Non-stomatal Sink Research Articles

Ozone deposition impact assessments for forest canopies require accurate ozone flux partitioning on diurnal timescales

Abstract. Dry deposition is an important sink of tropospheric ozone that affects surface concentrations and impacts crop yields, the land carbon sink, and the terrestrial water cycle. Dry deposition pathways include plant uptake via stomata and non-stomatal removal by soils, leaf surfaces, and chemical reactions. Observational studies indicate that ozone deposition exhibits substantial temporal variability that is not reproduced by atmospheric chemistry models due to a simplified representation of vegetation uptake processes in these models. In this study, we explore the importance of stomatal and non-stomatal uptake processes in driving ozone dry deposition variability on diurnal to seasonal timescales. Specifically, we compare two land surface ozone uptake parameterizations – a commonly applied big leaf parameterization (W89; Wesely, 1989) and a multi-layer model (MLC-CHEM) constrained with observations – to multi-year ozone flux observations at two European measurement sites (Ispra, Italy, and Hyytiälä, Finland). We find that W89 cannot reproduce the diurnal cycle in ozone deposition due to a misrepresentation of stomatal and non-stomatal sinks at our two study sites, while MLC-CHEM accurately reproduces the different sink pathways. Evaluation of non-stomatal uptake further corroborates the previously found important roles of wet leaf uptake in the morning under humid conditions and soil uptake during warm conditions. The misrepresentation of stomatal versus non-stomatal uptake in W89 results in an overestimation of growing season cumulative ozone uptake (CUO), a metric for assessments of vegetation ozone damage, by 18 % (Ispra) and 28 % (Hyytiälä), while MLC-CHEM reproduces CUO within 7 % of the observation-inferred values. Our results indicate the need to accurately describe the partitioning of the ozone atmosphere–biosphere flux over the in-canopy stomatal and non-stomatal loss pathways to provide more confidence in atmospheric chemistry model simulations of surface ozone mixing ratios and deposition fluxes for large-scale vegetation ozone impact assessments.

Ozone flux in plant ecosystems: new opportunities for long-term monitoring networks to deliver ozone-risk assessments.

Ozone (O3) is a photochemically formed reactive gas responsible for a decreasing carbon assimilation in plant ecosystems. Present in the atmosphere in trace concentrations (less than 100ppbv), this molecule is capable of inhibiting carbon assimilation in agricultural and forest ecosystems. Ozone-risk assessments are typically based on manipulative experiments. Present regulations regarding critical ozone levels are mostly based on an estimated accumulated exposure over a given threshold concentration. There is however a scientific consensus over flux estimates being more accurate, because they include plant physiology analyses and different environmental parameters that control the uptake-that is, not just the exposure-of O3. While O3 is a lot more difficult to measure than other non-reactive greenhouse gases, UV-based and chemiluminescence sensors enable precise and fast measurements and are therefore highly desirable for eddy covariance studies. Using micrometeorological techniques in association with latent heat flux measurements in the field allows for the partition of ozone fluxes into the stomatal and non-stomatal sinks along the soil-plant continuum. Long-term eddy covariance measurements represent a key opportunity in estimating carbon assimilation at high-temporal resolutions, in an effort to study the effect of climate change on photosynthetic mechanisms. Our aim in this work is to describe potential of O3 flux measurement at the canopy level for ozone-risk assessment in established long-term monitoring networks.

Partitioning ozone fluxes between canopy and forest floor by measurements and a multi-layer model

Ozone uptake by plant leaves is essential to studies investigating atmospheric air pollution and plant injury. A major challenge to these investigations is the up-scaling of leaf-level stomatal processes to the ecosystem level, the accounting for forest floor ozone removal mechanics, and resolving the numerous pathways responsible for non-stomatal ozone removal within the canopy sublayer. To progress on the first two and offer constraints on the third, the O3 fluxes above and within a boreal Scots pine forest in Southern Finland are explored using a combination of two-level eddy covariance fluxes and detailed within-canopy concentration profiles. The interpretation of the results is aided by a multi-layer canopy model (MLM), which couples radiation attenuation and turbulent transport within the canopy volume with leaf-level gas exchange, photosynthesis, and stomatal conductance. Validation of the MLM against measured turbulent CO2 and H2O fluxes within and above the canopy, as well as their concomitant mean concentration profiles suggest that the stomatal pathway is reasonably quantified via the proposed MLM approach. The results show that the stomatal pathway alone can explain some 80% of the daytime dry-canopy ecosystem uptake of O3. The non-stomatal O3 uptake is largest during nighttime and early morning hours when between one third and half of it seems to originate from below the overstory canopy. During daytime, almost all the non-stomatal uptake occurs in the sub-canopy region. Sub-canopy and/or understory processes contribute between 25–30% (nighttime) and 35–45% (daytime) ecosystem O3 uptake. In sub-canopy, the non-stomatal component exceeds the stomatal by a factor of 2–4 during daytime. Finally, the MLM was used to study some of the potential non-stomatal pathways, including cuticular and soil uptake, forest floor uptake and disequilibrium between photochemical O3 production and a first-order kinetic chemical destruction mechanism. The results indicate that the likely location of the non-stomatal sink is in the lower trunk-space close the forest floor but the soil surface uptake is insignificant. According to the results, a bulk gas-phase disequilibrium between O3 production (assumed to vary linearly with light at a given level inside the canopy) and destruction (assumed to vary exponentially with mean air temperature) is a plausible explanation for non-stomatal O3 removal inside the canopy.

