Street Dust Season Distorts Black Carbon Measurements: Helsinki Study Shows Coarse Particles Skew Absorption Data

A new study from Helsinki shows that spring street dust, the gritty mix of sand, mineral particles, and tire wear lifted off roads, can distort how instruments measure light absorption by urban aerosols. Researchers at an urban traffic site found that while fine combustion soot dominated absorption overall, opening the inlet to larger particles increased the measured equivalent black carbon by 41 percent during dust events. That extra signal comes from coarse mineral and iron-rich particles, not real black carbon, which can inflate reported concentrations and skew source apportionment. The study compared PM1, PM2.5, and PM7.2 size cuts and found that instruments with larger inlets effectively counted non-black-carbon absorbers like road dust as black carbon. This matters for cities and monitoring networks that compare black carbon data across different sites, since size cut-off choices can create large discrepancies. The authors and reviewers recommend accounting for size-selective sampling explicitly and conducting more campaigns during high-dust episodes to tighten the estimates. As tailpipe emissions decline with electrification, non-exhaust sources like brake and tire wear are becoming proportionally larger contributors to urban particulate matter. This study provides a template for separating those coarse, light-absorbing particles from combustion soot, which will help improve air quality monitoring and climate forcing estimates in cities.