Abstract. We present a comparison of three absorption photometers that measured the absorption coefficient (σ_abs) of ambient aerosol particles in 2012–2017 at SMEAR II, a measurement station located in a boreal forest in southern Finland. The comparison included an Aethalometer (AE31), a Multi Angle Absorption Photometer (MAAP), and a Particle Soot Absorption Photometer (PSAP). These optical instruments measured particles collected on a filter, which is a source for systematic errors, since in addition to the particles, also the filter fibers interact with the radiation. To overcome this problem, several algorithms have been suggested to correct the data measured by the AE31 and the PSAP. Our aim is to study how the different correction algorithms affected the derived optical properties. We applied different correction algorithms to the AE31 and PSAP data, and compared the results against the reference measurements conducted by the MAAP. The comparison between the MAAP and AE31 resulted to a multiple scattering correction factor (C_ref) used in the AE31 correction algorithms to compensate for the scattering by the filter fibers. The C_ref varies between different environments, and our results are applicable for measurements conducted in a boreal environment. We observed a clear seasonal cycle of C_ref, which was probably due to the variations in aerosol optical properties, such as the backscatter fraction and single-scattering albedo, and also due to the variations in the relative humidity (RH) even though the RH in the instruments were kept below 40 %. The results showed that the filter measurement methods seemed to be rather sensitive to the RH even if the RH was below the recommended value of 40 %. The instruments correlated well (R ≈ 0.98) but the slopes of the regression lines varied between the instruments and correction algorithms: compared to MAAP, the AE31 underestimated the σ_abs (the slopes varied between 0.93–0.97) and the PSAP overestimated the σ_abs (the slopes varied between 1.07–1.24). The instruments and correction algorithms had a notable influence on the absorption Ångström exponent: the median absorption Ångström exponent varied between 0.93–1.54 for the different algorithms and instruments.

中文翻译：

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北部森林地带的三种光学吸收光度计的比较–不同校正算法的影响

摘要。我们提出的所测量的吸收系数3分吸收光度计的比较（σ_绝对）2012-2017年SMEAR II的大气气溶胶颗粒），该测量站位于芬兰南部的北方森林中。比较中包括一个烟度计（AE31），一个多角度吸收光度计（MAAP）和一个颗粒煤烟吸收光度计（PSAP）。这些光学仪器测量收集在滤镜上的颗粒，该滤镜是系统误差的来源，因为除这些颗粒外，滤镜纤维还与辐射相互作用。为了克服这个问题，已经提出了几种算法来校正由AE31和PSAP测量的数据。我们的目的是研究不同的校正算法如何影响导出的光学特性。我们对AE31和PSAP数据应用了不同的校正算法，并将结果与​​MAAP进行的参考测量进行了比较。C _ref）用于AE31校正算法中，以补偿滤光纤维的散射。该Ç_裁判不同的环境之间变化，而我们的研究结果适用于在北方的环境中进行测量。我们观察到了一个清晰的C _ref季节性周期，这可能是由于气溶胶光学特性的变化，例如反向散射分数和单散射反照率，以及相对湿度（RH）的变化，即使RH在仪器中保持在40％以下。结果表明，即使RH低于推荐值40％，过滤器的测量方法对RH也相当敏感。仪器相关性很好（R ≈0.98），但仪器和校正算法之间变化的回归直线的斜率：相比MAAP，所述AE31低估了σ _ABS（斜坡0.93-0.97之间变化）和PSAP高估了σ _ABS（斜率1.07之间变化–1.24）。仪器和校正算法对吸收ngström指数有显着影响：不同算法和仪器的中值吸收ngström指数在0.93-1.54之间变化。