The direct effects of ozone on vegetation influence climate and feedback on tropospheric ozone concentration

Tropospheric ozone can cause significant damage to vegetation which can alter the partitioning of radiation into sensible and latent heat fluxes, directly influencing climate. The concentration of tropospheric ozone has approximately doubled since 1900 and is projected to continue increasing. Current global models and climate change projections account for emissions and the radiative warming associated with increasing ozone, however they do not account for the impact of increasing ozone on vegetation canopy fluxes and consequent impact on climate. Micrometeorological measurements at the Soybean Free Air Concentration Enrichment (SoyFACE; Champaign, IL) experiment have shown that soybean exposure to increasing ozone concentration result in warmer canopy temperatures and a greater Bowen Ratio (i.e. sensible/latent heat flux). This response was observed over multiple growing seasons, and showed a strong correlation with ozone over a wide range of concentrations and growing season conditions. While this experiment tested only soybean, it is likely that the response of other types of vegetation will have similar responses, as soybean employs the photosynthetic pathway commonly utilized among most temperate biomes. Because the rate of formation and destruction of ozone is affected by atmospheric temperature and humidity, the indirect effects of ozone on atmospheric conditions via plant canopies may affect ozone concentrations in ways that are not accounted for in current model predictions. We hypothesize that as ozone concentrations increase, an additional factor driven by ozone induced stress on vegetation could result in a key feedback on climate and consequently ozone concentrations. If so, this would imply that predictions of future ozone formation are underestimations. The goal of this project is to accurately represent the interaction between the climate, ozone and vegetation and to estimate the potential for a positive feedback on ozone concentration. To address this goal, we conducted simulations using the Weather Research and Forecasting (WRF) model coupled with a biophysically-based vegetation model. A sensitivity analysis was conducted to mimic the vegetation response observed for soybean. This was accomplished in two ways: 1) by perturbing the Bowen Ratio after it was calculated by the land surface model and 2) modifying key physiological parameters such as stomatal conductance, specific leaf area, and leaf greenness. A suite of simulations with varying levels of mimicked ozone responses were run using WRF as a single column model near two major Midwest US cities for a ten-day period in July. Our analysis indicates that the range of vegetation responses observed in the field was sufficient to cause appreciable changes in near-surface atmospheric state variables. Changes to atmospheric conditions induced by ozone damage to vegetation are greatest during midday, and the ozone effect is cumulative over time. Reduced latent heat flux and increased sensible heat flux have a warming and drying effect throughout the troposphere, with largest differences occurring in the planetary boundary layer. By the end of the ten-day simulation period, lower tropospheric and boundary layer temperatures are increased by up to 1.5 degrees Celsius and the boundary layer depth increases by ca. 200 m (i.e. 7%) while the water vapor mixing ratios decreases by ca. 0.4 g per kg (i.e. 5%). These results suggest the potential for a positive feedback on ozone formation through vegetation-induced alterations in atmospheric conditions; in particular, higher temperature favor the formation of ozone and lower humidity slows the destruction of ozone. Because ozone formation is highly dependent on temperature, changes on the order of 1 to 2 degrees Celsius are likely to have a significant impact on ozone concentration. Recent estimates indicate an ozone temperature sensitivity of approximately 2 to 5 ppb ozone per degree Celsius. To accurately predict the actual effect that the vegetation-climate response may have on ozone this work must be coupled with a full atmospheric chemistry model. The results of this experiment indicate that accurate global change predictions of ozone formation and other key climatic variables must consider the potential feedbacks between vegetation, climate and ozone.