Wildfires emit significant amounts of pollutants that degrade air quality. Plumes from three wildfires in the western U.S. were measured from aircraft during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) and the Biomass Burning Observation Project (BBOP), both in summer 2013. This study reports an extensive set of emission factors (EFs) for over 80 gases and 5 components of submicron particulate matter (PM1) from these temperate wildfires. These include rarely, or never before, measured oxygenated volatile organic compounds and multifunctional organic nitrates. The observed EFs are compared with previous measurements of temperate wildfires, boreal forest fires, and temperate prescribed fires. The wildfires emitted high amounts of PM1 (with organic aerosol (OA) dominating the mass) with an average EF that is more than 2 times the EFs for prescribed fires. The measured EFs were used to estimate the annual wildfire emissions of carbon monoxide, nitrogen oxides, total nonmethane organic compounds, and PM1 from 11 western U.S. states. The estimated gas emissions are generally comparable with the 2011 National Emissions Inventory (NEI). However, our PM1 emission estimate (1530 ± 570 Gg yr−1) is over 3 times that of the NEI PM2.5 estimate and is also higher than the PM2.5 emitted from all other sources in these states in the NEI. This study indicates that the source of OA from biomass burning in the western states is significantly underestimated. In addition, our results indicate that prescribed burning may be an effective method to reduce fine particle emissions.

Abstract. NASA's Orbiting Carbon Observatory-2 (OCO-2) has been measuring carbon dioxide column-averaged dryair mole fraction, X CO 2 , in the Earth's atmosphere for over 2 years. In this paper, we describe the comparisons between the first major release of the OCO-2 retrieval algorithm (B7r) and X CO 2 from OCO-2's primary ground-based validation network: the Total Carbon Column Observing Network (TC-CON). The OCO-2 X CO 2 retrievals, after filtering and bias correction, agree well when aggregated around and coincident with TCCON data in nadir, glint, and target observation modes, with absolute median differences less than 0.4 ppm and RMS differences less than 1.5 ppm. After bias correction, residual biases remain. These biases appear to depend on latitude, surface properties, and scattering by aerosols. It is thus crucial to continue measurement comparisons with TCCON to monitor and evaluate the OCO-2 X CO 2 data quality throughout its mission.

Gas-phase hydrogen (H) abstractions from molecules by free radicals have been studied extensively. They form the simplest class of elementary reactions and also play a key role in atmospheric chemistry and so are the centerpiece of models of reactivity. Despite intense scrutiny, two fundamental mechanistic issues remain unresolved:  (1) Do H abstractions proceed directly or indirectly? (2) Do thermodynamic or electronic interactions determine their reaction barrier? The thermoneutral identity reaction, OH + H2O → H2O + OH, provides an excellent opportunity to answer these questions. Several theoretically predicted H2O−HO complexes raise the possibility of an indirect mechanism, while no thermodynamic forcing influences the reaction barrier. To examine the various reactivity models, the isotopic scrambling reactions 18OH + H2 16O → H2 18O + 16OH and 16OD + H2 16O → H16OD + 16OH are studied in a high-pressure flow reactor. The measured rate constants are (2.3 ± 1.0) × 10-13 exp[−(2100 ± 250)/T] cm3 molecule-1 s-1 over the range 300−420 K ((2.2 ± 1.0) × 10-16 at 300 K) and (3 ± 1.0) × 10-16 cm3 molecule-1 s-1 at 300 K, respectively. The similarity between the room temperature rates indicates a small secondary isotope effect. While the strong temperature dependence reveals that the predicted complexes do not stabilize the isotope exchange transition state sufficiently to bring its energy below the reactants, the small preexponential factor indicates that the complexes pose entropic constraints. Therefore, the reaction mechanism appears to be indirect. This is clarified by tracing the evolution of reagent electronic interactions and geometrical transformations along the reaction path. Activation energies of isotope exchange reactions are used to constrain the thermoneutral intercept for themodynamically based reactivity models. These thermochemical models are shown to be unreliable. However, a correlation between theoretical (ab initio) and experimental reaction barriers does capture gross reactivity trends. These measurements also exclude kinetic fractionation by OH as an important contributor to the isotopic fractionation of water in the earth's atmosphere.

Wildfires are important contributors to atmospheric aerosols and a large source of emissions that impact regional air quality and global climate. In this study, the regional and nearfield influences of wildfire emissions on ambient aerosol concentration and chemical properties in the Pacific Northwest region of the United States were studied using real-time measurements from a fixed ground site located in Central Oregon at the Mt. Bachelor Observatory (∼2700 m a.s.l.) as well as near their sources using an aircraft. The regional characteristics of biomass burning aerosols were found to depend strongly on the modified combustion efficiency (MCE), an index of the combustion processes of a fire. Organic aerosol emissions had negative correlations with MCE, whereas the oxidation state of organic aerosol increased with MCE and plume aging. The relationships between the aerosol properties and MCE were consistent between fresh emissions (∼1 h old) and emissions sampled after atmospheric transport (6-45 h), suggesting that biomass burning organic aerosol concentration and chemical properties were strongly influenced by combustion processes at the source and conserved to a significant extent during regional transport. These results suggest that MCE can be a useful metric for describing aerosol properties of wildfire emissions and their impacts on regional air quality and global climate.

Abstract. We present data for the rate constant of the reaction NO2 q-OH -• HONO2 in nitrogen from 2 to 600 torr at 300 K. This is the first application of our high-pressure flow technique to a pressure-dependent reaction.