Measurements of peroxy radicals in a forested area of Portugal

Chemosphere ± Global Change Science 3 (2001) 327±338

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Measurements of peroxy radicals in a forested area of Portugal J. Burkert a, T. Behmann a, M.D. Andres Hern andez a, D. St obener a, M. Weiûenmayer a, D. Perner b, J.P. Burrows a,* a

Institute of Environmental Physics, University of Bremen, P.O. Box 330440, Kufsteiner Str., D-28334 Bremen, Germany b Max-Planck-Institut fur Chemie, Atmospheric Chemistry Division, P.O. Box 3060, D-55020 Mainz, Germany Received 18 March 1998; received in revised form 11 May 2000; accepted 22 February 2001

Importance of this Paper: Peroxy radicals have been observed in a forested rural site, originating from the photooxidation of isoprene. Large levels of peroxy radicals have been correlated with maximum O3 production rates, similar to that observed in urban smog episodes. No peroxy radicals were observed during early morning fog episodes, indicating e€ective removal of radicals in fog. Abstract As part of the FIELDVOC'94 study, ambient concentrations of peroxy radicals (RO2 , R ˆ H, CH3 , CH3 CO, etc.) were continuously measured at a forested site in Portugal, using the chemical ampli®cation technique. The amount of RO2 varied between 0 and 260 pptv with maximum values being obtained under clear sky conditions. The RO2 mixing ratio correlates with that of isoprene. The rate of production of ozone, O3 , indicates that the noon-time maximum values lay between 5 and 17 ppbv h 1 . On several days, the maximum O3 mixing ratio at this rural site was similar to that found in urban areas, which experience smog episodes (O3 mixing ratio > 90 ppbv). The RO2 estimated assuming a photostationary state RO2 ,(PSS) of NO2 , NO and O3 , was not found to have a signi®cant statistical correlation with the measured RO2 . The latter is best explained by the errors associated with the calculation of RO2 ,(PSS) at low NOx (NO + NO2 ) mixing ratios. The diurnal behavior of O3 in the boundary layer was estimated using a relatively simple source/sink parameterization. On days having high O3 mixing ratios, both predicted and observed O3 behavior agree well. For days having O3 mixing ratios < 75 ppbv, it was found that the simple parameterization needed to be extended to include entrainment of O3 after dawn from above the night-time inversion layer. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Peroxy radicals; Ozone production; Vertical transport; Photostationary state

1. Introduction The photolysis of O3 in the boundary layer results in the production of O(1 D), which, in spite of its quenching reactions with O2 and N2 , through its reaction with H2 O

* Corresponding author. Tel.: +49-421-2184548/4653; fax: +49-421-2184555. E-mail address: [email protected] (J.P. Burrows).

yields the most important daytime oxidizing agent, the OH radical (Levy, 1991): O3 ‡ hm ! O…1 D† ‡ O2

…1†

O…1 D† ‡ H2 O ! OH ‡ OH

…2a†

O…1 D† ‡ N2 ! O…3 P† ‡ N2

…2b†

O…1 D† ‡ O2 ! O…3 P† ‡ O2

…2c†

O3 itself also oxidizes a variety of trace gases (e.g., saturated hydrocarbons, sulfur-containing hydrocarbons,

1465-9972/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 5 - 9 9 7 2 ( 0 1 ) 0 0 0 1 4 - 9

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etc.) of both biogenic and anthropogenic origin. In consequence, the presence and abundance of O3 coupled with the amounts of UV-B radiation and NOx determines to a large extent the oxidizing capacity of the troposphere. Dependent on the amount of NOx present, O3 is produced or removed by the following chain reactions (Crutzen, 1973; Crutzen and Zimmermann, 1991) in the remote troposphere: Ozone production ± Chain A OH ‡ CO ‡ M ! CO2 ‡ HO2

…3†

HO2 ‡ NO ! OH ‡ NO2 NO2 ‡ hm ! NO ‡ O O ‡ O2 ‡ M ! O3 ‡ M ÐÐÐÐÐÐÐÐÐÐÐÐÐ

…4† …5† …6†

Net CO ‡ 2O2 ! CO2 ‡ O3 Ozone loss ± Chain B OH ‡ CO ‡ M ! HO2 ‡ CO2 HO2 ‡ O3 ! OH ‡ 2O2 ÐÐÐÐÐÐÐÐÐÐÐÐ

