The effect of human activities on the global climate may lead to large disturbances of the economic, social and political circumstances in the middle and long term. Understanding the dynamics of the Earth's climate is therefore of high importance and one of the major scientific challenges of our time. The estimation of the contribution of the Earth's climate system components needs observation and continuous monitoring of various atmospheric physical and chemical parameters. Temperature, water vapor and greenhouse gases concentration, aerosol and clouds loads, and atmospheric dynamics are parameters of particular importance in this respect. The quantification of the anthropogenic influence on the dynamics of these above-mentioned parameters is of crucial importance nowadays but still affected by significant uncertainties.
Aerosols can have their optical properties modified in the presence of water vapor with different classes of aerosols being modified in different manners. The need to identify hygroscopic properties from measurements is complicated by the fact that the most dramatic effects occur at high RH values which are not often seen at surface level but are manifested near clouds. Therefore, lidar measurements near cloud base offers a chance at looking at hygroscopic properties. In particular, the ability of a multiwavelength Raman lidar of performing simultaneous atmospheric measurements at three different wavelengths 355nm, 1064nm and 407nm (Raman water vapor) provides the opportunity of isolating the hygroscopic effect from the number density variations which plague the single channel approach since the normalized backscatter (σs(f)/ σs0 ) retrieved at 355 nm and 1064nm is independent of the total number of particles. To our knowledge this type of analyses have never been investigated so far. In addition, the longer wavelength channels allows us to more accurately validate the homogeneity of the aerosol layer as well as provide additional multi wavelength information that can be used to validate and modify the aerosol models underlying the hygroscopic trends observed in the Raman Channel.
To provide a quantitative measure of how the hygroscopic factor may be analyzed for different wavelength combinations, calculations of normalized extinction and absorption coefficients for different wavelengths are calculated based on existing hygroscopic models which provide analytical models which determine how the aerosol microphysical parameters of aerosols are modified by RH. The use of multiple channels also allows us to cancel out the particle number density effects which can be variable. In figure 1, we present typical measurements being made. In figure 1, we plot the normalized backscatter ratios of the 355nm backscattering to the 1064nm backscattering which are compared to different plausible models for an urban coastal area. Model 1 represents a clean continental aerosol, Model 3 is maritime aerosol Model 5 is urban aerosol and Model 6 is an aerosol model collected at high altitudes (1000m altitude). We see that the measurement data seems to be best matched to model 3 which seems to indicate a maritime aerosol which is consistent with sea-breeze conditions.
Finally, to assess the information content in the two channel approach, a parameterized hygroscopic model is used to explore the sensitivity of the 355/1064 backscatter ratio to specific growth factors in the aerosol.
Fig. 1 Normalized aerosol backscatter ratio of the 355nm and 1064nm lidar returns compared to theoretical models ACKNOWLEDGEMENTS This work is supported by grants from NOAA #NA17AE1625 and NASA #NCC-1-03009. modified by 66.65.178.63 on 8-9-2008-->
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