Ozone Flux Measurements Research Articles

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

Characteristics of Fine Particle Growth Events Observed Above a Forested Ecosystem in the Sierra Nevada Mountains of California

Atmospheric aerosols from natural and anthropogenic processes have both primary and secondary origins, and can influence human health, visibility, and climate. One key process affecting atmospheric concentrations of aerosols is the formation of new particles and their subsequent growth to larger particle sizes. A field study was conducted at the Blodgett Forest Research Station in the Sierra Nevada Mountains of California from May through September of 2002 to examine the effect of biogenic volatile organic compounds on aerosol formation and processing. The study included in-situ measurements of concentration and biosphere-atmosphere flux of VOCs, ozone, aerosol size distribution, aerosol physical and optical properties, and meteorological variables. Fine particle growth events were observed on approximately 30 percent of the 107 days with complete size distribution data. Average particle growth rates measured during these events were 3.8 ± 1.9 nm hr−1. Correlations between aerosol properties, trace gas concentrations, and meteorological measurements were analyzed to determine conditions conducive to fine particle growth events. Growth events were typically observed on days with a lesser degree of anthropogenic influence, as indicated by lower concentrations of black carbon, carbon monoxide, and total aerosol volume. Days with growth events also had lower temperatures, increased wind speeds, and larger momentum flux. Measurements of ozone concentrations and ozone flux indicate that gas phase oxidation of biogenic volatile organic compounds occur in the canopy, strongly suggesting that a significant portion of the material responsible for the observed particle growth are oxidation products of naturally emitted very reactive organic compounds.

Combined MU radar and ozonesonde measurements of turbulence and ozone fluxes in the tropo‐stratosphere over Shigaraki, Japan

Turbulent diffusivity and turbulent ozone fluxes in the tropo‐stratosphere are studied employing simultaneous observations with the Middle and Upper (MU) Atmosphere radar and ozonesondes in Shigaraki, Japan during April 16–24, 1998. A broad region around the tropopause was dynamically active. Maxima of turbulent diffusivity were observed at 8–14 km altitude. Such maxima may produce vertical turbulent ozone fluxes across the tropopause with magnitudes comparable to those required for the global ozone budget. Meso‐meteorological ozone intrusions may enhance the fluxes.

Testing and improving the EMEP ozone deposition module

This study evaluates the ozone deposition module developed for the European Monitoring and Evaluation Programme photochemical transport model. The module allows an estimation of both total and stomatal ozone flux to different vegetation types, taking into account plant phenology and environmental conditions. Based on a series of model tests against micrometeorological measurements of ozone fluxes, including data on stomatal conductances, the performance of the land-cover specific parameterisations was assessed and improvements implemented. Tests were carried out for coniferous forest, moorland and wetland ecosystems in Northern Europe, and for a wheat field and a grassland site located in Southern Europe. The improvements consisted of a needle age factor for coniferous forests, revised ground surface conductances, and a new leaf-to-canopy up-scaling method for wheat. These resulted in better agreement with observed surface conductances and deposition velocities, although a number of discrepancies are noted. In general, the parameterisation of non-stomatal deposition and the performance of the module under high soil moisture deficits were identified as key uncertainties requiring further investigation.

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%.

Five-year measurements of ozone fluxes to a Danish Norway spruce canopy

Abstract Ozone concentrations and fluxes have been measured continuously during 5 years (1996–2000) by the gradient method in a Norway spruce dominated forest stand in West Jutland, Denmark, planted in 1965. The method has been validated against other methodologies and a relatively good relationship was found. The data are analysed to quantify diurnal, seasonal and yearly fluxes, and non-stomatal and stomatal removal are estimated. Monthly means of climatic data are shown, and day and night values of the aerodynamic resistance, ra, viscous sub-layer resistance, rb, and the surface or canopy resistance, rc, are presented. The yearly ozone deposition is approximately 126 kg ha−1. The canopy ozone uptake is highest during the day and during the summer. This is interpreted as increased stomatal uptake and physical and chemical reactions. The daily means of ozone concentration and fluxes averaged over 5 years correlate, but the correlation is primarily based on two different uncoupled processes outside and inside the stomates: (1) The ozone destruction in the canopy occurring outside the stomates is much influenced by temperature, light and humidity, e.g. surface reactions, NO- and VOC-emissions. (2) The same factors have a strong influence on the stomatal opening, e.g. midday and night closure. Thus, looking at diurnal variations, the diurnal ozone concentration and ozone flux do not correlate at all during the growing season. The maximum diurnal difference for the ozone concentration is a factor 1.3 and the maximum diurnal difference for the ozone flux is a factor 3. From dawn to ca. 8:00 the ozone deposition increases and the ozone concentration decreases. The yearly stomatal uptake of ozone is estimated to minimum 21% of the total deposition, being highest in May–August (30–33%) and lowest in November–February (4–9%). The physiological ozone uptake per leaf area is estimated to 0.33 g ozone m−2 y−1.

