Relaxed Eddy Accumulation System Research Articles

A relaxed eddy accumulation (REA) LOPAP system for flux measurements of nitrous acid (HONO)

Abstract. In the present study a relaxed eddy accumulation (REA) system for the quantification of vertical fluxes of nitrous acid (HONO) was developed and tested. The system is based on a three-channel long-path absorption photometer (LOPAP) instrument, for which two channels are used for the updrafts and downdrafts, respectively, and a third one for the correction of chemical interferences. The instrument is coupled to a REA gas inlet, for which an ultrasonic anemometer controls two fast magnetic valves to probe the two channels of the LOPAP instrument depending on the vertical wind direction. A software (PyREA) was developed, which controls the valves and measurement cycles, which regularly alternates between REA, zero and parallel ambient measurements. In addition, the assignment of the updrafts and downdrafts to the physical LOPAP channels is periodically alternated, to correct for differences in the interferences of the different air masses. During the study, only small differences of the interferences were identified for the updrafts and downdrafts excluding significant errors when using only one interference channel. In laboratory experiments, high precision of the two channels and the independence of the dilution-corrected HONO concentrations on the length of the valve switching periods were demonstrated. A field campaign was performed in order to test the new REA-LOPAP system at the TROPOS monitoring station in Melpitz, Germany. HONO fluxes in the range of -4×1013 molecules m−2 s−1 (deposition) to +1.0×1014 molecules m−2 s−1 (emission) were obtained. A typical diurnal variation of the HONO fluxes was observed with low, partly negative fluxes during night-time and higher positive fluxes around noon. After an intensive rain period the positive HONO emissions during daytime were continuously increasing, which was explained by the drying of the uppermost ground surfaces. Similar to other campaigns, the highest correlation of the HONO flux was observed with the product of the NO2 photolysis frequency and the NO2 concentration (J(NO2)⋅[NO2]), which implies a HONO formation by photosensitized conversion of NO2 on organic surfaces, such as humic acids. Other postulated HONO formation mechanisms are also discussed but are tentatively ranked being of minor importance for the present field campaign.

Affordable relaxed eddy accumulation system to measure fluxes of H2O, CO2, CH4 and N2O from ecosystems

The relaxed eddy accumulation (REA) technique is frequently applied to measure fluxes of a large variety of atmospheric tracers above ecosystems. It is often the method of choice since the eddy covariance (EC) technique is limited to a few tracers due to the lack of fast response analysers, high financial costs and in some cases high power consumption. REA avoids the need for a fast response analyser by collecting air from up-drafts and down-drafts into separate reservoirs. After collecting the air over a predefined time period, trace gas concentrations in the reservoirs are analysed by a slow response analyser and the average fluxes can be calculated.We developed and tested a REA system that is capable of measuring CO2, CH4, N2O and H2O fluxes simultaneously with only one gas analyser (Picarro G2805). This system is compatible with virtually any gas analyser and thus supports the flux analysis of a wide range of other air constituents such as isotopes, aerosols and volatile organic compounds. Furthermore, the modular design and rugged casing makes the sampling system robust and portable, and with its 12 V DC operation it is suitable for a wide range of field campaigns. The performance of the REA system was tested during the growing seasons of 2018 and 2020 on a grassland on organic soil in central Sweden.The system has worked reliably during several months in the Nordic climate, covering ambient temperatures between -20°C and +30°C. Measured fluxes of CO2 and H2O agree well with fluxes measured independently by an EC system. The similarity in the technology and the determined detection limits made us confident that the REA system even captures fluxes of CH4 and N2O well.

Nitric acid dry deposition associated with equilibrium shift of ammonium nitrate above a forest by long-term measurement using relaxed eddy accumulation

