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We present airborne in-situ trace gas measurements which were performed on eight campaigns between November 2001 and July 2003 during the SPURT-project (SPURenstofftransport in der Tropopausenregion, trace gas transport in the tropopause region). The measurements on a quasi regular basis allowed an overview of the seasonal variations of the trace gas distribution in the tropopause region over Europe from 35°-75°N to investigate the influence of transport and mixing across the extratropical tropopause on the lowermost stratosphere. <P style="line-height: 20px;"> From the correlation of CO and O<sub>3</sub> irreversible mixing of tropospheric air into the lowermost stratosphere is identified. The CO distribution indicates that transport and subsequent mixing of tropospheric air across the extratropical tropopause predominantly affects a layer, which closely follows the shape of the local tropopause. In addition, the seasonal cycle of CO<sub>2</sub> illustrates the strong coupling of that layer to the extratropical troposphere. Both, horizontal gradients of CO on isentropes as well as the CO-O<sub>3</sub>-distribution in the lowermost stratosphere reveal that the influence of quasi-horizontal transport and subsequent mixing weakens with distance from the local tropopause. The mixing layer extends to about 25 K in potential temperature above the local tropopause exhibiting only a weak seasonality. <P style="line-height: 20px;"> However, at large distances from the tropopause a significant influence of tropospheric air is still evident. The relation between N<sub>2</sub>O and CO<sub>2</sub> indicates that a significant contribution of air originating from the tropical tropopause contributes to the background air in the extratropical lowermost stratosphere.
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Following recent observations of molecular iodine (I<sub>2</sub>) in the coastal marine boundary layer (MBL) (Saiz-Lopez and Plane, 2004), it has become important to determine the absolute absorption cross-section of I<sub>2</sub> at reasonably high resolution, and also to evaluate the rate of photolysis of the molecule in the lower atmosphere. The absolute absorption cross-section (σ) of gaseous I<sub>2</sub> at room temperature and pressure (295K, 760Torr) was therefore measured between 182 and 750nm using a Fourier Transform spectrometer at a resolution of 4cm<sup>-1</sup> (0.1nm at λ=500nm). The maximum absorption cross-section in the visible region was observed at λ=533.0nm to be σ=(4.24±0.50)x10<sup>-18</sup>cm<sup>2</sup>molecule<sup>-1</sup>. The spectrum is available as supplementary material accompanying this paper. The photo-dissociation rate constant (<i>J</i>) of gaseous I<sub>2</sub> was also measured directly in a solar simulator, yielding <i>J</i>(I<sub>2</sub>)=0.12±0.03s<sup>-1</sup> for the lower troposphere. This is in excellent agreement with the value of 0.12±0.015s<sup>-1</sup> calculated using the measured absorption cross-section, terrestrial solar flux for clear sky conditions and assuming a photo-dissociation yield of unity. A two-stream radiation transfer model was then used to determine the variation in photolysis rate with solar zenith angle (SZA), from which an analytic expression is derived for use in atmospheric models. Photolysis appears to be the dominant loss process for I<sub>2</sub> during daytime, and hence an important source of iodine atoms in the lower atmosphere.
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This is a study into the phase transitions of aerosol composed of the ternary system ammonium sulfate (AS) - malonic acid (MA) - water using infrared extinction spectroscopy. Twelve compositions were studied in both deliquescence and efflorescence mode experiments. The presence of a MA fraction, by dry mass, (<i>f</i><sub>MA</sub>) of 0.1 in an AS aerosol altered the relative humidity at which the phase transitions occur in an atmospherically significant manner. For compositions with 0.25<<i>f</i><sub>MA</sub><0.90, no distinct ``deliquescence" was observed, contrary to the observed behavior in the binary systems. The crystallization of both the MA and AS components is suppressed by the presence of the other component in the aerosol. At <i>f</i><sub>MA</sub>=0.9, the crystallization relative humidity of MA was lowered from <i>RH</i>=6% to less than 1%. Similarly, at <i>f</i><sub>MA</sub>=0.4, the AS component did not crystallize. The atmospheric implications of the results are discussed.
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This article, the first in the series, presents kinetic and photochemical data evaluated by the IUPAC Subcommittee on GasKinetic Data Evaluation for Atmospheric Chemistry. It covers the gas phase and photochemical reactions of O<sub>x</sub>, HO<sub>x</sub>, NO<sub>x</sub> and SO<sub>x</sub> species, which were last published in 1997, and were updated on the IUPAC website in late 2001. The article consists of a summary sheet, containing the recommended kinetic parameters for the evaluated reactions, and five appendices containing the data sheets, which provide information upon which the recommendations are made.
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Refractive and absorption indices in the UV and visible region of selected aqueous organic acids relevant to tropospheric aerosols are reported. The acids investigated are the aliphatic dicarboxylic acids oxalic, malonic, tartronic, succinic and glutaric acid. In addition we report data for pyruvic, pinonic, benzoic and phthalic acid. To cover a wide range of conditions we have investigated the aqueous organic acids at different concentrations spanning from highly diluted samples to concentrations close to saturation. The density of the investigated samples is reported and a parameterisation of the absorption and refractive index that allows the calculation of the optical constants of mixed aqueous organic acids at different concentrations is presented. The single scattering albedo is calculated for two size distributions using measured and a synthetic set of optical constants. The results show that tropospheric aerosols consisting of only these organic acids and water have a pure scattering effect.
