所以你想用植物来降低室内二氧化碳

背景与摘要

本文以幽默而严谨的科学计算，打破了人们试图通过在家中摆放绿植来降低室内二氧化碳水平的幻想。文章详细推算了人体每天产生的二氧化碳量，以及植物进行光合作用所需的最低能量、光照强度和叶片面积。结果显示，要中和一个人的二氧化碳排放，你需要相当于几千瓦的刺眼红色补光灯（产生相当于多个电暖器的热量）、至少17.6平方米密不透风的蕨类植物墙，并且每天还要处理掉约4.6公斤的新鲜植物组织。结论是：开窗通风更实在。

Summary

虽然使用室内植物来净化空气和中和呼出的二氧化碳的想法听起来很吸引人，但物理学定律使其在普通家庭中极其不切实际。为了抵消单个人类产生的二氧化碳 $\text{CO}_2$，您将需要大量的能量输入（相当于多个电暖器）、强烈的人造植物生长灯、至少 17.6 平方米密集种植的蕨类植物，以及一个每天能够收获并丢弃大约 4.6 公斤新鲜植物物质的系统。归根结底，你最好的做法就是简单地打开一扇窗户。

While the idea of using houseplants to clean the air and neutralize exhaled carbon dioxide sounds appealing, physics makes it wildly impractical for a normal home. To offset the $\text{CO}_2$ produced by a single human, you would need massive energy inputs (equivalent to multiple space heaters), intense grow lighting, at least 17.6 square meters of densely packed ferns, and a system capable of harvesting and discarding roughly 4.6 kilograms of fresh plant matter every single day. Ultimately, you are much better off simply opening a window.

The Dream vs. Reality

人类会产生二氧化碳。二氧化碳（有时被认为是）对认知有害。但是植物能将二氧化碳转化回氧气。而且植物是唯一名副其实的家居装饰策略。那么也许如果你养很多植物，你就能控制住二氧化碳，让你的大脑保持运转？

Humans make carbon dioxide. Carbon dioxide is (sometimes claimed to be) bad for cognition. But plants turn carbon dioxide back into oxygen. And plants are the one true home decoration strategy. So maybe if you get a lot of plants, you can keep carbon dioxide in check and keep your brain working?

这在理论上是可能的。在实践中它可能也只是勉强可行。但这将绝非易事。

It’s theoretically possible. It’s probably just barely possible in practice. But it won’t be easy.

The Math of Human Exhalation

人每天大约产生 1 公斤二氧化碳。那大约是 $5.7 \times 10^{23}$ 个分子，或者每小时 $0.948$ 摩尔。（你可能还记得高中学的，摩尔 (mole) 是一个为了避免到处都是 $10^{23}$ 这个因子而编造出来的巨大数字。）为了简单起见，我们称之为每小时一摩尔。

People produce roughly 1 kilogram of carbon dioxide per day. That’s around $5.7 \times 10^{23}$ molecules, or $0.948$ moles per hour. (You may remember from high school that a mole is a gigantic number made up to avoid having factors of $10^{23}$ everywhere.) Let’s keep it simple and call it one mole per hour.

同时，植物通过光合作用将二氧化碳转化为氧气——具体来说，就是以下化学反应：

Meanwhile, plants turn carbon dioxide into oxygen through photosynthesis—specifically, the chemical reaction of:

\[\text{6 } \text{H}_2\text{O} + \text{6 }\text{CO}_2 + \text{Energy} \longrightarrow \text{C}_6\text{H}_{12}\text{O}_6 + \text{6 }\text{O}_2$$ > $$\text{6 } \text{H}_2\text{O} + \text{6 }\text{CO}_2 + \text{Energy} \longrightarrow \text{C}_6\text{H}_{12}\text{O}_6 + \text{6 }\text{O}_2\]

通过这个反应将 1 摩尔二氧化碳转化为葡萄糖和氧气，物理上所需的最小能量大约是 477 千焦 (kilojoules)。

The minimum energy physically needed to convert 1 mole of carbon dioxide into glucose and oxygen via this reaction is roughly 477 kilojoules.

