The Fire Influence on Regional and Global Environments Experiment (FIREX) proposes to investigate the influence of fires in the western U.S. on climate and air quality, via an intensive, multi-platform, campaign. As part of this, we propose to track wildfire derived nitrogen oxides (NOx = NO+NO2) and their influence on the oxidative formation of nitrate (particulate NO3¯ and gaseous nitric acid (HNO3), and nitrous acid (HONO)). We will quantify the influence of biomass burning on atmospheric chemistry in the western U.S. using the concentration and isotopic composition of NOx (δ15N), NO3¯ (δ15N, δ18O, D17O), and HONO (δ15N, δ18O). The isotopes of these species offer a new tool for tracking the influence of biomass burning on the formation and chemistry of these important reactive species. Using a recently developed method that captures NOx without any fractionation effects, along with established methods for collecting soluble gases and aerosols, we will target daytime and nighttime air impacted directly by emissions from biomass fires. Our collections will take place via a mobile laboratory allowing us to remain flexible while mission focused. Laboratory studies using our new collection method reveal a direct correlation between the nitrogen isotopic composition (δ15N) of NOX and the δ15N of the biomass (fuel) N. A number of field-based studies link different NOx sources and the δ15-NO3¯ based upon a variety of supportive data (e.g., transport, correlation with other tracers), but none have ever undertaken a comprehensive isotopic characterization of NOx, HONO, and nitrate at the same time, and there are no direct studies of the isotopic composition of reactive N species in biomass burning plumes. We expect that the δ15N of NO3¯ and HONO will have a direct relationship with the δ15N-NOX that will be sensitive to biomass burning emissions and chemistry compared to air influenced by other sources. The oxygen isotopic composition (δ18O, D17O) of HONO and nitrate can track the relative abundances of oxidants (e.g., ozone, OH, RO2) responsible for the formation these reactive N species.