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A highly portable calibration source of nitric oxide (NO) based on the photolysis of nitrous oxide ( N2O ) supplied by 8 or 16 g disposable cartridges is demonstrated to serve as an accurate and reliable transfer standard for the calibration of NO monitors in the field. The instrument provides output mixing ratios in the range 0–1000 ppb with a precision and accuracy better than the greater of 3 ppb or 3 % of the target NO mixing ratio over a wide range of environmental conditions of ambient temperature (8.5–35.0 ∘ C), pressure (745–1015 mbar corresponding to 2.7–0.0 km of elevation), and relative humidity (0 %–100 % RH). The combination of the NO calibration source with a previously described ozone calibration source based on the photolysis of oxygen in air provides a new instrument capable of outputting calibrated mixing ratios of NO, ozone ( O3 ), and nitrogen dioxide ( NO2 ), where the NO2 is produced by the stoichiometric gas-phase reaction of NO with O3 . The portable NO 2 / NO / O 3 calibration source requires no external gas cylinders and can be used for calibrations of NO, NO2 , and O3 instruments for mixing ratios up to 1000, 500, and 1000 ppb, respectively. This portable calibrator may serve as a convenient transfer standard for field calibrations of ozone and NOx air pollution monitors.

Understanding nitrogen oxides (NOx = NO + NO2) measurement techniques is important as air-quality standards become more stringent, important sources change, and instrumentation develops. NOx observ...

Alkenes are reactive hydrocarbons that influence local and regional atmospheric chemistry by playing important roles in the photochemical production of tropospheric ozone and in the formation of secondary organic aerosols. The simplest alkene, ethene (ethylene), is a major plant hormone and ripening agent for agricultural commodities. The group of light alkenes (C2-C4) originates from both biogenic and anthropogenic sources, but their biogenic sources are poorly characterized, with limited field-based flux observations. Here we report net ecosystem fluxes of light alkenes and isoprene from a semiarid ponderosa pine forest in the Rocky Mountains of Colorado, USA using the relaxed eddy accumulation (REA) technique during the summer of 2014. Ethene, propene, butene and isoprene emissions have strong diurnal cycles, with median daytime fluxes of 123, 95, 39 and 17 µg m−2 h−1, respectively. The fluxes were correlated with each other, followed general ecosystem trends of CO2 and water vapor, and showed similar sunlight and temperature response curves as other biogenic VOCs. The May through October flux, based on measurements and modeling, averaged 62, 52, 24 and 18 µg m−2 h−1 for ethene, propene, butene and isoprene, respectively. The light alkenes contribute significantly to the overall biogenic source of reactive hydrocarbons: roughly 18 % of the dominant biogenic VOC, 2-methyl-3-buten-2-ol. The measured ecosystem scale fluxes are 40–80 % larger than estimates used for global emissions models for this type of ecosystem.

A new solid-phase scrubber for use in conventional ozone (O3) photometers was investigated as a means of reducing interferences from other UV-absorbing species and water vapor. It was found that when heated to 100–130 °C, a tubular graphite scrubber efficiently removed up to 500 ppb ozone and ozone monitors using the heated graphite scrubber were found to be less susceptible to interferences from water vapor, mercury vapor, and aromatic volatile organic compounds (VOCs) compared to conventional metal oxide scrubbers. Ambient measurements from a graphite scrubber-equipped photometer and a co-located Federal equivalent method (FEM) ozone analyzer showed excellent agreement over 38 days of measurements and indicated no loss in the scrubber's ability to remove ozone when operated at 130 °C. The use of a heated graphite scrubber was found to reduce the interference from mercury vapor to ≤ 3 % of that obtained using a packed-bed Hopcalite scrubber. For a series of substituted aromatic compounds (ranging in volatility and absorption cross section at 253.7 nm), the graphite scrubber was observed to consistently exhibit reduced levels of interference, typically by factors of 2.5 to 20 less than with Hopcalite. Conventional solid-phase scrubbers also exhibited complex VOC adsorption and desorption characteristics that were dependent upon the relative humidity (RH), volatility of the VOC, and the available surface area of the scrubber. This complex behavior involving humidity is avoided by use of a heated graphite scrubber. These results suggest that heated graphite scrubbers could be substituted in most ozone photometers as a means of reducing interferences from other UV-absorbing species found in the atmosphere. This could be particularly important in ozone monitoring for compliance with the United States (U.S.) Clean Air Act or for use in VOC-rich environments such as in smog chambers and monitoring indoor air quality.

A highly portable calibration source of nitric oxide (NO) based on photolysis of nitrous oxide (N2O) supplied by 8- or 16-g disposable cartridges is demonstrated to serve as an accurate and reliable transfer standard for the calibration of NO monitors in the field. The instrument provides output mixing ratios in the range 0–1,000 ppb with a precision and accuracy of better than the greater of 3 ppb or 3 % of the target NO mixing ratio over a wide range of environmental conditions of ambient temperature (8.5–35.0 °C), pressure (745–1,015 mbar corresponding to 2.7–0.0 km elevation) and relative humidity (0–100 % RH). Combination of the NO calibration source with a previously described ozone calibration source based on photolysis of oxygen in air provides a new instrument capable of outputting calibrated mixing ratios of NO, ozone (O3) and nitrogen dioxide (NO2), where the NO2 is produced by the stoichiometric gas-phase reaction of NO with O3. The portable NO2/NO/O3 calibration source requires no external gas cylinders and can be used for calibrations of NO, NO2 and O3 instruments for mixing ratios up to 1,000, 500, and 1,000 ppb, respectively. This portable calibrator may serve as a convenient transfer standard for field calibrations of ozone and NOx air pollution monitors.