An evaluation of ozone exposure metrics for a seasonally drought-stressed ponderosa pine ecosystem

Ozone stress has become an increasingly significant factor in cases of forest decline reported throughout the world. Current metrics to estimate ozone exposure for forest trees are derived from atmospheric concentrations and assume that the forest is physiologically active at all times of the growing season. This may be inaccurate in regions with a Mediterranean climate, such as California and the Pacific Northwest, where peak physiological activity occurs early in the season to take advantage of high soil moisture and does not correspond to peak ozone concentrations. It may also misrepresent ecosystems experiencing non-average climate conditions such as drought years. We compared direct measurements of ozone flux into a ponderosa pine canopy with a suite of the most common ozone exposure metrics to determine which best correlated with actual ozone uptake by the forest. Of the metrics we assessed, SUM0 (the sum of all daytime ozone concentrations>0) best corresponded to ozone uptake by ponderosa pine, however the correlation was only strong at times when the stomata were unconstrained by site moisture conditions. In the early growing season (May and June), SUM0 was an adequate metric for forest ozone exposure. Later in the season, when stomatal conductance was limited by drought, SUM0 overestimated ozone uptake. A better metric for seasonally drought-stressed forests would be one that incorporates forest physiological activity, either through mechanistic modeling, by weighting ozone concentrations by stomatal conductance, or by weighting concentrations by site moisture conditions.

Methodology for direct field measurements of ozone flux to foliage with shoot chambers

Values of ozone (O 3) flux measured on the shoot scale by enclosure techniques can provide detailed information about the partition and seasonality of the removal pathways on the foliage. However, O 3 reactivity makes such measurements potentially inaccurate, particularly in field conditions where ambient concentration is low. Special consideration should be given to the losses on the chamber wall material, which can rival in magnitude and dynamics the measure of shoot removal. We propose a way of taking into account this dynamic behaviour in the context of field monitoring with simple chambers based on dynamic principles. We accommodated an existing field monitoring gas exchange system for CO 2 and water vapour to measure simultaneous,but independent, O 3 fluxes. We report here the system's performance based on the analysis of 19-month data. The background noise of the measurements is mainly imposed by the analyser and is around 2 μg m −3. The resolution of the system is in the order of 10 −2 μg m −2 s −1. The disturbance due to wall-losses was corrected thoroughly but it was mostly needed when the shoot activity was low and the mass flow signals produced by the shoots and the walls came close to each other. In spite of the disfavouring combination of chamber material, O 3 reactivity and low concentrations, we have successfully followed diurnal and seasonal trends of O 3 removal in Scots pine foliage.

Areal Measurements of Ozone, Water, and Heat Fluxes Over Land With Different Surface Complexity, Using Aircraft

Contemporary models addressing issues of air quality and/oratmospheric deposition continue to exploit air-surface exchangeformulations originating from single-tower studies. In reality,these expressions describe situations that are rare in the realworld – nearly flat and spatially homogeneous. There have beenseveral theoretical suggestions about how to extend from single-point understanding to areal descriptions, but so far thecapability to address the problem experimentally has beenlimited. In recent years, however, developments in sensingtechnology have permitted adaptation of eddy-correlation methodsto low-flying aircraft in a far more cost-effective manner thanpreviously. A series of field experiments has been conducted,ranging from flat farmland to rolling countryside, employing arecently modified research aircraft operated by the US NationalOceanic and Atmospheric Administration (NOAA). The resultsdemonstrate the complexity of the spatial heterogeneity question,especially for pollutants (ozone in particular). In general, theuncertainty associated with the adoption of any single-pointformulation when describing areal averages is likely to be in therange 10% to 40%. In the case of sensible and latent heatfluxes, the overall behavior is controlled by the amount ofenergy available. For pollutant deposition, there is noconstraint equivalent to the net radiation limitation onconvective heat exchange. Consequently, dry deposition rates andair-surface exchange of trace gases in general are especiallyvulnerable to errors in spatial extrapolation. The resultsindicate that the susceptibility of dry deposition formulationsto terrain complexity depends on the deposition velocity itself. For readily transferred pollutants (such as HNO3), a factorof two error could be involved.