The dry deposition process of nitrogen compounds still has considerable uncertainty due to the complex compound behavior in the atmosphere. To enhance understanding of the nitric acid (HNO3) dry deposition process that largely contributes to the total nitrogen deposition, we associate this process with the equilibrium shift of ammonium nitrate (NH4NO3) to HNO3 and ammonia (NH3) near the surface. We obtained the flux of HNO3 and PM2.5 components above a forest in suburban Tokyo using a relaxed eddy accumulation (REA) system incorporating the denuder/filter-pack sampling technique. We conducted continuous samplings on a weekly basis over a long-term period, from October 14, 2016 to October 3, 2018, including leafy and leafless conditions. The measured deposition velocities of fine NO3− particles were significantly (p < 0.05) larger than those of fine SO42− particles and similar to those of HNO3, regardless of the leafy and leafless periods. The larger deposition velocities of fine NO3− particles caused by the equilibrium shift of NH4NO3 were found to be similar to previous measurements at the same site. The measured deposition velocities of HNO3 were low and included many emission cases compared to those of fine NO3− particles. The lower HNO3 deposition velocities were associated with the equilibrium shift of NH4NO3 to the gas phase, causing the suppressed deposition and apparent emission of HNO3, particularly during the leafless period. The emission of HNO3 is possibly due to high HNO3 concentration near the surface, which is attributed to a small removal of the leafless canopy. The turbulence enhanced both the upward and downward fluxes of HNO3. These results will contribute to the better understanding for the dynamics of the nitrogen compounds in the atmosphere and the accurate assessment for the nitrogen deposition.

A portable, low-cost relaxed eddy accumulation (REA) system for quantifying ecosystem-level fluxes of volatile organics

Quantification of biogenic volatile organic compound (BVOC) fluxes into the atmosphere is crucial to understand their role in atmospheric oxidation and biogeochemical cycles. BVOC flux measurements were carried out in nine forest ecosystems using a relaxed eddy accumulation (REA) based sampling system, which is easily transportable, simple to operate and designed to be low-cost and therefore can easily be deployed at multiple remote locations. The REA measurements were carried out during daytime between 06:00 and 18:30 (Local Time) with a flux averaging period of 30 min. A detailed description of the REA sampling setup, operational procedure and validation by comparison with full eddy covariance (EC) BVOC flux measurements is provided. BVOC flux measurements from established long-term carbon and water flux tower sites in nine forest ecosystems are compared including Manitou Forest Observatory in Colorado, USA (pine woodland forest), Niwot Ridge AmeriFlux site in Colorado, USA (subalpine forest), Deer Canyon Preserve in New Mexico, USA (pinyon-juniper forest), Lei bamboo forest site near Taihuyuan, China, Qianyanzhou ChinaFLUX site in China (pine forest), Baskett Wildfire Refuge MOFlux site in Missouri, USA (deciduous oak forest), University of Michigan Biological Station PROPHET site in Michigan, USA (mixed deciduous forest), Changbai Mountain Forest Research Station in China (mixed deciduous forest) and the Guyaflux site (GF-Guy) in French Guiana (tropical rainforest). BVOC flux measurements using our REA setup confirm dominance of 2,3,2- methylbutenol (2,3,2-MBO) at the Manitou Forest Observatory and Niwot Ridge sites in Colorado. Monoterpene fluxes measured by REA showed good agreement (within ±10%) with monoterpene fluxes measured by PTR-MS at the Manitou Forest Observatory. The MOFlux site in Missouri was dominated by isoprene emissions (average flux of ~ 9.5 mg m −2 h −1 ) whereas the Deer Canyon site was dominated by α-pinene emissions (average flux ~ 0.73 mg m −2 h −1 ). Mixed deciduous forest sites at the PROPHET Station in Michigan and Changbai Mountain Forest Research Station in China primarily emitted isoprene along with some α-pinene, β-pinene and d-Limonene. Isoprene and α-pinene were the dominant BVOCs emitted from the subtropical Lei bamboo plantation at the Taihuyuan site in China while the pine forest site at Qianyanzhou in China were dominated by α-pinene emissions along with significant isoprene. BVOC measurements across different seasons (during 2009–2011) at a tropical forest site in French Guiana (Guyaflux site) revealed the dominance of isoprene emissions during all seasons. Irrespective of the type of the forest ecosystem, α-pinene was among the dominant monoterpenes emitted from all nine forests.

Ammonia flux measurements above a corn canopy using relaxed eddy accumulation and a flux gradient system

