Depth‐resolved analysis of traffic soot sampled in the vicinity of a German motorway

Particulate air pollution is caused to a high extent by soot particles arising from traffic. The formation of soot particles and their interaction with the atmosphere usually results in an inhomogeneous depth distribution of its chemical inventory. Because the potential reactivity of the compounds strongly depends on their distance from the surface, the chemical composition at or near the surface is important. Therefore, airborne particles were sampled for depth-resolved analysis near a busy German motorway. The particles' sizes were classified with a multi-stage impactor and deposited on indium foil for analysis with plasma-based secondary neutral mass spectrometry (Plasma-SNMS), dynamic secondary ion mass spectrometry (SIMS) as well as XPS. At a distance of 2 m from the motorway, the fine particles smaller than 100 nm mainly consisted of carbon and organic hydrogen similar to virgin diesel soot particles produced by a test diesel engine. With increasing particle size, nitrogen, sulfur and oxygen occurred additionally in substantial amounts with a sulfur-oxygen ratio of 1:4. These elements were found in the surface region not deeper than 30 nm predominantly as ammonium sulfate. A minor fraction of the nitrogen was found to be part of nitro-organic compounds in the surface. Further away from the motorway no virgin soot particles were found. All soot particles (except for minor soil dust contributions) were found covered by the same surface layer as the motorway-near particles.