…3† …7†

RO2 ‡ NO2 ‡ M $ RO2 NO2 ‡ M

…15†

HO2 ‡ NO2 ‡ M $ HO2 NO2 ‡ M

…16†

HO2 ‡ HO2 ! H2 O2 ‡ O2

…17†

RO2 ‡ HO2 ! ROOH ‡ O2

…18†

RO2 ‡ RO2 ! ROOR ‡ O2

…19†

OH ‡ NO2 ‡ M ! HNO3 ‡ M

…20†

NO ‡ O3 ! NO2 ‡ O2

…21†

For typical tropospheric temperature ranges the only signi®cantly stable peroxyalkyl nitrate, RO2 NO2 , is the peroxyacetyl nitrate (PAN). Once the photochemical chain is initiated by the formation of OH, competition between the rates of termination and propagation in the air mass determines the length of the atmospheric chain production or loss of O3 . Provided that the free radicals and atoms achieve a stationary state (i.e., d‰X Š=dt ˆ 0), the photochemical production 

Net CO ‡ O3 ! CO2 ‡ O2 In the planetary urban and rural boundary layers, in addition to CO and CH4 , signi®cant amounts of reactive volatile organic compounds (VOC) from both natural and anthropogenic emissions are present. Similarly, sucient amounts of NOx from combustion sources are available for rapid oxidation to occur as an additional and ecient O3 -producing chain reaction exists: Ozone production ± Chain C OH ‡ RH ! H2 O ‡ R R ‡ O2 ‡ M ! RO2 ‡ M RO2 ‡ NO ! RO ‡ NO2 RO ‡ O2 ! R0 COR0 ‡ HO2 HO2 ‡ NO ! OH ‡ NO2 2…NO2 ‡ hm ! NO ‡ O† 2…O ‡ O2 ‡ M ! O3 ‡ M† ÐÐÐÐÐÐÐÐÐÐÐÐÐ Net RH ‡ 4O2 ! R0 COR0 ‡ 2O3 ‡ H2 O

…8† …9† …10† …11† …4† …5† …6†

During daylight the aldehydes or ketones, R0 COR0 (where R0 ˆ H, CH3 , CH3 CO, etc.) generate further RO2 radicals as their photolysis provides a source of R0 and R0 CO radicals: R0 COR0 ‡ hm ! R0 ‡ R0 CO R0 CO ‡ O2 ‡ M ! R0 C…O†O2 ‡ M

…12† …13†

R0 ‡ O2 ‡ M ! R0 O2 ‡ M

…14†

In this sense the reaction chain C is autocatalytic. Chain termination for A, B and C takes place by the loss of radicals via reactions such as

d‰O3 Š dt

 P

is given by: 

d‰O3 Š dt

 P

ˆ kRO2 ‡NO ‰RO2 Š‰NOŠ:

In order to estimate the amount of O3 at a given time and its diurnal behavior, it is necessary to understand both production and loss processes. In the boundary layer O3 is removed not only by both photochemical and chemical reactions but also by the process of dry deposition at the surface, the latter being most often the dominant loss process above land. Observations of RO2 , NO, O3 , and knowledge of the rate of O3 deposition at a site enable the observed and estimated amounts of O3 to be investigated. Comparison of the predicted and observed behavior of O3 provides insight into our current understanding of tropospheric chemistry. One of the primary aims of the FIELDVOC'94 study, which took place at the T abua site (40°190 N, 08°030 W) in Portugal from June 20 to July 12, 1994, was to investigate the oxidation of VOCs in a forested rural environment. In this study, measurements of RO2 made during FIELDVOC'94 are reported and analyzed in combination with NO, NO2 , O3 , isoprene and meteorological parameters, which were measured by di€erent groups within FIELDVOC'94 and are to be reported elsewhere (Bonsang et al., 2001). The objectives were to identify the amount and source of peroxy radicals and to determine the in¯uence of the production cycles A and C on the O3 present in the boundary layer.

