Water emissions put a damper on the coal-to-gas transition
Water emissions put a damper on the coal-to-gas transition
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DOI：
10.1073/pnas.2024360118
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发表时间：
2021-01
期刊：
影响因子：
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通讯作者：
G. Bowen;R. Fiorella
中科院分区：
文献类型：
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作者：
G. Bowen;R. Fiorella
人类决策对环境和生态造成的不可预见的后果至少可以追溯到农业的早期历史。从生物操纵（2）到替代能源技术（3），怀着最好的意图开展的活动都产生了意想不到的副作用，导致了有问题的、有时是长期的社会生态变化。随着全球采用更清洁和更可持续的能源系统的势头不断增长，有必要批判性地仔细评估这一转变的潜在间接后果。Xing等人（4）发表在PNAS上的工作强调了正在进行的天然气替代煤炭的副作用：天然气燃烧增加的水蒸气排放可能会导致局部空气污染增加（图1）。虽然水蒸气本身没有直接的健康后果，但水分是大气中许多化学反应的关键因素，这些化学反应可能产生危害人类健康和福祉的颗粒。世界卫生组织估计，全球每年有1700万人死于空气污染，但空气污染的影响超出了死亡率。最近的研究强调了细颗粒物浓度与GDP（6）和儿童入学率（7）等指标之间的联系。虽然其中一些颗粒物是从自然和人为来源直接排放到大气中的，但大多数通常是大气中化学反应产生的二次气溶胶。在这里，水蒸气的浓度可能至关重要。许多产生二次粒子的反应都是由非均匀气溶胶的存在介导的，这些气溶胶含有水和其他成分的混合物，并且这些气溶胶的丰度，大小和组成强烈依赖于大气湿度（8，9）。此外，湿度升高和由此导致的颗粒物增加本身可影响大气边界层光学和结构，潜在地产生进一步的气溶胶增强反馈（10）。水蒸气是大气中天然丰富的成分，其浓度在空间和时间上变化很大。虽然人类对水循环的改变是实质性的，但我们的行动仍然没有从水循环的大多数概念化（11）。在描述人类影响的地方，它们通常仅限于通过抽取地下水、灌溉和地表水蓄水等过程重新分配水。在某种程度上，这与我们物种在碳气候系统中有限作用的概念相似，这些概念在广泛认识到二氧化碳水平持续长期增加之前是常见的（12）（并且仍然可以在伪科学气候变化讨论组中找到回响在线）。化石燃料燃烧的过程支撑着人类对当代碳循环的无可辩驳的影响，然而，同样的过程也对当地的水循环产生了重大的扰动。含氧光合作用是大多数化石燃料的最终来源，在产生有机分子的过程中消耗水。当这些有机化合物后来被氧化时，无论是通过氧化风化还是自然或人为燃烧，都会产生新的水分子。与化石燃料产生的CO2相比，通过有机物氧化产生的全球水产量与进出大气的自然通量相比微不足道（13）。然而，化石燃料燃烧产生的水排放在空间和时间上高度局部化，因此在某些排放和大气循环条件下，燃烧可能会大大增加局部湿度。评估燃烧在何处以及何时明显改变大气湿度已被证明是一项挑战。燃烧产生的水蒸气排放的时空清单尚不存在。此外，控制大气水收支的其他过程，如蒸散、凝结和混合，可能是高度动态的，它们在大气中的表现形式也可能是动态的。
Unanticipated environmental and ecological consequences of human decision-making extend back at least as far as the early history of agriculture (1). Activities undertaken with the best of intentions, from biomanipulations (2) to alternative energy technologies (3), have had unexpected side effects leading to problematic, and sometimes long-term, socioecological change. As global momentum toward the adoption of cleaner and more sustainable energy systems grows, there is a need to critically and carefully evaluate the potential indirect consequences of this transition. Work by Xing et al. (4) published in PNAS highlights an under-studied side effect of the ongoing replacement of coal by natural gas: Increased water vapor emissions from combustion of natural gas may drive local increases in air pollution (Fig. 1). Although water vapor itself has no direct health consequences, moisture is a key contributor to many chemical reactions in the atmosphere that can produce particles that harm human health and well-being. The World Health Organization estimates that ∼7 million annual deaths globally are attributable to air pollution (5), but the effects of air pollution extend beyond mortality. Recent research has highlighted links between fine particulate concentrations and metrics as varied as GDP (6) and school attendance by children (7). Although some of these particles are emitted directly to the atmosphere from natural and anthropogenic sources, the majority are usually secondary aerosols produced by chemical reactions in the atmosphere. Here, the concentration of water vapor can be critically important. Many reactions that produce secondary particles are mediated by the presence of heterogeneous aerosols, which contain mixtures of water and other constituents, and the abundance, size, and composition of these aerosols depends strongly on atmospheric humidity (8, 9). In addition, elevated humidity and resulting increases in particulates can themselves affect atmospheric boundary layer optics and structure, potentially producing further aerosol-enhancing feedbacks (10). Water vapor is a naturally abundant constituent of the atmosphere, present in concentrations that are highly variable in space and time. Although human alteration of water cycling has been substantial, our actions remain absent frommost conceptualizations of the water cycle (11). Where human impacts are depicted, they are usually limited to the redistribution of water through processes like groundwater abstraction, irrigation, and impoundment of surface water. This parallels, to some degree, concepts of our species’ limited role in the carbon−climate system that were common prior to the widespread recognition of the ongoing secular increase in CO2 levels (12) (and can still be found reverberating within pseudoscientific climate change discussion groups online). The same process of fossil fuel burning that underpins much of humankind’s now irrefutable influence on the contemporary carbon cycle, however, has also produced locally significant perturbations of the water cycle. Oxygenic photosynthesis, the ultimate source of most fossil fuel, consumes water in the process of producing organic molecules. When these organic compounds are later oxidized, whether through oxidative weathering or natural or human-mediated combustion, new water molecules are produced. In contrast to fossil fuel-derived CO2 production, global rates of water production via organic matter oxidation are trivial compared to natural fluxes into and out of the atmosphere (13). Water emissions from fossil fuel combustion are highly localized in space and time, however, raising the possibility that combustion may significantly enhance local humidity under certain emissions and atmospheric circulation conditions. Assessing where and when combustion appreciably alters atmospheric humidity has proven to be a challenge. Spatiotemporal inventories of water vapor emissions from combustion do not yet exist. Moreover, other processes that govern atmospheric water budgets, like evapotranspiration, condensation, and mixing, can be highly dynamic, and their representation in