Studies of NH3 flux over agricultural ecosystems in the USA are limited by low temporal resolution (typically hours or days) and sparse spatial coverage, with no studies over corn in the Midwest USA. We report on NH3 flux measurements over a corn canopy in Central Illinois, USA, using the relaxed eddy accumulation (REA) and flux gradient (FG) methods, providing measurements at 4 h and 0.5 h intervals, respectively. The REA and FG systems were operated for the duration of the 2014 corn-growing season. Flux-footprint analysis was used to select data from both systems, resulting in 82 concurrent measurements. Mean NH3 flux of concurrent measurements was 205 ± 300 ng m−2 s−1 from REA and 110 ± 256 ng m−2 s−1 from FG for all concurrent samples. Results from both methods were not significantly different at a 95% confidence level for all concurrent measurements. The FG system resolved NH3 emission peaks at 0.5 h averaging time that were otherwise un-observed with 4 h REA averaging. Two early-season peak emission periods were identified (DOY 130-132 and 140-143), where the timing and intensity of such emissions were attributed to a combination of urease inhibitor, applied as a field-management decision, and localized soil temperature and precipitation. Given the dependence of NH3 fluxes on multiple parameters, this study further highlights the need for increased spatial coverage and high temporal resolution (e.g., <1 h) of measurements to better understand the impact of agricultural NH3 emissions on air quality and the global nitrogen cycle. Such measurements are also needed for evaluation of models describing surface-atmosphere exchange of NH3.

Fluxes of gaseous elemental mercury (GEM) in the High Arctic during atmospheric mercury depletion events (AMDEs)

Abstract. Measurements of gaseous elemental mercury (GEM) fluxes over snow surfaces using a relaxed eddy accumulation (REA) system are carried out at the High Arctic site at the Villum Research Station, Station Nord, in North Greenland. Simultaneously, CO2 fluxes are determined using the eddy covariance (EC) technique. The REA system with dual inlets and dual analyzers is used to measure fluxes directly over the snow. The measurements were carried out from 23 April to 12 May during spring 2016, where atmospheric mercury depletion events (AMDEs) took place. The measurements showed a net emission of 8.9 ng m−2 min−1, with only a few minor episodes of net depositional fluxes, from a maximum deposition of 8.1 ng m−2 min−1 to a maximum emission of 179.2 ng m−2 min−1. The data support the theory that gaseous oxidized mercury (GOM) is deposited during AMDEs followed by formation of GEM on surface snow and is re-emitted as GEM shortly after the AMDEs. Furthermore, observation of the relation between GEM fluxes and atmospheric temperature suggests that GEM emission partly could be affected by surface heating. However, it is also clear that the GEM emissions are affected by many parameters.

Comparative study of elemental mercury flux measurement techniques over a Fennoscandian boreal peatland

Quantitative estimates of the land-atmosphere exchange of gaseous elemental mercury (GEM) are biased by the measurement technique employed, because no standard method or scale in space and time are agreed upon. Here we present concurrent GEM exchange measurements over a boreal peatland using a novel relaxed eddy accumulation (REA) system, a rectangular Teflon® dynamic flux chamber (DFC) and a DFC designed according to aerodynamic considerations (Aero-DFC). During four consecutive days the DFCs were placed alternately on two measurement plots in every cardinal direction around the REA sampling mast. Spatial heterogeneity in peat surface characteristics (0–34 cm) was identified by measuring total mercury in eight peat cores (57 ± 8 ng g−1, average ± SE), vascular plant coverage (32–52%), water table level (4.5–14.1 cm) and dissolved gaseous elemental mercury concentrations (28–51 pg L−1) in the peat water. The GEM fluxes measured by the DFCs showed a distinct diel pattern, but no spatial difference in the average fluxes was detected (ANOVA, α = 0.05). Even though the correlation between the Teflon® DFC and Aero-DFC was significant (r = 0.76, p < 0.05) the cumulative flux of the Aero-DFC was a factor of three larger. The average flux of the Aero-DFC (1.9 ng m−2 h−1) and REA (2 ng m−2 h−1) were in good agreement. The results indicate that the novel REA design is in agreement for cumulative flux estimates with the Aero-DFC, which incorporates the effect of atmospheric turbulence. The comparison was performed over a fetch with spatially rather homogenous GEM flux dynamics under fairly consistent weather conditions, minimizing the effect of weather influence on the data from the three measurement systems. However, in complex biomes with heterogeneous surface characteristics where there can be large spatial variability in GEM gas exchange, the small footprint of chambers (<0.2 m2) makes for large coefficients of variation. Thus many chamber measurement replications are needed to establish a credible biome GEM flux estimate, even for a single point in time. Dynamic flux chambers will, however, be able to resolve systematic differences between small scale features, such as experimentally manipulated plots or small scale spatial heterogeneity.

