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During recent field campaigns, hydroxyl radical (OH) concentrations that were measured by laser-induced fluorescence (LIF) were up to a factor of ten larger than predicted by current chemical models for conditions of high OH reactivity and low NO concentration. These discrepancies, which were observed in forests and urban-influenced rural environments, are so far not entirely understood. In summer 2011, a series of experiments was carried out in the atmosphere simulation chamber SAPHIR in Jülich, Germany, in order to investigate the photochemical degradation of isoprene, methyl-vinyl ketone (MVK), methacrolein (MACR) and aromatic compounds by OH. Conditions were similar to those experienced during the PRIDE-PRD2006 campaign in the Pearl River Delta (PRD), China, in 2006, where a large difference between OH measurements and model predictions was found. During experiments in SAPHIR, OH was simultaneously detected by two independent instruments: LIF and differential optical absorption spectroscopy (DOAS). Because DOAS is an inherently calibration-free technique, DOAS measurements are regarded as a reference standard. The comparison of the two techniques was used to investigate potential artifacts in the LIF measurements for PRD-like conditions of OH reactivities of 10 to 30 s−1 and NO mixing ratios of 0.1 to 0.3 ppbv. The analysis of twenty experiment days shows good agreement. The linear regression of the combined data set (averaged to the DOAS time resolution, 2495 data points) yields a slope of 1.02 ± 0.01 with an intercept of (0.10 ± 0.03) × 106 cm−3 and a linear correlation coefficient of R2 = 0.86. This indicates that the sensitivity of the LIF instrument is well-defined by its calibration procedure. No hints for artifacts are observed for isoprene, MACR, and different aromatic compounds. LIF measurements were approximately 30–40% (median) larger than those by DOAS after MVK (20 ppbv) and toluene (90 ppbv) had been added. However, this discrepancy has a large uncertainty and requires further laboratory investigation. Observed differences between LIF and DOAS measurements are far too small to explain the unexpected high OH concentrations during the PRIDE-PRD2006 campaign.

[1] Recent studies have shown that measured OH under NOx-limited, high-isoprene conditions are many times higher than modeled OH. In this study, a detailed analysis of the HOx radical budgets under low-NOx, rural conditions was performed employing a box model based on the Master Chemical Mechanism (MCMv3.2). The model results were compared with HOx radical measurements performed during the international HOxComp campaign carried out in Julich, Germany, during summer 2005. Two different air masses influenced the measurement site denoted as high-NOx (NO, 1–3 ppbv) and low-NOx (NO, < 1 ppbv) periods. Both modeled OH and HO2 diurnal profiles lay within the measurement range of all HOx measurement techniques, with correlation slopes between measured and modeled OH and HO2 around unity. Recently discovered interference in HO2 measurements caused by RO2 cross sensitivity was found to cause a 30% increase in measured HO2 during daytime on average. After correction of the measured HO2 data, the model HO2 is still in good agreement with the observations at high NOx but overpredicts HO2 by a factor of 1.3 to 1.8 at low NOx. In addition, for two different set of measurements, a missing OH source of 3.6 ± 1.6 and 4.9 ± 2.2 ppb h−1 was estimated from the experimental OH budget during the low-NOx period using the corrected HO2 data. The measured diurnal profile of the HO2/OH ratio, calculated using the corrected HO2, is well reproduced by the MCM at high NOx but is significantly overestimated at low NOx. Thus, the cycling between OH and HO2 is better described by the model at high NOx than at low NOx. Therefore, similar comprehensive field measurements accompanied by model studies are urgently needed to investigate HOx recycling under low-NOx conditions.

A new method for stable carbon isotope ratio analysis of anhydrosugars from biomass burning aerosol particle source filter samples was developed by employing Thermal Desorption--2 Dimensional Gas Chromatography--Isotope Ratio Mass Spectrometry (TD-2DGC-IRMS). Compound specific isotopic measurements of levoglucosan, mannosan, and galactosan performed by TD-2DGC-IRMS in a standard mixture show good agreement with isotopic measurements of the bulk anhydrosugars, carried out by Elemental Analyzer--Isotope Ratio Mass Spectrometry (EA-IRMS). The established method was applied to determine the isotope ratios of levoglucosan, mannosan, and galactosan from source samples collected during combustion of hard wood, softwood, and crop residues. δ(13)C values of levoglucosan were found to vary between -25.6 and -22.2‰, being higher in the case of softwood. Mannosan and galactosan were detected only in the softwood samples showing isotope ratios of -23.5‰ (mannosan) and -25.7‰ (galactosan). The isotopic composition of holocellulose in the plant material used for combustion experiments was determined with δ(13)C values between -28.5 and -23.7‰. The difference in δ(13)C of levoglucosan in biomass burning aerosol particles compared to the parent fuel holocellulose was found to be -1.89 (±0.37)‰ for the investigated biomass fuels. Compound specific δ(13)C measurements of anhydrosugars should contribute to an improved source apportionment.

