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

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Abstract. Light absorption by urban aerosols is dominated by fine particles (particulate matter ≤2.5 µm; PM2.5), particularly from traffic exhaust and residential combustion. However, coarse particles (2.5–10 µm; PM2.5–10) can also contribute during spring street dust events in northern countries. This study investigated size-resolved optical properties and chemical composition of PM in an urban street canyon during spring. Hourly PM₁₀ concentrations averaged 27.1 µg m⁻³, while PM2.5 and PM1 (≤1 µm) concentrations averaged 7.1 and 4.3 µg m⁻³, respectively. Light absorption across PM₁, PM₂.₅, and PM₇.₂ fractions was dominated by submicron particles, predominantly at short wavelengths, while coarse particles enhanced absorption during dust-resuspension events. Equivalent black carbon (eBC) showed strong size dependence, with PM1 capturing most combustion-derived BC despite low campaign-mean concentrations (0.49 µg m⁻³). Relative to PM₁, hourly eBC increased by 14 % in PM2.5 and 41 % in PM7.2. Absorption Ångström exponent analysis revealed size- and source-dependent spectra differences, with stronger wavelength dependence during high PM10 dust events and values closer to unity under traffic-exhaust-dominated conditions. The inclusion of coarse particles complicated the interpretation of the AAE470/950 and related biomass burning contribution estimates. Elemental analysis revealed elevated concentrations of Si, Fe, and Al in coarse PM, suggesting contributions from crustal material and/or non-exhaust emissions that may also influence optical properties. These findings demonstrate that quantification and source identification of urban aerosol light absorption requires consideration of the selected particle size cut-off.

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