source (reactions (R13), (R14) and (R16) in Table 1), e.g., under polluted conditions and in the middle and upper troposphere [Wennberg et al., 1998; Cantrell et ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, D14308, doi:10.1029/2005JD007001, 2006
Influence of summertime deep convection on formaldehyde in the middle and upper troposphere over Europe A. Stickler,1 H. Fischer,1 J. Williams,1 M. de Reus,1,2 R. Sander,1 M. G. Lawrence,1 J. N. Crowley,1 and J. Lelieveld1 Received 16 December 2005; accepted 13 March 2006; published 22 July 2006.
[1] We present HCHO measurements performed during the Upper Tropospheric Ozone:
Processes Involving HOx and NOx (UTOPIHAN) II aircraft campaign over centralwestern Europe and the northwestern Mediterranean region in 2003 and 2004. We compare these with box model and three-dimensional (3-D) chemistry-transport model results for ‘‘background’’ and ‘‘convective’’ conditions. Generally, we obtain good agreement between the model and measurement data, although the 3-D model tends to underestimate HCHO near the tropopause, whereas the box model tends to slightly overestimate HCHO for the background case. Sensitivity simulations indicate that the most important factors determining HCHO concentrations are the abundances of NO, reactive hydrocarbons (e.g., isoprene), CH3OOH, CH3OH, and acetone and also the level of irradiance. In the middle and upper troposphere the most important HCHO production term is the reaction of CH3O2 with NO (68 ± 10%, including a contribution of 38 ± 7% from CH4 oxidation), followed by the destruction of CH3OOH by OH and photolysis (14 ± 2%), and the reaction of CH3OH with OH (14 ± 8%). HCHO loss is dominated by photolysis rather than by the reaction with OH, except in the upper tropospheric convective case, where the loss by OH contributes about 50%. Our measurements furthermore indicate that convective transport of H2O2 can be very efficient. Citation: Stickler, A., H. Fischer, J. Williams, M. de Reus, R. Sander, M. G. Lawrence, J. N. Crowley, and J. Lelieveld (2006), Influence of summertime deep convection on formaldehyde in the middle and upper troposphere over Europe, J. Geophys. Res., 111, D14308, doi:10.1029/2005JD007001.
1. Introduction [2] Formaldehyde is a key intermediate formed during the photochemical oxidation of volatile organic compounds (VOCs) in the troposphere and one of the most abundant carbonyls. Under ‘‘background’’ (i.e., not directly polluted) conditions its mixing ratio is considered to be determined mainly by the oxidation of CH4 (Table 1, reactions (R1) and (R2)). Reaction (2) is important at NO mixing ratios above approximately 0.05 ppbv, below which the reaction of CH3O2 with HO2 becomes dominant, forming CH3OOH. [3] Apart from methane, other organic precursors (both anthropogenic and biogenic) can play an important role. The photooxidation of CH3OH, CH3OOH, CH3C(O)CH3, alkenes (e.g., ethene and propene) and isoprene as well as the photolysis of CH3OOH and CH3C(O)CH3 generate HCHO as a main product (see Table 1, reactions (R3) – (R12)). In addition, formaldehyde can be formed by reactions of organic nitrates such as peroxy acetyl nitrate (PAN) and nitro-oxy acetaldehyde (H2C(NO3)CHO) with OH. 1 Department of Air Chemistry, Max Planck Institute for Chemistry, Mainz, Germany. 2 Now at Institute for Atmospheric Physics, Johannes Gutenberg University, Mainz, Germany.
