Site description This study was conducted from 19 July to 21 November 2008 at the AmeriFlux site in the UMBS domain (45.5932°N, 84.7130°W; Schmid et al., 2003). This site is located in an area rather distant from anthropogenic sources. The nearest major urban centers (population > 200,000) are Detroit, Michigan, ~350 km to the southeast, Milwaukee, Wisconsin, ~350 km to the southwest, and Chicago, Illinois, ~450 km also to the southwest (Thornberry et al., 2001). The UMBS forest falls in the transition zone between mixed hardwood and boreal forests with a mean annual (from 1979 to 2009) temperature of 6.7°C and rainfall of 803.4 mm (Vande Kopple, 2011). The pre-settlement forest, dominated by white pine, red pine, and hemlock, was cut around 1880. The area was disturbed repeatedly by fire until 1923. Today, within a 1 km radius of the Ameriflux tower, the forest is composed mainly of bigtooth aspen and trembling aspen, but there is also significant representation of maple, red oak, birch, and beech. In patches, there is a dense understory of young white pine, up to ~6 m high. The basal layer near the forest floor is dominated by bracken fern and saplings of red maple, red oak, beech, and white pine (Gough et al., 2007). Soils are well-drained and sandy, and they are classified as Entic Haplorthods of the Rubicon series (Lapin, 1990; United States Department of Agriculture Forest Service, 1990). The forest at UMBS has two distinctive layers: a crown layer and a basal layer (Fig. 1). The mean canopy height around the Ameriflux tower is ~22 m (Schmid et al., 2003). The averaged seasonal maximum (from 1999 to 2009) of the cumulative single-sided leaf area index (LAI, m2 m-2) was 3.5. LAI began to decrease in early-October, and it reached its averaged seasonal minimum of 1.5 by November. Instrumentation A UV absorbance monitor (DASIBI 1003-AH) was used to determine the mixing ratio profile of O3 through the canopy. Before installing the DASIBI at the site, a 5-point calibration was conducted against a TEI 49C monitor (Thermo Electron Corporation [TECO], Franklin, MA), which served as the transfer standard for the calibration. Brodin et al. (2010) describe the calibration of this transfer standard in detail. The calibration of the DASIBI resulted in a 1 ppbv offset with a 3% slope correction. The O3 data from the DASIBI were corrected for this difference. The detection limit of the DASIBI was 1 ppbv. The mixing ratio profile of NOx through the canopy was determined with a chemiluminescence analyzer (TEI 42C-TL; TECO). The TEI 42C-TL has two channels. The first channel measures nitric oxide (NO) via NO + O3 chemiluminescence. The second channel measures nitrogen dioxide (NO2) by redirecting air through a heated (325°C) molybdenum converter, which causes NO2—including other oxidized nitrogen compounds—to be converted to NO. NO2 is then determined by subtracting NO, obtained from the first channel, from the resulting NOx signal. There are several interferences in this NO2 measurement scheme (Steinbacher et al., 2007). The error in the NO2 measurement increases with rising levels of interfering gases such as nitrous acid (HONO), peroxyacetyl nitrate (PAN), and alkyl nitrates that contribute to the NO2-mode signal. However, in urban environments, NOx typically constitutes the largest fraction of oxidized nitrogen compounds (Spicer, 1982; Steinbacher et al., 2007); hence, NO2 concentrations obtained with the TEI 42C-TL will represent a reasonable estimate if the site is influenced by anthropogenic sources. Before the deployment of the TEI 42C-TL analyzer in the summer of 2008, the instrument was sent to TECO for preventive maintenance. TECO reported the instrument to have a NO2 conversion efficiency of 99.9% after servicing it. Ultra-zero air (Airgas Great Lakes, Inc., Royal Oak, MI) was used to establish baseline conditions and for dilution of a NIST-traceable 1 ppmv NO gas standard (Scott-Marrin, Inc., Riverside, CA) to 0.5 ppbv and 10 ppbv calibration gas levels. After propagating the uncertainties of the mass flow controllers and the NO gas