Trace Gas Concentrations Research Articles

Laboratory studies on remote method to assess the damage in underground mines after an emergency

Obtaining fast and accurate access to disaster information for underground mines is key in order to enact safe and effective emergency rescue measures. This study proposes a method to remotely evaluate the damage status in underground mines after a mine accident by using a tracer gas. This discussion includes the laws of migration, mixing and separating the tracer gas in the airway, and the superposition of the tracer gas concentration curve conforms to the Gaussian Mixture Model (GMM), which can be decomposed by the expectation-maximization (EM) algorithm. Based on the EM algorithm, the tracer gas method can assess the location and severity of the airway damage. The results show that the error between the calculated damage location and the actual damage location is very small. In addition, the area ratio of the concentration curve after the decomposition by the EM algorithms is highly correlated with the severity of the airway damage. This study demonstrates that the tracer gases can be used to remotely evaluate the location and severity of a mine accident, which is critical in order to perform successful rescue and emergency management.

Sulfuric acid–amine nucleation in urban Beijing

Abstract. New particle formation (NPF) is one of the major sources of atmospheric ultrafine particles. Due to the high aerosol and trace gas concentrations, the mechanism and governing factors for NPF in the polluted atmospheric boundary layer may be quite different from those in clean environments, which is however less understood. Herein, based on long-term atmospheric measurements from January 2018 to March 2019 in Beijing, the nucleation mechanism and the influences of H2SO4 concentration, amine concentrations, and aerosol concentration on NPF are quantified. The collision of H2SO4–amine clusters is found to be the dominating mechanism to initialize NPF in urban Beijing. The coagulation scavenging due to the high aerosol concentration is a governing factor as it limits the concentration of H2SO4–amine clusters and new particle formation rates. The formation of H2SO4–amine clusters in Beijing is sometimes limited by low amine concentrations. Summarizing the synergistic effects of H2SO4 concentration, amine concentrations, and aerosol concentration, we elucidate the governing factors for H2SO4–amine nucleation for various conditions.

Spontaneous Raman scattering at trace gas concentrations with a pressurized external multipass cavity

A setup is described with which the spontaneous Raman scattering rate from pressurized gas samples is greatly enhanced inside a near-concentric optical cavity. The pump source is a high power multimode semiconductor laser diode that receives optical feedback from the cavity by way of a volume Bragg grating, resulting in a linewidth of 80 pm and circulating power close to 100 W. Under pressurization (up to 8 bar) a limit of detection below 100 parts-per-billion was obtained for gaseous hydrogen, measured in ambient air and in breath. Furthermore, isotopic trace detection is demonstrated by headspace measurements of semiheavy water.

Retrieval of vertical profiles and tropospheric CO2 columns based on high-resolution FTIR over Hefei, China.

High-resolution solar absorption spectra, observed by ground-based Fourier Transform Infrared spectroscopy (FTIR), are used to retrieve vertical profiles and partial or total column concentrations of many trace gases. In this study, we present the tropospheric CO2 columns retrieved by mid-infrared solar spectra over Hefei, China. To reduce the influence of stratospheric CO2 cross-dependencies on tropospheric CO2, an a posteriori optimization method based on a simple matrix multiplication is used to correct the tropospheric CO2 profiles and columns. The corrected tropospheric CO2 time series show an obvious annual increase and seasonal variation. The tropospheric CO2 annual increase rate is 2.71 ± 0.36 ppm yr-1, with the annual peak value in January, and CO2 decreases to a minimum in August. Further, the corrected tropospheric CO2 from GEOS-Chem simulations are in good agreement with the coincident FTIR data, with a correlation coefficient between GEOS-chem model and FTS of 0.89. The annual increase rate of XCO2 observed from near-infrared solar absorption spectra is in good agreement with the tropospheric CO2 but the annual seasonal amplitude of XCO2 is only about 1/3 of dry-air averaged mole fractions (DMF) of tropospheric CO2. This is mostly attributed to the seasonal variation of CO2 being mainly dominated by sources near the surface.