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UV photolysis of H2 O and the PAN thermal decomposition were used to generate known amounts of HO2 and organic radical concentrations, respectively. These calibration methods have been described elsewhere (Hastie et al., 1991; Schultz et al., 1995). The average chain length, determined from all the radical calibrations performed in dry air during the measurement period, was 110  20. The detection limit is estimated to be 3±5 pptv RO2 . A more detailed description of the system can be found elsewhere (Hastie et al., 1991). The CA chain length has been shown to be dependent on the relative humidity present in the detector. This arises because of wall losses of HO2 and potential gas-phase reactions (Mihele et al., 1999). The detector used during the FIELDVOC'94 campaign was recalibrated in the laboratory to take into account the dependence of the chain length on relative humidity. The temperature of the detector walls were typically 9.9°C above ambient during the FIELDVOC'94 campaign, consequently the relative humidity in the detector was less than in ambient air. The total error on a single RO2 measurement is made up of several components but is approximately 30%. A second set of RO2 measurements was made during the FIELDVOC'94 campaign by the Max-Planck-Institut, Mainz (MPI) group, using a similar apparatus located directly on top of the measurement tower. In addition, the MPI system used a cover to protect it from rain. The two systems were calibrated at T abua using the same H2 O photolysis source of HO2 provided by the IUP group. During this campaign the RO2 -MPI detector used a silicon inlet system. Comparison of the two measurement sets showed a relatively good correlation (see Fig. 1) with r2 ˆ 0:75 but a systematic di€erence of approximately a factor of 3: the IUP-RO2 detector observing more radicals than the MPI-RO2 detector. This di€erence was apparently not dependent on wind speed or direction. The simplest explanation of this di€erent response of the detectors is that radicals in the MPI system were lost on the cover used to protect the inlet system from rain. In a subsequent measurement campaign, the MPI group noted that their cover and silicon inlet system seems to remove larger RO2 radicals (Volz-Thomas et al., 1998), providing further evidence for the above explanation. Consequently for the analysis presented here only the IUP-RO2 data are used.

2. Experimental The FIELDVOC'94 campaign was performed at T abua to investigate the photochemical transformations of VOCs and the associated peroxy radical chemistry. The site is located about 70 km east of the Atlantic coast. An overview characterizing the air masses encountered is given in Table 1. Most of the measurements were made on the top of a 20-m high tower in a eucalyptus forest area. The inlet of the RO2 -IUP (Institute of Environmental Physics, University of Bremen) instrument was located on top of the tower about 5 m above the canopy close to the inlet systems for the measurements of the other trace gases. Measurements of RO2 were made continuously from June 17 until July 12, 1994. The RO2 measurement technique (chemical ampli®cation, CA) is based on the conversion of peroxy radicals into NO2 by addition of NO and CO. An NO2 modulated signal results from the alternate addition of CO to the air sample at two different points of the reactor. In this study the latter was a PFA-Te¯on tube approximately 20 cm long, with NO being added continuously to the mixture at the upper point of CO addition. The CA measures the sum of RO and RO2 radicals. Only those RO2 which react with NO to produce NO2 are observed. RO2 which react with NO to produce RONO2 are not detected. Current literature (Hastie et al., 1991) implies that the CA detector should therefore primarily be sensitive to the lighter RO2 ; the larger RO2 react more slowly with NO and form signi®cant RONO2 . However, recent laboratory studies of the reaction of selected RO2 (C3 ±C5 ) with NO show that the rate coecient does not decrease with increasing carbon content of the RO2 (Eberhard and Howard, 1997). In addition, studies using the CA show that those larger RO2 studied appear to have similar chain lengths, i.e., the number of NO2 molecules produced per radical sampled to that P P of HO2 . In any case, in the troposphere, …RO2 †  …RO† and the CA signal, when correctly calibrated,P should therefore represent a good approximation to …RO2 †. The NO2 response of the instrument is calibrated regularly by addition of di€erent dilutions of the NO2 emission from permeation tubes. This approach has been validated by several studies (VolzThomas et al., 1998). A calibration method is required to determine the chain length of the CA detector. For this purpose the

Table 1 Overview of selected trace gases observed at Tabua during the FIELDVOC'94 measurement campaign CO (ppbv) Average Minimum Maximum

165.13 101.32 274.83

NO (ppbv) 0.13 <0.05 1.40

NO2 (ppbv)

NOy (ppbv)

Isoprene (ppbv)

1.49 0.25 5.25

3.19 0.88 6.68

2.04 0.01 12.32

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Fig. 1. Comparison between RO2 -IUP and RO2 -MPI for all data collected during the FIELDVOC'94 campaign.

