Surface Electric Fields of Aqueous Solutions of NH4NO3, Mg(NO3)2, NaNO3, and LiNO3: Implications for Atmospheric Aerosol Chemistry

Abstract

Ammonium (NH4+), magnesium (Mg2+), sodium (Na+), and nitrate (NO3–) ions are common constituents of ocean waters and found in abundance in marine atmospheric aerosols. Revealing the surface propensity (surface activity) of relevant ions provides insight into heterogeneous aerosol processes and potential impact on atmospheric chemistry. However, there is sparse surface data on NH4+, Mg2+, and little consensus for nitrate’s surface activity. Phase-resolved vibrational sum frequency generation (VSFG) experiments are highly well-suited for this task; yet, only aqueous NaNO3 salt has been studied by this technique to date. Here we investigate NH4+, Mg2+, and Na+, in addition to lithium (Li+), to fully understand the effect of countercations on nitrate but also to elucidate the surface propensity of the countercations themselves as this is the first phase-resolved heterodyne-detected (HD-)VSFG study to address nitrate with countercations beyond sodium. We show the presence of an interfacial electric field generated by surface-active NO3– and its counterions, including NH4+, a cation that is commonly identified with nitrate in tropospheric aerosol generated in urban regions. We find that NH4+, Li+, Na+, and Mg2+ are less surface-active relative to NO3–, with Mg2+ being the least so. The cation identity and the ion concentration determine the magnitude of this electric field, which decreases in the order of Mg2+ > Na+ > Li+ > NH4+ at the ∼2 M NO3– concentration (highly relevant to low water content aerosol). Important to note is that the surface charge density does not completely dictate the trend observed as one might expect.