Above-canopy Fluxes Research Articles

Oxidation of Volatile Organic Compounds as the Major Source of Formic Acid in a Mixed Forest Canopy.

Formic acid (HCOOH) is among the most abundant carboxylic acids in the atmosphere, but its budget is poorly understood. We present eddy flux, vertical gradient, and soil chamber measurements from a mixed forest and apply the data to better constrain HCOOH source/sink pathways. While the cumulative above-canopy flux was downward, HCOOH exchange was bidirectional, with extended periods of net upward and downward flux. Net above-canopy fluxes were mostly upward during warmer/drier periods. The implied gross canopy HCOOH source corresponds to 3% and 38% of observed isoprene and monoterpene carbon emissions and is 15× underestimated in a state-of-science atmospheric model (GEOS-Chem). Gradient and soil chamber measurements identify the canopy layer as the controlling source of HCOOH or its precursors to the forest environment; below-canopy sources were minor. A correlation analysis using an ensemble of marker volatile organic compounds suggests that secondary formation, not direct emission, is the major source driving ambient HCOOH.

Below-canopy contributions to ecosystem CO2 fluxes in a temperate mixed forest in Switzerland

Forests are structurally complex ecosystems with several canopy layers, each with distinct functional properties contributing differently to the net ecosystem CO2 exchange. A large number of studies have addressed ecosystem-scale net CO2 fluxes in forests, but only few studies have investigated the role of the forest understory. The goal of this study was to quantify the below-canopy contribution to ecosystem CO2 fluxes of a mixed deciduous forest in Switzerland. Below- and above-canopy eddy-covariance (EC) measurements were made continuously over two years, complemented by within canopy wind and CO2 concentration profile measurements.On an annual basis, the below-canopy fluxes indicated a net carbon source dominated by soil respiration, while the above-canopy fluxes were dominated by tree photosynthesis leading to a net carbon sink. Below-canopy fluxes showed a net carbon sink only in spring, with the early emergence of understory plants before overstory canopy leaf-out. Below-canopy respiration partitioned from the EC measurements agreed well with previous chamber-based soil respiration measurements. However, below- and above-canopy fluxes became decoupled under full canopy closure, thus leading to unaccounted below-canopy fluxes when measured only above the canopy. Wind and CO2 concentration profile measurements supported this finding. Decoupling was independent of low turbulence conditions and decoupled periods could be identified using the relationship between the below- and above-canopy standard deviation in vertical wind velocity. A decoupling correction was applied to the above-canopy measurements during decoupled periods and corrected annual net ecosystem production (NEP) agreed well with independent estimates from biomass inventory combined with models. Overall, the below-canopy fluxes contributed 79% to annual ecosystem respiration, but only 9% to annual ecosystem photosynthesis. The decoupling correction reduced annual NEP for the site from about 760 to 330gCm−2yr−1. Our results showed that below-canopy EC measurements are essential in this mixed deciduous forest, and likely in many other forests, to fully understand the carbon dynamics within structurally complex ecosystems.

Emissions of biogenic volatile organic compounds and subsequent formation of secondary organic aerosols in a <I>Larix kaempferi</I> forest

Abstract. We conducted simultaneous measurements of concentrations and above-canopy fluxes of isoprene and α-pinene, along with their oxidation products in aerosols in a Larix kaempferi (Japanese larch) forest in summer 2012. Vertical profiles of isoprene showed the maximum concentration near the forest floor with a peak around noon, whereas oxidation products of isoprene, i.e., methacrolein (MACR) and methyl vinyl ketone (MVK), showed higher concentrations near the canopy level of the forest. The vertical profile suggests large emissions of isoprene near the forest floor, likely due to Dryopteris crassirhizoma (a fern species), and the subsequent reaction within the canopy. The concentrations of α-pinene also showed highest values near the forest floor, with maximums in the early morning and late afternoon. The vertical profiles of α-pinene suggest its large emissions from soil and litter in addition to emissions from L. kaempferi leaves at the forest site. Isoprene and its oxidation products in aerosols exhibited similar diurnal variations within the forest canopy, providing evidence of secondary organic aerosol (SOA) formation via oxidation of isoprene most likely emitted from the forest floor. Although high abundance of α-pinene was observed in the morning, its oxidation products in aerosols showed peaks in daytime, due to a time lag between the emission and atmospheric reactions of α-pinene to form SOA. Positive matrix factorization (PMF) analysis indicated that anthropogenic influence is the most important factor contributing to the elevated concentrations of molecular oxidation products of isoprene- (> 64 %) and α-pinene-derived SOA (> 57 %). The combination of the measured fluxes and vertical profiles of biogenic volatile organic compounds (BVOCs) suggests that the inflow of anthropogenic precursors/aerosols likely enhanced the formation of both isoprene SOA and α-pinene SOA within the forest canopy even when the BVOC flux was relatively low. This study highlights the importance of intra-canopy processes that promote biogenic SOA formation in the presence of significant inflow of oxidants together with anthropogenic aerosols and their precursors.

