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  1. Abstract. We present the Fire Inventory from National Center for Atmospheric Research (NCAR) version 2.5 (FINNv2.5), a fire emissions inventory that provides publicly available emissions of trace gases and aerosols for various applications, including use in global and regional atmospheric chemistry modeling. FINNv2.5 includes numerous updates to the FINN version 1 framework to better represent burned area, vegetation burned, and chemicals emitted. Major changes include the use of active fire detections from the Visible Infrared Imaging Radiometer Suite (VIIRS) at 375 m spatial resolution, which allows smaller fires to be included in the emissions processing. The calculation of burned area has been updated such that a more rigorous approach is used to aggregate fire detections, which better accounts for larger fires and enables using multiple satellite products simultaneously for emissions estimates. Fuel characterization and emissions factors have also been updated in FINNv2.5. Daily fire emissions for many trace gases and aerosols are determined for 2002–2019 (Moderate Resolution Imaging Spectroradiometer (MODIS)-only fire detections) and 2012–2019 (MODIS + VIIRS fire detections). The non-methane organic gas emissions are allocated to the species of several commonly used chemical mechanisms. We compare FINNv2.5 emissions against other widely used fire emissions inventories. The performance of FINNv2.5 emissions as inputs to a chemical transport model is assessed with satellite observations. Uncertainties in the emissions estimates remain, particularly in Africa and South America during August–October and in southeast and equatorial Asia in March and April. Recommendations for future evaluation and use are given.

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  2. Abstract. While camphene is one of the dominant monoterpenesmeasured in biogenic and pyrogenic emission samples, oxidation of camphenehas not been well-studied in environmental chambers and very little is knownabout its potential to form secondary organic aerosol (SOA). The lack ofchamber-derived SOA data for camphene may lead to significant uncertaintiesin predictions of SOA from oxidation of monoterpenes using existingparameterizations when camphene is a significant contributor to totalmonoterpenes. Therefore, to advance the understanding of camphene oxidationand SOA formation and to improve representation of camphene in air qualitymodels, a series of experiments was performed in the University ofCalifornia Riverside environmental chamber to explore camphene SOA massyields and properties across a range of chemical conditions atatmospherically relevant OH concentrations. The experimental results werecompared with modeling simulations obtained using two chemically detailedbox models: Statewide Air Pollution Research Center (SAPRC) and Generatorfor Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A).SOA parameterizations were derived from the chamber data using both thetwo-product and volatility basis set (VBS) approaches. Experiments performedwith added nitrogen oxides (NOx) resulted in higher SOA mass yields (upto 64 %) than experiments performed without added NOx (up to 28 %).In addition, camphene SOA mass yields increased with SOA mass (Mo) atlower mass loadings, but a threshold was reached at higher mass loadings inwhich the SOA mass yields no longer increased with Mo. SAPRC modelingof the chamber studies suggested that the higher SOA mass yields at higherinitial NOx levels were primarily due to higher production of peroxyradicals (RO2) and the generation of highly oxygenated organicmolecules (HOMs) formed through unimolecular RO2 reactions. SAPRCpredicted that in the presence of NOx, camphene RO2 reacts with NOand the resultant RO2 undergoes hydrogen (H)-shift isomerizationreactions; as has been documented previously, such reactions rapidly addoxygen and lead to products with very low volatility (i.e., HOMs). The endproducts formed in the presence of NOx have significantly lowervolatilities, and higher O : C ratios, than those formed by initial campheneRO2 reacting with hydroperoxyl radicals (HO2) or other RO2.Further analysis reveals the existence of an extreme NOx regime, whereinthe SOA mass yield can be suppressed again due to high NO / HO2 ratios.Moreover, particle densities were found to decrease from 1.47 to 1.30 g cm−3 as [HC]0 / [NOx]0 increased and O : C decreased. Theobserved differences in SOA mass yields were largely explained by thegas-phase RO2 chemistry and the competition between RO2+HO2, RO2+ NO, RO2+ RO2, and RO2 autoxidationreactions. 
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  3. Understanding of the fundamental chemical and physical processes that lead to the formation and evolution of secondary organic aerosol (SOA) in the atmosphere has been rapidly advancing over the past decades. Many of these advancements have been achieved through laboratory studies, particularly SOA studies conducted in environmental chambers. Results from such studies are used to develop simplified representations of SOA formation in regional- and global-scale air quality models. Although it is known that there are limitations in the extent to which laboratory experiments can represent the ambient atmosphere, there have been no systematic surveys of what defines atmospheric relevance in the context of SOA formation. In this work, GEOS-Chem version 12.3 was used to quantitatively describe atmospherically relevant ranges of chemical and meteorological parameters critical for predictions of the mass, composition, and physical properties of SOA. For some parameters, atmospherically relevant ranges are generally well represented in laboratory studies. However for other parameters, significant gaps exist between atmospherically relevant ranges and typical laboratory conditions. For example, cold winter (less than 0 °C) and humid (greater than 70% RH) conditions are relatively common on the Earth’s surface but are poorly represented in published chamber data. Furthermore, the overlap in relative humidity and organic aerosol mass between chamber studies and ambient conditions is almost nonexistent. For parameters with significant gaps, extended laboratory studies and/or mechanistic models are needed to bridge these gaps. 
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  4. null (Ed.)