Ozone deposition into a boreal forest over a decade of observations: evaluating deposition partitioning and driving variables

Abstract. This study scrutinizes a decade-long series of ozone deposition measurements in a boreal forest in search for the signature and relevance of the different deposition processes. The canopy-level ozone flux measurements were analysed for deposition characteristics and partitioning into stomatal and non-stomatal fractions, with the main focus on growing season day-time data. Ten years of measurements enabled the analysis of ozone deposition variation at different time-scales, including daily to inter-annual variation as well as the dependence on environmental variables and concentration of biogenic volatile organic compounds (BVOC-s). Stomatal deposition was estimated by using multi-layer canopy dispersion and optimal stomatal control modelling from simultaneous carbon dioxide and water vapour flux measurements, non-stomatal was inferred as residual. Also, utilising the big-leaf assumption stomatal conductance was inferred from water vapour fluxes for dry canopy conditions. The total ozone deposition was highest during the peak growing season (4 mm s−1) and lowest during winter dormancy (1 mm s−1). During the course of the growing season the fraction of the non-stomatal deposition of ozone was determined to vary from 26 to 44% during day time, increasing from the start of the season until the end of the growing season. By using multi-variate analysis it was determined that day-time total ozone deposition was mainly driven by photosynthetic capacity of the canopy, vapour pressure deficit (VPD), photosynthetically active radiation and monoterpene concentration. The multi-variate linear model explained the high portion of ozone deposition variance on daily average level (R2 = 0.79). The explanatory power of the multi-variate model for ozone non-stomatal deposition was much lower (R2 = 0.38). The set of common environmental variables and terpene concentrations used in multivariate analysis were able to predict the observed average seasonal variation in total and non-stomatal deposition but failed to explain the inter-annual differences, suggesting that some still unknown mechanisms might be involved in determining the inter-annual variability. Model calculation was performed to evaluate the potential sink strength of the chemical reactions of ozone with sesquiterpenes in the canopy air space, which revealed that sesquiterpenes in typical amounts at the site were unlikely to cause significant ozone loss in canopy air space. The results clearly showed the importance of several non-stomatal removal mechanisms. Unknown chemical compounds or processes correlating with monoterpene concentrations, including potentially reactions at the surfaces, contribute to non-stomatal sink term.

Ozone deposition to an orange orchard: Partitioning between stomatal and non-stomatal sinks

Orange trees are widely cultivated in regions with high concentrations of tropospheric ozone. Citrus absorb ozone through their stomata and emit volatile organic compounds (VOC), which, together with soil emissions of NO, contribute to non-stomatal ozone removal. In a Valencia orange orchard in Exeter, California, we used fast sensors and eddy covariance to characterize water and ozone fluxes. We also measured meteorological parameters necessary to model other important sinks of ozone deposition. We present changes in magnitude of these ozone deposition sinks over the year in response to environmental parameters. Within the plant canopy, the orchard constitutes a sink for ozone, with non-stomatal ozone deposition larger than stomatal uptake. In particular, soil deposition and reactions between ozone, VOC and NO represented the major sinks of ozone. This research aims to help the development of metrics for ozone-risk assessment and advance our understanding of citrus in biosphere-atmosphere exchange.