High molecular weight (300–650 Da) naturally charged negative ions have previously been observed at a boreal forest site in Hyytiälä, Finland. The long-term measurements conducted in this work showed that these ions are observed practically every night between spring and autumn in Hyytiälä. The ambient mass spectral patterns could be reproduced in striking detail during additional measurements of α-pinene (C10H16) oxidation at low-OH conditions in the Jülich Plant Atmosphere Chamber (JPAC). The ions were identified as clusters of the nitrate ion (NO3−) and α-pinene oxidation products reaching oxygen to carbon ratios of 0.7–1.3, while retaining most of the initial ten carbon atoms. Attributing the ions to clusters instead of single molecules was based on additional observations of the same extremely oxidized organics in clusters with HSO4− (Hyytiälä) and C3F5O2− (JPAC). The most abundant products in the ion spectra were identified as C10H14O7, C10H14O9, C10H16O9, and C10H14O11. The mechanism responsible for forming these molecules is still not clear, but the initial reaction is most likely ozone attack at the double bond, as the ions are mainly observed under dark conditions. β-pinene also formed highly oxidized products under the same conditions, but less efficiently, and mainly C9 compounds which were not observed in Hyytiälä, where β-pinene on average is 4–5 times less abundant than α-pinene. Further, to explain the high O/C together with the relatively high H/C, we propose that geminal diols and/or hydroperoxide groups may be important. We estimate that the night-time concentration of the sum of the neutral extremely oxidized products is on the order of 0.1–1 ppt (~106–107 molec cm−3). This is in a similar range as the amount of gaseous H2SO4 in Hyytiälä during day-time. As these highly oxidized organics are roughly 3 times heavier, likely with extremely low vapor pressures, their role in the initial steps of new aerosol particle formation and growth may be important and needs to be explored in more detail in the future.

The secondary formation of HO(2) radicals following OH + aromatic hydrocarbon reactions in synthetic air under normal pressure and temperature was investigated in the absence of NO after pulsed production of OH radicals. OH and HO(x) (=OH + HO(2)) decay curves were recorded using laser-induced fluorescence after gas-expansion. The prompt HO(2) yields (HO(2) formed without preceding NO reactions) were determined by comparison to results obtained with CO as a reference compound. This approach was recently introduced and applied to the OH + benzene reaction and was extended here for a number of monocyclic aromatic hydrocarbons. The measured HO(2) formation yields are as follows: toluene, 0.42 ± 0.11; ethylbenzene, 0.53 ± 0.10; o-xylene, 0.41 ± 0.08; m-xylene, 0.27 ± 0.06; p-xylene, 0.40 ± 0.09; 1,2,3-trimethylbenzene, 0.31 ± 0.06; 1,2,4-trimethylbenzene, 0.37 ± 0.09; 1,3,5-trimethylbenzene, 0.29 ± 0.08; hexamethylbenzene, 0.32 ± 0.08; phenol, 0.89 ± 0.29; o-cresol, 0.87 ± 0.29; 2,5-dimethylphenol, 0.72 ± 0.12; 2,4,6-trimethylphenol, 0.45 ± 0.13. For the alkylbenzenes HO(2) is the proposed coproduct of phenols, epoxides, and possibly oxepins formed in secondary reactions with O(2). In most product studies the only quantified coproducts were phenols whereas only a few studies reported yields of epoxides. Oxepins have not been observed so far. Together with the yields of phenols from other studies, the HO(2) yields determined in this work set an upper limit to the combined yields of epoxides and oxepins that was found to be significant (≤0.3) for all investigated alkylbenzenes except m-xylene. For the hydroxybenzenes the currently proposed HO(2) coproducts are dihydroxybenzenes. For phenol and o-cresol the determined HO(2) yields are matching the previously reported dihydroxybenzene yields, indicating that these are the only HO(2) forming reaction channels. For 2,5-dimethylphenol and 2,4,6-trimethylphenol no complementary product studies are available.

The temperature dependence of the kinetic isotope effect (KIE) of beta-pinene ozonolysis was investigated experimentally at 258, 273 and 303 K in the AIDA atmospheric simulation chamber. Compound specific carbon isotopic analysis of gas phase samples was performed off-line with a Thermo Desorption-Gas Chromatography-Isotope Ratio Mass Spectrometry (TD-GC-IRMS) system. From the temporal behavior of the delta C-13 of beta-pinene a KIE of 1.00358 +/- 0.00013 was derived at 303 K, in agreement with literature data. Furthermore, KIE values of 1.00380 +/- 0.00014 at 273 K and 1.00539 +/- 0.00012 at 258 K were determined, showing an increasing KIE with decreasing temperature. A parameterization of the observed KIE temperature dependence was deduced and used in a sensitivity study carried out with the global chemistry transport model MOZART-3. Two scenarios were compared, the first neglecting, the second implementing the KIE temperature dependence in the simulations. beta-Pinene stable carbon isotope ratio and concentration were computed, with emphasis on boreal zones. For early spring it is shown that when neglecting the temperature dependence of KIE, the calculated average age of beta-pinene in the atmosphere can be up to two times over-or underestimated. The evolution of the isotopic composition of the major beta-pinene oxidation product, nopinone, was examined using Master Chemical Mechanism (MCM) simulations. The tested hypothesis that formation of nopinone and its associated KIE are the determining factors for the observed delta C-13 values of nopinone is supported at high beta-pinene conversion levels.