Step-by-Step Energy Requirements

1. The Theoretical Lower Bound

如果你拥有能以完美效率将所有射入能量转化为光合作用的魔法植物，它们每小时将需要大约 477 千焦的能量，这相当于持续消耗 132.5 瓦特 (watts)。[^1] 这比两个白炽灯泡所使用的能量稍微多一点——还不算太糟！

If you had magical plants that somehow channeled all incoming energy into photosynthesis with perfect efficiency, they would need ~477 kilojoules per hour, which converts to a continuous usage of 132.5 watts.[^1] That’s a bit more than what’s used by two incandescent light bulbs—not too bad!

2. The Biological Reality of Chloroplasts

真正的植物没有魔法；它们通过分为两个步骤的物理过程进行光合作用，每个步骤都需要四个电子吸收一个光子。这意味着每个二氧化碳分子你需要八个光子。

Real plants don't have magic; they perform photosynthesis through a physical process with two steps, each involving four electrons absorbing a photon. That means you need eight photons per carbon dioxide molecule.

如果你调节灯光以达到最大效率，给每个光子激发一个电子所需的最小能量（约 1.8 eV），你需要波长为 680 nm 的纯红光。这将导致持续消耗 386 瓦特。[^2] 没有任何使用叶绿体的物理系统能在低于这个数值的情况下中和你的 $\text{CO}_2$。

If you tune your lights for maximum efficiency, giving each photon the minimum energy necessary to excite an electron (~1.8 eV), you need pure red light with a wavelength of 680 nm. This results in a continuous usage of 386 watts.[^2] No physical system using chloroplasts can neutralize your $\text{CO}_2$ using less than that.

3. Optical Losses

你的室内植物无法抓住所有照射到它们身上的光子。在实践中，大约有 ~30% 的光子会从植物上反射出去，直接穿过植物，或者击中植物非叶绿体的部分。这将我们的需求推高到了 551 瓦特。[^3]

Your houseplants won’t be able to grab every single photon that hits them. In practice, ~30% of photons will reflect off the plant, pass right through it, or hit a non-chloroplast part of the plant. That bumps our requirement up to 551 watts.[^3]

4. Plant Respiration

在制造葡萄糖之后，它会怎么样？一部分用于生长更多的植物组织（永久封存碳），但很大一部分被植物为了维持生存而燃烧掉了，把碳重新释放回空气中。假设呼吸作用造成了标准的 ~40% 的损耗，[^4] 我们的需求攀升至 918 瓦特。[^5]

After making glucose, what happens to it? Some goes toward growing more plant (permanently sequestering carbon), but much of it is burned by the plant just to stay alive, releasing the carbon right back into the air. Assuming a standard ~40% loss due to respiration,[^4] our requirement climbs to 918 watts.[^5]

Living in a Tanning Booth

那听起来可能还不是那么糟糕。但想象一下，住在一个有 918 瓦纯红光的房间里是什么感觉。

That still might not sound that bad. But consider what it's like to live in a room with 918 watts of pure red light.

就辐射功率而言，这相当于 ~765 个白炽灯泡。[^6]
- In terms of radiant power, that is equivalent to ~765 incandescent lightbulbs.[^6]
现代 LED 植物生长灯的效率约为 50%，这意味着您实际上需要消耗 ~1836 瓦特。
- Modern LED grow bulbs are ~50% efficient, meaning you actually need to spend ~1836 watts.
把流失到房间里的光，以及使用正常白光频率而不是纯红光带来的更低效率考虑在内，你实际上面临的是 5,000 到 10,000 瓦特 的消耗。
- Factor in the light lost to the room and the lower efficiency of using normal white light frequencies instead of pure red, and you are realistically looking at 5,000 to 10,000 watts.

这些能量中的大部分会作为热量倾泻到你的生活空间中。想象一下，五个电暖器整天、每天都在开到最大档狂轰滥炸。

Most of that energy is dumped into your living space as heat. Imagine five space heaters blasting on high, all day, every day.

