Balloon validation of remotely sensed aerosol properties

...政府间气候变化专门委员会在其2007年的第四次评估报告中指出，人为气溶胶“仍然是辐射强迫的主要不确定性因素”。更多的气溶胶应该会将更多的阳光反射到太空中，并倾向于冷却气候，或者，如果气溶胶含有黑碳，它们可能会加热空气，导致云层蒸发，而更少的云层会反射更少的阳光到太空。更多的气溶胶可能会导致云中有更多但更小的水云滴，使它们看起来更白，并将更多的阳光反射回太空，此外，如果更小的水滴不太可能聚集、形成雨和分散，那么云的寿命可能会延长。对于冰云来说，情况甚至更加复杂和不确定。当温度降至冰点以下时，大多数水滴仍以液体形式存在，并处于过冷状态；只有极少数气溶胶粒子具有冰核的特性，并促进冰冻。人为气溶胶对冰核数量的影响基本上是未知的。迫切需要在全球和地方尺度上更好地观测气溶胶的粒子谱和化学成分，以便对上述过程进行研究和量化。我们需要知道气溶胶是在哪里产生的，它们是如何在水平和垂直方向上移动的，以及它们是如何通过降水去除的。我们需要知道它们是由黑碳组成的，因此会吸收辐射并加热空气，还是它们像硫酸铵一样具有吸湿性，可以作为有效的云凝结核，或者它们是否由沙漠尘埃组成，因此可能起到更好的冰核的作用。目前的全球气溶胶光学厚度气候是根据卫星对反射太阳光的观测得出的，而不是飞机乘客通过俯视雾霾层来测量气溶胶含量的方式，但要做到这一点，你必须假定气溶胶的大小分布和化学成分，此外，没有关于气溶胶垂直分布的信息。星载和陆基激光雷达通过发射光脉冲，然后在与距离相关的延迟时间后探测后向散射信号来提供垂直轮廓，但同样，气溶胶的大小和组成是什么？更复杂的激光雷达会产生额外的参数。如果使用两个频率，可以估计大小，形状可以从去极化的返回中推断出来；短波长激光雷达探测依赖于已知空气密度的分子后向散射，该信号的任何减小都是气溶胶光学厚度的直接测量。奇尔博顿的三个激光雷达在三个不同的波长，每30秒提供七个独立激光雷达参数的垂直剖面，分辨率为60米。最近发射的星载激光雷达CALIPSO有三个独立的参数：两个波长的后向散射和去偏振率。然而，目前用气溶胶的物理和化学性质来解释所有这些参数是不明确的。该项目的目的是在奇博尔顿的地面激光雷达上方的系留气球上飞行一个新的气雾剂包，并准确地确定可以从七个独立的激光雷达参数中推断出多少信息。这个气球最近已经获准飞行，三次试飞用一个简单的系统传递温度、湿度和压力，证明了这个气球系统的可行性。计划在两周内进行七到八次飞行，更广泛的地面气溶胶设备将在两个月内进行观测。如果成功，对激光雷达信号的解释将被置于一个更定量的框架内，并将导致对气溶胶特性剖面进行更准确的全球连续观测。

...The Intergovernmental Panel on Climate Change, in its fourth assessment report in 2007, states that anthropogenic aerosols 'remain the dominant uncertainty in radiative forcing'. More aerosols should reflect more sunlight out to space and tend to cool the climate, or, alternatively, if the aerosols contain black carbon they may heat the air and cause clouds to evaporate, and less cloud would reflect less sunlight to space. More aerosols could lead to clouds having more but smaller water cloud droplets so they would appear 'whiter' and reflect more sunshine back to space, in addition, if the smaller droplets are less likely to coalesce and form rain and disperse so cloud lifetime may increase. For ice clouds the situation is even more complex and uncertain. When the temperature falls below freezing most of the water drops remain as liquid and are 'supercooled'; only a very few aerosol particles have the property of being an 'ice nuclei' and promote freezing. The effect of anthropogenic aerosol on the number of ice nuclei is essentially unknown. Better observations of the size spectrum and chemical composition of aerosols are urgently needed on a global and local scale, so that the processes outlined above can be studies and quantified. We need to know where aerosols are produced, how they are moved around both horizontally and vertically, and how they are removed via precipitation. We need to know if they are made of black carbon and so will absorb radiation and heat the air, or if they are hygroscopic like ammonium sulphate and act as efficient cloud condensation nuclei, or if they are made of desert dust and so may act as better ice nuclei. Present global climatologies of aerosol optical depth are derived from satellite observations of reflected sunlight, rather in the manner that an airplane passenger gauges how much aerosol there is by looking down at the haze layer, but to do this you must assume an aerosol the size distribution and chemical composition of the aerosol, and, in addition, there is no information on the vertical distribution of the aerosol. Spaceborne and ground based lidars provide a vertical profile by emitting a pulse of light and then detecting the backscattered signal after a delay time which is related to the range, but again what is the size and composition of the aerosol? More complex lidars produce additional parameters. The size can be estimated if two frequencies are used, the shape can be inferred from the depolarised return; short wavelength lidars detect the molecular backscatter which depends upon the known air density, any reduction in this signal is a direct measure of the aerosol optical depth. Three lidars at Chilbolton at three different wavelengths provide vertical profiles of seven independent lidar parameters every 30 seconds with 60m resolution. The recently launched spaceborne lidar, CALIPSO, has three independent parameters: backscatter at two wavelengths and a depolarisation ratio. However, there currently there is an ambiguity in interpreting all these parameters in terms of the physical and chemical properties of the aerosols. The aim of this project is to fly a new aerosol package on a tethered balloon at Chibollton above the ground based lidars and to establish precisely how much information can be inferred from the seven independent lidar parameters. Permission has recently been granted to fly this balloon and three trail flights with a simple system transmitting temperature, humidity and pressure have demonstrated the viability of the balloon system. Seven or eight flights are planned over a two week period, with more extensive ground based aerosol equipment taking observations for a two month period. If successful, interpretation of the lidar signals would be placed in a more quantitative framework, and should lead to more accurate global continuous observations of the profiles of aerosol properties.