Aerosol hygroscopic models based on in situ measurements and lidar retrievals DANIELA VIVIANA VLADUTESCU, YONGHUA WU, BARRY GROSS, LEONA CHARLES, FRED MOSHARY, SAMIR AHMED Electrical Engineering Department City College of the City University of New York 140 th Street @ Convent Ave, The Grove School of Engineering, T 553 USA http://www-ee.ccny.cuny.edu/wwwa/web/vlad/index.html Abstract: - The affinity of aerosol particles to water, as measured by the relative humidity, plays an important role in several processes. It influences visibility reduction in the atmosphere, modifies aerosol gas chemistry through multiphase reactions, modifies the particles ability to act as cloud condensation nuclei resulting in an influence on the Earth’s radiation budget. In this paper, we examine the modeling needed to quantify optical scattering coefficients on physical models. The model uses as input hygroscopic growth factors, refractive indexes and dry air densities measured by Hanel(1976). The relative humidity was obtained from the water vapor mixing ratio obtained using the Raman lidar located at CCNY(City College of the City University of New York) and compared with the radiosonde measurements located at Brookhaven National Laboratory . The hygroscopic model results are then compared with CCNY lidar retrievals for validation. Key-Words: - Raman lidar, hygroscopic aerosol, hygroscopic growth factor, 1 Introduction 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 most dramatic effects occur at high relative humidity values which are not often seen at surface level but are manifested near clouds. Therefore lidar measurements near cloud base offer a chance at looking at hygroscopic properties. In particular, the ability of 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 of density variations which plague the single channel approach since the normalized backscatter (to the dry state) retrieved at 355nm and 1064nm is independent of the total number of particles (for small spatial ranges). In particular, we investigate the possibility of using the ratio optical scatter measurements which eliminate the inherent problem of variable particle number and illustrate the sensitivity of different hygroscopic aerosols to these measurements and find that combining both extinction and backscatter with color ratios improves the accuracy of the retrieval 2 Aerosol Hygroscopicity 2.1 Mathematical Modeling Hygroscopic models are a suitable way of quantitatively and qualitatively evaluate the backscatter outputs of lidars and other instruments that measure hygroscopic properties of aerosols [1,2]. The models created here are designed to determine both the color ratio of backscatter signals at 355nm and respectively 1064nm as function of altitude as well as the extinction – backscatter ratio at 355nm. It is expected that a combination of both ratios can provide improved retrieval of hygroscopic model parameters. The theory behind the model can be described by ratio of the normalized backscatter at two different wavelengths as given in equation 1 [3,4]. log * log ) ( * * , 2 log 2 * log ) ( * * , 2 log * log ) ( * * , 2 log 2 * log ) ( * * , 2 2 0 ) ( 1 0 ) ( 2 0 2 0 0 0 0 0 1 0 2 0 0 0 0 0 2 1 r d r r d r dn N m r Q r d r r d r dn N m r Q r d r r d r dn N m r Q r d r r d r dn N m r Q s RH s s RH s s s s s ∫ ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∫ ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∫ ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ∫ ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ = π λ π π λ π π λ π π λ π σ σ σ σ β β (1) 5th WSEAS Int. Conf. on ENVIRONMENT, ECOSYSTEMS and DEVELOPMENT, Tenerife, Spain, December 14-16, 2007 263