F. Rohrer

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (121)397.42 Total impact

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    ABSTRACT: Ozone concentrations in the Po Valley of northern Italy often exceed international regulations. As both a source of radicals and an intermediate in the oxidation of most volatile organic compounds (VOCs), formaldehyde (HCHO) is a useful tracer for the oxidative processing of hydrocarbons that leads to ozone production. We investigate the sources of HCHO in the Po Valley using vertical profile measurements acquired from the airship Zeppelin NT over an agricultural region during the PEGASOS 2012 campaign. Using a 1-D model, the total VOC oxidation rate is examined and discussed in the context of formaldehyde and ozone production in the early morning. While model and measurement discrepancies in OH reactivity are small (on average 3.4 +/- 13 %), HCHO concentrations are underestimated by as much as 1.5 ppb (45 %) in the convective mixed layer. A similar underestimate in HCHO was seen in the 2002-2003 FORMAT Po Valley measurements, though the additional source of HCHO was not identified. Oxidation of unmeasured VOC precursors cannot explain the missing HCHO source, as measured OH reactivity is explained by measured VOCs and their calculated oxidation products. We conclude that local direct emissions from agricultural land are the most likely source of missing HCHO. Model calculations demonstrate that radicals from degradation of this non-photochemical HCHO source increase model ozone production rates by as much as 0.6 ppb h(-1) (12 %) before noon.
    Atmospheric Chemistry and Physics 02/2015; 15(3):1289-1298. DOI:10.5194/acp-15-1289-2015 · 5.51 Impact Factor
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    ABSTRACT: During recent field campaigns, measured hydroxyl radical (OH) concentrations were up to a factor of ten larger than predicted by current chemical models for conditions of high OH reactivity and low nitrogen monoxide (NO) concentrations. These discrepancies were most often observed in forests, where concentrations of biogenic volatile organic compounds (BVOCs) were large. We investigated the radical budget during oxidation of VOCs with OH including a full set of accurate and precise radical measurements in the atmosphere simulation chamber SAPHIR in Juelich, Germany. The conditions during the chamber experiments were comparable to those during field campaigns with respect to radical and trace gas concentrations. In particular, OH reactivity was high and NO mixing ratios were as low as 200pptv. VOC species included the most important single compound isoprene, and its major oxidation products methacrolein and methyl vinyl ketone. Significant gaps between measured OH destruction and production pathways were found for isoprene and methacrolein. The additional OH needed to close the OH budget is consistent with recently proposed reaction mechanisms suggesting OH production from isomerization and decomposition reactions of organic peroxy radicals produced in the reaction of OH with VOCs.
    13th IGAC Science Conference, Natal; 09/2014
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    ABSTRACT: The hydroxyl radical (OH) is the main oxidation agent in the atmosphere during daytime. Recent field campaigns studying the radical chemistry in forests showed large differences between measured and modeled OH concentrations at low NOx concentration and when the OH reactivity was dominated by large concentrations of volatile organic compounds (VOC). These findings were only partially explained by the introduction of new efficient hydroxyl radical regeneration pathways in the isoprene oxidation mechanism. The question arises if other reactive VOCs with high global emission rates are also capable of additional OH recycling processes. In addition to isoprene, monoterpenes and 2-methyl-3-buten-2-ol (MBO) are the VOCs with the highest global emission rates. Due to their high reactivity towards OH they can dominate the radical chemistry in forested areas under certain conditions. The photochemical degradation of α-pinene, β-pinene, limonene, myrcene and MBO was investigated in the Jülich atmosphere simulation chamber SAPHIR in a dedicated series of experiments in 2012 and 2013. The chamber was equipped with instrumentation to measure radical concentrations (OH, HO2, RO2), the total OH reactivity, concentrations of all important OH precursors (O3, HONO, HCHO), of the parent VOC, its main oxidation products and photolysis frequencies to investigate the radical budget. All experiments were carried out under low NOx conditions (< 2ppb) and atmospheric terpenoid concentrations (< 5ppb) with and without addition of ozone into the SAPHIR chamber. For the investigation of the OH budget all measured OH production terms were compared to the measured OH destruction. Within the limits of accuracy of the instruments the OH budget was balanced in all cases. Consequently unaccounted OH recycling or primary OH production processes did not play a role for conditions of these experiments. In contrast to the analysis of the measured OH budget, numerical simulations of the conducted experiments using the Master Chemical Mechanism v3.2 showed an underestimation of the OH production in the α-pinene, β-pinene and limonene experiments. At the same time the measured OH destruction was overestimated by the numerical simulation. First sensitivity studies showed that these discrepancies are most likely related to a missing source of HO2 in the model.
