due to the sampling and measurement method

Table 4 Weekend–weekday comparison for the most abundant compounds

Compounds 2003 2004

Weekday Weekend Weekday Weekend

Ethane 1.49 1.52 1.33 1.43Ethene 0.23 0.31 0.19 0.27Propane 0.48 0.47 0.44 0.50Propene 0.10 0.12 0.07 0.09i-Butane 0.11 0.13 0.10 0.12n-Butane 0.20 0.22 0.18 0.22Acetylene 0.26 0.27 0.21 0.24i-Butene 0.10 0.11 0.07 0.07i-Pentane 0.09 0.09 0.08 0.09n-Pentane 0.07 0.07 0.05 0.05Isoprene 0.04 0.06 0.03 0.04Benzene 0.08 0.08 0.06 0.08Toluene 0.12 0.12 0.08 0.09Monoterpenes 0.07 0.08 0.05 0.06

Mean values (ppbv)

Fig. 4 Annual data compar-ison between the Valderejosite and the EMEP-CAMPstation


employed at the Campisábalos site. Also, significantdifferences can be seen on the average values ofhexane, c-2-pentene and toluene in year 2003 becausethe EMEP averages have been influenced by somehigh peak values measured, which may be due to thedirect influence on the site of some traffic or industrialsource.

The most reactive hydrocarbons

Among the list of measured compounds, many ofthem contribute to the formation of photochemicaloxidants, but each of them has a different contribu-tion, according to their concentration, reaction ratesand ozone formation potential. Therefore, it isinteresting to compile the information regardingconcentration and reactivity of each compound inorder to calculate the photochemical ozone formationpotential. This may be done by multiplying the

concentration of each compound by its maximumincremental reactivity (MIR). On this work, the MIRused was proposed by Carter (2000), which isexpressed as mol O3/mol VOC. The interpretation ofthis parameter is straightforward: The MIR value isthe number of additional ozone moles generated whenone mol of the VOC of interest is added to a VOCmixture. On Table 5, the compounds whose contri-bution to the ozone formation potential is higher atthe Valderejo site are summarized. Monoterpenes,light olefins, toluene and isoprene are the compoundswhich contribute the most to the ozone formation. It isimportant to notice the relevance of olefins andaromatics, which compose the majority of the list.

Daily evolution

On Figs. 5 and 6 the hourly averages for someselected compounds during 2004 are plotted. Only

Table 5 Top 10 reactive compounds and their contribution to the O3 formation

Year 2003 Year 2004

Compound O3 Formation (ppbv) Compound O3 Formation (ppbv)

Monoterpenes 1.45 Ethene 1.17Ethene 1.34 Monoterpenes 0.94Propene 1.06 Propene 0.81Toluene 0.89 Trans-2-butene 0.65i-Butene 0.80 Toluene 0.61Trans-2-butene 0.73 Isoprene 0.61Isoprene 0.72 i-Butene 0.52m and p-xylene 0.54 m and p-xylene 0.501,3-Butadiene 0.53 1,3-Butadiene 0.46

Fig. 5 Evolution of thehourly average concentra-tion of some selected VOCat the Valderejo site


two average daily cycles show a clear structure:Ethylene, with a cycle different from the one seen aturban sites, and isoprene, with a typical natural cycle.Remaining compounds have a little depression atmidday, probably associated to the increase of themixing layer height or as a result of their consumptionby reactions initiated by hydroxyl radicals.

Correlation among compounds

Statistical correlation analysis has been carried out inorder to classify the data into groups with similarcharacteristics. It can be seen on Tables 6 and 7 thePearson correlation coefficient matrices for the mostabundant species, for year 2003 and 2004, using5,693 and 6,338 valid data, respectively. Correlationcoefficients higher than 0.650 have been bolded. Thegroups of compounds with higher correlations are thefollowing:

– Light olefins: Ethene, propene, acetylene and 1,3-butadiene, generally present on tail-pipe emis-sions, are highly correlated among them (in somecases, correlation coefficients higher than 0.800),indicating a common origin. In general, thecoefficients obtained in year 2003 are higher thanthe ones for year 2004.