3. Results and discussion 3.1. RO2 measurements The measurement site at Tabua is characterized by a wind pattern corresponding to a combination of sea± land breeze circulation, channeled through the surrounding valley. This results in south±southeast breeze during the night and west±northwest breeze during daytime. Fig. 2 shows the observed behavior during the campaign. The large number of periods having low wind speeds (2±3 m s 1 ) indicate that local in¯uence might be of signi®cance at Tabua. This is con®rmed by the fact that on days where the air mass observed at Tabua have

Fig. 2. Wind distribution during the FIELDVOC campaign.

not a strong continental in¯uence, no signi®cant diurnal changes in the trace gas mixing ratios were observed (Bonsang et al., 2001). Twenty days of RO2 measurements were made during the campaign. Maximum mixing ratios are reached mostly between 13:00 and 15:00 (UT) and vary from 10 to 260 pptv. Meteorological conditions seem to in¯uence the diurnal evolution of RO2 concentrations. Sunny warm periods with clear skies are associated with local photochemical processes including the oxidation of CO, CH4 and other hydrocarbons, and with biogenic emissions, leading to elevated RO2 amounts. During the campaign, RO2 and isoprene mixing ratios are well correlated (Fig. 3). This is to be expected because of the high reactivity of isoprene with OH radicals and the subsequent oxidation mechanism, generating RO2 (Atherton and Penner, 1990; Pierotti et al., 1990; Paulson and Seinfeld, 1992). Isoprene has been found to be the major hydrocarbon emitted from adult eucalyptus trees with daily emission rates which increase with temperature up to 40°C. Isoprene emissions from eucalyptus alone can be an order of magnitude higher than anthropogenic VOC emissions in the months of July and August in the areas of Portugal relevant for this study (Pio et al., 1996). On several days during the campaign, periods of fog in the early morning were observed. Our measurements indicate that negligible (i.e., below our detection limit) amounts of RO2 are present during these fog episodes. Two representative days have been selected to illustrate this e€ect and are shown in Fig. 4. On June 28, RO2 production was observed when the fog vanished around 10:00, whereas on June 29, this occurred at 08:00 (UT). In general, the dispersion of fog correlates with the relative humidity dropping from 100%. Radicals were always observed at relative hu-

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331

Fig. 3. Correlation between isoprene and RO2 .

Fig. 4. Diurnal variation of O3 , RO2 and relative humidity for June 28 and 29, 1994.

midities 680% in the early morning at Tabua. Although the reduction of the CA chain length has been accounted for within experimental error resulting from its dependence on humidity, some additional losses may occur of radicals on the fog particles, which enter our detector. Nevertheless, the lack of any signi®cant CA RO2 signal during fog episodes is most likely explained by either a low rate of gas-phase formation of peroxy radicals or their rapid removal on the fog particles.

chemical measurements from the FIELDVOC'94 campaign to analyze the O3 production at the site. Several studies of the photostationary state (PSS) of NO, NO2 and O3 at di€erent measurement sites have been performed (Parrish et al., 1986; Trainer et al., 1991; Ridley et al., 1992). These studies assume that during daytime the interconversion of NO2 and NO forms a fast photochemical cycle and that a PSS involving reactions (5),(6) and (21) is achieved. The deviations of the calculated

3.2. Analysis of RO2 measured and predicted Although radicals were measured for 20 days during the FIELDVOC'94 campaign, simultaneous measurements of RO2 , NO, NO2 , O3 and the meteorological parameters are only available for 8 days. This selected data set has been used to analyze the measured and predicted RO2 amount. It was also combined with other

PSS ˆ

kNO‡O3 ‰NOŠ‰O3 Š jNO2 ‰NO2 Š

from 1.0 indicate the presence of NO oxidants other than O3 . RO2 radicals are considered to be the most important additional oxidizing agent converting NO to NO2 (reaction (10)).

