Indirect aerosol hygroscopic growth observations with a backscatter Lidar

Abstract

Aerosol Higroscopicity addresses a particular aspect of aerosol, namely, the extent to which they have an affinity for water vapor. The size increase of aerosol particles resulting from uptake of water vapor has important implications for the direct scattering of radiation and, under the right circumstances, to form cloud droplets. Ultimately this effect should have an important effect on the Earth's radiative budget and belongs to a category well known as aerosol indirect effect. For this purposed we have used a single-wavelength backscatter LIDAR (532 nm), combined with thermodynamics considerations, to derivate the hygroscopic growing factor of aerosols over Sao Paulo metropolitan region. To test this factor assessment we employed data obtained in a single day, namely on 11 September 2007, when a well characterized humidity intrusion is onset due the transport of water vapor by a sea-breeze phenomenon. For this data, we calculated the backscatter coefficient at 532 nm, and used this parameter to obtain the hygroscopic growing factor, assuming a well-mixed boundary layer where a cloud cap condition is present or a well defined and pronounced mixing layer boundary are present and other thermodynamic assumptions. These assumptions guarantee that any changes in the backscatter coefficient are mainly due to changes in relative humidity, rather than in aerosol size distribution. The results shown here should be regarded as a first step on an ongoing monitoring process of aerosol growth factor and will in the near future be merged with a Water Vapor Raman Lidar system in order to have a simultaneous water vapor mixing ratio profile together with the aerosol profile and it should be mainly used when clear, low-aerosol load conditions are available.