Ground-Based MAX-DOAS Observations of Tropospheric NO2 and HCHO During COVID-19 Lockdown and Spring Festival Over Shanghai, China

Reduced mobility and less anthropogenic activity under special case circumstances over various parts of the world have pronounced effects on air quality. The objective of this study is to investigate the impact of reduced anthropogenic activity on air quality in the mega city of Shanghai, China. Observations from the highly sophisticated multi-axis differential optical absorption spectroscope (MAX-DOAS) instrument were used for nitrogen dioxide (NO2) and formaldehyde (HCHO) column densities. In situ measurements for NO2, ozone (O3), particulate matter (PM2.5) and the air quality index (AQI) were also used. The concentration of trace gases in the atmosphere reduces significantly during annual Spring Festival holidays, whereby mobility is reduced and anthropogenic activities come to a halt. The COVID-19 lockdown during 2020 resulted in a considerable drop in vertical column densities (VCDs) of HCHO and NO2 during lockdown Level-1, which refers to strict lockdown, i.e., strict measures taken to reduce mobility (43% for NO2; 24% for HCHO), and lockdown Level-2, which refers to relaxed lockdown, i.e., when the mobility restrictions were relaxed somehow (20% for NO2; 22% for HCHO), compared with pre-lockdown days, as measured by the MAX-DOAS instrument. However, for 2019, a reduction in VCDs was found only during Level-1 (24% for NO2; 6.62% for HCHO), when the Spring Festival happened. The weekly cycle for NO2 and HCHO depicts no significant effect of weekends on the lockdown. After the start of the Spring Festival, the VCDs of NO2 and HCHO showed a decline for 2019 as well as 2020. Backward trajectories calculated using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model indicated more air masses coming from the sea after the Spring Festival for 2019 and 2020, implying that a low pollutant load was carried by them. No impact of anthropogenic activity was found on O3 concentration. The results indicate that the ratio of HCHO to NO2 (RFN) fell in the volatile organic compound (VOC)-limited regime.

Trace gas composition in landfill gas at Danish landfills receiving low-organic waste

In 1997, the landfilling of biodegradable waste was banned in Denmark, and currently Danish landfills receive mostly non-combustible waste with a low-organic content. This study aimed to investigate trace gas composition in landfill gas (LFG) at modern Danish landfills. Landfill gas samples were taken from waste cells containing shredder, mixed and aged waste from four Danish landfills. The highest trace gas concentrations were found in shredder waste cells (average concentration of 103 mg m−3), which were comparable with conventional municipal solid waste landfills receiving organic waste. Aliphatic hydrocarbons and aromatics were dominant in the shredder waste cells, most likely released through direct volatilisation from disposed waste products. Abundant oxygenated compounds were found in the shredder waste cell in one of the landfills, thereby indicating a higher level of organic fraction biodegradation. Benzene, toluene, ethylbenzene and xylenes (BTEXs) were measured in high concentrations in all shredder waste cells, contributing to more than 75% of total aromatics. Considerably lower concentrations of trace gases were measured in the mixed and aged waste cells, which were dominated by hydrogen sulphide and several aliphatic hydrocarbons. A constant concentration ratio was established between aliphatic hydrocarbons together with aromatics and methane in shredder waste cells, which was then used in an LFG generation model to estimate trace gas production. The production rates of BTEXs from two landfills were estimated at 272 and 73 kg yr−1 in 2020, which were not considered to pose a significant risk to the environment or to human health.

Design and experiment of light field shaping system for three-dimensional extended light source used in photoacoustic spectrometer