– Lighter hydrocarbons: Ethane and propane, theleast reactive compounds. Both compounds sharea common origin, the use of natural gas. Theircorrelation coefficient of 0.730, is the same inyear 2003 and 2004, giving an indication of thequality of the measurements carried out.

– Biogenic compounds: Isoprene and terpenes.Isoprene, as well as monoterpenes, does notcorrelate well with any of the most abundanthydrocarbons, even with negative correlationcoefficients. The only good correlation is betweenthem, confirming their biogenic (common) origin.

– C4–C5 paraffins: i-pentane, n-pentane y n-butane.Those compounds are usually related to emis-sions due to traffic, mainly by gasoline evapora-tion. In this case, the correlations from year 2004are higher than the ones from year 2003, as it canbe seen on Table 7.

Biogenic VOC (BVOC)

The relevance of the BVOC in photochemicalprocesses is an interesting research subject nowadays,i.e. the work of Tao et al. (2003) in the USA con-tinental area, Derognat et al. (2003) in the Paris region,

Fig. 6 Evolution of thehourly average concentra-tion of some selectedhydrocarbons at the Valder-ejo site

Table 6 Correlation matrices of light olefins at the Valderejo site

2003 2004

Ethene Propene Acetyl. Ethene Propene Acetyl.

Ethene 1.000 1.000Propene 0.893 1.000 0.789 1.000Acetylene 0.742 0.642 1.000 0.776 0.587 1.0001,3-Butadiene 0.873 0.862 0.679 0.565 0.580 0.257


and the results from Thunis and Cuvelier (2000) at theMediterranean. Simultaneously, several inventories ofbiogenic emissions have been developed, amongthem, the works from Velasco (2003) in México,Wang et al. (2003) in Beijing (China), Parra et al.(2004) in Catalonia (Spain), and Xu et al. (2002) inthe USA.

As in many other rural areas (Hopkins et al. 2005),isoprene shows higher values during the summermonths. As it can be seen on Figs. 7 and 8, bothBVOC isoprene and monoterpenes show a seasonalevolution decoupled from the one of anthropogeniccompounds: Maxima are found during summertime,and minima, close to the detection limit, duringwintertime. BVOC emissions are highly dependenton seasonal-related factors, such as temperature,photosynthetic active radiation, (PAR), and foliardensity, among others (Guenther et al. 1995), whosemaxima are achieved during summer. Even thoughboth compounds show similar trends, there aresignificant differences: Isoprene shows significantvalues between May and September, where thedeciduous tree species with high isoprene production(mainly Quercus spp.) have their maximum activity.

For rest of the year, the emission is negligible, and ispractically below the detection limit. Monoterpenesalso show maxima during the summer months, buttheir concentration is significant along the whole year(Fig. 8).

Figure 9 summarizes the evolution of an averageday for each month of year 2004. Several interestingfacts can be noticed: (1) ozone does not deplete atnight, which can be explained when considering thatthis is a rural background sampling station; (2) ozonemaxima can be observed during June and July,whereas the maxima for isoprene and monoterpenesis found during August, a sign that those compoundscontribute to ozone formation, but other parametersmay have a bigger influence, and (3) the daily cyclesfor isoprene and monoterpenes show trends apparent-ly contrary to each other, not following the expectedtheoretical emissions for both compounds, especiallyduring the midday hours.

In order to estimate the theoretical emission forsuch compounds, it has been used the BVOC emissionmodel of Guenther (EEA 2003; Guenther et al. 1995;Moukhtar et al. 2005; Parra et al. 2004; Sabillón2001). This model predicts the BVOC emission flux,E , in μg m−2 h−1, for a given vegetation species asfollows: E=egD, where e (in mg g�1

dw h�1) is thestandard emission rate for a reference temperature of303 K and an PAR of 1,000 μmol m−2 s−1, g is adimensionless environmental correction factordepending on the emission type and D (gdw m−2) isa foliar density factor.

