Hourly mixing ratios of HNO3, NH3 and various other trace gas and aerosol species were determined at Harvard Forest in central Massachusetts between 1991 and 1995 in order (1) to ascertain the representativeness of this composite data set, (2) to identify general seasonal and diurnal trends in the mixing ratios of HNO3 and NH3 at this site, (3) to establish how these mixing ratios depend on characteristic meteorological parameters such as wind direction and temperature, and (4) to examine HNO3 relative to the other reactive nitrogen species measured. Harvard Forest receives air masses from both urban and rural source regions resulting in mean midday HNO3 mixing ratios 4 times higher when surface winds were from the SW (~2000 parts per trillion by volume (pptv)) as opposed to the NW (~ 500 pptv) wind sector. The HNO3 diel cycle provides evidence of entrainment of HNO3 from aloft as the nocturnal inversion breaks down. Gaseous NH3 mixing ratios are typically 200--300 pptv and on average exhibit little diel variability. High levels of atmospheric sulfate consistently suppress NH3 concentrations below the predicted NH3 compensation point of the canopy, even during periods when the sum of NH3 and NH4+(NHx) mixing ratios are quite high. The aerosol SO42- regulation of NHx partitioning changes as a function of temperature. At the same SO42- mixing ratio, colder ambient temperatures result in lower NH3/NHx. On average HNO3 makes up about 20% of NOy at midday. The sum of the measured NOy species (NO, NO2, HNO3, and particulate NO3-) typically account for 60--80% of NOy, suggesting that peroxyacetylnitrate and other organic nitrates are a significant fraction of NOy at this predominantly oak forested site. ¿ 1999 American Geophysical Union