Nitric acid (HNO3) and total reactive nitrogen (NOy) were measured at the Harvard Forest Long Term Ecological Research site in central Massachusetts along with micrometeorological and supporting data during the summer and fall of 2000. The concentration of HNO3 was measured using a tunable diode laser absorption spectrometer (TDLAS) installed on a tower above the forest canopy. The inlet was designed to keep the residence time short, to minimize wall effects, and to exclude aerosols from the sample flow. The TDLAS specifically and quantitatively measured gas-phase HNO3, eliminating interferences from other reactive nitrogen species such as PAN, hydroxy alkyl nitrates, NOx, and fine aerosols which are included in the NOy measurement. The hourly deposition velocity of HNO3 was estimated using a dry deposition inferential method (DDIM) in which the deposition velocity, Vd, is modeled as a set of resistances in series depending on meteorological and site-specific conditions. The flux was computed as the product of measured HNO3 concentration and inferred Vd. Diel correlations between HNO3 concentration and Vd did not introduce a substantial bias in the daily and weekly inferred flux when compared to the hourly values integrated over the same interval. The measurements confirm that HNO3 is often the primary NOy depositor and suggest conditions at Harvard Forest where other species are important. Under unpolluted, background flow conditions when winds are from the Northwest, HNO3 deposition only falls short of NOy deposition during morning daytime hours, in late summer and early fall, by approximately 20-30% of FNOy . During Southwesterly flow when the site is subject to warmer transport from polluted source regions, the deposition flux of HNO3 accounts for half or less of measured NOy deposition, leaving as much as 2-5 mu mol/m2/hr of the reactive nitrogen flux in the form of species not individually measured.