Understanding nitrogen oxides (NO

A highly portable ozone (O3) calibration source that can serve as a U.S. EPA level 4 transfer standard for the calibration of ozone analyzers is described and evaluated with respect to analytical figures of merit and effects of ambient pressure and humidity. Reproducible mixing ratios of ozone are produced by the photolysis of oxygen in O3-scrubbed ambient air by UV light at 184.9 nm light from a low-pressure mercury lamp. By maintaining a constant volumetric flow rate (thus constant residence time within the photolysis chamber), the mixing ratio produced is independent of both pressure and temperature and can be varied by varying the lamp intensity. Pulse width modulation of the lamp with feedback from a photodiode monitoring the 253.7 nm emission line is used to maintain target ozone mixing ratios in the range 30–1000 ppb. In order to provide a constant ratio of intensities at 253.7 and 184.9 nm, the photolysis chamber containing the lamp is regulated at a temperature of 40 ∘C. The resulting O3 calibrator has a response time for step changes in output ozone mixing ratio of σp) of 0.4 % of the output mixing ratio for 10 s measurements (e.g., σp=±0.4 ppb for 100 ppb of O3). Ambient humidity was found to affect the output mixing ratio of ozone primarily by dilution of the oxygen precursor. This potential humidity interference could be up to a few percent in extreme cases but is effectively removed by varying the lamp intensity to compensate for the reduced oxygen concentration based on feedback from a humidity sensor.

We describe and characterize a modular folded tubular photometer for making direct measurements of the concentrations of nitrogen dioxide (NO2) and specify how this method could be extended to measure other pollutants such as sulfur dioxide (SO2), ozone (O3), and black carbon particulate matter. Direct absorbance measurements using this photometer can be made across the spectral range from the ultraviolet (UV) to the near infrared. The absorbance cell makes use of modular components (tubular detection cells and mirror cubes) that allow construction of path lengths of up to 2 m or more while maintaining low cell volumes. The long path lengths and low cell volumes enable sensitive detection of ambient air pollutants down to low part-per-billion levels for gas species and aerosol extinctions down to 1 Mm−1, corresponding to ∼ 0.1 µg m−3 for black carbon particulates. Pressure equalization throughout the stages of the absorbance measurement is shown to be critical to accurate measurements of analyte concentrations. The present paper describes the application of this photometer to direct measurements of nitrogen dioxide (NO2) and the incorporation of design features that also enable measurement of nitric oxide (NO) in the same instrument. Excellent agreement for ambient measurements along an urban roadside was found for both NO2 and NO measured by the folded tubular photometer compared to existing standard techniques. Compared to commonly used methods for measurements of NOx species, the advantages of this approach include (1) an absolute quantification for NO2 based on the Beer–Lambert law, thereby greatly reducing the frequency at which calibrations are required; (2) the direct measurement of NO2 concentration without prior conversion to NO as is required for the commonly used chemiluminescence method; (3) the use of modular components that allow construction of absorbance detection cells of varying lengths for extending the dynamic range of concentrations that can be measured; (4) a more economical instrument than other currently available direct measurement techniques for NO2; and (5) the potential for simultaneous detection of additional species such as SO2, O3, and black carbon in the same instrument. In contrast to other commercially available direct NO2 measurements, such as cavity-attenuated phase-shift spectroscopy (CAPS), the folded tubular photometer also measures NO simultaneously in the same apparatus by quantitatively converting NO to NO2 with ozone, which is then detected by direct absorbance.

In this study, we evaluated three conceptually similar ozone gas deposition models. These dry deposition models are frequently used with chemical transport models for calculations over large spatial domains. However, large scale applications of surface-atmosphere exchange of reactive gases require modeling results as accurate as possible to avoid nonlinear accumulation of errors in the spatially representative results. In this paper, model evaluation and comparison against measured data over a coniferous forest at Niwot Ridge AmeriFlux site (Colorado, USA) is carried out. At this site, no previous model calibration took place for any of the models, therefore, we can test and compare their performances under similar conditions as they would perform in a spatial application. Our results show systematic model errors in all the three cases, model performance varies with time of the day, and the errors show a pronounced seasonal pattern as well. The introduction of soil moisture content stress in the model improved model performance regarding the magnitude of fluxes, but the correlation between measured and modeled ozone deposition values remains low. Our results suggest that ozone dry deposition model results should be interpreted carefully in large scale applications, where the accuracy can vary with land cover sometimes are biased. © 2018, Hungarian Meteorological Service. All rights reserved.

Alkenes are reactive hydrocarbons that influence local and regional atmospheric chemistry by playing important roles in the photochemical production of tropospheric ozone and in the formation of secondary organic aerosols. The simplest alkene, ethene (ethylene), is a major plant hormone and ripening agent for agricultural commodities. The group of light alkenes (C2-C4) originates from both biogenic and anthropogenic sources, but their biogenic sources are poorly characterized, with limited field-based flux observations. Here we report net ecosystem fluxes of light alkenes and isoprene from a semiarid ponderosa pine forest in the Rocky Mountains of Colorado, USA using the relaxed eddy accumulation (REA) technique during the summer of 2014. Ethene, propene, butene and isoprene emissions have strong diurnal cycles, with median daytime fluxes of 123, 95, 39 and 17 μg mg-2 hg-1, respectively. The fluxes were correlated with each other, followed general ecosystem trends of CO2 and water vapor, and showed similar sunlight and temperature response curves as other biogenic VOCs. The May through October flux, based on measurements and modeling, averaged 62, 52, 24 and 18 μg mg-2 hg-1 for ethene, propene, butene and isoprene, respectively. The light alkenes contribute significantly to the overall biogenic source of reactive hydrocarbons: roughly 18 % of the dominant biogenic VOC, 2-methyl-3-buten-2-ol. The measured ecosystem scale fluxes are 40-80 % larger than estimates used for global emissions models for this type of ecosystem.

