TEMPERATURE-DEPENDENT AND FREQUENCY-DEPENDENT OPTICAL-CONSTANTS FOR NITRIC-ACID DIHYDRATE FROM AEROSOL SPECTROSCOPY

Frequency-dependent complex refractive indices have been determined fr om the aerosol infrared extinction spectra of laboratory-generated nit ric acid dihydrate (NAD) particles. The spectra reported here were rec orded at several temperatures (160, 180, and 190 K) over the mid-infra red region of the spectrum (700-4700 cm(-1)). The optical constants re trieved from these spectra are overall in reasonable agreement with re sults from thin-film experiments conducted at 184 K, although there ar e significant differences in the intensities of several absorption ban ds located below 1500 cm(-1). We attribute these to the different meth ods used to make the samples in the two types of experiments and consi der the substrate-free aerosol results to be unperturbed NAD. The temp erature dependence of the NAD aerosol optical constants was found to b e quite weak between 160 and 190 K, in contrast to the strong temperat ure dependence observed previously for the optical constants of water ice.