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Secondary organic aerosol (SOA) plays a critical, yet uncertain, role in air quality and climate. Once formed, SOA is transported throughout the atmosphere and is exposed to solar UV light. Information on the viscosity of SOA, and how it may change with solar UV exposure, is needed to accurately predict air quality and climate. However, the effect of solar UV radiation on the viscosity of SOA and the associated implications for air quality and climate predictions is largely unknown. Here, we report the viscosity of SOA after exposure to UV radiation, equivalent to a UV exposure of 6 to 14 d at midlatitudes in summer. Surprisingly, UV-aging led to as much as five orders of magnitude increase in viscosity compared to unirradiated SOA. This increase in viscosity can be rationalized in part by an increase in molecular mass and oxidation of organic molecules constituting the SOA material, as determined by high-resolution mass spectrometry. We demonstrate that UV-aging can lead to an increased abundance of aerosols in the atmosphere in a glassy solid state. Therefore, UV-aging could represent an unrecognized source of nuclei for ice clouds in the atmosphere, with important implications for Earth’s energy budget. We also show that UV-aging increases the mixing times within SOA particles by up to five orders of magnitude throughout the troposphere with important implications for predicting the growth, evaporation, and size distribution of SOA, and hence, air pollution and climate. 

Biomass burning organic aerosol (BBOA) in the atmosphere contains many compounds that absorb solar radiation, called brown carbon (BrC). While BBOA is in the atmosphere, BrC can undergo reactions with oxidants such as ozone which decrease absorbance, or whiten. The effect of temperature and relative humidity (RH) on whitening has not been well constrained, leading to uncertainties when predicting the direct radiative effect of BrC on climate. Using an aerosol flow-tube reactor, we show that the whitening of BBOA by oxidation with ozone is strongly dependent on RH and temperature. Using a poke-flow technique, we show that the viscosity of BBOA also depends strongly on these conditions. The measured whitening rate of BrC is described well with the viscosity data, assuming that the whitening is due to oxidation occurring in the bulk of the BBOA, within a thin shell beneath the surface. Using our combined datasets, we developed a kinetic model of this whitening process, and we show that the lifetime of BrC is 1 d or less below ∼1 km in altitude in the atmosphere but is often much longer than 1 d above this altitude. Including this altitude dependence of the whitening rate in a chemical transport model causes a large change in the predicted warming effect of BBOA on climate. Overall, the results illustrate that RH and temperature need to be considered to understand the role of BBOA in the atmosphere. 

Individual atmospheric particles can contain mixtures of primary organic aerosol (POA), secondary organic aerosol (SOA), and secondary inorganic aerosol (SIA). To predict the role of such complex multicomponent particles in air quality and climate, information on the number and types of phases present in the particles is needed. However, the phase behavior of such particles has not been studied in the laboratory, and as a result, remains poorly constrained. Here, we show that POA+SOA+SIA particles can contain three distinct liquid phases: a low-polarity organic-rich phase, a higher-polarity organic-rich phase, and an aqueous inorganic-rich phase. Based on our results, when the elemental oxygen-to-carbon (O:C) ratio of the SOA is less than 0.8, three liquid phases can coexist within the same particle over a wide relative humidity range. In contrast, when the O:C ratio of the SOA is greater than 0.8, three phases will not form. We also demonstrate, using thermodynamic and kinetic modeling, that the presence of three liquid phases in such particles impacts their equilibration timescale with the surrounding gas phase. Three phases will likely also impact their ability to act as nuclei for liquid cloud droplets, the reactivity of these particles, and the mechanism of SOA formation and growth in the atmosphere. These observations provide fundamental information necessary for improved predictions of air quality and aerosol indirect effects on climate. 

Abstract. Secondary organic aerosol (SOA) constitutes a largefraction of atmospheric aerosol. To assess its impacts on climate and airpollution, knowledge of the number of phases in internal mixtures ofdifferent SOA types is required. Atmospheric models often assume thatdifferent SOA types form a single phase when mixed. Here, we present visualobservations of the number of phases formed after mixing differentanthropogenic and biogenic SOA types. Mixing SOA types generated inenvironmental chambers with oxygen-to-carbon (O/C) ratios between 0.34 and 1.05, we found 6 out of 15 mixtures of two SOA types to result in two phase particles. We demonstrate that the number of phases depends on thedifference in the average O/C ratio between the two SOA types (Δ(O/C)). Using a threshold Δ(O/C) of 0.47, we can predict the phasebehavior of over 90 % of our mixtures, with one- and two-phase particlespredicted for Δ(O/C)<0.47 and Δ(O/C)≥0.47,respectively. This threshold ΔO/C value provides a simple parameterto predict whether mixtures of fresh and aged SOA form one- or two-phase particles in the atmosphere. In addition, we show that phase-separated SOAparticles form when mixtures of volatile organic compounds emitted from realtrees are oxidized. 

