O3 Deposition Research Articles

Ozone production and transport over the Amazon Basin during the dry-to-wet and wet-to-dry transition seasons

Abstract. The Regional Carbon Balance in Amazonia (BARCA) campaign provided the first Amazon Basin-wide aircraft measurements of ozone (O3) during both the dry-to-wet (November and December 2008) and wet-to-dry (May 2009) transition seasons. Extremely low background values (< 20 ppb) were observed to the west and north of Manaus in both seasons and in all regions during the wet-to-dry transition. On the other hand, elevated O3 levels (40–60 ppb) were seen during the dry-to-wet transition to the east and south of Manaus, where biomass burning emissions of O3 precursors were present. Chemistry simulations with the CCATT-BRAMS and WRF-Chem models are within the error bars of the observed O3 profiles in the boundary layer (0–3 km a.s.l.) in polluted conditions. However, the models overestimate O3 in the boundary layer in clean conditions, despite lacking the predominant NO source from soil. In addition, O3 simulated by the models was either within the error bars or lower than BARCA observations in mid-levels (3–5 km a.s.l.), and lower than total tropospheric O3 retrieved from the OMI/MLS instruments, which is primarily comprised of middle troposphere O3 and thus reflects long-range transport processes. Therefore, the models do a relatively poor job of representing the free troposphere-boundary layer gradient in O3 compared with aircraft and satellite observations, which could be due to missing long-range and convective transport of O3 at mid-levels. Additional simulations with WRF-Chem showed that the model O3 production is very sensitive to both the O3 deposition velocities and the NOx emissions, which were both about one-half of observed values. These results indicate the necessity of more realistic model representations of emissions, deposition, and convective processes for accurate monitoring and prediction of increases in O3 production in the Amazon Basin as the regional population grows.

Estimation of biogenic CH4 and CO2 emissions and dry deposition of O3 using 222Rn measurements in TROICA expeditions

On the basis of simultaneous measurements of surface concentrations of CH4, CO2, O3, and 222Rn during 6 TROICA (TRanscontinental Observations Into the Chemistry of the Atmosphere) expeditions along the Trans-Siberian Railway from Moscow to Vladivostok in 1999–2008, we have estimated the biogenic emissions of CH4 and CO2 from terrestrial ecosystems and the nighttime sink of O3 on the underlying surface from data on the accumulation rate of 222Rn under conditions of near-surface temperature inversion. The regional emissions of CH4 were the highest in summer in the Far East (0.87 ± 0.52 μg m−2 s−1) and in West Siberia (0.77 ± 0.41 μg m−2 s−1), which is associated with significant methane flows from wetlands and moist soils that are typical for these regions. The biogenic emissions of CO2 vary on average from 0.18 ± 0.04 μg m−2 s−1 in West Siberia to 0.89 ± 0.07 μg m−2 s−1 in East Siberia. The nighttime sink of O3 has a seasonal maximum in summer and varies from 0.05 ± 0.01 μg m−2 s−1 in West Siberia to 0.07 ± 0.01 μg m−2 s−1 in Central Siberia; the dry deposition rate of O3 varies from 0.10 ± 0.08 cm s−1 in West Siberia to 0.33 ± 0.21 cm s−1 in East Siberia and the Far East.

Ozone enhancement in western US wildfire plumes at the Mt. Bachelor Observatory: The role of NOx

We looked at 19 wildfire events that were observed in the summers of 2012 and 2013 at the Mt. Bachelor Observatory (MBO, 2.7 km a.s.l.), a mountaintop site located in central Oregon. We identified wildfire plumes using enhancement ratios (ΔY/ΔX), which we calculated for each plume by taking the Reduced Major Axis linear regression slope of various species. We reported the calculated enhancement ratios and explored their relationship with ozone production. We observed a negative correlation between ΔO3/ΔCO and ΔNOx/ΔNOy (r = −0.72). This showed that the degree of NOx oxidation is a key predictor of ozone production. The highest ΔNOx/ΔNOy (0.57 pptv/pptv) was associated with ozone loss (ozone titration). Low ΔNOx/ΔNOy values (ranging from 0.049 to 0.15 pptv/pptv) are generally associated with ozone enhancement. We also found that even if ΔO3/ΔCO is low, ΔO3 may still be significant if CO enhancements are large. We then explored events that are not associated with any O3 enhancement/loss. Out of 19 fire events, 3 belong to this category. We discovered that these events are either BL-influenced (O3 deposition), associated with low ΔNOy/ΔCO ratios, and/or associated with minimal photochemistry (due to nighttime transport). Absolute ozone enhancements ranged from 3.8 to 32 ppbv, while ozone production efficiencies (OPEs) ranged from 2.1 to 17. However, because PAN comprises most of the reactive nitrogen in fire plumes, the calculated OPEs underestimated the true ozone mixing ratios. OPEs may therefore be misleading indicators of ozone production in wildfires. Finally, we segregated the data into plume/non-plume time periods. From this we found that the average O3 mixing ratio was significantly higher in fire plumes compared to non-plume time periods, and the noontime NO/NO2 ratios were also higher. This later result gives insight into the photochemical environment in the fire plumes.