    Abstract. Acid–base clusters and stable salt formation are critical drivers of new particle formation events in the atmosphere. In this study, we explore salt heterodimer (a cluster of one acid and one base) stability as a function of gas-phase acidity, aqueous-phase acidity, heterodimer proton transference, vapor pressure, dipole moment and polarizability for salts comprised of sulfuric acid, methanesulfonic acid and nitric acid with nine bases. The best predictor of heterodimer stability was found to be gas-phase acidity. We then analyzed the relationship between heterodimer stability and J4×4, the theoretically predicted formation rate of a four-acid, four-base cluster, for sulfuric acid salts over a range of monomer concentrations from 105 to 109 molec cm−3 and temperatures from 248 to 348 K and found that heterodimer stability forms a lognormal relationship with J4×4. However, temperature and concentration effects made it difficult to form a predictive expression of J4×4. In order to reduce those effects, heterodimer concentration was calculated from heterodimer stability and yielded an expression for predicting J4×4 for any salt, given approximately equal acid and base monomer concentrations and knowledge of monomer concentration and temperature. This parameterization was tested for the sulfuric acid–ammonia system by comparing the predicted values to experimental data and was found to be accurate within 2 orders of magnitude. We show that one can create a simple parameterization that incorporates the dependence on temperature and monomer concentration on J4×4 by defining a new term that we call the normalized heterodimer concentration, Φ. A plot of J4×4 vs. Φ collapses to a single monotonic curve for weak sulfate salts (difference in gas-phase acidity >95 kcal mol−1) and can be used to accurately estimate J4×4 within 2 orders of magnitude in atmospheric models. 
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  5. Abstract. Camphene, a dominant monoterpene emitted from both biogenic and pyrogenicsources, has been significantly understudied, particularly in regard tosecondary organic aerosol (SOA) formation. When camphene represents asignificant fraction of emissions, the lack of model parameterizations forcamphene can result in inadequate representation of gas-phase chemistry andunderprediction of SOA formation. In this work, the first mechanistic study of SOA formation from camphene was performed using the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A). GECKO-A was used to generate gas-phase chemical mechanisms for camphene and two well-studied monoterpenes, α-pinene and limonene, as well as to predict SOAmass formation and composition based on gas/particle partitioning theory. Themodel simulations represented observed trends in published gas-phase reactionpathways and SOA yields well under chamber-relevant photooxidation and darkozonolysis conditions. For photooxidation conditions, 70 % of thesimulated α-pinene oxidation products remained in the gas phasecompared to 50 % for limonene, supporting model predictions andobservations of limonene having higher SOA yields than α-pinene underequivalent conditions. The top 10 simulated particle-phase products in theα-pinene and limonene simulations represented 37 %–50 % ofthe SOA mass formed and 6 %–27 % of the hydrocarbon mass reacted. Tofacilitate comparison of camphene with α-pinene and limonene, modelsimulations were run under idealized atmospheric conditions, wherein thegas-phase oxidant levels were controlled, and peroxy radicals reacted equallywith HO2 and NO. Metrics for comparison included gas-phasereactivity profiles, time-evolution of SOA mass and yields, andphysicochemical property distributions of gas- and particle-phaseproducts. The controlled-reactivity simulations demonstrated that (1)in the early stages of oxidation, camphene is predicted to form very low-volatility products, lower than α-pinene and limonene, which condenseat low mass loadings; and (2) the final simulated SOA yield for camphene(46 %) was relatively high, in between α-pinene (25 %) andlimonene (74 %). A 50 % α-pinene + 50 % limonene mixture was then used as a surrogate to represent SOA formation from camphene; while simulated SOA mass and yield were well represented, the volatility distribution of the particle-phase products was not. To demonstrate the potential importance of including a parameterized representation of SOA formation by camphene in air quality models, SOA mass and yield were predicted for three wildland fire fuels based on measured monoterpene distributions and published SOA parameterizations for α-pinene and limonene. Using the 50/50 surrogate mixture to represent camphene increased predicted SOA mass by 43 %–50 % for black spruce and by 56 %–108 % for Douglas fir. This first detailed modeling study of the gas-phase oxidation of camphene and subsequent SOA formation highlights opportunities for future measurement–model comparisons and lays a foundation for developing chemical mechanisms and SOA parameterizations for camphene that are suitable for air quality modeling. 
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  6. null (Ed.)
    The basicity constant, or p K b , is an intrinsic physical property of bases that gives a measure of its proton affinity in macroscopic environments. While the p K b is typically defined in reference to the bulk aqueous phase, several studies have suggested that this value can differ significantly at the air–water interface (which can have significant ramifications for particle surface chemistry and aerosol growth modeling). To provide mechanistic insight into surface proton affinity, we carried out ab initio metadynamics calculations to (1) explore the free-energy profile of dimethylamine and (2) provide reasonable estimates of the p K b value in different solvent environments. We find that the free-energy profiles obtained with our metadynamics calculations show a dramatic variation, with interfacial aqueous dimethylamine p K b values being significantly lower than in the bulk aqueous environment. Furthermore, our metadynamics calculations indicate that these variations are due to reduced hydrogen bonding at the air–water surface. Taken together, our quantum mechanical metadynamics calculations show that the reactivity of dimethylamine is surprisingly complex, leading to p K b variations that critically depend on the different atomic interactions occurring at the microscopic molecular level. 
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  7. null (Ed.)