Snowpack temperatures from a subalpine forest below Niwot Ridge, Colorado, are examined with respect to atmospheric conditions and the 30-min above-canopy and subcanopy eddy covariance fluxes of sensible Qh and latent Qe heat. In the lower snowpack, daily snow temperature changes greater than 1°C day−1 occurred about 1–2 times in late winter and early spring, which resulted in transitions to and from an isothermal snowpack. Though air temperature was a primary control on snowpack temperature, rapid snowpack warm-up events were sometimes preceded by strong downslope winds that kept the nighttime air (and canopy) temperature above freezing, thus increasing sensible heat and longwave radiative transfer from the canopy to the snowpack. There was an indication that water vapor condensation on the snow surface intensified the snowpack warm-up. In late winter, subcanopy Qh was typically between −10 and 10 W m−2 and rarely had a magnitude larger than 20 W m−2. The direction of subcanopy Qh was closely related to the canopy temperature and only weakly dependent on the time of day. The daytime subcanopy Qh monthly frequency distribution was near normal, whereas the nighttime distribution was more peaked near zero with a large positive skewness. In contrast, above-canopy Qh was larger in magnitude (100–400 W m−2) and primarily warmed the forest–surface at night and cooled it during the day. Around midday, decoupling of subcanopy and above-canopy air led to an apparent cooling of the snow surface by sensible heat. Sources of uncertainty in the subcanopy eddy covariance flux measurements are suggested. Implications of the observed snowpack temperature changes for future climates are discussed.

Ecosystem level ozone (O3) fluxes during four different years were examined at a subalpine forest site in the Colorado Rocky Mountains. The local mountain–valley wind system and the proximity of the Denver Metropolitan area leads to high summertime ozone episodes on many afternoons. The timing between these episodes and the ecosystem processes controlling photosynthesis during the growing season plays a critical role in determining the amount of ozone deposition. Light and vapor pressure deficit (VPD) were the most dominant environmental drivers controlling the deposition of O3 at this site through their influence on stomatal conductance. 81% of the daytime O3 uptake was predicted to occur through the stomata. Stomatal uptake decreased at high VPD and temperatures leading to an overall decrease in O3 flux; however, we did observe a non-stomatal conductance for O3 that increased slightly with temperature before leveling off at higher values. During the growing season, O3 deposition fluxes were enhanced after midday precipitation events and continued at elevated levels throughout the following night, implying a role for surface wetness. From nighttime data, evidence for both the presence of water films on the needles and non-closure of the plant stomata were observed. During the winter (nongrowing) season, the ozone deposition velocity showed a consistent dependency on the latent heat flux. Although the mechanism is unclear, it is apparent that precipitation events play a role here through their influence on latent heat flux.

CO2 transport processes relevant for estimating net ecosystem exchange (NEE) at the Niwot Ridge AmeriFlux site in the front range of the Rocky Mountains, Colorado, USA, were investigated during a pilot experiment. We found that cold, moist, and CO2rich air was transported downslope at night and upslope in the early morning at this forest site situated on a � 5% east-facing slope. We found that CO2 advection dominated the total CO2 transport in the NEE estimate at night although there are large uncertainties because of partial cancellation of horizontal and vertical advection. The horizontal CO2 advection captured not only the CO2 loss at night, but also the CO2 uptake during daytime. We found that horizontal CO2 advection was significant even during daytime especially whenturbulentmixingwas notsignificant, such as inearly morningand evening transitionperiodsand withinthe canopy. Similar processes can occur anywhere regardless of whether flow is generated by orography, synoptic pressure gradients, or surface heterogeneity as long as CO2 concentration is not well mixed by turbulence. The long-term net effect of all the CO2 budget terms on estimates of NEE needs to be investigated. # 2007 Elsevier B.V. All rights reserved.

Direct measurements of winter water loss due to sublimation were made in a sub-alpine forest in the Rocky Mountains of Colorado. Above-and below-canopy eddy covariance systems indicated substantial losses of winter-season snow accumulation in the form of snowpack (0·41 mm d−1) and intercepted snow (0·71 mm d−1) sublimation. The partitioning between these over and under story components of water loss was highly dependent on atmospheric conditions and near-surface conditions at and below the snow/atmosphere interface. High above-canopy sensible heat fluxes lead to strong temperature gradients between vegetation and the snow-surface, driving substantial specific humidity gradients at the snow surface and high sublimation rates. Intercepted snowfall resulted in rapid response of above-canopy latent heat fluxes, high within-canopy sublimation rates (maximum = 3·7 mm d−1), and diminished sub-canopy snowpack sublimation. These results indicate that sublimation losses from the sub-canopy snowpack are strongly dependent on the partitioning of sensible and latent heat fluxes in the canopy. This compels comprehensive studies of snow sublimation in forested regions that integrate sub-canopy and over-story processes. Copyright © 2007 John Wiley & Sons, Ltd.