Volatile organic compound conversion by ozone, hydroxyl radicals, and nitrate radicals in residential indoor air: Magnitudes and impacts of oxidant sources

Indoor chemistry may be initiated by reactions of ozone (O3), the hydroxyl radical (OH), or the nitrate radical (NO3) with volatile organic compounds (VOC). The principal indoor source of O3 is air exchange, while OH and NO3 formation are considered as primarily from O3 reactions with alkenes and nitrogen dioxide (NO2), respectively. Herein, we used time-averaged models for residences to predict O3, OH, and NO3 concentrations and their impacts on conversion of typical residential VOC profiles, within a Monte Carlo framework that varied inputs probabilistically. We accounted for established oxidant sources, as well as explored the importance of two newly realized indoor sources: (i) the photolysis of nitrous acid (HONO) indoors to generate OH and (ii) the reaction of stabilized Criegee intermediates (SCI) with NO2 to generate NO3. We found total VOC conversion to be dominated by reactions both with O3, which almost solely reacted with d-limonene, and also with OH, which reacted with d-limonene, other terpenes, alcohols, aldehydes, and aromatics. VOC oxidation rates increased with air exchange, outdoor O3, NO2 and d-limonene sources, and indoor photolysis rates; and they decreased with O3 deposition and nitric oxide (NO) sources. Photolysis was a strong OH formation mechanism for high NO, NO2, and HONO settings, but SCI/NO2 reactions weakly generated NO3 except for only a few cases.

Coupling dry deposition to vegetation phenology in the Community Earth System Model: Implications for the simulation of surface O3

Dry deposition is an important removal process controlling surface ozone. We examine the representation of this ozone loss mechanism in the Community Earth System Model. We first correct the dry deposition parameterization by coupling the leaf and stomatal vegetation resistances to the leaf area index, an omission which has adversely impacted over a decade of ozone simulations using both the Model for Ozone and Related chemical Tracers (MOZART) and Community Atmospheric Model‐Chem (CAM‐Chem) global models. We show that this correction increases O3 dry deposition velocities over vegetated regions and improves the simulated seasonality in this loss process. This enhanced removal reduces the previously reported bias in summertime surface O3 simulated over eastern U.S. and Europe. We further optimize the parameterization by scaling down the stomatal resistance used in the Community Land Model to observed values. This in turn further improves the simulation of dry deposition velocity of O3, particularly over broadleaf forested regions. The summertime surface O3 bias is reduced from 30 ppb to 14 ppb over eastern U.S. and 13 ppb to 5 ppb over Europe from the standard to the optimized scheme, respectively. O3 deposition processes must therefore be accurately coupled to vegetation phenology within 3‐D atmospheric models, as a first step toward improving surface O3 and simulating O3 responses to future and past vegetation changes.

High tolerance of subalpine grassland to long-term ozone exposure is independent of N input and climatic drivers

In a seven-year study, we tested effects of increased N and O3 deposition and climatic conditions on biomass of subalpine grassland. Ozone risk was assessed as exposure (AOT40) and as stomatal flux (POD0,1). We hypothesized that productivity is higher under N- and lower under O3 deposition, with interactions with climatic conditions.Aboveground biomass was best correlated with growing-degree days for May (GDDMay). Nitrogen deposition increased biomass up to 60% in the highest treatment, and 30% in the lowest addition. Also belowground biomass showed a positive N-response. Ozone enrichment had no effect on biomass, and no interaction between O3 and N was observed. Growth response to N deposition was not correlated to GDDMay or precipitation, but indicated a cumulative effect over time.Productivity of subalpine grassland is tolerant to increasing ozone exposure, independent of N input and climatic drivers. N deposition rates at current critical loads, strongly increase the grassland yield.