Seasonal and interannual variations in whole-ecosystem BVOC emissions from a subtropical plantation in China

Measurements of BVOC emissions, ozone concentration and environmental parameters were carried out from May 2013 to January 2016 in a subtropical Pinus plantation in China. Isoprene and monoterpene emissions were measured using a relaxed eddy accumulation (REA) system and a gradient technique on an above-canopy tower. In 2013, isoprene comprised 21.2% of total terpenoid emissions, while α-pinene, camphene, β-pinene and limonene constituting 51.5%, 2.4%, 9.1%, and 13.0% of total emissions, respectively. Monoterpenes together were the dominant VOCs measured contributing 71.6%. α-pinene, camphene, β-pinene and limonene constituted 67.7%, 3.2%, 11.9%, 17.2% of total monoterpene emissions. Isoprene and monoterpene emissions displayed strong diurnal variations, with lower emissions in the morning and late evening, and the highest emissions around noon. BVOC peak emissions typically occurred a few hours after the noon PAR peak. Isoprene and monoterpene emissions varied with season and were the highest in summer, contributing more than half of the total annual emission, and the lowest emissions were in winter. Evident interannual variations of isoprene, monoterpenes and total BVOCs were observed. Compared to 2013, annual BVOC emissions decreased in 2015, associated with decreases of PAR, Temperature, water vapor, and an increase of all substances in gas, liquid and solid phases in the atmospheric column (e.g., S/Q, the ratio of solar scattered radiation to global radiation). Ozone concentration showed clear diurnal variation with PAR, higher around noon and lower in the early morning and late evening. Generally, there were no evident correlations between ozone concentrations and BVOC emissions, or the vertical gradients of ozone concentrations and BVOC concentrations. Under all sky conditions (including cloudy skies), no strong correlations at a high confidence level or very similar variation patterns were observed between any two following parameters, BVOC emissions, PAR, temperature, water vapor, and S/Q. The major factors controlling BVOC emissions were PAR and temperature but biomass burning smoke and phenology (pine florescence) may also play a role. The mean emission factors at standard conditions determined using the MEGAN model emission algorithms and empirical model of BVOC emissions were 0.71 and 1.19 mg m−2 h−1 for isoprene and 1.39 and 1.65 mg m−2 h−1 for total monoterpenes, respectively.

Season-long ammonia flux measurements above fertilized corn in central Illinois, USA, using relaxed eddy accumulation

The objective of this research is to quantify NH3 flux above an intensively managed cornfield in the Midwestern United States to improve understanding of NH3 emissions and evaluations of new and existing emission models. A relaxed eddy accumulation (REA) system was deployed above a corn canopy in central Illinois, USA (40°3′46.209″N, 88° 11′46.0212″W) from May through September 2014 (day of year 115–273) to measure NH3 fluxes due to chemical fertilizer application. NH3 flux was measured in four-hour periods during mornings and afternoons. Mean atmospheric NH3 concentration during the complete measurement period was 2.6±2.0μgm−3. Larger upward fluxes of gaseous NH3 were measured during the first 30days after fertilization, with variations observed throughout the field campaign. Measured NH3 fluxes ranged from −246.0ngm−2s−1 during wintertime background measurements to 799.6ngm−2s−1 within two weeks of fertilization (where negative flux indicates deposition). Mean positive flux was 233.3±203.0ngm−2s−1 in the morning and 260.0±253.3ngm−2s−1 in the afternoon while mean negative flux was −45.3±38.6ngm−2s−1 in the morning and −78.35±74.9ngm−2s−1 in the afternoon. NH3 volatilization during the first 21days after fertilization accounted for 79% of total nitrogen loss during the growing season. Such measurements are critical to improve understanding of agricultural NH3 emissions in managed agricultural ecosystems dominated by rotations of highly fertilized corn and moderately to lightly fertilized soybeans, such as the plot studied herein. These measurements are also important to improve understanding of how managed agricultural ecosystems impact air quality, and contribute to the global nitrogen cycle, and to evaluate current NH3 emission models.