Compared with infrared laser sources, the three-dimensional incoherent extended light source has the advantages of high power, wide spectral range, and low cost. It has extremely wide applications in high-precision and multi-component photoacoustic spectrometers. However, it encounters some problems about poor directivity, low energy density, irregular shape, light field shaping needed in the design of optical system. The photoacoustic spectrometer is required to collect and optimize the radiation of the centimeter-level three-dimensional extended light source to the whole space in a small volume. Through using a series of wavelength and frequency modulation elements, the final cylindrical light field distribution with millimeter-level radius and centimeter-level length is realized. According to the concept of optical expansion and the principle of edge light, this paper breaks through the traditional design mode based on point light source in the process of optical system design and optimization. The concept of extended light source is used throughout the design process. The luminous characteristics of the three-dimensional extended light source are directly acquired by the self-designed measurement method and device which is accurately reflected in the three-dimensional extended light source model in the form of micro-element. The design of the light field shaping system of the three-dimensional extended light source for the photoacoustic spectrometer is realized by the aspheric surface, and the relevant experimental verification is carried out. Taking the Hawkeye IR-Si272 light source for example, the experimental value of the light power at the entrance of the photoacoustic cell and the sidewall noise rate of the photoacoustic spectrometer have a small deviation from their corresponding simulation values. Compared with the original condenser system, the self-designed photoacoustic spectrometer light source system increases the value of the light power at the entrance of the photoacoustic cell from 0.86W to 1.32W, and reduces the value of the sidewall noise rate from 50.3% to 19.7%. The lower limit of detection of the concentration of trace gas in the order of ppm (parts per million) is also achieved.

Gas Concentration Sensing Based on Fiber Loop Ring-Down Spectroscopy: A Review

Gas detection plays an important role in many fields, such as atmospheric environment monitoring, biological monitoring, gas leakage monitoring, gas decomposition products detection, and so on. Nowadays, there are many kinds of gas sensors. Compared with normal gas sensors, optical sensors have the advantages of antielectromagnetic interference, high sensitivity, and compact size. Especially in certain extreme conditions, such as in aerospace applications and oil exploration application, optical fiber sensors are more attractive. This article presents a critical review on fiber loop ring-down spectroscopy (FLRDS), which can be used for quantitative detection of concentrations of trace gases. Fiber cavity is the core component of FLRDS system. The principles, loss characteristics, and application ranges of different types of fiber cavities are discussed. This article brings a variety of FLRDS techniques and introduces the basis for each technique, recent developments in methods, and performance limitations in detail. Finally, the existing technical barriers and future prospects of the FLRDS system for gas sensing are presented.

Year-long Concurrent MAX-DOAS Observations of Nitrogen Dioxide and Formaldehyde at Pune: Understanding Diurnal and Seasonal Variation Drivers

Year-long observations of nitrogen dioxide (NO2) and formaldehyde (HCHO) using the Multi-Axis Differential Absorption Spectroscopy (MAX-DOAS) technique are reported from Pune City, India. We studied the diurnal and seasonal variations, effect of biomass burning and the weekend effect on both species. NO2 diurnal profiles displayed a traffic induced peak at ~09:00 hrs. HCHO also showed a morning peak ~10:00 hrs due to production from oxidation of VOCs in the presence of solar radiation. Both NO2 and HCHO show the highest average concentrations during the winter (October, November, December, January and February—ONDJF), with mean mixing ratios of 2.0 ± 1.4 ppb and 3.0 ± 1.4 ppb, respectively. These observations suggest that a lower boundary layer (BL) height during ONDJF leads to higher concentrations of trace gases. During June, July, August, and September (JJAS), both trace gases show a minimum in their concentrations, with average mixing ratios for NO2 and HCHO being 0.9 ± 0.6 ppb and 1.1 ± 0.7 ppb, most likely due to removal by wet deposition. There was no significant difference in both the trace gases on weekdays and weekends. Using back-trajectory analysis, we conclude that air parcels coming from regions of biomass burning increased the concentrations in Pune. Emissions from nearby industrial areas of Bhosari and Pimpri-Chinchwad increased NO2 concentrations in Pune city. Finally, we compared the observations with previous reports over India and found that both HCHO and NO2 concentrations are lower in Pune compared to the other large cities in India.

