Measurements at a rural site in central Ontario, Canada, in August 1988 showed gradual increases in particle nitrate (&rgr;NO-3) concentration from 1800 hours to midnight and a decrease thereafterward toward the early morning. A time-dependent Lagrangian box model, which included homogeneous and heterogeneous chemical reactions of nitrogen species, dry deposition of O3, NO2, HNO3 and &rgr;NO-3, and sources of NOx, was used to stimulate the observed nightime increase of pNO-3 for four nights in August. The model results indicate that the nighttime pNO-3 was mainly formed by reactions of the nitrate radical (NO3) and N2O5 with particles, with these species contributing on average more than 80% and about 10% of the total nightly pNO-3 formation, respectively. Conversion of HNO3 to pNO-3 was found to be negligible compared to these two processes, contributing less than 5%. By assuming steady-state concentrations, a simplified approach to fit nighttime pNO-3 concentrations using only NOx and O3 observations gave results similar to the box model. As expected, the pseudo-first-order rate constants for the formations of pNO-3 on particles from NO3 and N2O5 were found to be linear functions of the particle mass.

When asumming an extended Junge distributions for the particles, the rate constants indicated a near unity sticking coefficient (α1) for NO3 but a substantially smaller sticking coefficient (α2) of about 0.02 for N2O5, consistent with results from laboratory studies. Sensitivity tests showed that the ranges of 0.8--1.0 for α1 and 0.01--0.04 for α2 gave resonable model fits for pNO-3 and HNO3. The box model showed that the heterogeneous reactions were the major sinks for NO3 and N2O5. The dry deposition rate velocities for O3, NO2, pNO-3, and HNO3 derived from the model simulations were similar to observed values over similar vegetation. The dry deposition rate constant of pNO-3 appeared to be a function of the near-surface wind speed, although the dry deposition velocity of pNO-3 was quite variable. The box model predicted similar amounts of dry deposition for HNO3 and pNO-3 during each of the four nights. ¿American Geophysical Union 1993