In this study we investigated the secondary formation of HO(2) following the benzene + OH reaction in N(2) with variable O(2) content at atmospheric pressure and room temperature in the absence of NO. After pulsed formation of OH, HO(x) (= OH + HO(2)) and OH decay curves were measured by means of a laser-induced fluorescence technique (LIF). In synthetic air the total HO(2) yield was determined to be 0.69 ± 0.10 by comparison to results obtained with CO as a reference compound. HO(2) is expected to be a direct product of the reaction of the intermediately formed OH-benzene adduct with O(2). The HO(2) yield is slightly greater than the currently recommended yield of the proposed HO(2) co-product phenol (∼53%). This hints towards other, minor HO(2) forming channels in the absence of NO, e.g. the formation of epoxide species that was proposed in the literature. For other test compounds upper limits of HO(2) yields of 0.10 (isoprene) and 0.05 (cyclohexane) were obtained, respectively. In further experiments at low O(2) concentrations (0.06-0.14% in N(2)) rate constants of (2.4 ± 1.1) × 10(-16) cm(3) s(-1) and (5.6 ± 1.1) × 10(-12) cm(3) s(-1) were estimated for the OH-benzene adduct reactions with O(2) and O(3), respectively. The rate constant of the unimolecular dissociation of the adduct back to benzene + OH was determined to be (3.9 ± 1.3) s(-1). The HO(2) yield at low O(2) was similar to that found in synthetic air, independent of O(2) and O(3) concentrations indicating comparable HO(2) yields for the adduct + O(2) and adduct + O(3) reactions.

Stress-induced volatile organic compound (VOC) emissions from transgenic Grey poplar modified in isoprene emission potential were used for the investigation of photochemical secondary organic aerosol (SOA) formation. In poplar, acute ozone stress induces the emission of a wide array of VOCs dominated by sesquiterpenes and aromatic VOCs. Constitutive light-dependent emission of isoprene ranged between 66 nmol m−2 s−1 in non-transgenic controls (wild type WT) and nearly zero (−2 s−1) in isoprene emission-repressed plants (line RA22), respectively. Nucleation rates of up to 3600 cm−3 s−1 were observed in our experiments. In the presence of isoprene new particle formation was suppressed compared to non-isoprene containing VOC mixtures. Compared to isoprene/monoterpene systems emitted from other plants the suppression of nucleation by isoprene was less effective for the VOC mixture emitted from stressed poplar. This is explained by the observed high efficiency of new particle formation for emissions from stressed poplar. Direct measurements of OH in the reaction chamber revealed that the steady state concentration of OH is lower in the presence of isoprene than in the absence of isoprene, supporting the hypothesis that isoprenes' suppressing effect on nucleation is related to radical chemistry. In order to test whether isoprene contributes to SOA mass formation, fully deuterated isoprene (C5D8) was added to the stress-induced emission profile of an isoprene free poplar mutant. Mass spectral analysis showed that, despite the isoprene-induced suppression of particle formation, fractions of deuterated isoprene were incorporated into the SOA. A fractional mass yield of 2.3% of isoprene was observed. Future emission changes due to land use and climate change may therefore affect both gas phase oxidation capacity and new particle number formation.

We performed MAX-DOAS measurements during the PRiDe-PRD2006 campaign in the Pearl River Delta region 50 km north of Guangzhou, China, for 4 weeks in June 2006. We used an instrument sampling at 7 different elevation angles between 3° and 90°. During 9 cloud-free days, differential slant column densities (DSCDs) of O4 (O2 dimer) absorptions between 351 nm and 389 nm were evaluated for 6 elevation angles. Here, we show that radiative transfer modeling of the DSCDS can be used to retrieve the aerosol extinction and the height of the boundary layer. A comparison of the aerosol extinction with simultaneously recorded, ground based nephelometer data shows excellent agreement.

For CareBeijing-2006, two sites were established in urban and suburban regions of Beijing in summer 2006. Observations of O-3 and its precursors together with meteorological parameters at both sites are presented. Gross ozone production rate P(O-3) and sensitivity to nitric oxides (NOx) and volatile organic compounds (VOCs) were investigated using an observation-based photochemical box model (OBM). P(O-3) varied from nearly zero to 120 and 50 ppb h(-1) for urban and suburban sites, respectively. These rates were greater than the accumulation rates of the observed oxidant (O-3 + NO2) concentrations. The O-3 episodes typically appeared under southerly wind conditions with high P(O-3), especially at the urban site. Sensitivity studies with and without measured nitrous acid (HONO) as a model constraint suggested that the estimated P(O-3) at both sites was strongly enhanced by radical production from HONO photolysis. Both NOx- and VOC-sensitive chemistries existed over time scales from hours to days at the two sites. The variation in O-3-sensitive chemistry was relatively well explained by the ratio of the average daytime total VOC reactivity (k(TVOC)) to NO, with the transition chemistry corresponding to a k(TVOC)/NO value of 2-4 s(-1) ppb(-1). Pronounced diurnal variations in the O-3 production regime were found. In the morning, conditions were always strongly VOC-limited, while in the afternoon, conditions were variable for different days and different sites. The model-calculated results were tested by measurements of H2O2, HNO3, total OH reactivity, and HOx radicals. The OBM was generally capable of correctly simulating the levels of P(O-3), although it might tend to overpredict the VOC-sensitive chemistry.