    13th IGAC Science Conference, Natal; 09/2014
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    ABSTRACT: The removal of trace gases from the troposphere is, in most cases, initialized by reactions with hydroxyl radicals, and the products of these reactions are eventually deposited on the Earth's surface. The concentration of these hydroxyl radicals is therefore a measure of atmospheric self-cleansing. In theory, hydroxyl-radical concentrations can be enhanced by the recycling of some of the reaction products. The only known efficient recycling process involves nitrogen oxide and leads to production of ozone, yet observations in regions with high hydrocarbon and low nitrogen oxide concentrations show substantially elevated hydroxyl-radical concentrations, up to ten times higher than expected. If we normalize observed hydroxyl-radical concentrations to the maximum achievable in model calculations with variable nitrogen oxide concentrations, this photochemical coordinate system uncovers a common feature in almost all of these observations: even in the presence of inadequate amounts of nitrogen oxides, hydroxyl-radical concentrations are enhanced to the theoretical maximum obtainable at very much higher nitrogen oxide concentrations. This means that this important part of the self-cleansing capability of the atmosphere is working at maximum efficiency even in regions with a high burden of biogenic hydrocarbons and low nitrogen oxide concentration. Since these processes do not involve nitrogen oxides, tropospheric ozone production is greatly reduced compared with the expectation from current theory.
    Nature Geoscience 07/2014; 7(8):559-563. DOI:10.1038/ngeo2199 · 11.67 Impact Factor
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    ABSTRACT: UV radiation drives the processes of O 1 D formation from O 3 and NO 2 photolysis which are very important for atmospheric chemistry. Measurements of photolysis frequencies by filter radiometry and by actinic flux density measurements are rare worldwide, due to limited instrumentation. More recently, techniques have been developed for the retrieval of the photolysis frequencies from surface irradiance data. The method proposed includes the determination of J(O 1 D) and J(NO 2) as a function of solar zenith angle, by the use of global irradiance and empirical relationships, instead of the direct way using actinic flux densities. Here we present a method using UV irradiance products from satellite data, in order to calculate the photolysis frequencies in global scale. A second-order polynomial was used to convert solar global irradiance at certain wavelengths (305, 380nm) to photolysis frequencies. Results were validated using ground measurements from two sites (Greece, Ireland) where times series of measured photolysis frequencies of J(NO2) and J(O1D) were available. The validation process shows relatively good agreement when meteorological parameters agreed in both data sets and larger deviations in case of altered estimation of clouds.
    12th International Conference on Meteorology, Climatology and Atmospheric Physics COMECAP 2014, Herakleion, Greece; 05/2014
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    ABSTRACT: Formation and evolution of secondary organic aerosols (SOA) from biogenic VOCs influences the Earth's radiative balance. We have examined the photo-oxidation and aging of boreal terpene mixtures in the SAPHIR simulation chamber. Changes in thermal properties and chemical composition, deduced from mass spectrometric measurements, were providing information on the aging of biogenic SOA produced under ambient solar conditions. Effects of precursor mixture, concentration and photochemical oxidation levels (OH exposure) were evaluated. OH exposure was found to be the major driver in the long term photochemical transformations, i.e. reaction times of several hours up to days, of SOA and its thermal properties, whereas the initial concentrations and terpenoid mixtures had only minor influence. The volatility distributions were parameterized using a sigmoidal function to determine TVFR0.5 (the temperature yielding a 50% particle volume fraction remaining) and the steepness of the volatility distribution. TVFR0.5 increased by 0.3±0.1% (ca. 1 K), while the steepness increased by 0.9±0.3% per hour of 1 × 10(6) cm(-3) OH exposure. Thus, aging reduces volatility and increases homogeneity of the vapor pressure distribution, presumably because highly volatile fractions become increasingly susceptible to gas phase oxidation, while less volatile fractions are less reactive with gas phase OH.
