Ozone Deposition Research Articles

Comparison of ozone deposition measured with the dynamic chamber and the eddy covariance method

Nowadays, eddy covariance is the state-of-the-art method to quantify turbulent exchange fluxes in the surface boundary layer. In the absence of instruments suitable for high-frequency measurements, fluxes can also be determined using e.g., chamber techniques. However, up to date fluxes of depositing compounds were rarely determined using chamber techniques, mainly due to a modification of the aerodynamic conditions for the trace gas transport within the chamber. In this study, we present ozone (O3) deposition fluxes measured by the dynamic chamber technique and validate them against the eddy covariance (EC) method for a natural grassland site in Germany. The chamber system presented in Pape et al. (2009) was used and optimized to (i) reduce the likelihood of non-stationarities, (ii) yield 30min averages of flux measurements, and (iii) supply simultaneous profile measurements. The raw O3 fluxes of the dynamic chamber were corrected for gas-phase chemistry in the chamber volume and for the modification of the aerodynamic and boundary layer resistances. Simultaneously measured carbon dioxide and water vapor fluxes by both methods compared well during daytime documenting an equal vegetation activity inside and outside the chambers. The final corrected O3 deposition fluxes of both methods deviated on average by only 11% during daytime. The findings demonstrate the capability of the dynamic chamber method to capture representative O3 deposition fluxes for grassland ecosystems, even when the canopy height is similar to the chamber height. The canopy resistance to O3 was assessed by both methods and showed a characteristic diurnal cycle with minimum hourly median values of 180sm−1 (chambers) and 150sm−1 (EC) before noon. During nighttime the fluxes and resistances showed a higher uncertainty for both methods due to frequent low wind associated with non-stationary conditions at the experimental site. Canopy resistances for nitrogen dioxide (NO2) deposition were determined analogously with the chambers and were on average 86% higher than for O3.

DO3SE model applicability and O3 flux performance compared to AOT40 for an O3-sensitive tropical tree species (Psidium guajava L. 'Paluma').

Phytotoxic ozone (O3) levels have been recorded in the Metropolitan Region of São Paulo (MRSP). Flux-based critical levels for O3 through stomata have been adopted for some northern hemisphere species, showing better accuracy than with accumulated ozone exposure above a threshold of 40 ppb (AOT40). In Brazil, critical levels for vegetation protection against O3 adverse effects do not exist. The study aimed to investigate the applicability of O3 deposition model (Deposition of Ozone for Stomatal Exchange (DO3SE)) to an O3-sensitive tropical tree species (Psidium guajava L. 'Paluma') under the MRSP environmental conditions, which are very unstable, and to assess the performance of O3 flux and AOT40 in relation to O3-induced leaf injuries. Stomatal conductance (g s) parameterization for 'Paluma' was carried out and used to calculate different rate thresholds (from 0 to 5 nmol O3 m(-2) projected leaf area (PLA) s(-1)) for the phytotoxic ozone dose (POD). The model performance was assessed through the relationship between the measured and modeled g sto. Leaf injuries were analyzed and associated with POD and AOT40. The model performance was satisfactory and significant (R (2) = 0.56; P < 0.0001; root-mean-square error (RMSE) = 116). As already expected, high AOT40 values did not result in high POD values. Although high POD values do not always account for more injuries, POD0 showed better performance than did AOT40 and other different rate thresholds for POD. Further investigation is necessary to improve our model and also to check if there is a critical level of ozone in which leaf injuries arise. The conclusion is that the DO3SE model for 'Paluma' is applicable in the MRSP as well as in temperate regions and may contribute to future directives.

