Heterogeneous nighttime production and deposition of particle nitrate at a rural site in North America during summer 1988

Measurements at a rural site in central Ontario, Canada, in August 1988 showed gradual increases in particle nitrate ( p 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 O 3 , NO 2 , HNO 3 and p NO 3 − , and sources of NO x , was used to simulate the observed nighttime increase of p NO 3 − for four nights in August. The model results indicate that the nighttime p NO 3 − was mainly formed by reactions of the nitrate radical (NO 3 ) and N 2 O 5 with particles, with these species contributing on average more than 80% and about 10% of the total nightly p NO 3 − formation, respectively. Conversion of HNO 3 to p NO 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 p NO 3 − concentrations using only NO x and O 3 observations gave results similar to the box model. As expected, the pseudo‐first‐order rate constants for the formation of p NO 3 − on particles from NO 3 and N 2 O 5 were found to be linear functions of the particle mass. When assuming an extended Junge distribution for the particles, the rate constants indicated a near unity sticking coefficient (α 1 ) for NO 3 but a substantially smaller sticking coefficient (α 2 ) of about 0.02 for N 2 O 5 , consistent with results from laboratory studies. Sensitivity tests showed that the ranges of 0.8–1.0 for α 1 and 0.01–0.04 for α 2 gave reasonable model fits for p NO 3 − and HNO 3 . The box model showed that the heterogeneous reactions were the major sinks for NO 3 and N 2 O 5 . The dry deposition velocities for O 3 , NO 2 , p NO 3 − , and HNO 3 derived from the model simulations were similar to observed values over similar vegetation. The dry deposition rate constant of p NO 3 − appeared to be a function of the near‐surface wind speed, although the dry deposition velocity of p NO 3 − was quite variable. The box model predicted similar amounts of dry deposition for HNO 3 and p NO 3 − during each of the four nights.