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Abstract. OH reactivity (OHR) is an important control on the oxidative capacity in the atmosphere but remains poorly constrained in many environments, such asremote, rural, and urban atmospheres, as well as laboratory experiment setups under low-NO conditions. For an improved understanding of OHR, itsevolution during oxidation of volatile organic compounds (VOCs) is a major aspect requiring better quantification. We use the fully explicitGenerator of Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) model to study the OHR evolution in the NO-free photooxidationof several VOCs, including decane (an alkane), m-xylene (an aromatic), and isoprene (an alkene). Oxidation progressively produces more saturated and functionalized species. Total organic OHR (including precursor and products, OHRVOC) first increases for decane (as functionalization increases OH rate coefficients) and m-xylene (as much more reactive oxygenated alkenes are formed). For isoprene, C=C bond consumption leads to a rapid drop in OHRVOC before significant production of the first main saturated multifunctional product, i.e., isoprene epoxydiol. The saturated multifunctional species in the oxidation of different precursors have similar average OHRVOC per C atom. The latter oxidation follows a similar course for different precursors, involving fragmentation of multifunctional species to eventual oxidation of C1 and C2 fragments to CO2, leading to a similar evolution of OHRVOC per C atom. An upper limit of the total OH consumption during complete oxidation to CO2 is roughly three per C atom. We also explore the trends in radical recycling ratios. We show that differences in the evolution of OHRVOC between the atmosphere and an environmental chamber, and between the atmosphere and an oxidation flow reactor (OFR), can be substantial, with the former being even larger, but these differences are often smaller than between precursors. The Teflon wall losses of oxygenated VOCs in chambers result in large deviations of OHRVOC from atmospheric conditions, especially for the oxidation of larger precursors, where multifunctional species may suffer substantial wall losses, resulting in significant underestimation of OHRVOC. For OFR, the deviations of OHRVOC evolution from the atmospheric case are mainly due to significant OHR contribution from RO2 and lack of efficient organic photolysis. The former can be avoided by lowering the UV lamp setting in OFR, while the latter is shown to be very difficult to avoid. However, the former may significantly offset the slowdown in fragmentation of multifunctional species due to lack of efficient organic photolysis. 

Abstract. Volatility and viscosity are important properties of organic aerosols (OA),affecting aerosol processes such as formation, evolution, and partitioning ofOA. Volatility distributions of ambient OA particles have often beenmeasured, while viscosity measurements are scarce. We have previouslydeveloped a method to estimate the glass transition temperature (Tg) ofan organic compound containing carbon, hydrogen, and oxygen. Based onanalysis of over 2400 organic compounds including oxygenated organiccompounds, as well as nitrogen- and sulfur-containing organic compounds, weextend this method to include nitrogen- and sulfur-containing compoundsbased on elemental composition. In addition, parameterizations are developedto predict Tg as a function of volatility and the atomicoxygen-to-carbon ratio based on a negative correlation between Tg andvolatility. This prediction method of Tg is applied to ambientobservations of volatility distributions at 11 field sites. Thepredicted Tg values of OA under dry conditions vary mainly from 290 to 339 Kand the predicted viscosities are consistent with the results of ambientparticle-phase-state measurements in the southeastern US and the Amazonianrain forest. Reducing the uncertainties in measured volatility distributionswould improve predictions of viscosity, especially at low relative humidity.We also predict the Tg of OA components identified via positive matrixfactorization of aerosol mass spectrometer (AMS) data. The predicted viscosity ofoxidized OA is consistent with previously reported viscosity of secondary organic aerosols (SOA) derivedfrom α-pinene, toluene, isoprene epoxydiol (IEPOX), and diesel fuel.Comparison of the predicted viscosity based on the observed volatilitydistributions with the viscosity simulated by a chemical transport modelimplies that missing low volatility compounds in a global model can lead tounderestimation of OA viscosity at some sites. The relation betweenvolatility and viscosity can be applied in the molecular corridor orvolatility basis set approaches to improve OA simulations in chemicaltransport models by consideration of effects of particle viscosity in OAformation and evolution. 

Abstract To quantify the volatility of organic aerosols (OA), a comprehensive campaign was conducted in the Chinese megacity. Volatility distributions of OA and particle‐phase organic nitrate (pON) were estimated based on five methods: (a) empirical method and (b) kinetic model based on the measurement of a thermodenuder (TD) coupled with an aerosol mass spectrometer; (c) Formula‐based SIMPOL model‐driven method; (d) Element‐based estimations using molecular formula measurements of OA; and (e) gas/particle partitioning. Our results demonstrate that the ambient OA volatility distribution shows good agreement between the two heating methods and the formula‐based method when assuming ambient OA was mainly composed of organic nitrate (pON), organic sulfate and acid groups using the SIMPOL model. However, the element‐based method tends to overestimate the volatility of OA compared to the above three methods, suggesting large uncertainties in the parameterizations or in the representativeness of the molecular measurements that need further refinement. The volatility of ambient OA is generally lower than that of the laboratory‐derived secondary OA, emphasizing the impact of aging. A large fraction at the higher and lower volatility ranges (approximately logC* ≤ −9 and ≥2 μg m⁻³) was found for pON, implying the importance of both extremely low volatile and semi‐volatile species. Overall, this study evaluates different methods for volatility estimation and gives new insight into the volatility of OA and pON in urban areas.