Insights into ozone deposition patterns from decade-long ozone flux measurements over a mixed temperate forest

Long-term fluxes of ozone (O(3)) were measured over a mixed temperate forest using the aerodynamic gradient method. The long-term average O(3) flux (F) was -366 ng m(-2) s(-1) for the period 2000-2010, corresponding to an average O(3) concentration of 48 μg m(-3) and a deposition velocity v(d) of 9 mm s(-1). Average nocturnal ozone deposition amounted to -190 ng m(-2) s(-1), which was about one third of the daytime flux. Also during the winter period substantial O(3) deposition was measured. In addition, total O(3) fluxes were found to differ significantly among canopy wetness categories. During the day, highest deposition fluxes were generally measured for a dry canopy, whereas a rain-wetted canopy constituted the best sink at night. Flux partitioning calculations revealed that the stomatal flux (F(s)) contributed 20% to the total F but the F(s)/F fraction was subject to seasonal and diurnal changes. The annual concentration-based index AOT40 (accumulated dose over a threshold of 40 ppb) and the Phytotoxic Ozone Dose (POD(1) or accumulated stomatal flux above a threshold of 1 nmol m(-2) s(-1)) were related in a curvilinear way. The O(3) deposition was found to be largely controlled by non-stomatal sinks, whose strength was enhanced by high friction velocities (u(*)), optimizing the mechanical mixing of O(3) into the canopy and the trunk space. The long-term geometrical mean of the non-stomatal resistance (R(ns)) was 136 s m(-1) but lower R(ns) values were encountered during the winter half-year due to higher u(*). The R(ns) was also subject to a marked diurnal variability, with low R(ns) in the morning hours, when turbulence took off. We speculate that non-stomatal deposition was largely driven by scavenging of ozone by biogenic volatile organic compounds (BVOCs) and especially NO emitted from the crown or the forest floor.

Long-term (13 years) measurements of SO2 fluxes over a forest and their control by surface chemistry

Long-term fluxes of sulphur dioxide (SO2) have been measured over a mixed suburban forest subjected to elevated SO2 concentrations. The net exchange was shown to be highly dynamic with substantial periods of both upward and downward fluxes observed in excellent conditions for flux measurement. Upward fluxes constituted 30% of selected fluxes and appeared more frequently when the canopy was acidic. Upward fluxes were shown to be due to desorption from a drying surface or when ambient levels declined after periods of increased SO2 exposure.The long term average SO2 flux (F) was −59ngSO2m−2s−1 for the period 1997–2009 corresponding to an average SO2 concentration of 12.3μgSO2m−3 and a deposition velocity υd of 5mms−1. The smallest deposition fluxes and υd were measured in dry conditions (−42ngm−2s−1 and 3.5mms−1, resp.), which represented 57% of all cases. Wet canopies were more efficient sinks for SO2 and a dew-wetted canopy had a smaller υd (6mms−1) than a rain-wetted canopy (ca 10mms−1). Seasonal variability reflected differences in chemical climate or canopy buffering properties. During the summer half-year when surface acidity was low due to higher NH3/SO2 ratios, a higher deposition efficiency (υd/υdmax) and lower non-stomatal resistance (Rw) were observed compared to winter conditions. Comparisons of Rc for different combinations of canopy wetness and surface acidity categories emphasized the importance of both factors in regulating the non-stomatal sinks of SO2. Increased surface water acidity gradually led to a lower υd/υdmax and an increased Rc for all considered canopy wetness categories. The smallest υd/υdmax ratio and highest Rc were obtained for a dry canopy with high surface acidity. Conversely, a rain-wetted canopy was the most efficient sink for SO2. The canopy sink strength was further enhanced by high friction velocities (u*), optimizing the mechanical mixing into the canopy. Long-term trends were strongly coupled to changes in the NH3/SO2 ratio, which has clearly enhanced the deposition efficiency of SO2 in recent years.

Stomatal and non-stomatal fluxes of ozone to a northern mixed hardwood forest

Measurements of ozone, sensible heat, and latent heat fluxes and plant physiological parameters were made at a northern mixed hardwood forest located at the University of Michigan Biological Station in northern Michigan from June 27 to September 28, 2002. These measurements were used to calculate total ozone flux and partitioning between stomatal and non-stomatal sinks. Total ozone flux varied diurnally with maximum values reaching 100 μmol m-2 h-1 at midday and minimums at or near zero at night. Mean daytime canopy conductance was 0.5 mol m-2 s-1. During daytime, non-stomatal ozone conductance accounted for as much as 66% of canopy conductance, with the non-stomatal sink representing 63% of the ozone flux. Stomatal conductance showed expected patterns of behaviour with respect to photosynthetic photon flux density (PPFD) and vapour pressure defecit (VPD). Non-stomatal conductance for ozone increased monotonically with increasing PPFD, increased with temperature (T) before falling off again at high T, and behaved similarly for VPD. Day-time non-stomatal ozone sinks are large and vary with time and environmental drivers, particularly PPFD and T. This information is crucial to deriving mechanistic models that can simulate ozone uptake by different vegetation types

Foliage surface ozone deposition: a role for surface moisture?