    Environmental Science & Technology 05/2014; 48(11). DOI:10.1021/es405412p · 5.48 Impact Factor
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    ABSTRACT: The hydroxyl radical (OH) is the main oxidation agent in the atmosphere during daytime. Recent field campaigns studying the radical chemistry in forested areas showed large discrepancies between measured and modeled OH concentration at low NOx conditions and when OH reactivity was dominated by VOC. These observations were only partially explained by the evidence for new efficient hydroxyl radical regeneration pathways in the isoprene oxidation mechanism. The question arises if also other reactive VOCs with high global emission rates are also capable of additional OH recycling. Beside isoprene, monoterpenes and 2-methyl-3-buten-2-ol (MBO) are the volatile organic compounds (VOC) with the highest global emission rates. Due to their high reactivity towards OH monoterpenes and MBO can dominate the radical chemistry of the atmosphere in forested areas under certain conditions. In the present study the photochemical degradation mechanism of α-pinene, β-pinene, limonene, myrcene and MBO was investigated in the Jülich atmosphere simulation chamber SAPHIR. The focus of this study was in particular on the investigation of the OH budget in the degradation process. The photochemical degradation of these terpenoids was studied in a dedicated series of experiments in the years 2012 and 2013. The SAPHIR chamber was equipped with instrumentation to measure radicals (OH, HO2, RO2), the total OH reactivity, all important OH precursors (O3, HONO, HCHO), the parent VOC and its main oxidation products and photolysis frequencies to investigate the radical budget in the SAPHIR chamber. All experiments were carried out under low NOx conditions (≤ 2ppb) and atmospheric terpene concentrations (≤ 5ppb) with and without addition of ozone into the SAPHIR chamber. For the investigation of the OH budget all measured OH production terms were compared to the measured OH destruction. Within the limits of accuracy of the instruments the OH budget was balanced in all cases. Conse- quently unaccounted OH recycling or primary OH production processes did not play a role for conditions of these experiments. Despite the OH budget was closed in these experiments simulation results from the Master Chemical Mechanism v3.2 showed that the OH production was underestimated by the model in the α-pinene, β-pinene and limonene experiments. The measured total OH reactivity was overestimated by the numerical simulation.
    European Geosciences Union General Assembly, Vienna; 04/2014
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    ABSTRACT: Gaseous nitrous acid (HONO) is an important precursor of tropospheric hydroxyl radicals (OH). OH is responsible for atmospheric self-cleansing and controls the concentrations of greenhouse gases like methane and ozone. Due to lack of measurements, vertical distributions of HONO and its sources in the troposphere remain unclear. Here, we present a set of observations of HONO and its budget made onboard a Zeppelin airship. In a sunlit layer separated from Earth's surface processes by temperature inversion, we found high HONO concentrations providing evidence for a strong gas-phase source of HONO consuming nitrogen oxides and potentially hydrogen oxide radicals. The observed properties of this production process suggest that the generally assumed impact of HONO on the abundance of OH in the troposphere is substantially overestimated.
    Science 04/2014; 344(6181):292-6. DOI:10.1126/science.1248999 · 31.48 Impact Factor
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    ABSTRACT: Current photochemical models developed to simulate the atmospheric degradation of aromatic hydrocarbons tend to underestimate OH radical concentrations. In order to analyse OH budgets, we performed experiments with benzene, toluene, p-xylene, and 1,3,5-trimethylbenzene in the atmosphere simulation chamber SAPHIR. Experiments were conducted under low-NO conditions (typically 0.1–0.2 ppb) and high-NO conditions (typically 7–8 ppb), and starting concentrations of 6–250 ppb of aromatics, dependent on OH rate constants. For the OH budget analysis a steady-state approach was applied where OH production and destruction rates (POH and DOH) have to be equal. The POH were determined from measurements of HO2, NO, HONO, and O3 concentrations, considering OH formation by photolysis and recycling from HO2. The DOH were calculated from measurements of the OH concentrations and total OH reactivities. The OH budgets were determined from DOH / POH ratios. The accuracy and reproducibility of the approach were assessed in several experiments using CO as a reference compound where an average ratio DOH / POH = 1.13 ± 0.19 was obtained. In experiments with aromatics, these ratios ranged within 1.1–1.6 under low-NO conditions and 0.9–1.2 under high-NO conditions. The results indicate that OH budgets during photo-oxidation experiments with aromatics are balanced within experimental accuracies. Inclusion of a further, recently proposed OH production via HO2 + RO2 reactions led to improvements under low-NO conditions but the differences were small and insignificant within the experimental errors.