Comparisons of Measured and Modelled Ozone Deposition to Forests in Northern Europe

The performance of a new dry deposition module, developedfor the European-scale mapping and modelling of ozone flux to vegetation, was tested against micrometeorological ozone and water vapour flux measurements. The measurement data are for twoconiferous (Scots pine in Finland, Norway spruce in Denmark) and one deciduous forest (mountain birch in Finland). On average, themodel performs well for the Scots pine forest, if local inputdata are used. The daytime deposition rates are somewhat over-predicted at the Danish site, especially in the afternoon. The mountain birch data indicate that the generic parameterisationof stomatal responses is not very representative of this northernspecies. The module was also tested by using modelled meteorological data that constitute the input for a photochemical transport model.

The Role of Vertical Mixing in the Temporal Evolution of Ground-Level Ozone Concentrations

Aircraft measurements taken during the North American Research Strategy for Tropospheric Ozone-Northeast field study reveal the presence of ozone concentration levels in excess of 80 ppb on a regional scale in the nocturnal residual layer during ozone episodes. The air mass containing increased concentrations of ozone commonly is found on a horizontal spatial scale of about 600 km over the eastern United States. The diurnal variation in ozone concentrations at different altitudes, ozone flux measurements, and vertical profiles of ozone suggest that ozone and its precursors trapped aloft in the nocturnal residual layer can influence the ground-level ozone concentrations on the following day as the surface-based inversion starts to break up. A simple onedimensional model, treating both meteorological and chemical processes, has been applied to investigate the relative contributions of vertical mixing and photochemical reactions to the temporal evolution of the groundlevel ozone concentration during the daytime. The results demonstrate that the vertical mixing process contributes significantly to the ozone buildup at ground level in the morning as the mixing layer starts to grow rapidly. When the top of the mixing layer reaches the ozone-rich layer aloft, high ozone concentrations are brought down into the mixing layer, rapidly increasing the ground-level ozone concentration because of fumigation. As the mixing layer grows further, it contributes to dilution while the chemical processes continue to contribute to ozone production. Model simulations also were performed for an urban site with different amounts of reduction in the ground-level emissions as well as a 50% reduction in the concentration levels of ozone and its precursors aloft. The results reveal that a greater reduction in the ground-level ozone concentration can be achieved by decreasing the concentrations of ozone and precursors aloft than can be achieved from a reduction of local emissions. Given the regional extent of the polluted dome aloft during a typical ozone episode in the northeastern United States, these results demonstrate the necessity and importance of implementing emission reduction strategies on the regional scale; such regionwide emission controls would reduce effectively the long-range transport of pollutants in the Northeast.

On-line measurements of ozone surface fluxes: Part II. Surface-level ozone fluxes onto the Sahara desert

Surface-level ozone concentrations, the vertical turbulent ozone flux as well as the fluxes of sensible and latent heat were continuously monitored by the eddy covariance method in the Lybian desert, 30 km south of the Dakhla Oasis in Egypt, from 23 March until 9 April 1993. An automatic station powered by a photovoltaics generator system was used to measure the vertical turbulent ozone flux to the desert ecosystem. Fairly high ozone volume fractions up to 60 ppb were recorded when northerly winds prevailed. When southerly winds were blowing, the ozone volume fractions were lower and reached maximum values slightly above 40 ppb. On-line eddy correlation measurements of the vertical turbulent ozone flux to the desert were performed with a novel fast-response ozone sensor. The fairly small ozone fluxes were corrected for effects of micro-turbulent density fluctuations caused by the concomitant fluxes of heat and water vapour in the air volume (Webb correction). While ozone fluxes to the desert ecosystem are below 2 ppb cm s − in the night, maximum daytime ozone fluxes of 20 ppb cm s −1 were measured which yielded a maximum daily dry deposition velocity of 0.15 cm s −1. During the whole measurement campaign of 16 d a mean deposition velocity of V d = 0.065 cm s −1 for ozone is calculated. For global numerical models in which the sources and sinks of ozone in the troposphere are taken into account, a daytime V d of 0.1 cm s −1 and a nighttime value of 0.04 cm s −1 are recommended for the desert ecosystem.

Ultraviolet Irradiance over Mexico City

Abstract This article compiles and analyzes information on the interaction of both ground-level ultraviolet radiation and anthropogenic ozone in the extremely polluted atmosphere of Mexico City. Simultaneous anthropogenic ozone and global flux (sun and sky) measurements were recorded in near ultraviolet radiation (295-385 nm) for a 30-month period (March 1990 to August 1992). Results reveal that under clear sky conditions, high concentrations of ozone (and presumably of other pollutants) impede UV-fluxes from reaching their maximum values. Furthermore, heavy cloud cover causes ozone concentrations to reach their minimum values together with UV-minimum fluxes. Seasonal variation in UV-irradiance is also discussed. Findings indicate that maximum UV-fluxes occur in spring and summer and that minimum values occur in autumn and winter. Significant daily local reductions in UV-fluxes (up to 50%) during afternoon hours in Mexico City can be attributed to air pollution. Finally, the growing incidence of rickets i...