Going Faster The concentrations of most tropospheric pollutants and trace gases are kept in check by their reactions with hydroxyl radicals (OH). OH is a short-lived, highly reactive species that is produced in the atmosphere by photochemical processes, and regenerated in the chain of chemical reactions that follows the oxidative destruction of those molecules. These regeneration mechanisms were thought to be fairly well understood, but now Hofzumahaus et al. (p. 1702 , published online 4 June) present evidence of a pathway not previously recognized. In a study of atmospheric composition in the Pearl River Delta, a highly polluted region of China, greatly elevated OH concentrations were observed without the correspondingly high levels of ozone expected from current models. Thus, OH concentrations may be augmented by a process that speeds the regeneration of OH without producing ozone.

This study was part of the international field measurement Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing-2006). We investigated a new particle formation event in a highly polluted air mass at a regional site south of the megacity Beijing and its impact on the abundance and properties of cloud condensation nuclei (CCN). During the 1-month observation, particle nucleation followed by significant particle growth on a regional scale was observed frequently (similar to 30%), and we chose 23 August 2006 as a representative case study. Secondary aerosol mass was produced continuously, with sulfate, ammonium, and organics as major components. The aerosol mass growth rate was on average 19 mu g m(-3) h(-1) during the late hours of the day. This growth rate was observed several times during the 1-month intensive measurements. The nucleation mode grew very quickly into the size range of CCN, and the CCN size distribution was dominated by the growing nucleation mode ( up to 80% of the total CCN number concentration) and not as usual by the accumulation mode. At water vapor supersaturations of 0.07-0.86%, the CCN number concentrations reached maximum values of 4000-19,000 cm(-3) only 6-14 h after the nucleation event. During particle formation and growth, the effective hygroscopicity parameter kappa increased from about 0.1-0.3 to 0.35-0.5 for particles with diameters of 40-90 nm, but it remained nearly constant at similar to 0.45 for particles with diameters of similar to 190 nm. This result is consistent with aerosol chemical composition data, showing a pronounced increase of sulfate.

Ground-based measurements of aerosol and trace gases were conducted at a rural site in Beijing in the summer of 2006 as a part of the Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing 2006). The size-resolved chemical composition of submicron aerosol was measured using an Aerodyne quadrupole aerosol mass spectrometer (AMS). The data obtained from 15 August to 10 September 2006 are presented. Meteorological analysis shows that the measurement period can be characterized as a cycle of low wind speed periods over the course of a few days (stagnant periods) followed by rapid advection of clean air from the north./northwest mostly due to passage of a midlatitude cyclone. Mass concentrations of total measured aerosol with diameters less than 1 mu m (total PM1), which is defined as the sum of elemental carbon and nonrefractory components measured by the AMS, were highly variable, ranging from similar to 2 to similar to 100 mu g m(-3). Large variability of the PM1 composition and drastic changes in the sulfate (SO42-) size distribution were observed to be associated with the cycle of stagnant periods and advection of clean air, indicating that both chemical and physical properties of aerosols were significantly altered on a time scale of a few days. We have found the dominance of organic aerosol at lower total PM1 mass loadings and that of SO42- at higher mass loadings, which may have important implications for the PM control strategy in Beijing. Possible factors affecting the evolution of the mass concentration and size distribution of SO42- are discussed.

A chamber study was carried out to investigate the stable carbon isotopic composition (delta C-13) of secondary organic aerosol (SOA) formed from ozonolysis of beta-pinene. beta-Pinene (600 ppb) with a known delta C-13 value (-30.1%) and 500 ppb ozone were injected into the chamber in the absence of light and the resulting SOA was collected on preheated quartz fiber filters. Furthermore, delta C-13 values of the gas-phase beta-pinene and one of its oxidation products, nopinone, were measured using a gas chromatograph coupled to an isotope ratio mass spectrometer (GC-IRMS). beta-Pinene was progressively enriched with the heavy carbon isotope due to the kinetic isotope effect (KIE). The KIE of the reaction of beta-pinene with ozone was measured to be 1.0026 ((O3)epsilon 2.6 +/- 1.5 parts per thousand). The delta C-13 value of total secondary organic aerosol was very similar to that of its precursor (average = -29.6 +/- 0.2 parts per thousand) independent of experiment time. Nopinone, one of the major oxidation products of beta-pinene, was found in both the gas and aerosol phases. The gas-phase nopinone was heavier than the initial beta-pinene by 1.3 parts per thousand but lighter than the corresponding aerosol-phase nopinone. On average, the gas-phase nopinone was lighter by 2.3 parts per thousand than the corresponding aerosol- phase nopinone. The second product found in the SOA was detected as acetone, but it desorbed from the filter at a higher temperature than nopinone, which indicates that it is a pyrolysis product. The acetone showed a much lower delta C-13 (-36.6%) compared to the initial beta-pinene delta C-13.