Representative sampling analysis of radioactive airborne effluent for the stack system in a molten salt reactor

During the operation of the molten salt reactor, the radioactive gas and radioactive aerosol particles will be generated, and should be discharged into the atmospheric environment through ventilation ducts and stacks after filtration and adsorption. In order to determine whether the radioactive airborne effluent emissions would meet the requirements of the National management limits (ISO 2889–2010), it is necessary to calculate and monitor the radioactive airborne effluents in the stack system. Therefore, a three-dimensional computational fluid dynamics (CFD) model is built to evaluate the flowing and mixing state for radioactive airborne effluent in the stack system of the molten salt reactor. The uniformities of velocity, cyclonic flow, tracer gas concentration, and aerosol particle concentration at different sampling heights in the stack are analyzed by CFD-Fluent. The results show that, the airflow and concentration of tracer gas easily tend to blend well at high flow speed, while there is a little random distribution effect for the aerosol particles due to their large densities, which needs a longer transport process to increase the uniformity. When the sampling height is above 11 m from the bottom of the stack, the variation coefficients of velocity, cyclonic flow, tracer gas concentration, and aerosol particle concentration are below 11.6%, 4.4%, 0.4% and 20% correspondingly, which all meet the well-mixed criteria and the requirement of representativeness sampling location in ISO 2889–2010. If the variation coefficient of aerosol particle concentration can be allowed to be less than 30%, then sampling height above 6 m will satisfy the ISO standard. The results of this study provide a good reference for the detection, sampling and analysis of radioactive gases and aerosol particles in the stack of the molten salt reactor.

Approach to enhance trace gas determinations through multi-satellite data fusion

The imaging instruments on the polar-orbiting S-NPP and NOAA-20 satellite platforms [e.g., Visible Infrared Imaging Radiometer Suite (VIIRS)] and on geostationary GOES and Himawari platforms [e.g., Advanced Baseline Imager (ABI) and Advanced Himawari Imager (AHI)] have high horizontal spatial resolution but coarse vertical information about tropospheric gases and temperatures. An approach for fusing a high spatial resolution imager (e.g., VIIRS, ABI, and AHI) and a high information content sounder or trace gas monitor [e.g., Cross-track Infrared Sounder (CrIS) and TROPOspheric Monitoring Instrument (TROPOMI)] offers the opportunity to construct retrieval products (e.g., trace gas concentrations of CO, SO2, and NH3) via product fusion at high spatial resolution. Furthermore, these products can also be provided at high temporal resolution when geostationary imager (ABI and AHI) data are used in the product fusion. Promising results from VIIRS/CrIS, VIIRS/TROPOMI, and AHI/TROPOMI fusion are presented.

Dynamic measurement of the airflow rate in a two-zones dwelling, from the CO2 tracer gas-decay method using the Kalman filter

The measurement of natural airflows is challenging, because of low pressures involved in natural ventilation, and because of their variability. The tracer gas decay method is the most used method to assess natural airflows. It is, however, not adapted to multi-zone dwellings. A protocol based on the decay method is tested here involving the Kalman filter, and a correction procedure of concentrations. Both intend to allow the measurement of the global airflow-rate of a multi-zone passive-stack ventilated dwelling. The correction procedure of concentrations inhibits infiltrations that would have an impact on the accuracy of the tracer decay method implemented in a multi-zone dwelling. The Kalman filter allows measuring a dynamic airflow-rate, while the background tracer gas concentration varies which would bias the conventional decay method. It is a key issue when implementing the CO2 decay method in a multi-zone dwelling as, depending on its sources, the CO2 concentration in any room of the dwelling is likely to vary. The robustness of the proposed protocol was tested through a parametrical analysis in laboratory twincells and it showed a significantly lower sensitivity compared to the conventional 2-points decay method. Maximum deviations on the airflow-rate were systematically lower from 4 to 11 points. The concentration correction procedure allowed increasing the accuracy up to 190 points. The ability of the Kalman filter to assess the dynamic airflow-rate was questioned. It was strongly influenced by the process noise variance associated to state parameters of the Kalman filter, which are defined by the experimenter.