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Fig. 5. Correlation between P(O3 ) measured and P(O3 †PSS calculated for all data with jNO2 > 0:008 s 1 .

In Fig. 5 the correlation between the O3 production, measured and calculated from the PSS assumption, i.e., P…O3 † ˆ

dO3 ˆ jNO2 ‰NO2 Š dt

kNO‡O3 ‰NOŠ‰O3 Š

is shown. Midday periods having clear sky and maximum photochemical activity, selected by using the values of jNO2 simultaneously measured (i.e., periods with jNO2 > 0:008 s 1 † are shown. Although the calculated and measured values both increase together, there is no signi®cant statistical correlation between these data sets. The RO2 mixing ratios that could explain the di€erence between observed and calculated [NO2 ]/[NO] have also been calculated assuming that ‰RO2 …PSS†Š ˆ

jNO2 ‰NO2 Š kNO‡RO2 ‰NOŠ

kNO‡O3 ‰O3 Š: kNO‡RO2

Provided that HO2 and CH3 O2 are roughly equal and the dominant RO2 radicals, then the assumption that a kNO‡RO2;aver ˆ 8:0  10 12 cm3 molecule 1 s 1 (DeMore et al., 1994) for these air masses is reasonable. In Fig. 6 the calculated RO2 (PSS) and the measured RO2 are compared. The values of the statistical parameters indicate that no signi®cant correlation between the two datasets exist. In contrast to the observations at T abua, in selected days of data from the ROSE experiment in Alabama (Cantrell et al., 1992), where the chemical ampli®er technique was also applied to measure RO2 , a signi®cant correlation between observed RO2 and calculated RO2 (PSS) exists. However, in a subsequent work on the RO2 data from the entire ROSE campaign, Cantrell et al. (1993) found that RO2 (PSS) could be up to a factor of two higher than the RO2 measured. In a separate experiment using the same RO2

Fig. 6. Correlation between RO2 measured and RO2 (calculated, PSS) for all data with jNO2 > 0:008 s 1 .

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detection system during the MLOPEX campaign, the RO2 (PSS) in the free troposphere was found to be higher than the RO2 measured (Cantrell et al., 1996). In conclusion, previous reported comparisons of measured RO2 and calculated RO2 (PSS) show no clear relationship. Although it appears that PSS may explain the observed RO2 , NO, NO2 , O3 behavior at Tabua, it is important to analyze the error in the RO2 (PSS) calculation to understand the discrepancies between calculated and measured RO2 . When calculating RO2 (PSS), it is important to note that the accuracy of the measurements of O3 , jNO2 , NO and NO2 and their ratio is a decisive factor in determining the reliability and precision of RO2 (PSS). Assuming an accuracy for the jNO2 detector of 10% and for O3 , NO and NO2 of 5%, a maximum accumulated error of about 25% is obtained. However, as a result of the non-linearity of the error associated with the NO2 /NO ratio, the estimation is much more sensitive to inaccuracies at low NOx levels, where the majority of the RO2 (PSS) has been calculated for this studies. Overall, the uncertainties in both the measured RO2 and calculated RO2 (PSS) (particularly at low NOx ) are such that the larger error of RO2 (PSS) overlaps with that of the measured RO2 . More accurate measurements and calculations (i.e., measurements of the parameters jNO2 , NO, NO2 and O3 ) are required to probe more deeply the question of whether further oxidants, in addition to RO2 , such as the halogen oxides or other species, are present in the observed air mass.