Copyright 2006 by the American Geophysical Union. 0148-0227/06/2005JD007001
[4] As listed in Table 1, there are four main reactions which destroy HCHO in the gas phase (note that we do not consider halogen radical reactions). These are the reaction pathways with OH or NO3 (reactions (R13) and (R14)) and two photolysis channels to either molecular (reaction (R15)) or radical products (reaction (R16)), in each case forming CO. [5] HCHO is not only photochemically produced and destroyed as described above, but also emitted directly into the atmosphere by biomass burning, incomplete combustion, industrial processes and by vegetation [e.g., Carlier et al., 1986, and references therein]. HCHO removal can take place through dry deposition, heterogeneous loss on aerosol particles [e.g., Tie et al., 2001], cloud chemistry and precipitation scavenging [e.g., Lelieveld and Crutzen, 1990; Heikes, 1992]. Even though these processes have been studied in laboratory experiments, their relative contributions to the HCHO budget in the middle and upper troposphere remain quite uncertain. [6] Formaldehyde can be an important HOx (OH + HO2) source (reactions (R13), (R14) and (R16) in Table 1), e.g., under polluted conditions and in the middle and upper troposphere [Wennberg et al., 1998; Cantrell et al., 2003]. It therefore significantly affects the oxidizing power of the atmosphere [Lelieveld and Crutzen, 1990; Crutzen et al., 1999] and the partitioning of the HOx and NOx (NO + NO2) families involved in tropospheric ozone chemistry.
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Table 1. Key Source and Sink Reactions of Formaldehyde in the Model Reactiona (R1) (R2) (R3) (R4) (R5) (R6) (R7) (R8) (R9) (R10) (R11) (R12) (R13) (R14) (R15) (R16)
CH4 + OH (+O2) ! CH3O2 + H2O CH3O2 + NO (+O2) ! HCHO + NO2 + HO2 CH3OH + OH (+O2) ! HCHO + HO2 + H2O CH3OOH + OH (+O2) ! 0.7 CH3O2 + 0.3 HCHO + 0.3 OH + H2O CH3COCH3 + OH (+O2) ! CH3COCH2O2 + H2O CH3COCH2O2 + NO ! NO2 + CH3C(O)O2 + HCHO C3H6 + OH (+O2) ! CH3CH(O2)CH2OH CH3CH2 (O2)CH2OH + NO ! 0.98 CH3CHO + 0.98 HCHO + 0.98 HO2 + 0.98 NO2 + 0.02 ONIT ISOP + OH (+O2) ! ISO2 ISO2 + NO (+O2) ! 0.88 NO2 + 0.88 MVK + 0.88 HCHO + 0.88 HO2 + 0.12 ISON CH3OOH + hn (+O2) ! HCHO + OH + HO2 CH3COCH3 + hn (+O2) ! CH3C(O)O2 + CH3O2 HCHO + OH (+O2) ! CO + H2O + HO2 HCHO + NO3 (+O2) ! HNO3 + CO + HO2 HCHO + hn ! H2 + CO HCHO + hn + O2 ! CO + 2 HO2
a ONIT, organic nitrates from higher alkyl nitrates and C3H6 + NO3; ISOP, isoprene; ISO2, isoprene (hydroxyl) peroxy radical; ISON, organic nitrates from ISO2 and ISOP + NO3; MVK, methyl vinyl ketone; branching ratios of all reactions presented here are taken from the MECCA v0.1p chemical mechanism (see electronic supplement of Sander et al. [2005]).
[7] Because of its relatively short atmospheric lifetime, HCHO is considered to be a good tracer for local and regional pollution. It is a sensitive indicator of photochemical activity and an especially useful marker to test the representation of organic chemistry in models, being probably more suited than ozone [Carlier et al., 1986] and even HOx [Crawford et al., 1999]. [8] A number of studies have demonstrated that the fast upward transport of oxidized VOCs like CH3OOH, aldehydes (especially HCHO) and acetone can cause strong local HOx enhancements in the upper troposphere [Prather and Jacob, 1997; Lee et al., 1998; Cohan et al., 1999; Mu¨ller and Brasseur, 1999; Wang and Prinn, 2000; Ravetta et al., 2001], also affecting net ozone production (NOP) [e.g., Pickering et al., 1989; Prather and Jacob, 1997; Brunner et al., 1998; Wennberg et al., 1998]. Near the tropopause, ozone concentration changes can significantly modify the radiation budget, thus affecting the climate system [e.g., Wang et al., 1986]. Furthermore, an enhancement in NOP and photochemical activity can be significant during long-distance transport of pollutants in the upper troposphere, also because of the much longer lifetime of NOx in this altitude region compared to the boundary layer [Ehhalt et al., 1992]. [9] In prior comparisons of measured HCHO mixing ratios in the free troposphere with model results, significant discrepancies have been found. In some cases the modelmeasurement deviations were positive [Liu et al., 1992; Zhou et al., 1996; Heikes et al., 1996; de Reus et al., 2005], while in others they were found to be negative [Arlander et al., 1995; Jaegle´ et al., 1997, 2000; Heikes et al., 2001; Ravetta et al., 2001; Frost et al., 2002; Kormann et al., 2003]. Differences were assigned to the uncertainty in the measurements/estimates of precursors (hydrocarbons, CH3OOH, OH, NO) as well as HCHO itself [Liu et al., 1992; Jaegle´ et al., 1997; Hauglustaine et al., 1998;