    Atmospheric Chemistry and Physics 03/2014; 2014(14):5535-5560. DOI:10.5194/acpd-14-5535-2014 · 5.51 Impact Factor
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    ABSTRACT: Hydroxyl radicals (OH) are the most important reagent for the oxidation of trace gases in the atmosphere. OH concentrations measured during recent field campaigns in isoprene rich environments were unexpectedly large. A number of studies showed that unimolecular reactions of organic peroxy radicals (RO2) formed in the initial reaction step of isoprene with OH play an important role for the OH budget in the atmosphere at low mixing ratios of nitrogen monoxide (NO) of less than 100 pptv. It has also been suggested that similar reactions potentially play an important role for RO2 from other compounds. Here, we investigate the oxidation of methacrolein (MACR), one major oxidation product of isoprene, by OH in experiments in the simulation chamber SAPHIR under controlled atmospheric conditions. The experiments show that measured OH concentrations are approximately 50% larger than calculated by current chemical models for conditions of the experiments (NO mixing ratio of 90 pptv). The analysis of the OH budget reveals a so far unaccounted OH source, which is correlated with the production rate of RO2 radicals from MACR. In order to balance the measured OH destruction rate, (0.77±0.3) OH radicals need to be additionally reformed from each OH that has reacted with MACR. The strong correlation of the missing OH source with the production of RO2 radicals is consistent with the concept of OH formation from unimolecular isomerization and decomposition reactions of RO2. The comparison of observations with model calculations gives a lower limit of 0.03 s−1 for the reaction rate constant, if the OH source is attributed to an isomerization reaction of one RO2 species formed in the MACR+OH reaction as suggested in literature. This fast isomerization reaction would be competitive to the reaction of this RO2 species with minimum 150 pptv NO.
    Atmospheric Chemistry and Physics 02/2014; 2014(14):5197-5231. DOI:10.5194/acpd-14-5197-2014 · 5.51 Impact Factor
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    ABSTRACT: The Zeppelin NT airship is an airborne platform capable of flying at low speed throughout the entire planetary boundary layer (PBL) and an ideal platform for the study of chemical processes in the lowest atmospheric layers with high spatial resolution. Secondary atmospheric aerosols have a long lifetime, up to several days, so their observation at a location can result not only from local production but also from transport processes. The aim of this project is to characterize the origin (local production or transported) of the observed aerosols as well as their precursors. We analyze: ● one North-South transect flight (between the Apennin and San Pietro Capofiume), and ● one vertical profiling flight (near the super-site San Pietro Capofiume) to determine the regions which contributed to the air-mass under observation. The analysis is done using the EURopean Air pollution Dispersion and Inverse Modeling (EURAD-IM) system, driven by meteorology given by the Weather Research and Forecasting model (WRF). The method applied follows: 1. A forward run of the chemistry transport model is performed, until a time after the observations. 2. Then, a backward (adjoint) run is performed, in which, the evolution of artificial perturbations (discrepancy between model and observation at the time/location of the observations) are tracked backwards in time. Methodology Introduction Back-plume schematic Comparison: model-observations Observations During the height profiling flights of the south-bound PEGASOS campaign, increased mass concentration of nitrate and ammonium was observed at narrow height levels. Back-plume examples for 20-21.06.2012 The model performance is being checked with respect to vertical profiling performed in 20.06.2012: ● In mixed atmospheric conditions, flight F028 (8:44-12:29UTC), no variance is observed with height, NOX and O3 appear slightly overestimated and underestimated respectively. ● In not mixed atmospheric conditions, flight F027 (4:30-8:25UTC), NOX is underestimated (possibly weak ground emissions taken from emission inventories), while the modeled O3 meets the correct mean value but shows smaller amplitude with respect to height variations. Summary and Acknowledgment 12-hour back-plumes were performed for the 20 and 21.06.2012. The adjoint concentration represents the area that influences the zeppelin observations. The F027 and F029 flight tracks have been used. ● An effort is done to identify the sources of observed trace species. ● A modified version of the chemistry transport model EURAD-IM is developed accounting for back-plumes (identical to adjoint model calculations). ● Using the method of back-plumes one can follow the adjoint concentration along the wind path and take into account gas and aerosol chemistry ● In order to succeed, first the model-observation discrepancies, during the planetary boundary layer evolution, should be explained. We acknowledge the financial support by the EU-FP7 project PEGASOS (project no.