Molecular composition, viscosity, and liquid–liquid phase separation (LLPS) were investigated for secondary organic aerosol (SOA) derived from synthetic mixtures of volatile organic compounds (VOCs) representing emission profiles for Scots pine trees under healthy and aphid-herbivory stress conditions. Model “healthy plant SOA” and “stressed plant SOA” were generated in a 5 m 3 environmental smog chamber by photooxidation of the mixtures at 50% relative humidity (RH). SOA from photooxidation of α-pinene was also prepared for comparison. Molecular composition was determined with high resolution mass spectrometry, viscosity was determined with the poke-flow technique, and liquid–liquid phase separation was investigated with optical microscopy. The stressed plant SOA had increased abundance of higher molecular weight species, reflecting a greater fraction of sesquiterpenes in the stressed VOC mixture compared to the healthy plant VOC mixture. LLPS occurred in both the healthy and stressed plant SOA; however, stressed plant SOA exhibited phase separation over a broader humidity range than healthy plant SOA, with LLPS persisting down to 23 ± 11% RH. At RH ≤25%, both stressed and healthy plant SOA viscosity exceeded 10 8 Pa s, a value similar to that of tar pitch. At 40% and 50% RH, stressed plant SOA had the highest viscosity, followed by healthy plant SOA and then α-pinene SOA in descending order. The observed peak abundances in the mass spectra were also used to estimate the SOA viscosity as a function of RH and volatility. The predicted viscosity of the healthy plant SOA was lower than that of the stressed plant SOA driven by both the higher glass transition temperatures and lower hygroscopicity of the organic molecules making up stressed plant SOA. These findings suggest that plant stress influences the physicochemical properties of biogenic SOA. Furthermore, a complex mixture of VOCs resulted in a higher SOA viscosity compared to SOA generated from α-pinene alone at ≥25% RH, highlighting the importance of studying properties of SOA generated from more realistic multi-component VOC mixtures. 

Abstract. Information on liquid–liquid phase separation (LLPS) and viscosity (ordiffusion) within secondary organic aerosol (SOA) is needed to improvepredictions of particle size, mass, reactivity, and cloud nucleatingproperties in the atmosphere. Here we report on LLPS and viscosities withinSOA generated by the photooxidation of diesel fuel vapors. Diesel fuelcontains a wide range of volatile organic compounds, and SOA generated bythe photooxidation of diesel fuel vapors may be a good proxy for SOA fromanthropogenic emissions. In our experiments, LLPS occurred over the relativehumidity (RH) range of ∼70 % to ∼100 %,resulting in an organic-rich outer phase and a water-rich inner phase. Theseresults may have implications for predicting the cloud nucleating propertiesof anthropogenic SOA since the presence of an organic-rich outer phase athigh-RH values can lower the supersaturation with respect to water requiredfor cloud droplet formation. At ≤10 % RH, the viscosity was ≥1×108 Pa s, which corresponds to roughly the viscosity of tarpitch. At 38 %–50 % RH, the viscosity was in the range of 1×108 to 3×105 Pa s. These measured viscosities areconsistent with predictions based on oxygen to carbon elemental ratio (O:C)and molar mass as well as predictions based on the number of carbon,hydrogen, and oxygen atoms. Based on the measured viscosities and theStokes–Einstein relation, at ≤10 % RH diffusion coefficients oforganics within diesel fuel SOA is ≤5.4×10-17 cm2 s−1 and the mixing time of organics within 200 nm diesel fuel SOAparticles (τmixing) is 50 h. These small diffusion coefficientsand large mixing times may be important in laboratory experiments, where SOAis often generated and studied using low-RH conditions and on timescales ofminutes to hours. At 38 %–50 % RH, the calculated organic diffusioncoefficients are in the range of 5.4×10-17 to 1.8×10-13 cm2 s−1 and calculated τmixing values arein the range of ∼0.01 h to ∼50 h. These valuesprovide important constraints for the physicochemical properties ofanthropogenic SOA.