Comparison of conventional and green building materials in respect of VOC emissions and ozone impact on secondary carbonyl emissions

Building materials (BMs) are major contributors to indoor emission sources of volatile organic compounds (VOCs). In this study, 8 kinds of BMs (including conventional and green) for ceiling, cabinetry, and flooring commonly used indoors were tested in a 216 L chamber. Primary emissions of carbonyls (C1 to C8 aldehydes and ketones) at 48 h were 75–673 μg m−2 h−1 from conventional BMs, and 62–151 μg m−2 h−1 from green BMs. Primary emissions of BTEX at 48 h were 59–264 μg m−2 h−1 from conventional BMs, and 37–56 μg m−2 h−1 from green BMs. Ozone initiated molar yields of carbonyls were 0.10–2.36 from conventional BMs, and 0.13–0.86 from green BMs. Secondary emissions of carbonyls were 7–150 μg m−2 h−1 from conventional BMs, and 4–73 μg m−2 h−1 from green BMs. Green BMs had lower emissions than conventional analogs, especially for wooden flooring and gypsum board. BMs with mineral content are the most promising materials in this study, given moderately high ozone deposition velocity but generating the least byproducts. Secondary emissions determined in this study demonstrate that moderate indoor ozone concentrations may lead to increased concentrations of carbonyls, especially formaldehyde and acetaldehyde that potentially increase adverse chronic health effects.

Deposition velocities and impact of physical properties on ozone removal for building materials

This study aims to estimate the ozone deposition velocities of eight commonly used building materials (BMs) which include calcium silicate board (CSB), green calcium silicate board (GCSB), mineral fiber ceiling (MFC), green mineral fiber ceiling (GMFC), gypsum board (GB), green gypsum board (GGB), wooden flooring (WF) and green wooden flooring (GWF). In addition, the impact of physical properties (specific surface area and total pore volume of BM) on ozone removal ability was also explored and discussed. Studies were conducted in a small-scale environmental stainless steel chamber. CSB and GCSB showed the highest ozone deposition velocities, while WF and GWF showed the lowest ozone deposition velocities among test BMs materials. All reaction probabilities were estimated to fall within the order of magnitude of 10−6. Green BMs showed lower reaction probabilities with ozone comparing with non-green BMs except for GGB. Consistent with the trends for deposition velocity, fleecy and porous materials exhibit higher reaction probabilities than smooth, non-porous surfaces. Specific surface area of BM is more closely related to ozone removal than total pore volume of BM with R2 of 0.93 vs. R2 of 0.84. Discussion of Thiele modulus for all test BMs indicates surface reactions are occurring quickly relative to internal diffusion and ozone removal is internal diffusion-limited.

Comparing i-Tree modeled ozone deposition with field measurements in a periurban Mediterranean forest

Ozone flux estimates from the i-Tree model were compared with ozone flux measurements using the Eddy Covariance technique in a periurban Mediterranean forest near Rome (Castelporziano). For the first time i-Tree model outputs were compared with field measurements in relation to dry deposition estimates. Results showed generally a good agreement between predicted and measured ozone fluxes (least sum square = 5.6 e−4) especially when cumulative values over the whole measurement campaign are considered. However at daily and hourly time-step some overestimations were observed in estimated values especially in hot dry periods. The use of different m values in the Ball–Berry formula in the different periods, produced the best fit between predicted and measured ozone fluxes. This suggests that a variable value for the coefficient m accounting for water availability may be appropriate to improve model estimates for Mediterranean and drought prone regions.

Simultaneous measurements of above and below canopy ozone fluxes help partitioning ozone deposition between its various sinks in a Mediterranean Oak Forest

Castelporziano Estate is a coastal forest 25km from downtown Rome. It is an ideal site to study interactions between Mediterranean forest ecosystems and a polluted atmosphere. Two eddy covariance systems were used to simultaneously measure water vapour and ozone fluxes above and below a canopy of Holm Oak (Quercus ilex). Additional measurements of environmental parameters allowed to calculate stomatal ozone fluxes in order to parameterize atmospheric models and new algorithms for discriminating ozone deposition into its three more significant sinks: soil, stomata, and cuticles. Results showed that stomata explained almost the totality of ozone fluxes during the winter season, and <60% during the warm seasons under condition of drought stress. Soils removed up to 30% of ozone, suggesting the importance of this sink in this forest ecosystem. This study spanning all seasons over a 2-year period advanced our understanding about the contribution of a representative Mediterranean Oak forest to biosphere–atmosphere exchange.