Volatile organic compound emissions from Miscanthus and short rotation coppice willow bioenergy crops

Miscanthus × giganteus and short rotation coppice (SRC) willow (Salix spp.) are increasingly important bioenergy crops. Above-canopy fluxes and mixing ratios of volatile organic compounds (VOCs) were measured in summer for the two crops at a site near Lincoln, UK, by proton transfer reaction mass spectrometry (PTR-MS) and virtual disjunct eddy covariance. The isoprene emission rate above willow peaked around midday at ∼1 mg m−2 h−1, equivalent to 20 μg gdw−1 h−1 normalised to 30 °C and 1000 μmol m−2 s−1 PAR, much greater than for conventional arable crops. Average midday peak isoprene mixing ratio was ∼1.4 ppbv. Acetone and acetic acid also showed small positive daytime fluxes. No measurable fluxes of VOCs were detected above the Miscanthus canopy. Differing isoprene emission rates between different bioenergy crops, and the crops or vegetation cover they may replace, means the impact on regional air quality should be taken into consideration in bioenergy crop selection.

Contribution of leaf and needle litter to whole ecosystem BVOC fluxes

Biogenic volatile organic compound (BVOC) emissions come from a variety of sources, including living above-ground foliar biomass and microbial decomposition of dead organic matter at the soil surface (litter and soil organic matter). There are, however, few reports that quantify the contributions of each component. Measurements of emission fluxes are now made above the vegetation canopy, but these include contributions from all sources. BVOC emission models currently include detailed parameterization of the emissions from foliar biomass but do not have an equally descriptive treatment of emissions from litter or other sources. We present here results of laboratory and field experiments to characterize the major parameters that control emissions from litter.Litter emissions are exponentially dependent on temperature. The moisture content of the litter plays a minor role, except during and immediately following rain events. The percentage of carbon readily available for microbial and other decomposition processes decreases with litter age. These 3 variables are combined in a model to explain over 50% of the variance of individual BVOC emission fluxes measured. The modeled results of litter emissions were compared with above-canopy fluxes. Litter emissions constituted less than 1% of above-canopy emissions for all BVOCs measured. A comparison of terpene oil pools in litter and live needles with above-canopy fluxes suggests that there may be another canopy terpene source in addition to needle storage or that some terpene emissions may be light-dependent.Ground enclosure measurements indicated that compensation point concentrations of BVOCs (equilibrium between BVOC emission and deposition) were usually higher than ambient air concentrations at the temperature of the measurements.

Methanol and other VOC fluxes from a Danish beech forest during late springtime

In-canopy mixing ratio gradients and above-canopy fluxes of several volatile organic compounds (VOCs) were measured using a commercial proton transfer reaction mass spectrometer (PTR-MS) in a European beech (Fagus sylvatica) forest in Denmark. Fluxes of methanol were bidirectional: Emission occurred during both day and night with highest fluxes (0.2 mg C m−2 h−1) during a warm period; deposition occurred dominantly at daytime. Confirming previous branch-level measurements on beech, the forest’s monoterpene emissions (0–0.5 mg C m−2 h−1), and in-canopy mixing ratios showed a diurnal cycle consistent with light-dependent emissions; a result contrasting temperature-only driven emissions of most conifer species. Also emitted was acetone, but only at ambient temperatures exceeding 20°C. Slow deposition dominated at lower temperatures. Our in-canopy gradient measurements contrast with earlier results from tropical and pine forest ecosystems in that they did not show this beech ecosystem to be a strong sink for oxygenated VOCs (OVOCs). Instead, their gradients were flat and only small deposition velocities (<0.2 cm s−1) were observed to the onsite soil. However, as methanol soil uptake was consistent and possibly related to soil moisture, more measurements are needed to evaluate its soil sink strength. In turn, as canopy scale fluxes are net fluxes with stomatal emissions from photosynthesizing leaves potentially affecting non-stomatal oxygenated VOC uptake, only independent, controlled laboratory experiments may be successful in separating gross fluxes.

