| Reactive nitrogen trace gases are central to the photochemistry of the troposphere: they participate in powerful catalytic cycles, stimulate ozone (O3) production, affect the concentration of the hydroxyl radical (OH), terminate reaction chains, and contribute to acid precipitation. They also have profound influences on the biosphere, since the form of nitrogen available to plants via wet and dry deposition affects nutrient loading, acid stress, and toxicity.;We quantify the relative and absolute contributions of nitric acid (HNO 3) and NOx (nitric oxide (NO) + nitrogen dioxide (NO 2)) to total reactive nitrogen deposition at a typical Northeastern U.S. forest under a wide range of seasonal, meteorological, and transport conditions. A new tunable diode laser absorption spectrometer (TDLAS) was designed, built, and deployed on top of a 22 m tower at Harvard Forest to measure HNO3 and NO2 concentrations, with 1 s detection limits of several hundred pmol mol-1. The TDLAS NO 2 data were used to determine the eddy covariance flux. Other observations at the site included concentrations and fluxes of total odd nitrogen (NO y), NO, sensible heat and water vapor; and peroxyacetyl nitrate (PAN) concentration.;At night, NO2 deposition depended quadratically on NO 2 concentration. During the day, eddy flux observations of coupled downward NO and upward NO2 fluxes above the forest were driven by gradients of light and eddy diffusivity through the canopy, and represented no net NO x flux. NO2 flux can be parameterized as a simple function of light, concentration (linear and quadratic terms), and stomatal conductance. Average net NOx deposition velocity was 0.2 cm s-1 with little variation between day and night.;A dry deposition inferential method (DDIM) was used to estimate the hourly deposition velocity of HNO3. Weekly aggregates (simulating standard dry deposition network procedures) introduced noise but no significant bias into the HNO3 flux. HNO3 concentrations were slightly lower under clean (NW flow) conditions compared to more polluted (SW); inferred Vd(HNO3) was nearly the same for the two flow regimes, as low as 2 cm s-1 at night and up to 8 cm s-1 during the day. The concentration and flux budgets of NOy were essentially closed for the NW, and unaccounted by up to 50% for the SW. The deposition velocity of the unmeasured species (SW) was approximately 3 cm s-1. |
