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  1. Abstract. Organic nitrate (RONO2) formation in the atmosphere represents a sink of NOx(NOx = NO + NO2) and termination of the NOx/HOx(HOx = HO2 + OH) ozone formation and radical propagation cycles, can act as a NOx reservoirtransporting reactive nitrogen, and contributes to secondary organic aerosol formation. While some fraction of RONO2 is thought to reside in the particle phase, particle-phase organic nitrates (pRONO2) are infrequently measured and thus poorly understood. There is anincreasing prevalence of aerosol mass spectrometer (AMS) instruments, which have shown promise for determining the quantitative total organic nitratefunctional group contribution to aerosols. A simple approach that relies on the relative intensities of NO+ and NO2+ ions inthe AMS spectrum, the calibrated NOx+ ratio for NH4NO3, and the inferred ratio for pRONO2 hasbeen proposed as a way to apportion the total nitrate signal to NH4NO3 and pRONO2. This method is increasingly beingapplied to field and laboratory data. However, the methods applied have been largely inconsistent and poorly characterized, and, therefore, adetailed evaluation is timely. Here, we compile an extensive survey of NOx+ ratios measured for variouspRONO2 compounds and mixtures from multiple AMS instruments, groups, and laboratory and field measurements. All data and analysispresented here are for use with the standard AMS vaporizer. We show that, in the absence of pRONO2 standards, thepRONO2 NOx+ ratio can be estimated using a ratio referenced to the calibrated NH4NO3 ratio, aso-called “Ratio-of-Ratios” method (RoR = 2.75 ± 0.41). We systematically explore the basis for quantifyingpRONO2 (and NH4NO3) with the RoR method using ground and aircraft field measurements conducted over a largerange of conditions. The method is compared to another AMS method (positive matrix factorization, PMF) and other pRONO2 andrelated (e.g., total gas + particle RONO2) measurements, generally showing good agreement/correlation. A broad survey of ground andaircraft AMS measurements shows a pervasive trend of higher fractional contribution of pRONO2 to total nitrate with lower totalnitrate concentrations, which generally corresponds to shifts from urban-influenced to rural/remote regions. Compared to ground campaigns,observations from all aircraft campaigns showed substantially lower pRONO2 contributions at midranges of total nitrate(0.01–0.1 up to 2–5 µg m−3), suggesting that the balance of effects controlling NH4NO3 and pRONO2formation and lifetimes – such as higher humidity, lower temperatures, greater dilution, different sources, higher particle acidity, andpRONO2 hydrolysis (possibly accelerated by particle acidity) – favors lower pRONO2 contributions for thoseenvironments and altitudes sampled. 
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  4. Abstract. Glyoxal (CHOCHO), the simplest dicarbonyl in thetroposphere, is a potential precursor for secondary organic aerosol (SOA)and brown carbon (BrC) affecting air quality and climate. The airbornemeasurement of CHOCHO concentrations during the KORUS-AQ (KORea–US AirQuality study) campaign in 2016 enables detailed quantification of lossmechanisms pertaining to SOA formation in the real atmosphere. Theproduction of this molecule was mainly from oxidation of aromatics (59 %)initiated by hydroxyl radical (OH). CHOCHO loss to aerosol was found to bethe most important removal path (69 %) and contributed to roughly∼ 20 % (3.7 µg sm−3 ppmv−1 h−1,normalized with excess CO) of SOA growth in the first 6 h in SeoulMetropolitan Area. A reactive uptake coefficient (γ) of∼ 0.008 best represents the loss of CHOCHO by surface uptakeduring the campaign. To our knowledge, we show the first field observationof aerosol surface-area-dependent (Asurf) CHOCHO uptake, which divergesfrom the simple surface uptake assumption as Asurf increases in ambientcondition. Specifically, under the low (high) aerosol loading, the CHOCHOeffective uptake rate coefficient, keff,uptake, linearly increases(levels off) with Asurf; thus, the irreversible surface uptake is areasonable (unreasonable) approximation for simulating CHOCHO loss toaerosol. Dependence on photochemical impact and changes in the chemical andphysical aerosol properties “free water”, as well as aerosol viscosity,are discussed as other possible factors influencing CHOCHO uptake rate. Ourinferred Henry's law coefficient of CHOCHO, 7.0×108 M atm−1, is ∼ 2 orders of magnitude higher than thoseestimated from salting-in effects constrained by inorganic salts onlyconsistent with laboratory findings that show similar high partitioning intowater-soluble organics, which urges more understanding on CHOCHO solubilityunder real atmospheric conditions. 
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  5. 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. 
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