Lag time determination in DEC measurements with PTR-MS

The disjunct eddy covariance (DEC) method has emerged as a popular technique for micrometeorological flux measurements of volatile organic compounds (VOCs). It has usually been combined with proton transfer reaction mass spectrometry (PTR-MS), an online technique for VOC concentration measurements. However, the determination of the lag time between wind and concentration measurements has remained an important challenge. To address this issue, we studied the effect of different lag time methods on DEC fluxes. The analysis was based on both actual DEC measurements with PTR-MS and simulated DEC data derived from high frequency H<sub>2</sub>O measurements with an infrared gas analyzer. Conventional eddy covariance fluxes of H<sub>2</sub>O served as a reference in the DEC simulation. The individual flux measurements with PTR-MS were rather sensitive to the lag time methods, but typically this effect averaged out when the median fluxes were considered. The DEC simulation revealed that the maximum covariance method was prone to overestimation of the absolute values of fluxes. The constant lag time methods, one based on a value calculated from the sampling flow and the sampling line dimensions and the other on a typical daytime value, had a tendency to underestimate. The visual assessment method and our new averaging approach utilizing running averaged covariance functions did not yield statistically significant errors and thus fared better than the habitual choice, the maximum covariance method. Given this feature and the potential for automatic flux calculation, we recommend using the averaging approach in DEC measurements with PTR-MS. It also seems well suited to conventional eddy covariance applications when measuring fluxes near the detection limit

Technical Note: Quantitative long-term measurements of VOC concentrations by PTR-MS ? measurement, calibration, and volume mixing ratio calculation methods

International audienceProton transfer reaction mass spectrometry (PTR-MS) is a technique for online measurements of atmospheric concentrations, or volume mixing ratios, of volatile organic compounds (VOCs). The aim of this paper is to give a detailed description of our measurement, calibration, and volume mixing ratio calculation methods, which have been designed for long-term stand-alone field measurements by PTR-MS. We also show how the information obtained from a calibration can be used to determine the instrument specific relative transmission curve, which enables quantitative mixing ratio calculation for VOCs which are not present in a calibration gas standard. To illustrate the functionality of our measurement, calibration, and calculation methods, we present a one-month period of ambient mixing ratio data measured in a boreal forest ecosystem at the SMEAR II station in southern Finland. During the measurement period 27 March?26 April 2007, the hourly averages of mixing ratios were 0.1?0.5 ppbv for formaldehyde, 0.2?3.0 ppbv for methanol, 0.04?0.39 ppbv for benzene, and 0.03?1.25 ppbv for monoterpenes

Hydrocarbon fluxes above a Scots pine forest canopy: Measurements and modeling

International audienceWe measured the fluxes of several hydrocarbon species above a Scots pine (Pinus sylvestris) stand using disjunct eddy covariance technique with proton transfer reaction ? mass spectrometry. The measurements were conducted during four days in July at SMEAR II research station in Hyytiälä, Finland. Compounds which showed significant emission fluxes were methanol, acetaldehyde, acetone, and monoterpenes. A stochastic Lagrangian transport model with simple chemical degradation was applied to assess the sensitivity of the above canopy fluxes to chemistry. According to the model, the chemical degradation had a minor effect on the fluxes measured in this study but has a major effect on the vertical flux profiles of more reactive compounds, such as sesquiterpenes. The monoterpene fluxes followed the traditional exponential temperature dependent emission algorithm but were considerably higher than the fluxes measured before at the same site. The normalized emission potential (30°C) was 2.5 ?g gdw?1 h?1 obtained using the temperature dependence coefficient of 0.09°C?1

Carbonyl compounds in boreal coniferous forest air in Hyytiälä, Southern Finland

International audienceA variety of C1-C12 carbonyl compounds were measured in the air of a boreal coniferous forest located in Hyytiälä, Southern Finland. 24-h samples were collected during March and April in 2003 using DNPH (2,4-dinitrophenyl hydrazine) coated C18-cartridges and analyzed by liquid chromatography-mass spectrometry (LC-MS). Altogether 22 carbonyl compounds were quantified. The most abundant carbonyls were acetone (24-h average 1340 ng/m3), formaldehyde (480 ng/m3) and acetaldehyde (360 ng/m3). In contrast, scaling of concentrations against reactivity with the hydroxyl (OH) radical significantly increased the contribution of larger aldehydes and ketones (e.g. decanal, octanal and 6-methyl-5-hepten-2-one). Concentrations of monoterpene reaction products nopinone (9 ng/m3) and limona ketone (5 ng/m3) were low compared to the most abundant low molecular weight carbonyls. The total concentration of carbonyl compounds in Hyytiälä in April/March 2003 was much higher than the concentration of aromatic hydrocarbons and monoterpenes in April 2002. Lifetimes of the measured carbonyls with respect to reactions with OH radicals, ozone (O3), and nitrate (NO3) radicals as well as photolysis were estimated. The main sinks for most of the carbonyl compounds in Hyytiälä in springtime are expected to be reactions with the OH radical and photolysis. For 6-methyl-5-hepten-2-one and limona ketone also reactions with ozone are important. The sources of carbonyl compounds are presently highly uncertain. Due to the relatively short lifetimes of aldehydes and ketones, secondary biogenic and anthropogenic sources, that is oxidation of volatile organic compounds, and primary biogenic sources are expected to dominate in Hyytiälä

Effect of chemical degradation on fluxes of reactive compounds – a study with a stochastic Lagrangian transport model

