Ozone 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 ozonepolluted 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 Deposition Figure 1 Forest ecosystem ozone removal pathways 3 Stomatal Uptake 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 bypass controls 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 in a concentric circle pattern air is drawn out near the center of the chamber and sent to the ozone analyzer Dasibi 1020 at the rate of 0 5 L min The total system flow is always greater than the ozone analyzer s flow so as to create an overpressure inside the chamber thus preventing outside air contamination Air escapes from the chamber via the perforations so the chamber remains at atmospheric pressure A probe Campbell Scientific model HMP45C inside the chamber measures RH and T The chamber is placed on top of a Pelletier cooler and surrounded by an insulated foam box To lower T the box is covered and the Pelletier turned on to raise T a 660W heat lamp suspended 30cm above the chamber is turned on Bypass Valve Needle Valve H20 Flowmeter Flowmeter O3 Chamber O3 Sensor Clean Air Generator Figure 2 Ozone system layout Atmosphere Atmosphere Switch Valve Data from the
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