Condensed-phase biogenic–anthropogenic interactions with implications for cold cloud formation

Abstract

Anthropogenic and biogenic gas emissions contribute to the formation of secondary organic aerosol (SOA). When present, soot particles from fossil fuel combustion can acquire a coating of SOA. We investigate SOA-soot biogenic-anthropogenic interactions and their impact on ice nucleation in relation to the particles' organic phase state. SOA particles were generated from the OH oxidation of naphthalene, a-pinene, longifolene, or isoprene, with or without the presence of sulfate or soot particles. Corresponding particle glass transition (T g ) and full deliquescence relative humidity (FDRH) were estimated using a numerical diffusion model. Longifolene SOA particles are solid-like and all biogenic SOA sulfate mixtures exhibit a core-shell configuration (i.e. a sulfate-rich core coated with SOA). Biogenic SOA with or without sulfate formed ice at conditions expected for homogeneous ice nucleation, in agreement with respective T g and FDRH. a-pinene SOA coated soot particles nucleated ice above the homogeneous freezing temperature with soot acting as ice nuclei (IN). At lower temperatures the a-pinene SOA coating can be semisolid, inducing ice nucleation. Naphthalene SOA coated soot particles acted as ice nuclei above and below the homogeneous freezing limit, which can be explained by the presence of a highly viscous SOA phase. Our results suggest that biogenic SOA does not play a significant This journal is © The Royal Society of Chemistry 2017Faraday Discuss. Faraday Discussions View Article OnlineView Journal role in mixed-phase cloud formation and the presence of sulfate renders this even less likely. However, anthropogenic SOA may have an enhancing effect on cloud glaciation under mixed-phase and cirrus cloud conditions compared to biogenic SOA that dominate during pre-industrial times or in pristine areas.

Temperature‐ and Humidity‐Dependent Phase States of Secondary Organic Aerosols

“…However, α-pinene-derived SOA viscosities vary greatly between studies, and this is discussed below. The diversity of results is also apparent with ice nucleation measurements for SOA at cirrus temperatures (Charnawskas et al, 2017;Ignatius et al, 2016;Wagner et al, 2017), which is thought to be controlled in part by viscosity (e.g., Baustian et al, 2013;Berkemeier et al, 2014;Murray et al, 2010). The wide range of results for SOA from one precursor complicates comparisons between precursors.…”

Indirect Effects of Secondary Organic Aerosol on Cirrus Clouds

“…The calculation of the radiative effect of SOA on cirrus presented here is limited by the knowledge of the ability of SOA to act as an INP. The temperature at which SOA‐containing particles form ice is uncertain since it is probably dependent on the chemical composition, particle size, and mixing state of SOA (Baustian et al, 2013; Berkemeier et al, 2014; Charnawskas et al, 2017), which were not considered in the model. A better quantification of the role of different organic functional groups in ice formation is needed to improve the estimation of the number concentration of INPs from SOA and its radiative effects.…”

Modelling the effect of condensed-phase diffusion on the homogeneous nucleation of ice in supercooled water

“…However, recent work has highlighted that anthropogenic sources also produce highly viscous aerosols that exist at much higher temperatures and relative humidities (Reid et al, 2018), which could have implications under the conditions of mixed phase cloud formation (Charnawskas et al, 2017). Then there are more complex multi-component systems to consider, containing a mixture of SOA and inorganic salts have been observed to form liquid-liquid phase separations, which can effect cloud particle 16…”