Balloon validation of remotely sensed aerosol properties

...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.