Atmospheric Environment (v.40, #S1)

Experiments on forest/atmosphere exchange: Climatology and fluxes during two summer campaigns in NE Bavaria by O. Klemm; A. Held; R. Forkel; R. Gasche; H.-J. Kanter; B. Rappenglück; R. Steinbrecher; K. Müller; A. Plewka; C. Cojocariu; J. Kreuzwieser; J. Valverde-Canossa; G. Schuster; G.K. Moortgat; M. Graus; A. Hansel (3-20).
During two summer field campaigns in 2001 and 2002, biosphere/atmosphere exchange fluxes of energy, gases, and particles were quantified in a Norway spruce forest in NE Bavaria at 775 m a.s.l. The overall goal of the BEWA campaigns was to study the influence of the emissions of reactive biogenic volatile organic compounds (BVOCs) on chemical and physical processes in the atmosphere, and an overview over the meteorological conditions, experimental frame, and the achieved results is provided. A rigorous quality assurance/quality control plan was implemented. From analysis of meteorological conditions and experimental success, golden day periods were selected for coordinated data analysis. These periods cover typical summertime conditions with various wind directions, NO x mixing ratios between 2 and 10 ppb, and O3 mixing ratios ranging between 13 and 98 ppb. Diurnal patterns of trace gas concentrations resulted from the dynamics of the boundary layer, from regional atmospheric processes (for example production of O3 in the atmosphere), and deposition. Turbulence also exhibited a diurnal pattern indicating thermal production during daytime and calm conditions during nighttime. However, in many cases, turbulence was often well developed during the nights. Horizontal advection of air masses into the trunk space occurred due to the patchiness of the forest. Nevertheless, for most conditions, the application of a one-dimensional model to describe the vertical exchange processes was appropriate. Therefore, the use of one single meteorological tower to study biosphere/atmosphere exchange is valid. Measured turbulent vertical exchange fluxes were estimated to be representative within an error of less than 25%. The results for VOC concentrations and fluxes were rather heterogeneous. Both model and measurements demonstrated that the Norway spruce trees acted as a weak source of formaldehyde.
Keywords: Atmospheric boundary layer; Biogenic volatile organic compounds; BVOC; Norway spruce; Quality assurance of flux data; Surface exchange fluxes; Ozone fluxes; Turbulent exchange;

Evaluation of meteorological parameters over a coniferous forest in a single-column chemistry-climate model by L. Ganzeveld; O. Klemm; B. Rappenglück; J. Valverde-Canossa (21-27).
The simulated micrometerology by a single-column chemistry-climate model (SCM) has been evaluated by comparison with BEWA2000 field campaign measurements over a coniferous forest, July–August 2001. This comparison indicates the limitations in the representation of the SCM's micrometeorological parameters that control atmosphere–biosphere peroxide exchanges. The evaluation of the micrometeorology also indicates that the validity of the so-called nudging technique to force climate models such as the SCM to the observed meteorology is sensitive to the applied timestep. For the typical timestep used in global scale climate models, numerical instabilities in the calculation of surface exchanges occur due to a misrepresentation of the surface–atmosphere temperature gradient. However, using a 1-min timestep shows good agreement between the simulated and observed meteorology of a wet and cold period during the campaign whereas the model underestimates daytime and nocturnal turbulent exchanges during a following warm and dry period.
Keywords: Chemistry-climate model; Atmosphere–biosphere exchanges; Micrometeorology; Turbulence; Nocturnal;