Emission inventories for ships in the arctic based on satellite sampled AIS data

This paper presents a detailed BC, NOx and SO2 emission inventory for ships in the Arctic in 2012 based on satellite AIS data, ship engine power functions and technology stratified emission factors. Emission projections are presented for the years 2020, 2030 and 2050. Furthermore, the BC, SO2 and O3 concentrations and the deposition of BC are calculated for 2012 and for two arctic shipping scenarios – with or without arctic diversion routes due to a possible polar sea ice extent in the future.In 2012, the largest shares of Arctic ships emissions are calculated for fishing ships (45% for BC, 38% for NOx, 23% for SO2) followed by passenger ships (20%, 17%, 25%), tankers (9%, 13%, 15%), general cargo (8%, 11%, 12%) and container ships (5%, 7%, 8%). In 2050, without arctic diversion routes, the total emissions of BC, NOx and SO2 are expected to change by +16%, −32% and −63%, respectively, compared to 2012. The results for fishing ships are the least certain, caused by a less precise engine power – sailing speed relation.The calculated BC, SO2, and O3 surface concentrations and BC deposition contributions from ships are low as a mean for the whole Arctic in 2012, but locally BC additional contributions reach up to 20% around Iceland, and high additional contributions (100–300%) are calculated in some sea areas for SO2. In 2050, the arctic diversion routes highly influence the calculated surface concentrations and the deposition of BC in the Arctic. During summertime navigation contributions become very visible for BC (>80%) and SO2 (>1000%) along the arctic diversion routes, while the O3 (>10%) and BC deposition (>5%) additional contributions, respectively, get highest over the ocean east of Greenland and in the High Arctic.The geospatial ship type specific emission results presented in this paper have increased the accuracy of the emission inventories for ships in the Arctic. The methodology can be used to estimate shipping emissions in other regions of the world, and hence may serve as an input for other researchers and policy makers working in this field.

Diurnal variation of ozone flux over corn field in Northwestern Shandong Plain of China

High concentration ground-level ozone (O3) has adverse effects on plant growth and photosynthesis. Compared to the O3 concentration-based index, the O3 flux-based (especially stomatal O3 uptake) index has been considered the better criterion for assessing the impact of ozone on vegetation and ecosystems. This paper reports on a study of O3 flux using the eddy covariance technique over a corn field in the Northwestern Shandong Plain of China. Diurnal variation of atmospheric O3 concentration, deposition velocity and flux, and their relationships to environmental factors are analyzed. The results show that: (1) During the observation period (9 August-28 September, 2011), there was a strong diurnal variation of O3 concentration, with low (16.5 nL L−1) and high (60.1 nL L−1) O3 mean concentrations observed around 6:30 and 16:00, respectively. Mean O3 concentrations during daytime (6:00–18:00) and nighttime (18:00–6:00) were 39.8±23.1 and 20.7±14.1 nL L−1 (mean±std), respectively. The maximum observed concentration was 97.5 nL L−1. The concentration was mainly affected by solar radiation and air temperature. (2) Whether daytime or nighttime, ground-level O3 flux is always downward. The diurnal course of mean deposition velocity was divided into 4 phases: a low and stable process during nighttime, fast increasing in early morning, relatively large and steady changes around noon, and quickly decreasing in later afternoon. Daytime and nighttime mean deposition velocities were 0.29 and 0.09 cm s−1, respectively. The maximum deposition velocity was 0.81 cm s−1. The magnitude of deposition velocity was influenced by the corn growth period, and its diurnal variation was significantly correlated with global radiation and relative humidity. (3) O3 flux was affected by variations of both O3 concentration and deposition velocity, with mean O3 fluxes −317.7 and −70.2 ng m−2 s−1 during daytime and nighttime, respectively. There was strong correlation between O3 flux and CO2 flux or latent heat flux. By comparing the deposition velocities of daytime and nighttime, we infer that stomatal uptake was probably the main sink of ground-level O3.

Modelling the impact of megacities on local, regional and global tropospheric ozone and the deposition of nitrogen species

