Factors affecting stomatal uptake of ozone by different canopies and a comparison between dose and exposure

A database of modeled gridded dry deposition velocities for 45 gaseous species and three particle size ranges across North America

The dry deposition process refers to the flux loss of an atmospheric pollutant due to uptake of the pollutant by the earth's surfaces. Dry deposition flux of a chemical species is typically calculated as the product of its surface-layer concentration and its dry deposition velocity (V_d). Field measurement based V_d data are very scarce or do not exist for many chemical species considered in chemistry transport models. In the present study, gaseous and particulate dry deposition schemes were applied to generate a database of hourly V_d for 45 gaseous species and three particle size ranges for two years (2016-2017) at a 15 km by 15 km horizontal resolution across North America. Hourly V_d of the 45 gaseous species ranged from < 0.001 to 4.6 cm/sec across the whole domain, with chemical species-dependent median (mean) values being in the range of 0.018-1.37 cm/sec (0.05-1.43 cm/sec). The spatial distributions of the two-year average V_d showed values higher than 1-3 cm/sec for those soluble and reactive species over certain land types. Soluble species have the highest V_d over water surfaces, while insoluble but reactive species have the highest V_d over forests. Hourly V_d of PM_2.5 across the whole domain ranged from 0.039 to 0.75 cm/sec with median (mean) value of 0.18 (0.20) cm s^-1, while the mean V_d for PM_2.5-10 is twice that of PM_2.5. Uncertainties in the modeled V_d are typically on the order of a factor of 2.0 or larger, which needs to be considered when applying the dataset in other studies.

Dry Deposition of Ozone over Land: Processes, Measurement, and Modeling

Dry deposition of ozone is an important sink of ozone in near surface air. When dry deposition occurs through plant stomata, ozone can injure the plant, altering water and carbon cycling and reducing crop yields. Quantifying both stomatal and nonstomatal uptake accurately is relevant for understanding ozone's impact on human health as an air pollutant and on climate as a potent short-lived greenhouse gas and primary control on the removal of several reactive greenhouse gases and air pollutants. Robust ozone dry deposition estimates require knowledge of the relative importance of individual deposition pathways, but spatiotemporal variability in nonstomatal deposition is poorly understood. Here we integrate understanding of ozone deposition processes by synthesizing research from fields such as atmospheric chemistry, ecology, and meteorology. We critically review methods for measurements and modeling, highlighting the empiricism that underpins modeling and thus the interpretation of observations. Our unprecedented synthesis of knowledge on deposition pathways, particularly soil and leaf cuticles, reveals process understanding not yet included in widely-used models. If coordinated with short-term field intensives, laboratory studies, and mechanistic modeling, measurements from a few long-term sites would bridge the molecular to ecosystem scales necessary to establish the relative importance of individual deposition pathways and the extent to which they vary in space and time. Our recommended approaches seek to close knowledge gaps that currently limit quantifying the impact of ozone dry deposition on air quality, ecosystems, and climate.

Dry deposition of O3 and SO2 estimated from gradient measurements above a temperate mixed forest

Vertical profiles of O3 and SO2 concentrations were monitored at the Borden Forest site in southern Ontario, Canada from May 2008 to April 2013. A modified gradient method (MGM) was applied to estimate O3 and SO2 dry deposition fluxes using concentration gradients between a level above and a level below the canopy top. The calculated five-year mean (median) dry deposition velocity (Vd) were 0.35 (0.27) and 0.59 (0.54) cm s(-1), respectively, for O3 and SO2. Vd(O3) exhibited large seasonal variations with the highest monthly mean of 0.68 cm s(-1) in August and the lowest of 0.09 cm s(-1) in February. In contrast, seasonal variations of Vd(SO2) were smaller with monthly means ranging from 0.48 (May) to 0.81 cm s(-1) (December). The different seasonal variations between O3 and SO2 were caused by the enhanced SO2 uptake by snow surfaces in winter. Diurnal variations showed a peak value of Vd in early morning in summer months for both O3 and SO2. Canopy wetness increased the non-stomatal uptake of O3 while decreasing the stomatal uptake. This also applied to SO2, but additional factors such as surface acidity also played an important role on the overall uptake.

Characteristics of surface O₃ over Qinghai Lake area in Northeast Tibetan Plateau, China

Surface O3 was monitored continuously during Aug. 12, 2010 to Jul. 21, 2011 at a high elevation site (3,200 m above sea level) in Qinghai Lake area (36°58'37″N, 99°53'56″E) in Northeast Tibetan Plateau, China. Daily average O3 ranged from 21.8 ppbv to 65.3 ppbv with an annual average of 41.0 ppbv. Seasonal average of O3 followed a decreasing order of summer>autumn>spring>winter. Diurnal variations of O3 showed low concentrations during daytime and high concentrations during late night and early morning. An intensive campaign was also conducted during Aug. 13-31, 2010 to investigate correlations between meteorological or chemical conditions and O3. It was found that O3 was poorly correlated with solar radiation due to the insufficient NOx in the ambient air, thus limiting O3 formation under strong solar radiation. In contrast, high O3 levels always coincided with strong winds, suggesting that stratospheric O3 and long range transport might be the main sources of O3 in this rural area. Back-trajectory analysis supported this hypothesis and further indicated the transport of air masses from northwest, northeast and southeast directions.

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 (15 min) 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.

Implementation of a module for risk of ozone impacts assessment to vegetation in the Integrated Assessment Modelling system for the Iberian Peninsula. Evaluation for wheat and Holm oak

A module to estimate risks of ozone damage to vegetation has been implemented in the Integrated Assessment Modelling system for the Iberian Peninsula. It was applied to compute three different indexes for wheat and Holm oak; daylight AOT40 (cumulative ozone concentration over 40 ppb), cumulative ozone exposure index according to the Directive 2008/50/EC (AOT40-D) and POD(Y) (Phytotoxic Ozone Dose over a given threshold of Y nmol m(-2) s(-1)). The use of these indexes led to remarkable differences in spatial patterns of relative ozone risks on vegetation. Ozone critical levels were exceeded in most of the modelling domain and soil moisture content was found to have a significant impact on the results. According to the outputs of the model, daylight AOT40 constitutes a more conservative index than the AOT40-D. Additionally, flux-based estimations indicate high risk areas in Portugal for both wheat and Holm oak that are not identified by AOT-based methods.