    EGU; 01/2014
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    ABSTRACT: Oxidation by hydroxyl radical (OH) and ozonolysis are the two major pathways of daytime biogenic volatile organic compound (BVOC) oxidation and secondary organic aerosol (SOA) formation. In this study, we investigated the particle formation of several common monoterpenes (α-pinene, β-pinene and limonene) by OH-dominated oxidation, which has seldom been investigated. OH oxidation experiments were carried out in the SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction) chamber in Jülich, Germany, at low NOx (0.01 ~ 1 ppbV) and low ozone (O3) concentration (< 20 ppbV). OH concentration and total OH reactivity (kOH) were measured directly, and through this the overall reaction rate of total organics with OH in each reaction system was quantified. Multi-generation reaction process, particle growth, new particle formation (NPF), particle yield and chemical composition were analyzed and compared with that of monoterpene ozonolysis. Multi-generation products were found to be important in OH-dominated SOA formation. The relative role of functionalization and fragmentation in the reaction process of OH oxidation was analyzed by examining the particle mass and the particle size as a function of OH dose. We developed a novel method which quantitatively links particle growth to the reaction rate of OH with total organics in a reaction system. This method was also used to analyze the evolution of functionalization and fragmentation of organics in the particle formation by OH oxidation. It shows that functionalization of organics was dominant in the beginning of the reaction (within two lifetimes of the monoterpene) and fragmentation started to play an important role after that. We compared particle formation from OH oxidation with that from pure ozonolysis. In individual experiments, growth rates of the particle size did not necessarily correlate with the reaction rate of monoterpene with OH and O3. Comparing the size growth rates at the similar reaction rates of monoterpene with OH or O3 indicates that, generally, OH oxidation and ozonolysis had similar efficiency in particle growth. The SOA yield of α-pinene and limonene by ozonolysis was higher than that of OH oxidation. Aerosol mass spectrometry (AMS) shows SOA elemental composition from OH oxidation follows a slope shallower than −1 in the O / C vs. H / C diagram, also known as Van Krevelen diagram, indicating that oxidation proceeds without significant loss of hydrogen. SOA from OH oxidation had higher H / C ratios than SOA from ozonolysis. In ozonolysis, a process with significant hydrogen loss seemed to play an important role in SOA formation.
    Atmospheric Chemistry and Physics 01/2014; 14(9):12591-12634. DOI:10.5194/acpd-14-12591-2014 · 4.88 Impact Factor
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    ABSTRACT: Two gas-phase formaldehyde (HCHO) measurement techniques, a modified commercial wet-chemical instrument based on Hantzsch Fluorimetry and a custom-built instrument based on Fiber-Laser Induced Fluorescence (FILIF), were deployed at the atmospheric simulation chamber SAPHIR to compare the instruments' performances under a range of conditions. Thermolysis of para-HCHO and ozonolysis of 1-butene were used as HCHO sources, allowing for calculations of theoretical HCHO mixing ratios. Calculated HCHO mixing ratios are compared to measurements, and the two measurements are also compared. Experiments were repeated under dry and humid conditions (RH < 2% and RH > 60%) to investigate the possibility of a water artifact in the FILIF measurements. The ozonolysis of 1-butene also allowed for the investigation of an ozone artifact seen in some Hantzsch measurements in previous intercomparisons. Results show that under all conditions the two techniques are well correlated (R2 ≥ 0.997), and linear regression statistics show measurements agree with within stated uncertainty (15% FILIF + 5% Hantzsch). No water or ozone artifacts are identified.