A dual-inlet, single detector relaxed eddy accumulation system for long-term measurement of mercury flux

Abstract. The fate of anthropogenic emissions of mercury (Hg) to the atmosphere is influenced by the exchange of elemental Hg with the earth surface. This exchange holds the key to a better understanding of Hg cycling from local to global scales, which has been difficult to quantify. To advance research about land–atmosphere Hg interactions, we developed a dual-inlet, single detector relaxed eddy accumulation (REA) system. REA is an established technique for measuring turbulent fluxes of trace gases and aerosol particles in the atmospheric surface layer. Accurate determination of gaseous elemental mercury (GEM) fluxes has proven difficult due to technical challenges presented by extremely small concentration differences (typically &lt; 0.5 ng m−3) between updrafts and downdrafts. We present an advanced REA design that uses two inlets and two pairs of gold cartridges for continuous monitoring of GEM fluxes. This setup reduces the major uncertainty created by the sequential sampling in many previous designs. Additionally, the instrument is equipped with a GEM reference gas generator that monitors drift and recovery rates. These innovations facilitate continuous, autonomous measurement of GEM flux. To demonstrate the system performance, we present results from field campaigns in two contrasting environments: an urban setting with a heterogeneous fetch and a boreal peatland during snowmelt. The observed average emission rates were 15 and 3 ng m−2 h−1, respectively. We believe that this dual-inlet, single detector approach is a significant improvement of the REA system for ultra-trace gases and can help to advance our understanding of long-term land–atmosphere GEM exchange.

Seasonal variations in whole-ecosystem BVOC emissions from a subtropical bamboo plantation in China

Abstract Isoprene and monoterpene emissions and environmental conditions were measured over a six month period in a Lei bamboo (Phyllostachys violascens) forest in a subtropical region in China. Isoprene and monoterpene emissions were measured using a relaxed eddy accumulation (REA) system on an above-canopy tower. From July to November of 2012, isoprene contributed 99.1% of terpenoid emissions. α-pinene, constituting 0.8% of total observed terpenoid emissions, was the only monoterpene for which a significant flux was detected. Emissions of the sesquiterpenes longifolene and α-cedrene were observed at very low rates. Isoprene and α-pinene emissions exhibited strong diurnal variations, with lower emissions in the morning and late evening, and the highest emissions around noon. BVOC peak emissions typically occurred a few hours after the noon PAR peak and coincided with the daily temperature peak. This behavior can be described reasonably well by the MEGANv2.1 biogenic emission model. During the campaign (i.e., from 7 July, 2012 to 19 Jan., 2013), the mean (and maximum) emission fluxes (mg m −2 h −1 ) were 0.95 (10.32) for isoprene, 0.010 (0.176) for α pinene, 0.001 (0.063) for longifolene, and 2.6 × 10 −4 (0.009) for α-cedrene, respectively. During the winter season, when the ground was covered by organic mulch to increase soil temperature and to increase the yield of bamboo shoot, there was no evident impact on BVOC emissions. The observed seasonal variation followed the general behavior predicted by the MEGANv2.1 model, with lower emissions associated with cooler conditions, but the magnitude of the emission decrease was greater than expected indicating driving variables are missing from the model. Emission factors, representing the emission expected for a Leaf Area Index of 5 at a temperature of 30 °C and PAR of 1500 μmol m −2 s −1 , during the peak growing season for this site were 0.008 mg m −2 h −1 for α-pinene and 3.3 mg m −2 h −1 for isoprene. The isoprene emission factor is similar to the value (3.6 mg m −2 h −1 ) for this location in the MEGANv2.1 global biogenic emission model. A second bamboo plantation, containing Moso bamboo ( Phyllostachys heterocycla ), was investigated and found to have similar isoprene and monoterpene emission rates as Lei bamboo forest. The emission data obtained in this study are the first canopy-scale flux measurements reported for bamboo plantations and demonstrate the potential importance of bamboo isoprene emissions for regional ozone and organic aerosol production.

Background concentrations and fluxes of atmospheric ammonia over a deciduous forest

Natural ammonia (NH3) fluxes over terrestrial ecosystems are difficult to quantify because most measurement sites are influenced by nearby anthropogenic NH3 sources. Furthermore, measuring the net exchange of NH3 is challenging due to bi-directionality of the flux and the high reactivity of NH3. In this study, we present two months of half-hourly NH3 fluxes and concentrations measured using a Relaxed Eddy Accumulation system during late summer and fall 2013 above a remote forest site in the central Midwest in USA. Supplementary nitric acid (HNO3) flux and size-resolved aerosol-N measurements are used to quantify the phase-partitioning and diagnose possible causes of upward NH3 fluxes. Data from 2013 combined with previous NH3 studies at this site indicate a seasonal background NH3 concentration of spring: 0.92±0.95, summer: 0.30±0.39, autumn: 0.20±0.26, and winter: 0.26±0.1μgNH3-Nm−3. Air mass back trajectories from the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model did not indicate any directional bias in the measured NH3 concentration confirming the absence of strong local sources of NH3. The NH3 fluxes were mainly upward (emission) and had a magnitude of up to 0.11μgNH3-Nm−2s−1. The wetness of the forest surfaces (assessed using a proxy of time since precipitation) was found to be crucial in controlling both deposition and emission of atmospheric NH3. Size resolved aerosol concentrations (of NH4+, NO3−, Cl− and SO42−) indicated that the aerosol and gas phase concentration of ammonia/ammonium were of similar magnitude but that the aerosol phase typically dominated. Nitric acid flux measurements showed periods of apparent upward fluxes, but they were not preferentially associated with upwards NH3 fluxes indicating they may not have a common causality.

