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.

Accurate fire emissions inventories are crucial to predict the impacts of wildland fires on air quality and atmospheric composition. Two traditional approaches are widely used to calculate fire emissions: a satellite-based top-down approach and a fuels-based bottom-up approach. However, these methods often considerably disagree on the amount of particulate mass emitted from fires. Previously available observational datasets tended to be sparse, and lacked the statistics needed to resolve these methodological discrepancies. Here, we leverage the extensive and comprehensive airborne in situ and remote sensing measurements of smoke plumes from the recent Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign to statistically assess the skill of the two traditional approaches. We use detailed campaign observations to calculate and compare emission rates at an exceptionally high resolution using three separate approaches: top-down, bottom-up, and a novel approach based entirely on integrated airborne in situ measurements. We then compute the daily average of these high-resolution estimates and compare with estimates from lower resolution, global top-down and bottom-up inventories. We uncover strong, linear relationships between all of the high-resolution emission rate estimates in aggregate, however no single approach is capable of capturing the emission characteristics of every fire. Global inventory emission rate estimates exhibited weaker correlations with the high-resolution approaches and displayed evidence of systematic bias. The disparity between the low resolution global inventories and the high resolution approaches is likely caused by high levels of uncertainty in essential variables used in bottom-up inventories and imperfect assumptions in top-down inventories.

Wildfire smoke influences on air quality and atmospheric chemistry have been underscored by the increasing fire prevalence in recent years, and yet, the connection between fire, smoke emissions, and the subsequent transformation of this smoke in the atmosphere remains poorly constrained. Toward improving these linkages, we present a new method for coupling high-time-resolution satellite observations of fire radiative power (FRP) with in situ observations of smoke aerosols and trace gases. We apply this technique to thirteen fire plumes comprehensively characterized during the recent FIREX-AQ mission and show that changes in FRP directly translate into changes in conserved smoke tracers (CO2, CO, and black carbon aerosol) observed in the downwind smoke plume. The correlation is particularly strong for CO2 (mean r>0.9). This method is important for untangling the competing effects of changing fire behavior versus the influence of dilution and atmospheric processing on the down-wind evolution of measured smoke properties.