An evaluation of ozone dry deposition simulations in East Asia

Abstract. We use a 3-D regional atmospheric chemistry transport model (WRF-Chem) to examine ozone dry deposition in East Asia, which is an important but uncertain research area because of insufficient observation and numerical studies focusing on East Asia. Here we compare two widely used dry deposition parameterization schemes, the Wesely and M3DRY schemes, which are used in the WRF-Chem and Community Multiscale Air Quality (CMAQ) models, respectively. Simulated ozone dry deposition velocities with the two schemes under identical meteorological conditions show considerable differences (a factor of 2) owing to surface resistance parameterization discrepancies. Resulting ozone concentrations differ by up to 10 ppbv for a monthly mean in May when the peak ozone typically occurs in East Asia. An evaluation of the simulated dry deposition velocities shows that the Wesely scheme calculates values with more pronounced diurnal variation than the M3DRY and results in a good agreement with the observations. However, we find significant changes in simulated ozone concentrations using the Wesely scheme but with different surface type data sets, indicating the high sensitivity of ozone deposition calculations to the input data. The need is high for observations to constrain the dry deposition parameterization and its input data to improve the use of air quality models for East Asia.

Emissions of terpenoids, benzenoids, and other biogenic gas-phase organic compounds from agricultural crops and their potential implications for air quality

Abstract. Agriculture comprises a substantial, and increasing, fraction of land use in many regions of the world. Emissions from agricultural vegetation and other biogenic and anthropogenic sources react in the atmosphere to produce ozone and secondary organic aerosol, which comprises a substantial fraction of particulate matter (PM2.5). Using data from three measurement campaigns, we examine the magnitude and composition of reactive gas-phase organic carbon emissions from agricultural crops and their potential to impact regional air quality relative to anthropogenic emissions from motor vehicles in California's San Joaquin Valley, which is out of compliance with state and federal standards for tropospheric ozone PM2.5. Emission rates for a suite of terpenoid compounds were measured in a greenhouse for 25 representative crops from California in 2008. Ambient measurements of terpenoids and other biogenic compounds in the volatile and intermediate-volatility organic compound ranges were made in the urban area of Bakersfield and over an orange orchard in a rural area of the San Joaquin Valley during two 2010 seasons: summer and spring flowering. We combined measurements from the orchard site with ozone modeling methods to assess the net effect of the orange trees on regional ozone. When accounting for both emissions of reactive precursors and the deposition of ozone to the orchard, the orange trees are a net source of ozone in the springtime during flowering, and relatively neutral for most of the summer until the fall, when it becomes a sink. Flowering was a major emission event and caused a large increase in emissions including a suite of compounds that had not been measured in the atmosphere before. Such biogenic emission events need to be better parameterized in models as they have significant potential to impact regional air quality since emissions increase by several factors to over an order of magnitude. In regions like the San Joaquin Valley, the mass of biogenic emissions from agricultural crops during the summer (without flowering) and the potential ozone and secondary organic aerosol formation from these emissions are on the same order as anthropogenic emissions from motor vehicles and must be considered in air quality models and secondary pollution control strategies.

Resistive switching in HfO2-based atomic layer deposition grown metal–insulator–metal structures

Abstract We prepared Pt/HfO 2 /TiN metal–insulator–metal structures for resistive switching experiments. The HfO 2 films were prepared by thermal, ozone and plasma assisted atomic layer deposition. The deposition techniques yielded HfO 2 films that were conducive to stable and reproducible bipolar resistive switching. We observed that the forming voltage scaled with the HfO 2 film thickness. The structures did not show degradation after 10 4 consecutive resistive switching operations in a millisecond pulsed regime.

