Experimental Determination of the Relationship Between Organic Aerosol Viscosity and Ice Nucleation at Upper Free Tropospheric Conditions

Abstract

Cirrus clouds play an important role in Earth's energy budget by reflecting the incoming solar radiation into space which leads to cooling, and by reducing the infrared longwave coming from the Earth surface which leads to warming (Chen et al., 2000;Heymsfield et al., 2017;Storelvmo & Herger, 2014). The radiative properties of cirrus clouds depend on the ice crystal number concentration, which in turn depends on the aerosol that nucleate ice at these conditions. Cirrus cloud ice crystals form either through homogeneous processes, requiring no ice nucleating particle (INP) or active site, or heterogeneously where an INP is involved (Hoose & Möhler, 2012;Kanji et al., 2017;Vali et al., 2015). Heterogeneous ice nucleation (IN) can occur via deposition of ice on particle surfaces, condensation in pores and subsequent freezing (Campbell et al., 2017;David et al., 2019;Marcolli, 2014), through immersion freezing of droplets as they cool, or condensation freezing as particles simultaneously uptake water and cool. Another freezing path is contact freezing where a particle approaches the air-water interface from either the outside of the droplet or from inside of the droplet (Durant & Shaw, 2005;Kanji et al., 2017). Homogeneous freezing of liquid aerosol is predominantly controlled by water activity (or ambient relative humidity (RH)), temperature, particle size, and hygroscopicity (Baumgartner et al., 2022;Koop et al., 2000;Schneider et al., 2021). Among these, temperature and RH dominate. Chemical composition influences homogeneous IN by modulating hygroscopicity (Junge, 1953;M. D. Petters & Kreidenweis, 2007). However, the overall influence of chemical composition on homogeneous freezing nucleation is small (Kreidenweis et al., 2009). In contrast, heterogeneous IN strongly depends on the chemical composition and morphology (Hiranuma et al., 2014). Ambient measurements of cirrus ice crystal residuals show a wide range of chemical composition, including organic particles, black carbon, mineral dust, lead and other metal bearing particles, sulfate particles, and salt particles

Viscosity, Glass Formation, and Mixing Times within Secondary Organic Aerosol from Biomass Burning Phenolics

“…For example, the long-range transport of pollutants depends on SOA viscosity. − In addition, predictions of gas-particle partitioning (and hence, aerosol mass and size distributions) depend on the viscosity of SOA. − Similarly, heterogeneous chemical reactions ,− and photochemical reactions , in SOA depend on the mobility and uptake of reactants and products, which can be inhibited at high viscosity. Furthermore, some studies suggest that SOA particles in a glassy phase state (η > 10 12 Pa s) might be good heterogeneous ice nuclei and thus affect the microphysical and radiative properties of clouds. − However, this is an area of active debate, and a recent study reported that heterogeneous ice formation on glassy SOA may be uncommon for tropospheric conditions …”

“…52−58 However, this is an area of active debate, and a recent study reported that heterogeneous ice formation on glassy SOA may be uncommon for tropospheric conditions. 59 In addition to the gaseous components that can lead to SOA formation, biomass burning also directly emits organic aerosols into the atmosphere, referred to as biomass burning primary organic aerosol (BB-POA). The viscosity of BB-POA from the pyrolysis of pine wood has been studied previously and was found to depend strongly on relative humidity (RH) and temperature.…”

Direct measurement of the viscosity of ternary aerosol mixtures

“…However, we note that dicarboxylic acids and secondary organic aerosol generated from oxidation ow reactors have shown evaporation just a few degrees warmer than the transition temperature. 15,20,27 A related question is whether particles achieve equilibrium with respect to viscosity during transit in the thermal conditioner. Modelling studies with the KM-GAP model suggest that equilibration timescales for sucrose and citric acid are <11 s for the 10 6 Pa s phase transition, if the phase transition occurs at T > ∼ −25 °C.…”

“…7 This viscosity typically occurs at temperatures ∼20-30 K warmer than the glass transition temperature for complex organic mixtures. 20 For reference the diffusion coefficient implies an intraparticle mixing time of ∼9.6 hours for 300 nm particles and ∼5.8 min for 30 nm particles. Zhang et al, 16 Järvinen et al, 14 and Bell et al 17 measured the sintering of agglomerates formed by natural coagulation.…”

“…21,22 Applications using the dual tandem DMA have thus been limited to particles generated by atomization 15,[23][24][25] and secondary organic aerosols generated from oxidation ow reactors. 20,26,27 Measurements of ambient particle sources or particles generated inside an environmental chamber have not been possible. Online methods that work at low number concentration and with non-agglomerated particles are not yet available.…”