Purpose of Review

The direct radiative effect of brown carbon (BrC) absorption predicated by climate-modeling studies is highly uncertain, with values ranging between +0.03 W/m² and + 0.57 W/m². This review strives to identify sources of this uncertainty stemming from challenges in translating measurements into model inputs and to draw lessons from recent advances that lead to improved BrC representation in models.

Recent Findings

Previously thought to absorb only short-visible and UV light, BrC was recently shown to comprise components that are strongly absorptive in the mid- and long-visible wavelengths, with light-absorption efficiencies approaching that of black carbon. The classic picture of biomass and biofuel combustion being the major sources of atmospheric BrC still holds, with recent measurements indicating a strong correlation between BrC optical properties and combustion conditions. Other combustion sources of BrC, currently not accounted for in models, include low-efficiency coal combustion and ship engines utilizing heavy fuel oil. Gas-phase, aqueous, and particle-phase reactions in the atmosphere produce secondary BrC and bleach/darken the primary BrC. Climate-modeling studies revealed that predicted BrC radiative effects are sensitive to the assumed optical properties and atmospheric aging mechanisms.

Summary

BrC can be grouped into four optical classes, each separated by an order of magnitude in mid-visible light absorption. The classes are approximately mapped to BrC sources, with secondary BrC being the least absorbing and BrC from high-temperature combustion the most absorbing. There is evidence that each class exhibits characteristic physicochemical properties (molecular size, volatility, and solubility), which can be leveraged to design measurements that quantify distributions of BrC across classes as well as rates of photobleaching/darkening for each class. Utilizing this framework to develop BrC parameterizations promises to enhance its representation in climate models.

中文翻译：

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从测量到模型：在气候计算中准确表达棕色碳

审查目的

由气候建模研究预测棕色碳（BRC）吸收直接辐射效果是非常不确定的，具有值0.03瓦/米之间的范围内²和+ 0.57瓦/米²。这篇综述力图找出将不确定性的根源归因于将测量转化为模型输入中的挑战，并从最近的进展中汲取了教训，这些进展导致改进的BrC表示在模型中。

最近的发现

以前认为BrC仅吸收近可见光和UV光，最近发现BrC包含在中可见光和长可见光波长具有强吸收性的成分，光吸收效率接近黑碳。生物质和生物燃料燃烧是大气中BrC的主要来源的经典图片仍然存在，最近的测量表明BrC光学特性与燃烧条件之间存在很强的相关性。目前尚未在模型中说明的BrC的其他燃烧源包括低效燃煤和使用重质燃油的船舶发动机。大气中的气相，水相和颗粒相反应会生成二级BrC，并使一级BrC漂白/变暗。

概要

BrC可以分为四个光学类别，每个类别在中可见光吸收方面相隔一个数量级。这些类别大致映射到BrC源，其中次要BrC吸收最少，而来自高温燃烧的BrC吸收最多。有证据表明，每个类别都具有特征性的理化性质（分子大小，挥发性和溶解性），可以利用这些特征来设计度量，以量化各个类别中BrC的分布以及每个类别的光漂白/变暗率。利用该框架开发BrC参数化有望增强其在气候模型中的表示。