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Kormann et al., 2003; de Reus et al., 2005], or to a lack of quantitative knowledge of important parameters or processes determining the HCHO mixing ratio, including surface deposition [Zhou et al., 1996; de Reus et al., 2005], multiphase chemistry of HCHO, the abundance of precursor gases (e.g., CH3OOH or CH3OH) [Jacob et al., 1996; Jaegle´ et al., 2000] and missing chemical pathways and sources [Frost et al., 2002; Kormann et al., 2003]. Especially the uncertainties associated with the modeling of transport, convection and precipitation may account for disagreements in direct comparisons of models with measurement data [Heikes et al., 2001]. Furthermore, the assumption of chemical steady state after rapid transport processes and the parameterization of cloud properties, sometimes through a constant cloud correction factor (CCF), can contribute to discrepancies [Fried et al., 2003b; Olson et al., 2004]. [10] Despite these potential difficulties, some box model and 3-D chemistry-transport model (CTM) results do however agree well with measured free tropospheric mixing ratios of HCHO [Brasseur et al., 1996; Zhou et al., 1996; Singh et al., 2000; Wang et al., 2001; Singh et al., 2001; Cantrell et al., 2003; Fried et al., 2003a, 2003b; Kormann et al., 2003; Olson et al., 2004; Singh et al., 2004]. [11] The objective of this paper is to evaluate gas phase chemistry and transport effects on HCHO by comparing measured mixing ratios with box model and 3-D CTM results, focusing on the controlling factors in the middle and upper troposphere under undisturbed background and convectively influenced conditions. In particular, we aim at quantifying the influence of convection on the budget of HCHO, and the consequences for O3 and more generally the photochemical activity in the upper troposphere.
2. Measurements and Instrumentation [12] As part of the measurement component of the Upper Tropospheric Ozone: Processes Involving HOx and NOx: The Impact of Aviation and Convectively Transported Pollutants in the Tropopause Region (UTOPIHAN-ACT) project (see also http://www.mpch-mainz.mpg.de/reus/ utopihan), an intensive field campaign was performed in July 2003 (UTOPIHAN II). The aircraft used was a GFD Learjet 35A (Gesellschaft fu¨r Flugzieldarstellung, Hohn; see also http://www.enviscope.de/home_operation.html) based at Oberpfaffenhofen airport southwest of Munich in the south of Germany. [13] The aim of the aircraft measurements was to provide a detailed data set for the continental upper troposphere for a large suite of partially oxidized hydrocarbons (POH), associated trace gases (e.g., ozone, NO etc.) and radiation, necessary, e.g., for the calculation of HOx formation rates. Concurrently, atmospheric models representing hydrocarbon-HOx-NOx-O3 chemistry were developed, so that the impact of convectively transported boundary layer pollution on the photochemistry of the tropopause region could be assessed. [14] The main focus was first to characterize the background upper troposphere and tropopause region in summer over Europe in cloud-free anticyclonic conditions, and secondly to sample the outflow of single convective cells (Cumulus congestus (Cu con) or Cumulonimbus (Cb)
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clouds) as well as organized convection in frontal or frontlike (convergence line) systems associated with cyclonic conditions. In general, the measurements concentrated on the 7 – 13 km altitudinal range. Flight planning was conducted with the help of a daily ‘‘chemical weather forecast’’ [Lawrence et al., 2003] using a 3-D CTM (MATCH-MPIC) [von Kuhlmann et al., 2003] and additional meteorological forecast products from Technical University of Munich (TU Mu¨nchen, see Stohl et al. [2004]) and the Royal Netherlands Meteorological Institute (KNMI). 