The team set out to answer a deceptively simple question: when instruments measure how much light urban aerosol particles absorb, how much of that signal actually comes from the fine soot particles everyone expects, and how much comes from the coarser mineral and road-dust particles that dominate during resuspension events? The distinction matters because light absorption by aerosols affects climate forcing, visibility, and the way cities estimate their black carbon emissions. Most monitoring networks sample only the fine fraction, typically particles smaller than 1 or 2.5 micrometers, on the assumption that nearly all absorption comes from combustion-derived black carbon carried by those small particles.

To test that assumption, the researchers deployed a suite of instruments in an urban street canyon during the spring dust season. Air was drawn through several size-selective inlets, cutting the aerosol at diameters of 1, 2.5, and 7.2 micrometers, and each fraction was fed into Aethalometer-type instruments that measure light attenuation through a filter and convert it into an equivalent black carbon concentration. Supporting measurements included a multi-angle absorption photometer, an optical particle spectrometer for size distributions, and X-ray fluorescence analysis to determine the elemental composition of the collected particles. Over the campaign, hourly PM10 concentrations averaged 27.1 micrograms per cubic meter, while PM2.5 and PM1 averaged only 7.1 and 4.3 micrograms per cubic meter respectively, a stark illustration of how much mass resides in the coarse fraction during dust season.

The headline finding was that submicron particles nonetheless dominated light absorption across all three measured size fractions, particularly at short wavelengths where black carbon and brown carbon absorb most strongly. Even though coarse particles carried the bulk of the mass, the fine combustion aerosol remained the principal absorber under most conditions. Campaign-mean equivalent black carbon was a modest 0.49 micrograms per cubic meter, and the PM1 fraction captured most of the combustion-derived black carbon, confirming that exhaust emissions from the surrounding traffic were overwhelmingly fine-mode. For cities that monitor black carbon with PM1 or PM2.5 inlets, this part of the result is reassuring.

But the picture changed when the researchers looked at the increments between size cuts. Relative to the PM1 measurement, hourly equivalent black carbon increased by 14 percent when the inlet was opened to PM2.5, and by 41 percent at PM7.2. In other words, a substantial share of the absorption signal recorded by instruments with larger inlets did not come from classic combustion soot at all. The enhancement was most pronounced during dust-resuspension events, when dry road surfaces and passing vehicles lofted clouds of mineral-rich grit into the street canyon. During those episodes, coarse particles measurably brightened the absorption signal, meaning that instruments calibrated for fine soot were effectively counting non-black-carbon absorbers as if they were black carbon.

Elemental analysis offered a clue to the culprit. The coarse fraction was enriched in silicon, iron, and aluminum, elements characteristic of crustal minerals and resuspended road material, and possibly of non-exhaust traffic emissions such as brake and tire wear. Iron oxides are known to absorb light across the visible spectrum, and mineral dust itself has wavelength-dependent optical properties. This chemistry helps explain a second key result: the absorption Ångström exponent, a spectral parameter used to distinguish black carbon from brown carbon and to estimate biomass burning contributions, behaved very differently depending on the size cut. During high-PM10 dust events the exponent showed stronger wavelength dependence, while under traffic-exhaust-dominated conditions its values approached unity, the signature expected for pure black carbon. When coarse particles entered the measurement, the exponent at 470 and 950 nanometers became difficult to interpret, and the biomass burning contribution estimates derived from it became unreliable.

The study has attracted detailed scrutiny through the open peer review process, and the referees’ comments underscore why the findings matter beyond one street canyon. One reviewer noted that the fixed multiple-scattering correction used by the Aethalometer was calibrated for submicron, black-carbon-dominated aerosols, and that applying it to coarse, mineral-laden particles may introduce systematic biases, potentially overestimating absorption in the coarse fraction. Another reviewer cautioned that the extra equivalent black carbon measured with larger inlets should not be read as an increase in actual black carbon mass, because the instrument converts attenuation into concentration using a black-carbon-specific mass absorption coefficient that mineral dust and other non-black-carbon absorbers do not share. The reviewers also flagged the difficulty of separating resuspended mineral dust from brake, tire, and rail wear using elemental data alone, since iron can originate from several sources and X-ray fluorescence does not reveal its chemical form.

These caveats do not undermine the central message; they sharpen it. The authors and reviewers agree that the choice of size cut-off is not a technical footnote but a first-order control on what absorption instruments actually see. A city measuring with a PM1 inlet will report a clean, combustion-dominated black carbon signal. The same street, sampled with a PM10 inlet during a dust event, will report absorption figures inflated by mineral particles, with skewed spectral parameters and distorted source apportionment. For networks that aim to harmonize equivalent black carbon datasets across Europe, and for modelers who feed absorption measurements into climate and air quality simulations, the implication is that size cut-off variability must be explicitly accounted for, or coarse-mode absorption will remain a poorly constrained term in the urban radiation budget.

The research also arrives at a moment of growing interest in non-exhaust traffic emissions. As tailpipe emissions fall with electrification, brake, tire, and road wear are becoming proportionally larger contributors to urban particulate matter, and several of these sources produce coarse, potentially light-absorbing particles. The Helsinki dataset, with its simultaneous optical and elemental measurements across three size fractions, provides a template for quantifying that contribution. The authors acknowledge that their analysis of high-dust episodes rests on a limited number of events, and both reviewers recommend future campaigns capturing additional dust-resuspension episodes to place the statistics on firmer ground. Uncertainty propagation between the paired instruments used to derive the coarse-fraction signal by subtraction is another priority, since the incremental absorption between size cuts is small enough that modest instrument offsets can matter.

For now, the study stands as a warning wrapped in a dataset: the invisible hand of street dust reaches into instruments designed to track soot. As northern cities warm and winters shorten, the window of the dust season may shift, but the physics will not change. Coarse particles will keep resuspending from the roads, and any measurement that ignores them risks mistaking ground-up minerals and brake debris for the black carbon of combustion. The researchers’ conclusion is straightforward: quantifying and attributing urban aerosol light absorption requires deliberate consideration of the particle size cut-off selected, because in a street canyon during dust season, what you measure depends very much on what you let through the inlet.

Subject of Research: Size-resolved light absorption by fine and coarse aerosol particles in an urban street canyon during spring street dust season

Article Title: Contributions of Fine and Coarse Particles to Light Absorption at an Urban Traffic Site during Street Dust Season

Article References: Barreira, L. M. F., Li, D., Hoivala, J., Aurela, M., Virkkula, A., Niemi, J. V., Manninen, H. E., Rönkkö, T., Timonen, H., & Saarikoski, S. (2026). Contributions of Fine and Coarse Particles to Light Absorption at an Urban Traffic Site during Street Dust Season. https://doi.org/10.5194/ar-2026-28

Image Credits: AI Generated

DOI: 10.5194/ar-2026-28

Keywords: black carbon, street dust, coarse particles, light absorption, urban aerosol, PM10, absorption Ångström exponent, traffic emissions, non-exhaust emissions, aethalometer, Helsinki, air quality