It’s not just the heat and the drought: the role of ozone air pollution in the 2012 heat wave

S!#$%. – Weather conditions which characterize heat waves (e.g. high temperatures, severe drought, increased solar radiation, stagnation of air masses, increased presence of biogenic volatile compounds) are highly conductive to the formation and persistence of photochemically-produced tropospheric ozone. However, it is the actual dose of ozone uptake through stomata rather than mere exposure that determines the ozone impact to vegetation. There are evidences that drought protects plants from ozone injury by stomatal closure, which restricts ozone influx into the leaves. Conversely, in natural ecosystems and in non-irrigated crops dry deposition of ozone, which is mainly due to stomatal absorption, is curtailed. As a consequence, during heat wave episodes more ozone is produced and less ozone is depleted by dry deposition: the positive contribute of vegetation to improve air quality fails when we need it most! So, ozone is a major risk factor for living organisms (especially human beings) during extreme summer weather conditions. In addition, ozone is a powerful greenhouse gas and its increase contributes to further air warming. The paper reports some ozone data captured in Tuscany during the summer of 2012 and the results of biomonitoring campaigns performed during the same period. More frequent (and intense) exceedances of dangerous ground level ozone thresholds would be expected in Southern Europe at the current emission levels if summer weather conditions such as those of 2003 and 2012 will become a rule in the coming decades. I'+$/7!9+;/'. – Ozone is since long the most import ant air pollutant in Europe and in many parts of the world. Its concentrations in the troposphere increased dramatically during the last century and show worldwide relevant peak values in urban, suburban and remote areas (K<=% et al. , 1999). Today more than 95% of the total European Union urban population is exposed to ozone levels above World Health Organizations guidelines. Ozone is formed in the troposphere by the interaction of nitrogen dioxide (NO 2) and the UV radiation from the sun. NO 2 absorbs UV radiation to decompose the molecule into nitrogen monoxide (NO) and an activated oxygen atom (O ∙- ). On contact with molecular oxygen (O 2), ozone (O 3) is formed. The reaction is reversible when O 3 and NO react to

Simultaneous leaf- and ecosystem-level fluxes of volatile organic compounds from a poplar-based SRC plantation

Emission of carbon from ecosystems in the form of volatile organic compounds (VOC) represents a minor component flux in the global carbon cycle that has a large impact on ground-level ozone, particle and aerosol formation and thus on air chemistry and quality. This study reports exchanges of CO2 and VOC between a poplar-based short rotation coppice (SRC) plantation and the atmosphere, measured simultaneously at two spatial scale, one at stand level and another at leaf level. The first technique combined Proton Transfer Reaction “Time-of-Flight” mass spectrometry (PTR–TOF–MS) with the eddy covariance method, to measure fluxes of a multitude of VOC. Abundant fluxes of isoprene, methanol and, to a lesser extent, fluxes of other oxygenated VOC such as formaldehyde, isoprene oxidation products (methyl vinyl ketone and methacrolein), methyl ethyl ketone, acetaldehyde, acetone and acetic acid, were measured. Under optimal environmental conditions, isoprene flux was mostly controlled by temperature and light. Differently, methanol flux underwent a combined enzymatic and stomatal control, together involving environmental drivers such as vapour pressure deficit (VPD), temperature and light intensity. Moreover fair weather condition favoured ozone deposition to the poplar plantation.The second technique involved trapping the VOCs emitted from leaves followed by gas chromatography-mass spectrometry (GC–MS) analysis. These leaf-level measurements showed that emission of isoprene in adult leaves and of monoterpenes in juvenile leaves are widespread across poplar genotypes. Detection of isoprene oxidation products (iox) emission with leaf-level measurements confirmed that a fraction of isoprene may be already oxidized within leaves, possibly when isoprene copes with foliar reactive oxygen species (ROS) formed during warm and sunny days.

Role of Ozone Deposition in the Occurrence of the Spring Maximum

A hypothesis has been formulated on the basis of experimental data presented in this article. According to the hypothesis, occurrence of the spring surface ozone maximum at mid-latitudes results from a delay in snow-cover melt. The data were collected at ozone stations in Minsk (Belarus) and Preila (Lithuania). Because the measurements of surface ozone concentration are quite different, despite the close proximity of the stations, a conclusion can be drawn about the significant influence of meteorological parameters on measurements. In addition to a rather subjective and poorly defined parameter—time of snow melt—the difference between the average March temperature and a climatological mean may be treated as a criterion for the presence or absence of the spring ozone maximum.

