Abstract. The Arctic absorbing aerosols have a high potential to accelerate global warming. Accurate and sensitive measurements of their concentrations, variability and atmospheric mixing are needed. Filter-based aerosol light absorption measurement methods are the most widely applied in the Arctic. Those will be the focus of this study. Aerosol light absorption was measured during one month field campaign in June–July 2019 at the Pallas Global Atmospheric Watch (GAW) station in northern Finland. The campaign provided a real-world test for different absorption measurement techniques supporting the goals of the EMPIR BC metrology project in developing aerosol absorption standard and reference methods. Very low aerosol concentrations prevailed during the campaign which imposed a challenge for the instruments detection. In this study we compare the results from five filter-based absorption techniques: Aethalometer models AE31 and AE33, Particle Soot Absorption Photometer (PSAP), Multi Angle Absorption Photometer (MAAP) and Continuous Soot Monitoring System (COSMOS), and from one indirect method called Extinction Minus Scattering (EMS). The sensitivity of the filter-based techniques was adequate to measure aerosol light absorption coefficients down to around 0.05 Mm⁻¹ levels. The average value measured during the campaign using MAAP was 0.09 Mm⁻¹ (at wavelength of 637 nm). When data were averaged for > 1 h, an agreement of around 20 % was obtained between instruments. COSMOS measured systematically the lowest absorption coefficient values, which was expected due to the sample pre-treatment in COSMOS inlet. PSAP showed the best linear correlation with MAAP (R² = 0.85), followed by AE33 and COSMOS (R² = 0.84). The noisy data from AE31 resulted in a slightly lower, yet a significant, correlation with MAAP (R² = 0.46). In contrast to the filter-based techniques, the sensitivity of the indirect EMS method to measure aerosol absorption was not adequate at such low concentrations levels. An absorption coefficient on the order of > 1 Mm⁻¹ was estimated as the lowest limit, to reliably distinguish the signal from the noise. Throughout the campaign the aerosol was highly scattering with an average single-scattering albedo of 0.97. Two different air-mass origins could be identified: the north-east and from the north-west. The north-eastern air masses contained higher fraction of thickly coated light absorbing particles than the westerly air masses. Aerosol scattering, absorption and the particle coating thickness increased on the last ten days of the campaign during the north-eastern air flow. The simultaneous changes in aerosol source region, mixing state, concentration and particle optical size were reflected in the instruments' response in a complex way. The observed decrease in aerosol size suggested additional activation of secondary particle formation mechanisms. The results demonstrate the challenges encountered in the Arctic absorbing aerosol measurements. The applicability and uncertainties of different techniques are discussed and new knowledge on the absorbing aerosol characteristics in summer Arctic air masses reference to the source region is provided.

中文翻译：

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利用多仪器观测表征北极吸收气溶胶

摘要。北极吸收气溶胶具有加速全球变暖的巨大潜力。需要对其浓度，变异性和大气混合进行准确而灵敏的测量。基于过滤器的气溶胶吸光度测量方法在北极地区应用最广泛。这些将是本研究的重点。在2019年6月至7月的一个月野外活动期间，对芬兰北部的Pallas全球大气监测（GAW）站进行了为期一个月的野外测量。该活动为不同的吸收测量技术提供了实际测试，以支持EMPIR BC计量项目制定气溶胶吸收标准和参考方法的目标。整个运动期间气溶胶浓度非常低，这对仪器检测提出了挑战。在这项研究中，我们比较了五种基于过滤器的吸收技术的结果：烟气测量仪模型AE31和AE33，颗粒碳烟吸收光度计（PSAP），多角度吸收光度计（MAAP）和连续碳烟监测系统（COSMOS），以及一种间接方法称为消光负散射（EMS）。基于过滤器的技术的灵敏度足以测量低至0.05 Mm左右的气溶胶光吸收系数^-1级。在竞选期间使用MAAP测得的平均值为0.09 Mm ^-1（在637 nm波长下）。当平均数据> 1 h时，仪器之间获得约20％的一致性。COSMOS系统地测量了最低的吸收系数值，这是由于在COSMOS进样口中进行了样品预处理所预期的。PSAP与MAAP的线性相关性最好（R ² = 0.85），其次是AE33和COSMOS（R ² = 0.84）。AE31的嘈杂数据导致与MAAP（R ²= 0.46）。与基于过滤器的技术相反，在如此低的浓度水平下，间接EMS方法测量气溶胶吸收的灵敏度不足。大于1 Mm ^-1的吸收系数被估计为最低限度，以可靠地区分信号与噪声。在整个运动过程中，气溶胶高度分散，平均单散射反照率为0.97。可以确定两个不同的空气质量起源：东北和西北。东北气团比西气团含有更高比例的厚涂层吸光颗粒。运动的最后十天，在东北气流期间，气溶胶的散射，吸收和颗粒涂层厚度增加。气溶胶源区域，混合状态，浓度和颗粒光学尺寸的同时变化以复杂的方式反映在仪器的响应中。观察到的气溶胶尺寸的减小表明次级颗粒形成机制的额外活化。结果证明了在北极吸收气溶胶测量中遇到的挑战。讨论了不同技术的适用性和不确定性，并提供了参考源区的夏季北极气团吸收气溶胶特征的新知识。