The location of the Niwot Ridge Ameriflux site within the rocky mountains subjects it to airflows which are common in mountainous terrain. In this study, we examine the effects of some of these mesoscale features on local turbulent flux measurements; most notably, the formation of valley/mountain flows and mountain lee-side waves. The valley/mountain flows created local non-stationarities in the wind flow caused by the passage of a lee-side convergence zone (LCZ) in which upslope and downslope flows met in the vicinity of the measurement tower. During June–August, 2001, possible lee-side convergences were flagged for ∼26% of all half-hour daytime flux measurement periods. However, there was no apparent loss of flux during these periods. On some relatively stable, summer nights, turbulence (designated via σw), and scalar fluctuations (temperature and CO2, for example) exhibited periodicities that appeared congruent with passage of low frequency gravity waves (τ ∼ 20 min). Spectral peaks at 0.0008 Hz (20 min) in both vertical velocity and scalar spectra were observed and indicated that 25–50% of the total scalar covariances were accounted for by the low frequency waves. During some periods of strong westerly winds (predominantly in winter), large mountain gravity waves were observed to form. Typically, the flux tower resided within a region of downslope “shooting flow”, which created high turbulence, but had no detrimental effect on local flux measurements based on valid turbulence statistics and nearly complete energy budget closure. Periodically, we found evidence for re-circulating, rotor winds in the simultaneous time series of wind data from the Ameriflux tower site and a second meteorological site situated 8 km upslope and to the West. Only 14% of the half-hour time periods that we examined for a 4 month period in the winter of 2000–2001 indicated the possible existence of rotor winds. On average, energy budget closure was ∼20% less during periods with rotor occurrence compared to those without. Results from this study demonstrate that the potential exists for relatively rare, yet significant influences of mesoscale wind flow patterns on the local half-hour flux measurements at this site. Occurrence of these events could be detected through examination of normal turbulence statistical parameters.

Components of the energy budget were measured above a subalpine coniferous forest over two complete annual cycles. Sensible and latent heat fluxes were measured by eddy covariance. Bowen ratios ranged from 0.7 to 2.5 in the summer (June–September) depending upon the availability of soil water, but were considerably higher (∼3–6) during winter (December–March). Energy budget closure averaged better than 84% on a half-hourly basis in both seasons with slightly greater closure during the winter months. The energy budget showed a dependence on friction velocity (u ∗ ), approaching complete closure at u ∗ values greater than 1 m s −1 . The dependence of budget closure on u ∗ explained why energy balance was slightly better in the winter as opposed to summer, since numerous periods of high turbulence occur in winter. It also explained the lower degree of energy closure (∼10% less) during easterly upslope flow since these periods were characterized by low wind speeds ( U< 4ms −1 ) and friction velocities (u ∗ < 0. 5ms −1 ). Co-spectral analysis suggests a shift of flux density towards higher frequencies under conditions where closure was obtained. It is suggested that low frequency contributions to the flux and advection were responsible for the lack of day-time energy budget closure. These effects were reduced at high friction velocities observed at our site. Our ability to close the energy budget at night was also highly dependent on friction velocity, approaching near closure (∼90%) at u ∗ values between 0.7 and 1.1 m s −1 . Below this range, the airflow within the canopy becomes decoupled with the flow above. Above this range, insufficient temperature resolution of the sonic anemometer obscured the small temperature fluctuations, rendering measurements intractable. © 2002 Elsevier Science B.V. All rights reserved.

The dry deposition velocity of nitric acid, Vd(HNO3), over a 12-m (mean height) spruce–fir forest at Niwot Ridge, Colorado was estimated during 13 daytime periods using the flux-gradient approach. Turbulence intensity at this site is high (mean u ∗ of 0.65 m s−1 with u of 2.9 m s−1) and contributed to the large observed Vd(HNO3). The overriding contributor is identified to be the small aerodynamic needle width of the conifer trees. Two cases had inflated Vd(HNO3) due to height-differentiated nitric acid loss to soil-derived particle surfaces. Not considering these cases, the mean Vd(HNO3) was 7.6 cm s−1. The mean laminar boundary layer resistance (Rb) was found to be 7.8 s m−1 (of similar magnitude to that of the aerodynamic resistance, 8.5 s m−1). The data-determined Rb is bracketed by two theoretical estimates of the mean Rb, 5.9 and 8.6 s m−1, that include consideration of the small canopy length scale (aerodynamic needle width), 1 mm or less, at this conifer forest. However, the poor correlation of data-determined Rb values with both sets of theoretical estimates indicates that measurement error needs to be reduced and/or improved formulations of theoretical Rb values are in order. The large observed Vd(HNO3) at this conifer forest site is attributed to high turbulence intensity, and, especially, to small aerodynamic needle width.

The spatial variability of turbulent flow statistics in the roughness sublayer (RSL) of a uniform even-aged 14 m (=h) tall loblolly pine forest was investigated experimentally. Using seven existing walkup towers at this stand, high frequency velocity, temperature, water vapour and carbon dioxide concentrations were measured at 15.5 m above the ground surface from October 6 to 10 in 1997. These seven towers were separated by at least 100 m from each other. The objective of this study was to examine whether single tower turbulence statistics measurements represent the flow properties of RSL turbulence above a uniform even-aged managed loblolly pine forest as a best-case scenario for natural forested ecosystems. From the intensive space-time series measurements, it was demonstrated that standard deviations of longitudinal and vertical velocities (u;w )a nd temper- ature (T ) are more planar homogeneous than their vertical flux of momentum (u 2 ) and sensible heat (H ) counterparts. Also, the measured H is more horizontally homogeneous when compared to fluxes of other scalar entities such as CO 2 and water vapour. While the spatial variability in fluxes was significant (>15%), this unique data set confirmed that single tower measurements represent the 'canonical' structure of single-point RSL turbulence statistics, especially flux-variance relationships. Implications to extending the 'moving-equilibrium' hypothesis for RSL flows are discussed. The spatial variability in all RSL flow variables was not constant in time and varied strongly with spatially averaged friction velocity u, especially whenu was small. It is shown that flow properties derived from two-point temporal statistics such as correlation functions are more sensitive to local variability in leaf area density when compared to single point flow statistics. Specifically, that the local relation- ship between the reciprocal of the vertical velocity integral time scale (Iw) and the arrival frequency of organized structures (N u=h) predicted from a mixing-layer theory exhibited dependence on the local leaf area index. The broader implications of these findings to the measurement and modelling of RSL flows are also discussed.