Trace gas exchange and gas phase chemistry in a Norway spruce forest: A study with a coupled 1-dimensional canopy atmospheric chemistry emission model by Renate Forkel; Otto Klemm; Martin Graus; Bernhard Rappenglück; William R. Stockwell; Wolfgang Grabmer; Andreas Held; Armin Hansel; Rainer Steinbrecher (28-42).
Numerical modelling is an efficient tool to investigate the role of chemical degradation of biogenic volatile organic compounds (BVOC) and the effect of dynamical processes on BVOC and product mixing ratios within and above forest canopies. The present study shows an application of the coupled canopy-chemistry model CACHE to a Norway spruce forest at the Waldstein (Fichtelgebirge, Germany). Simulated courses of temperature, trace gas mixing ratios, and fluxes are compared with measurements taken during the BEWA2000 field campaigns. The model permits the interpretation of the observed diurnal course of ozone and VOC by investigating the role of turbulent exchange, chemical formation and degradation, emission, and deposition during the course of the day. The simulation results show that BVOC fluxes into the atmosphere are 10–15% lower than the emission fluxes on branch basis due to chemical BVOC degradation within the canopy. BVOC degradation by the NO3 radical was found to occur in the lower part of the canopy also during daytime. Furthermore, the simulations strongly indicate that further research is still necessary concerning the emission and deposition of aldehydes and ketones.
Keywords: Biogenic VOC; Net emission; Chemical transformation; Canopy-chemistry model CACHE;

A relaxed-eddy-accumulation method for the measurement of isoprenoid canopy-fluxes using an online gas-chromatographic technique and PTR-MS simultaneously by Martin Graus; Armin Hansel; Armin Wisthaler; Christian Lindinger; Renate Forkel; Karin Hauff; Matthias Klauer; Andreas Pfichner; Bernhard Rappenglück; Dominik Steigner; Rainer Steinbrecher (43-54).
A relaxed-eddy-accumulation set-up using an online gas-chromatographic technique and proton-transfer-reaction mass spectrometry was applied to determine isoprenoid fluxes above a Norway spruce forest in July 2001/2002. The system was quality assured and its suitability for determination of canopy fluxes of isoprenoids was demonstrated. Flux measurements of oxygenated hydrocarbons failed the data quality check due to artefacts presumably arising from line and ozone-scrubber effects. Observations of turbulent fluxes of isoprenoids during the two field experiments show good agreements with primary flux data derived from enclosure measurements and modelling results using a canopy-chemistry emission model (CACHE). For monoterpenes maximum daytime fluxes of more than 2 nmol m - 2 s - 1 were observed, isoprene fluxes reached 1– 1.5 nmol m - 2 s - 1 .
Keywords: Biogenic volatile organic compounds (BVOC); Fluxes; Proton-transfer-reaction-mass spectrometry (PTR-MS); Relaxed-eddy-accumulation (REA); Norway spruce;

First measurements of H2O2 and organic peroxides surface fluxes by the relaxed eddy-accumulation technique by J. Valverde-Canossa; L. Ganzeveld; B. Rappenglück; R. Steinbrecher; O. Klemm; G. Schuster; G.K. Moortgat (55-67).
The relaxed eddy-accumulation (REA) technique was specially adapted to a high-performance liquid chromatographer (enzymatic method) and scrubbing coils to measure concentrations and fluxes of hydrogen peroxide (H2O2) and organic peroxides with a carbon chain ⩽C4, of which only methylhydroperoxide (MHP) and hydroxymethylhydroperoxide (HMHP) were detected. Flux measurements were carried out above the canopy of a Norway spruce forest in Germany (775 m a.s.l.) in NE Bavaria, Germany, during the BEWA2000 research cluster in summer 2001. This period was characterised by H2O2 maximum mixing ratios of 1 ppb and mixing ratios of organic peroxides below 200 ppt. Daily mean H2O2 deposition fluxes of −0.8±0.3 nmol m−2  s−1, MHP of −0.03±0.03 nmol m−2  s−1 and HMHP of −0.7±0.5 nmol m−2  s−1 were obtained. Profile measurements were performed as a qualitative comparison of the controlling mechanism of the surface exchanges. The REA as well as the profile measurements show that during daylight the surface exchanges of H2O2 and MHP over coniferous forest are mainly controlled by dry deposition. The high H2O2 dry-deposition velocity suggests a negligible surface uptake resistance for H2O2, whereas the significantly smaller MHP-deposition velocity indicates a significant surface uptake resistance. However, nighttime surface exchanges of these compounds should be further investigated since the in-canopy ozonolysis reaction is expected to affect exchanges. HMHP REA flux measurements show mainly deposition, whereas the ones based on the profile method suggest in-canopy chemical production.
Keywords: Deposition velocity; Organic peroxides; Hydrogen peroxide; Ozonolysis; Flux divergence;