Abstract. We examine the effects of ozone precursor emissions from megacities on present-day air quality using the global chemistry–climate model UM-UKCA (UK Met Office Unified Model coupled to the UK Chemistry and Aerosols model). The sensitivity of megacity and regional ozone to local emissions, both from within the megacity and from surrounding regions, is important for determining air quality across many scales, which in turn is key for reducing human exposure to high levels of pollutants. We use two methods, perturbation and tagging, to quantify the impact of megacity emissions on global ozone. We also completely redistribute the anthropogenic emissions from megacities, to compare changes in local air quality going from centralised, densely populated megacities to decentralised, lower density urban areas. Focus is placed not only on how changes to megacity emissions affect regional and global NOx and O3, but also on changes to NOy deposition and to local chemical environments which are perturbed by the emission changes. The perturbation and tagging methods show broadly similar megacity impacts on total ozone, with the perturbation method underestimating the contribution partially because it perturbs the background chemical environment. The total redistribution of megacity emissions locally shifts the chemical environment towards more NOx-limited conditions in the megacities, which is more conducive to ozone production, and monthly mean surface ozone is found to increase up to 30% in megacities, depending on latitude and season. However, the displacement of emissions has little effect on the global annual ozone burden (0.12% change). Globally, megacity emissions are shown to contribute ~3% of total NOy deposition. The changes in O3, NOx and NOy deposition described here are useful for quantifying megacity impacts and for understanding the sensitivity of megacity regions to local emissions. The small global effects of the 100% redistribution carried out in this study suggest that the distribution of emissions on the local scale is unlikely to have large implications for chemistry–climate processes on the global scale.

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.

Understanding of the formation mechanisms of ozone and particulate matter at a fine scale over the southeastern U.S.: Process analyses and responses to future-year emissions

Ozone (O3) and fine particle (PM2.5) formation over the southeastern U.S. are of a major concern due to high emissions of precursors and special weather conditions that are conducive to their formation. In this study, the Community Multiscale Air Quality (CMAQ) modeling system is applied to simulate the formation of major air pollutants over an area in the southeastern U.S. at a 4-km horizontal grid resolution for January, April, July, and October in 2002 and 2018. Model performance evaluation shows an overall satisfactory performance for O3 in all months and for PM2.5 in January and October at rural sites and in January, April, and October at urban sites. Large underpredictions in PM2.5 concentrations occur in April and July at rural sites and in July at urban sites, because of biases in meteorological predictions and underestimation of emissions of precursors. The model performance at 4-km in terms of O3, PM2.5 and PM2.5 components show some improvements but overall are not always better than that at 12-km. O3 chemistry is VOC-limited in urban areas and NOx-limited over the west of the mountain regions and the southern Georgia throughout the year, and VOC-limited over the rest of areas in January but NOx-limited in other months. Among all photochemical indicators examined, PH2O2/PHNO3 and O3/NOy are the most robust indicators. The domain is NH3-rich or neutral in all months, indicating a high potential for NH4NO3 formation and the sensitivity of PM2.5 formation to the emissions of SO2, NOx, and NH3. Surface O3 is accumulated primarily through vertical transport in urban, rural and coastal areas and both horizontal and vertical transport in mountain regions and produced via gas-phase chemistry at non-urban sites during daytime. The loss of O3 is attributed to gas-phase chemistry via NO titration in urban areas, and dry deposition and transport processes in rural and mountain areas. PM2.5 is produced by primary emissions and PM processes and lost through vertical and horizontal transport in urban areas. The combined effects of transport, emissions, and PM processes influence PM concentrations in other areas. The 2018 simulations project a decrease in PM2.5 concentrations and an improvement in visibility over almost the entire domain, slight decreases in O3 mixing ratios in urban areas in July and most non-urban areas in April and October but large increases in the rest of areas in other months, and a decrease in total N deposition fluxes in most areas except for central and eastern North Carolina and northern Georgia. The development of integrated emission control strategies should consider region-specific seasonality and differences in the responses of O3, PM2.5, visibility, and nitrogen deposition.

Demonstration of a diel trend in sensitivity of Gossypium to ozone: a step toward relating O3 injury to exposure or flux

Plant injury by ozone (O3) occurs in three stages, O3 entrance through stomata, overcoming defences, and attack on bioreceptors. Concentration, deposition, and uptake of O3 are accessible by observation and modelling, while injury can be assessed visually or through remote sensing. However, the relationship between O3 metrics and injury is confounded by variation in sensitivity to O3. Sensitivity weighting parameters have previously been assigned to different plant functional types and growth stages, or by differentially weighting O3 concentrations, but diel and seasonal variability have not been addressed. Here a plant sensitivity parameter (S) is introduced, relating injury to O3 dose (uptake) using three independent injury endpoints in the crop species, Pima cotton (Gossypium barbadense). The diel variability of S was determined by assessment at 2h intervals. Pulses of O3 (15min) were used to assess passive (constitutive) defence mechanisms and dose was used rather than concentration to avoid genetic or environmental effects on stomatal regulation. A clear diel trend in S was apparent, with maximal sensitivity in mid-afternoon, not closely related to gas exchange, whole leaf ascorbate, or total antioxidant capacity. This physiologically based sensitivity parameter provides a novel weighting factor to improve modelled relationships between either flux or exposure to O3, and O3 impacts. This represents a substantial improvement over concentration- or phenology-based weighting factors currently in use. Future research will be required to characterize the variability and metabolic drivers of diel changes in S, and the performance of this parameter in prediction of O3 injury.