Dynamics of Trace Methane Diffusion/Flow Into Hollow Core Fiber Using Laser Absorption Spectroscopy

A technique is described for measurement of the flow and diffusion rates of trace gases in atmosphere into and out of hollow core fiber (HCF) using tunable diode laser absorption spectroscopy (TDLAS). Both diffusion and pressure-induced flow for methane through the fiber end faces obey simple equations. Measurements of the TDLAS signal of the trace gas optical absorption through the fiber as a function of time for small pressure differentials around atmospheric pressure across the fiber end faces can easily determine the times to fill and purge the fiber. The slope of these times as a function of the inverse pressure differential is a function of fiber core diameter, fiber length, and the dynamic viscosity of air. The data are in excellent agreement with the theory. Furthermore, it is found that highly sensitive trace gas concentration measurements are possible using relatively short lengths of HCF (< 1 m) which can be filled with external gas samples using modest pressure differentials (~10 kPa or 100 mbar) within minutes or even seconds. This result is important for remote trace gas sensing using HCF.

Pollution trace gas distributions and their transport in the Asian monsoon upper troposphere and lowermost stratosphere during the StratoClim campaign 2017

Abstract. We present the first high-resolution measurements of pollutant trace gases in the Asian summer monsoon upper troposphere and lowermost stratosphere (UTLS) from the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) during the StratoClim (Stratospheric and upper tropospheric processes for better climate predictions) campaign based in Kathmandu, Nepal, 2017. Measurements of peroxyacetyl nitrate (PAN), acetylene (C2H2), and formic acid (HCOOH) show strong local enhancements up to altitudes of 16 km. More than 500 pptv of PAN, more than 200 pptv of C2H2, and more than 200 pptv of HCOOH are observed. Air masses with increased volume mixing ratios of PAN and C2H2 at altitudes up to 18 km, reaching to the lowermost stratosphere, were present at these altitudes for more than 10 d, as indicated by trajectory analysis. A local minimum of HCOOH is correlated with a previously reported maximum of ammonia (NH3), which suggests different washout efficiencies of these species in the same air masses. A backward trajectory analysis based on the models Alfred Wegener InsTitute LAgrangian Chemistry/Transport System (ATLAS) and TRACZILLA, using advanced techniques for detection of convective events, and starting at geolocations of GLORIA measurements with enhanced pollution trace gas concentrations, has been performed. The analysis shows that convective events along trajectories leading to GLORIA measurements with enhanced pollutants are located close to regions where satellite measurements by the Ozone Monitoring Instrument (OMI) indicate enhanced tropospheric columns of nitrogen dioxide (NO2) in the days prior to the observation. A comparison to the global atmospheric models Copernicus Atmosphere Monitoring Service (CAMS) and ECHAM/MESSy Atmospheric Chemistry (EMAC) has been performed. It is shown that these models are able to reproduce large-scale structures of the pollution trace gas distributions for one part of the flight, while the other part of the flight reveals large discrepancies between models and measurement. These discrepancies possibly result from convective events that are not resolved or parameterized in the models, uncertainties in the emissions of source gases, and uncertainties in the rate constants of chemical reactions.

Flow and mixing characteristics of dual parallel plane jets subject to acoustic excitation

The effects of jet pulsation intensity and excitation Strouhal number on the flow and dispersion characteristics of dual parallel plane jets at a low Reynolds number of 200 were experimentally studied so that the transition from laminar to turbulence was focused. A hot-wire anemometer was used to record the jet velocities. The flow patterns were obtained using the laser-light sheet assisted smoke flow visualization technique. Jet spread widths were derived from the long-exposure images using the binary edge detection method. The jet fluid dispersion characteristics were examined using the tracer-gas concentration detection technique. Four characteristic flow modes (discrete coherent vortices, contiguous coherent vortices, early vortex breakup, and lateral dispersion) were identified in the domain of the jet pulsation intensity and excitation Strouhal number. At small jet pulsation intensities, discrete and contiguous coherent vortices appeared at low and high excitation Strouhal numbers, respectively. The vortices appearing in the jet shear layers broke up slowly and caused small jet expansions and turbulent intensities, thus inducing a low efficiency of jet fluid dispersion. The early vortex breakup mode was induced by the low-frequency back-and-forth flow motions in the near region at large jet pulsation intensities and low excitation Strouhal numbers. Large turbulence fluctuations caused by the early vortex breakup increased the jet dispersion significantly. At large jet pulsation intensities and excitation Strouhal numbers, high-frequency back-and-forth flow motions in the region near the jet exits significantly reduced the time-averaged axial momentum and induced large lateral jet expansion and jet fluid dispersions.