The Space Problem: Photon Flux Density

植物不能吸收无限量的光。叶绿体在能够吸收更多光子之前，需要时间来“重置”。你的宠物蕨类植物每平方米叶面积只能吸收大约 52 瓦的能量。[^7]

Plants can’t absorb infinite amounts of light. Chloroplasts take time to "reset" before they can absorb more photons. Your pet fern can only absorb roughly 52 watts of energy per square meter of leaf surface area.[^7]

无论您产生多少光，中和您每天的 $\text{CO}_2$ 排放量都需要：

No matter how much light you produce, neutralizing your daily $\text{CO}_2$ output requires:

\[\frac{918 \text{ watts}}{52 \text{ W/m}^2} = 17.6 \text{ square meters of fern leaf}$$ > $$\frac{918 \text{ watts}}{52 \text{ W/m}^2} = 17.6 \text{ square meters of fern leaf}\]

想象一堵 4.2 米见方的墙，上面密密麻麻地塞满了蕨类植物。如果有任何空隙、茎干、土壤或墙壁露出来，它的面积就得更大。这是绝对的物理最小值。

Picture a 4.2-meter square wall packed solid with ferns. If there are any gaps, stems, soil, or wall showing, it needs to be even larger. That is the absolute physical minimum.

Where Does the Carbon Go?

如果植物从空气中去除了碳，它们必须把它放到某个地方。它唯一能去的地方（除了回到空气中）就是进入植物的物理结构中。

If plants remove carbon from the air, they must put it somewhere. The only place it can go (other than back into the air) is into the physical structure of the plant.

你每天产生的 1 公斤 $\text{CO}_2$ 含有 273 克元素碳。
- The 1 kg of $\text{CO}_2$ you produce daily contains 273 grams of elemental carbon.
干燥的植物物质只含有大约 50% 的碳。
- Dry plant matter is only ~50% carbon.
对于每克干燥植物物质，植物体内会保留 5–10 克的游离水（因物种而异）。
- For each gram of dry plant matter, plants hold 5–10 grams of water (depending on the species).

为了封存你制造的所有碳，你的室内花园必须生长出：

To sequester all the carbon you make, your indoor garden must grow:

\[\begin{aligned} & 1 \text{ kg CO}_2 \\ \times \quad & 0.273 \text{ kg carbon / kg CO}_2 \\ \times \quad & 2 \text{ kg dry plant / kg carbon} \\ \times \quad & 8.5 \text{ kg actual plant / kg dry plant} \\ \mathbf{= \quad} & \mathbf{4.6 \text{ kg of actual plant per day}} \end{aligned}$$ > $$\begin{aligned} > & 1 \text{ kg CO}_2 \\ > \times \quad & 0.273 \text{ kg carbon / kg CO}_2 \\ > \times \quad & 2 \text{ kg dry plant / kg carbon} \\ > \times \quad & 8.5 \text{ kg actual plant / kg dry plant} \\ > \mathbf{= \quad} & \mathbf{4.6 \text{ kg of actual plant per day}} > \end{aligned}\]

那就是每个月 140 公斤 (308 磅)。你必须修剪、收获并把所有这些植物物质丢弃在你的家门外；否则，这些碳最终会分解并重新返回到你的室内空气中。

That is 140 kg (308 lbs) per month. You must prune, harvest, and discard all of this plant matter outside your home; otherwise, the carbon eventually decomposes and returns right back to your indoor air.

Conclusion: How to Use Plants for $\text{CO}_2$ Reduction

建立一个工业化的室内农场。
1. Build an industrial indoor farm.
称称它的重量。
1. Weigh it.
等待两周。
1. Wait two weeks.
再称一次重量。
1. Weigh it again.
用增加的重量除以你自己的体重。
1. Divide the increase in weight by your own body mass.
那就是你实际上从环境中去除的 $\text{CO}_2$ 的比例。
1. That is the fraction of your $\text{CO}_2$ you're actually removing from the environment.
打开一扇窗。
1. Open a window.