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The formation and detailed composition of secondary organic aerosol (SOA) from the gas phase ozonolysis of α- and β-pinene has been simulated using the Master Chemical Mechanism version 3 (MCM v3), coupled with a representation of gas-to-aerosol transfer of semivolatile and involatile oxygenated products. A kinetics representation, based on equilibrium absorptive partitioning of ca. 200 semivolatile products, was found to provide an acceptable description of the final mass concentrations observed in a number of reported laboratory and chamber experiments, provided partitioning coefficients were increased by about two orders of magnitude over those defined on the basis of estimated vapour pressures. This adjustment is believed to be due, at least partially, to the effect of condensed phase association reactions of the partitioning products. Even with this adjustment, the simulated initial formation of SOA was delayed relative to that observed, implying the requirement for the formation of species of much lower volatility to initiate SOA formation. The inclusion of a simplified representation of the formation and gas-to-aerosol transfer of involatile dimers of 22 bi- and multifunctional carboxylic acids (in addition to the absorptive partitioning mechanism) allowed a much improved description of SOA formation for a wide range of conditions. The simulated SOA composition recreates certain features of the product distributions observed in a number of experimental studies, but implies an important role for multifunctional products containing hydroperoxy groups (i.e. hydroperoxides). This is particularly the case for experiments in which 2-butanol is used to scavenge OH radicals, because [HO<sub>2</sub>]/[RO<sub>2</sub>] ratios are elevated in such systems. The optimized mechanism is used to calculate SOA yields from α- and β-pinene ozonolysis in the presence and absence of OH scavengers, and as a function of temperature.
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A variety of C<sub>1</sub>-C<sub>12</sub> carbonyl compounds were measured in the air of a boreal coniferous forest located in Hyytiälä, Southern Finland. 24-hour samples were collected during March and April in 2003 using DNPH (2,4-dinitrophenyl hydrazine) coated C<sub>18</sub>-cartridges and analyzed by liquid chromatography-mass spectrometry (LC-MS). <P style="line-height: 20px;"> Altogether 22 carbonyl compounds were quantified. The most abundant carbonyls were acetone (24-hour average 1340ng/m<sup>3</sup>), formaldehyde (480ng/m<sup>3</sup>) and acetaldehyde (360ng/m<sup>3</sup>). Concentrations of monoterpene reaction products nopinone (9ng/m<sup>3</sup>) and limona ketone (5ng/m<sup>3</sup>) were low compared to the most abundant low molecular weight carbonyls. Trajectory analysis showed that highest concentrations of carbonyls were measured in the air masses coming from the East and the lowest in the air masses cycled long time over Scandinavia. The total concentration of carbonyl compounds in Hyytiälä in March/April 2003 was much higher than the concentration of aromatic hydrocarbons and monoterpenes in April 2002. <P style="line-height: 20px;"> Scaling the concentrations against reactivity with the OH-radical showed, that in spite of relatively low ambient concentrations higher molecular weight aldehydes contribute significantly to the total OH-reactive mass of carbonyls. The impact of carbonyl compounds on OH-radical chemistry is important. Contribution of carbonyls as an OH sink is comparable to that of NO<sub>2</sub> and higher than monoterpenes and aromatic hydrocarbons. <P style="line-height: 20px;"> Lifetimes of the measured carbonyls with respect to reactions with OH radicals, ozone (O<sub>3</sub>), and nitrate (NO<sub>3</sub>) radicals as well as photolysis were estimated. The main sink reactions for most of the carbonyl compounds in Hyytiälä in springtime are expected to be reactions with the OH radical and photolysis. For 6-methyl-5-hepten-2-one and limona ketone also reactions with ozone are important. The sources of carbonyl compounds are presently highly uncertain. Based on the comparisons with urban concentrations the direct anthropogenic emissions are not as important as secondary biogenic and anthropogenic sources or primary biogenic sources in Hyytiälä.
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The OHP Temperature and Ozone Intercomparison Campaign (OTOIC) took place at the Observatoire de Haute Provence, France, from 1–18 July 1997. The NASA Goddard Space Flight Center (GSFC) mobile lidar system was deployed at the Observatoire de Haute Provence (OHP) during a blind intercomparison as a part of the continuous validation process within the Network for the Detection of Stratospheric Change. The GSFC measurements were compared to two lidars permanently deployed at OHP and operated by the Centre National de la Recherche Scientifique (CNRS), one measuring ozone and the other measuring temperature.
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From high latitude lidar observations, quite precise information is extracted about the temporal evolution and vertical distribution of volcanic aerosol in the high latitude lower stratosphere following the eruption of Mount Pinatubo. Irreversible mixing of lower stratospheric aerosol, to the arctic pole during early 1992, is demonstrated, as a function of potential temperature and time. This work complements previous studies, which either identify vortex intrusions - without demonstrating irreversible transport, or use lower resolution satellite observations. The observed transport is associated tentatively with the vortex disturbance during late January, 1992. A very large number of high resolution lidar observations of Mount Pinatubo aerosol are analysed, without any data averaging. Averaging in measurement or analysis can cause tracer mixing to be overestimated. Averaging in the analysis can also require assumptions about which quantity has the dominant error (in this case, the equivalent latitude coordinate or the measurement), and which part of the data contains real structure. The method below attempts to avoid such assumptions.
