Biomass burning is a primary emission source for a host of gas- and aerosol-phase compounds, which can damage environmental and human health. During the FIREX-AQ campaign in July and August of 2019, we measured reactive nitrogen species (NOx, NO2, HONO, HNO3 and p-NO3-), in wildfire plumes aboard NASA Langley’s Mobile Aerosol Characterization Laboratory (MACH-2). Nitrous acid (HONO) and nitric acid (HNO3) mixing ratios were measured at nominal 5-minute resolution using a dual mist chamber-ion chromatograph from five separate areas of fire in the western US and are the primary focus of this paper. Average HONO mixing ratios were significantly higher in young daytime smoke compared to young nighttime smoke, while no statistical differences were observed between young versus aged smoke during the day or night. In the largest fire sampled during the day, UV-A irradiation was highly correlated (R2 = 0.91) with HONO to nitrogen dioxide (NO2) ratios indicating that photo-enhanced heterogeneous NO2 to HONO conversion, likely facilitated by ground surfaces (e.g. soil, foliage, and dust), more than compensated for rapid photolytic loss of HONO.