Mercury vapor air–surface exchange measured by collocated micrometeorological and enclosure methods – Part II: Bias and uncertainty analysis

Abstract. Dynamic flux chambers (DFCs) and micrometeorological (MM) methods are extensively deployed for gauging air–surface Hg0 gas exchange. However, a systematic evaluation of the precision of the contemporary Hg0 flux quantification methods is not available. In this study, the uncertainty in Hg0 flux measured by the relaxed eddy accumulation (REA) method, the aerodynamic gradient method (AGM), the modified Bowen ratio (MBR) method, as well as DFC of traditional (TDFC) and novel (NDFC) designs, are assessed using a robust data set from two field intercomparison campaigns. The absolute precision in Hg0 concentration difference (ΔC) measurements is estimated at 0.064 ng m−3 for the gradient-based MBR and AGM systems. For the REA system, the parameter is Hg0 concentration (C) dependent at 0.069 + 0.022C. During the campaigns, 57 and 62 % of the individual vertical gradient measurements are found to be significantly different from 0, while for the REA technique, the percentage of significant observations is lower. For the chambers, non-significant fluxes are confined to a few night-time periods with varying ambient Hg$^{0}$ concentrations. Relative bias for DFC-derived fluxes is estimated to be ~ ±10, and ~ 85% of the flux bias is within ±2 ng m−2 h−1 in absolute terms. The DFC flux bias follows a diurnal cycle, which is largely affected by the forced temperature and irradiation bias in the chambers. Due to contrasting prevailing micrometeorological conditions, the relative uncertainty (median) in turbulent exchange parameters differs by nearly a factor of 2 between the campaigns, while that in ΔC measurement is fairly consistent. The estimated flux uncertainties for the triad of MM techniques are 16–27, 12–23 and 19–31% (interquartile range) for the AGM, MBR and REA methods, respectively. This study indicates that flux-gradient-based techniques (MBR and AGM) are preferable to REA in quantifying Hg0 flux over ecosystems with low vegetation height. A limitation of all Hg0 flux measurement systems investigated is their inability to obtain synchronous samples for the calculation of ΔC. This reduces the precision of flux quantification, particularly in the MM systems under non-stationarity of ambient Hg0 concentration. For future applications, it is recommended to accomplish ΔC derivation from simultaneous collected samples.

A high‐frequency response relaxed eddy accumulation flux measurement system for sampling short‐lived biogenic volatile organic compounds

AbstractA second‐generation relaxed eddy accumulation system was built and tested with the capability to measure vertical biogenic volatile organic compound (VOC) fluxes at levels as low as 10 µg C m−2 hr−1. The system features a continuous, integrated gas‐phase ozone removal procedure to allow for the measurement of highly reactive species such as β‐caryophyllene and polar terpenoids such as linalool. A two‐component internal standard continuously added to the accumulators was used to correct for switching‐induced volumetric errors and as a check on VOC losses exceeding accumulator tube adsorption limits. In addition, the internal standards were used to demonstrate that accumulators quickly return to target flow rates at segregation valve switching frequencies up to at least 0.8 Hz. The system was able to measure daytime hourly fluxes of individual biogenic VOC including oxygenated terpenoids, monoterpenes, and sesquiterpenes.