2.1. HCHO and Peroxides [15] During the UTOPIHAN campaign, HCHO and H2O2 mixing ratios as well as an upper estimate of the CH3OOH mixing ratio were determined using modified and optimized versions of commercially available analyzers (AEROLASER, Garmisch-Partenkirchen, Germany, models AL 4021 and AL 2001CA) based on the fluorescence techniques first described by Kelly and Fortune [1994] and Lazrus et al. [1985, 1986]. [16] Formaldehyde, after having been quantitatively stripped to the liquid phase (0.025 M H2SO4, produced from H2SO4 95– 98%, very pure, and later 95– 97% p.a., both VWR International, Darmstadt, Germany) at 10C in a stainless steel stripping coil, reacts with an amine and 2,4pentanedione (acetylacetone) at a regulated low pH (CH3C(O)OH, 100% p.a., VWR) in a second temperature stabilized stainless steel coil at 65C to form a,a0-dimethylb,b0-diacetyl-pyridine (Hantzsch reaction), which is excited at 400 nm with an Hg lamp. This produces a fluorescence radiation detected 90 off axis at 510 nm with a photomultiplier tube (PMT) (H957-01, Hamamatsu Photonics Deutschland, Herrsching, Germany). A low pH with SO2 4 is used to prevent ambient SO2, a possible interfering species, from entering the stripping solution. [17] In order to measure the peroxides, a two-channel technique was used with one channel detecting all peroxides (in general with a stripping efficiency between 60% (CH3OOH) and 100% (H2O2)) and the second one the organic peroxides only (by selective catalytic destruction of H2O2 with catalase). The peroxides are first stripped to a buffered solution (potassium hydrogen phthalate/NaOH) in a glass stripping coil before a conditioning solution (concentrated buffer solution containing catalase for the second channel) is added. The buffer solution also contains ethylene diamine tetra-acetic acid (EDTA) to scavenge metal ions and HCHO to form HOCH2SO3 with dissolved SO2 to inhibit potential interferences caused by these substances. This is followed by reaction with p-hydroxyphenylacetic acid catalyzed by peroxidase to form the fluorescent dimer 6,60-dihydroxy-3,30-biphenyldiacetic acid. To optimize fluorometric detection, the pH is raised after the reaction by adding NaOH solution. The product of the reaction is excited with a Cd lamp at 326 nm and detected at 400 – 420 nm, using in principle the same technique as for HCHO. By subtracting the signals obtained from the two channels the H2O2 mixing ratio can be inferred. The catalase efficiency, i.e., the relative fraction of total H2O2 destroyed in the second channel, determined for each liquid calibration, was 92– 95%. An upper estimate for the mixing ratio of CH3OOH was calculated on the basis of the assumption that all organic peroxide was present in the
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form of methyl hydroperoxide and correcting for the lower sampling efficiency. [18] For the preparation of all reaction solutions and liquid standards mentioned below ultrapure water of Milli-Q1 grade (Millipore, Eschborn, Germany) or p.a. grade (VWR) was used. The solutions were stored in hermetically sealed DURAN1 glassware (Schott, Mainz, Germany) in refrigerators (except stripping solutions, which were only kept in the dark in barrels (CurTec Deutschland, Mo¨nchengladbach, Germany)) and filled into chemically inert PFA jars (Semadeni, Du¨sseldorf, Germany), packed into cooling units prior to the flight. The jars were connected to the measurement devices by 1/1600 PFA tubing passing through bulkhead unions (Metron Technology, Feldkirchen, Germany) at the rear of a metal drawer in which they were placed. They were arranged together with a small canister to collect the waste liquid. To prevent HCHO or H2O2 in the cabin air from entering the reagents, the air sucked into the jars through a second connector for venting reasons was drawn over small cartridges filled with hopcalite and Purafil1 respectively. [19] Both instruments were connected to a common backward sampling inlet system (see Figure 1), which consisted of approximately 2 m 1=4 00 PFA tubing (Metron), 2 Teflon-coated membrane pumps (KNF Neuberger KN 828 KNDC, Freiburg, Germany; Vacuubrand MD 4C VARIO SP, Wertheim, Germany), one small 12 V pump (ASF Thomas, Wuppertal, Germany; only for UTOPIHAN III) to keep a bypass used for gas zeroing continuously flushed with ambient air at about 2 l/min, two 24 V PTFE-coated 3-way valves (Teqcom, Santa Ana, USA), two pressure sensors, an electric fail-open valve (Bu¨rkert, Ingelfingen, Germany) to the cabin and an electric proportional valve, as part of the constant pressure inlet. The pressure in front of the instruments (p2) was held constant at 1050 hPa except when the pressure before the pumps (p1) dropped below 135 hPa. In this case, the flow through both devices (1 slm (1 l/min at STP) for HCHO, 2 slm for H2O2) remained constant until p2 reached 775 hPa and the flow for the HCHO measurement could not be sustained, so that these data had to be discarded. However, this did not occur during flights 4 (‘‘convection’’) and 7 (background) primarily used for the present study. Additionally, the measurement results