Abstract. This paper addresses the potential role of surface wetness in ozone deposition to plant foliage. We studied Scots pine foliage in field conditions at the SMEARII field measurement station in Finland. We used a combination of data from flux measurement at the shoot (enclosure) and canopy scale (eddy covariance), information from foliage surface wetness sensors, and a broad array of ancillary measurements such as radiation, precipitation, temperature, and relative humidity. Environmental conditions were defined as moist during rain or high relative humidity and during the subsequent twelve hours from such events, circumstances that were frequent at this boreal site. From the measured fluxes we estimated the ozone conductance using it as the expression of the strength of ozone removal surface sink or total deposition. Further, we estimated the stomatal contribution and the remaining deposition was interpreted and analysed as the non-stomatal sink. The combined time series of measurements showed that both shoot and canopy-scale ozone total deposition were enhanced when moist conditions occurred. On average, the estimated stomatal deposition accounted for half of the measured removal at the shoot scale and one third at the canopy scale. However, during dry conditions the estimated stomatal uptake predicted the behaviour of the measured deposition, but during moist conditions there was disagreement. The estimated non-stomatal sink was analysed against several environmental factors and the clearest connection was found with ambient relative humidity. The relationship disappeared under 70% relative humidity, a threshold that coincides with the value at which surface moisture gathers at the foliage surface according to the leaf surface wetness measurements. This suggests the non-stomatal ozone sink on the foliage to be modulated by the surface films. We attempted to extract such potential modulation with the estimated film formation via the theoretical expression of adsorption. Whereas this procedure could predict the behaviour of the non-stomatal sink, it implied a chemical sink that was not accountable as simple ozone decomposition. We discuss the existence of other mechanisms whose relevance in the removal of ozone needs to be clarified, in particular: a significant nocturnal stomatal aperture neglected in the estimations, and a potentially large chemical sink offered by reactive biogenic organic volatile compounds.

Formaldehyde and acetaldehyde exchange during leaf development of the Amazonian deciduous tree species Hymenaea courbaril

Abstract The effect of leaf age on the formaldehyde (HCHO) and acetaldehyde (CH3CHO) exchange pattern of the deciduous Amazonian tree species Hymenaea courbaril was investigated under field conditions. Branch enclosure measurements on senescent, young, and mature leaves showed that leaf development had a pronounced impact on the aldehyde exchange behavior with respect to both the direction and the magnitude of the exchange. The emission activity of senescent leaves was associated with a negative CO2 balance, even during daytime, indicative of a catabolic metabolism and decomposition processes leading to an increased aldehyde production within the leaf. The low exchange rates observed in young leaves were attributed to low stomatal conductance, while in mature leaves stomatal conductance and metabolic activities allowed efficient uptake. Within each leaf class the diurnal variations in the exchange of both aldehyde compounds were found to be mainly dependent on the respective ambient air concentrations. High ambient air concentrations resulted in decreased emission rates of senescent leaves and in enhanced uptake in young and mature leaves. CH3CHO compensation points decreased from 1.8 to 1.1 ppb with leaf maturation. We provide evidence that leaf-age-dependent variations in the stomatal conductance can account for the major share of differences in the CH3CHO deposition velocity. The results indicate that leaf surfaces of young and mature leaves may represent an effective additional non-stomatal sink for atmospheric aldehydes.

Measurements of ozone removal by Scots pine shoots: calibration of a stomatal uptake model including the non-stomatal component

The calculation of ozone uptake to vegetation requires the determination of both the stomatal uptake and the non-stomatal sinks. This paper analyses the relative contribution of stomatal and non-stomatal sinks at the shoot level based on direct measurements of ozone flux that included both components. Ozone deposition was estimated for Scots pine shoots enclosed in chambers, connected to a gas-exchange system, at the SMEAR II field station, in southern Finland. For the analysis of stomatal conductance, we utilised the formulation within the dry deposition module of the photochemical EMEP model, for which we revised the parameterisation for Scots pine. In addition, we introduced modifications in the treatment of spring phenology and parameter estimation. Provided these modifications, the parameter values estimated from our data fell close to the default values, although presenting a less sensitive VPD response during the summer. A major proportion of the measured removal was due to the non-stomatal component. We introduced a non-stomatal term dependent on the ambient relative humidity, which significantly improved the agreement between the calculated and measured values of total shoot conductance. The non-stomatal contribution to total conductance was in the order of 50%.