For isoprene, giso is a product of two factors, PARdependant (CL) and temperature dependant (CT): giso=CLCT. For monoterpenes: gmts ¼ e b T�Tsð Þ, where b is

Table 7 Correlation matrices of C4–C5 paraffins at theValderejo site

2003 2004

n-Butane i-Pentane n-Butane i-Pentane

n-Butane 1.000 1.000i-Pentane 0.692 1.000 0.770 1.000n-Pentane 0.684 0.892 0.796 0.924

Fig. 7 Time series (hourlyvalues) of isoprene (years2003–2004) at Valderejostation (12,024 valid data)


an empirical coefficient. Guenther et al. (1995) suggest0.09 K−1 as a reasonable estimate for most plants. T isthe leaf surface temperature, in K, and Ts the referencetemperature (303 K).

Therefore, for both compounds, for a specific site,and for a short period of time, the daily evolution ofthe emissions is directly proportional to the variationof g. Thus, in order to compare the evolution patternswith the ambient concentration measured at Valderejo,it is not required to exactly calculate all emissions ofisoprene and monoterpenes; an analysis of thetemporal variation of such parameters is more thanenough for our task.

Those calculations were carried out for each day ofyears 2003 and 2004. As an example, model results aswell as hourly concentration values for isoprene canbe seen on Figs. 10 and 11 for one full week of June2003. It can be noticed the general trend is acceptable,especially during the morning rise. Later, two dis-crepancies can be noticed: first, the measured isoprene

concentration decreases while the emission continues,an effect which may be due to the photochemicaldecomposition of isoprene, mainly by reaction withOH radicals and ozone; and second, the daily max-imum concentration measured appears, when theemission is close to zero. This may be due to theisoprene transport from other close-by areas, as well asthe decrease of the photochemical activity at such timeof the day.

The measured concentration of monoterpenes doesnot apparently fit with the calculated theoreticalemissions. From the qualitative point of view, whenthe theoretical emission is low, the measured concen-tration is also low. But, when the model predicts highmonoterpene emissions, the maxima and minima donot fit between theoretical emissions and measuredvalues: they are shifted.

This fact, also observed by other authors (Lindingeret al. 1998; Holzinger et al. 2005; Lee et al. 2005), canbe explained by the much higher reactivity of mono-

Fig. 9 Monthly mean day(year 2004) of isoprene,monoterpenes, and ozone atValderejo station

Fig. 8 Time series (hourlyvalues) of monoterpenes(years 2003–2004) at theValderejo station (10,040valid data)


terpenes, having lifetime of minutes to hours, whichcause a very efficient and fast consumption during theday, even more than the one of isoprene. During thenight, the continued emission of monoterpenes (not ofisoprene) and the reduction of the number of availableradicals for reaction causes their accumulation, alsofavoured by the reduction of the mixing layer height,the surface inversion, and the low wind speed, typicalduring the night time.

Regional scale transport episodes

The maximal concentration of ozone and otheroxidants are highly dependent of the interaction

between dispersive processes at a regional scale andother phenomena with lower spatial scale, such asurban plumes, and point, linear and area sources. As arule of thumb, concentration maxima are foundbetween 30 and 100 km downwind from the sourceareas of precursors. Effects of the most reactive VOC,which react closer to the source, are not limited tosuch area; when there is an oxidation product withlonger residence time than its precursor, it may act asa secondary-effect propagating agent later in time orin space.

On Fig. 12 it can be seen a seven day evolution ofthe concentration of some selected aromatic com-pounds (summer 2003). During the early morning

Fig. 10 Measured concen-trations (C) of isopreneand calculated giso for asummer week of 2003 at theValderejo site

Fig. 11 Measured concen-trations (C) of monoter-penes and calculated gmts

for a summer week of 2003at the Valderejo site


hours of Friday and Saturday it can be seen an increaseof such compounds, but their toluene/benzene ratioindicates that it was an aged air mass, for benzeneconcentration was higher than toluene, a sign that thoseemissions were of anthropogenic origin, and trans-ported from medium or large distances.