    Atmospheric Measurement Techniques 12/2013; 7(1). DOI:10.5194/amtd-7-233-2014 · 3.21 Impact Factor
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    ABSTRACT: Diurnal and seasonal variations of gaseous sulfuric acid (H2SO4) and methane sulfonic acid (MSA) were measured in N.E. Atlantic air at the Mace Head atmospheric research station during the years 2010 and 2011. The measurements utilized selected ion/chemical ionization mass spectrometry (SI/CIMS) with a detection limit for both compounds of 4.3 × 10 4 cm-3 at 5 min signal integration. The H2SO4 and MSA gas-phase concentrations were analysed in conjunction with the condensational sink for both compounds derived from 3 nm-10 μm (diameter) aerosol size distributions. Accommodation coefficients of 1.0 for H2SO4 and 0.12 for MSA were assumed leading to estimated atmospheric lifetimes of the order of 7 min and 25 min, respectively. With the SI/CIMS instrument in OH measurement mode alternating between OH signal and background (non-OH) signal evidence was obtained for the presence of one or more unknown oxidants of SO2 in addition to OH. Depending on the nature of the oxidant(s) their ambient concentration may be enhanced in the CIMS inlet system by additional production. The apparent unknown SO2 oxidant was additionally confirmed by direct measurements of SO2 in conjunction with calculated H2SO4 concentrations. The calculated concentrations were consistently lower than the measured concentrations by a factor 4.8 ± 3.4 when considering the oxidation of SO2 by OH as the only source of H2SO4. Both the OH and the background signal were also observed to increase significantly during daytime aerosol nucleation events, independent of the ozone photolysis frequency, J(O1D), and were followed by peaks in both H2SO4 and MSA concentrations. This suggests a strong relation between the unknown oxidant(s), OH chemistry, and the atmospheric photo-oxidation of biogenic iodine compounds. As to the identity of the oxidant(s), we have been able to exclude ClO, BrO, IO, and OIO as possible candidates based on ab initio calculations. Stabilized Criegee intermediates (sCI) produced from ozonolysis of alkenes potentially contribute to the oxidation efficiency of the coastal and marine atmosphere. However, analysis of the CIMS background signal in context with recently published kinetic data currently suggests that larger Criegee intermediates produced from ozonolysis play no significant role for SO2 oxidation in the marine atmosphere. The possibility of H2SO4 formation without SO2 as precursor or from SO2 oxidation by small sCI produced photolytically should be explored.
    Atmospheric Chemistry and Physics 12/2013; 14(1). DOI:10.5194/acpd-14-1159-2014 · 4.88 Impact Factor
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    ABSTRACT: Nighttime HOx chemistry was investigated in two ground-based field campaigns (PRIDE-PRD2006 and CAREBEIJING2006) in summer 2006 in China by comparison of measured and modelled concentration data of OH and HO2. The measurement sites were located in a rural environment in the Pearl River Delta (PRD) under urban influence and in a suburban area close to Beijing, respectively. In both locations, significant nighttime concentrations of radicals were observed under conditions with high total OH reactivities of about 40-50 s-1 in PRD and 25 s-1 near Beijing. For OH, the nocturnal concentrations were within the range of (0.5-3) × 106 cm s-3 implying a signficant nighttime oxidation rate of pollutants in the order of several ppb per hour. The measured nighttime concentration of HO2 was about (0.2-5) × 108 cm -3 containing a significant, model-estimated contribution from RO2 as an interference. A chemical box model based on an established chemical mechanism is capable to reproduce the measured nighttime values of the measured peroxy radicals and kOH, but underestimates in both field campaigns the observed OH by about one order of magnitude. Sensitivity studies with the box model demonstrate that the OH discrepancy between measured and modelled nighttime OH can be resolved, if an additional \chem{RO_x} production process (about 1 ppb h s-1) and additional recycling (RO2 → HO2 → OH) with an efficiency equivalent to 1 ppb NO is assumed. The additional recycling mechanism was also needed to reproduce the OH observations at the same locations during daytime for conditions with NO mixing ratios below 1 ppb. This could be an indication that the same missing process operates at day and night. In principle, the required primary c source can be explained by ozonolysis of terpenoids, which react faster with ozone than with OH in the nighttime atmosphere. However, the amount of these highly reactive biogenic VOC would require a strong local source, for which there is no direct evidence. A more likely explanation for an additional s-1 source is the vertical downward transport of radical reservoir species in the stable nocturnal boundary layer. Using a simplified 1-dimensional two-box model, it can be shown that ground-based NO emissions could generate a large vertical gradient causing a downward flux of PAN and MPAN. The downward transport and the following thermal decomposition of these compounds can produce up to 0.3 ppb h-1 radicals in the atmospheric layer near the ground. Although this rate is not sufficient to explain the complete OH discrepancy, it indicates the potentially important role of vertical transport in the lower nighttime atmosphere.
