Photolysis of nitrate ion in the natural environment produces NO, NO2, and O3, releasing these toxic gases into the atmosphere. Work done by other groups has shown ionic strength dependence of the ratio of products from photolysis of aqueous nitrate ion. To better understand the kinetic mechanisms of nitrate photolysis, the effects that ionic strength in solution have on nitrate ion symmetry breaking are needed. Different solvation environments induce nitrate bonding motifs that may be correlated to the product ratio. Fourier-transform infrared spectra of aqueous nitrate–ion solutions were obtained over a range of temperatures for several total electrolyte concentrations. The electric fields (arising from water molecules and ions in solution) in aqueous potassium nitrate solution distort the trigonal planar shape of the nitrate ion, which may favor a specific initial path of the decomposition of nitrate during photolysis. Van’t Hoff plots of the relative peak areas corresponding to the formally-degenerate asymmetric stretching mode reveal the relative energies of the two solvation geometries. The difference in energy between the two geometries is linearly proportional to the ionic strength of the solution. Electronic structure calculations suggest that the more symmetric geometry has an increased stability relative to the less-symmetric geometry in high ionic strength solutions. Thus, the relative amounts of the nitrate ion solvation geometries can be correlated to the amount of products produced during photolysis to help explain the ionic-strength dependence of the product yields. Nitrate geometries at the water—CCl4 interface and aqueous carbonate ion bonding motifs are being investigated to identify pure-water effects. Preliminary results suggest that the more symmetric geometry nitrate is stabilized at the water—CCl4 interface and the lesssymmetric carbonate solvation geometry has an increased stability relative to the more symmetric geometry in high ionic strength solutions.

This work provides improved procedures for spectrophotometric carbon system measurements. Indicator dyes used for routine spectrophotometric pH measurements in seawater suffer from impurity issues, which introduce vendor-specific systematic errors in pH determinations. The magnitude of these errors for several vendors was investigated for meta Cresol Purple (mCP) and Cresol Red (CR). Flash chromatography procedures were developed to obtain purified mCP and CR on a bulk scale in order to supply the oceanographic research community with the indicators. Easy access to the purified indicators ensures global intercomparability of spectrophotometric pH determinations. Internal consistency of marine inorganic carbon system measurements was studied using datasets obtained on two large coastal ocean acidification research cruises. In both cases, purified mCP was used to obtain the pH measurements, thereby improving accuracy relative to previous studies in which measurements were obtained with unrefined mCP. Based on this internal consistency study, recommendations are made for selecting the parameter pairs used for saturation state calculations. Direct spectrophotometric methods for measuring carbonate ion concentrations in seawater were improved by (a) using a higher concentration of lead as the carbonate indicator and (b) altering the carbonate computational algorithm based on high quality field data. Measurements of DIC and pH (using purified mCP) were used to calculate carbonate ion concentrations for comparison with spectrophotometrically measured carbonate ion concentrations (i.e., via spectrophotometric measurements of Pb(II) spectra in the ultraviolet). Minor changes in the computational algorithm substantially improved agreement between measured and calculated carbonate ion concentrations.