Evaluation of peroxide exchanges over a coniferous forest in a single-column chemistry-climate model by L. Ganzeveld; J. Valverde-Canossa; G.K. Moortgat; R. Steinbrecher (68-80).
A single-column chemistry-climate model has been applied to evaluate peroxide exchanges measured over a coniferous forest during the BEWA2000 field campaign, July–August 2001. Simulations indicate that for suppressed nocturnal turbulent mixing, the H2O2 mixing ratios are sensitive to the representation of sources and sinks, e.g., non-stomatal uptake and chemical transformations, the latter tightly linked to atmosphere–biosphere NO x exchanges through its control on HO2 production. Comparison of observed and simulated H2O2 fluxes suggests that the commonly applied method to estimate uptake resistances results in a significant underestimation of the dry deposition flux. By using a very small surface uptake resistance, as observed, the modeled surface fluxes are still too low due to an underestimation of the simulated turbulent transport. Further, a reasonable agreement between simulated and observed methylhydroperoxide and hydroxymethylhydroperoxide mixing ratios in and above the canopy air is observed. Our analysis indicates the important role of daytime as well as nocturnal turbulent exchanges, which control the efficiency of dry deposition and downward transport of peroxides that are chemically produced higher up in the boundary layer. In turn, this chemical production depends on the upward transport of emitted precursor gases and their oxidization products. This demonstrates that improved simulations of atmosphere–biosphere peroxide exchanges rely heavily on improved model representations of boundary layer and canopy turbulent exchanges.
Keywords: Atmosphere–biosphere peroxide exchanges; Dry deposition; Non-stomatal uptake; Turbulence; Nocturnal;

Biogenic carbonyl compounds within and above a coniferous forest in Germany by Konrad Müller; Sylvia Haferkorn; Wolfgang Grabmer; Armin Wisthaler; Armin Hansel; Jürgen Kreuzwieser; Cristian Cojocariu; Heinz Rennenberg; Hartmut Herrmann (81-91).
Diurnal mixing ratios of aldehydes and ketones were investigated during two joint experiments in summer months to identify biogenic contributions from coniferous forests to tropospheric chemistry. In a Norway spruce forest, the diurnal variation of carbonyl compounds was measured at 12 m (in the treetop) and at 24 m (above the canopy). The main findings of the experiment are that acetone (up to 9.1 ppbv), formaldehyde (up to 6.5 ppbv), acetaldehyde (up to 5.5 ppbv) and methyl ethyl ketone (MEK, up to 1.8 ppbv) were found in highest concentrations. For all major compounds with the exception of MEK, primary emissions are supposed. From α-pinene oxidation, pinonaldehyde was found with its peak concentrations (up to 0.15 ppbv) during the early morning hours. The diurnal variation of concentrations for most other compounds shows maximum concentrations near midday in 2,4-dinitrophenylhydrazine (DNPH) measurements but not for proton-transfer reaction mass spectrometry (PTR-MS) measurements of acetaldehyde and acetone. A clear correlation of carbonyl compound concentration to the radiation intensity and the temperature (R 2=0.66) was found. However, formaldehyde did not show distinct diurnal variations. A very high correlation was observed for both heights between mixing ratios of acetaldehyde and acetone (R 2=0.84), acetone and MEK (R 2=0.90) as well as acetaldehyde and MEK (R 2=0.88) but not for formaldehyde and the others. For the most time, the observed carbonyl compound concentrations above the canopy are higher than within the forest stand. This indicates an additional secondary formation in the atmosphere above the forest.The differences of acetone and acetaldehyde mixing ratios detected by DNPH technique and the PTR-MS could not be fully clarified by a laboratory intercomparison.
Keywords: Norway spruce atmosphere; Aldehydes; Ketones; DNPH analysis; PTR-MS; Diurnal variation;