Solar tracker with optical feedback and continuous rotation

Abstract. Solar trackers are often used by spectrometers to measure atmospheric trace gas concentrations using direct sun spectroscopy. The ideal solar tracker should be sufficiently accurate, highly reliable, and with a longevity that exceeds the lifetime of the spectrometer that it serves. It should also be affordable, easy to use, and not too complex should maintenance be required. In this paper we present a design that fulfils these requirements using some simple innovations. Our altitude–azimuth design features a custom coaxial power transformer, enabling continuous 360∘ azimuth rotation. This increases reliability and avoids the need to reverse the tracker each day. In polar regions, measurements can continue uninterrupted through the summer polar day. Tracking accuracy is enhanced using a simple optical feedback technique that adjusts error offset variables while monitoring the edges of a focused solar image with four photodiodes. Control electronics are modular, and our software is written in Python, running as a web server on a recycled laptop with a Linux operating system. Over a period of 11 years we have assembled four such trackers. These are in use at Lauder (45∘ S), New Zealand, and Arrival Heights (78∘ S), Antarctica, achieving a history of good reliability even in polar conditions. Tracker accuracy is analysed regularly and can routinely produce a pointing accuracy of 0.02∘.

Effects of pulsation intensity on the flow and dispersion of pulsed dual plane jets

The effects of jet pulsation intensity on the flow and dispersion characteristics of dual parallel plane jets were experimentally studied. The jet pulsations were generated by acoustic excitations. The laser-light-sheet-assisted smoke flow visualization method was used to study the flow evolution processes. The lateral spreading characteristics of the jets were measured using the binary edge detection method. A hotwire anemometer was used to measure the velocity pulsations, mean velocities, turbulence intensities, and Lagrangian turbulent length and time scales. The dispersion characteristics of the jet fluids were measured by the tracer-gas concentration detection method. The jet Reynolds number and excitation Strouhal number were fixed at 600 and 0.106, respectively. The jet pulsation intensities were varied from 0 to 1.6. As the jets were pulsed, the turbulence intensity, lateral spread width, and jet fluid dispersion capability markedly increased compared with those of the non-pulsed jets. The larger the pulsation intensity was, the higher the turbulence intensity, lateral spread width, and jet fluid dispersion. The Lagrangian time and length scales of the turbulent eddies decreased as the jet pulsation intensity increased because the mushroom-shaped structures generated by the jet pulsations broke up into small turbulent eddies caused by the strong vortex-stretching effect.

Trace gas emissions from municipal solid waste landfills: A review

Trace gas emissions from municipal solid waste (MSW) landfills have received increasing attention in recent years. This paper reviews literature published between 1983 and 2019, focusing on (i) the origin and fate of trace gas in MSW landfills, (ii) sampling and analytical techniques, (iii) quantitative emission measurement techniques, (iv) concentration and surface emission rates of common trace compounds at different landfill units and (v) the environmental and health concerns associated with trace gas emissions from MSW landfills. Trace gases can be produced from waste degradation, direct volatilisation of chemicals in waste products or from conversions/reactions between other compounds. Different chemical groups dominate the different waste decomposition stages. In general, organic sulphur compounds and oxygenated compounds are connected with fresh waste, while abundant hydrogen sulphide, aromatics and aliphatic hydrocarbons are usually found during the methane fermentation stage. Selection of different sampling, analytical and emission rate measurement techniques might generate different results when quantifying trace gas emission from landfills, and validation tests are needed to evaluate the reliability of current methods. The concentrations of trace gases and their surface emission rates vary largely from site to site, and fresh waste dumping areas and uncovered waste surfaces are the most important fugitive emission sources. The adverse effects of trace gas emission are not fully understood, and more emission data are required in future studies to assess quantitatively their environmental impacts as well as health risks.