On Fig. 13 it can be seen the evolution of theconcentration of isoprene, 1,3-butadiene, and ozone.From the analysis of the isoprene concentrationprofiles it may be concluded that isoprene had anactive participation on the ozone process formationduring the day period. During days 4 and 5, eventhough the emission is at a maximum, the isoprene

measured concentration does not fit: it starts risingduring the early morning, later to decrease sharply.Isoprene is emitted, but most of it disappears byreaction with OH radicals generated by the ozonephotolysis. Those reactions lead to the regeneration ofthe ozone concentration, showing an additional peakof about 40 μg m−3 in the late afternoon, and highlyvisible during days 5, 6 and 7.

Some periods have been marked on the graph,corresponding with simultaneous increases in theconcentration of isoprene and 1,3-butadiene at night.The meteorological analysis shows a high pressurecentre over Europe and the Atlantic, inducing over the

Fig. 12 Evolution of select-ed aromatic compoundsduring August 1–7, 2003

Fig. 13 Isoprene and 1,3-butadiene concentrationsmeasured on August 1–7,2003. Periods where airmasses with high anthropo-genic VOC content weretransported into the areahave been marked


Basque Country synoptic E–ESE winds coming fromthe Ebro Valley (Fig. 14). This situation favours thetransport of oxidant-laden aged air masses either fromthe Mediterranean or from the nearest coastal areas(Bay of Biscay), especially during the night (Gangoitiet al. 2002, 2005).

As 1,3-butadiene is of anthropogenic origin (Currenet al. 2006), and isoprene nocturnal local emissionsare not expected, it may be concluded that bothisoprene and 1,3-butadiene arriving during dawn ofdays 5 and 6 are anthropogenic and they have thesame origin. This fact reinforces the idea of theimportance of anthropogenic isoprene on the emissioninventories to be used as an input of photochemicalmodels, since a small fraction of the isoprene mea-sured in summer, and almost all the isoprenemeasured in winter is of such origin (Reimann et al.2000; Borbon et al. 2001).

Conclusions

In rural background areas, of the 59 NMHC morethan 20 have mean concentration values lower thantheir detection limit. The most abundant compoundsare the less reactive anthropogenic species (paraffins),and the most reactive ones (monocyclic aromatics) areless abundant in terms of percentage in rural areasthan in urban areas.

The seasonal evolution of the main biogeniccompounds (isoprene and monoterpenes) has beenassessed, and maxima were found between themonths of May and September.

Comparisons between the measured ambient con-centration and their estimated emissions, calculatedwith the Guenther’s algorithms for some selectedBVOC show that, for isoprene, discrepancies arederived from its reactivity and its transport fromshort-range areas. For monoterpenes, due to their veryhigh reactivity, it is not possible to obtain a good fitbetween concentration measured and emission dataunless numerical models including the chemicaldegradation mechanisms are used: Monoterpenesmaxima occur during the night, when the availabilityof OH radicals is lower, and, thus, they do not seem tofit on an emission model based on temperature as amain factor.

The episode observed during the night of days 5and 6 of August, 2003 has been studied andsimulated. The relative ratio of compounds withdifferent reactivity indicates that the sampling sitewas subjected to the influence of aged air massestransported from medium to large distances. This typeof transport is very important, since it “additionally”increases the surface ozone concentrations by 30–40 μg/m3, surpassing, in some periods, the thresholdlimit for warning to the population, in spite of the factthat the sampling site is at a rural background area.

Acknowledgements The authors wish to thank the personnelfrom the Valderejo Natural Park Center, for their friendlinessand logistic support; to the Viceconsejería de Medio Ambienteof the Basque Government for his support, the supply ofcomplementary meteorological and air quality data; and to theSpanish Ministry of Science and Technology (MEC), for theirfinancial aid for the project entitled “Advanced measurementmethods of volatile organic compounds in the atmosphere”(MAMECOVA) REN2003-03973.

Fig. 14 Reanalysis datafrom August 5 and 6, 2003,at 00:00 UTC


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