Direct measurements of particle number fluxes by eddy covariance (EC) were carried out in the years 2001 and 2002 during the BEWA2000 field experiments. An EC system combining a sonic anemometer and two condensation particle counters was set up and successfully applied above a Norway spruce forest in NE Bavaria, Germany. Particle deposition clearly dominated over emission, with the strongest deposition fluxes occurring during particle formation events identified from submicron particle size distributions. Typical deposition velocities derived from these measurements ranged from −37 to +23 mm s−1. The ultrafine particle fraction (UFP) (3–11 nm diameter) showed different concentration patterns and larger deposition velocities as compared to the particle fraction with larger diameters. Also, particle deposition occurring before noon could be attributed mainly to the UFP, whereas larger particles contributed to turbulent deposition in the afternoon.
Keywords: Eddy covariance; Particle deposition; Deposition velocity; Ultrafine particles; Norway spruce;

Biogenic contributions to the chemical composition of airborne particles in a coniferous forest in Germany by Antje Plewka; Thomas Gnauk; Erika Brüggemann; Hartmut Herrmann (103-115).
Airborne particles in and above the canopy of a middle European spruce forest were investigated in summer 2001 and in day/night rhythm in summer 2002 near Waldstein site (Fichtelgebirge, Germany).The particles were size-segregated collected and analyzed for main components (inorganic ions, elemental and organic carbon) as well as oxalic acid and alkanes. A mass closure for the chemical composition including water was performed successfully for both years. For analysis of other organic compounds high volume (HV) samplers were used in order to obtain more particle mass. The HV filter particles were measured with GC/MS after extraction and derivatisation. The highest concentrations were found for the sugars and the dicarboxylic acids. Four terpene acids, pinonaldehyde and isoprene oxidation products were detected. Differences between day and night samples were found for pinonaldehyde (night: 13.7 ng m−3; day: 2.7 ng m−3), for pinic acid (night: 3.2 ng m−3; day: 9.5 ng m−3) and also for the 2-methyltetrols (night: 4.0 ng m−3; day: 8 ng m−3).The detected terpene and isoprene oxidation products account only for a small part of the measured organic carbon content of particles. In all cases oxalic acid accounts for the major fraction of the speciated organic carbon. The origin of the main components like inorganic ions, OC, and EC is associated with the origin of air masses.
Keywords: Terpene and isoprene oxidation products; Biogenic organic compounds; Airborne particles;

Reactions of NO3 radicals with limonene and α-pinene: Product and SOA formation by M. Spittler; I. Barnes; I. Bejan; K.J. Brockmann; Th. Benter; K. Wirtz (116-127).
Monoterpenes are ubiquitous in forested environments and are subject to reactions with OH radicals, NO3 radicals and O3 both within and above forest canopies. While reactions of monoterpenes with OH and O3 have received considerable attention with regard to their reaction kinetics, reaction products and secondary organic aerosol (SOA) formation those with NO3 have received relatively little consideration. To redress this imbalance and to try and assess the possible importance of NO3+monterpene reactions within forest canopies investigations have been performed on selected monterpenes. The products and aerosol formation from the NO3 radical initiated oxidation of limonene and α-pinene have been investigated in the EUPHORE photoreactor facility, and the secondary chemistry of the observed major products was then investigated in a large volume laboratory photoreactor. In addition to large yields of yet unidentified organic nitrates, pinonaldehyde and endolim have been identified as the major reaction products of the NO3 radical initiated oxidation of α-pinene and limonene, respectively. The aerosol formation has been investigated in the presence of different seed aerosols and also water vapour. The reactions lead to the formation of SOA and the results indicate the importance of the chemical character of the seed aerosol in determining the SOA yield in NO3+monoterpene reactions. The possible pathways leading to SOA formation and also their potential importance are discussed based on the laboratory results on the reaction of NO3 with the two major products.
Keywords: Aerosol; Limonene; Nitrate radical; Products; α-Pinene;