were not influenced by p2 down to at least 800 hPa as shown in a laboratory test. A bypass (about 1 m 1=4 00 PFA tubing with two filter cartridges, IAH-432 filled with hopcalite type IAC-330 and IAH-434 filled with silica gel type IAC-502, Infiltec, Speyer, Germany) was used to produce peroxide and HCHO free zero gas from ambient air. The response time (10 – 90% response on a steplike concentration rise) including the inlet system was about 105 s for the peroxide and 135 s for the HCHO device. The mass flow through the inlet was maintained at the highest possible rate (exhaust rate of the pumps) and as low as necessary (maximum flow through the proportional valve at 1050 hPa) by regulating the pump voltages on a software loop feeding back on the value of p1 to minimize inlet effects. [20] Liquid calibrations using liquid standards of 27.0 mg l1 (HCHO) and 35.5 mg l1 (H2O2) (UTOPIHAN II) were performed before and after each flight. These standards were produced by serial dilution from stock solutions kept in a refrigerator (HCHO: 5.625 102 M, H2O2:
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Figure 1. Schematic of the inlet system used for the HCHO and peroxide measurements during UTOPIHAN; the connection from the bypass via the 12 V pump to the outlet was introduced only for UTOPIHAN III to constantly flush the bypass while not in use for zeroing. 1.015 102 M). The HCHO standard was calibrated by reaction with SO 3 in excess and titration against I2 using the method of de Jong [Deutsches Institut fu¨r Normung e. V., 1988] as well as titration of the produced OH against an HCl standard (all chemicals VWR). Regular checks for concentration changes were performed by extinction measurements at 412 nm using the Hantzsch reaction and according to the national compendium of research techniques [Deutsches Akkreditierungssystem Pru¨fwesen GmbH, 1985]. H2O2 was titrated regularly (every 2 – 4 weeks) against KMnO4 standards (VWR). [21] Gas calibrations using permeation devices were performed once a day, before or after the flights though not during the flights, because of a lack of space in the aircraft and the relatively long time to reach a stable signal needed to calculate sensitivity as compared to the flight time of about 3 hours. The permeation source used is described in detail by Wagner [2000], Kormann et al. [2002], and Wagner et al. [2001]. For HCHO, commercially available permeation tubes (VICI, Schenkon, Switzerland; AEROLASER) were employed. The ovens were held at constant temperatures of 70C (HCHO) and 40C (H2O2). Calibration gas mixing ratios were 4.96 ± 0.07 ppbv (HCHO, UTOPIHAN II) and 7.77 ± 0.13 ppbv (H2O2, UTOPIHAN II), respectively. These calibrations were used together with the zeroing (see below) and the liquid calibrations to calculate an inlet efficiency giving an estimate of inlet losses or secondary sources of the measured trace gases. For UTOPIHAN II, this efficiency was found to be relatively
constant at 85 ± 4% (H2O 2) and 69 ± 2% (HCHO). Laboratory tests after the measurement campaign showed that probably more than 50% of the loss in sampling efficiency for HCHO was due to either reduced stripping efficiency (to a value of about 75% instead of 100%) or losses of gaseous formaldehyde presumably on the walls of the stainless steel stripping coil. The HCHO permeation rate was determined gravimetrically, that of H2O2 by bubbling the gas standard through washing flasks and measuring the concentration in the liquid phase by titration with MnO 4 and by extinction measurement with TiCl4 [Pilz and Johann, 1974]. These H2O2 calibrations were cross-checked with the fluorometric detector. [22] Additionally, before, during (2 – 3 times for UTOPIHAN II) and after the flights gas zeroing was performed as described above, i.e., with zero air produced from ambient air, as well as with zero air from an air purifier (CAP60, HEADLINE Filters, Speyer, Germany; only on the ground). This, together with measurements of the calibration gas drawn through the filter cartridges of the onboard gas zeroing system, allowed for checking the scrubbing efficiency of the hopcalite filter system. The efficiency was 99.5 ± 0.6% (HCHO) and 99.3 ± 1.7% (H2O2) during UTOPIHAN II. For flights 7, 8 and 9 an additional liquid zeroing was applied shortly after one of the in-flight gas zero determinations to cross-check the drift in the gas zero and liquid signal. Liquid zeroing, which is also regularly carried out in the liquid calibration process, is carried out with an internal hopcalite (HCHO) or Purafil (H2O2) filter.