Fluxes and concentrations of volatile organic compounds from a South-East Asian tropical rainforest

Abstract. As part of the OP3 field study of rainforest atmospheric chemistry, above-canopy fluxes of isoprene, monoterpenes and oxygenated volatile organic compounds were made by virtual disjunct eddy covariance from a South-East Asian tropical rainforest in Malaysia. Approximately 500 hours of flux data were collected over 48 days in April–May and June–July 2008. Isoprene was the dominant non-methane hydrocarbon emitted from the forest, accounting for 80% (as carbon) of the measured emission of reactive carbon fluxes. Total monoterpene emissions accounted for 18% of the measured reactive carbon flux. There was no evidence for nocturnal monoterpene emissions and during the day their flux rate was dependent on both light and temperature. The oxygenated compounds, including methanol, acetone and acetaldehyde, contributed less than 2% of the total measured reactive carbon flux. The sum of the VOC fluxes measured represents a 0.4% loss of daytime assimilated carbon by the canopy, but atmospheric chemistry box modelling suggests that most (90%) of this reactive carbon is returned back to the canopy by wet and dry deposition following chemical transformation. The emission rates of isoprene and monoterpenes, normalised to 30 °C and 1000 μmol m−2 s−1 PAR, were 1.6 mg m−2 h−1 and 0.46mg m−2 h−1 respectively, which was 4 and 1.8 times lower respectively than the default value for tropical forests in the widely-used MEGAN model of biogenic VOC emissions. This highlights the need for more direct canopy-scale flux measurements of VOCs from the world's tropical forests.

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.

The effect of canopy gaps on subcanopy ventilation and scalar fluxes in a tropical forest

Forest gaps may provide conduits that preferentially vent moist, CO 2-rich subcanopy air to the atmosphere. We measured the above-canopy fluxes of momentum, sensible heat (H), CO 2, and water vapor (E t), and the vertical profiles of CO 2 and water vapor, from two 67-m meteorological towers in a selectively logged Brazilian rainforest. The logging removed ∼3.5 trees ha −1, and increased the incidence of gaps by a factor of 3 over nearby undisturbed forest. One tower was located in an intact patch of forest within the selectively logged area; the other was 400 m upwind in a large gap created by the logging. During daytime the subcanopy air in the intact patch of forest had more CO 2, more water vapor, and was cooler than the air at comparable altitudes in the gap. Meanwhile, the daytime CO 2 flux was less negative (reduced CO 2 uptake) above the gap than above the intact forest, the daytime E t was greater above the gap than the intact forest, and the daytime H was lower above the gap than the intact forest. These patterns cannot be explained fully by the local loss of canopy gas exchange in the gap, but are consistent with the horizontal transport into the gap, and subsequent vertical transport out of the gap, of high-CO 2, humid, cool air from the forest understory. The understory was drier and warmer during daytime after the logging, which would be expected to increase flammability. Further measurement and modeling efforts are needed to better understand the effect of canopy gaps on the local CO 2 and energy exchange, as well as the flux footprint.

Inferring profiles of energy fluxes within a soybean canopy using Lagrangian analysis

Two one-dimensional Lagrangian analytical dispersion models were used to estimate scalar sources, sinks and fluxes within a soybean canopy from concentration profile measurements. The two models were also used to separate soil evaporation and crop transpiration in terms of the inverse analysis. The results indicate that these two models produced similar source/sink distributions most of the time, but the values of within-canopy source distributions are dependent on the parameterizations of the turbulence statistics. The performance of these two models was similar and comparable in terms of the cumulative fluxes at the top of the canopy. The inferred above-canopy fluxes are sensitive to the parameterization of turbulence statistics, but not to the choice of model. The inferred above-canopy fluxes were in agreement with eddy covariance measurements for two turbulence schemes, while overestimated (1.8–2.6 times) when using a third turbulence scheme. Results also showed that the predicted ratio of soil evaporation to total evapotranspiration is coincident with the expected trend for different growth stages of soybean, but the value is highly dependent on the choice of turbulence statistics. The variation of daily ratio of soil evaporation to total evapotranspiration with leaf area index (LAI) from the inverse analysis is in rough agreement with that estimated from an evaporation model.

Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature)

Abstract. Reactive gases and aerosols are produced by terrestrial ecosystems, processed within plant canopies, and can then be emitted into the above-canopy atmosphere. Estimates of the above-canopy fluxes are needed for quantitative earth system studies and assessments of past, present and future air quality and climate. The Model of Emissions of Gases and Aerosols from Nature (MEGAN) is described and used to quantify net terrestrial biosphere emission of isoprene into the atmosphere. MEGAN is designed for both global and regional emission modeling and has global coverage with ~1 km2 spatial resolution. Field and laboratory investigations of the processes controlling isoprene emission are described and data available for model development and evaluation are summarized. The factors controlling isoprene emissions include biological, physical and chemical driving variables. MEGAN driving variables are derived from models and satellite and ground observations. Tropical broadleaf trees contribute almost half of the estimated global annual isoprene emission due to their relatively high emission factors and because they are often exposed to conditions that are conducive for isoprene emission. The remaining flux is primarily from shrubs which have a widespread distribution. The annual global isoprene emission estimated with MEGAN ranges from about 500 to 750 Tg isoprene (440 to 660 Tg carbon) depending on the driving variables which include temperature, solar radiation, Leaf Area Index, and plant functional type. The global annual isoprene emission estimated using the standard driving variables is ~600 Tg isoprene. Differences in driving variables result in emission estimates that differ by more than a factor of three for specific times and locations. It is difficult to evaluate isoprene emission estimates using the concentration distributions simulated using chemistry and transport models, due to the substantial uncertainties in other model components, but at least some global models produce reasonable results when using isoprene emission distributions similar to MEGAN estimates. In addition, comparison with isoprene emissions estimated from satellite formaldehyde observations indicates reasonable agreement. The sensitivity of isoprene emissions to earth system changes (e.g., climate and land-use) demonstrates the potential for large future changes in emissions. Using temperature distributions simulated by global climate models for year 2100, MEGAN estimates that isoprene emissions increase by more than a factor of two. This is considerably greater than previous estimates and additional observations are needed to evaluate and improve the methods used to predict future isoprene emissions.

Canopy CO 2 assimilation, energy balance, and water use efficiency of an alfalfa crop before and after cutting

Canopy photosynthesis, energy balance and evapotranspiration (ET), and water use efficiency of an alfalfa ( Medicago sativa L.) crop were followed by measuring the above-canopy fluxes with the Bowen ratio/energy balance/CO 2 gradient technique and correcting for CO 2 released by the soil and roots via respiration. Measurements were taken late in the season, just before the last cutting, after cutting, during regrowth, and during the initial senescence phase near the end of autumn. Before cutting, canopy photosynthesis rate was similar to those for other high yielding crop species and ET was very close to reference ET rate, with most of the energy for evaporation coming from net radiation and a small part from sensible heat. Net canopy CO 2 assimilation rate ( A c) reached 1.7 mg (39 μmol) CO 2 m −2 s −1 around noon on sunny days and up to 2.4 mg (55 μmol) CO 2 m −2 s −1 in periods between clouds on a windy and relatively cool day. A c increased with increase in photosynthetic active radiation up to 1700 μmol m −2 s −1 with no sign of light saturation. Immediately after cutting, ET was reduced markedly; A c was reduced even more drastically but remained positive. Energy balance changed so that only a minor fraction of the net radiation was used for ET, and the major part was dissipated as sensible heat and as storage heat flux into the soil. Evapotranspiration dropped to about 1 mm per day, but started to increase after cutting over a period of about 2 weeks. A c also increased and reached 84% of pre-cutting value about 3 weeks after cutting. Seven diurnal curves of A c, ET, and parameters associated with energy fluxes are presented, spanning pre-cutting, immediately after cutting, regrowth and the start of senescence. Before cutting, water use efficiency of the alfalfa normalized for the evaporative demand was between 49.1 and 52.8 g CO 2 m −2 per day (approximately 10.5–11.3 g dry matter m −2 per day), declined to 7.5 g CO 2 m −2 per day after cutting, but increased afterwards continuously, reaching 46.3 g m −2 per day 3 weeks later, before declining under less favorable growing conditions as the end of autumn approached. Under favorable conditions, alfalfa water use efficiency appears to be as high as most other crops when the cost in assimilates for symbiotic nitrogen fixation and mycorrhizal fungi, larger proportion of root biomass, and higher protein content are taken into account. Its reputation as a water spender is not deserved.