VOC emissions from Norway spruce (Picea abies L. [Karst]) twigs in the field—Results of a dynamic enclosure study by W. Grabmer; J. Kreuzwieser; A. Wisthaler; C. Cojocariu; M. Graus; H. Rennenberg; D. Steigner; R. Steinbrecher; A. Hansel (128-137).
During the 2002 summer intensive field campaign of BEWA2000 a proton-transfer-reaction mass spectrometer (PTR-MS) was used for online determination of volatile organic compounds (VOC) emitted by Norway spruce (Picea abies L. [Karst]) twigs in a dynamic sampling enclosure. Emissions of isoprenoids (isoprene and monoterpenes) and oxygenated VOC (OVOC; acetaldehyde, acetone, methanol, and ethanol) were investigated. Emissions showed clear diurnal patterns with high daytime emission rates amounting to 1.8 μg C g−1  dwt h−1 for the sum of monoterpenes and in the range of 0.1 to 0.6 μg C g−1  dwt h−1 for isoprene>acetone>ethanol>methanol. Data were used to validate existing models on isoprene and monoterpene emissions and to discuss environmental and physiological factors affecting VOC emissions. Isoprene and acetaldehyde emission rates were best modelled applying the Guenther 1993 temperature and solar radiation algorithm. Emissions of monoterpenes, acetone and ethanol were best described by a temperature-only exponential algorithm. Using these model approaches a maximum emission variability of 66% was covered (isoprene). Poor r 2 values ranging from 0.15 to 0.42 were typical for oxygenated VOC emission modelling indicating the need for model improvement e.g. development of process-based models describing the emission as a result of biochemical de novo synthesis as well as physico-chemical transport properties inside the leaves.
Keywords: Isoprenoids; OVOC; PTR-MS; BVOC; Emission algorithms;

Photosynthesis and substrate supply for isoprene biosynthesis in poplar leaves by E. Magel; S. Mayrhofer; A. Müller; I. Zimmer; R. Hampp; J.-P. Schnitzler (138-151).
Gray poplar leaves emit high amounts of isoprene. In this context, we investigated the degree to which photosynthesis delivers necessary precursors for chloroplast isoprene biosynthesis, and whether this energy-consuming pathway could be involved in protecting the photosynthetic electron transport system. Such protection could result from consumption of a surplus in ATP and NADPH, generated under constricted net assimilation caused by high leaf temperatures and high light intensities.During the course of the day triose phosphate (TP) and dimethylallyl diphosphate (DMADP) concentrations showed pronounced diurnal variations closely related to net assimilation and isoprene emission rates, while other variables, e.g. energy (ATP/ADP) and redox (NADPH/NADP) ratio, as well as phosphoenolpyruvate (PEP) and pyruvate strongly scattered related to changing temperature and light intensities.Intra-day positive correlations were found mainly between leaf concentrations of TP and DMADP, and sucrose, ATP/ADP ratio and net assimilation rates. Under non-saturating light (200–400 μmol photons m−2  s−1), leaf DMADP pools were positively correlated mainly with PEP, starch, and fructose 2,6-bisphosphate (F26BP). Under saturating light, correlations improved and additionally involved sucrose, TP, and the ratio of NADPH/NADP.Study of temperature response curves showed that net assimilation and isoprene emission were negatively correlated to each other. This disconnection was mostly visible by the transient change of DMADP contents with maximum levels at 25 °C. At higher temperatures, declining pools of DMADP, TP and pyruvate indicated that DMADP consumption overcompensated DMADP production resulting in highest isoprene emission rates at declining pool sizes of precursors. In parallel to the reduction of net assimilation increases of NADPH/NADP and ATP/ADP ratios also portended that the MEP pathway dissipates a surplus of ATP and NADPH which cannot be used for carbon reduction under limited CO2 supply.Taken together we thus assume that DMADP/isoprene biosynthesis constitutes a way to avoid damage to the photosynthetic electron transport chain when CO2 reduction limits photosynthetic electron transport.
Keywords: Isoprene emission; Dimethylallyl diphosphate; Photosynthetic intermediates; Photosynthesis; Populus×canescens;

