|Title||Stomatal and non-stomatal fluxes of ozone, NOx, and NOy to a northern mixed hardwood forest|
|Year of Publication||2007|
|Degree||Doctor of Philosophy|
|Number of Pages||192 pp.|
|University||University of Michigan|
|City||Ann Arbor, MI|
This is the first multi-year study partitioning ozone fluxes between stomatal and non-stomatal components in a temperate deciduous forest. This is important because of the predominance of this type of forest in midlatitude regions where ozone exposure can be elevated above background [USDA Forest Service, 2001]. This study compares measurements at the same site over portions of four growing seasons, looking at stomatal and non-stomatal behavior year to year. It also estimates growing season cumulative ozone burdens. Despite differences in stomatal conductances year to year (a reflection of the changes in photosythetic drivers), ozone fluxes did not vary greatly over these four years, nor did the canopy level ozone conductances, the sum of stomatal and non-stomatal conductances, which varied between 0.39 and 0.52 mol m-2 s-1. In contrast, the stomatal component was more variable, from 0.17 to 0.40 mol m-2 s-1. These measurements present an upper limit for stomatal ozone uptake to be estimated. Average daily ozone uptake data can be expanded over a growing season to estimate the seasonal ozone burden of the stomata in the forest. If the growing season is defined as 90 days (June 15 to September 15, the peak of the growing season), the ozone burden would have been 2.65 x107 nmol m-2 in the 2002 growing season, 5.30 x107 nmol m-2 in the 2003 growing season, 5.92 x107 nmol m-2 in the 2004 growing season, and 3.72 x107 nmol m-2 in the 2005 growing season. Despite the number of variables affecting these estimates, and a twofold maximum to minimum difference, they are on the same order of magnitude. These numbers don+t address episodic high ozone effects, but do represent the chronic ozone exposure when looking at a cumulative seasonal burden. The year to year variability in seasonal (90 day) non-stomatal burden is even smaller: 3.74 x107 nmol m-2 in the 2002 growing season, 1.78 x107 nmol m-2 in the 2003 growing season, 3.16 x107 nmol m-2 in the 2004 growing season, and 3.17 x107 nmol m-2 in the 2005 growing season. 4.2 Reactive nitrogen deposition This is the first NOx and NOy flux study at this site, one of only a few done in a temperate deciduous forest. There was no net NOx flux, but NOy flux behaved diurnally, strongest in the middle of the day. Having measured an NOy flux consisting, most likely, of HNO3 flux and PAN flux, this measurement puts an upper limit on both at this site. In the simplest assumed case of delivery of nutrient nitrogen to this nitrogen-limited forest, if all the NOy deposition in this study were HNO3 deposition, it would increase the available nutrient nitrate input to the forest by 8% over measured wet NO3 #NAME? Clearly, a longer NOy dataset, now that a functioning NOx and NOy flux system has been built, would be advantageous. Preferably over a larger variety of conditions, and in conjunction with PAN, HNO3, and other reactive nitrogen flux measurements, these measurements would join with the ongoing wet deposition measurements or, ideally, improved measurements of N deposition that can better discern dissolved organic nitrogen in precipitation [Hill, et al., 2005]. Further investigation of the fate of nitrogen species via studies of leaf-level and in-canopy nitrogen fluxes will complement abovecanopy studies, and parallel investigations involving gradient profiles below-canopy and nutrient use studies involving isotopically labeled nitrogen species are needed to better understand the fate and speciation of nitrogen. While leaf level uptake or emissions of NO, NO2, HNO3, and PAN have begun to be characterized, closing the nitrogen budget will require measurement of NOy component species in conjunction with an NOy measurement, as well as throughfall and soil emissions. 4.3 Flux system improvements To continue making flux measurements with this system, upgrading the UMMCI software to collect data at 10Hz would avoid the currently necessary correction from 1 Hz to 10 Hz data collection. The instrument reaction vessels, under current operating conditions, have residence times of 15 Hz for ozone and 4 Hz for NO, both better than the data acquisition software rate, so this correction, usually on the order of 10%, could be avoided. Additionally, the NO2 fluxes sampled through a shuttered photolytic converter should be examined, in case the six second residence time in the photolytic conversion cell is introducing smearing that is losing the information contained in higher frequency eddies. UMBS, in its century of existence, has amassed a long term record of climate data, and more recently a host of flux, trace gas, and biometric data. There are opportunities to synthesize this wealth of information, mine it for detailed relationships, and even, potentially, apply an ecosystem process model to investigate interactions among the drivers and constraints of regional change in these forests, through the carbon and nitrogen cycles and their connections to water, trace gas, and energy budgets. With the results of the studies described in this dissertation, a number of additional investigations may be undertaken, for example: #NAME? velocity can be modelled, then it can be applied to previous years when fluxes were not measured, in order to estimate fluxes and thus deposition for either O3 or NOy. This could also allow for the simpler ambient measurement of these compounds, avoiding the labor-intensive eddy covariance flux processing. #NAME? humidity, could be used to model stomatal conductance, expanding our knowledge of ozone deposition at other sites. #NAME? nitrogen deposition on carbon sequestration, using an ecosystem model. Overall, further measurements that report more detailed NOy speciation will further limit and clarify how much HNO3, PAN, and other reactive nitrogen oxides are delivered during each growing season to the UMBS forest ecosystem. This will also make it easier to determine the eventual fate of NOy components, and whether they are intercepted on canopy surfaces, taken up by stomata, delivered to the soil, or lost through chemical reactions in the canopy atmosphere. Additionally, more detailed speciation will also help inform hypotheses regarding the benefits of assimilation of atmospheric nitrogen might yield for this forest, as well as the damage that can be caused by reactive nitrogen oxides. The data presented here also suggest that the assumption made by many ozone deposition models v that non-stomatal ozone conductance is constant and low v is untenable, and emphasizes the need for improvements in modeling the partitioning of ozone flux. While this study leaves unanswered questions as to the mechanisms of nonstomatal conductance, is important in the formulation of mechanistic models of ozone fluxes to quantify their magnitude and identify patterns with respect to major environmental factors in order to determine impacts of tropospheric ozone in forest ecosystems. Further exploration of the year-to-year differences in gs and gns in this data set may reveal parts of this mechanism. Additionally, experiments are needed to partition non-stomatal fluxes into reactions occurring at surfaces or in gas-phase, to learn if ozone is lost between the top of the tower and the canopy surfaces. Finally, leaf-level studies, in chambers or in the canopy, are needed to test the hypothesis of a BVOC emission driving non-stomatal ozone flux, and if it is happening on or near the leaf surface, or farther from the canopy.