The rate at which isoprene is emitted by a forest depends on an array of environmental variables, the forest’s biomass, and its species composition. At present it is unclear whether errors in canopy-scale and process-level isoprene emission models are due to inadequacies in leaf-to-canopy integration theory or the imperfect assessment of the isoprene-emitting biomass in the flux footprint. To address this issue, an isoprene emission model (CANVEG) was tested over a uniform aspen stand and a mixed-species, broad-leaved forest. The isoprene emission model consists of coupled micrometeorological and physiological modules. The micrometeorological module computes leaf and soil energy exchange, turbulent diffusion, scalar concentration profiles, and radiative transfer through the canopy. Environmental variables that are computed by the micrometeorological module, in turn, drive physiological modules that calculate leaf photosynthesis, stomatal conductance, transpiration and leaf, bole and soil/root res...

A modification to the relaxed eddy accumulation (REA) flux measurement technique is proposed which maximizes the scalar mixing ratio difference in updrafts and downdrafts. This technique was developed with the goal of measuring the stable isotope ( 13 C/ 12 C and 18 O/ 16 O) ratios of updraft and downdraft air and thus the net fluxes of 13 C 16 O2 and 12 C 18 O 16 O. Current mass spectrometer precision is small relative to measured isotopic gradients in CO2 in the Earth's boundary layer, and the conventional REA approach is likely to be ineffective. The new technique, which we refer to as hyperbolic relaxed eddy accumulation (HREA), uses the conditional sampling concept of hyperbolic hole analysis to control sampling of air during only those turbulent events which contribute most strongly to the flux. Instead of basing updraft/downdraft sampling decisions strictly on vertical wind velocity, CO2 mixing ratio ((CO2)) fluctuations or those of another scalar are also used. Simulations using 10-Hz data show that a wind-based/ scalar-based sampling threshold can achieve a factor of 2.7 increase in scalar updraft/downdraft (CO2) differences over simple REA. During midday periods with strong photosynthetic fluxes, up/down (CO2) differences with HREA of 8 -10 ppm are possible, compared with 3-5 ppm for the best conventional REA case. Corresponding isotopic differences can likely be resolved with current mass spectrometers using this approach.

The micrometeorological flux measurement technique known as relaxed eddy accumulation (REA) holds promise as a powerful new tool for ecologists. The more popular eddy covariance (eddy correlation) technique requires the use of sensors that can respond at fast rates (10 Hz), and these are unavailable for many ecologically relevant compounds. In contrast, the use of REA allows flux measurement with sensors that have much slower response time, such as gas chromatography and mass spectrometry. In this review, relevant micrometeorological details underlying REA are presented, and critical analytical and system design details are discussed, with the goal of introducing the technique and its potential applications to ecologists. The validity of REA for measuring fluxes of isoprene, a photochemically reactive hydrocarbon emitted by several plant species, was tested with measurements over an oak-hickory forest in the Walker Branch Watershed in eastern Tennessee. Concurrent eddy covariance measurements of isoprene flux were made using a newly available chemiluminesence instrument. Excellent agreement was obtained between the two techniques (r 2 = 0.974, n = 62), providing the first direct comparison between REA and eddy covariance for measuring the flux rate of a reactive compound. The influence of a bias in vertical wind velocity on the accuracy of REA was examined. This bias has been thought to be a source of significant error in the past. Measurements of normalized bias ( ) alone would lead us to think that a large potential error exists at this site. However, with our isoprene data and through simulations of REA with fast-response H2O and CO2 data, we conclude that accurate REA flux measurements can be made even in the presence of a bias in w.

The rate coefficient for the IO + BrO → products (1) reaction was measured using pulsed laser photolysis with a discharge flow tube for radical production and pulsed laser-induced fluorescence and UV absorption for detection of IO and BrO radicals, respectively. Reaction 1 was studied under pseudo-first-order conditions in IO with an excess of BrO between 204 and 388 K at total pressures of 6−15 Torr. The Arrhenius expression obtained for non-iodine atom producing channels is k1a(T) = (2.5 ± 1.0) × 10-11 exp[(260 ± 100)/T] cm3 molecule-1 s-1 independent of pressure. The rate coefficient for the reaction BrO + BrO → products (2) and the UV absorption cross sections of BrO as a function of temperature were also determined as part of this study. The implications of these results to the loss rate of stratospheric ozone are discussed.

The rate coefficient, k1 (298 K) = (5.0 ± 1.1) × 10−11 cm3 molecule−1 s−1, for the reaction, Br + IO → I + BrO (1) was determined by monitoring the temporal profiles of IO in an excess of Br. The rate coefficient for the reverse reaction, I + BrO → IO + Br(−1), k−1 (298 K) = (1.9 ± 1.1) × 10−11 cm3 molecule−1 s−1, was determined by modeling IO temporal profiles, measured in the presence of BrO and O3, in which the regeneration of IO was observed. From these rate coefficients, the difference in the heats of formations of IO and BrO, ΔfH° (298 K) (BrO) − ΔfH° (298 K) (IO), was calculated to be 0.5 ± 0.6 kcal mol−1.