In the analyses of VOC fluxes measured above plant canopies, one usually assumes the flux above canopy to equal the exchange at the surface. Thus one assumes the chemical degradation to be much slower than the turbulent transport. We used a stochastic Lagrangian transport model in which the chemical degradation was described as first order decay in order to study the effect of the chemical degradation on above canopy fluxes of chemically reactive species. With the model we explored the sensitivity of the ratio of the above canopy flux to the surface emission on several parameters such as chemical lifetime of the compound, friction velocity, stability, and canopy density. Our results show that friction velocity and chemical lifetime affected the loss during transport the most. The canopy density had a significant effect if the chemically reactive compound was emitted from the forest floor. We used the results of the simulations together with oxidant data measured during HUMPPA-COPEC-2010 campaign at a Scots pine site to estimate the effect of the chemistry on fluxes of three typical biogenic VOCs, isoprene, α-pinene, and β-caryophyllene. Of these, the chemical degradation had a major effect on the fluxes of the most reactive species β-caryophyllene, while the fluxes of α-pinene were affected during nighttime. For these two compounds representing the mono- and sesquiterpenes groups, the effect of chemical degradation had also a significant diurnal cycle with the highest chemical loss at night. The different day and night time loss terms need to be accounted for, when measured fluxes of reactive compounds are used to reveal relations between primary emission and environmental parameters

Temperature dependent sound velocity in hydrodynamic equations for relativistic heavy-ion collisions

We analyze the effects of different forms of the sound-velocity function cs(T) on the hydrodynamic evolution of matter formed in the central region of relativistic heavy-ion collisions. At high temperatures (above the critical temperature Tc) the sound velocity is calculated from the recent lattice simulations of QCD, while in the low temperature region it is obtained from the hadron gas model. In the intermediate region we use different interpolations characterized by the values of the sound velocity at the local maximum (at T = 0.4 Tc) and local minimum (at T = Tc). In all considered cases the temperature dependent sound velocity functions yield the entropy density, which is consistent with the lattice QCD simulations at high temperature. Our calculations show that the presence of a distinct minimum of the sound velocity leads to a very long (about 20 fm/c) evolution time of the system, which is not compatible with the recent estimates based on the HBT interferometry. Hence, we conclude that the hydrodynamic description is favored in the case where the cross-over phase transition renders the smooth sound velocity function with a possible shallow minimum at Tc.Comment: 6 pages, 3 figures, talk given at SQM'07 Levoca, Slovaki

Annual cycle of volatile organic compound exchange between a boreal pine forest and the atmosphere

Long-term flux measurements of volatile organic compounds (VOC) over boreal forests are rare, although the forests are known to emit considerable amounts of VOCs into the atmosphere. Thus, we measured fluxes of several VOCs and oxygenated VOCs over a Scots-pine-dominated boreal forest semi-continuously between May 2010 and December 2013. The VOC profiles were obtained with a proton transfer reaction mass spectrometry, and the fluxes were calculated using vertical concentration profiles and the surface layer profile method connected to the Monin-Obukhov similarity theory. In total fluxes that differed significantly from zero on a monthly basis were observed for 13 out of 27 measured masses. Monoterpenes had the highest net emission in all seasons and statistically significant positive fluxes were detected from March until October. Other important compounds emitted were methanol, ethanol+ formic acid, acetone and isoprene+ methylbutenol. Oxygenated VOCs showed also deposition fluxes that were statistically different from zero. Isoprene+ methylbutenol and monoterpene fluxes followed well the traditional isoprene algorithm and the hybrid algorithm, respectively. Emission potentials of monoterpenes were largest in late spring and autumn which was possibly driven by growth processes and decaying of soil litter, respectively. Conversely, largest emission potentials of isoprene+ methylbutenol were found in July. Thus, we concluded that most of the emissions of m/z 69 at the site consisted of isoprene that originated from broadleaved trees. Methanol had deposition fluxes especially before sunrise. This can be connected to water films on surfaces. Based on this assumption, we were able to build an empirical algorithm for bi-directional methanol exchange that described both emission term and deposition term. Methanol emissions were highest in May and June and deposition level increased towards autumn, probably as a result of increasing relative humidity levels leading to predominance of deposition.Peer reviewe

Scaling of hadronic transverse momenta in a hydrodynamic treatment of relativistic heavy ion collisions

The transverse momenta of hadrons in central nucleus-nucleus collisions are evaluated in a boost invariant hydrodynamics with transverse expansion. Quark gluon plasma is assumed to be formed in the initial state which expands and cools via a first order phase transition to a rich hadronic matter and ultimately undergoes a freeze-out. The average transverse momentum of pions, kaons, and protons is estimated for a wide range of multiplicity densities and transverse sizes of the system. For a given system it is found to scale with the square-root of the particle rapidity density per unit transverse area, and consistent with the corresponding values seen in $p\overline{p}$ experiments at 1800 GeV, suggesting a universal behaviour. The average transverse momentum shows only an approximate scaling with multiplicity density per nucleon which is at variance with the $p\overline{p}$ data.Comment: 6 pages including 9 figure

Modeling Eddy Current Losses in HTS Tapes Using Multiharmonic Method

Due to the highly nonlinear electrical resistivity of high temperature superconducting (HTS) materials, computing the steady-state eddy current losses in HTS tapes, under time-periodic alternating current excitation, can be time consuming when using a time-transient method (TTM). The computation can require several periods to be solved with a small time-step. One alternative to the TTM is the multiharmonic method (MHM) where the Fourier basis is used to approximate the Maxwell fields in time. The method allows obtaining the steady-state solution to the problem with one resolution of the nonlinear problem. In this work, using the finite element method with the H−φ formulation, the capabilities of the MHM in the computational eddy current loss modeling of HTS tapes are scrutinized and compared against the TTM.publishedVersionPeer reviewe