Total atmospheric OH reactivities (kOH) have been measured as reciprocal OH lifetimes by a newly developed instrument at a rural site in the densely populated Pearl River Delta (PRD) in Southern China in summer 2006. The deployed technique, LP-LIF, uses laser flash photolysis (LP) for artificial OH generation and laser-induced fluorescence (LIF) to measure the time-dependent OH decay in samples of ambient air. The reactivities observed at PRD covered a range from 10 s−1 to 120 s−1, indicating a large load of chemical reactants. On average, kOH exhibited a pronounced diurnal profile with a mean maximum value of 50 s−1 at daybreak and a mean minimum value of 20 s−1 at noon. The comparison of reactivities calculated from measured trace gases with measured kOH reveals a missing reactivity of about a factor of 2 at day and night. The reactivity explained by measured trace gases was dominated by anthropogenic pollutants (e.g., CO, NOx, light alkenes and aromatic hydrocarbons) at night, while it was strongly influenced by local, biogenic emissions of isoprene during the day. Box model calculations initialized by measured parameters reproduce the observed OH reactivity well and suggest that the missing reactivity is contributed by unmeasured, secondary chemistry products (mainly aldehydes and ketones) that were photochemically formed by hydrocarbon oxidation. Overall, kOH was dominated by organic compounds, which had a maximum contribution of 85% in the afternoon. The paper demonstrates the usefulness of direct reactivity measurements, emphasizes the need for direct measurements of oxygenated organic compounds in atmospheric chemistry studies, and discusses uncertainties of the modelling of OVOC reactivities.

The LOPAP (long path absorption) technique has been shown to be very sensitive for the detection of nitrous acid (HONO) in the atmosphere. However, current instruments were mainly built for ground based applications. Therefore, we designed a new LOPAP instrument to be more versatile for mobile measurements and to meet the requirements for airborne application. The detection limit of the new instrument is below 1 ppt at a time resolution of 5 to 7 min. As a first test, the instrument was successfully employed during the ZEPTER-1 campaign in July 2007 on board of the Zeppelin NT airship. During 15 flights on six days we measured HONO concentration profiles over southwest Germany, predominantly in the range between 100 m and 650 m above ground level. On average, a mixing ratio of 34 ppt was observed, almost independently of height. Within a econd campaign, ZEPTER-2 in fall 2008, higher HONO mixing ratios were observed in the Lake Constance area.

This paper presents results from the first large-scale in situ intercomparison of oxygenated volatile organic compound (OVOC) measurements. The intercomparison was conducted blind at the large (270 m(3)) simulation chamber, Simulation of Atmospheric Photochemistry in a Large Reaction Chamber (SAPHIR), in Julich, Germany. Fifteen analytical instruments, representing a wide range of techniques, were challenged with measuring atmospherically relevant OVOC species and toluene (14 species, C-1 to C-7) in the approximate range of 0.5-10 ppbv under three different conditions: (1) OVOCs with no humidity or ozone, (2) OVOCs with humidity added (r.h. approximate to 50%), and (3) OVOCs with ozone (approximate to 60 ppbv) and humidity (r.h. approximate to 50%). The SAPHIR chamber proved to be an excellent facility for conducting this experiment. Measurements from individual instruments were compared to mixing ratios calculated from the chamber volume and the known amount of OVOC injected into the chamber. Benzaldehyde and 1-butanol, compounds with the lowest vapor pressure of those studied, presented the most overall difficulty because of a less than quantitative transfer through some of the participants' analytical systems. The performance of each individual instrument is evaluated with respect to reference values in terms of time series and correlation plots for each compound under the three measurement conditions. A few of the instruments performed very well, closely matching the reference values, and all techniques demonstrated the potential for quantitative OVOC measurements. However, this study showed that nonzero offsets are present for specific compounds in a number of instruments and overall improvements are necessary for the majority of the techniques evaluated here.

We performed MAX-DOAS measurements during the PRiDe-PRD2006 campaign in the Pearl River Delta region 50 km north of Guangzhou, China, for 4 weeks in June 2006. We used an instrument which simultaneously sampled the wavelength range from 292 nm to 443 nm at 7 different elevation angles between 3° and 90°. Here we show that the O4 (O2 dimer) absorption at 360 nm can be used to retrieve the aerosol extinction and the height of the boundary layer. A comparison with simultaneously recorded, ground based nephelometer data shows an excellent agreement.

Column abundances of OClO were measured during the Airborne Arctic Stratospheric Expedition from January 6 to February 9,1989 by near UV absorption spectroscopy. OClO was detected during AASE inside the vortex and at the edge of the vortex with column abundances of 3 – 10 × 1012 cm−2 under twilight conditions and up to 1 × 1014 cm−2 during the night. For comparison, in the Antarctic winter nighttime column abundances of OClO ranged from 1 – 2.5 × 1014 cm−2, while those at mid-latitudes were below the detection limit, i.e. by at least a factor of 10 lower (Solomon et al., 1988). The measurements are broadly consistent with observed BrO column abundances and chemical model calculations.

