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

Abstract. The dynamic behavior of nitrogen oxides (NO_x = NO + NO₂) and ozone (O₃) above and within the canopy at the University of Michigan Biological Station AmeriFlux (UMBS Flux) site was investigated by continuous multi-height vertical gradient measurements during the summer and the fall of 2008. A daily maximum in nitric oxide (NO) mixing ratios was consistently observed during the morning hours between 06:00 and 09:00 EST above the canopy. Daily NO maxima ranged between 0.1 and 2 ppbv (with a median of 0.3 ppbv), which were 2 to 20 times above the atmospheric background. The sources and causes of the morning NO maximum were evaluated using NO_x and O₃ measurements and synoptic and micrometeorological data. Numerical simulations with a multi-layer canopy-exchange model were done to further support this analysis. The observations indicated that the morning NO maximum was caused by the photolysis of NO₂ from non-local air masses, which were transported into the canopy from aloft during the morning breakup of the nocturnal boundary layer. The analysis of simulated process tendencies indicated that the downward turbulent transport of NO_x into the canopy compensates for the removal of NO_x through chemistry and dry deposition. The sensitivity of NO_x and O₃ concentrations to soil and foliage NO_x emissions was also assessed with the model. Uncertainties associated with the emissions of NO_x from the soil or from leaf-surface nitrate photolysis did not explain the observed diurnal behavior in NO_x (and O₃) and, in particular, the morning peak in NO_x mixing ratios. However, a ~30% increase in early morning NO_x and NO peak mixing ratios was simulated when a foliage exchange NO₂ compensation point was considered. This increase suggests the potential importance of leaf-level, bidirectional exchange of NO₂ in understanding the observed temporal variability in NO_x at UMBS.