Atmospheric reactive nitrogen concentration and flux budgets at a Northeastern U.S. forest site

We report concentrations of atmospheric NOx, nitric acid (HNO3), peroxyacetyl nitrate (PAN), and NOy; eddy covariance fluxes of NOx and NOy; inferred fluxes of HNO3 at the mixed deciduous Harvard Forest field site, June–November 2000. A novel Tunable Diode Laser Absorption Spectrometer (TDLAS) produced sensitive, hourly HNO3 concentration data, which were used to evaluate systematic error in the Dry Deposition Inferential Method (DDIM), often employed to estimate weekly HNO3 flux at deposition monitoring network sites. Due to the weak diurnal variation in HNO3 concentration at Harvard Forest, no systematic bias was found in the application of this method to compute daily and weekly average fluxes. The sum of individually measured reactive nitrogen species concentrations and fluxes were approximately equal to total NOy concentrations and fluxes for clean Northwesterly flows, but fell short of the total NOy values for the more polluted Southwesterly transport regime. The concentration and deposition velocity of the unmeasured reactive nitrogen compounds were consistent with prior estimates and recent measurements of alkyl- and hydroxyalkyl nitrates, suggesting that these compounds play an important role in reactive nitrogen deposition processes where anthropogenic NOx emissions and natural hydrocarbons are present.