    Atmospheric Chemistry and Physics 12/2013; 13(12):31311-31361. DOI:10.5194/acpd-13-31311-2013 · 4.88 Impact Factor
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    ABSTRACT: Thirty years of balloon-borne measurements over Boulder (40°N, 105°W) are used to investigate the water vapor trend in the tropopause region. This analysis extends previously published trends, usually focusing on altitudes greater than 16 km, to lower altitudes. Two new concepts are applied: (1) Trends are presented in a thermal tropopause (TP) relative coordinate system from –2 km below to 10 km above the TP, and (2) sonde profiles are selected according to TP height. Tropical (TPz > 14 km), extratropical (TPz < 12 km), and transitional air mass types (12 km < TPz < 14 km) reveal three different water vapor reservoirs. The analysis based on these concepts reduces the dynamically induced water vapor variability at the TP and principally favors refined water vapor trend studies in the upper troposphere and lower stratosphere. Nonetheless, this study shows how uncertain trends are at altitudes –2 to +4 km around the TP. This uncertainty in turn has an influence on the uncertainty and interpretation of water vapor radiative effects at the TP, which are locally estimated for the 30 year period to be of uncertain sign. The much discussed decrease in water vapor at the beginning of 2001 is not detectable between –2 and 2 km around the TP. On lower stratospheric isentropes, the water vapor change at the beginning of 2001 is more intense for extratropical than for tropical air mass types. This suggests a possible link with changing dynamics above the jet stream such as changes in the shallow branch of the Brewer-Dobson circulation.
    10/2013; 118(19):11269-11284. DOI:10.1002/jgrd.50831
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    ABSTRACT: Most pollutants in the Earth's atmosphere are removed by oxidation with highly reactive hydroxyl radicals. Field measurements have revealed much higher concentrations of hydroxyl radicals than expected in regions with high loads of the biogenic volatile organic compound isoprene. Different isoprene degradation mechanisms have been proposed to explain the high levels of hydroxyl radicals observed. Whether one or more of these mechanisms actually operates in the natural environment, and the potential impact on climate and air quality, has remained uncertain. Here, we present a complete set of measurements of hydroxyl and peroxy radicals collected during isoprene-oxidation experiments carried out in an atmospheric simulation chamber, under controlled atmospheric conditions. We detected significantly higher concentrations of hydroxyl radicals than expected based on model calculations, providing direct evidence for a strong hydroxyl radical enhancement due to the additional recycling of radicals in the presence of isoprene. Specifically, our findings are consistent with the unimolecular reactions of isoprene-derived peroxy radicals postulated by quantum chemical calculations. Our experiments suggest that more than half of the hydroxyl radicals consumed in isoprene-rich regions, such as forests, are recycled by these unimolecular reactions with isoprene. Although such recycling is not sufficient to explain the high concentrations of hydroxyl radicals observed in the field, we conclude that it contributes significantly to the oxidizing capacity of the atmosphere in isoprene-rich regions.
