Stomatal and non-stomatal fluxes of ozone, nitrogen oxides, and total reactive nitrogen to a northern mixed hardwood forest

Measurements of ozone, sensible heat, and latent heat fluxes and plant physiological parameters were made at a northern mixed hardwood forest located at the University of Michigan Biological Station (LIMBS) in northern Michigan during the growing seasons 2002 through 2005. NO and NO2 fluxes were measured during the 2005 growing season, and NOy fluxes were measured in August 2005.;The ozone measurements were used to calculate total ozone flux and partitioning between stomatal and non-stomatal sinks. Total ozone flux varied diurnally with downward flux reaching -100 mumol m-2 h-1 at midday, at or near zero at night. Mean daytime canopy conductance varied over the four years from 0.39 mol m-2 s-1 (2002) to 0.52 mol m-2 s-1 (2004). Stomatal conductance showed expected patterns of behavior with respect to photosynthetic photon flux density (PPFD) and vapor pressure deficit (VPD). Estimated peak growing season stomatal ozone burden was 2.65x107 nmol m -2 in 2002, 5.30x107 nmol m-2 in 2003, 5.92x107 nmol m-2 in 2004, and 3.72x107 nmol m-2 in 2005. Non-stomatal conductance for ozone increased monotonically with increasing PPFD, and increased with temperature before falling off again at high temperature. Daytime nonstomatal ozone sinks were large and varied with time and environmental drivers. Daytime non-stomatal ozone conductance accounted for as much as 31% (2003) to 61% (2002) of canopy conductance, with the non-stomatal partition representing 2.0x105 nmol m-2 (2003) to 4.2x10 5 nmol m-2 (2002). Non-stomatal ozone conductance was strongly diurnal and a significant proportion of total canopy conductance.;NOx flux was essentially zero throughout the day. NO y flux was diurnal with strongest mid-day flux of -6 mumol m -2 hr-1 and nighttime flux at or near zero. With a downward NOy flux, the atmosphere at UMBS deposited nitrogen in non-NOx form. Integrating the diurnal average of NOy deposition over the three weeks of measurement indicates it was 10.6 mumol NOy-N m-2 day-1. If all NOy-N flux consisted of HNO3, this NOy deposition would potentially increase the available nutrient nitrate input to the forest by 8% over measured wet NO3- deposition.