Modification of Ozone Deposition and I2 Emissions at the Air–Aqueous Interface by Dissolved Organic Carbon of Marine Origin

The reaction between gaseous ozone (O3) and aqueous iodide (I(-)) at the surface microlayer (SML) is believed to be a major chemical contributor to the oceanic dry deposition of O3 over open ocean waters and has also recently been shown to produce environmentally significant quantities of gaseous molecular iodine (I2). Here we investigate how this reaction is affected by the presence of dissolved organic carbon (DOC) of marine origin, using a heterogeneous flow reactor and detection of gaseous I2 by solvent trapping and UV/vis spectroscopy. Ozone deposition measurements over coastal seawater implied an O3 reactivity (λ) toward coastal marine DOC of ∼500 (420-580) s(-1), 2-5 times higher than that toward iodide at typical ocean concentrations (∼0.5-1 × 10(-7) M). We added varying amounts of highly concentrated DOC extracted from coastal seawater to I(-) solutions (1 × 10(-5) M) such that the relative reactivities of DOC and I(-) toward O3 (λDOC/λI) were in the expected range for natural seawater. The evolution of gaseous I2 and the loss of aqueous I(-) both reduced as DOC concentrations increased, with an overall suppression of I2 emissions of about a factor of 2 under conditions of λDOC/λI representative of open ocean waters (0.5-1). A kinetic model of the SML suggested that neither competition of DOC with I(-) for reaction with interfacial O3, nor direct loss of I2 and hypoiodous acid (HOI) through reaction with increasing quantities of DOC, can fully explain these results. We conclude that the suppression of I2 emissions by DOC is largely a physical effect arising from a decrease in the net transfer of I2 from the aqueous to gas phase, as suggested by recent laboratory studies.

Scorched Earth: how will changes in the strength of the vegetation sink to ozone deposition affect human health and ecosystems?

Abstract. This study investigates the effect of ozone (O3) deposition on ground level O3 concentrations and subsequent human health and ecosystem risk under hot summer "heat wave" type meteorological events. Under such conditions, extended drought can effectively "turn off" the O3 vegetation sink leading to a substantial increase in ground level O3 concentrations. Two models that have been used for human health (the CMAQ chemical transport model) and ecosystem (the DO3SE O3 deposition model) risk assessment are combined to provide a powerful policy tool capable of novel integrated assessments of O3 risk using methods endorsed by the UNECE Convention on Long-Range Transboundary Air Pollution. This study investigates 2006, a particularly hot and dry year during which a heat wave occurred over the summer across much of the UK and Europe. To understand the influence of variable O3 dry deposition three different simulations were investigated during June and July: (i) actual conditions in 2006, (ii) conditions that assume a perfect vegetation sink for O3 deposition and (iii) conditions that assume an extended drought period that reduces the vegetation sink to a minimum. The risks of O3 to human health, assessed by estimating the number of days during which running 8 h mean O3 concentrations exceeded 100 μg m−3, show that on average across the UK, there is a difference of 16 days exceedance of the threshold between the perfect sink and drought conditions. These average results hide local variation with exceedances between these two scenarios reaching as high as 20 days in the East Midlands and eastern UK. Estimates of acute exposure effects show that O3 removed from the atmosphere through dry deposition during the June and July period would have been responsible for approximately 460 premature deaths. Conversely, reduced O3 dry deposition will decrease the amount of O3 taken up by vegetation and, according to flux-based assessments of vegetation damage, will lead to a reduction in the impact of O3 on vegetation across the UK. The new CMAQ-DO3SE model was evaluated by comparing observation vs. modelled estimates of various health related metrics with data from both urban and rural sites across the UK; although these comparisons showed reasonable agreement there were some biases in the model predictions with attributable deaths at urban sites being over predicted by a small margin, the converse was true for rural sites. The study emphasises the importance of accurate estimates of O3 deposition both for human health and ecosystem risk assessments. Extended periods of drought and heat wave type conditions are likely to occur with more frequency in coming decades, therefore understanding the importance of these effects will be crucial to inform the development of appropriate national and international policy to mitigate against the worst consequences of this air pollutant.

