Author: Anam Munir Khan

Publisher:

Published: 2024

Total Pages: 0

ISBN-13:

Tropospheric ozone (O3) is a secondary air pollutant formed through photochemical reactions involving biogenic and anthropogenic emissions of volatile organic compounds (VOC) and nitrogen oxides (NOx). Of particular importance to forests and agriculture is the dry deposition of O3 through plant stomata. Stomatal uptake of O3 creates oxidative stress in plants reducing net photosynthesis and biomass of major forest tree species and agricultural crops with cascading impacts to the global carbon and water cycling. Historically, O3 risk assessments to forests and other ecosystems have been carried out using measurements of ambient O3 concentrations. However, an increasing body of literature strongly advocates for using the stomatal uptake (or flux) of O3 for risk assessment because it represents a more biologically meaningful quantity. The stomatal uptake also represents a significant portion of O3 dry deposition directly impacting the tropospheric O3 budget. The dual significance of stomatal O3 flux as an indicator of the phytotoxic dose to vegetation and as a tropospheric O3 loss pathway warrants continued monitoring of ecosystem O3 flux. Furthermore, many ecosystems face accumulating stomatal O3 uptake under predicted increases in aridity, droughts, and heatwaves which has the potential to further decouple stomatal uptake from O3 concentrations. This further stresses the importance of understanding how the simulation of moisture stress in current stomatal conductance schemes implemented in the dry deposition components of chemical transport and air quality models impacts the simulation of the stomatal component of O3 dry deposition.Existing O3 flux measurements have facilitated various model comparisons and the development of dry deposition schemes. The have also helped us develop an understanding of the dynamics of O3 exposure to ambient concentrations and the stomatal uptake of O3 across forested and agricultural sites. However, long-term O3 flux measurements remain spatiotemporally sparse hindering our ability to understand exposure-uptake dynamics across poorly represented system like vegetation utilizing the C4 photosynthetic pathway. This is partly because measuring O3 flux has largely utilized instruments that are difficult to operate and maintain in the field. Furthermore, while various disparate model intercomparisons of O3 deposition and the stomatal component exist, there has yet to be a multi-site multi-model comparison of the simulated stomatal component with observed flux-based estimates of the stomatal component with a specific focus on the simulation of moisture stress. This dissertation studies the stomatal component of O3 flux measurements across forested, shrubland, and agricultural sites to understand how plant ecophyisology impacts the dry deposition of O3 and how the simulated stomatal moisture stress in dry deposition schemes impacts the simulation of the stomatal component. The simulated stomatal component is compared with observed flux-based estimates of the stomatal component to identify sources of disagreement between the two. For Chapter 1, O3 flux measurements were measured using two UV absorption-based instruments that are easier to maintain in the field compared to the instruments that are typically used, the NASA ROZE and 2B Technology's dual-beam O3 monitor with an experimental upgrade to sample at 10 Hz, over the growing season in a maize agricultural field in central Illinois, United States. After analyzing the cospectra of, and lag covariance between vertical wind and O3 from both instruments, the 2B O3 monitor was unable to achieve reasonable flux while ROZE showed a much clearer flux signal. Chapter 3 used NASA ROZE O3 flux measurements to study how crop ecophysiology and structure impact the component sinks in the bulk O3 flux. ROZE ozone flux was partitioned into stomatal and non-stomatal fractions using an inversion of the Penman-Monteith equation with latent heat flux and a stomatal optimality-based model with the net ecosystem exchange (NEE) of carbon dioxide partitioned into gross primary productivity (GPP). The stomatal flux can make up to 50 - 80% of the total flux of O3 to the field during peak leaf area index and canopy height. The deposition velocity of O3 was coupled with stomatal conductance indicating that stomatal conductance is a strong driver of deposition velocity to the field. Chapter 3 is an intercomparison of the stomatal component of O3 dry deposition (egs) from various air quality and chemical transport models at four forested sites, one shrubland site, and one grassland site. Egs was also estimated using the observed flux of CO2, H2O, and O3 that were not used as forcing data to compare with the simulated egs. Finally, sensitivity simulations that perturbed model parameters controlling the impact of moisture stress on stomatal regulation were conducted to identify sources of disagreements among models during times of water stress. Multiyear monthly means from the observed flux-based estimates fall within the interquartile range (IQR) of multiyear means from single point models throughout the growing season across most sites with disagreements during the later part of the growing season. Interannual variation in egs reveals that disagreements between single point modeled egs and observed flux-based egs can result from the treatment of soil moisture stress in the models. Single point modeled soil moisture effects on stomatal conductance are too strong in a light and temperature limited northern forest where high light and temperatures favor high net ecosystem productivity. The single point models also struggle with simulating the phenology of egs in an ecosystem where seasonal water availability effects the seasonality of stomatal conductance.