An evaluation of the regional acid deposition model surface module for ozone uptake at three sites in the San Joaquin Valley of California

Plants and soils act as major sinks for the destruction of tropospheric ozone, especially during daylight hours when plant stomata open and are thought to provide the dominant pathway for the uptake of ozone. The present study, part of the California Ozone Deposition Experiment, compares predictions of the regional acid deposition model ozone surface conductance module with surface conductance data derived from eddy covariance measurements of ozone flux taken at a grape, a cotton, and a grassland site in the San Joaquin Valley of California during the summer of 1991. Results indicate that the model (which was developed to provide long‐term large‐area estimates for the eastern United States) significantly overpredicts the surface conductance at all times of the day for at least two important types of plant cover of the San Joaquin Valley and that it incorrectly partitions the ozone flux between transpiring and nontranspiring components of the surface at the third site. Consequently, the model either overpredicts or inaccurately represents the observed deposition velocities. Other results indicate that the presence of dew does not reduce the rate of ozone deposition, contradicting to model assumptions, and that model assumptions involving the dependency of stomata upon environmental temperature are unnecessary. The effects of measurement errors and biases, arising from the presence of the roughness sublayer and possible photochemical reactions, are also discussed. A simpler model for ozone surface deposition (at least for the San Joaquin Valley) is proposed and evaluated.

Airborne flux measurements and budget estimates of trace species over the Amazon Basin during the GTE/ABLE 2B expedition

In situ airborne measurements of turbulent heat, moisture, momentum, ozone, and carbon monoxide fluxes in a convective boundary layer were obtained over a tropical rain forest between 1100 and 1630 LT on May 4, 1987. The aircraft flight path was chosen so as to fly over the tower site at the Ducke Forest Reserve near Manaus, Amazonas, Brazil. Both turbulence statistics and mean quantities were used to study the budgets of heat, water vapor, ozone, and carbon monoxide. Residuals were found in the heat and water vapor budgets and are largely explainable in terms of the bandwidth of the respective sensors used for making flux measurements. The confidence of the findings for the O3 and CO budgets were found to be limited by the error in the determination of the along‐track advective component. The ozone budget study shows an accumulation rate in the boundary layer of 0.3±0.2 ppbv h−1. The surface resistance to ozone during this flight was determined to be 0.06±0.03 s cm−1, while the aerodynamic resistance was 0.14–0.17 s cm−1. Results from the CO budget analysis show a midday accumulation rate of 0.6±0.3 ppbv h−1 in the Amazonian boundary layer. The evidence suggests production of CO in the PBL. A source of CO may exist below the lowest flight level (≈ 150 m), although it was not possible to determine what part of the flux at flight level was due to chemical production and what part may be due to surface emission.

Solar ultraviolet radiation induced variations in the stratosphere and mesosphere

Solar ultraviolet induced perturbations of the middle atmosphere occurring on the solar cycle time scale have received the most theoretical attention in the past because of the need for comparison with predicted anthropogenic trends or for evaluation of possible climatological consequences. However, short‐term perturbations occurring on time scales comparable to the solar rotation period are more readily observable at present. Studies of these perturbations allow basic tests of our understanding of the relevant physics and chemistry that are needed for more accurate long‐term model predictions. Detection of short‐term solar UV induced ozone and/or temperature responses is hindered even at low latitudes by endogenic dynamical forcing which results in an inverse phase relationship (for either ozone or temperature) with higher‐latitude variations for many events. Nevertheless, consistent correlative evidence for contributions of solar UV variability to ozone temporal behavior in the upper stratosphere and lower mesosphere has been obtained in recent years. The mean amplitude of the ozone response at low latitudes reaches a maximum near the 3‐mbar level of approximately 0.5% for a 1% change in the solar flux at 205 nm. The phase lag of the ozone response relative to the 205‐nm flux increases with decreasing altitude and is positive below 3 mbar. Above 3 mbar, increasingly negative lags are measured (i.e., the ozone maximum leads the UV maximum). The increasingly negative lags imply the existence of a positive temperature perturbation following the UV maximum that acts to reduce the amplitude of the ozone response during the latter part of the ozone response cycle. Weak correlative evidence for the presence of the inferred temperature perturbations has been obtained in some studies. It is shown that these temperature perturbations have approximately the correct amplitudes and phase lags needed to explain the negative ozone lags. The derived positive temperature phase lags are approximately twice as large as would be predicted by one‐dimensional models that consider only radiative‐photochemical coupling. The importance of dynamical coupling in producing these temperature perturbations is therefore indicated. A possible major source of dynamical coupling is alteration of the reflection‐transmission properties of planetary waves, a process for which some observational evidence exists on the solar rotation time scale. Finally, in regions where transport effects on ozone concentration may be neglected, simultaneous measurements of ozone, temperature, and solar UV flux can be combined to calculate parameters that are directly relatable to photochemical theory. These are the odd oxygen photochemical relaxation time (approximately half of the conventional odd oxygen lifetime) and the chemical sensitivity of odd oxygen to local temperature changes. It is therefore possible that observed upper stratospheric and lower mesospheric responses to measured solar irradiance changes will be useful to constraining photochemical theory.

