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The dynamic behavior of nitrogen oxides (NOx = NO + NO2) and ozone(O3) above and within the canopy at the University of MichiganBiological Station AmeriFlux (UMBS Flux) site was investigated by continuousmulti-height vertical gradient measurements during the summer and the fall of2008. A daily maximum in nitric oxide (NO) mixing ratios wasconsistently observed during the morning hours between 06:00 and 09:00 ESTabove the canopy. Daily NO maxima ranged between 0.1 and 2 ppbv (witha median of 0.3 ppbv), which were 2 to 20 times above the atmosphericbackground. The sources and causes of the morning NO maximum wereevaluated using NOx and O3 measurements and synoptic andmicrometeorological data. Numerical simulations with a multi-layercanopy-exchange model were done to further support this analysis. Theobservations indicated that the morning NO maximum was caused by thephotolysis of NO2 from non-local air masses, which were transportedinto the canopy from aloft during the morning breakup of the nocturnalboundary layer. The analysis of simulated process tendencies indicated thatthe downward turbulent transport of NOx into the canopy compensatesfor the removal of NOx through chemistry and dry deposition. Thesensitivity of NOx and O3 concentrations to soil and foliageNOx emissions was also assessed with the model. Uncertaintiesassociated with the emissions of NOx from the soil or fromleaf-surface nitrate photolysis did not explain the observed diurnal behaviorin NOx (and O3) and, in particular, the morning peak inNOx mixing ratios. However, a ~30% increase in earlymorning NOx and NO peak mixing ratios was simulated when afoliage exchange NO2 compensation point was considered. This increasesuggests the potential importance of leaf-level, bidirectional exchange ofNO2 in understanding the observed temporal variability in NOxat UMBS

Dynamics of nitrogen oxides and ozone above and within a mixed hardwood forest in northern Michigan