什么是黑碳？概述、影响和缓解策略

--我们可以做些什么来解决全球变暖的第二大原因。

来源于: https://www.treehugger.com/what-is-black-carbon-5201028

作者：David M. Kuchta         出版时间：2021/10/26                翻译:刘敏

黑碳(Black Carbon, BC)作为烟尘和烟雾的主要成分之一，是木柴或化石燃料等含碳物质不完全燃烧的产物。在恰当的地方，黑碳是土壤重要的天然肥料，这也是人们几千年来实行刀耕火种的原因。在不恰当的地方，当黑碳沉积在肺部深处，可导致过早死亡，当黑碳沉积在雪地上，可增加灾难性洪水的风险。悬浮在大气中的黑碳是继二氧化碳(CO₂)之后导致全球变暖的第二大因素。由于黑碳对弱势群体的过度影响，解决黑碳问题是一个环境正义问题。

一、黑碳来源

工业革命之前，自然或人为活动引起的火灾是黑碳的主要来源。作为自然碳循环的一部分，生物质燃烧产生的固态黑碳（生物炭）多于空气中的黑碳（烟尘）^[1]。火灾更多的将碳封存在土壤中而不是排入大气中，排入大气中的碳可被植物重新吸收^[2]。

土壤中高达40%的有机碳是黑碳^[3]，主要作用是增加土壤的肥力。即使现在，人们也常使用生物炭来提高因密集型工业农业而退化土壤的肥力^[4]。

自十八世纪末工业化开始以来，煤炭（最脏的化石燃料^[5]）取代生物燃料成为黑碳排放的主要来源^[6]。大气中的黑碳（烟尘）增加了七倍，并在二十世纪初达到顶峰。

1952年的伦敦大烟雾主要是黑碳. Fox Photos/Hulton Archive/Getty Images

然而，生物质的燃烧仍在继续，特别是在低收入国家的农村地区。全世界有20亿人使用生物质(木材、粪便或作物残渣)作为取暖和烹饪的主要燃料^[8]。事实上，随着20世纪人口的快速增长^[9]，生物质的燃烧增加了一倍，低效的炉灶是一个主要来源^[10]。

全球范围内化石燃料的碳排放大约是生物质源的两倍，约占所有黑碳排放的25%^[11]。不同源对大气黑碳的贡献依地区的工业化和城市化而有所不同，生物质在农村地区贡献更多的黑碳，而化石燃料在城市地区贡献更多^[12]。

继化石燃料和生物质之后，来自汽车尾气、刹车和轮胎磨损的路尘是黑碳的第三排放源。现今，柴油机尾气排放了交通部门90%的黑碳^[13]，高于任何其他单一来源。作为城市颗粒物（PM_2.5）的重要组成部分，道路附近的黑碳水平可能高出50%至200%^[14]。在燃煤电厂周围，沉积在道路上或附近的烟尘会重新悬浮在空气中。

二、黑碳的危害

黑碳的影响既是一个地方性问题，也是一个全球性问题，其影响取决于排放源以及排放地点。生物质源的黑碳对人类健康有局部影响，而化石燃料所产生的黑碳会造成更大的全球性问题，如增加自然灾害和全球变暖的风险。

1.      对人体健康的影响

虽然黑碳在大气中停留时间较短(几天-十几天)，但它对人类健康的影响巨大^[16]。根据两项研究，在农村地区，来自炉灶的家庭黑碳空气污染对妇女和幼儿的影响不成比例^[17]。在城市地区，道路灰尘，特别是靠近煤厂和港口设施的灰尘，也有类似的风险，低收入家庭和有色人种接触黑碳的机会明显增加^[18]。例如，在底特律地区的一项研究中，弱势和有色人种社区的近道路黑碳浓度比其他地方高35%-40%^[19]。

黑碳已被确定为温室气体排放的 "第二大 "来源^[20]。来自化石燃料燃烧的黑碳全球变暖潜力是生物质源的两倍^[21]。由于黑碳吸收而不是反射辐射，它阻挡了通常会逃逸太空的能量离开地球大气层，从而促进了全球变暖。

无论黑碳是降落地面还是悬浮在大气中，情况都是如此。当黑碳降落于雪地，其影响巨大，变黑的雪不再将光线反射至太空，而是吸收更多的热能。根据最近的研究，黑碳可解释冰川和融雪加速50%以上的原因。在极地地区，这是海平面上升的一个直接原因^[22]。

2.      自然灾害

在冰川等常年结冰的区域，黑碳的存在增加了洪水风险。喜马拉雅山脉的冰川融化增加了生活在恒河和雅鲁藏布江流域的7800万人的洪水风险^[23]。黑碳与中国北方干旱和中国南方洪水的频率增加有关，也与源自阿拉伯海的热带气旋的强度增加有关^[24]。

三、技术解决方案

黑碳主要影响的是生活在贫困中、发展中国家和世界各地的有色人群，因此黑碳是一个环境正义问题。需要指出的是，目前已存在缓解黑碳排放的方法。实施这些措施可以改善人类健康，并在2050年前可将全球变暖降低0.2摄氏度^[25]。黑碳和CO₂通常在相同燃烧过程中排放（如柴油燃烧），因此许多减少CO₂排放的努力也会产生减少黑碳的效果。然而，一些缓解的技术方案对于减少黑碳排放水平特别重要。

更清洁的燃烧炉灶。如太阳能灶有可能减少农村黑碳排放，减缓森林砍伐，改善人类健康，并提高教育水平。因为儿童如将大量时间收集木柴，将削减他们的教育机会^[26]。

赞比亚的太阳能炊具。Tina Stallard/Getty Images

再生农业。包括将碳和其他营养物质返回到土壤中以维持土壤健康。黑碳在土壤中持续稳定了几千年^[27]，将其作为生物炭返回土壤也可以作为一种碳耕作或 "负排放 "的形式^[28]。

混合动力汽车和电动汽车。主要依靠再生制动而不是摩擦制动来降低道路灰尘水平，而摩擦制动产生的颗粒物约占道路交通产生颗粒物的20%^[29]。

更少和更清洁的交通可减少黑碳暴露。低排放区（LEZs）也是有效的，伦敦的低排放区减少了40%-50%的黑碳。 减少卡车的柴油污染也可以改善低收入和弱势社区的健康状况^[30]。加州长滩港这样一个项目赢得了美国环保局的环境正义成就奖。

更清洁的航运。由于黑碳只在大气中停留几天，减少船舶在极地等敏感地区的黑碳排放，对减少融雪和海平面上升有很大影响^[31]。

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