Direct measurements of nitrogen oxides and ozone fluxes over grassland

Using the eddy correlation method, fluxes of nitric oxide, nitrogen dioxide, ozone, water, and sensible heat were measured at a site 20 km north of Denver, Colorado over mature crested wheat grass, 0.75 m high in late June and early July. During this period the weather was fair with no synoptic disturbances. In the early morning a well-mixed diluted urban pollution plume traversed the site, by late morning aged pollution had mixed downward into the local boundary layer, and by afternoon the air came from a relatively unpolluted area of the high plains. The mean trace gas concentrations reflect this repeated pattern of local air flow. The fluxes of the trace gases were influenced both by the variation of the means and by other factors including temperature and biological activity. Ozone fluxes were found to be always negative and proportional to the mean, with an average deposition velocity for this case of about 0.006 m s-1. For the oxides of nitrogen this simple treatment was not appropriate. Both deposition and emission were observed, generally deposition predominated in the morning and emission in the afternoon with observed variations in the fluxes of NOx=NO+NO2 from −0.3 to +0.2 ppbv m s-1.

Concurrent measurements of ozone dry deposition using eddy correlation and profile flux methods

Simultaneous surface layer ozone flux measurements by eddy flux and profile methods are reported with supporting micrometeorological data. The results show a close correspondence between the eddy diffusivities of ozone and sensible heat rather than momentum eddy diffusivity. Ozone deposition velocities are computed both from the eddy flux and profile fluxes of ozone. As with eddy diffusivities, these results clearly show the sensible heat analogy giving the better fit with the eddy flux values. These sets of deposition velocities also have a consistent temporal variation, with peak values of up to 1 cm/s at midday changing to less than 0.1 cm/s in the evening.

Ozone concentration studies and ozone flux measurements near the ground at Poona

Using a modified Brewer bubbler ozone sensor, continuous measurements of the ozone concentration near the ground were made at Poona (18°N, 73°E) for one year 1969–1970. The surface ozone concentration shows a pronounced seasonal variation, with a minimum during the monsoon months and a maximum during the pre-monsoon summer months. There is also a marked diurnal variation in surface ozone concentration which clearly follows the diurnal variation of temperature and is again a maximum during the summer months and a minimum during the monsoon. A secondary maximum in ozone concentration occurs in the forenoon during the winter months, associated with the temperature inversions that occur near the ground in this season.

Ozone concentration studies near the ground at Poona- Part I: Diurnal, seasonal and other time variations and ozone flux measurements

Using a Brewer bubbler ozone sensor, designed and developed in the Instruments Research Laboratories of the India Met. Dep. continuous measurements of ozone concentration near the ground were made at Poona (18°N, 73°E) for one year 1969,70. There is a marked diurnal variation in surface ozone concentration which clearly follows the diurnal variation of temperature and is again a maximum during the pre-monsoon hot season months and a minimum during the monsoon. The surface ozone concentration also shows a pronounced seasonal variation, with a minimum during the monsoon months and a maximum during the pre-monsoon hot weather months. A secondary maximum in ozone concentration occurs in the forenoon during the winter months, associated with the breakdown of temperature inversion that occurs near the ground.
 From continuous measurements of surface ozone made with three electrochemical sensors exposed at three levels, 0,15 and 35 m above the ground, the ozone flux was calculated using the rate of decay method used by Kroening and Ney, and Regener's profile method. The profile method gives values of 1. 71 to 7. 04X 1011 mol/cm2/sec and the rate of decay method gives 4.2 to5.6 x 10l1mol/cm2/sec which are in good agreement with the flux values reported by other investigators.