Footnotes

[^1]: 见证神奇的算术：$(1 \text{ mole CO}_2 / \text{hour}) \times (477 \text{ kJ} / \text{mole CO}_2) = 132.5 \text{ watts}$。 ↩

[^1]: Behold the power of arithmetic: $(1 \text{ mole CO}_2 / \text{hour}) \times (477 \text{ kJ} / \text{mole CO}_2) = 132.5 \text{ watts}$. ↩ [^2]: 换算单位：$(1 \text{ mole CO}_2 / \text{hour}) \times (8 \text{ photons} / \text{CO}_2 \text{ molecule}) \times (1.8 \text{ eV} / \text{photon}) = 385.94 \text{ watts}$。因此，叶绿体将光能转化为光合作用的效率最高约为 ~34% ($132.5 / 385.94$)。 ↩ [^2]: Using units: $(1 \text{ mole CO}_2 / \text{hour}) \times (8 \text{ photons} / \text{CO}_2 \text{ molecule}) \times (1.8 \text{ eV} / \text{photon}) = 385.94 \text{ watts}$. Chloroplasts are thus at most ~34% ($132.5 / 385.94$) efficient at channeling light energy into photosynthesis. ↩ [^3]: 这个 30% 的损耗数字来源于被过滤到 400–700 nm 范围内的阳光。如果你使用纯 680 nm 的光以及非常密集的植物，你也许能把这个损耗降到 10–20%。 ↩ [^3]: This 30% loss figure comes from sunlight filtered to the 400–700 nm range. If you use pure 680 nm light with very densely packed plants, you might drop this loss to 10–20%. ↩ [^4]: 维基百科引用的数据指出，仅仅叶片本身的呼吸作用就会带来 35–45% 的损耗，而这篇论文显示，根据物种和生长速度的不同，这个数字从 30% 到 56% 不等。 ↩ [^4]: Wikipedia cites a 35–45% loss just for respiration in the leaf itself, while this paper shows numbers ranging from 30% to 56% depending on species and growth rate. ↩ [^5]: 这给出的总体效率为 $132.5 / 918 \approx 14.4\%$。考虑到真实的阳光包含理想范围之外的波长这一事实，维基百科估计真实世界叶片的效率更接近 5.4%，这与这些物理限制非常吻合。 ↩ [^5]: This gives an overall efficiency of $132.5 / 918 \approx 14.4\%$. Accounting for the fact that real sunlight contains wavelengths outside the ideal range, Wikipedia estimates real-world leaf efficiency closer to 5.4%, aligning closely with these physical constraints. ↩ [^6]: 传统的“60 瓦”白炽灯泡的额定值基于功率输入，但只有 ~2% 转化为可见光。因此，918 瓦的红光相当于大概 $918 / 60 / 0.02 = 765$ 个灯泡。请注意，因为人眼对 680 nm 的光不是很敏感，所以感知到的亮度不会感觉那么刺眼。 ↩ [^6]: A traditional "60-watt" incandescent bulb's rating is based on power input, but only ~2% is converted to visible light. Therefore, 918 watts of red light equates to roughly $918 / 60 / 0.02 = 765$ lightbulbs. Note that because human eyes aren't very sensitive to 680 nm light, the perceived brightness won't feel nearly as blinding. ↩ [^7]: 植物的饱和点对于耐阴的室内植物通常测量在 ~300 $\mu\text{mol/m}^2/\text{s}$。在 680 nm 处，300 $\mu\text{mol}$ 的光子每平方米叶面积每秒携带约 51.92 焦耳的能量，即 52 瓦。 ↩ [^7]: The saturation point of plants is typically measured at ~300 $\mu\text{mol/m}^2/\text{s}$ for shade-tolerant houseplants. At 680 nm, 300 $\mu\text{mol}$ of photons carries ~51.92 joules of energy per square meter of leaf surface per second, or 52 watts. ↩