Assessment of the total, stomatal, cuticular, and soil 2 year ozone budgets of an agricultural field with winter wheat and maize crops

AbstractThis study evaluates ozone (O3) deposition to an agricultural field over a period of 2 years. A two‐layer soil‐vegetation‐atmosphere‐transfer (Surfatm‐O3) model is used to partition the O3 flux between the soil, the cuticular, and the stomatal pathways. The comparison between measured and modeled O3 fluxes exhibited a good agreement, independently of the canopy structure and coverage and the climatic conditions, which implicitly validates the O3 flux partitioning. The total, soil, cuticular, and stomatal O3 budgets are then established from the modeling. Total ecosystem O3 deposition over the 2 year period was 87.5 kg ha−1. Clearly, nonstomatal deposition dominates the deposition budget, especially the soil component which represented up to 50% of the total deposition. Nevertheless, the physiological and phenological differences of maize and winter wheat induced large difference in the stomatal deposition budgets of these two crops. Then, the effect of simplified parameterizations for soil and cuticular resistances currently used in other models on the O3 budget is tested. Independently, these simplified parameterizations cause an underestimation of the O3 deposition ranging between 0% and 11.2%. However, the combination of all simplifications resulted in an underestimation of the total O3 deposition by about 20%. Finally, crop yield loss was estimated to be 1.5–4.2% for the winter wheat, whereas maize was not affected by O3.

Evaluation of three common green building materials for ozone removal, and primary and secondary emissions of aldehydes

Ozone reactions that occur on material surfaces can lead to elevated concentrations of oxidized products in the occupied space of buildings. However, there is little information on the impact of materials at full scale, especially for green building materials. Experiments were completed in a 68 m3 climate-controlled test chamber with three certified green building materials that can cover large areas in buildings: (1) recycled carpet, (2) perlite-based ceiling tile and (3) low-VOC paint and primer on recycled drywall. Ozone deposition velocity and primary and secondary emission rates of C1 to C10 saturated carbonyls were determined for two chamber mixing conditions and three values of relative humidity. A direct comparison was made between ozone deposition velocities and carbonyl yields observed for the same materials analyzed in small (10 L) chambers. Total primary carbonyl emission rates from carpet, ceiling tile and painted drywall ranged from 27 to 120 μg m−2 h−1, 13 to 40 μg m−2 h−1, 3.9 to 42 μg m−2 h−1, respectively. Ozone deposition velocity to these three materials averaged 6.1 m h−1, 2.3 m h−1 and 0.32 m h−1, respectively. Total secondary carbonyl emissions from these materials ranged from 70 to 276 μg m−2 h−1, 0 to 12 μg m−2 h−1, and 0 to 30 μg m−2 h−1, respectively. Carbonyl emissions were determined with a transient approximation, and were found to be in general agreement with those found in the literature. These results suggest that care should be taken when selecting green building materials due to potentially large differences in primary and secondary emissions.

Probabilistic estimation of future emissions of isoprene and surface oxidant chemistry associated with land-use change in response to growing food needs