[1] Reaction rate coefficients and product yields in the gas phase reaction of O-3 with the short-chained alkenes ethene, propene, 1-butene, isobutene, (E)-butene, and (Z)-butene were determined by Simulation of Atmospheric Photochemistry in a Large Reaction Chamber (SAPHIR). In a first set of experiments, reaction rate coefficients were acquired in an absolute reaction rate study from the measured concentration time profiles of ozone and the alkenes with side reactions being suppressed by adding a radical scavenger. The rate coefficients obtained agree well with literature data; for all but one alkene, the deviation was less than 10%. In a second set of experiments, OH yields were derived from the additional alkene turnover in the absence of a radical scavenger. In contrast to other studies, the OH yields determined in the dry chamber (propene, 0.10 +/- 0.07; 1-butene, 0.00 +/- 0.08, isobutene, 0.30 +/- 0.14; (Z)-butene, 0.18 +/- 0.09; and (E)-butene, 0.70 +/- 0.12) differed from the yields obtained under humid conditions (propene, 0.30 +/- 0.08; 1-butene, 0.30 +/- 0.09; isobutene, 0.80 +/- 0.10; ( Z)-butene, 0.40 +/- 0.05; and (E)-butene, 0.60 +/- 0.12). The only exception was ethene ozonolysis, where no OH production was observed. HO2 yields (propene, 1.50 +/- 0.75; 1-butene, 1.60 +/- 0.80; and isobutene, 2.00 +/- 1.00) estimated from the additional ozone turnover compared to the experiments where radicals were not scavenged are reported here for the first time. Furthermore, the yields of the stable ozonolysis products CO, acetaldehyde, and formaldehyde were acquired by monitoring the concentration time profile of the respective compound.

Aircraft are used to investigate the atmosphere and the ground by visual observations and by measurements with instruments on board the aircraft. They allow for: Targeted measurement of atmospheric properties above ground with in-situ instruments and with airborne remote sensing instruments along a given flight path; Measurements at high temporal/spatial resolution; Data for process studies; Data for validation of models; Test and validation of remote sensing; Measurements of otherwise not measurable properties; Simultaneous measurements of many parameters. This lecture is based on experiences gained within several European research projects with research aircraft, such as POLINAT, EULINOX, INCA, and TROCCINOX. Instead The paper describes the European research aircraft fleet and the methodological aspect of airborne measurements.

The results from a simulation chamber study on the formaldehyde (HCHO) absorption cross section in the UV spectral region are presented. We performed 4 experiments at ambient HCHO concentrations with simultaneous measurements of two DOAS instruments in the atmosphere simulation chamber SAPHIR in Jülich. The two instruments differ in their spectral resolution, one working at 0.2 nm (broad-band, BB-DOAS), the other at 2.7 pm (high-resolution, HR-DOAS). Both instruments use dedicated multi reflection cells to achieve long light path lengths of 960 m and 2240 m, respectively, inside the chamber. During two experiments HCHO was injected into the clean chamber by thermolysis of well defined amounts of para-formaldehyde reaching mixing rations of 30 ppbV at maximum. The HCHO concentration calculated from the injection and the chamber volume agrees with the BB-DOAS measured value when the absorption cross section of Meller and Moortgat (2000) and the temperature coefficient of Cantrell (1990) were used for data evaluation. In two further experiments we produced HCHO in-situ from the ozone + ethene reaction which was intended to provide an independent way of HCHO calibration through the measurements of ozone and ethene. However, we found an unexpected deviation from the current understanding of the ozone + ethene reaction when CO was added to suppress possible oxidation of ethene by OH radicals. The reaction of the Criegee intermediate with CO could be 240 times slower than currently assumed. Based on the BB-DOAS measurements we could deduce a high-resolution cross section for HCHO which was not measured directly so far.

Ye et al . have determined a maximum nitrous acid (HONO) yield of 3% for the reaction HO 2 ·H 2 O + NO 2 , which is much lower than the yield used in our work. This finding, however, does not affect our main result that HONO in the investigated Po Valley region is mainly from a gas-phase source that consumes nitrogen oxides.

[1] Absolute reaction rate studies of NO3 radicals with 4 aldehydes were performed in the atmosphere simulation chamber SAPHIR at the Research Center Julich. Rate coefficients (ethanal: 2.6 ± 0.5, propanal: 5.8 ± 1.0, butanal: 11.9 ± 1.4, benzaldehyde: 2.2 ± 0.6; in 10−15 cm3 s−1 at 300 K) were determined from measured concentration–time profiles of aldehydes and NO3 at near ambient conditions. The values for the aliphatic aldehydes are in good agreement with the most recent recommendations (IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry: Evaluated kinetic and photochemical data for atmospheric chemistry, 2005, available at http://www.iupac-kinetic.ch.cam.ac.uk). The measured concentration-time profiles of precursor aldehydes, NO3, NO2, and of product aldehydes were compared to model calculations based on the MCM v3 (Jenkin et al., 2003; Saunders et al., 2003). Differences between measurements and model are attributed to a major interference of the GC system to peroxyacyl nitrates. In addition modifications to the rate constants in the MCM are suggested.