standard, we estimated the instrument to have a 5% accuracy error. The signal noise was 0.05 ppbv, which resulted in a detection limit of ~0.1 ppbv. Sampling Mixing ratio profiles of NOx and O3 were measured from the AmeriFlux tower at 4, 15, 21, 25, 34, and 40 m above the ground (Fig. 1). Sampling through each inlet was done sequentially from the 40 m level down to the 4 m level. The sampling inlet at a particular height was selected through a manifold constructed of an array of six two-way solenoid valves with polytetrafluoroethylene (PTFE) body seals (Norgren USA, Littleton, CO). Each sampling interval was 5 min long with gas concentrations determined in this flow every minute. A complete cycle took 30 min, thus there were 48 cycles per day. Perfluoroalkoxy (PFA) inlet funnels with 1 mm grids (Savillex Co., Minnetonka, MN) were used to prevent large debris from being drawn into the sampling line. Single stage 47 mm PFA filter clamps (Savillex Co.) with 5 mm PTFE membrane filter (Millipore Co., Bellerica, MA) were placed upstream of the instrument inlet to prevent fine particles from interfering with NOx and O3 measurements. The filters were replaced biweekly. All sampling lines, valves, and filters were conditioned for three days with a flow of 2 L min-1 of air containing 200 ppbv of O3 prior to installation. This was done to minimize the loss of O3 in the manifold during subsequent field sampling. Six 61 m-long PFA Teflon® tubes with outer diameter of 6.4 mm and inner diameter of 3.6 mm (Parker Hannifin, Cleveland, OH) were used as sampling lines. The flow rate through the DASIBI was 1.8 L min-1, and the TEI 42C-TL flow rate was 1.2 L min-1. Therefore, the total flow rate through each sampling line was 3 L min-1. The theoretical transport time of air samples from the inlet to the gas analyzers was calculated (using tubing dimensions, manifold volume, and purge rate) to be 15 s. Bias in the sampling lines An intercomparison was conducted by bringing all sampling inlets to the 15 m height of the tower to determine the potential measurement bias due to inherent differences in the sampling lines. Concentrations of NO and O3 and line pressure were monitored through each line for 5 min over a two-day period. The sampling lines varied < 0.1 ppbv in NO, < 1 ppbv in O3, and < 2 kPa in pressure against each other. Correcting for the loss of NO in the sampling lines Nitric oxide undergoes rapid oxidation through its reaction with O3 and other free radicals, e.g. hydroperoxy (HO2) and alkylperoxy (RO2), in the atmosphere. Therefore, it is necessary to correct for the loss of NO during the transport in the sampling line to the analyzer. Since ambient air HO2 and RO2 levels are two to three orders of magnitude smaller than NO (Fuchs et al., 2008), it was assumed they would not affect the sampled NO concentrations. The loss of NO due to oxidation by O3 alone was considered in the correction. In the absence of light, NO is oxidized to NO2 by, (1) where k is the reaction rate constant ( [cm3 molecules-1 s-1], for T between 195 and 308 K; Atkinson et al., 2004). The reaction rate constants were calculated using ambient temperature recorded when the air sample was collected. The conversion rate of NO was then determined from reaction (1) using the O3 concentration measured at any given moment from the same inlet. From this conversion rate, the percentage of NO lost after 15 s, which was the residence time of the air sample in the tube, was calculated in the air sample. Up to 32% of the NO was converted to NO2 by O3 depending on the air sample temperature, O3 mixing ratio, and line pressure. The NO mixing ratio was corrected for this loss. The NO2 concentrations were recalculated accordingly by subtracting the corrected NO mixing ratio from the 42C-TL’s output of NOx. Ancillary data Meteorological instrumentation on the AmeriFlux tower provided the ancillary data used in the analyses. Wind speed, wind direction, turbulence, global radiation, air temperature, and atmospheric pressure were collected according to protocols described by Schmid et al. (2003).