Concentrations of ammonia and nitrogen dioxide at roadside verges, and their contribution to nitrogen deposition

Abstract

Bimonthly integrated measurements of NO 2 and NH 3 have been made over one year at distances up to 10 m away from the edges of roads across Scotland, using a stratified sampling scheme in terms of road traffic density and background N deposition. The rate of decrease in gas concentrations away from the edge of the roads was rapid, with concentrations falling by 90% within the first 10 m for NH 3 and the first 15 m for NO 2. The longer transport distance for NO 2 reflects the production of secondary NO 2 from reaction of emitted NO and O 3. Concentrations above the background, estimated at the edge of the traffic lane, were linearly proportional to traffic density for NH 3 (μg NH 3 m −3 = 1 × 10 −4 × numbers of cars per day), reflecting emissions from three-way catalysts. For NO 2, where emissions depend strongly on vehicle type and fuel, traffic density was calculated in terms of ‘car equivalents’; NO 2 concentrations at the edge of the traffic lane were proportional to the number of car equivalents (μg NO 2 m −3 = 1 × 10 −4 × numbers of car equivalents per day). Although absolute concentrations (μg m −3) of NH 3 were five times smaller than for NO 2, the greater deposition velocity for NH 3 to vegetation means that approximately equivalent amounts of dry N deposition to road side vegetation from vehicle emissions comes from NH 3 and NO 2. Depending on traffic density, the additional N deposition attributable to vehicle exhaust gases is between 1 and 15 kg N ha −1 y −1 at the edge of the vehicle lane, falling to 0.2–10 kg N ha −1 y −1 at 10 m from the edge of the road.