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<p>We have developed a novel single-beam photothermal interferometer and present here its application for the measurement of aerosol light absorption. The use of only a single laser beam allows for a compact optical set-up and significantly easier alignment compared to standard dual-beam photothermal interferometers, making it ideal for field measurements. Due to a unique configuration of the reference interferometer arm, light absorption by aerosols can be determined directly – even in the presence of light-absorbing gases. The instrument can be calibrated directly with light-absorbing gases, such as <span class="inline-formula">NO₂</span>, and can be used to calibrate other light absorption instruments. The detection limits (1<span class="inline-formula"><i>σ</i></span>) for absorption for 10 and 60 s averaging times were determined to be 14.6 and 7.4 <span class="inline-formula">Mm⁻¹</span>, respectively, which for a mass absorption cross section of 10 <span class="inline-formula">m² g⁻¹</span> leads to equivalent black carbon concentration detection limits of 1460 and 740 <span class="inline-formula">ng m⁻³</span>, respectively. The detection limit could be reduced further by improvements to the isolation of the instrument and the signal detection and processing schemes employed.</p>