333

HO2 and CH3 O2 to be the dominant Ri O2 , vd represents the O3 deposition velocity, and HBL the boundary layer height. This height is assumed to increase linearly from a nocturnal value of 200 m at sunrise to a maximum of 1000 m, remaining constant till sunset (Poisson et al., 2001). A diurnal variation of vd having a value of a 0.05 cm s 1 minimum at night and 0.6 cm s 1 at midday maximum was assumed, on the basis of a recent assessment (Ganzeveld and Lelieveld, 1995, and references therein). In spite of its simplicity, the results obtained with this parameterization have been shown to yield good agreement for measurements at T abua and also in Bremen (Germany) (unpublished data), indicating the predominance of the local production of O3 over other possible processes (vertical and horizontal transport) for days having the relevant anti-cyclonic conditions. For di€erent meteorological systems (e.g., strong advection or convection, etc.), the parameterization needs further modi®cation for it to optimally describe the diurnal O3 behavior. Biogenic sources of radicals, especially isoprene, with emission rates dependent on the temperature, are considered to be more abundant in T abua than in Bremen, where the anthropogenic in¯uence on the measurement site is more important. These di€erences are re¯ected in the relatively low NO/RO2 ratios at T abua. Similar O3 concentrations are however obtained, although relatively late in the afternoon, con®rming the in¯uence of the temperature on the biogenic sources of radicals. The evolution of the boundary layer is also likely to be di€erent.

3.3. Analysis of production and loss of O3 In order to understand the RO2 role in the O3 formation, the measured diurnal variation of O3 needs to be parameterized. The mixing ratio of O3 at the site is assumed to be controlled by transport, photochemical production and losses by deposition. As a ®rst-order approximation, the O3 detected was assumed to arise only from the local production by RO2 and loss by deposition. This restricts ambient conditions to low wind speeds, high insolation and negligible vertical transport. The resulting parameterization for O3 is: Z t d‰O3 Š ‰O3 Š ˆ dt dt 0 ! # ( " " Z t X Z t d ‰O3 ŠBackground ‡ ki ‰Ri O2 Š‰NOŠ dt ˆ 0

0



‰NO2 Š

i

#, )   dt 1 ‰NO2 ŠBackground

vd  dt; HBL

where [O3 ]Background and [NO2 ]Background represent the corresponding concentrations at dawn, [Ri O2 ] is the concentration of the peroxy radical Ri O2 , ki is the rate coecient for the reaction of NO with Ri O2 , assuming

3.3.1. Cases having high maximum O3 On three of the 8 days where all measurements are available, relatively high amounts of O3 were produced: on June 23 approximately 90 ppbv, on June 29 a maximum of around 95 ppbv, and on July 7, 75 ppbv were obtained. For these three days, the parameterization agrees well with the observed O3 behavior (see Fig. 7). This implies that the production of O3 following the reaction of NO with RO2 and the loss by deposition are the most important source and sink, respectively, for O3 . 3.3.2. Cases having O3 maximum less than 75 ppbv For days having lower production of O3 , the in¯uence of additional processes such as transport, deposition, entrainment, etc. becomes of importance. The limitations of the simple parameterization for the estimation of O3 are the likely source for the deviations between the observed and estimated O3 . Examples of this behavior for June 25 and July 4 are shown in Fig. 7. On some of the days, O3 is overestimated in the evening, suggesting the possible underestimation of deposition or some additional O3 losses. Information about the diurnal evolution of the boundary layer and explicit

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Fig. 7. Comparison of the diurnal behavior of O3 measured and calculated for all data.

measurements of O3 deposition for this site is required to con®rm our assumed behavior. Fig. 8 shows a plot of O3 estimated using the parameterization scheme described above versus measured O3 for data recorded from 05:00 to 24:00 (UT). The break-up of the night-time boundary and the subsequent entrainment of gases from the free troposphere in¯uences the O3 production. Ideally, a regression coecient of one is expected. At least two factors are considered as

potential explanations for the observed regression coef®cient being less than one. These are discussed below. 3.3.2.1. The mixing of trace gases from the planetary boundary layer and the ®rst inversion layer during the break-up of the nocturnal surface inversion. In Fig. 9 the diurnal evolution of the boundary layer is schematically depicted. After dusk, the formation of surface inversion layer as a result of the rapid cooling of the earth isolates

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335

Fig. 8. Correlation between O3 measured and O3 calculated, before breaking up of the boundary layer.