Below-canopy and soil CO 2 fluxes in a ponderosa pine forest

Below-canopy eddy covariance measurements of CO 2 flux ( F cb) and soil surface CO 2 flux measurements ( F s) were made seasonally in a ponderosa pine forest in central Oregon in 1996 and 1997. The forest ecosystem has a very open canopy, and it is subject to drought and high vapor pressure deficits in summer. Below-canopy flux measurements in March, May, and August 1997 showed increasing effluxes from the forest floor as soils warmed. In July 1996, daytime F cb measurements appeared to have been influenced by photosynthetic uptake of CO 2 by ground vegetation. We did not see a similar diurnal trend in F cb data in August 1997, probably because photosynthesis may have decreased with senescence of ∼1/3 of the pine canopy and the herbaceous species. On 4 days in August 1997, the mean nocturnal F s (2.6 ± 0.08 μmol m −2 s −1) was lower than nocturnal F cb (3.5 ± 0.28 μmol m −2 s −1) by 26%, and daytime F s was lower than nocturnal F cb by 18%, possibly because F cb includes respiration by understory and the lower portions of trees. The mean nocturnal NEE calculated from above-canopy flux and storage in the canopy airspace ( F ca + F stor) at this time was 2.8 ± 0.40 μmol m −2 s −1, 23% lower than ecosystem respiration calculated from chamber measurements on soils, wood, and foliage. The largest difference was observed on a more a turbulent night ( u * = 0.30 m s −1) when F ca + F stor was even significantly less than F cb and F s. Our hypothesis is that under calm conditions (e.g. u * < 0.15 m s −1 as observed on three of the nights), F ca is negligible and has no impact on the CO 2 budget. Under weak wind conditions (e.g. u * = 0.30 m s −1), F ca begins to become significant and fluxes missed by the above-canopy eddy correlation system degrade the CO 2 budget. Under windy conditions, the above-canopy eddy correlation measurement is a good approximation and the CO 2 budget improves again. Below-canopy flux measurements provided useful temporal information for understanding seasonal differences in diel patterns, while the chambers allowed us to characterize spatial variation in CO 2 fluxes. It is important to measure below-canopy fluxes along with above-canopy fluxes throughout the year to understand CO 2 exchange components and annual contributions to the carbon budget of open canopy forest systems.

Energy budgets and resistances to energy transport in sparsely vegetated rangeland

Partitioning available energy between plants and bare soil in sparsely vegetated rangelands will allow hydrologists and others to gain a greater understanding of water use by native vegetation, especially phreatophytes. Standard methods of conducting energy budget studies result in measurements of latent and sensible heat fluxes above the plant canopy which therefore include the energy fluxes from both the canopy and the soil. One-dimensional theoretical numerical models have been proposed recently for the partitioning of energy in sparse crops. Bowen ratio and other micrometeorological data collected over phreatophytes growing in areas of shallow ground water in central Nevada were used to evaluate the feasibility of using these models, which are based on surface and within-canopy aerodynamic resistances, to determine heat and water vapor transport in sparsely vegetated rangelands. The models appear to provide reasonably good estimates of sensible heat flux from the soil and latent heat flux from the canopy. Estimates of latent heat flux from the soil were less satisfactory. Sensible heat flux from the canopy was not well predicted by the present resistance formulations. Also, estimates of total above-canopy fluxes were not satisfactory when using a single value for above-canopy bulk aerodynamic resistance.