[1] Nitrogen oxides play a key role in tropospheric chemistry; to study the distribution patterns of the corresponding anthropogenic emissions ( fossil, industrial, waste), we use three independent data sources: GOME measurements of the tropospheric NO2 column density fields, the EDGAR 3 emission inventory as an estimation of the anthropogenic NOx emissions and nighttime images of worldwide human settlements seen by the DMSP OLS satellite instrument as a proxy for these emission patterns. The uncertainties are not known precisely for any of the fields. Using the MOZART-2 CTM, tropospheric column density fields are calculated from the emission estimates, and transformations are developed to turn the GOME columns into anthropogenic emission fields. Assuming the errors of the three data sources ( GOME, EDGAR, lights) to be independent, we are able to determine ranges for the pattern errors of the column density fields and values for the pattern errors of the source fields by a correlation analysis that connects relative error (co) variances and correlation coefficients. That method was developed for this investigation but can generally be used to calculate relative error variances of data sets, if the errors of at least three of them can be assumed to be independent. We estimate the pattern error of the EDGAR 3 anthropogenic NOx emission field as ( 27 +/- 5)%, which can be reduced by combining all fields to ( 15 +/- 3)%. By determining outliers, we identify locations with high uncertainty that need further examination.

International audience; HONO formation has been proposed as an important OH radical source in simulation chambers for more than two decades. Besides the heterogeneous HONO formation by the dark reaction of NO2 and adsorbed water, a photolytic source has been proposed to explain the elevated reactivity in simulation chamber experiments. However, the mechanism of the photolytic process is not well understood so far. As expected, production of HONO and NOx was also observed inside the new atmosphere simulation chamber SAPHIR under solar irradiation. This photolytic HONO and NOx formation was studied with a sensitive HONO instrument under reproducible controlled conditions at atmospheric concentrations of other trace gases. It is shown that the photolytic HONO source in the SAPHIR chamber is not caused by NO2 reactions and that it is the only direct NOy source under illuminated conditions. In addition, the photolysis of nitrate which was recently postulated for the observed photolytic HONO formation on snow, ground, and glass surfaces, can be excluded in the chamber. A photolytic HONO source at the surface of the chamber is proposed which is strongly dependent on humidity, on light intensity, and on temperature. An empirical function of the form S(HONO)=a1,2×J(NO2)×(1+(RH/RH0)2)×exp(?T0/T) describes these dependencies and reproduces the observed HONO formation rates to within 10%. It is shown that the photolysis of HONO represents the dominant radical source in the SAPHIR chamber for typical tropospheric O3/H2O concentrations. For these conditions, the HONO concentrations inside SAPHIR are similar to recent observations in ambient air.

Daytime concentrations of HONO, NOx, OH and photolysis frequencies were measured during the ECHO field campaign in a mixed deciduous forest near Julich, West-Germany, in summer 2003. Midday measurements show clear evidence for a large, yet unexplained daytime source of HONO (similar to 500 pptv/h), which represents an important net source of OH radicals due to ongoing HONO photolysis. The evidence for a large HONO daytime source is for the first time completely constrained by measured parameters, needed to determine the daytime budget of HONO. The large contribution of 33% to the primary OH production during noon at the top of the forest canopy suggests that the unexplained source of HONO could have an important impact on the photochemical transformation of biogenically emitted volatile organic compounds (VOCs) by OH into partly oxidized VOCs and secondary organic aerosols during their release from forest regions into the troposphere.

Kinetic studies conducted in the new atmosphere simulation chamber SAPHIR at the Research Center Julich allow a thorough investigation of oxidation of isoprene induced by O 3 and the OH radical under atmospheric conditions. Rate coefficients for the 03-isoprene and OH-isoprene reaction are determined from measured concentration-time profiles. For the reaction of O 3 with isoprene the rate coefficient is determined to be (9.6 ± 0.7) x 10 -18 cm 3 molecule -1 s -1 at 286 K. The rate coefficient for the reaction OH + isoprene is (10.0 ± 1.2) x 10 -11 cm 3 molecule -1 s -1 at 294 K. The kinetic parameters determined in SAPHIR at atmospheric concentrations agree well with recent recommendations.

[1] Surface-active organic compounds are common constituents of atmospheric aerosol particles. In this work, the effects of organic coatings on the heterogeneous hydrolysis of N2O5 was investigated. For this purpose aqueous NH4HSO4 particles were coated in situ in a large aerosol chamber. The experiments show that films composed of the ozonolysis products of α-pinene can reduce the reaction probability by more than an order of magnitude. Model calculations indicate that the main cause for the retardation of the hydrolysis is a slower uptake possibly due to a lower solubility of N2O5 in the organic film.