The rate coefficients for the reactions of O(3P) with CF3I (1) and CH3I (2) were measured between 213 and 364 K to be: k1(T) = (7.9 ± 0.8) x 10-12 exp[−(175 ± 40)/T] and k2(T) = (1.0 ± 0.2) x 10-11 exp[(160 ± 50)/T] cm3 molecule-1 s-1. The rate coefficients for the reaction of O(3P) with CD3I, (CH3)2CHCH2I, (CH3)2CHI, CF3CH2I, CF3CHFI, and CF3CF2I at 298 K were also measured. The yields of the IO and CF3O products in reaction 1 at 298 K were found to be 0.83 ± 0.09 and

The technique of pulsed laser photolysis/pulsed laser-induced fluorescence was used to determine products of the following reactions: OH + CH3SCH3 → products (1) and CH3S(OH)CH3 + O2 → products (5). The non-adduct-forming channel of reaction 1 was observed to proceed via hydrogen abstraction (branching ratio, Φabs = 0.84 ± 0.26) at 298 K. Direct formation of CH3S from reaction 1 was not observed (ΦCH3S < 0.04). A small channel for producing CH3 and CH3SOH in the non-adduct-forming route could not be ruled out. HO2 was found to be produced from reaction 5 with a yield of 0.50 ± 0.15 at 234 and 258 K. An upper limit for the production of OH from reaction 5 was determined to be ΦOH < 0.05. The atmospheric implications of the results from this and the accompanying paper are discussed.

The formation of a weakly bound adduct in the reaction of OH with DMS-d6 was observed between 217 and 240 K using the technique of pulsed laser photolysis/pulsed laser-induced fluorescence. The equilibrium constant for this process, OH + DMS-d6 ↔ OH·DMS-d6, was measured as a function of temperature. The bond strength of this adduct was determined to be 10.7 ± 2.5 kcal mol-1. The weakly bound adduct was observed to react rapidly with O2. The rate constant for the reaction OH·DMS-d6 + O2 → products was determined to be (1.00 ± 0.33) × 10-12 cm3 molecules-1 s-1, independent of pressure and temperature. The atmospheric implications of the formation of this adduct and its reaction with O2 to the mechanism of DMS oxidation in the atmosphere are discussed.

Laser induced fluorescence (LIF) from the (0, 0), (2, 0), (3, 0) and (2, 1) bands of the A 2 Π 3 2 → X Π 3 2 system of IO was detected. Using LIF detection of IO, the rate coefficients for I + O3 ← IO + O2 (k1) and IO + NO → I + NO2 (k2) reactions were measured between 240 and 370 K to be k1(T) = (2.3 ± 0.7 × 10−11 exp[−(860 ± 100)/T] and k2(T) = (1.02 ± 0.31) × 10−11 exp[(185) ± 70)/T] cm3 molecule−1 s−1.

Exchange Model (Noah-GEM) were only 0.2 cm s � 1 and 0.6 cm s � 1 , respectively. The observations also showed considerable PAN deposition at night with typical Vd values of 0.2–0.6 cm s � 1 , while the estimated values from both models were less than 0.1 cm s � 1 . Noah-GEM modeled more realistic stomatal resistance (Rs) than WDDM, as compared with observations of water vapor exchange fluxes. The poor performance of WDDM for stomatal uptake is mainly due to its lack of dependence on leaf area index. Thermal decomposition was found to be relatively unimportant for measured PAN fluxes as shown by the lack of a relationship between measured total surface conductance and temperature. Thus, a large part of the underprediction in Vd from both models should be caused by the underestimation of nonstomatal uptake, in particular, the cuticle uptake. Sensitivity tests on both stomatal and nonstomatal resistances terms were conducted and some recommendations were provided.

Utilizing the technique of pulsed laser photolysis/pulsed laser induced fluorescence, we have investigated the atmospherically important reactions of the (trifluoromethyl)peroxy radical, CF[sub 3]O, with several alkanes. The reaction rate coefficients for CF[sub 3]O + CH[sub 4] (k[sub 3]), C[sub 2]H[sub 6] (k[sub 4]), C[sub 3]H[sub 8] (k[sub 5]), (CH[sub 3])[sub 3]CH (k[sub 6]), and CD[sub 4] (k[sub 7]) were measured, as functions of temperature, to be k[sub 3] = (1.92 [+-] 0.33) x 10[sup [minus]12] exp[[minus](1370 [+-] 85)/T] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1], k[sub 4] = (4.84 [+-] 1.11) x 10[sup [minus]12] exp[-(400 [+-] 70)/T] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1], k[sub 5] = (5.12 [+-] 1.12) x 10[sup [minus]12] exp[-(15 [+-] 30)/T] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1], k[sub 6] = (4.32 [+-] 0.42) x 10[sup [minus]12] exp[(135 [+-] 30)/T] cm[sup 3] molecule[sup 1] s[sup [minus]1], and k[sub 7] = (0.91 [+-] 0.31) x 10[sup [minus]12] exp[-(1540 [+-] 100)] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1], respectively. These kinetic data are compared with results from previous studies. The atmospheric implications of these findings are discussed. 28 refs., 7 figs., 4 tabs.