Measurement report: Leaf-scale gas exchange of atmospheric reactive trace species (NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;, NO, O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;) at a northern hardwood forest in Michigan

Abstract. During the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) campaign from 21 July to 3 August 2016, field experiments on leaf-level trace gas exchange of nitric oxide (NO), nitrogen dioxide (NO2), and ozone (O3) were conducted for the first time on the native American tree species Pinus strobus (eastern white pine), Acer rubrum (red maple), Populus grandidentata (bigtooth aspen), and Quercus rubra (red oak) in a temperate hardwood forest in Michigan, USA. We measured the leaf-level trace gas exchange rates and investigated the existence of an NO2 compensation point, hypothesized based on a comparison of a previously observed average diurnal cycle of NOx (NO2+NO) concentrations with that simulated using a multi-layer canopy exchange model. Known amounts of trace gases were introduced into a tree branch enclosure and a paired blank reference enclosure. The trace gas concentrations before and after the enclosures were measured, as well as the enclosed leaf area (single-sided) and gas flow rate to obtain the trace gas fluxes with respect to leaf surface. There was no detectable NO uptake for all tree types. The foliar NO2 and O3 uptake largely followed a diurnal cycle, correlating with that of the leaf stomatal conductance. NO2 and O3 fluxes were driven by their concentration gradient from ambient to leaf internal space. The NO2 loss rate at the leaf surface, equivalently the foliar NO2 deposition velocity toward the leaf surface, ranged from 0 to 3.6 mm s−1 for bigtooth aspen and from 0 to 0.76 mm s−1 for red oak, both of which are ∼90 % of the expected values based on the stomatal conductance of water. The deposition velocities for red maple and white pine ranged from 0.3 to 1.6 and from 0.01 to 1.1 mm s−1, respectively, and were lower than predicted from the stomatal conductance, implying a mesophyll resistance to the uptake. Additionally, for white pine, the extrapolated velocity at zero stomatal conductance was 0.4±0.08 mm s−1, indicating a non-stomatal uptake pathway. The NO2 compensation point was ≤60 ppt for all four tree species and indistinguishable from zero at the 95 % confidence level. This agrees with recent reports for several European and California tree species but contradicts some earlier experimental results where the compensation points were found to be on the order of 1 ppb or higher. Given that the sampled tree types represent 80 %–90 % of the total leaf area at this site, these results negate the previously hypothesized important role of a leaf-scale NO2 compensation point. Consequently, to reconcile these findings, further detailed comparisons between the observed and simulated in- and above-canopy NOx concentrations and the leaf- and canopy-scale NOx fluxes, using the multi-layer canopy exchange model with consideration of the leaf-scale NOx deposition velocities as well as stomatal conductances reported here, are recommended.

Time-averaged flow characteristics and mixing properties of double-concentric jets

We examined the flow behaviors and mixing characteristics of double-concentric jets using laser-assisted smoke flow visualization method to analyze typical flow patterns and binary boundary detection technique to investigate jet spread width. Time-averaged velocity vectors, streamline patterns, velocity distributions, turbulence properties, and vorticity contours were analyzed using Particle Image Velocimetry (PIV). Topological flow patterns were analyzed to interpret the vortical flow structures. Mixing properties were investigated using a tracer-gas concentration detection method. Four characteristic modes were observed: annular flow dominated mode, transition mode, central jet dominated mode-low shear, and central jet dominated mode-high shear. The jets’ mixing properties were enhanced by two major phenomena: the merging of annular flow and central jet at the centerline and the large turbulence fluctuations produced in the flow field. The merging of the jets induced stagnation points on the central axis in the annular flow dominated mode, which caused reverse flow on the central axis and drastic turbulence fluctuations of the near field region. When the central jet penetrated the recirculation region in the other three modes, the stagnation points on the central axis and the reverse flow vanished. Therefore, the mixing behaviors were prominently enhanced in the annular flow dominated mode.