On the use of UV photo-oxidation for the determination of total nitrogen in rainwater and water-extracted atmospheric aerosol

Three methods are currently in use for the determination of total N within rainwater and water-extracted aerosols—UV photo-oxidation, persulfate digestion, and high-temperature combustion. Among these methods, UV photo-oxidation has received scrutiny for its reported inability to digest “refractory” organic N compounds. In this manuscript we utilized inorganic and organic nitrogen standards to carefully assess the ability of a specific UV digestor, the Metrohm 705 UV digestor, to correctly digest organic N within dilute solutions (such as rainwater and atmospheric aerosol). We also discuss the negative aspects of the three methods listed above, comment on the photochemical reactions and chemical products produced by exposure to UV light, and make suggestions concerning the proper use of UV digestion technique when determining total N. Our tests indicate that at a digestion time of 2 h and a temperature of 85°C the system is satisfactory for determining the total amount of organic N within dilute solutions. The upper concentration limit using this UV system was found to be 50 μM N of organic N. Diluted samples were found to carry a larger burden of analytical error due to the combined effects of UV blanks (that can range from 0.1–0.6 μM N) and the uncertainty associated with the chromatographic ion analysis (for nitrate, nitrite, and ammonium). This error can often lead to negative values when reporting organic N. We suggest that negative values for organic N be included within the mean and standard deviation of organic N measurements rather than excluding them by rounding negative values to zero. Only by including all analytical error within measurements will true concentrations of organic N within rainwater and atmospheric aerosols and its relevance to total N budgets be known.