Influence of humidity on the aerosol scattering coefficient and its effecton the upwelling radiance during ACE-2

Aerosol scattering coefficients (sigma(sp)) have been measured over the oce an at different relative humidities (RH) as a Function of altitude in the r egion surrounding the Canary Islands during the Second Aerosol Characteriza tion Experiment (ACE-2) in June and July 1997. The data were collected by t he University of Washington passive humidigraph (UWPH) mounted on the Pelic an research aircraft. Concurrently, particle size distributions, absorption coefficients and aerosol optical depth were measured throughout 17 flights . A parameterization of sigma(sp) as a function of RH was utilized to asses s the impact of aerosol hydration on the upwelling radiance (normalized to the solar constant and cosine of zenith angle). The top of the atmosphere r adiance signal was simulated at wavelengths corresponding to visible and ne ar-infrared bands of the EOS-AM ("Terra") detectors, MODIS and MISR. The UW PH measured sigma(sp) at 2 RHs, one below and the other above ambient condi tions. Ambient sigma(sp) was obtained by interpolation of these 2 measureme nts. The data were stratified in terms of 3 types of aerosols: Saharan dust , clean marine (marine boundary layer background) and polluted marine aeros ols (i.e., 2- or 1-day old polluted aerosols advected from Europe). An empi rical relation for the dependence of sigma(sp) on RH, defined by sigma(sp) (RH) = k. (1 - RH/100)(-gamma), was used with the hygroscopic exponent gamm a derived from the data. The following gamma values were obtained for the 3 aerosol types: gamma(dust) = 0.23 +/- 0.05, gamma(clean marine) = 0.69 +/- 0.06 and gamma(polluted marine) = 0.57 +/- 0.06. Based on the measured gam ma's, the above equation was utilized to derive aerosol models with differe nt hygroscopicities. The satellite simulation signal code 6S was used to co mpute the upwelling radiance corresponding to each of those aerosol models at several ambient humidities. For the prelaunch estimated precision of the sensors and the assumed viewing geometry of the instrument, the simulation s suggest that the spectral and angular dependence of the reflectance measu red by MISR is not sufficient to distinguish aerosol models with various di fferent combinations of values for dry composition, gamma and ambient RH. A similar behavior is observed for MODIS at visible wavelengths. However, th e 2100 nm band of MODIS appears to be able to differentiate between at leas t same aerosol models with different aerosol hygroscopicity given the MODIS calibration error requirements. This result suggests the possibility of re trieval of aerosol hygroscopicity by MODIS.