Process-based modelling of isoprenoid emissions from evergreen leaves of Quercus ilex (L.) by R. Grote; S. Mayrhofer; R.J. Fischbach; R. Steinbrecher; M. Staudt; J.-P. Schnitzler (152-165).
Monoterpenes play an important role in regulating the trace gas composition of the lower troposphere. Therefore, realistic estimates of the daily as well as seasonal variations of monoterpene emission source strength on the Earth surface are required. Monoterpenes are emitted by Holm oak (Quercus ilex L.) and other species lacking specific foliar terpene storage structures and their development is dependent on light and temperature. In the present work we describe a process-based emission model taking into account the physiological/phenological state of Holm oak leaves and biochemical processes leading to the formation of monoterpenes. The model ‘seasonal isoprenoid synthase model–biochemical isoprenoid biosynthesis model’ (SIM–BIM2) is developed based on a previous version which was used to simulate isoprene emissions from deciduous oaks. The current model considers additional enzymatic reactions in Holm oak chloroplasts that lead to the formation of monoterpenes. The comparison of simulated and measured biochemical properties as well as emission rates displayed that the ability of the model to dynamically adjust monoterpene biosynthesis capacity by modulating the amount of monoterpene synthase activities in dependence of the weather pattern led to realistic simulations of light-dependent monoterpene emission rates. Differences to simulation results obtained by a widely used alternative model [Guenther, A.B., Zimmerman, P.R., Harley, P.C., Monson, R.K., Fall, R., 1993. Isoprene and monoterpene emission rate variability—model evaluations and sensitivity analyses. Journal of Geophysical Research 98, 12609–12617] are discussed.
Keywords: Monoterpene emission rates; Seasonality; SIM–BIM2; Monoterpene synthases; Model development; Quercus ilex;

Biogenic emissions are strongly governed by temperature and light. One key issue for air quality modelling is to quantify the emission source strength variation of biogenic volatile organic compounds (BVOC) on different temporal and spatial scales. In Germany, this variation has been investigated for forests (the major source of BVOC) at country, state, and local scales over 10 years. Hourly emission rates for each vegetation period (April to October) from 1994 until 2003 were calculated in 10 km resolution. A database was compiled containing land cover, forest distribution, foliar biomass density, leaf area index (LAI) and plant specific emission potential. Meteorological input parameters were obtained using the non-hydrostatic meteorological mesoscale model MM5. The semi-empirical BVOC model (seBVOC) uses these parameters as input. SeBVOC considers in addition seasonality of the emission potentials, light extinction within the canopy as well as leaf temperature, air humidity, wind speed, and solar angle. The results show an average BVOC emission of 366 Gg, with 40 Gg isoprene, 188 Gg monoterpenes, and 138 Gg other VOC (OVOC), for the period 1994–2003. The dominating emitter type is Norway spruce (Picea abies) contributing approximately 40% to the total BVOC. In the years 1994–2003, the annual BVOC emissions varied in the range of ± 20 % . However, in specific episodes and at specific locations the variation is as high as 150% compared to average emissions. Also the BVOC composition emitted is location specific. Using a sensitivity analysis, the contribution of uncertainties in estimating foliar biomass, emission potential, as well as temperature to the overall uncertainty of calculated potential surface BVOC emission rates is addressed.
Keywords: Isoprene; Monoterpene; BVOC; Inventory; Emission modelling;