Investigating discrepancies in heat, CO2 fluxes and O3 deposition velocity over maize as measured by the eddy-covariance and the aerodynamic gradient methods

The eddy covariance (EC) method is widely considered as the reference method for heat and trace gas exchange flux measurements. However, for some species of interest, the aerodynamic gradient method (AG) is still a valuable method. Furthermore, some useful datasets are based on the AG method. In this study we compare the EC and the AG methods for latent (LE) and sensible (H) heat, carbon dioxide (Fc) fluxes and ozone deposition velocity (Vd O3) over a maize field near Paris. The AG method gave roughly 15% smaller Fc and LE, similar H, and 40% larger Vd O3 than the EC method. The differences between the two methods are discussed. In particular, the effects of the displacement height and heights of measurements on the AG fluxes are explored and the similarity among heat, CO2, H2O and O3 is tested. Furthermore, the vertical divergence of the flux above the canopy is estimated with the AG method.

DO<sub>3</sub>SE modelling of soil moisture to determine ozone flux to forest trees

Abstract. The DO3SE (Deposition of O3 for Stomatal Exchange) model is an established tool for estimating ozone (O3) deposition, stomatal flux and impacts to a variety of vegetation types across Europe. It has been embedded within the EMEP (European Monitoring and Evaluation Programme) photochemical model to provide a policy tool capable of relating the flux-based risk of vegetation damage to O3 precursor emission scenarios for use in policy formulation. A key limitation of regional flux-based risk assessments has been the assumption that soil water deficits are not limiting O3 flux due to the unavailability of evaluated methods for modelling soil water deficits and their influence on stomatal conductance (gsto), and subsequent O3 flux. This paper describes the development and evaluation of a method to estimate soil moisture status and its influence on gsto for a variety of forest tree species. This DO3SE soil moisture module uses the Penman-Monteith energy balance method to drive water cycling through the soil-plant-atmosphere system and empirical data describing gsto relationships with pre-dawn leaf water status to estimate the biological control of transpiration. We trial four different methods to estimate this biological control of the transpiration stream, which vary from simple methods that relate soil water content or potential directly to gsto, to more complex methods that incorporate hydraulic resistance and plant capacitance that control water flow through the plant system. These methods are evaluated against field data describing a variety of soil water variables, gsto and transpiration data for Norway spruce (Picea abies), Scots pine (Pinus sylvestris), birch (Betula pendula), aspen (Populus tremuloides), beech (Fagus sylvatica) and holm oak (Quercus ilex) collected from ten sites across Europe and North America. Modelled estimates of these variables show consistency with observed data when applying the simple empirical methods, with the timing and magnitude of soil drying events being captured well across all sites and reductions in transpiration with the onset of drought being predicted with reasonable accuracy. The more complex methods, which incorporate hydraulic resistance and plant capacitance, perform less well, with predicted drying cycles consistently underestimating the rate and magnitude of water loss from the soil. A sensitivity analysis showed that model performance was strongly dependent upon the local parameterisation of key model drivers such as the maximum gsto, soil texture, root depth and leaf area index. The results suggest that the simple modelling methods that relate gsto directly to soil water content and potential provide adequate estimates of soil moisture and influence on gsto such that they are suitable to be used to assess the potential risk posed by O3 to forest trees across Europe.

Atmosphere‐ocean ozone fluxes during the TexAQS 2006, STRATUS 2006, GOMECC 2007, GasEx 2008, and AMMA 2008 cruises