Abstract. We quantify the impact of land-use change, determined by our growing demand for food and biofuel production, on isoprene emissions and subsequent atmospheric oxidant chemistry in 2015 and 2030, relative to 1990, ignoring compound climate change effects over that period. We estimate isoprene emissions from an ensemble (n = 1000) of land-use change realizations from 1990–2050, broadly guided by the IPCC AR4/SRES scenarios A1 and B1. We also superimpose land-use change required to address projected biofuel usage using two scenarios: (1) assuming that world governments make no changes to biofuel policy after 2009, and (2) assuming that world governments develop biofuel policy with the aim of keeping equivalent atmospheric CO2 at 450 ppm. We present the median and interquartile range (IQR) statistics of the ensemble and show that land-use change between −1.50 × 1012 m2 to +6.06 × 1012 m2 was found to drive changes in the global isoprene burden of −3.5 to +2.8 Tg yr−1 in 2015 and −7.7 to +6.4 Tg yr−1 in 2030. We use land-use change realizations corresponding to the median and IQR of these emission estimates to drive the GEOS-Chem global 3-D chemistry transport model to investigate the perturbation to global and regional surface concentrations of isoprene, nitrogen oxides (NO+NO2), and the atmospheric concentration and deposition of ozone (O3). We show that across subcontinental regions the monthly surface O3 increases by 0.1–0.8 ppb, relative to a zero land-use change calculation, driven by increases (decreases) in surface isoprene in high (low) NOx environments. At the local scale (4° × 5°) we find that surface O3 increases by 5–12 ppb over temperate North America, China and boreal Eurasia, driven by large increases in isoprene emissions from short-rotation coppice crop cultivation for biofuel production.

Tropospheric ozone reduces carbon assimilation in trees: estimates from analysis of continuous flux measurements

High ground-level ozone concentrations are typical of Mediterranean climates. Plant exposure to this oxidant is known to reduce carbon assimilation. Ozone damage has been traditionally measured through manipulative experiments that do not consider long-term exposure and propagate large uncertainty by up-scaling leaf-level observations to ecosystem-level interpretations. We analyzed long-term continuous measurements (>9 site-years at 30min resolution) of environmental and eco-physiological parameters at three Mediterranean ecosystems: (i) forest site dominated by Pinus ponderosa in the Sierra Mountains in California, USA; (ii) forest site composed of a mixture of Quercus spp. and P. pinea in the Tyrrhenian sea coast near Rome, Italy; and (iii) orchard site of Citrus sinensis cultivated in the California Central Valley, USA. We hypothesized that higher levels of ozone concentration in the atmosphere result in a decrease in carbon assimilation by trees under field conditions. This hypothesis was tested using time series analysis such as wavelet coherence and spectral Granger causality, and complemented with multivariate linear and nonlinear statistical analyses. We found that reduction in carbon assimilation was more related to stomatal ozone deposition than to ozone concentration. The negative effects of ozone occurred within a day of exposure/uptake. Decoupling between carbon assimilation and stomatal aperture increased with the amount of ozone pollution. Up to 12-19% of the carbon assimilation reduction in P. ponderosa and in the Citrus plantation was explained by higher stomatal ozone deposition. In contrast, the Italian site did not show reductions in gross primary productivity either by ozone concentration or stomatal ozone deposition, mainly due to the lower ozone concentrations in the periurban site over the shorter period of investigation. These results highlight the importance of plant adaptation/sensitivity under field conditions, and the importance of continuous long-term measurements to explain ozone damage to real-world forests and calculate metrics for ozone-risk assessment.

Enhanced vertical mixing associated with a nocturnal cold front passage and its impact on near‐surface temperature and ozone concentration

AbstractA sudden rise in surface temperature is sometimes observed during the nighttime hours with the passage of cold fronts. The physics contributing to such nocturnal warming events and their potential impacts on atmospheric chemistry are not yet fully understood. In this study, a nocturnal warming event associated with a cold front passage in Oklahoma on 3 April 2006 is simulated with the Weather Research and Forecasting model with Chemistry (WRF/Chem). During the prefrontal period under clear‐sky and calm conditions, surface radiative cooling resulted in a decoupled shallow surface layer in which air temperature and wind speed decreased quickly and ozone was removed efficiently by chemical reactions. During the passage of the cold front, strong wind shear enhanced turbulent mixing, which weakened the temperature inversion near the surface. Warmer and ozone‐richer air from aloft was mixed downward to the surface. Thus, a sudden warming and nocturnal secondary ozone maxima were observed near the surface. Dry deposition of ozone at the surface was also enhanced in this warming event.

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.