The technique of pulsed laser photolysis/pulsed laser-induced fluorescence detection of CF[sub 3]O was used to study the stratospherically important reactions of CF[sub 3]O radicals with NO (k[sub 5]), O[sub 3] (k[sub 4]), and O[sub 2] (k[sub 6]) and that of CF[sub 3]-OO with NO (k[sub 2]) and O[sub 3](k[sub 3]). Over the temperature range 233-360, K, k[sub 5](T) = (3.34 [+-] 0.68) x 10[sup [minus]11] exp[(160 [+-] 45)/T] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1]. At 298 K, k[sub 2] = (1.57 [+-] 0.38) x 10[sup [minus]11] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1] in agreement with past work. The reactions of CF[sub 3]O and CF[sub 3]OO with O[sub 3] were found to be slow with rate coefficients at 298 K of k[sub 4] = (2.5[sub [minus]1.5][sup 0.7]) x 10[sup [minus]14] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1] and k[sub 3] [le] 7 x 10[sup [minus]15] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1], respectively. No reaction of CF[sub 3]O with O[sub 2] was observed at 298 or 373 K, leading to an upper limit of k[sub 6] [le] 4 x 10[sup [minus]17] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1] at 373 K. The implications of these results to the chemistry of CF[sub 3]O[sub x] radicalsmore » and O[sub 3] in the stratosphere are discussed. 32 refs., 7 figs., 1 tab.« less

Hydrofluorocarbons, many of which contain a CF(3) group, are one of the major substitutes for the chlorofluorocarbons and halons that are being phased out because they contribute to stratospheric ozone depletion. It is critical to ensure that CF(3) groups do not cause significant ozone depletion. The rate coefficients for the key reactions that determine the efficiency of the CF(3) radical as a catalyst for ozone loss in the stratosphere have been measured and used in a model to calculate the possible depletion of ozone. From these results, it is concluded that the ozone depletion potentials related to the presence of the CF(3) group in hydrofluorocarbons are negligibly small.

Hourly measurements of O3, NO, NO2, PAN, HNO3 and NOy concentrations, and eddy-covariance fluxes of O3 and NOy over a temperate deciduous forest from June to November, 2000 were used to evaluate the dry deposition velocities (Vd) estimated by the WRF-Chem dry deposition module (WDDM), which adopted Wesely (1989) scheme for surface resistance (Rc), and the Noah land surface model coupled with a photosynthesis-based Gas-exchange Evapotranspiration Model (Noah-GEM). Noah-GEM produced better Vd(O3) variations due to its more realistically simulated stomatal resistance (Rs) than WDDM. Vd(O3) is very sensitive to the minimum canopy stomatal resistance (Ri) which is specified for each seasonal category assigned in WDDM. Treating Sep-Oct as autumn in WDDM for this deciduous forest site caused a large underprediction of Vd(O3) due to the leafless assumption in ‘autumn’ seasonal category for which an infinite Ri was assigned. Reducing Ri to a value of 70 s m �1 , the same as the default value for the summer season category, the modeled and measured Vd(O3) agreed reasonably well. HNO3 was found to dominate the NOy flux during the measurement period; thus the modeled Vd(NOy )w as mainly controlled by the aerodynamic and quasi-laminar sublayer resistances (Ra and Rb), both being sensitive to the surface roughness length (z0). Using an appropriate value for z0 (10% of canopy height), WDDM and Noah-GEM agreed well with the observed daytime Vd(NOy). The differences in Vd(HNO3) between WDDM and Noah-GEM were small due to the small differences in the calculated Ra and Rb between the two models; however, the differences in Rc of NO2 and PAN between the two models reached a factor of 1.1e1.5, which in turn caused a factor of 1.1e1.3 differences for Vd. Combining the measured concentrations and modeled Vd ,N Ox, PAN and HNO3 accounted for 19%, 4%, and 70% of the measured NOy fluxes, respectively.

The atmospherically important reactions of both CH 3 S and CH 3 SOO were examined using the technique of pulsed laser photolysis/laser-induced fluorescence. Rate coefficients for the reactions CH 3 S+O 3 →products (1) and CH 3 S+NO 2 →CH 3 SO+NO (2) were measured to be k 1 (T)=(1.98±0.38) X 10 -12 exp[(290±40)/T] cm 3 molecule -1 s -1 and k 2 (T)=(2.06±0.44) X 10 -11 exp[(320±40)/T] cm 3 molecule -1 s -1

The atmospheric lifetimes of the fluorinated gases CF(4), C(2)F(6), c-C(4)F(8), (CF(3))(2)c-C(4)F(6), C(5)F(12), C(6)F(14), C(2)F(5)Cl, C(2)F(4)C(12), CF(3)Cl, and SF(6) are of concern because of the effects that these long-lived compounds acting as greenhouse gases can have on global climate. The possible atmospheric loss processes of these gases were assessed by determining the rate coefficients for the reactions of these gases with O((1)D), H, and OH and the absorption cross sections at 121.6 nanometers in the laboratory and using these data as input to a two-dimensional atmospheric model. The lifetimes of all the studied perfluoro compounds are2000 years, and those of CF(3)Cl, CF(3)CF(2)Cl, and CF(2)ClCF(2)Cl are300 years. If released into the atmosphere, these molecules will accumulate and their effects will persist for centuries or millennia.

This report observes for the first time the reversible addition of: CH{sub 3}S + O{sub 2} {r_equilibrium} CH{sub 3}SOO, in the gas phase (T = 216-258 K) by detecting CH{sub 3}S radicals. Also briefly described is the thermochemical and kinetic information of this system and its importance to the atmospheric oxidation of reduced sulfur compounds.

