Accurate estimation of black carbon (BC) from the widely used optical attenuation technique is important for the reliable assessment of their climatic impact. The optical instruments use Mass Absorption Cross-section (MAC) for converting light attenuation records to BC mass concentrations and Aethalometer is a widely used optical instrument for BC estimation. Several studies have shown large variability in MAC values. It is thus necessary to examine the accuracy and consistency of MAC values obtained using Aethalometer over distinct geographic locations and seasons. In the present study, MAC values are derived using simultaneous observations (2014–2017) from an EC-OC analyzer and an Aethalometer (AE-42) over a high altitude central Himalayan site at Nainital (29.4^oN, 79.5^oE, 1958 a.m.s.l). The observations reveal that the annual mean value of MAC (5.03 ± 0.03 m²g^− 1 at 880nm) is significantly lower than the constant value used by the manufacturer (16.6 m²g^− 1 at 880nm). The estimated MAC values also showed significant seasonal variation, spanning over a range from 3.7 to 6.6 m²g^− 1. It is found that the seasonal variability of elemental carbon (EC), air mass variation and meteorological parameters play an important role in the changes in MAC values over this region. Multi-wavelength determination of MAC shows the contribution of absorption by species other than EC at shorter wavelengths. MAC does not show a clear diurnal variation, unlike EC and absorption coefficient. The slope of EC vs. corrected equivalent black carbon (eBC) showed a significant improvement during all seasons when compared with uncorrected eBC. This lends credibility to the fact that the use of site-specific MAC leads to more reliable estimates of eBC over the central Himalayan region. It is found that, instead of using the site specific MAC value, had we used the one supplied by the instrument, we would have underestimated the radiative forcing by about 7.8Wm^− 2 which amounts to a reduction by 24 %.

从广泛使用的光学衰减技术中准确估算黑碳（BC）对于可靠评估其气候影响非常重要。光学仪器使用质量吸收横截面（MAC）将光衰减记录转换为BC质量浓度，而Aethalometer是一种广泛用于BC估算的光学仪器。多项研究表明，MAC值存在较大差异。因此，有必要检查在不同地理位置和季节使用Aethalometer获得的MAC值的准确性和一致性。在本研究中，MAC值是使用EC-OC分析仪和高度计（AE-42）在奈尼塔尔（29.4 ^o N，79.5 ^oE，1958 amsl）。观察结果表明，MAC的年平均值（在880nm处为5.03±0.03 m ² g ^-1）明显低于制造商使用的恒定值（在880nm处为16.6 m ² g ^-1）。MAC的估计值还显示出明显的季节性变化，范围从3.7到6.6 m ² g ^{− 1}。发现元素碳（EC）的季节变化，空气质量变化和气象参数在该区域的MAC值变化中起重要作用。MAC的多波长测定表明，在短波长下，除EC以外的其他物种对吸收的贡献。与EC和吸收系数不同，MAC没有明显的昼夜变化。与未校正的eBC相比，EC与校正后的等效黑碳（eBC）的斜率在所有季节中均显示出显着改善。这为以下事实提供了可信度：使用特定于站点的MAC可以对喜马拉雅中部地区的eBC进行更可靠的估计。我们发现，如果我们使用仪器提供的值，而不是使用特定于站点的MAC值，^{− 2}，减少了24％。