Dry deposition of individual nitrogen species at eight Canadian rural sites

Abstract

Three nitrogen species in air, HNO 3, particle nitrate (pNO 3 -), and particle ammonium (pNH 4+), have been routinely measured for estimating nitrogen dry deposition in the Canadian Air and Precipitation Monitoring Network (CAPMoN) and the U.S. Clean Air Status and Trends Network (CASNET) for several decades. To investigate the relative contributions of other nitrogen species to total nitrogen dry and dry + wet deposition, 14 short-term field campaigns were conducted between 2001 and 2005 at eight selected rural sites across eastern Canada. Air concentrations were measured for the total oxidized nitrogen (NO y) and major species comprising NO y (NO, NO 2, PAN, PPN, HNO 3, pNO 3 -), and for the two reduced nitrogen species (NH 3, pNH 4 +). Dry deposition fluxes of NO y and NH x (= NH 3 + pNH 4 +) were then estimated by combining measured concentrations of individual nitrogen species with their respective dry deposition velocities estimated from big-leaf models using on-site meteorological measurements. Nitrogen dry deposition were estimated to be 0.8-4.0 kg N ha -1 a -1, depending on location, with 60-75% from NO y and 25-40% from NH x. HNO 3 and NO 2 dominated NO y dry deposition while NH 3 and pNH 4 contributed equally to NH x dry deposition. The pNO 3 -, PAN, and unidentified NO y species also contributed appreciable amounts to NO y dry deposition. Nitrogen dry + wet deposition from NO y + NH x was estimated at 4.3-11 kg N ha -1 a -1, with dry deposition accounting for 10-50%. The routinely monitored species accounted for less than 50% of total N dry deposition; thus, total dry + wet deposition from the Canadian monitoring network had a lower bias by 5-25% at most of the sites.