Max HenkleIntroduction(1) WindMethodsThis study quantifies the flux of ozone onto the following surfaces: Ponderosa Pine needles and bark. Because only surface flux is being studied, all other removal pathways need to excluded. Therefore, the stomata need to be closed, and the samples neeTo quantify the chamber surface loss term, chambeDiscussionIt appears that bark and needle surfaces perform similarly under manipulation of concentration, temperature, and relative humidity. Bark flux is significantly higher under all conditions compared to needles, partly because the bark samples were still prOzone Deposition on Pinus Ponderosa Surfaces Max Henkle Abstract Stratospheric ozone is a secondary compound defined by the EPA as one of six “primary pollutants.” Recently, high ozone concentrations have damaged stands of Pinus ponderosa found in California’s Sierra Nevada mountain range. While the flux of ozone entering these forest ecosystems is well quantified, the relative contribution of each ozone removal mechanism is poorly understood. Surface deposition is one such removal mechanism. Surface deposition rates of ozone onto Pinus ponderosa needle and bark surfaces were measured in a laboratory chamber under varied conditions of relative humidity, temperature, and ozone concentration. Statistically significant correlations could not be established between surface deposition rate and ozone concentration, surface deposition rate and temperature, and surface deposition rate and relative humidity. The measured rates, when compared to the amount of area these surfaces represent, indicate surface deposition does not play a significant role in removing ozone from the ecosystem.Introduction Tropospheric ozone is a compound produced by the reaction of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) in the presence of sunlight (Haggen-Smit, 1952). Elevated tropospheric ozone concentrations can damage vegetation by oxidizing material in their stomates (Wesley et al., 1978). This leads to an eventual decrease in carbon accumulation (Arbaugh et al., 1998). Ozone has been defined by the Environmental Protection Agency as one of six criteria air pollutants (USEPA, 1996). Because forest ecosystems act as a removal mechanism for ozone-polluted air basins it is important to understand the removal mechanisms within the forest. The Sierra Nevada mountain region is a forested ecosystem that has been defined by the EPA as “serious” non-attainment area for high ozone concentrations (USEPA, 1996). Pinus ponderosa (Ponderosa pine) forest accounts for 8% of the vegetated area in the Sierra Nevada region (SNEP, 1996). Ponderosa Pine forest is being studied for two reasons: (1) As a common plantation tree, damage from ozone carries substantial economic consequences and (2) Ponderosas are more sensitive to ozone damage than other species (Miller and McBride, 1988). Prevailing winds blow ozone precursors from the Sacramento metropolitan area into the Sierra Nevadas, where they react and create ozone. This ozone is removed from the forest ecosystem through four major pathways, as illustrated in Figure 1: (1) by wind that carries some ozone away from the ecosystem, (2) by reactions where certain gases in the atmosphere are oxidized by ozone, (3) by ozone entering leaf stomata, and (4) by ozone deposition onto forest surfaces. (1) Wind ↑ O3→ Forest Ecosystem→ (2) Chemistry ↓ ↓ (4) Surface (3) Stomatal Deposition Uptake Figure 1. Forest ecosystem ozone removal pathways.Stomatal uptake is a major removal pathway (Grantz et al. 1997), but other major removal pathways of ozone in this forest ecosystem are poorly understood (Bauer et al., 2000). Data provided by Goldstein and Kuripus in the ESPM department at UC Berkeley indicates that removal pathways other than stomatal uptake represent a non-negligible amount of ozone uptake. The data from Goldstein and Kuripus indicates that at certain times over 50% of ozone loss in ponderosa pine forest is due to chemical reactions between ozone, nitrogen oxide (NO), and several VOCs that occur in the air within the canopy rather than plant and soil surfaces. Additional removal of ozone may be occurring via deposition directly onto the ecosystem’s surfaces. The goal of my research is to identify and quantify the deposition of ozone on the predominant surfaces one would find in a Ponderosa Pine. Uncertainty in other data collection methods has failed to accurately address the magnitude of surface deposition, so it is unclear whether the effect is small or large. Care is taken to examine each of the variables that are known to affect ozone surface chemistry: concentration, temperature, relative humidity, and time. The ultimate goal is to create a model that accurately predicts ozone deposition behavior. Methods This study quantifies the flux of ozone onto the following surfaces: Ponderosa Pine needles and bark. Because only surface flux is being studied, all other removal pathways need to excluded. Therefore, the stomata need to be closed, and the samples need to not be producing volatile organic compounds (VOCs) that would influence ozone loss. Dead needles were tested in a proton transfer reaction-mass spectrometer (Ionicon Analytik PTR-MS) to evaluate their emissions of ozone-reactive mono- and sesquiterpenes. The needles are baked at 50˚C for 24 hours in order to remove these compounds. The laboratory ozone chamber consists of a 6-liter ring of Pyrex capped on both sides by perforated Teflon film. A UVP Pen-Ray UV lamp in a separate Pyrex tube generates ozone, and air from a clean air generator (CAG) is metered to the lamp chamber by a MKS Mass-Flo controller. Thus [O3] is adjusted by changing the ratio between air flowing to the lamp chamber and total flow through the system. For the main system flow (3L/min), clean air passes through a bubbler that humidifies the air and through a bypass. The relative humidity (RH) of the air exiting the bubbler is nearly 100%. Thus adjusting the relative flows of the bubbler and bypasscontrols humidity. For instance, if one needed 50% RH, flow between the bubbler and bypass would be split evenly. The humidified air and ozonated air are mixed just prior to chamber injection (see Figure 2). The chamber is designed in such a manner as to minimize turbulence: air injected into the chamber flows along the sides
View Full Document
Unlocking...