Investigation of the uncertainties of simulated optical properties of brown carbon in two Asian sites using a modified bulk aerosol optical scheme of the Community Atmospheric Model version 5.3 (CAM5.3)

Recent studies have suggested that brown carbon (BrC), an absorbing component in organic aerosol (OA), has strong absorption in the near-ultraviolet wavelengths, and contributes to regional and global radiative forcing. However, the inclusion of BrC in global climate models leads to significant uncertainties in estimated radiative forcing, mainly attributed to uncertain BrC properties and relevant BrC parameters assigned in the model. In this study, we modified the bulk aerosol optical scheme (BAOS) in the Community Atmospheric Model version 5.3 (CAM5.3) by including BrC absorption and evaluated the performance of the modified BAOS by comparing the simulated aerosol absorption with two-year surface observational data in two Asian cities, Kanpur, India and Nanjing, China. The mean relative errors in the simulated total aerosol absorption (Babs) and absorption Angstrom exponent (AAE) in modified BAOS are around 30% in Kanpur and even below 20% in Nanjing. Our results show that the inclusion of BrC remedies the underestimated total aerosol absorption by 20% and 14% on average at Kanpur and Nanjing, respectively, exhibiting a better agreement with ground-based observations of aerosol absorption at both sites. We also conducted a series of sensitivity experiments to quantify the uncertainties caused by varying parameters related to BrC. The model simulations suggest that the imaginary refractive index of BrC is the most significant factor contributing to the uncertainties in aerosol optical properties calculated in BAOS at the Kanpur site. While in the Nanjing site, both the particle size distribution and mixing state have dominant impacts on the calculated aerosol optical properties.

近期研究表明，棕碳（brown carbon, BrC）作为有机气溶胶（organic aerosol, OA）中的吸收组分，在近紫外波段具有显著吸收，并对区域及全球辐射强迫形成贡献。然而，在全球气候模式中纳入棕碳会导致辐射强迫估算结果出现显著不确定性，这主要源于模式中棕碳的相关属性及参数设置存在不确定性。本研究通过纳入棕碳吸收过程，对社区大气模式5.3版本（Community Atmospheric Model version 5.3, CAM5.3）中的整体气溶胶光学方案（bulk aerosol optical scheme, BAOS）进行了改进，并通过对比印度坎普尔、中国南京两座亚洲城市两年的地面观测气溶胶吸收数据，评估了改进后整体气溶胶光学方案的性能。改进后的整体气溶胶光学方案中，模拟的总气溶胶吸收量（total aerosol absorption, Babs）与吸收埃指数（absorption Angstrom exponent, AAE）的平均相对误差在坎普尔约为30%，在南京甚至低于20%。研究结果显示，纳入棕碳后，坎普尔与南京站点的总气溶胶吸收量估算低估偏差分别被平均修正了20%与14%，两地的模拟结果与地基气溶胶吸收观测数据的吻合度均得到显著提升。本研究还开展了一系列敏感性试验，以量化棕碳相关参数变化带来的模拟不确定性。模式模拟结果表明，在坎普尔站点，棕碳的虚部折射率是影响整体气溶胶光学方案计算的气溶胶光学特性不确定性的最关键因素；而在南京站点，粒径分布与混合状态均对气溶胶光学特性的计算结果具有显著主导作用。