Fig. 9. Theoretical diurnal evolution of the boundary layer.

the air mass close to the surface from the air above, which is then separated from the in¯uence of boundary layer emissions and deposition processes. Chemical species above the nocturnal inversion layer do not su€er signi®cant depletion by deposition, whereas those con®ned to the inversion layer do. Dependent on the mesoscale conditions, horizontal transport in this layer may be important. At sunrise the solar heating of the earth results in convection, which leads to the e€ective mixing of the nocturnal boundary layer and the layer above it. This results in the dilution of those components having higher amounts aloft. Thus a net transport of O3 from

the layer aloft and a rapid increase in the surface O3 concentrations are expected early in the morning. For FIELDVOC'94 the early morning fog delays the warming and the associated convection processes, which lead to destruction of the nocturnal surface inversion. In order to study the possible in¯uence of the entrainment of O3 from the layers aloft, the calculations of O3 were repeated using di€erent starting times with respect to the break-up of the night-time boundary layer. No signi®cant di€erence is observed on days dominated by local O3 production (June 23 and 29, and July 7) but on June 25, 26 and 28, and July 4, the agreement between mea-

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Fig. 10. Correlation between O3 measured and O3 calculated, after breaking up of the boundary layer.

sured and estimated O3 production improves when started after the break-up of the boundary layer. In Figs. 8 and 10, linear regressions have been plotted corresponding to Section 3.3.1 data, all data and Section 3.3.2 data. The calculations are initialized with the background concentrations at dawn (Fig. 8) and at 09:00 (UT) (Fig. 10) in order to separate the in¯uence of the break-up of the night-time boundary layer. The spread of the three linear regressions is reduced in Fig. 10 compared to Fig. 8, indicating that the parameterization has been improved when entrainment is e€ectively taken into account. It can therefore be concluded that there is a disturbance in the ozone concentration related to early morning convection and the diurnal variation in the boundary layer height. This indicates that the entrainment of O3 from above, as the nocturnal inversion layer disperses, is responsible on average for up to 20% of the maximum O3 observed. This explains why the O3 maximum increases from day to day during stable summer anti-cyclonic periods. 3.3.2.2. The possible presence in the sampled air mass of signi®cant amounts of a peroxy radical, which reacts with NO at higher rate than the dominant radicals HO2 + CH3 O2 . Peroxyacetyl radicals (PA) are generated from the thermal decomposition of PAN, one of the products of the oxidation of non-methane hydrocarbons (NMHC). The rate constant for the reaction of PA and NO is k ˆ 2:0  10 11 cm3 molecule 1 s 1 . There is no evidence for the formation of acetyl nitrate in this reaction (Lightfoot et al., 1992). Should PA be present in signi®cant amounts in an air mass with respect to HO2 , then the simple O3 parameterization used above is likely to underestimate O3 .

An estimation of an upper limit for the amount of PA can therefore be made by solving the expression ‰RO2 rest Š‰NOŠkaverage ‡ ‰PAŠ‰NOŠkPA ˆ ‰RO2 total Š‰NOŠkaverage =X ; where [RO2 total ] is all peroxy radicals measured by the chemical ampli®er, [RO2 rest Š ˆ ‰RO2 total Š ‰PAŠ, kaverage ˆ 8:0  10 12 cm3 molecule 1 s 1 , kPA ˆ 2:0  10 11 cm3 molecule 1 s 1 and X is the slope of the correlation O3 measured versus O3 calculated. The resulting upper limit 17% of the RO2 is potentially PA. This implies that the maximum PA concentrations are between 4 and 19 pptv. This is not dissimilar in magnitude to the mixing ratio estimated from NO2 and PAN measurements in a similar forested area at Scotia, Pennsylvania (Trainer et al., 1991). In summary, the break-up of the nocturnal boundary layer after dawn and subsequent entrainment of gases from above is clearly playing an important role in determining the amount of O3 measured in the early morning at T abua. Agreement between the predicted and measured O3 improves when this is taken into account. An upper limit for the amount of CH3 COO2 was established from an analysis of the observed di€erence between the real and ideal behavior of the observed and predicted O3 .

