The structural independence of Raman scattering cross sections of ν₁(NO₃^-) and ν(H₂O)

The Raman scattering cross section (RSCS) is an important parameter in the applications of Raman spectroscopy to make quantitative analysis. To date, the dependence of the RSCS on concentration has remained unclear. Nitrate aerosols can easily achieve a supersaturated state, which provides a way to obtain the RSCS especially under this state. In this study, Raman spectra of NaNO ₃ and Mg(NO₃)₂ solutions are obtained with molar water-to-solute ratios (WSRs) ranging from 84.2 to 2.30 and 93.8 to 7.32, respectively. With decreasing WSR, a shift to higher wavenumbers of the symmetric stretching band of nitrate ion, i.e. ν₁(NO ₃^-), is observed, indicating the formation of various ion pairs. Meanwhile, the area ratio between the strongly and weakly hydrogen-bonded components of water O-H stretching envelope, i.e. ν(H₂O), reduces as the WSR decreases, implying the transformation of water molecules from strong hydrogen-bonding structures to the weak ones. However, a good linear relationship is revealed between the integrated intensity ratio of the ν(H₂O) band to ν₁(NO₃^-) band and WSR. The results suggest that the RSCSs of NO₃^- and H₂O are insensitive to the structures of both ion pairs and hydrogen-bonding structures. This observation points to the possibility of conducting quantitative analysis through the area ratio of the ν(H ₂O) band to the ν₁(NO₃^-) band with Raman spectra without considering the formation of ion pairs and the variation of the hydrogen-bonding structure.