Quantifying the impact of atmospheric aerosol on diffuse radiation and the Amazon biosphere

亚马逊地区的树木越来越大，可以吸收大气中的大量二氧化碳，几乎相当于人为温室气体排放量的10%。这种“碳汇”减缓了大气中二氧化碳的积累，因此在调节气候变化方面起着重要作用。目前，我们不明白为什么亚马逊森林会有这样的行为。一个潜在的原因是大气中称为气溶胶的微小颗粒物的数量发生了变化。我们知道，人类活动极大地改变了亚马逊地区的气溶胶数量：每个旱季，人类都会在亚马逊盆地南部地区引发森林大火(即所谓的森林砍伐弧线)。这些大火向大气中排放了大量的烟雾气溶胶，散射并吸收了来自太阳的辐射。太阳光的这种散射减少了到达表面的直接辐射量，但增加了漫射辐射量。人们很早就知道，增加漫射辐射可以提高植物生产力。例如，园丁在温室内放置“漫射板”，以类似于大气颗粒物的方式增加漫射辐射。漫射辐射之所以能提高植物生产力，是因为它能深入树冠，照亮原本在阴凉处的树叶。这些以前遮蔽的叶子可以进行更多的光合作用。最近的一项研究表明，这种效应可能非常重要，它使全球森林吸收的碳量增加了25%。因此，砍伐森林火灾(二氧化碳排放的一个主要来源)可能至少在一定程度上导致了亚马逊盆地未受干扰的森林中的二氧化碳沉没。到目前为止，人们还没有仔细研究过亚马逊森林的这种影响。首先，人们并不清楚亚马逊地区的气溶胶颗粒物到底发生了多大的变化。其次，没有人量化气溶胶颗粒的这种变化如何改变了漫射辐射量。最后，我们不知道漫射辐射的这些变化是否足以解释亚马逊地区森林生物量的任何观察到的变化。在这个项目中，我们将使用一套大气气溶胶、辐射和陆地-地表模型来探索这个问题。我们将使用一个全球大气气溶胶模型来模拟过去几十年来亚马逊地区气溶胶分布的变化。我们将把气溶胶中的这些变化输入大气辐射模型，以量化漫射辐射的量是如何变化的。最后，我们将使用漫射辐射的这些变化作为陆地表面模型的输入，以量化它对亚马逊森林的影响。在这一过程的每一步，我们将使用来自地面站点和卫星传感器的观测来测试模型对观察到的变化的解释能力。该项目将使人们更好地了解亚马逊气溶胶是如何变化的，以及这种变化是如何影响漫射辐射和生物圈的。

Trees in the Amazon are getting bigger absorbing large amounts of carbon dioxide from the atmosphere; equivalent to almost 10% of anthropogenic greenhouse gas emissions. This "carbon sink" slows the buildup of atmospheric carbon dioxide and therefore plays an important role in regulating climate change. At the moment we do not understand why the Amazon forest is behaving in this way. One potential reason is changes to the amount of tiny particles in the atmosphere called aerosols. We know that human activity has significantly changed the amount of aerosol over the Amazon: each dry season, humans start forest fires across southern parts of the Amazon basin (the so called "Arc of deforestation"). These fires emit large quantities of smoke aerosol into the atmosphere, scattering and absorbing radiation from the sun. This scattering of the sun's rays decreases the amount of direct radiation reaching the surface but increases the amount of diffuse radiation. It has been known for a long time that increases in diffuse radiation can increase plant productivity. For example, gardeners place "diffuser sheets" inside greenhouses that increase diffuse radiation in the similar way to atmospheric particles. The reason why diffuse radiation increases plant productivity is that it penetrates deeper into the canopy and illuminates leaves that would otherwise be in the shade. These previously shaded leaves can do more photosynthesis. A recent study showed that this effect can be very important, increasing the amount of carbon taken up by forests globally by 25%. It is therefore likely that deforestation fires (a major source of carbon dioxide emissions) are at least partly contributing to the sink of carbon dioxide in undisturbed forests in the Amazon basin. So far this effect has not been carefully studied for forests in the Amazon. Firstly, it is not well known exactly how much the amount of aerosol particles over the Amazon has changed. Second, no one has quantified how this change in aerosol particles has altered the amount of diffuse radiation. And finally we do not know whether these changes in diffuse radiation are sufficient to explain any of the observed changes in forest biomass across the Amazon. In this project we will use a suite of atmospheric aerosol, radiation and land-surface models to explore the problem. We will use a global model of atmospheric aerosol to simulate how aerosol distributions have changed over the Amazon over the last few decades. We will input these changes in aerosol into an atmospheric radiation model to quantify how amounts of diffuse radiation have changed. Finally, we will use these changes in diffuse radiation as input to a land-surface model to quantify how it impacts Amazon forests. At each step in the process we will use observations from ground-sites and satellite sensors to test how well the models explain observed changes. The project will lead to a much better understanding of how Amazon aerosol has changed and how this impacts diffuse radiation and the biosphere.