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[23] The detection limits (DLs) were determined to be 0.043 ppbv (HCHO) and 0.029 ppbv (H2O2), assuming the average 1s value of the zero signal (internal measurement values integrated over 1 s, but taken only every 3 s without further integration; zero air generator) of two measurements in the laboratory. A similar calculation for liquid zero and gas zero measurements including the inlet efficiency correction made during flight 7 of UTOPIHAN II (the background flight defined in section 4; same integration time) yields DLs of 0.032/0.055 ppbv (HCHO, liquid/gas zeroing) and 0.024/0.008 ppbv (H2O2) respectively, confirming or even outperforming the numbers obtained in the laboratory. Precisions (1s of the calibration signal) were about 14.3% (HCHO) and 9.7% (H2O2) at 300 pptv mixing ratio with a total estimated accuracy of 20.6% and 11.1% at 300 pptv and 50 ppbv O3, including errors of liquid standards, inlet efficiency and of an O3 interference correction that had to be applied to the data. To determine the origin and the magnitude of this ozone interference (7 104 ppbv/ ppbv for HCHO and 6 104 ppbv/ppbv for H2O2 (UTOPIHAN II)), extensive laboratory experiments were carried out after the campaign. These showed that for H2O2 and most probably also for HCHO part of the interference takes place in the inlet system. For H2O2, this external interference was found to be even more important than the internal one. The interference gives rise to an additional estimated systematic error of less than +25% for H2O2 and less than +29% for HCHO. For future campaigns, at least for the HCHO measurement, this correction, which represents a large part of the total uncertainty, can be avoided by adding excess NO to the inlet (e.g., Tanner et al. [1986] or Keuken et al. [1988]), but as the same inlet was used for H2O2 and the latter device possibly suffers from an NO interference [Ischiropoulos et al., 1996], this possibility was rejected for UTOPIHAN. 2.2. Other Species [24] NO, NOy (all nitrogen species with an oxidation number larger than I) and O3 were measured using a chemiluminescence detector (ECO PHYSICS SR790H) and a heated Au converter in the presence of CO for NOy [Lange et al., 2002]. The precision of the NO data was determined as 6.5%, the accuracy for flight 4 (‘‘convective’’ case) was estimated conservatively as 25%, that for flight 7 (background case) as 18.5% with a detection limit of less than 10 pptv. O3 precision was 1%, accuracy 5%. J(NO2) data were collected with a filter radiometer (Meteorologieconsult, Glashu¨tten, Germany) both upward and downward. The precision of the J(NO2) measurements was 1%, the accuracy 15%. [25] CO was measured by a TDLAS system based on a lead salt diode laser in continuous wave (CW) operation in the midinfrared (4.7 mm), described in detail by Kormann et al. [2005]. The detection limit was at 0.26 ppbv (30 s) and the reproducibility of calibration 3.6% (6 s) at a calibration mixing ratio of 68.9 ± 0.4 ppbv. Calibrations were performed in flight about every 10 minutes, and the standard used was compared to a secondary standard certified with respect to the NIWA scale [Brenninkmeijer et al., 2001]. [26 ] Twenty-four electropolished and prehumidified stainless steel canisters of 0.8 l volume were filled in flight