The primary quantum yields of OH(X 2Π),H(2S), and oxygen atoms [O(1D)+O(3P)] produced in the photodissociation of H2O2 at 193 and 222 nm have been measured at 298 K. At 193 nm, the primary quantum yields were observed to be 1.51±0.18, 0.16±0.04, and

The quantum yields for OH, O(3P), O(1D), and H(2S) from the photolysis of HNO3 have been determined at 248, 222, and 193 nm at 298 K. The quantum yield for OH was observed to be near unity at 248 [Φ(OH)=0.95±0.09] and 222 nm [Φ(OH)=0.90±0.11]. However, at 193 nm the quantum yield for OH was found to be 0.33±0.06. The quantum yield for O atoms [O(3P)+O(1D)], Φ(O), was observed to be 0.031±0.010, 0.20±0.03, 0.81±0.13, at 248, 222, and 193 nm, respectively. Both O(3P) and O(1D) were observed at 222 and 193 nm, but only O(3P) was detected at 248 nm. It was observed that 40% of the O atoms formed at both 193 and 222 nm were O(1D) atoms. The upper limits for H(2S) quantum yields: Φ(H)≤0.002 at 248 nm, ≤0.01 at 222 nm, and ≤0.012 at 193 nm were also determined. This present work quantifies the photolysis channel producing HONO+O and show that it accounts for a large fraction at HNO3 photolysis at 193 nm.

The kinetics of the BrO + ClO reaction was studied over the temperature range 234-406 K by discharge flow/mass spectrometry. The total rate constant for the reaction was found to have a significant negative activation energy and is given by k{sub 1}(T) = (2.59 {plus minus} 0.36) {times} 10{sup {minus}12} exp((445 {plus minus} 84)/T) cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}. The reaction has three product channels: (a) ClOO + Br, (b) OClO + Br, and (c) BrCl + O{sub 2}. The branching ratios for the three channels as a function of temperature were measured, and it was found that the channel leading to formation of OClO becomes dominant at lower temperatures (branching ratio of 0.51 at 250 K). Evidence for the direct formation of BrCl was found, yielding a branching ratio of 0.09-0.10 for the BrCl channel over the temperature range 250-406 K. The temperature-dependent rate coefficients for the three channels are given by k{sub 1a} = (2.1 {plus minus} 0.4) {times} 10{sup {minus}12} exp((258 {plus minus} 56)/T), k{sub 1b} = (6.7 {plus minus} 1.0) {times} 10{sup {minus}13} exp((622 {plus minus} 94)/T), and k{sub 1c} = (1.9 {plus minus} 0.6) {times} 10{sup {minus}13} exp((513 {plus minus} 86)/T) cm{sup 3} molecule{supmore » {minus}1} s{sup {minus}1}. These results are discussed relative to other recent studies and their importance in understanding the stratospheric chemistry of the Antarctic ozone hole.« less

The eddy covariance technique, which is used in the determination of net ecosystem CO2 exchange (NEE), is subject to significant errors when advection that carries CO2 in the mean flow is ignored. We measured horizontal and vertical advective CO2 fluxes at the Niwot Ridge AmeriFlux site (Colorado, USA) using a measurement approach consisting of multiple towers. We observed relatively high rates of both horizontal (F(hadv)) and vertical (F(vadv)) advective fluxes at low surface friction velocities (u(*)) which were associated with downslope katabatic flows. We observed that F(hadv) was confined to a relatively thin layer (0-6 m thick) of subcanopy air that flowed beneath the eddy covariance sensors principally at night, carrying with it respired CO2 from the soil and lower parts of the canopy. The observed F(vadv) came from above the canopy and was presumably due to the convergence of drainage flows at the tower site. The magnitudes of both F(hadv) and F(vadv) were similar, of opposite sign, and increased with decreasing u(*), meaning that they most affected estimates of the total CO2 flux on calm nights with low wind speeds. The mathematical sign, temporal variation and dependence on u(*) of both F(hadv) and F(vadv) were determined by the unique terrain of the Niwot Ridge site. Therefore, the patterns we observed may not be broadly applicable to other sites. We evaluated the influence of advection on the cumulative annual and monthly estimates of the total CO2 flux (F(c)), which is often used as an estimate of NEE, over six years using the dependence of F(hadv) and F(vadv) on u(*). When the sum of F(hadv) and F(vadv) was used to correct monthly F(c), we observed values that were different from the monthly F(c) calculated using the traditional u(*)-filter correction by--16 to 20 g C x m(-2) x mo(-1); the mean percentage difference in monthly Fc for these two methods over the six-year period was 10%. When the sum of F(hadv) and F(vadv) was used to correct annual Fc, we observed a 65% difference compared to the traditional u(*)-filter approach. Thus, the errors to the local CO2 budget, when F(hadv) and F(vadv) are ignored, can become large when compounded in cumulative fashion over long time intervals. We conclude that the "micrometeorological" (using observations of F(hadv) and F(vadv)) and "biological" (using the u(*) filter and temperature vs. F(c) relationship) corrections differ on the basis of fundamental mechanistic grounds. The micrometeorological correction is based on aerodynamic mechanisms and shows no correlation to drivers of biological activity. Conversely, the biological correction is based on climatic responses of organisms and has no physical connection to aerodynamic processes. In those cases where they impose corrections of similar magnitude on the cumulative F(c) sum, the result is due to a serendipitous similarity in scale but has no clear mechanistic explanation.