4. Conclusions In this study peroxy radicals have been successfully measured above the canopy of a eucalyptus forest at the T abua site in Portugal as part of the FIELDVOC'94 study by the CA technique. Similar to other recent studies, the observed RO2 and that predicted by as-

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sumption of a PSS of NO, NO2 and O3 are not well correlated. The most likely explanation for this behavior is that the precision of the measurements of jNO2 and the trace gases, at relatively low values of NOx is insucient to test the accuracy of the RO2 measurements. This implies that RO2 measurements are required and the PSS theory is inadequate as a surrogate source of RO2 information. The measured RO2 does correlate well with the amount of isoprene present in an air mass. This indicates that at this rural site, the dominant source of RO2 is the oxidation of isoprene. No RO2 was observed during the early morning fog episodes indicating either a lack of UV-B radiation or the e€ective removal of gasphase radicals in fog. The dominant source of the photochemical-induced O3 at the site appears to be the reaction of RO2 with NO. This implies that chemical reaction chain C is in the boundary layer of great importance. During periods of signi®cant O3 production, which are associated with low wind speeds, the most important O3 loss process at this boundary layer site appears to be deposition. A simple parameterization of the O3 behavior based on these two processes agreed well with the measured O3 for high maximum values of O3 . For maximum O3 amounts below 75 ppbv, it is necessary to account for the entrainment of O3 after dawn from the layer above the nighttime boundary layer, to obtain an adequate description of the early morning O3 behavior. This implies that entrainment needs to be considered on all days but the error, which occurs by ignoring this process on days having high O3 production, is relatively small. Finally, the correlation between measured O3 and predicted O3 was investigated. One tentative explanation of the deviation of the slope of a linear regression from unity is that peroxyacetyl radicals (CH3 COO2 ), which are expected to be generated in the isoprene oxidation, are present in relatively large amounts at around the 15% level. Acknowledgements This work was in part funded by the University of Bremen, the State of Bremen, the European Union and the German Ministry of Research and Technology (BMBF). We would like to thank A. Volz-Thomas (FA J ulich, Germany), H. Harder (MPI for Chemistry, Mainz, Germany), C. Pio and T. Nunes (University Aveiro, Portugal) for the provision of their data. References Atherton, C.S., Penner, J.E., 1990. The e€ects of biogenic hydrocarbons on the transformation of nitrogen oxides in the troposphere. J. Geophys. Res. 95, 14027±14038.

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Thomas Behmann was born in Bremen, Germany, and studied Physics at the University of Bremen. From 1993 to 2000 he was a Ph.D. student at the Institute of Environmental Physics at the University of Bremen and participated in the ®eld campaigns of the EU-projects Fieldvoc, Octa-Izana, and Price. Currently he is training to be a physics grammar school teacher. Maria Dolores Andres Hernandez was born in Madrid, Spain, where she studied Chemistry at the Universidad Complutense. After undertaking her Ph.D. which investigated the behaviour of HONO and HNO3 in the troposphere, at the Alfred Wegener Institute for Marine and Polar Research in Germany, she joined the Institute of Environmental Physics at the University of Bremen. Her main scienti®c interest is the study of the production and loss mechanisms of O3 in the troposphere. Dirk Stoebener was born in Bremen, Germany. He obtained his degree in Physics at the Institute of Environmental Physics of the University of Bremen. Presently he is a Ph.D. student at the Institute of Industrial Technology and Applied Work Science of the University of Bremen (BIBA). His main research topic is the use of ultrasonic for the high resolution measurement of geometric sizes. Michael Weiûenmayer was born in Speyer, Germany and studied Chemistry at the University of Mainz, Germany. He undertook his Ph.D. at the Max Planck Institute of Chemistry in Mainz. After post-doctoral research at the Institute of Environmental Physics of the University of Bremen, he joined the Environment Department in Koblenz, Germany. Presently he is working at the Environment Department in Mainz, Germany. Dieter Perner has worked for more than 40 years in research. He invented the long path DOAS measurement technique. He is now a retired member of the Max Planck Institute for Chemistry in Mainz. John P. Burrows was born near Liverpool, UK and educated at Trinity College, University of Cambridge, where he obtained a Ph.D. degree in Physical Chemistry. After post-doctoral research at the Harvard Smithsonian Center for Astrophysics, he worked at the UKAEA and the PCL, Oxford University, before joining the Max Planck Institute of Chemistry in Mainz. He has been a Professor of Atmospheric Physics at the University of Bremen, where he founded in 1992 the Institute of Environmental Physics and Remote Sensing. His research group works on problems in Atmospheric Physics and Chemistry, specializing in laboratory studies of the spectroscopy and kinetics of atmospheric trace species, in situ atmospheric measurements and remote sensing.