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automatically inside a wing pod and subsequently analyzed for VOCs and greenhouse gases (HCFC-134a, propane, HCFC-22, CFC-12, chloromethane, HCFC-142b, CFC114, butane, methyl bromide, chloroethane, isopentane, CFC-11, pentane, CH3I, CFC-113, CH2Cl2, CHCl3, dichloroethane, benzene, CCl4, trichloroethene, CH2Br2, toluene, C2Cl4, ethyl benzene, m- and p-xylene, CHBr3 and oxylene). For a detailed description of the system used, see Gros et al. [2003]. The sampling system used 1=4 00 stainless steel tubing and connectors, and the inlets and outlets were regulated by computer-controlled stainless steel pneumatic valves. The analysis was carried out directly after the flights using a GC-MS (modified GC/MS 6890/5973, Agilent Technology, Palo Alto, USA) with a mean detection limit of 0.05 pptv, a precision of 5 – 15% and an estimated accuracy of 15% based on the accuracy and stability of the calibration standard. [27] Six gases with masses attributed to protonated acetone (m59), methanol (m33), acetaldehyde (m45), acetonitrile (m42), benzene (m79) and toluene (m93) were measured with a Proton Transfer Reaction Mass Spectrometer (PTR-MS) system. A seventh mass (m71) was attributed to the sum of the isoprene oxidation products methyl vinyl ketone and methylacrolein, as in previous studies [Williams et al., 2001], while isoprene itself (m69) is determined only qualitatively in this study because of the relatively high noise on the instrument signal. The accuracy, excluding precision, for all species except isoprene was determined to be about 20% on the basis of the accuracy and stability of the calibration standard and systematic errors in the calibration procedure as well as in the measurement of the mass spectrometer ion transmission, while the precision was 21% for CH3OH (with a mean mixing ratio of 1.96 ppbv), 73% for acetonitrile (mean 0.177 ppbv), 34% for CH3CHO (mean 0.537 ppbv) and 7% for acetone (mean 1.84 ppbv). Here, the precision is calculated conservatively as two times the standard deviation of the stable section of the background flight expressed as a percentage of the campaign mean. In most cases mass 71, benzene and toluene were below the detection limit (DL). Measurements of each species were made circa every 30 s with dwell times of between 0.5 and 2 s. The detection limits for the unsmoothed data were determined to be 0.410 ppbv (methanol), 0.130 ppbv (acetonitrile), 0.183 ppbv (acetaldehyde), 0.134 ppbv (acetone), 0.171 ppbv (mass 71), 0.261 ppbv (benzene) and 0.219 ppbv (toluene). They were again specified using a signal-to-noise ratio of 2. The configuration of the PTRMS used in this study has been discussed in detail previously [Salisbury et al., 2003]. [28] H2O was measured with a commercially available LI-COR infrared analyzer adapted to aircraft measurements. The inlet was forward facing (3/800 stainless steel) while the instrument only detected gaseous water. The response time constant was about 3 s with a total uncertainty of 5%, being at least 100 ppmv. [29] Flight data (latitude, longitude, aircraft heading, wind speed, wind angle, drift angle, cabin temperature, static pressure, cabin pressure, aircraft generator DC voltage, altitude, static air temperature and true air speed) were acquired from the aircraft flight management system and the Enviscope data acquisition system (Enviscope, Frankfurt,
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Table 2. Synopsis of the Parameters Measured During UTOPIHAN II Parameter
Measurement Technique
DL, ppbv
Precision, %
Accuracy, %
Systematic Error, %
HCHO H2O2 NO NOy O3 J(NO2) CO VOCs/GHGs CH3OH acetonitrile CH3CHO CH3COCH3
derivatization plus fluorescence derivatization plus fluorescence chemiluminescence chemiluminescence chemiluminescence filter radiometer TDLAS GC-MS PTR-MS PTR-MS PTR-MS PTR-MS
0.043 0.029
