Abstract In this study, the applicability of a benchtop polar photometer (PP_UniMI) to retrieve multi-wavelength aerosol absorption coefficient data by off-line measurements of the Multi-Angle Absorption Photometer (MAAP) sample spots is presented. MAAP is a widespread single wavelength online instrument providing the aerosol absorption coefficient and the equivalent black carbon concentration. In this work, MAAP samples collected during four different campaigns were analysed off-line with PP_UniMI. First of all, data from PP_UniMI and MAAP were compared to investigate contributions to measurement uncertainties. In particular, the role of the following assumptions performed in the MAAP was investigated: - reconstructing angular distribution of light scattered by filter samples from measurements at three fixed angles using analytical functions; - setting the fraction of the back-scattered radiation by the blank filter (BM) at a fixed value BM = 0.7: - assuming a fixed value for the asymmetry factor g = 0.75. Samples collected at several sites showed an agreement within 5% when data from the two instruments were retrieved using the same approximations (i.e. backscattered radiation from the filter matrix BM set at a fixed value, phase functions reconstructed by analytical functions from measurements at 3 angles, same asymmetry factor) in the data retrieval algorithm. Conversely, larger differences (14% on average) between off-line measurements and averaged MAAP data were obtained when the high-angular resolved information available by PP_UniMI was exploited. By analysing the role of MAAP assumptions for σap retrieval, it resulted that fixing BM = 0.7 was the main responsible for the detected differences. Indeed, high-angular resolved off-line measurements by PP_UniMI allow to directly measure BM, obtaining BM = 0.88 on white spots. It is noteworthy that the observed results were similar at all considered sites, so they proved to be independent of the aerosol type and can likely be attributed to instrumental effects. Moreover, a sensitivity test was carried out also to check the impact of the fixed value used for the asymmetry factor (set at g = 0.75 in both instruments). Varying g values within the typical range for ambient aerosol (0.50–0.75) the estimate of aerosol absorbance ABS (directly obtained from PP_UniMI measurements and linked to σap) was affected by 8% at most, thus being a minor source of uncertainty in the calculation. The effect of the variability in blank spots used for off-line analyses was also evaluated and resulted in a contribution smaller than 3% to the uncertainty of the methodology employed. Finally, the possibility of exploiting multi-wavelength assessment of absorption coefficients is an added value of PP_UniMI; indeed, it allows to estimate the contribution of different aerosol sources and components to the absorption coefficient using MAAP tapes used in present or past campaigns.

中文翻译：

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台式多波长极性光度计对多角度吸收光度计 (MAAP) 样品离线测量的适用性

摘要 在这项研究中，提出了台式极坐标光度计 (PP_UniMI) 通过离线测量多角度吸收光度计 (MAAP) 样品点来检索多波长气溶胶吸收系数数据的适用性。MAAP 是一种广泛使用的单波长在线仪器，可提供气溶胶吸收系数和等效黑碳浓度。在这项工作中，使用 PP_UniMI 离线分析了在四个不同活动中收集的 MAAP 样本。首先，比较来自 PP_UniMI 和 MAAP 的数据，以研究对测量不确定性的贡献。特别地，研究了在 MAAP 中执行的以下假设的作用： - 使用分析函数从三个固定角度的测量中重建由滤光片样本散射的光的角分布；- 将空白滤光片 (BM) 的反向散射辐射部分设置为固定值 BM = 0.7： - 假设不对称因子 g = 0.75 的固定值。当使用相同的近似值（即滤波器矩阵 BM 的反向散射辐射设置为固定值，通过 3 个角度的测量值通过分析函数重建的相位函数）检索来自两个仪器的数据时，在多个地点收集的样本显示出 5% 以内的一致性，相同的不对称因素）在数据检索算法中。相反，当利用 PP_UniMI 可用的高角度解析信息时，离线测量和平均 MAAP 数据之间的差异更大（平均 14%）。通过分析 MAAP 假设对 σap 检索的作用，得出固定 BM = 0。7 是检测到差异的主要原因。事实上，PP_UniMI 的高角度解析离线测量允许直接测量 BM，在白点上获得 BM = 0.88。值得注意的是，所有考虑地点的观测结果都相似，因此证明它们与气溶胶类型无关，并且可能归因于仪器效应。此外，还进行了灵敏度测试以检查用于不对称因子的固定值的影响（在两种仪器中均设置为 g = 0.75）。在环境气溶胶的典型范围 (0.50–0.75) 内改变 g 值，气溶胶吸收率 ABS 的估计值（直接从 PP_UniMI 测量获得并与 σap 关联）最多受 8% 的影响，因此是计算中不确定性的次要来源. 还评估了用于离线分析的空白点可变性的影响，并导致对所采用方法的不确定性的贡献小于 3%。最后，利用吸收系数的多波长评估的可能性是 PP_UniMI 的附加值；事实上，它允许使用当前或过去活动中使用的 MAAP 磁带来估计不同气溶胶来源和成分对吸收系数的贡献。