Automated, High-resolution Mobile Collection System for the Nitrogen Isotopic Analysis of NO(x)

Nitrogen oxides (NO(x) = NO + NO(2)) are a family of atmospheric trace gases that have great impact on the environment. NO(x) concentrations directly influence the oxidizing capacity of the atmosphere through interactions with ozone and hydroxyl radicals. The main sink of NO(x) is the formation and deposition of nitric acid, a component of acid rain and a bioavailable nutrient. NO(x) is emitted from a mixture of natural and anthropogenic sources, which vary in space and time. The collocation of multiple sources and the short lifetime of NO(x) make it challenging to quantitatively constrain the influence of different emission sources and their impacts on the environment. Nitrogen isotopes of NO(x) have been suggested to vary amongst different sources, representing a potentially powerful tool to understand the sources and transport of NO(x). However, previous methods of collecting atmospheric NO(x) integrate over long (week to month) time spans and are not validated for the efficient collection of NO(x) in relevant, diverse field conditions. We report on a new, highly efficient field-based system that collects atmospheric NO(x) for isotope analysis at a time resolution between 30 min and 2 hr. This method collects gaseous NO(x) in solution as nitrate with 100% efficiency under a variety of conditions. Protocols are presented for collecting air in urban settings under both stationary and mobile conditions. We detail the advantages and limitations of the method and demonstrate its application in the field. Data from several deployments are shown to 1) evaluate field-based collection efficiency by comparisons with in situ NO(x) concentration measurements, 2) test the stability of stored solutions before processing, 3) quantify in situ reproducibility in a variety of urban settings, and 4) demonstrate the range of N isotopes of NO(x) detected in ambient urban air and on heavily traveled roadways.