Urban Railroad Yards: An Underappreciated Source of Urban Black Carbon Particles

Railroads in the United States of America (USA) remain a major provider of freight transport. For most of the country, railroad engines are diesel-powered and frequently relatively old, creating local emission hotspots along the tracks and facilities within urban areas. Within cities, railroad yards concentrate activities such as switching railcars among trains and loading and unloading of the railcars. The switching of individual cars from one train to another or to sidings is performed by switcher engines, which repeatedly start and stop under load, emitting substantial particulate matter. Consequently, a one-year study was conducted using two AethLabs MA200 instruments, one upwind and one downwind of the yard. The measurement campaign around the yard in Rochester (ROC), NY, measured higher black carbon (BC) concentrations, even on the upwind side of the yard, where black deposits were observed on windowsills. Higher concentrations were often recorded during evening hours (20:00–00:00), coinciding with active car movements. Increased BC concentrations were consistently associated with low to moderate winds from western to southwestern sectors, implicating local combustion sources, including rail operations and nearby traffic corridors, with amplified impacts under limited dispersion conditions. Across all sites, elevated concentrations were absent under high wind speeds, indicating that dilution dominates over long-range transport. These findings highlight the combined role of source proximity, emissions, and dispersion regime in controlling urban BC and brown carbon BrC contributions to light absorption, with implications for exposure assessment of nearby residents and air quality management.