A ship‐based eddy covariance ozone flux system was deployed to investigate the magnitude and variability of ozone surface fluxes over the open ocean. The flux experiments were conducted on five cruises on board the NOAA research vesselRonald Brownduring 2006–2008. The cruises covered the Gulf of Mexico, the southern as well as northern Atlantic, the Southern Ocean, and the persistent stratus cloud region off Chile in the eastern Pacific Ocean. These experiments resulted in the first ship‐borne open‐ocean ozone flux measurement records. The median of 10 min oceanic ozone deposition velocity (vd) results from a combined ∼ 1700 h of observations ranged from 0.009 to 0.034 cm s−1. For the Gulf of Mexico cruise (Texas Air Quality Study (TexAQS)) the median vd (interquartile range) was 0.034 (0.009–0.065) cm s−1 (total number of 10 min measurement intervals, Nf = 1953). For the STRATUS cruise off the Chilean coast, the median vd was 0.009 (0.004–0.037) cm s−1 (Nf = 1336). For the cruise from the Gulf of Mexico and up the eastern U.S. coast (Gulf of Mexico and East Coast Carbon cruise (GOMECC)) a combined value of 0.018 (0.006–0.045) cm s−1 (Nf = 1784) was obtained (from 0.019 (−0.014–0.043) cm s−1, Nf = 663 in the Gulf of Mexico, and 0.018 (−0.004–0.045) cm s−1, Nf = 1121 in the North Atlantic region). The Southern Ocean Gas Exchange Experiment (GasEx) and African Monsoon Multidisciplinary Analysis (AMMA), the Southern Ocean and northeastern Atlantic cruises, respectively, resulted in median ozone vd of 0.009 (−0.005–0.026) cm s−1 (Nf = 2745) and 0.020 (−0.003–0.044) cms−1 (Nf = 1147). These directly measured ozone deposition values are at the lower end of previously reported data in the literature (0.01–0.12 cm s−1) for ocean water. Data illustrate a positive correlation (increase) of the oceanic ozone uptake rate with wind speed, albeit the behavior of the relationship appears to differ during these cruises. The encountered wide range of meteorological and ocean biogeochemical conditions is used to investigate fundamental drivers of oceanic O3 deposition and for the evaluation of a recently developed global oceanic O3 deposition modeling system.

The Effects of Tropospheric Ozone on Net Primary Productivity and Implications for Climate Change

Tropospheric ozone (O(3)) is a global air pollutant that causes billions of dollars in lost plant productivity annually. It is an important anthropogenic greenhouse gas, and as a secondary air pollutant, it is present at high concentrations in rural areas far from industrial sources. It also reduces plant productivity by entering leaves through the stomata, generating other reactive oxygen species and causing oxidative stress, which in turn decreases photosynthesis, plant growth, and biomass accumulation. The deposition of O(3) into vegetation through stomata is an important sink for tropospheric O(3), but this sink is modified by other aspects of environmental change, including rising atmospheric carbon dioxide concentrations, rising temperature, altered precipitation, and nitrogen availability. We review the atmospheric chemistry governing tropospheric O(3) mass balance, the effects of O(3) on stomatal conductance and net primary productivity, and implications for agriculture, carbon sequestration, and climate change.

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.

Measurement and modelling of tropospheric reactive halogen species over the tropical Atlantic Ocean

Abstract. Although tropospheric reactive halogen chemistry is well studied in coastal and polar environments, the presence of halogens over the open ocean environment has not been widely reported. The impacts of halogens on the tropical open ocean marine boundary layer (MBL), in particular, are not well characterised. This paper describes observations of iodine monoxide (IO) and bromine oxide (BrO) over eight months in the tropical open ocean MBL, on the north-eastern side of São Vicente (Cape Verde Islands, 16.85° N, 24.87° W). The highest BrO mixing ratio observed was 5.6±1 pmol mol−1, while the maximum observed IO mixing ratio was 3.1±0.4 pmol mol−1. The average values seen between 09:00–17:00 GMT were ~2.8 pmol mol−1 for BrO and ~1.5 pmol mol−1 for IO; these averages showed little variability over the entire campaign from November 2006 to June 2007. A 1-dimensional chemistry and transport model is used to study the evolution of iodine species and quantify the combined impact of iodine and bromine chemistry on the oxidising capacity of the MBL. It appears that the measured fluxes of iodocarbons are insufficient to account for the observed levels of IO, and that an additional I atom source is required, possibly caused by the deposition of O3 onto the ocean surface in the presence of solar radiation. Modelling results also show that the O3 depletion observed at Cape Verde cannot be explained in the absence of halogen chemistry, which contributes ~45% of the observed O3 depletion at the height of measurements (10 m) during summer. The model also predicts that halogens decrease the hydroperoxy radical (HO2) concentration by ~14% and increase the hydroxyl radical (OH) concentration by ~13% near the ocean surface. The oxidation of dimethyl sulphide (DMS) by BrO takes place at a comparable rate to oxidation by OH in this environment. Finally, the potential of iodine chemistry to form new particles is explored and conditions under which particle formation could be important in the remote MBL are discussed.