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Abstract. The hydroxyl (OH), hydroperoxy (HO2), and organic peroxy (RO2)radicals play important roles in atmospheric chemistry. In the presence ofnitrogen oxides (NOx), reactions between OH and volatile organiccompounds (VOCs) can initiate a radical propagation cycle that leads to theproduction of ozone and secondary organic aerosols. Previous measurements ofthese radicals under low-NOx conditions in forested environmentscharacterized by emissions of biogenic VOCs, including isoprene andmonoterpenes, have shown discrepancies with modeled concentrations. During the summer of 2016, OH, HO2, and RO2 radical concentrationswere measured as part of the Program for Research on Oxidants:Photochemistry, Emissions, and Transport – Atmospheric Measurements ofOxidants in Summer (PROPHET-AMOS) campaign in a midlatitude deciduousbroadleaf forest. Measurements of OH and HO2 were made by laser-inducedfluorescence–fluorescence assay by gas expansion (LIF-FAGE) techniques,and total peroxy radical (XO2) mixing ratios were measured by the Ethane CHemical AMPlifier (ECHAMP) instrument. Supporting measurements ofphotolysis frequencies, VOCs, NOx, O3, and meteorological datawere used to constrain a zero-dimensional box model utilizing either theRegional Atmospheric Chemical Mechanism (RACM2) or the Master ChemicalMechanism (MCM). Model simulations tested the influence of HOxregeneration reactions within the isoprene oxidation scheme from the LeuvenIsoprene Mechanism (LIM1). On average, the LIM1 models overestimated daytimemaximum measurements by approximately 40 % for OH, 65 % for HO2,and more than a factor of 2 for XO2. Modeled XO2 mixing ratioswere also significantly higher than measured at night. Addition of RO2 + RO2 accretion reactions for terpene-derived RO2 radicals tothe model can partially explain the discrepancy between measurements andmodeled peroxy radical concentrations at night but cannot explain thedaytime discrepancies when OH reactivity is dominated by isoprene. Themodels also overestimated measured concentrations of isoprene-derivedhydroxyhydroperoxides (ISOPOOH) by a factor of 10 during the daytime,consistent with the model overestimation of peroxy radical concentrations.Constraining the model to the measured concentration of peroxy radicalsimproves the agreement with the measured ISOPOOH concentrations, suggestingthat the measured radical concentrations are more consistent with themeasured ISOPOOH concentrations. These results suggest that the models maybe missing an important daytime radical sink and could be overestimating therate of ozone and secondary product formation in this forest. 

Abstract. Extensive airborne measurements of non-methane organic gases (NMOGs), methane, nitrogen oxides, reduced nitrogen species, and aerosol emissions from US wild and prescribed fires were conducted during the 2019 NOAA/NASA Fire Influence on Regional to Global Environments and Air Quality campaign (FIREX-AQ). Here, we report the atmospheric enhancement ratios (ERs) and inferred emission factors (EFs) for compounds measured on board the NASA DC-8 research aircraft for nine wildfires and one prescribed fire, which encompass a range of vegetation types. We use photochemical proxies to identify young smoke and reduce the effects of chemical degradation on our emissions calculations. ERs and EFs calculated from FIREX-AQ observations agree within a factor of 2, with values reported from previous laboratory and field studies for more than 80 % of the carbon- and nitrogen-containing species. Wildfire emissions are parameterized based on correlations of the sum of NMOGs with reactive nitrogen oxides (NOy) to modified combustion efficiency (MCE) as well as other chemical signatures indicative of flaming/smoldering combustion, including carbon monoxide (CO), nitrogen dioxide (NO2), and black carbon aerosol. The sum of primary NMOG EFs correlates to MCE with an R2 of 0.68 and a slope of −296 ± 51 g kg−1, consistent with previous studies. The sum of the NMOG mixing ratios correlates well with CO with an R2 of 0.98 and a slope of 137 ± 4 ppbv of NMOGs per parts per million by volume (ppmv) of CO, demonstrating that primary NMOG emissions can be estimated from CO. Individual nitrogen-containing species correlate better with NO2, NOy, and black carbon than with CO. More than half of the NOy in fresh plumes is NO2 with an R2 of 0.95 and a ratio of NO2 to NOy of 0.55 ± 0.05 ppbv ppbv−1, highlighting that fast photochemistry had already occurred in the sampled fire plumes. The ratio of NOy to the sum of NMOGs follows trends observed in laboratory experiments and increases exponentially with MCE, due to increased emission of key nitrogen species and reduced emission of NMOGs at higher MCE during flaming combustion. These parameterizations will provide more accurate boundary conditions for modeling and satellite studies of fire plume chemistry and evolution to predict the downwind formation of secondary pollutants, including ozone and secondary organic aerosol.