The purpose of this report prepared on behalf of the European Commission is to review the current understanding of chemical and dynamical processes in the upper troposphere and lower stratosphere, and to assess how these processes could be perturbed as a result of current and future aircraft emissions. Specifically, perturbations in the atmospheric abundance of ozone and in climate forcing, as predicted by atmospheric models, will be presented. The goal is to compile and evaluate scientific information related to the atmospheric impact of subsonic and supersonic aircraft emissions and to review the state of knowledge concerning the various aspects of this problem. In Section 2, the issues and scientific questions relevant to the problem of aircraft perturbations will be presented. The key physical and chemical processes occurring in the troposphere and stratosphere will be discussed in Section 3. Estimates of air traffic and aircraft emissions will be given in Section 4. Section 5 and 6 will review the understanding of the atmospheric impact of aircraft emissions at small and large scale, respectively. The effect of aircraft emissions on climate forcing will be discussed in Section 7. Finally, conclusions will be provided in Section 8.

The 2.4-dinitrophenylhydrazine coated silica cartridge technique (DSC) was used for the measurements of HCHO and CH3CHO during the POPCORN campaign in August 1994. A total number of 505 measurements was carried out using an automatic sampling system. The sampling time for each measurement was 30 minutes. During the first two weeks of the campaign samples were taken every 3 hours and during the last two weeks every 30 minutes. No significant diurnal variation of HCHO and CH3CHO was observed. The average mixing ratios of HCHO and CH3CHO were 1.8 ± 1.0 ppb and 1.4 ± 1.3 ppb. The results for HCHO are in a good agreement with simultaneous measurements by differential optical absorption spectroscopy (DOAS). The absence of a strong diurnal variation of the HCHO mixing ratio can be explained by production and destruction processes during day and night. The measured mixing ratios of HCHO and CH3CHO, especially the mixing ratios during night, are a strong indication that during the POPCORN campaign the maize was a local source of HCHO and CH3CHO.

Publisher Summary Heterogeneous reactions of trace gases on aerosol surfaces play an important role in the chemistry of the troposphere. Heterogeneous processes on aerosol surfaces provide an important pathway for the removal of nitrogen oxides from the troposphere. Thus, the conversion of N 2 0 5 to HNO 3 on wet aerosol is the largest single removal mechanism for NO x during night and winter time. The heterogeneous conversion of N 2 0 5 to HNO 3 on wet NaNO 3 aerosol is investigated by oxidizing NO 2 by O 3 with and without the presence of aerosol and observing the decay and building of NO 2 , O 3 , N 2 O 5, and HNO 3 . The data are interpreted using model calculations. The chemical box model includes night-time nitrogen oxide chemistry and the heterogeneous conversion of N 2 O 5 to HNO 3 approximated as a gas kinetic collision of N 2 O 5 with NaNO 3 aerosol surface. From comparison of the experimental results with the model, an uptake coefficient of N 2 O 5 on NaNO 3 aerosol is determined.

An understanding of the distribution, variation, and photochemical interactions of reactive species in the global troposphere is fundamental to questions about the budgets and trends of tropospheric ozone and other trace gases. These are among the major objectives of the IGAC-GLOCHEM Activity. Recently, several field campaigns have measured reactive trace gases and radical species in the remote troposphere to test models of transport and chemical processes. The experimental programs described here are STRATOZ III, TROPOZ II, and MLOPEX II. Examples from each of these programs will highlight some of the results.

A liquid jet technique is used to measure gas phase loss and liquid phase uptake of nitrogen dioxide (NO2) and nitrous acid (HNO2) caused by heterogeneous reactions on liquid surfaces. The experimental results are compared with two-dimensional model calculations. In the case of NO2 the predictions of the model are in agreement with the experimental observations for both gas and liquid phase (i.e., conservation of mass) assuming an effective Henry constant at least eight times larger as the physical Henry constant. In addition the comparison yields a mass accommodation coefficient (alpha) >= 2 * 10-4 for NO2 and (alpha) > 1 * 10-3 for HNO2.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In June 1984 several French and German research groups joined in the STRATOZ III aircraft campaign to measure the global distribution of a number of trace gases. The first part of that campaign led across the Northern Atlantic with stops at Prestwick, Scotland; Keflavik, Iceland; Sondrestrom, Greenland, and Goose Bay, Canada, all falling in the zonal belt between roughly 55° N and 65° N latitude (see Figure 1). During the ascents from and descents to these airports vertical profiles were measured for NO by Drummond et al., [1988] and for O3, CO, CH4 by Marenco et al. [1989a,b]. Figure 2 presents the data coverage projected onto the altitude by longitude plane at 60° N latitude. Also measured was PAN but with a much lower sampling density [Rudolph et al., 1987]. The measurements took place between the early morning on June 4, 1984, beginning with the descent to Prestwick, and the late morning of June 6, 1984, ending with the ascent from Goose Bay, and resulted in altogether 8 vertical profiles. As indicated in Figure 2 the aircraft, while gradually climbing or descending, also covered large horizontal distances. Thus, the term “vertical profile” is somewhat of a figure of speech. The flight pattern, however, is useful in more evenly filling the altitude by longitude plane with data points.