Nitric acid and ammonia at a rural northeastern U.S. site

Hourly mixing ratios of HNO 3 , NH 3 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 HNO 3 and NH 3 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 HNO 3 relative to the other reactive nitrogen species measured. Harvard Forest receives air masses from both urban and rural source regions resulting in mean midday HNO 3 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 HNO 3 diel cycle provides evidence of entrainment of HNO 3 from aloft as the nocturnal inversion breaks down. Gaseous NH 3 mixing ratios are typically 200–300 pptv and on average exhibit little diel variability. High levels of atmospheric sulfate consistently suppress NH 3 concentrations below the predicted NH 3 compensation point of the canopy, even during periods when the sum of NH 3 and NH 4 + (NH X ) mixing ratios are quite high. The aerosol SO 4 2− regulation of NH X partitioning changes as a function of temperature. At the same SO 4 2− mixing ratio, colder ambient temperatures result in lower NH 3 /NH X . On average HNO 3 makes up about 20% of NO Y at midday. The sum of the measured NO Y species (NO, NO 2 , HNO 3 , and particulate NO 3 − ) typically account for 60–80% of NO Y , suggesting that peroxyacetylnitrate and other organic nitrates are a significant fraction of NO Y at this predominantly oak forested site.