10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Secondary Organic Aerosol Yield, Volatility, and Viscosity from Smog Chamber and Flow Reactor Experiments
WYATT CHAMPION, Sarah Suda Petters, Nicholas Rothfuss, Markus Petters, Andrew Grieshop, North Carolina State University
Abstract Number: 1653 Working Group: Aerosol Chemistry
Abstract Vapor pressure and viscosity of pure organic compounds are controlled by molecular weight and functional group composition. Secondary organic aerosols (SOA) can be mixtures of thousands of compounds. Although it is not obvious that the pure component relationship applies to complex mixtures, one would expect experimental measures of aerosol yield, volatility, and viscosity to relate to experimental measures of chemical composition. These parameters have substantial uncertainties to which laboratory SOA generation systems contribute. Yield experiments utilizing smog chambers introduce bias due to vapor wall losses, and extended aging is difficult due to relatively low oxidant exposures. Yield experiments with flow reactors utilize high oxidant concentrations that may promote fragmentation reactions as opposed to functionalization (leading to higher volatility products and lower yield) (Bruns et al., 2015), and low residence times that may not allow slower processes to occur (e.g., condensation). Here, we apply a combination of methods to probe the ability of each to give insight into the properties of SOA formed from alpha-pinene (α-pinene), and possibly other well-studied precursors, in several laboratory reactors. Objectives of this study are to: 1) compare volatility and viscosity properties of SOA formed in flow reactors to that produced in ‘traditional’ chamber experiments, and 2) explore relationships between these fundamental aerosol properties.
Organic aerosol mass concentration and chemical composition are measured using an Aerosol Chemical Speciation Monitor. Flow reactor experiments utilize both pure ozonolysis and oxidation driven by low-pressure UV lamps emitting at 254 and 185 nm (OFR-185) oxidation regimes. Smog chamber ozonolysis experiments are also conducted for the same precursors (e.g., α-pinene, naphthalene). For all systems, aerosol yield is quantified using traditional methods. Additionally, volatility distributions are parametrized with the dual thermodenuder (2TD) approach (Saha et al., 2015) in which varied heating perturbations constrain the thermodynamic (heat of vaporization ΔHvap, and saturation concentration, C*) and kinetic (evaporation coefficient, γe) properties dictating aerosol volatility. Temperature and RH dependency of viscosity is inferred from the coalescence time scale of synthesized dimers using two differential mobility analyzers of opposite polarity.
Volatility results from 2TD (25-180°C) experiments indicate a substantial presence (>20%) of very low volatility organics (LVOC) in SOA, and evaporation kinetics consistent with moderate kinetic limitation to evaporation (evaporation coefficient ~ 0.1).
Observations of room-temperature dilution-driven evaporation are consistent with the 2TD-derived volatility parameterizations. These distributions are also consistent with others’ observations of higher yields and prevalence of LVOCs via chemical analysis. Coalescence experiment results show that the temperature dependence of dry secondary organic aerosol viscosity is similar to that of coal tar pitch, citric acid, and sorbitol. Glass transition temperatures range from –10 to 20°C. Strong correlations of viscosity with O:C and H:C indicate that viscosity is sensitive to changes in composition.
Relationships between oxidation state, yield, volatility distributions, temperature-dependent aerosol viscosity, and enthalpy of vaporization will be explored for single precursor systems at varying levels of oxidation using an oxidation flow reactor at low and high OH exposures. Similarly, the same relationships are explored across systems from different precursors and using different oxidation methods. This work will ultimately allow for more representative estimates of atmospheric organic aerosols in chemical transport models by providing a more robust representation of physicochemical properties of the particles.
References: [1] Bruns, E. A., et al. Inter-Comparison of Laboratory Smog Chamber and Flow Reactor Systems on Organic Aerosol Yield and Composition. Atmos. Meas. Tech. 2015, 8 (6). [2] Saha, P. K., et al. Determining Aerosol Volatility Parameters Using A “dual Thermodenuder” System: Application to Laboratory-Generated Organic Aerosols. Aerosol Sci. Technol. 2015, 49 (8).