Temperature- and frequency-dependent optical constants for nitric acid dihydrate from aerosol spectroscopy

Frequency-dependent complex refractive indices have been determined from the aerosol infrared extinction spectra of laboratory-generated nitric acid dihydrate (NAD) particles. The spectra reported here were recorded at several temperatures (160, 180, and 190 K) over the mid-infrared region of the spectrum (700-4700 cm(-1)). The optical constants retrieved from these spectra are overall in reasonable agreement with results from thin-film experiments conducted at 184 K, although there are significant differences in the intensities of several absorption bands located below 1500 cm(-1). We attribute these to the different methods used to make the samples in the two types of experiments and consider the substrate-free aerosol results to be unperturbed NAD. The temperature dependence of the NAD aerosol optical constants was found to be quite weak between 160 and 190 K, in contrast to the strong temperature dependence observed previously for the optical constants of water ice.