姜唯 台湾环境资讯协会
一篇刊登在《科学》期刊上的新研究发现，空气中的微小粒子会导致强大的风暴形成，对天气的影响力超乎过去所知。只要小于人类头发宽度千分之一的粒子，就可能导致风暴加剧、云量和降雨增加。
这篇研究聚焦于“气溶胶”（大气气胶，或称悬浮微粒，Aerosols）的能力。气溶胶是空气中的微小粒子，可能来自城市、工业空气污染或自然产生如野火。
过去虽然已经掌握到，气溶胶可能影响天气和气候，但这次科学家选在未受工业污染的巴西亚马逊热带雨林进行实验，精确辨识出悬浮微粒造成的效应，发现这些细小颗粒的威力不容小觑。
细小颗粒可能是造成部分风暴如此强烈的原因之一。图片来源：Montanus Photography
“我们证实这些粒子是造成部分风暴如此强烈并产生大量雨水的原因之一。在温暖潮湿，且一般来说天气多为晴朗的地区，空气中的细小颗粒可能会产生相当大的影响。”硏究主要作者、美国能源部西北太平洋国家实验室学者范季文（Jiwen Fan，暂译）说。
该研究以Green Ocean Amazon计画搜集来的独特资料为基础。在这个计画中，科学家于2014-2015年期间进行了气候相关的地面和空中测量。
Green Ocean Amazon研究计画选在巴西亚马逊地区进行。图片来源：美国能源部 ARM Climate Research Facility
研究所利用的资料来自亚马逊地区，因为这里除了马瑙斯（Manaus）周边地区外，几乎未受污染。马瑙斯是亚马逊最大的城市，人口超过200万。
亚马逊热带雨林为气溶胶研究提供了独特的天然实验室。在这里科学家难得可以在工业化前的环境中观察污染对大气过程的影响，并精确辨识出除了温度和湿度等其他因素外，悬浮微粒造成的效应。
科学家研究了风暴发展过程中，50奈米以下超细颗粒的作用。
科学家过去已经了解，较大的悬浮颗粒会促使强大上升气流形成云，进而形成水滴，最终降雨。但过去一直不曾观察到的是小于50奈米的颗粒（如车辆和工业过程产生的颗粒）是否具有相同的效果。
这项研究结果显示，超细微粒刺激云层的威力比稍大的颗粒更强。
藉由精细的电脑模拟，科学家展示了超细微粒如何影响暴风雨云的产生——温暖潮湿的环境中没有较大颗粒的存在，超细微粒就有机会形成云滴（cloud droplets）；而在相对湿度超过100％的温暖潮湿高空环境中，低浓度的大颗粒会导致大量多余的水蒸气，也会使超细颗粒转变成云滴。
这些超细颗粒尺寸虽小但数量庞大，因此可以形成许多小水滴，让过量的水蒸气在其上凝结。
如此的凝结过程将释放更多的热量，使得上升气流更加强大。更多的暖空气被拉入云中，将更多的水滴拉到高空，产生更多的冰珠、霰、闪电和雨水。
这样的结果，范季文称之为“强化对流”（invigorated convection），意味着产生更强的风暴。
“我们证实在晴朗潮湿的条件下，像是海面上和部分热带地区，微小的气胶对天气和气候有很大的影响，并且会加剧风暴威力。”范季文说，“更广泛而言，这样的结果显示，从前工业时代至今，人类活动可能剧烈地改变了这些地区的暴风雨特性。”
研究主要作者范季文。圖片來源：Andrea Starr/PNNL
Ultrafine Nanoparticles Form Intense Storms
RICHLAND, Washington, February 19, 2018 (ENS)
Tiny particles fuel powerful storms and influence weather much more than has been previously understood, finds a new study published in the journal “Science.”
The research focuses on the power of minute airborne particles known as aerosols, which can come from urban and industrial air pollution as well as wildfires.
Scientists have known that aerosols may play an important role in shaping weather and climate, but the new study shows that the smallest of particles have an outsize effect.
Particles smaller than 1,000th the width of a human hair can cause storms to intensify, clouds to grow and more rain to fall, the researchers have learned.
“We showed that the presence of these particles is one reason why some storms become so strong and produce so much rain. In a warm and humid area where atmospheric conditions are otherwise very clean, the intrusion of very small particles can make quite an impact,” said lead author Jiwen Fan of the U.S. Department of Energy’s Pacific Northwest National Laboratory in Richland.
The findings are based on unique data made possible by the GoAmazon research campaign, where scientists made ground-based and airborne measurements related to climate during 2014-2015.
The study capitalized on data from an area of the Amazon that is pristine except for the region around Manaus. With a population of more than two million people it is the largest city in the Amazon.
The Amazon rainforest provided a unique natural lab to study effects of aerosols. The setting gave scientists the rare opportunity to look at the impact of pollution on atmospheric processes in a pre-industrial environment and pinpoint the effects of the particles apart from other factors such as temperature and humidity.
The scientists studied the role of ultrafine particles less than 50 nanometers wide in the development of thunderstorms.
Similar but larger particles are known to play a role in feeding powerful, fast-moving updrafts of air from the land surface to the atmosphere, creating the clouds that play a central role in the formation of water droplets that fall as rain.
But scientists had not observed until now that particles smaller than 50 nanometers, such as the particles produced by vehicles and industrial processes, could have the same effects.
The new study revealed that these small particles can invigorate clouds in a much more powerful way than their larger counterparts.
Through detailed computer simulations, the scientists showed how the smaller particles have a powerful impact on storm clouds.
When larger particles are not present high in a warm and humid environment, it opens opportunities for the smaller particles to form cloud droplets.
A low concentration of large particles high in a warm and humid environment with relative humidity beyond 100 percent, contributes to high levels of excessive water vapor, allowing ultrafine particles to transform into cloud droplets.
While the particles are small in size, they are large in number, and they can form many small droplets on which the excess water vapor condenses.
That condensation releases more heat, which makes the updrafts much more powerful. More warm air is pulled into the clouds, pulling more droplets aloft and producing more ice and snow pellets, lightning, and rain.
The result is what Fan calls “invigorated convection,” producing stronger storms.
“We’ve shown that under clean and humid conditions, like those that exist over the ocean and some land in the tropics, tiny aerosols have a big impact on weather and climate and can intensify storms a great deal,” said Fan, an expert on the effects of pollution on storms and weather.
“More broadly, the results suggest that from pre-industrial to the present day, human activity possibly may have changed storms in these regions in powerful ways,” he said.
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