Nocturnal isoprene emission from mature trees and diurnal acceleration of isoprene oxidation rates near Quercus serrata Thunb. leaves

Quercus serrata Thunb. ex Murray is a widespread deciduous oak in China, the Korean Peninsula, and Japan, and a strong isoprene emitter. Establishing accurate inventories of this species and estimating net carbon budgets, including biogenic volatile organic compounds (BVOC), necessitates detailed evaluation of BVOC emission and oxidation characteristics. Emissions of isoprene, the most abundant BVOC, presumably contribute to atmospheric chemistry through the formation of photochemical oxidants and secondary organic aerosols. We built an isoprene flux monitoring system to simultaneously reveal characteristics of the flux and fate of isoprene at multiple locations in Q. serrata forests. We used proton transfer reaction mass spectrometry (PTR-MS) and an automated closed chamber to measure isoprene emissions from soil and leaves in a warm-temperate Q. serrata forest. We used a relaxed eddy accumulation system with PTR-MS to simultaneously measure the canopy flux. In continuous foliage chamber measurements, we observed daily variations of isoprene emissions and continuous nocturnal emissions from leaves. Nocturnal emissions exceeded 25 % of total daily leaf emissions and were relatively high at sunset and low at sunrise. These results suggest that nocturnal emissions from mature trees may not be negligible. When leaf emissions were high in the daytime, the canopy isoprene flux tended to plateau at an upper limit. Observations of isoprene concentrations and gradients suggest that the plateau was caused by acceleration of isoprene oxidation, and sequential formation of secondary organic aerosols may occur near the leaf just after emission. Elucidation of these linkages may require continuous field measurements with a simultaneous multi-flux monitoring system.

Measurement of methane flux over an evergreen coniferous forest canopy using a relaxed eddy accumulation system with tuneable diode laser spectroscopy detection

Very few studies have conducted long-term observations of methane (CH4) flux over forest canopies. In this study, we continuously measured CH4 fluxes over an evergreen coniferous (Japanese cypress) forest canopy throughout 1 year, using a micrometeorological relaxed eddy accumulation (REA) system with tuneable diode laser spectroscopy (TDLS) detection. The Japanese cypress forest, which is a common forest type in warm-temperate Asian monsoon regions with a wet summer, switched seasonally between a sink and source of CH4 probably because of competition by methanogens and methanotrophs, which are both influenced by soil conditions (e.g., soil temperature and soil moisture). At hourly to daily timescales, the CH4 fluxes were sensitive to rainfall, probably because CH4 emission increased and/or absorption decreased during and after rainfall. The observed canopy-scale fluxes showed complex behaviours beyond those expected from previous plot-scale measurements and the CH4 fluxes changed from sink to source and vice versa.

Characteristics of the flux of isoprene and its oxidation products in an urban area

[1] We measured the mixing ratios and fluxes of isoprene and its oxidation products, methacrolein (MACR) and methyl vinyl ketone (MVK) from a tall flux tower in metropolitan Houston, Texas, during summertime using a relaxed eddy accumulation (REA) system combined with a dual-channel GC-FID instrument. We show that isoprene was affected dominantly by biogenic emission sources during daytime, but also that tail-pipe emission sources (alongside 2-pentenes) are contributing during the rush hours and at night. The observed daytime mixing ratios of isoprene were much lower than over forested areas due to a comparatively low density of isoprene emitting trees in the tower's footprint area. Daytime isoprene fluxes displayed the expected light and temperature driven emission characteristics, and a detailed isoprene emissions model explained average fluxes fairly well. Our investigation of isoprene's oxidation products MACR and MVK showed that both anthropogenic and isoprene oxidation sources exist for MACR, while MVK was strongly dominated by isoprene oxidation between its emission and sampling points due to presumably high local OH radical concentrations. While biogenic emission modeling appears to work well for this urban environment when supplied with accurate input data, emission inventories used for ozone modeling may need to consider tailpipe isoprene (and MACR) emissions to properly account for urban concentrations, particularly higher morning abundances.

A relaxed eddy accumulation system for measuring vertical fluxes of nitrous acid

Abstract. A relaxed eddy accumulation (REA) system combined with a nitrous acid (HONO) analyzer was developed to measure atmospheric HONO vertical fluxes. The system consists of three major components: (1) a fast-response sonic anemometer measuring both vertical wind velocity and air temperature, (2) a fast-response controlling unit separating air motions into updraft and downdraft samplers by the sign of vertical wind velocity, and (3) a highly sensitive HONO analyzer based on aqueous long path absorption photometry that measures HONO concentrations in the updrafts and downdrafts. A dynamic velocity threshold (±0.5σw, where σw is a standard deviation of the vertical wind velocity) was used for valve switching determined by the running means and standard deviations of the vertical wind velocity. Using measured temperature as a tracer and the average values from two field deployments, the flux proportionality coefficient, β, was determined to be 0.42 ± 0.02, in good agreement with the theoretical estimation. The REA system was deployed in two ground-based field studies. In the California Research at the Nexus of Air Quality and Climate Change (CalNex) study in Bakersfield, California in summer 2010, measured HONO fluxes appeared to be upward during the day and were close to zero at night. The upward HONO flux was highly correlated to the product of NO2 and solar radiation. During the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX 2009) at Blodgett Forest, California in July 2009, the overall HONO fluxes were small in magnitude and were close to zero. Causes for the different HONO fluxes in the two different environments are briefly discussed.

