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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 amore »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.« less

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 humiditymore »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.« less

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.

Abstract Wintertime episodes of high aerosol concentrations occur frequently in urban and agricultural basins and valleys worldwide. These episodes often arise following development of persistent cold-air pools (PCAPs) that limit mixing and modify chemistry. While field campaigns targeting either basin meteorology or wintertime pollution chemistry have been conducted, coupling between interconnected chemical and meteorological processes remains an insufficiently studied research area. Gaps in understanding the coupled chemical-meteorological interactions that drive high pollution events make identification of the most effective air-basin specific emission control strategies challenging. To address this, a September 2019 workshop occurred with the goal of planning a future research campaign to investigate air quality in Western U.S. basins. Approximately 120 people participated, representing 50 institutions and 5 countries. Workshop participants outlined the rationale and design for a comprehensive wintertime study that would couple atmospheric chemistry and boundary-layer and complex-terrain meteorology within western U.S. basins. Participants concluded the study should focus on two regions with contrasting aerosol chemistry: three populated valleys within Utah (Salt Lake, Utah, and Cache Valleys) and the San Joaquin Valley in California. This paper describes the scientific rationale for a campaign that will acquire chemical and meteorological datasets using airborne platforms with extensive range, coupledmore »to surface-based measurements focusing on sampling within the near-surface boundary layer, and transport and mixing processes within this layer, with high vertical resolution at a number of representative sites. No prior wintertime basin-focused campaign has provided the breadth of observations necessary to characterize the meteorological-chemical linkages outlined here, nor to validate complex processes within coupled atmosphere-chemistry models.« less

ABSTRACT To explore the various couplings across space and time and between ecosystems in a consistent manner, atmospheric modeling is moving away from the fractured limited-scale modeling strategy of the past toward a unification of the range of scales inherent in the Earth system. This paper describes the forward-looking Multi-Scale Infrastructure for Chemistry and Aerosols (MUSICA), which is intended to become the next-generation community infrastructure for research involving atmospheric chemistry and aerosols. MUSICA will be developed collaboratively by the National Center for Atmospheric Research (NCAR) and university and government researchers, with the goal of serving the international research and applications communities. The capability of unifying various spatiotemporal scales, coupling to other Earth system components, and process-level modularization will allow advances in both fundamental and applied research in atmospheric composition, air quality, and climate and is also envisioned to become a platform that addresses the needs of policy makers and stakeholders.

Abstract. Western US wildlands experience frequent and large-scale wildfires which arepredicted to increase in the future. As a result, wildfire smoke emissionsare expected to play an increasing role in atmospheric chemistry whilenegatively impacting regional air quality and human health. Understanding theimpacts of smoke on the environment is informed by identifying andquantifying the chemical compounds that are emitted during wildfires and byproviding empirical relationships that describe how the amount andcomposition of the emissions change based upon different fire conditions andfuels. This study examined particulate organic compounds emitted from burningcommon western US wildland fuels at the US Forest Service Fire ScienceLaboratory. Thousands of intermediate and semi-volatile organic compounds(I/SVOCs) were separated and quantified into fire-integrated emission factors(EFs) using a thermal desorption, two-dimensional gas chromatograph withonline derivatization coupled to an electron ionization/vacuum ultraviolethigh-resolution time-of-flight mass spectrometer(TD-GC × GC-EI/VUV-HRToFMS). Mass spectra, EFs as a function ofmodified combustion efficiency (MCE), fuel source, and other definingcharacteristics for the separated compounds are provided in the accompanyingmass spectral library. Results show that EFs for total organic carbon (OC),chemical families of I/SVOCs, and most individual I/SVOCs span 2–5 orders ofmagnitude, with higher EFs at smoldering conditions (low MCE) than flaming.Logarithmic fits applied to the observations showed that log (EFs) forparticulate organic compoundsmore »were inversely proportional to MCE. Thesemeasurements and relationships provide useful estimates of EFs for OC,elemental carbon (EC), organic chemical families, and individual I/SVOCs as afunction of fire conditions.« less