    Nature Geoscience 10/2013; 6(12):1023-1026. DOI:10.1038/ngeo1964 · 11.67 Impact Factor
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    ABSTRACT: The impact of nitrogen oxides (NOx = NO + NO2) on new particle formation (NPF) and on photochemical ozone production from real plant volatile organic compound (BVOC) emissions was studied in a laboratory set up. At high NOx conditions (BVOC/NOx < 7, NOx > 23 ppb) no new particles were formed. Instead photochemical ozone formation was observed resulting in higher hydroxyl radical (OH) and lower nitrogen monoxide (NO) concentrations. As soon as [NO] was reduced to below 1 ppb by OH reactions, NPF was observed. Adding high amounts of NOx caused NPF orders of magnitude slower than in analogous experiments at low NOx conditions (NOx ~ 300 ppt), although OH concentrations were higher. Varying NO2 photolysis enabled showing that NO was responsible for suppression of NPF suggesting that peroxy radicals are involved in NPF. The rates of NPF and photochemical ozone production were related by power law dependence with an exponent of approximately -2. This exponent indicated that the overall peroxy radical concentration must have been the same whenever NPF appeared. Thus permutation reactions of first generation peroxy radicals cannot be the rate limiting step in NPF from monoterpene oxidation. It was concluded that permutation reactions of higher generation peroxy radical like molecules limit the rate of new particle formation. In contrast to the strong effects on the particle numbers, the formation of particle mass was less sensitive to NOx concentrations, if at all. Only at very high NOx concentrations yields were reduced by about an order of magnitude.
    Atmospheric Chemistry and Physics 10/2013; 13(10):25827-25870. DOI:10.5194/acpd-13-25827-2013 · 4.88 Impact Factor
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    ABSTRACT: Ozone is produced in the lower troposphere by the OH-initiated photooxidation of volatile organic compounds in the presence of NOx. Aromatic hydrocarbons from anthropogenic sources are a major contributor to the OH-reactivity and thus to ozone formation in urban areas [1]. Moreover, their degradation leads to formation of secondary organic aerosol. Aromatic compounds are therefore important trace constituents with regard to air quality. We will present the results of photooxidation experiments which were conducted in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich. The experiments were designed to investigate the degradation mechanisms of benzene and p-xylene, which are among the most abundant aromatics in urban air samples. Benzene and p-xylene were selected because they have high structural symmetry which limits the number of potential isomers of secondary products. The experiments were performed under low-NOx-conditions (
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    ABSTRACT: An inhomogeneous mixing of reactants causes a reduction of their chemical removal compared to the homogeneously mixed case in turbulent atmospheric flows. This can be described by the intensity of segregation IS being the covariance of the mixing ratios of two species divided by the product of their means. Both terms appear in the balance equation of the mixing ratio and are discussed for the reaction between isoprene and OH for data of the field study ECHO 2003 above a deciduous forest. For most of these data, IS is negatively correlated with the fraction of mean OH mixing ratio reacting with isoprene. IS is also negatively correlated with the isoprene standard deviation. Both findings agree with model results discussed by Patton et al. (2001) and others. The correlation coefficient between OH and isoprene, and, therefore, IS increases with increasing mean reaction rate. In addition, the balance equation of the covariance between isoprene and OH is applied for the analysis of the same field data. The storage term is small, and, therefore, a diagnostic equation for this covariance can be derived. The chemical reaction term Rij is dominated by the variance of isoprene times the quotient of mixing ratios of OH and isoprene. In addition a diagnostic equation for IS is formulated. Comparing different terms of this equation, IS and Rij show a relation also to the normalized isoprene standard deviation. It is shown that not only chemistry, but also turbulent and convective mixing and advection - considered in a residual term - influence IS. Despite this finding, a detection of the influence of coherent eddy transport above the forest according to Katul et al. (1997) on IS fails, but a relation with the turbulent transport of isoprene variance is determined. In addition, largest values of IS are found for most unstable conditions with increasing buoyancy. These results are compared to model results by Ouwersloot et al. (2011).
    Atmospheric Chemistry and Physics 05/2013; 13(5):12913-12989. DOI:10.5194/acpd-13-12913-2013 · 4.88 Impact Factor