Flux measurements of volatile organic compounds by the relaxed eddy accumulation method combined with a GC-FID system in urban Houston, Texas

A tall tower flux measurement setup was established in metropolitan Houston, Texas, to measure trace gas fluxes from emission sources in the urban surface layer. We describe a new relaxed eddy accumulation (REA) system combined with a dual-channel GC-FID used for VOC flux measurements, focusing on benzene, toluene, ethylbenzene and xylenes (BTEX) results. Ambient air sampled from 60 m above the ground next to a sonic anemometer was subsampled by a membrane pump and pushed into an REA valve system with two Teflon bag reservoirs, then transferred to two preconcentration units for thermal desorption. We discuss the performance of our system and the selected BTEX measurement results using approximately 8 weeks of data (May 22–July 22, 2008), presenting diurnal variations of concentrations and fluxes of these traffic tracers. The measured values exhibited diurnal cycles with dominant morning and midday peaks during weekdays related to rush hour traffic and additional weekday daytime toluene and xylenes emissions. Local evaporative emissions, likely from solvent usage, significantly contributed to the measured fluxes. We upscaled measured emissions to the county level using a high resolution land cover data set and compared the results with EPA’s National Emission Inventory (NEI).

Dynamics of ammonia exchange with cut grassland: synthesis of results and conclusions of the GRAMINAE Integrated Experiment

Abstract. Improved data on biosphere-atmosphere exchange are fundamental to understanding the production and fate of ammonia (NH3) in the atmosphere. The GRAMINAE Integrated Experiment combined novel measurement and modelling approaches to provide the most comprehensive analysis of the interactions to date. Major inter-comparisons of micrometeorological parameters and NH3 flux measurements using the aerodynamic gradient method and relaxed eddy accumulation (REA) were conducted. These showed close agreement, though the REA systems proved insufficiently precise to investigate vertical flux divergence. Grassland management had a large effect on fluxes: emissions increased after grass cutting (−50 to 700 ng m−2 s−1 NH3) and after N-fertilization (0 to 3800 ng m−2 s−1) compared with before the cut (−60 to 40 ng m−2 s−1). Effects of advection and air chemistry were investigated using horizontal NH3 profiles, acid gas and particle flux measurements. Inverse modelling of NH3 emission from an experimental farm agreed closely with inventory estimates, while advection errors were used to correct measured grassland fluxes. Advection effects were caused both by the farm and by emissions from the field, with an inverse dispersion-deposition model providing a reliable new approach to estimate net NH3 fluxes. Effects of aerosol chemistry on net NH3 fluxes were small, while the measurements allowed NH3-induced particle growth rates to be calculated and aerosol fluxes to be corrected. Bioassays estimated the emission potential Γ = [NH4+]/[H+] for different plant pools, with the apoplast having the smallest values (30–1000). The main within-canopy sources of NH3 emission appeared to be leaf litter and the soil surface, with Γ up to 3 million and 300 000, respectively. Cuvette and within-canopy analyses confirmed the role of leaf litter NH3 emission, which, prior to cutting, was mostly recaptured within the canopy. Measured ammonia fluxes were compared with three models: an ecosystem model (PaSim), a soil vegetation atmosphere transfer model (SURFATM-NH3) and a dynamic leaf chemistry model (DCC model). The different models each reproduced the main temporal dynamics in the flux, highlighting the importance of canopy temperature dynamics (Surfatm-NH3), interactions with ecosystem nitrogen cycling (PaSim) and the role of leaf surface chemistry (DCC model). Overall, net above-canopy fluxes were mostly determined by stomatal and cuticular uptake (before the cut), leaf litter emissions (after the cut) and fertilizer and litter emissions (after fertilization). The dynamics of ammonia emission from leaf litter are identified as a priority for future research.